Aluminum alloys for electrical engineering: a review

Abstract

High-performance conductors are essential for economically and environmentally sustainable ways of electricity transfer in modern infrastructure, manufacturing and transportation, including electric vehicles. This report reviews the aluminum conductors, their fundamentals, classification and utilization markets, focusing on metallurgical characteristics of present commercial solutions and the strategy of future development directions. The inherent features of aluminum, both beneficial and detrimental, for electrical engineering are emphasized along with alloying concepts that provide the accelerated decomposition of matrix solid solution to minimize the electron scattering. Development activities are assessed of new generation of aluminum conductors that in addition to alloying utilize novel processing techniques such as ultra-fast crystallization, severe plastic deformation and complex thermomechanical treatments aiming at grain reduction to nanometer scale, crystallographic texture control and grain boundary engineering. Transition metals and rare earths are considered as the promising alloying candidates for high-strength conductors having superior thermal stability with extra importance given to immiscible systems of Al–Ce, Al–La and Al–Y along with multiply additions, combined to generate the synergy effects. The composites with cladding configuration and particulate reinforcement including via carbon-type strengtheners are discussed as the effective solutions of advanced conductors. A variety of strategies that aim at overcoming the strength–conductivity trade-off in conductor materials are presented throughout the report.

Graphical abstract

Introduction

Electricity becomes the major energy market with power demand expected to double every ten years to support the road transport electrification and green hydrogen production [1]. For global consumption of over 20 trillion kWh per year [2], the high-efficiency conductors are paramount to improving the transmission/distribution of electricity that enable the clean energy transition, provide enormous energy savings and reduce harmful emissions. Thus, to minimize the resistance-related energy losses, estimated at about 7%, there is a continuous search for advanced room-temperature conductors that would outperform the present solutions [3, 4].

Although electrical engineering utilizes several metal conductors, two most common ones remain copper and aluminum. Despite a long history of copper as the metal of choice for conducting electricity, aluminum has strategic advantages that secure its usage in high-volume applications and continuously attract new opportunities. Aluminum is essential for the wire and cable industries, being most widely used for electricity transmission and distribution that reached 4.2 and 5.0 million metric tons, respectively, that totals to over 14% of 64.2 million metric tons of aluminum consumed globally in 2021 [5]. According to the American Electric Power (AEP) transmission data, over 320,000 km (200,000 miles) of transmission lines will need replacement over the next 10 years across North American Electric Reliability Corporation (NERC) regions [6]. The ability to upgrade the existing conductors of the transmission network and raise loading limits transforms the grid into more resilient one, increases transmission capacity, thus eliminates congestion and enables the cost-effective integration of clean energy [7].

Aluminum is two–three times less expensive and three orders of magnitude more abundant in Earth crust than copper. Having 61% of copper conductivity but 30% of its specific weight, aluminum conductor is about 50% lighter than copper with the equivalent electrical resistance. It is, therefore, advantageous in applications where the conductor weight is of primary importance. Lightweighting contributes to aluminum considerations for next generation wiring in electric vehicles. Automotive wiring harness market was worth USD 50.2 billion in 2021 and its predicted compound annual growth rate (CAGR) is 5.7% till 2029 [8]. It is anticipated that for electric vehicles the change from heavy copper to the less expensive and lighter aluminum is a viable option and for a typical passenger car may reduce the wiring weight from 25 to 10 kg [9]. At present, there are commercially available aluminum cables, designed specifically to handle the higher voltage and electrical currents required in hybrid and electric vehicles [10] and replacing the copper with aluminum winding in automotive scale electric motors is researched [11]. The novel application opportunities are created by certain components of electric vehicles such as traction motor, rotor or inverter where aluminum castings with high conductivity are required for the equipment performance [12].

This report provides the comprehensive review of aluminum alloys destined for electrical engineering, focusing on metallurgical aspects of present commercial solutions and development strategies of new generation conductors. In addition to transmission and distribution of electricity markets, and variety of conventional applications, novel opportunities for aluminum conductors in electric vehicles are assessed.

Electrical conductivity—microstructure relationship in metals

Understanding the charge carrier transport and its interaction with structural features of an electrical conductor is essential for improving the present and developing future materials for electrical engineering. Although the primary goal in conductor development is maximizing its electrical conductivity, in engineering practice reaching the optimum between the conductivity and mechanical properties is the real target.

Conductivity and resistivity

Electrical conductivity (or specific conductance), defined as the ratio of the current density to the electric field strength, indicates the material ability to carry a current. The conductivity SI unit is Siemens per meter (S/m), but its values are often reported as %IACS, being an acronym for the empirically established International Annealed Copper Standard. According to this standard, commercially pure annealed copper has a conductivity of 100% IACS at 20 °C. The conductivity is temperature-dependent, and for most materials, it decreases as temperature increases. Electrical resistivity (or specific electrical resistance) with the SI units being the ohm meter (Ω⋅m) is the reciprocal of electrical conductivity and indicates how strongly a material resists electric current. Basic characteristics of top electrical metal conductors are listed in Table 1.

Table 1 Basic characteristics of top metal electrical conductors (based on [258, 259])

The value of material conductivity may be calculated by measuring the resistance, area and length of the test sample that typically has a boxlike shape. The test technique for determining electrical conductivity using the electromagnetic, Eddy current, method is specified by the ASTM E1004 [13] and the DIN EN 50994 [14] standards. For thin films, the van der Pauw method of resistivity measurements is used [15].

Electrical versus thermal conductivity of metals

The theory that explains the electrical and thermal conductivity of metals was introduced by Paul Drude in 1900, within three years of discovery of electron by Joseph John Thompson [16, 17]. It assumes that a gas of electrons moving freely in the periodic lattice of immobile ionic cores is responsible for the electrical conductivity of metals. The density of the conduction electrons (number of electrons per volume, n/V) is expressed as:

$$n = {N_{\text{A}}}\frac{{Z{\rho_m}}}{{\text{A}}}$$

(1)

with N_A = 6 $\times$ 10²³, the Avogadro number, Z the number of conduction electrons per atom, ρ_m the mass density in gram per cm³ and A the atomic mass of the element. Therefore, ρ_m is the number of moles per cm³. Typical densities are 10²² electrons per cm³.

Since both the heat and electrical transport in metals involve free electrons, good electrical conductors are also good thermal conductors. As described by the Wiedemann–Franz law, the ratio of thermal conductivity to electrical conductivity is proportional to the temperature:

$$\frac{K}{\sigma } = {\text{LT}}$$

(2)

where K is the electrical component of thermal conductivity, σ is the thermal conductivity, T is the absolute temperature and L is the Lorentz number, which has a value of 2.44 × 10^–8 WΩK⁻². The specific equations that link both parameters for major class of metals are available [18]. The thermal and electrical conductivities of pure aluminum were measured about five decades ago using both state-of-the-art equipment for thermal conductivity and the Lorentz number determinations [19]. Examples of relationships between thermal and electrical conductivities for selected alloys are shown in Fig. 1. At present, there are manufacturing strategies capable of inverting the proportionality of thermal and electrical conductivity to produce materials with, e.g., extremely low thermal conductivity and high electrical conductivity [20].

Figure 1

Plot of electrical conductivity versus thermal conductivity for selected metals pointing the location of aluminum.

Role of metal microstructure in conductivity

In the classical conductivity theory, electron is considered as a very small particle with a certain mass and electric charge. According to the free electron theory, electrons move through the metal lattice freely following the Newton's laws of motion and Maxwell–Boltzmann statistics with electron mean free path being inversely proportional to the probability of electron scattering upon each encounter.

Lattice defects and electron scattering

An interaction of electrons with lattice and structural defects leads to scattering of electron waves and an increase in the electrical resistivity. In a metal lattice, point defects as vacancies, self-interstitials and atomic impurities, linear defects as dislocation lines and dislocation loops, and planar defects as stacking faults, grain boundaries and crystal surfaces act as scattering sites.

According to the Matthiessen's rule [21], the total electrical resistivity of metals (ρ) is affected by thermal (ρ_T) and structural disorder caused by impurities (ρ_imp) and deformation (ρ_def).

$$\rho = {\rho_{\text{T}}} + {\rho_{{\text{imp}}}} + {\rho_{{\text{def}}}}$$

(3)

Hence, it is the sum of contributions from thermal vibrations, roughly proportional to the temperature, and a microstructure part controlled by foreign atoms present in solid solutions, precipitates of second phases, defects as dislocations, vacancies and grain boundaries. The latter are seen as the dominant defects that increase the resistivity of alloys.

Role of grain boundaries in electron scattering

Grain boundaries in metals usually increase the electrical resistivity due to their less regular atomic arrangement compared to the grain interior and the impact depends on the grain boundary structure. In metal conductors, the grain boundary structure is controlled by the crystallographic texture, orientation of the neighboring grains, alloy composition and temperature. The influence of microstructure and its modification during processing on metal conductivity is shown schematically in Fig. 2a,b,c. In deformed state, where the electrons are mainly scattered by grain boundaries and dislocations, after annealing they are passing through the super-long oriented grain channels avoiding scattering.

Figure 2

Effect of structural disorder on electrical conductivity of metals: a, b high scattering of electrons on grain boundary of deformed state; c low scattering on elongated grains with defects free interior; d resistivities of different coincident site lattice (CSL) grain boundaries and low-angle grain boundaries (LAGBs); d reproduced from [23] under CC-BY 4.0 license.

Understanding the role of grain boundaries in electron scattering allows an enhancement of their conductivity through grain boundary engineering during material processing, e.g., through selecting the deformation or heat treatment scenarios that generate the crystallographic texture that is characterized by the favorable grain boundary character distribution. Hence, overcoming this challenge is essential for developing the high-efficiency conductors.

Quantitative determination of the relative contributions of surface and grain boundary scattering to resistivity is very challenging [22]. The direct and local resistivity measurements of grain boundaries in copper, combined with structural characterization using analytical microscopy and molecular dynamics simulation, showed that to a first approximation the metal electrical resistivity is correlated with the boundary excess volume and excess energy [23]. As shown in Fig. 2d, the resistivities of low-angle grain boundaries with misorientations of more than 10° are the highest among the tilt grain boundaries, due to the high dislocation density and resulting strain fields.

The most resistive grain boundaries are in the high range of low-angle grain boundaries (14°–18°) with twice the resistivity of high-angle tilt grain boundaries, due to the high dislocation density and corresponding strain fields. Separating the resistivities of grain boundary and grain interior in dilute Fe-alloyed copper thin films showed that GB resistivity increased by an order of magnitude while the grain interior had only a minor contribution [24].

Conductivity mechanisms in aluminum

An early review identified mechanisms affecting the electrical conductivity of aluminum [25]. In aluminum, specific volume defects, such as Guinier–Preston zones and intermetallic precipitates, significantly influence its electrical resistivity [26] 27. The atomistic details of electrical conductivity in crystalline aluminum and the role of point and extended lattice defects can be derived from modeling based on a framework of density functional theory (DFT) that reveals the spatial nature of thermal fluctuations [28]. The modeling concluded that well below the Debye temperature, a classical thermal molecular dynamics simulation reproduces the form of temperature-dependent conductivity predicted by the Bloch–Grüneisen formula [29]. Although the description has a limited theoretical value, it describes the temperature dependence of conductivity.

Strength–conductivity trade-off

The electrical conductivity and mechanical strength are the most important properties of conductor materials that are directly related to energy losses and safety. The common engineering efforts aim at enhancing the mechanical performance through alloying and thermomechanical treatment that increases density of electron scattering centers. This causes energy dissipation and a decrease in conductivity, determined by the speed of free electrons movement without being disturbed. Hence, designing the conductor materials for simultaneous low resistance and high strength is not trivial task. The trade-off between the strength and electrical conductivity is the present knowledge barrier and this is essentially true for all metal conductors including aluminum [30] 31. There is an analogy to the strength–ductility trade-off dilemma during optimizing the mechanical properties of alloys [32] 3334. Examples of strength–conductivity relation in selected aluminum alloys, contributing factors and role of solute atoms are shown in Fig. 3.

Figure 3

Strength–electrical conductivity trade-off in metals: a strength–conductivity plot for a variety of conductors along with aluminum alloys, reproduced from [31] with permission from Springer; b strength–conductivity plot for selected wrought aluminum alloys; c modeled contribution to strength and electrical resistivity in AA6201 alloy from precipitates (mean precipitate spacing = 50 nm), dislocations (density = 1015 m⁻²), grain boundaries (grain size = 400 nm) and solutes (Mg = 0.3 wt.% and Si = 0.3 wt.%), reproduced from [102] with permission from Springer; d detrimental effect of alloying elements on conductivity of 99.99% Al, reproduced from [33] with permission from Springer.

Controlling the conductivity and strength of conductors to achieve the optimum required by the design is of great scientific and engineering interest. A variety of metallurgical methods of improving both the strength and conductivity have been exploited through the microstructure control with a few being successful [35]. In precipitation-hardened alloys, the reduction in the interfacial energy between the second phase and matrix can induce the uniform dispersion of ultra-fine second phase in a matrix, increasing at the same time strength, ductility and conductivity. For that strengthening mode, the methodology was developed to predict the optimum trade-off between strength and conductivity by combining the Nordheim relation of conductivity and dissolved solute, the Gibbs–Thomson equation of dissolved solute to particle size and the Orowan theory relating the particle size/spacing to yield strength [36].

Applications of aluminum conductors

In addition to conventional applications in power transmission and distribution, aluminum is increasingly used in modern, renewable energy solutions including solar systems, electric vehicles and charging stations. The global aluminum wire market was valued at USD 50.7 billion in 2021 and is projected to reach USD 69.7 billion by 2031 [37] with major application sectors described below.

Transmission and distribution lines

Aluminum is the most widely used material for electricity transmission and distribution lines since its superior conductivity-to-weight ratio allows utilities to run more wire with fewer supporting structures (Fig. 4a). The classification of commercial overhead conductors is listed in Fig. 4b. All aluminum conductors (AAC) are a refined aluminum stranded conductors with a minimum metal purity of 99.7% that is primarily used for overhead transmission and distribution services where spacing is short and supports are close. All aluminum alloy conductors (AAAC) are used as a bare conductor cable on aerial circuits that require a larger mechanical strength than AAC. The aluminum conductor steel-reinforced (ACSR) has steel contents ranging from 6 to 40% for an additional strength with galvanizing, acting as sacrificial anode for the steel core. The benefits of AAAC conductors include lower electrical resistance, higher strength-to-weight ratio and better resistance to corrosion than the ACSR counterparts. The aluminum conductor alloy-reinforced (ACAR) is a concentric-lay-stranded conductor, manufactured of the AA1350-H19 and AA6201-T81 strands that usually make up the core with the AA1350 wires stranded around them.

Figure 4

Applications of aluminum conductors: a high-voltage transmission line; b classification of overhead line conductors; c high-voltage bus bars, reproduced from [260]; d EV battery bus bar, reproduced from [261]; e power transformer with aluminum winding [262]; f AA1350 strip for transformer winding, reproduced from [263]; g quadruple 600 V rated cable of ruggedized design with 350 MCM-4/0 AWG size of AA1350-H16/H26 conductors—MCM means thousand (M) circular (C) mils (M).

Aluminum-based ceramic fiber core conductor (ACCR) and carbon fiber ACCC, collectively referred to as the composite core aluminum conductors, exhibit the best overall performance of present commercial solutions. They not only have a wide operating temperature range and lower linear expansion coefficient, but also have the lower unit weight. The ACCC design breaks the traditional conductor structure, showing many technical advantages, and represents an important innovation in transmission conductors [38]. The electrical conductor grade aluminum alloy designated as AA1350 is used as a strand in the AAC, ACSR and ACCC designs [39].

The conventional ACSR and AAAC conductors are currently designed to operate at maximum temperature of 85° C and 95° C, respectively. In contrast, high-performance conductors can operate continuously at elevated temperatures, above 150 °C and up to 210 °C. To increase the operating temperature, Al–Zr alloys are used in conductors for transmission lines termed as thermal-resistant Al alloy (TAl), ultra-thermal-resistant Al alloy (ZTAl) and high-strength thermal-resistant Al alloy (KTAl). TAI can withstand up to 150 °C and can carry 1.6 times current capacity of AA1350 alloy when ZTAI can withstand up to 210 °C and can carry 2.0 times current capacity of AA1350 alloy [40, 41].

GTACSR—gap-type thermal-resistant aluminum alloy conductor steel-reinforced—and GZTACSR—gap-type super-thermal-resistant aluminum alloy conductor steel-reinforced—have unique design, featuring a small gap between the steel core, and (super)-thermal-resistant aluminum alloy layer that offers excellent sag and current-carrying characteristics [41]. To avoid friction between the steel core and aluminum inner layer, the gap is filled with thermal-resistant grease. As another example, Invar conductors—ZTACIR—with continuous operating temperature up to 210 °C use a combination of aluminum-clad Invar (Fe–Ni alloy) for the core and super-thermal-resistant Al–Zr alloy for the conductive layer [42].

In a search for advanced conductors, the high-strength, lightweight composite core with carbon fiber or metal matrix composite coreis considered instead of the traditional steel cores. This leads to strength increase, allowing the incorporation of about 30% more aluminum content without necessity to increase weight or diameter [43]. The added aluminum content that reduces conductor resistance also lowers the line losses by 25 to 40%, depending upon electrical load [44]. Transmission lines that use advanced conductors exhibit more current capacity and provide better mechanical performance at higher operating temperatures. Depending on the type of conductor, the cost of high-performance designs may reach up to 5 times the cost of conventional ACSR/AAAC conductors.

Rigid electrical conduits

Aluminum is widely used as rigid electrical conduit called bus bars that allow to surpass the use of hardwired power distribution. The bus bars are applied in generating or receiving stations on which power is concentrated for distribution in transformers, generators, switchgears in smelters and electrochemical plants, high-power rectifier units or direct current power supply systems (Fig. 4c). There is a trend of material change in bus bars from copper to aluminum; compared to copper, aluminum bus bars weight requires an increase in cross-sectional area of about 62% so if there is no size limitation, aluminum is the lighter and more cost-efficient choice [45].

In electric vehicles, bus bars serve as conductors to power-generating components within the electrical system by transferring the electric current from batteries to the inverter. In a typical vehicle battery pack, there could be up to two dozen bus bars. The bus bar design is also vital for thermal management of electric vehicle that prevents overheating by heat generated in the battery [46]. The common aluminum alloys for electrical bus bars include the AA1060, 1100, 1350, 6063 and 6101 grades (Fig. 4d).

Winding in transformers

The transformer manufacturers considered the aluminum wire windings versus copper for decades with choice depending on the specific needs of the facility (Fig. 4e, f). The performance of aluminum and copper in transformers is seen similar at intermediate frequencies, around 80–120 kHz due to significant Eddy current losses [47]. The alloys applicable for transformer windings contain over 99.5% of Al and include the AA1350, 11,050, 1060 or 1070 grades depending on transformer voltage. A comprehensive comparison of use of copper and aluminum windings in distribution transformers is available in the literature [48].

Conductors in solar applications

The photovoltaic (PV) cable is a single conductor wire rated at either 600 V, 1 kV or 2 kV, used to connect PV panels in solar power generation systems [49]. The global solar cable market size was USD 787 million in 2022 with predicted growth of 12.12% CAGR to reach USD 1.54 billion in 2028 [50]. Both aluminum and copper PV cables are used in surface and unground photovoltaic power systems, particularly in their interconnection wiring both indoors and outdoors [51]. The aluminum PV wire requires the larger gauge to ensure the same ampacity, i.e., the maximum current that a conductor can carry without exceeding its temperature rating, as its copper counterpart. The solar cables may also be used in other applications (Fig. 4g).

Wiring in transport vehicles

Aircraft wiring

In aircraft, aluminum wiring is mostly avoided due to safety considerations linked to a negative experience with home wiring. An industry standard does not exist to allow a common use throughout the aerospace and aircraft industries. At present, aluminum wiring is used in some European aircraft where weight savings come at the cost of more parts and more work, both in production and later in service. It is anticipated that ongoing research will improve confidence in quality management and maintenance of aluminum conductors. For example, NASA JSC seeks to advance the technology readiness of aluminum wiring for aerospace, aircraft and other applications where weight is important [52]. To determine whether aluminum electrical conductors can be used in the space environment, the wiring must be thoroughly tested including connectors, crimping techniques and other elements of the wiring system.

Automotive wiring

An application of aluminum cables in automotive industry was initiated in 2000, and later in 2013, the international standard ISO 6722-2 was created [53] (Fig. 5a). In automotive applications, the wire harness is an organized collection of cables and connectors that send power and data throughout the vehicle. Electric vehicles employ almost double the number of wires as compared to a traditional internal combustion engine vehicle what significantly increases the weight of wiring. Moreover, in EV the working current amplitude in the high-voltage wire harness can reach up to 300A, increasing the electrical performance requirements [54]. Increasing the driving range of EV requires more powerful batteries, resulting in replacing the smaller electrical conductors with larger ones to handle the additional current. There are intensive research activities aimed at replacing copper with aluminum in wiring harness to reduce the EV weight and cost.

Figure 5

Aluminum wiring in automotive vehicles: a time line of application of aluminum in automotive wiring, reproduced from [53]; b high-strength aluminum alloy wire installed in the engine harness, reproduced from [57]; c die-cast aluminum coil for motor winding with seven turns and a conductor height of approx. 1.5 mm along with d coil arrangement, reproduced from [264]; e hairpin motor using aluminum V-cat windings, reproduced from [59]; f insulated and bare AA1350 aluminum of 7 AWG square tested for hairpin winding of electric motors and copper wire for comparison along with stress vs strain elongation curves, reproduced from [11].

Aluminum is used for starter and battery cables in ICE vehicles for last two decades mainly in Europe, and it is anticipated that much of the high-current wiring in EVs will soon be made entirely of aluminum as well. The aluminum wire conductor between the high-voltage battery and the inverters, and a power cable and direct connector that connect the inverters to the motors are available commercially [55]. As a production method of aluminum wires for an automotive wiring harness, an alternate drawing procedure was developed, which can provide the high ductility [56].

Pure aluminum, e.g., AA1060, in an annealed state has strength of about of 70 MPa, being insufficient for automotive harness to withstand the influence of vibrating parts of the engine. The aluminum alloy wire developed for EV harness exhibits high-strength and high-bending properties exceeding those of copper wire and being twice higher than the standard aluminum wire [57]. In the present commercial design of engine harness, the Al wire fills 20% of the total number of wires and reduces the harness weight and cost as compared to copper (Fig. 5b).

Winding in traction motors of electric vehicles

Advanced research is in progress to apply the aluminum windings in electric motors to generate the electromagnetic field. The coreless axial flux permanent magnet (AFPM) machines attract an increasing attention due to their compact structure and high torque density with the largest portion of motor mass being contributed by copper coils. An application of aluminum windings may reduce the weight by 66% for the same power output and enhance the motor torque-to-weight ratio [58] (Fig. 5c,d).

The hairpin motors are increasingly used in electric vehicles since they are more efficient, have a higher power density and thermal performance, and are simpler to manufacture than previous types of electric motors (Fig. 5e) [59]. The feasibility studies that aimed at replacing copper with aluminum conductors in automotive scale electric motors revealed that aluminum exhibited formability and windability advantages over copper, especially regarding the springback, accommodation of elongation during high-speed winding, and repeated absorption of bends and twists during winding [11]. At the same time, insulation adhesion and delamination issues occurred for elongation beyond 10%, due to incompatibility of properties between the polymer coating and aluminum wire (Fig. 5f).

Applications of aluminum benefiting from its electrical/thermal conductivity

In addition to typical aluminum conductors, there are applications in electrical engineering, where aluminum does not serve as conductor. While some applications benefit from high heat transfer, for others the electrical conductivity of aluminum is of importance [60].

Heat transfer in electrical/energy applications

The capability of aluminum to fast transfer heat makes it the perfect choice for heat dissipation components (heat sinks) by automotive and communication industries in devices such as semiconductors, transformer tanks, transistors and central processing units (CPUs) (Fig. 6a). Heating elements, laminated between self-adhesive aluminum foil sheets, are used in a wide range of applications where temperature is limited to approximately 130 °C and covers particularly large, heated areas [61].

Figure 6

Applications of aluminum that require high electrical/thermal conductivity: a electric transformer heat sink, reproduced from [265]; b aluminum electrolytic capacitors, reproduced from [64]; c HPDC cast heat sink of Castasil-21 (AlSi9Sr) alloy, reproduced from [266]; d cutaway of BMW’s e-drive system in its fifth generation, reproduced from [267]; e strength–conductivity requirement by Tesla and a selection of reference commercial alloys, reproduced from [12].

Aluminum casings protect electrical components from temperature and moisture. During manufacturing of electronic devices, aluminum alloys are used for base panels and parts for smartphones providing lightweight, high strength and efficient heat radiation [62]. The new energy sector is increasingly turning to aluminum to power its innovations in housings that hold solar panels and wind turbines.

Aluminum electrolytic capacitors

Aluminum electrolytic capacitors can store massive amounts of energy in compact packages (Fig. 6b). They explore anode electrode of a pure aluminum foil, e.g., AA1235, 1070, 1100, 3003 with thickness of 0.005–0.055 mm, with an etched surface that forms a thin anodized layer of alumina that acts as the dielectric of the capacitor [63]. Applications range from solar power converters to miniature power supplies for highly complex processing cores. As advantages, low cost with high capacitance values for filtering lower frequencies and higher energy density than film or ceramic capacitors are listed. In addition to liquid electrolyte, the conductive polymer and hybrid electrolytic material systems were introduced what minimized previous limitations of aluminum electrolytic technologies [64]. The global market of aluminum electrolytic capacitors reached in 2022 USD 5.18 billion with CAGR of 3.60% by 2030 [65].

Components of EV drive system

To achieve the high-power output and efficiency, certain parts of electric vehicles, like traction motor, a rotor or an inverter, require aluminum alloys that have a combination of high electrical conductivity and strength [66] (Fig. 6c, d).

Out of cast commercial alloys that have high conductivity, the Castasil-21 F alloy, used in general automotive market (Fig. 6c), with conductivity of 44% IACS and yield strength of 85 MPa has the electrical and mechanical properties that are closest to those needed for the above EV parts. It should be noted that due to shape of these parts, casting is the manufacturing choice, what eliminates the wrought grades (Fig. 6d). Since properties of present aluminum alloys are still insufficient for growing requirements of EV, Tesla developed alloys with high-yield strength and conductivity while being resistant to hot tearing. The alloys were designed to achieve yield strength of 90 to 150 MPa and electrical conductivity of 40% IACS to 60% IACS [67], 12 (Fig. 6e).

In another research, the Al–Ce–Si–Mg alloy was developed with the yield strength of over 120 MPa and the electrical conductivity of at least 50%IACS after T5 and T6 heat treatments. The alloy is seen as promising alternative to the near pure Al or Cu-based alloys that are currently used in induction motor rotor castings [68]. To optimize the chemistry of cast Al–Si–Cu–Ni–Sr alloys, the optimal combination of Al6Si3Cu0.5Ni0.003Sr (wt.%) led to strength of 247.5 MPa and electrical conductivity of 38.0%IACS [69]. The potential alternatives such as Al–Fe, Al–Ni and Al–Fe–Ni–Mg–Si alloying systems are also considered [70]. The aluminum cast alloys for motor applications in electric vehicles along with details of Fe effect are analyzed in recent review [71].

Factors affecting the performance of aluminum conductors

Degradation mechanisms contributing to the failure of aluminum conductors are complex, as shown for aluminum strand of the AA1350 alloy used in marine environment [72]. Detailed monitoring technologies for overhead transmission lines along with issues and challenges of controlling the thermal stress were developed [73]. Although many requirements control the endurance of bare conductors of the overhead lines and power cable lines with insulation apply to aluminum use in automotive wiring, some of them are unique and require special considerations (Fig. 7a,b).

Figure 7

Factors controlling the performance of aluminum conductors: a classification of degradation mechanisms of ACSR conductor, reproduced from [72]; b statistics of different causes of conductor failure in unassisted failure mode in overhead distribution network, reproduced from [268]; c heat transfer diagram of multi-conductor bundle divided into thermal conduction, thermal convection and thermal radiation, reproduced from [269] with permission from Elsevier; d ACSR conductor after 40 years of exposure, showing the corrosion attack on galvanized strand, reproduced from [83].

Conductor temperature

The conductor temperature increases due to current flow and is also influenced by ambient temperature, i.e., weather. The higher temperature leads to increased electrical resistance and wire elongation that in extreme cases may affect the conductor structural stability. The temperature limit is the barrier for increasing the transmission capacity of a cable that prevents an increase in current load. The load rate of electrical conductors during service should not result in the temperature exceeding the maximum allowed by the specification.

The thermal linear coefficient of expansion of aluminum is 36% greater than copper. For transmission lines, the maximum temperature is generally limited by allowable conductor sag. Excessive sagging adds weight on power lines that may lead to failure of poles and towers. Some conductors can operate at temperatures exceeding 200 °C without compromising the design requirements. At full rated current, the temperature of the core can be significantly higher than the conductor surface.

Power cables, located in free air, can be exposed to wind and solar radiation (Fig. 7c). Both aspects significantly affect the current-carrying capacity of the cables with solar radiation reducing their current rating [74].

Thermal stability of conductor alloys during service

Thermal stability is the key design feature that determines a suitability of conductor materials for specific applications and has a particular meaning for aluminum alloys [75]. As documented throughout the decades, practically all aluminum alloys are thermally unstable with their properties being affected by heat [76]. Extending thermal stability to higher temperatures is the technology and knowledge barrier that prevents the substantial expansion of application scope of aluminum alloys in general that also applies to electrical conductors. There is extensive research aimed at development of heat-resistant aluminum alloys for electrical engineering, in particular, to understand the effect of alloying additives [77], 78.

Stress-related behavior

The aluminum alloys experience much higher creep than copper with creep rates being up to 25 times higher under the extreme loading and temperature conditions. For overhead line conductors, the permanent elongation is caused by creep due to either high stress over short time or due to modest stress, acting during longer time periods [79]. The conductor creep is always greater than that of the individual wires and after long exposures generates a substantial sagging defined as vertical distance between two points of conductor support. The excessive sagging may cause grid outages. Since wind adds some weight to the conductor in a horizontal direction, it increases sagging. The excessive current contributes to thermal sagging.

The creep phenomenon also affects life of winding conductors in distribution transformers.

Transformer winding consists turns of coil bundles connected with each other. As a result of creep, the separation between winding conductors may exceed the allowable minimum safety clearance. An analysis of failed high-tension windings shows that 70% cracks originated in areas of maximum stress. Also, frequent energization (switching) of distribution transformers affects their creep life [80].

Conductor corrosion

Corrosion of aluminum conductors occurs due to long-term exposure to air under the action of water vapor, chemical gas and salts at service temperature, often higher than the ambient temperature. The level of corrosion is closely related to the chemical composition and manufacturing process of the wire. In coastal environments, high-voltage electrical cables are prone to saline corrosion. The corrosion types of conductor wires include notch corrosion, electrochemical corrosion and pitting corrosion with electrochemical corrosion being the dominant one [81].

The corrosion behavior of aluminum steel-reinforced conductors, ACSR, is much complex as evidenced throughout decades of service (Fig. 7d) [82, 83]. An investigation of the joint effect of marine and industrial pollutants on the atmospheric corrosion showed a correlation between weight loss, attack depth, tensile strength to rupture and service time as well as pollutants contents on performance of aluminum alloys in high-power electrical conductors [84]. The main cause of the conductor degradation in overhead transmission lines is atmospheric corrosion imposed by highly aggressive species such as Cl- and SO₂ [85]. In marine environment, the conductor of the aluminum alloy (Al–0.5 wt.% Mn)-clad steel wire as the tension support showed 1.6 to 2 times higher corrosion resistance than that of the conventional ACSR/AC conductor [86].

One of factors limiting the use of aluminum wires is galvanic corrosion at connections. Since aluminum causes galvanic corrosion when in contact with other metals in an electrolyte, it is essential to prevent this phenomenon from occurring at crimped terminals [87].

Conductor oxidation

High affinity of aluminum for oxygen results in quick formation of protective but highly resistant oxide on its surface [88]. Although the oxide is important for the corrosion protection of aluminum, it deteriorates the outer surface of the wire conductor, degrades its conductivity and is not desirable for electrical applications. When an oxidized conductor is connected without any pre-treatment, the increased contact resistance results in temperature increase what accelerates oxidation [89]. The thick oxide layer further increases the resistance and temperature of electrical connection what may lead to fire.

It should be noted that there are also beneficial aspects of aluminum oxidation, being explored in strip conductors for use in transformer coils [90]. In that case, the anodic oxide film is used as insulating barrier that replaces the polymer or tape films. The aluminum oxide with its high melting point of 2072 °C makes the coils highly compact and increases the transformer operating temperature.

Connections and terminations control

Connections and terminations of aluminum conductors represent the serious drawback to their application expansion and the biggest factor in their bad reputation developed during the housing wire crisis of the 1960 and 1970. As a result, by the late 1970s aluminum was eliminated from building wiring. The poor performance of aluminum connections results from (i) corrosion, oxidation and (ii) creep and thermal expansion.

Aluminum has higher stress relaxation than copper, experiences cold flow under moderate stress, has a higher creep rate and is more prone to fatigue. This behavior leads to a decrease in the connection clamping pressure during thermal cycling, which in turn increases the contact resistance, resulting in the connection failure.

High-voltage connections

To prevent the resistivity increase at aluminum connections, oxidation inhibitors are recommended. The anti-oxidation compounds, mostly available in a grease form, mitigate oxidation, allowing to maintain a low-impedance connection to the lug. The effectiveness of applying an antioxidant to the aluminum wiring and the role of temperature are assessed in Ref. [91].

Aluminum alloy connections require the same maintenance as copper-based [92]. The connectors manufactured in accordance with UL standards are intended to be tightened to the recommended value during installation and then operate undisturbed for many years. Re-tightening attempts to verify whether connections came loose may lead to damage and failure. Since loose connections generate excessive heat at the contact surface, their maintenance should include the visual inspection and thermal scanning. These methods can detect a problem, and follow-up investigation can help determine the root cause of the overheating. If the connection becomes loose, the root cause must be identified and corrected.

Low-voltage automotive connections

In automotive aluminum wiring, the specifically developed terminals are used where most often wires are connected by crimping, which is commonly used in harness manufacturing [93]. In addition to many modifications of conventional crimp, the connection techniques investigated include different welding and soldering methods such as friction welding, ultrasonic welding, resistance welding or plasma soldering [94]. Since the conventional TIG process is difficult to implement at the assembly line, friction and ultrasonic welding are utilized. Both welding techniques can break through the oxide film with no need of flux or shielding gas and join aluminum wire to a compatible lug without overheating the surrounding area [95].

Researching the commercial conductor alloys

The characteristics of presently used electrical conductors are provided by standard specification of ASTM International with a selection listed in Table 2. There are continuous efforts to increase strength and electrical conductivity of commercial Al conductors through optimization of main alloying elements such as Si and Mg, additions of minor alloying elements, molten alloy treatment, solid-state thermomechanical treatments, including deformation during the conductor wire drawing [96]. In regard to improvement expectations, an increase in the conductivity of the order of 1% IACS is seen relevant. To maximize deformation, cold drawing, severe plastic deformation via equal-channel angular pressing, accumulated roll bonding and high-pressure torsion area are used. This section describes major research efforts aimed at improving the commercial conductors of the AA1350/AA1370, AA6000 and AA8000 alloy series.

Table 2 Selected standard specifications of ASTM International, Sect. 02.03: Electrical Conductors

AA1350/AA1370 alloys

The AA1350/AA1370 alloys are especially important for electrical rod products, accounting for 80% of the total aluminum rod market [97]. For cold-drawn AA1370 electrical conductor, Fe-rich precipitates play a significant role by pinning the grain boundary movement, and prevent recrystallization and grain growth at elevated temperatures, leading to the 〈0 0 1〉 fiber texture (Fig. 8) [98]. The systematic investigation of the microstructure strength relationship of a cold-drawn commercially pure Al conductor revealed that the texture evolution, dislocation recovery and sub-grain growth during recrystallization caused the conductor strength degradation [99].

Figure 8

Optimizing properties of AA1370 conductors: a effect of Fe and Si in solid solution (Fe_ss and Si_ss) on electrical conductivity with wire microstructure in cold-drawn state shown in the inset; b tensile strength–electrical conductivity relation of the AA1370 alloy after various processing with literature data for dilute and commercial aluminum alloys. Reproduced from [98] under CC-BY license.

The screw extruded wire at 450 °C had an advantage reaching the conductivity of 64.2% IACS and strength of 65 MPa, being lower than the cold-drawn wire with conductivity of 61.9%IACS but higher strength of 164 MPa. According to Ref. [100], in commercially pure aluminum wire (99.6% Al), both the grain boundaries and texture influenced the strength–electrical conductivity relationship where highly elongated grains having the strong < 111 > texture improved both the strength by 74.2 MPa and electrical conductivity by 0.31%IACS. Using deformation method to generate elongated grains having the strong < 111 > texture, higher electrical conductivity of 63.0%IACS was achieved as compared to 62.0%IACS of conventionally processed wire. Using the computer-aided design, new processing method was proposed for the AA3105 alloy that resulted higher creep resistance compared to the conventionally processed alloy [101].

AA6000 alloys

The AA6000 Al–Mg–Si conductor alloys are of commercial importance due to their excellent mechanical properties, high conductivity and low cost [102]. This is the precipitation-hardened grade with thermomechanical treatment being the effective processing route. The precipitates and dislocations generated during treatment significantly change the alloy performance [103].

Due to processing optimization the new low-cost, scalable AA6000 conductors were developed with increased strengthening and minimized conductivity reduction [4]. Two examples include (i) Al0.7 Mg0.3Si0.08Bi aged at 200 °C for 7 h with strength of 426 MPa and conductivity of 52.7%IACS) and (ii) Al0.7 Mg0.3Si0.01Sn aged at 200 °C for 4 h with strength of 445 MPa and conductivity of 48.2%IACS.

Researching the effect of dislocations on electrical conductivity and mechanical properties of AA6201 wires concluded that the recrystallized grains were coarsened by reducing the dislocation density, and the dislocations were more randomly arranged, resulting in a decrease in the electrical conductivity [104]. In another study, the extrusion ratio was found to affect the performance of AA6101 conductors [105].

Although the AA6201-T81 alloy wires have higher fatigue resistance than the AA1350-H19 wires, their fatigue life was lower than that of the AA1350-H19 wires reinforced with steel [39]. The research revealed that while the increased tensile strength of the AA6201-T81 alloy enabled the critical wires in the conductor to withstand higher stress levels, its resistance to fretting fatigue falls short in compensating the damaging effects of elevated stresses. As a result, greater fatigue damage occurred in the AA6201-T81 wires, ultimately leading to a reduction in their overall durability.

To optimize the AA6201 conductor performance, modifications of chemical composition and processing routes were researched. The experimental validation of Mg/Si ratio and aging treatment concluded that Mg/Si ratio of about 1.5 allowed to reach the optimum of strength and electrical conductivity of 54–56% IACS (Fig. 9a, b) [106]. Another investigation concluded that the excessive Si benefited the high strength and high conductivity while excessive Mg deteriorated the strength and conductivity of AA6101 wires [107]. The excessive Si content promoted both the precipitation rate and quantity of β″ thus increasing the tensile strength. At the same time, the high Si also helped decreasing the lattice distortion thus improving conductivity.

Figure 9

Optimizing strength and electrical conductivity of the AA6201 (Al–Mg–Si) alloys through Mg/Si ratios: a tensile strength as a function of Mg/Si ratio; b hardness as a function of conductivity for Mg/Si ratio shown in (a). Reproduced from [106].

The study that aimed at determining the role of Si and Fe in strength and electrical conductivity of dilute Al–Mg–Si alloys concluded that Fe additions improved electrical conductivity of the Al0.5Mg0.35Si alloy [108]. Moreover, the electrical conductivity and mechanical properties of the Al0.5Mg0.4Si0.2Fe (wt.%) alloy could further be improved through the homogenization treatment. In another study of the diluted Al1Mg0.5Si0.8Cu (wt.%) alloy, the modified thermomechanical process enhanced the removal of solutes from Al matrix and through the work-hardening effect compensated the loss of age hardening caused by the precipitates coarsening. As a result, an improvement in strength/conductivity combination was achieved [109]. The similar conclusion was reached for dilute Al–Si alloys where the electrical conductivity was improved by additions of Fe, which controlled the harmful effect of Si [110].

AA8000 alloys

The AA8000 alloys were designed for use in secondary distribution circuits within buildings to replace the AA1350 grades. Since 1987, the National Electrical Code (NEC) has specifically required that the electrical AA8000 series being used for a number of insulated conductors and cables [92]. The building wires are now manufactured according to the ASTM-B800 standard. Small additions of Fe, Mg and Cu in the AA8000 alloys (Fe in AA8030 and AA8176) benefit the microstructural stability and creep resistance [111].

The extensive investigation of the role of Cu, Fe and Mg in creep behavior of AA8000 electrical conductor alloys revealed that the creep threshold stress increased from 24.6 MPa to 33.9 MPa with increasing Fe contents from 0.3 to 0.7 wt.% (Fig. 10) [112], 113. Additions of Cu positively influenced the short-term creep resistance but provided no advantage to the long-term creep resistance; it substantially decreased the primary creep strain but had a negligible effect on the minimum creep rate. The small addition of Mg greatly reduced both the primary creep strain and minimum creep rate, resulting in a significant improvement of the creep resistance of AA8000 conductors. Moreover, small additions of Cu and Mg improved the strength, but slightly reduced electrical conductivity. To further improve the creep resistance and stress relaxation resistance, additions of rare earth were proposed that do not substantially affected the electrical conductivity [114].

Figure 10

Optimizing strength and conductivity of AA8000 conductor alloys by minor Cu and Mg additions: a orientation image mapping showing the extruded microstructure of the alloys: a Al0.3Fe, b Al0.3Fe0.29Cu, c Al0.7Fe, d Al0.7Fe0.18Cu0.1 Mg (wt.%); e inversed pole figure coloring; f effect of Cu; g effect of Mg; h tensile strength and conductivity of alloys with different Cu and Mg, arrows show the increasing Cu and Mg additions to their corresponding regions. Reproduced from [113].

Since the AA8176 alloy is not age-hardenable, cold deformation was tested for the strength improvement [115]. After equal-channel angular pressing at room temperature, its electrical conductivity exceeded 60% IACS, being close to that of pure aluminum. At the same time, there was an improvement of yield stress which increased sharply from 46 MPa to nearly 200 MPa after 8 deformation passes.

Rare earths in aluminum conductors

The Al-based immiscible systems such as Al–Y, Al–Ce and Al–La are promising conductor candidates due to the negligible solid-state solubility of rare earths in aluminum. The solid-state solubility values for earth earths were determined both experimentally and using first-principles calculations [116]. At the same time, the fine and uniformly distributed intermetallic phases containing rare earths improve the mechanical properties and thermal stability without degrading the electrical conductivity. Despite the overall positive effect, excessive amounts of immiscible element compounds have detrimental effects on electrical conductivity.

Yttrium

The mutual solubilities between Al and Y were determined to be smaller than 0.1 wt.% [117]. The general conclusion is that additions of Y to pure aluminum improve its strength and electrical conductivity.

The behavior of the Al–Y binary system may be understood based on examination of the Al7.5Y (wt.%) composition. After annealing, the alloy preserved fine grains due to the β-Al₃Y intermetallic phases present at grain boundaries that promoted grain nucleation and prevented grain growth [118]. Although the β-Al₃Y phase spheroidized during annealing the < 111 > fiber texture and stacking faults remained stable. An improvement of electrical conductivity was attributed to the release of interfacial energy during spheroidization of the β-Al₃Y phase and decrease in interfacial scattering at β-Al₃Y/α-Al interfaces. For the Al7.5Y (wt.%) alloy, cold drawing increased the ultimate tensile strength from 126 to 232 MPa and the conductivity from 49.94% IACS to 52.24% IACS [119].

Cerium, Lanthanum

Cerium additions are utilized for decades in aluminum electrical conductors. Cerium has negligible solid-state solubility in aluminum and does not form strengthening precipitates after age hardening [120] 121. In Al alloys with Ce contents up to 15wt.%Ce, the Al–Al₁₁Ce₃ eutectic provides the limited strengthening [122]. Alternatively, additions of Ce-rich mischmetal to aluminum base alloys are practiced, enhancing the conductivity. The nanoscale Ce-containing phases at grain boundaries are effective in reducing the resistivity due to impeding the dislocation motion. The nanoscale secondary phases with better matching interfaces with the matrix have no positive effect on the scattering motion of electrons [123].

Controlling the Al solid solution through Ce additions

The primary effect of Ce on electrical conductivity is through its influence on the Al solid solution. Cerium provides deoxidizing and dehydrogenizing effect through reacting with impurities in aluminum, resulting in purification of the solid solution [124]. Moreover, small additions of Ce decrease the solid-state solubility of impurities in aluminum [125].

Additions of Ce reduce the solid-state solubility of Si in the aluminum matrix and its negative effect on aluminum conductivity [126]. An example includes Al alloys with additions of 0.55 to 1.2 wt.% Fe and 0.2 to 1.5 wt.% Ce. The presence of cerium improved the alloy electrical conductivity due to a reduction of Fe and Si in the α-Al solution [127]. The solvent content reduction is attributed to formation of the binary, ternary or quaternary compounds of Ce, Si, Fe and Al [128]. It is believed that Ce alleviates the lattice static distortion of Al solution and expands the average free path of electrons. As a result, Ce-induced change of electron energy band structure may increase the effective number of electrons that participate in conduction [129].

A very small increase in electrical conductivity was reported after Ce additions of 0.1 and 0.5 wt.% [130]. The same positive effect is specified in Ref. [131], where additions of Ce-rich mischmetal to aluminum-based alloys either enhanced the conductivity when compared to the commercial conductor grade or provided equivalent conductivity when utilizing grades of aluminum containing higher impurity levels.

At the same time, there are also data claiming that Ce additions have no significant effect on both the strength and conductivity of aluminum. For example, additions of 0.2 wt.% Ce to 99.9% pure cold-drawn aluminum, reduced its strength from 200 to 184 MPa and electrical conductivity from 61.1% IACS to 60.9% IACS [132]. The same addition of 0.2 wt.% Mg increased strength to 252 MPa but reduced conductivity to 59.1% IACS, while 0.2 wt.% of Co slightly increased strength to 203 MPa but reduced conductivity to 60.5% IACS. For alloys with additions of Co and Ce, better thermal stability resulted from the formation of new phases: Al_9.02Co_1.51Fe_0.47 and Al₄Ce, respectively.

The extraordinary effect of cerium on electrical conductivity of aluminum is also reported (Fig. 11a,b) [133]. The first action involves the reduction in impurity contents in Al solid solution. The second action assumes that the Ce-induced alteration of electron energy band structure may intensify the number of effective electrons that participate in conduction. So far, however, there is no convincing experimental evidence supporting the letter role of Ce.

Figure 11

Effect of Ce and La on tensile strength (hardness) and conductivity of pure Al and its alloys: a, b Ce in Al0.3Si0.2 Mg (wt.%) alloy, reproduced from [133] with permission from Springer; c, d Ce, La in 99.7% pure Al, reproduced from [134] with permission from Elsevier; e, f properties of Al–(Ce + La) alloys vs. Ce + La concentration for different annealing temperatures: gray-colored area shows the interval for optimum strength/conductivity combination, reproduced from [137] with permission from Elsevier.

Comparing the Ce and La effects

Experiments show that at the same additions level, there is a higher purification effect of Al solid solution by Ce than by La (Fig. 11,c,d) [134]. An increase in electrical conductivity of commercial purity aluminum by about 1% of IACS by an addition of lanthanum is reported. In as-cast conditions, the electrical conductivity of the Al–Ce alloy was higher than that of Al–La alloy at the same addition content. Moreover, Ce exhibited better ability than La to remove Fe and Si impurities from aluminum solid solution . The higher conductivity of as-cast Al–Ce was attributed to the stronger tendency of Ce to form the Al–Fe–Si–Ce compounds, thus preventing the elements from forming the solid-state solutions with aluminum. In as-cast Al–Ce alloys with low Ce content, the precipitates had the short rod morphology distributed within grains. For larger Ce contents, precipitates were distributed at grain boundaries as well. This contrasts with Al–La alloys, where precipitates were located only at grain boundaries. However, after heating at 300 °C for 24 h, for both the Al–La and Al–Ce alloys most of the solid solution atoms entirely precipitated from the aluminum matrix, resulting in similar conductivity.

Ce and La combinations

A combination of cerium and lanthanum exerts the enhanced effect on electrical conductivity of aluminum. Additions of 5.4 wt.% Ce and 3.1 wt.% La to aluminum alloy processed by high-pressure torsion (HPT) resulted in homogeneous ultra-fine-grained microstructure after fragmentation of RE-rich intermetallic particles to the nanoscale and deformation-induced supersaturated solid solution of RE atoms in the Al matrix [135]. Although the HPT processing induced the significant reduction in the electrical conductivity, it was partly restored by annealing because of clustering of RE atoms, reduction in the dislocation density and grain growth.

A combination of Ce and La at lower concentration of total 4.5 wt.% after severe plastic deformation by HPT and 1 h annealing at 230 °C resulted in optimum of ultimate tensile strength of 430 MPa and conductivity of 55.9% IACS (Fig. 11e,f) [136] 137. The optimum results were attributed to the morphology of intermetallic Al₁₁RE₃ phase and Al/Al₁₁RE₃ interphase surface area.

In recent developments, the enhanced effect of simultaneous additions La + Ce and La + Er was showed to overcome the trade-off between strength and electrical conductivity of Al–Mg–Si alloy conductors [138]. The La + Ce additions showed tendency to co-precipitate and generate dislocations around the co-precipitated phases. This structural effect resulted in the ultimate tensile strength of 223 MPa, 28.4% higher than the same single-doped alloy with La. The other structural effect of Si solute atoms reduction by forming ErFeSi phase improved the electrical conductivity. The electrical conductivity of 52.35%IACS reached by the La/Er co-doped alloy was higher than that of the La/Ce co-doped alloy and similar to the La single-doped alloy.

Scandium

There are two different classifications of scandium where it is considered either the rare earth or transition metal [121]. In this review, the first option of scandium as rare earth is used. The Al–Sc system offers an improvement in tensile strength per atomic % of scandium much higher than for any other alloying element added to aluminum. The strengthening effect is attributed to the coherent, nanoscale L1₂-ordered Al₃Sc precipitates. The maximum solid solubility of scandium in aluminum of 0.38 wt.% (0.23 at.%) occurs at the eutectic temperature, being only 1 °C below the melting point of pure aluminum [139]. In practice, this low value is increased to over 0.6 wt.% Sc due to non-equilibrium solidification conditions. An investigation of the solid solution of as-cast microstructure of the Al–Sc–Zr, Al–Mg–Si–Sc–Zr and Al–Mg–Si–Cu–Sc–Zr model alloys revealed the supersaturation with Sc and Zr after casting, even in the presence of Mg, Si and Cu [140]. This finding is of importance for the electrical conductivity control.

Al–Sc–Zr alloys

The Al–Sc–Zr system has a great potential in conductor development due to its high-temperature creep resistance [141, 142]. However, Sc and Zr solute atoms cause sharp decrease in electrical conductivity, which is attributed to electron scattering caused by point defects and lattice distortion. In Al–Sc–Zr–Si alloys, the Sc–Si clusters improve thermal stability without enhancement of electrical conductivity. Separate additions of Sc and Zr in Al0.16Zr and Al0.16Sc (wt.%) wires with a diameter of 9.5 mm produced by continuous rheo-extrusion led to improved strength but reduced electrical conductivity as compared to pure Al (99.996%) [143]. The combined alloying with 0.12% Sc and 0.04% Zr in Al0.12Sc0.04Zr (wt.%) along with the optimal heat treatment resulted in the highest tensile strength of 160 MPa and electrical conductivity of 64.03% IACS.

There are examples of Al–Sc–Zr alloys with strength reaching 200 MPa and conductivity exceeding 60 IACS. The Al0.35Sc0.2Zr (wt.%) alloy exhibited a tensile strength of 210 MPa, elongation of 7.6% and electrical conductivity of 34.9 MS/m (60.2% IACS) [144]. A low-scandium-content Al0.06Sc0.23 Zr (wt.%) alloy exhibited a tensile strength of 194 MPa and electrical conductivity of 61% IACS (35.4 MS/m) [145] (Fig. 12). For the Al–Sc–Zr system, no positive effect of Fe and Mg on strength and conductivity was reported. The Al–Sc–Zr–Fe conductor wires manufactured by continuous rheo-extrusion after optimized thermomechanical treatment reached the tensile strength of 165.7 MPa, elongation of 7.3% and electrical conductivity of 60.26% IACS [146].

Figure 12

Properties of Al0.06Sc0.23 Zr (wt.%) conductors: a electrical conductivity–strength after processing routes indicated; b TEM micrograph of the hot extruded alloy showing fine recrystallized grains; and c Al₃(Sc, Zr) precipitates inside grain. Reproduced from [145] with permission from Elsevier.

Al–Sc–RE alloys

To reduce the prohibitive cost barrier of scandium, additions of other rare earths and transition metals are used to substitute a part of scandium in the L1₂ precipitates [147] 148. As an example, the lower-cost alloy Al–Sc–Zr–Er,  containing Er in a combination with Zr was proposed to replace a portion of Sc [149].

A comparative study of Sc and Er influence on mechanical properties and electrical conductivity of Al using the Al0.2Zr (wt.%) base showed some advantages of Sc [150]. In particular, the Al0.2Zr01Sc (wt.%) conductors reached a microhardness of 575 MPa, electrical conductivity of 34.5 MS/m after 36-h annealing. As a reference, the Al0.2Zr0.1Er (wt.%) conductors reached a microhardness of 550 MPa and an electrical conductivity of 33.6 MS/m after 8-h annealing. The better electrical conductivity of the Al0.2Zr0.1Sc (wt.%) alloy than the Al0.2Zr0.1Er (wt.%) was attributed to higher effectiveness of Sc to inhibit recrystallization, reducing density of grain boundaries, what in turn, lowered the electron scattering.

Multiply RE additions: synergy effect of co-doping

A review of mixed elemental additions of rare earths to Al shows the general trend of their enhanced effect on the strength/conductivity optimum. For example, Sc and Y in the Al0.2Y0.05Sc and Al0.2Y0.2Sc (wt.%) alloys led to good thermal stability of the hardness and tensile properties after annealing of rolled alloys at 200 and 300 °C [151]. The latter composition reached the yield stress of 177–183 MPa, ultimate tensile strength of 199–202 MPa, elongation of 15.2–15.8% and electrical conductivity of 60.8%-61.5% IACS. The improvement in thermal stability was attributed to blocking the dislocations and grain boundary movement by L1₂ precipitates of Al₃(Sc_xY_y) that nucleated homogenously within the aluminium matrix and heterogeneously on dislocations and the Al3Y eutectic phase.

For the Al0.2Y0.2Sc0.3Er (wt.%) alloy, the elemental composition and thermomechanical treatment allowed optimizing the strength/conductivity combination [152]. In particular, after pre-aging at 300 °C, hot and cold rolling, followed by annealing at 300 °C that resulted in aluminum solid solution and τ₂ (Al_75–76Er_11–17Y_7–14) eutectic phase particles with a size of 50–200 nm, the alloy reached the yield strength of 191 MPa, tensile strength of 207 MPa, elongation of 14% and electrical conductivity of 59.7% IACS.

The Al–Ag–Sc–Zr alloy with ultra-fine grains processed by accumulative continuous extrusion reached high electrical conductivity due to partial replacing of low conductive Sc and Zr solute atoms with highly conductive Ag solute atoms [153]. The wires reached electrical conductivity of 53.5 ± 0.4%IACS, significantly higher than the Al–Sc–Zr alloy with similar atomic concentration of alloying elements.

During study of several systems, including Al–Ce, Al–Ce–Zr and Al–Ce–(Sc)–(Y), the best combination of strength and conductivity was reached by the Al–Ce–Y configuration, which does not contain scandium [154]. Additions of Sc were also tested in Al–Ce–Sc and Al–Ce–Sc–Y alloys manufactured by die casting, high-temperature homogenization, hot extrusion and cold drawing [155]. A presence of Sc and Y in the Al0.2Ce (wt.%) alloy eliminated the dendritic segregation, leading to equiaxed grains with dispersed and coherent nanosize Al₃Sc precipitates improving strength, elongation and thermal stability. Moreover, additions of 0.1 wt.% Y to the Al0.2Ce0.2Sc (wt.%) alloy contributed to the high strength, elongation and electrical conductivity after annealing, caused by the lower density of dislocations, stacking faults and sub-grain boundaries, and the larger size of Al₃Sc precipitates.

Transition metals in aluminum conductors

To overcome the high cost of rare earths, an alternative way of aluminum alloying for electrical engineering is explored via additions of eutectic-forming elements with low solid-state solubility in aluminum. Most research efforts utilize for this purpose various contents of transition metals, mainly Zr, Cr, Mn, Ni, Fe and Ag. At the initial stage, the computational modeling methods may be used for electrical conductivity prediction. An example of applying the integrated computational materials engineering (ICME) and density functional theory (DFT) to select candidates of Al–Zn–TM systems is shown in Fig. 13 [156].

Figure 13

Estimating the electrical conductivity for multi-element alloy systems of Al–transition metal using integrated computational materials engineering (ICME) and the density functional theory (DFT): a comparison of trends between experiment (precipitation and pure solid solution) and DFT, excluding pure Al, the alloy composition for each data point is Al–0.926 at% TM; b electrical conductivity per aluminum of Al–Zn–TM formed across transition metals in the periodic table. Reproduced from [156] with permission from Elsevier

Zirconium

The Al–Zr system offers potentials for developing thermally stable lightweight electrical conductors [157] 158159160. Although the maximum solubility of Zr in α-Al is 0.28 wt.% at 667 °C, at the aging temperature of 400 °C it is practically negligible and Al–Zr alloys do not show age-hardening behavior [161]. However, due to the precipitation of Al₃Zr metastable phase with the L1₂ structure, even very small additions of Zr could produce a significant precipitation strengthening effect. The emphasis is on controlling the Al₃Zr precipitation, and two major techniques explored the supersaturated solid solutions of Zr in Al.

Supersaturation through alloy processing

The rapid crystallization of alloys with Zr content of about 0.5 wt.%, being much higher than the solubility limit of Zr in Al, is the effective way of generating the matrix supersaturation. When the solidification is slow the Al₃Zr particles become larger. This approach is not suitable to wires with small diameters since the primary, large Al₃Zr particles may cause the wire breaking during drawing or rolling at room temperature [158].

As another method, the gas-assisted continuous casting of the Al0.81Zr (wt.%) alloy led to non-equilibrium solidification that increased the solid solubility of Zr in Al to 0.353 wt.% [162]. After aging at 350℃ for 275 h, the alloy reached the tensile strength of 200 MPa and electrical conductivity of 59.6% IACS. After 94% deformation via cold drawing the strength increased to 278 MPa but conductivity reduced to 55.2% IACS. It appears that an optimum temperature and time combinations for aging treatments exists, as documented for the Al0.25 wt.% Zr alloy [163].

A supersaturation of the Al–Zr solid solution can also be achieved through severe plastic deformation. As example, high-pressure torsion shear of the Al–Zr alloy with shear strains up to 40,000, generated supersaturated solid solution where subsequent aging caused the precipitation of AlZr compounds with Bf orthorhombic structure at grain boundaries and the coherent metastable Al₃Zr precipitates within grain interiors with L1₂ cubic structure [164]. Although severe plastic deformation causes significant degradation in electrical conductivity, the subsequent aging and solid solution decomposition leads to an increase in conductivity. These microstructural features led to hardness of 148 HV, thermal stability up to 300 °C and electrical conductivity up to 35% IACS. About 30% of hardening was caused by precipitation strengthening when remaining was due to nanograin formation, grain boundary segregation and dislocation accumulation (Fig. 14).

Figure 14

Properties of Al–Zr conductors with different processing routes: a–e age-hardenable Al5Zr (wt.%) alloy processed by ultra-severe plastic deformation/high-pressure torsion: a schematics of microstructure change after casting, severe plastic deformation and aging at 230 °C for 48 h; b STEM-HAADF micrographs for samples processed by high-pressure torsion of N = 1000 turns; c, d microstructure after N = 1000 turns and aging at 230 °C for 48 h; e electrical conductivity versus hardness plot after deformation and aging at parameters indicated, reproduced from [164] with permission from Elsevier; f–i: Al0.1Zr (wt%) alloy after different processing routes and aging temperature of 265 or 400 °C: hardness (f), tensile strength (g) and electrical conductivity (h); (i) electrical conductivity vs tensile strength for conditions indicated reproduced from [165] under CC-BY license.

To overcome the strength–electrical conductivity trade-off in the microalloyed Al0.1Zr (wt.%) conductor, a nanostructuring strategy was found effective [165]. The optimum processing route consisted of solutionizing at 550 °C for 48 h with cold water quenching, aging at 265 °C for 24 h or at 400 °C for 24 h and cold drawing to a diameter of 3.84 mm. As a result, the Al–Zr conductor achieved the highest electrical conductivity (59% IAST) and at the same time the highest strength (̰220 MPa). The key objective  of the processing was to create the nanosize intragranular coherent Al₃Zr precipitates to provide strengthening and minimizing the local strain field to reduce the scattering of electrons.

Supersaturation via alloying with third element

The alternative way of generating the supersaturated Al–Zr state is through alloying with third elements, typically rare earths or transition metals. The alloying results in accelerated precipitation of Al₃Zr nanoparticles that stabilize the non-equilibrium microstructure. An investigation of bimetallic fine-grained wires from Al0.25Zr–(Sc,Hf) (wt.%) alloys revealed formation of the Al₃(Sc,Hf) particles leading to the maximum values of microhardness around  500–520 MPa and ultimate strength of 195–235 MPa after annealing at 500 °C for wires made from Al with 0.05–0.1Sc (wt.%) [166].

The combination of integrated computational materials engineering and experiment found the Al–Zn–Zr and Al–Zn–Ni alloys as promising for high electrical conductivity and strength [156]. It was demonstrated that Ni and Zr were not present in the matrix as solute atoms but were completely used up by forming the Al_3-xZn_xNi and Al_3-xZn_xZr (L1₂ type) precipitates in respective alloys. The key outcome was that the electrical conductivity predictions from density functional theory (DFT) simulations agreed well with experiment when the solubility limits of the solutes in aluminum were not considered and all the solute atoms were assumed to remain in solid solution with no precipitation.

Cr, Mn, Ni

The studies of small additions (up to 1 wt.%) of Mn, Ni and Cr on mechanical properties and electrical conductivity of the Al–Fe–Si-based alloys showed mixed results. While additions of nickel increased the electrical conductivity by 3%, chromium and manganese reduced it by 5–10% with the difference being attributed to the elements content in solid solution [77].

Manganese, the low-cost transition metal, has a relatively low solid-state solubility in aluminum with the maximum of 0.62 at.% (1.25 wt.%) at the eutectic temperature of 658 °C and the Al₆Mn phase providing strengthening. The eutectic reaction L ↔ (Al) + Al₆Mn occurs at the composition of 1.0 ± 0.1 at.% Mn [167]. In the conventional wrought AA3000 and AA7000 alloys, additions of 0.05–1.20 wt.% Mn increased strength by about 50% [168]. When low amounts of 0.05 wt.% Mn had little effect on the electrical conductivity of as-cast AA3000 grades, increased Mn to 1.04 wt.% caused the conductivity reduction by 30% IACS.

The simultaneous increase in the electrical conductivity and hardness was achieved in the Al–1.5 wt.% Mn as-cast ingots and cold-rolled sheets by additions of 1.5 wt.% Cu and 0.5 wt.% Zr [169]. The Al solid solution with low level of solutes and the Al₂₀Cu₂Mn₃ and Al₃Zr (L1₂) strengthening nanoparticles increased hardness twice and conductivity up to 30% as compared to the Al–1.5Mn (wt.%) base, making it better than the AA6201-type grades (Fig. 15).

Figure 15

Simultaneous increase in the electrical conductivity and hardness of Al–1.5 wt.% Mn alloy by addition of 1.5 wt.% Cu and 0.5 wt.% Zr: a conductivity vs temperature plot; b hardness vs temperature plot; c TEM image of Al–Cu–Mn precipitates; d TEM image of Al–Zr precipitates. Reproduced from [169] under CC-BY license.

Iron

The Al–Fe immiscible systems, similarly as Al–RE, are promising for electrical conductors due to negligible solid-state solubility of Fe in Al and the presence of thermally stable Fe-containing compounds. The maximum solid-state solubility of Fe in Al reaches 0.052 wt.% at 650 °C [170].

Improving strength of the Al–Fe–X ₁–Xn alloys through deformation

Since the Al–Fe alloys are not age-hardenable, to improve their strength without compromising electrical conductivity the cold deformation is frequently used.

During researching of the Al1.7Fe (wt.%) alloy, two-stage processing by equal-channel angular pressing and cold rolling led to tensile strength of 257 MPa along with electrical conductivity of 53.3% IACS after continuous casting and 298 MPa and 51.3% IACS after continuous casting into  an electromagnetic crystallizer [171]. The study of the Al0.5Fe and Al1.7Fe (wt.%) alloys produced by casting into an electromagnetic crystallizer showed that the latter one reached the same thermal stability as that of the Al–Zr and Al–RE conductors [172]. The electrical conductivity—strength of the Al0.5Fe (wt.%)  was 58.4 %IACS 204 MPA, respectively,  reducing for the Al1.7Fe (wt.%) to 52.0 %IACS with substantial strength increase to 295 MPa. 

In case of the AA8176 Al–Fe alloy, the temperature of equal-channel angular pressing affected the structure/conductivity optimum [173]. The elevated temperature deformation produced the ultra-fine-grained alloy with micron-size Al₃Fe and Al₆Fe precipitates where their strong dissolution was demonstrated in this immiscible Al–Fe alloy systems inducing high density of dislocations and accelerated the grains refinement. The modified treatment resulted in high strength with yield stress of 178.0 MPa along the electrical conductivity of 61.55% IAST.

In the Al1.13 wt.% Fe wire, fabricated by cold drawing with area reduction of over 61.9% the high strength was accompanied by high electrical conductivity [174]. The simultaneous increase in the strength and electrical conductivity was attributed to the elongated grains, the < 111 > texture, the low solubility of Fe and the nanoscale precipitates. The conductivity increase was explained by the fact that most of 1.13 wt.% of Fe added to Al formed precipitates during aging and did not remain in Al solid solution (Fig. 16).

Figure 16

Abnormal strength–electrical conductivity relation for Al1.13Fe0.03Si0.2Cu (wt.%) alloy wire: axial orientation distribution maps of the Al–Fe wire with the area reductions of a 4.7%, (b) 61.9%, c 80.0% and d 86.3%; e relation between the area reduction, yield strength and electrical conductivity; f mechanisms of the abnormal strength–electrical conductivity relation. Reproduced from [174] with permission from Elsevier.

Al–Fe–RE systems

The brittle nature of Fe-containing particles that act as crack initiation sites during tensile loading is reduced by changing their morphology, size and distribution through rare-earth additions, serving as the common modifiers. After rare-earth additions, the Al–Re eutectics are formed changing the crystallization process, thus refining grains and the primary Al₃Fe phase.

Since the common rare-earth modifiers have the adverse effect on strength and electrical conductivity, there are trends to replace them with Zr, Sc and Er. It was revealed that both Zr and Er could inhibit recrystallization of the Al0.4 wt.% Fe alloy by formation of Al₃Zr or Al₃Er particles. The Al alloys containing Zr are more recrystallization-resistant than those containing Er due to the preferential precipitation of Al₃Zr. While testing the Al0.4Fe (wt.%) alloy, Er additions were more beneficial and the Al0.4Fe0.2Er (wt.%) alloy after cold rolling reached the yield strength of 145 MPa, elongation at fracture of 8% and electrical conductivity of 61.2%IACS [175]. The microstructure of Al0.7Fe (wt.%) alloy was refined by Sc, through the Al₃Sc particles formed during solidification, and the creep resistance was also significantly improved [176].

In contrast, in Al–RE alloys, Fe is used to improve strength while minimizing the electrical conductivity reduction [177]. For example, additions of 0.25 to 0.75 wt.% of Fe to Al–1.0 wt.%RE (presumably mischmetal) increased volume fraction of second phases and caused the electrical conductivity reduction from 60.29 to 59.13% IACS, accordingly. At the same time, additions of 0.75 wt.% Fe increased the alloy strength to 77.5 MPa. A similar mechanism was explored through Ce modification of the Al2Fe (wt.%) binary alloy where Ce reduced the coarsening temperature of Al–Fe eutectic structure, improved the morphology and distribution of Fe-containing phases, and simultaneously increased both the conductivity and mechanical properties [178].

In wrought Al alloys containing 0.4–3.3 wt.% Ce, additions of 0.1–1.0 wt.% Fe combined with deformation and heat treatment increased the alloy strength to 172–179 MPa and specific electrical resistance to 0.0291–0.0300 Ohm‧mm²/m [179, 180]. Additions of 0.4–0.6wt.% Ce and 0.2–0.3 wt.% La increased the electrical conductivity through formation of ternary Al₄(Ce, La), AlFeSi and quaternary Al(Ce, La)Fe phases, reduced Fe and Si in the solid solution. In a presence of Zr, nanosize coherent Al₃Zr and dispersed Al₄(Ce, La), Al(Ce, La)Fe compounds, uniformly distributed in the aluminum matrix, improved the conductor thermal stability [155].

Silver

Silver is used as addition to low-current and high-power transmission due to its outstanding electrical conductivity. The practical usage of Al–Ag alloys is very limited due to relatively high cost of Ag in comparison with other alloying elements. The solubility of Ag in Al is high, reaching 56 wt.% at the eutectic temperature of 567 °C but at 100 °C it changes from 0.2 to 0.8 wt.%. The Al–Ag alloys with low Ag content are used to increase the current-carrying capacity, especially in the cases of ACSS conductors with soft wires operating at 150–240°C [181].

The Ag additions are also used in multi-component alloys. Additions of 0.1 and 2.0 wt.% Ag to the high-silicon Al10Si (wt.%) alloy were tested as an alternative solution for as-cast components of electric vehicles [182]. In particular, the Al10Si0.1Ag (wt.%) alloy exhibited tensile strength of 160 MPa, elongation of 12% and electrical conductivity of 34% IACS.

The mechanical properties and microstructure of Al–Mg–Cu–Ag alloys, obtained mainly due to fine and uniformly distributed Ω phase, are very sensitive to heat treatment and deformation conditions [183]. In case of the Al4Cu (wt.%) alloy, additions of Ag slowed down the aging rate  at the low temperature limit and hardening took place by the expanding aging time [184]. Thus, to achieve the optimum of strength and electrical conductivity in addition to conventional thermomechanical treatments, comprising of quenching from the solutionizing temperature and cold wire drawing followed by post-aging treatment, the modified, more complex processing scenarios are employed.

Transition metal impurities and the boron treatment

Transition metal impurities in aluminum solid solution that originate from raw materials as alumina and coke as well as operational practices significantly impede its electrical conductivity, and their detrimental effect was shown earlier in Fig. 2d. Of several impurities such as titanium, zirconium, vanadium and chromium, two latter ones exert the greatest effect and their presence in the solid solution increases the aluminum resistivity by a factor of 10 to 20 [185]. The transformation of solute atoms into precipitated phase (Al(V, Ti, Cr…)B₂) is the essence of treatment aimed at conductivity improvement.

In industrial practice, transition metal impurities are removed in molten state by additions of Al–B master alloys in a form of waffles, ingots or rods that contain AlB₂ or AlB₁₂ phases. During treatment in liquid state, impurities combine with boron, creating borides that are subsequently separated through the gravity settling or downstream filters [186]. Knowledge of thermodynamics and kinetics of borides formation in a molten aluminum can help to identify the ways of designing the more effective boron treatments [187].

Boron treatment in liquid state

The liquid-state boron treatment is applicable to conductors of technically pure Al and to its alloys with larger contents of other elements.

During experiments with the AA1070 alloy, 0.2% addition of the Al5Ti0.8B0.2C (wt.%) master alloy led to the electrical conductivity of 60.7% IACS [188]. After treatment with 1% of Al6B (wt.%), its electrical conductivity reached 64% IACS, i.e., 5.3% improvement. After treatment with 0.2% of Al6B (wt.%) and 0.5% Al5Ti0.8B0.2C (wt.%) additions, the electrical conductivity of the AA1070 alloy reached 63.2% IACS. At the same time, the alloy tensile strength reached 85 MPa with elongation of 58%, improvement by 26.9% and 9.4%, accordingly as compared to the untreated state. Treatment of the same AA1070 alloy using 3% Al8B2C (wt.%) master alloy resulted in thermal conductivity of 62.06% IACS [189]. The alloy mechanical properties improved due to dispersion strengthening and grain refinement, caused by the Al₃BC particles. A slight increase in the alloy resistance due to the aluminum-rich phase Al₃BC can be offset by AlB₂ particles in the Al8B2C (wt.%) master alloy because of the inoculation of transition elements. To improve electrical conductivity of 99.6% pure commercial aluminum with high concentration of transition elements: 0.013% Ti, 0.011% V and 0.006% Cr, an inoculation by aluminum–boron alloys in the phases of 5% AlB₁₂, 4% AlB₁₂ and 3% AlB₂ was found effective [190].

To improve the efficiency of boron treatment and increase a purity of Al matrix, a strategy was proposed that involved deliberate additions of trace amounts of transition metals during boron treatment (Fig. 17) [191].

Figure 17

Improving efficiency of the boron treatment of AA6101 alloy by trace additions of TM (Ti, Zr, V)-: a schematic diagram of the mechanism of boride formation by Zr-assisted boron treatment; b changes in conductivity of AA6101 after the addition of different contents of TM (Ti, Zr, V)-assisted with boron treatment. Reproduced from [191] with permission from Elsevier.

An optimum addition of 0.09 wt.% B was recorded during treatment of the AA6101 alloy that led to the maximum conductivity value of 53.4% IACS; lower and higher amounts of boron additions were less effective. Analysis of the boron reaction products revealed that the borides of transition metals developed a core–shell structure with a core remaining as an unreacted AlB₂ compounds, contributing to low efficiency of the treatment. For 0.09 wt.% B, additions of 0.05 wt.% Zr, 0.06 wt.% V and 0.03 wt.% Ti resulted in the AA6101 electrical conductivities of 54.2% IACS, 53.9% IACS and 53.7% IACS, respectively, suggesting that increased contents of transition metals broke the core–shell structure of the AlB₂ compound, improving the treatment efficiency. Of elements tested, Zr was the most effective and the electrical conductivity of the AA6101 alloy was 8.2% better than that without treatment, being 50.1%IACS. It appears that Zr can replace the transition metals in boride particles and the replaced transition metals continue to react with the unreacted AlB₂ in the boride center.

Boron treatment in solid state

In addition to liquid-state treatments, there are reports confirming the effectiveness of solid-state treatment in boron-containing environments on aluminum conductivity. For example, in case of aluminum single crystals, grown by the Bridgman method, the resistivity at room temperature decreased by 11.5% after heat treatment in a boron environment at 600 °C, i.e., well below its melting temperature [192]. The effect was explained by the boron-induced formation of distorted regions at the surface of aluminum crystals. These regions were of 30–50 μm in size and comprised of finer grains with a diameter of 5 μm, separated by low-angle grain boundaries. The conductivity improvement was attributed to the getter effect, i.e., the removal of the impurity atoms from the bulk crystal to the distorted surface regions by the outward diffusion.

Combining the boron treatment with other forms of processing

The boron treatment is often combined with other techniques of liquid and solid metal processing. For example, a combination of boron treatment, Sr modification and hot extrusion deformation of the hypoeutectic Al4Si (wt.%) alloy resulted in electrical conductivity of 59.2% IACS, ultimate tensile strength of 206 MPa and elongation of 26.7% [193]. The treatment not only removed transition metals from the Al matrix but also modified the eutectic Si morphology from large needle-/platelike to fine particles and changed its distribution from a segregated along grain boundaries to uniform one and generated the extrusion texture.

A combination of boron treatment and grain refinement improved the electrical conductivity of low-alloying aluminum conductors [194]. At the optimum addition of 0.12 wt.% B, the electrical conductivity of Al0.5 Mg0.35Si, Al0.5Fe0.2Si and Al0.8Fe0.2Cu (wt.%) reached 55.0%IACS, 58.3% IACS and 59.8% IACS, improving by 3.8%, 10% and 6.4%, respectively. After combining the boron treatment with grain refinement, the electrical conductivities of Al0.5Mg0.35Si, Al0.5Fe0.2Si and Al0.8Fe0.2Cu (wt.%) improved further to 56.5% IACS, 60.4% IACS and 61.8% IACS.

The composite boron treatment that was assisted by titanium additions helped overcoming some limitations of conventional boron treatment [195]. In experiments using the AA1070 alloy, the content of excessive B was neutralized to 66 ppm from 266 ppm and alloy reached the electrical conductivity of 66.9% IACS compared to 62.4% IACS of the untreated state. Another composite boron treatment, accompanied by alloying, hot extrusion and heat treatment applied to the Al4Si0.8Mg0.6Fe (wt.%) alloy,  resulted in purification of the Al matrix with large amount of platelike Si being transformed into equiaxed forms with rodlike shape [196]. The alloy reached strength of 227 MPa and electrical conductivity of 52.1%IACS.

The combined boron treatment of the A356 alloy that consisted of liquid-state additions of 0.6 wt.% nano-AlN_p and 0.06 wt.% B resulted in electrical conductivity of 46.2% IACS and tensile strength of 300 MPa [197]. During treatment, boron neutralized the detrimental transition metals while nano-AlN_p decreased the size and the mutual overlap of eutectic silicon, which reduced the electrons scattering (Fig. 18).

Figure 18

Simultaneous increase in the strength and conductivity of the A356 alloy by boron coupling nano-AlN_p: a effect of different contents of A1-3B master alloy on conductivity; b effect of aging time at 180 °C on the electrical conductivity of samples which were solution treated at 500 °C for 3 h (AB alloy with 0.6% nano-AlN_p addition, B alloy after only boron treatment); c synergistic effect between nano-AlN_p and AlB₂ without AlN_p and AlB₂; and d with AlN_p and AlB₂. Reproduced from [197] with permission from Elsevier.

Nanostructured aluminum conductors

Grain boundaries in metals usually reduce the electrical conductivity due to their distinct atomic arrangement compared to the grain interior. It would be anticipated, therefore, that the grain refinement, especially to the nanometer size, would lead to the conductivity reduction. This challenge has been overcome through novel manufacturing techniques of nanostructured conductors.

Concept of conductivity improvement by grain refinement

The design strategy of aluminum conductors with nanosize grains aims at simultaneous increasing the alloy strength and electrical conductivity, in contrast to their coarse-grained counterparts [26]. The solid-state grain refinement of aluminum relies on recrystallization, which can be achieved by conventional thermomechanical treatment or severe plastic deformation. The latter one is the promising method of obtaining a significant increase in alloy strength and electrical conductivity [198]. The solid-state deformation could be implemented by a variety of specific techniques including multi-directional forging, equal-channel angular pressing, cyclic extrusion–compression or high-pressure torsion.

The main strategy of Al grain refinement for electrical conductors relies on grain boundary engineering to create grain boundaries with reduced resistivity. Severe plastic deformation leads to the formation of an ultra-fine-grained structure with high population of grain boundaries with special crystallographic structure that differ from the state of grain boundaries in coarse-grained materials. According to this approach, controlling the shape and orientation of grains could break the trade-off relation between the strength and conductivity [100] 199.

In Al alloys, the strategy involves a combination of grain refinement with an accelerated formation of nanosize precipitates during severe plastic deformation. An increase in electrical conductivity is caused by migration of alloying elements from the solid solution into very fine precipitates, while the higher strength results from grain size refinement and precipitation contribution.

Grain refinement in pure Al conductors

For pure Al, severe plastic deformation is seen as the effective strategy to break the trade-off between the strength and the electrical conductivity and a number of fundamental studies of nanostructured conductors involved pure aluminum. For example, an ultra-fine pure Al wire with a diameter of around 0.2 mm manufactured by two-stage cold drawing with intermediate annealing, generated ultra-fine and long grains with super-strong < 111 > texture. The conductor achieved the high strength of 228.3 MPa and the conductivity of 63.14%IACS [200]. It was concluded that the ultra-fine grains in the radial section together with the super-strong < 111 > texture provided grain boundary strengthening and texture strengthening while the axial long grains weakened the electron scattering due to the reduction in volume of transverse grain boundaries (Fig. 19a–d).

Figure 19

Improving the conductivity and strength through grain refinement and grain boundary design. Ultra-fine grain pure aluminum wire: orientation distribution maps of a radial direction at low magnification, b radial direction at high magnification and c axial direction; d ultimate tensile strength versus electrical conductivity of pure Al conductors prepared by different processes, (CD: cold drawing; HE: hot extrusion; RS: rotary swaging; ECAP: equal-channel angular pressing; DDT: dual-direction torsion; SDT: single-direction torsion; CDDT: cryogenic dual-direction torsion and CSDT: cryogenic single-direction torsion), reproduced from [200] with permission from Elsevier. Al–Si–Mg alloy: e, f STEM image of microstructure of AA6101 alloy processed up to 20 turns by high-pressure torsion at room temperature followed by aging at 130 °C during 48 h; g general prediction of the combination of hardness and electrical conductivity for Al–Si–Mg alloys as a function of the grain size and with 0.2 wt.% of solute left in solid solution, reproduced from [204] with permission from Elsevier.

For commercially pure ultra-fine-grained aluminum, annealing within a temperature range of 90–200 °C anomalously increased the microhardness by 6 to 13%, likely due to annihilation of mobile dislocations in high-angle grain boundaries leading to an increase in yield stress to activate the new dislocation sources [201]. At the same time, the electrical conductivity increased by 4–8%. An analysis of grain boundary nature in this nanostructured alloy revealed that a non-equilibrium state associated with strain-distorted grain boundary structure strongly affected the electrical resistivity [202]. Moreover, the resistivity of non-equilibrium grain boundaries was found to be at least 50% higher than the resistivity of the equilibrium grain boundaries in a conventional coarse-grained structure.

Grain refinement in Al–X ₁–X _n alloy conductors

In nanostructured Al alloys, second phases co-influence the strength–conductivity relation in addition to high density of grain boundaries. The higher strength is generally accompanied by lower conductivity, but grain refinement allows to improve that relation. It is commonly observed that ultra-fine grains and nanoscale precipitates optimize the combination of strength and electrical conductivity.

For example, the AA6101 wire rods deformed at 130 °C via equal-channel angular pressing exhibit the reduced grain size to 400–600 nm along with nanoscale spherical metastable β′ and stable β second-phase precipitates. The wire with such structure reached increased strength to 308 MPa at the electrical conductivity level of 53.1% IACS [203]. The strength-to-conductivity ratio can further be improved by artificial aging at 170 °C through additional decomposition of solid solution. Another study, exploring the AA6101 and AA6201 alloys, found that refining the grain size and precipitates to nanoscale level via severe plastic deformation and post-processing precipitation optimized the combination of strength and electrical conductivity (Fig. 19e–g) [204].

It should be noted that many observations made for Al–Si alloys are also valid for Al alloys with complex composition. For example, the Al0.25Zr0.25Er0.20Hf0.15Si (wt.%) alloy, deformed via equal-channel angular pressing and rotary swaging, subjected to long-term annealing at 300 °C reached the optimum combination of microhardness of 480 HV and electrical conductivity of 59.8% IACS [205]. The optimum of hardness and electrical conductivity resulted from nucleation of two types of non-coherent particles of nano- and submicron-sized Al₃(Zr,Hf) compounds and small submicron Al₃Er with additions of Si.

Conductors of Al composites and gradient structures

In present commercial applications, the composite conductor (composite-cored conductors) is understood as the design where the composite material within the core provides the mechanical strength. Two types of fiber composites are used in overhead conductors: polymer matrix composites (PMCs) and metal matrix composites (MMCs). The aluminum matrix composites provide an opportunity to achieve a combination of mechanical properties and electrical conductivity beyond the limits of conventional alloys. They are generally heavier and have a slightly lower sag performance than PMCs [206]. Several research directions aim at modifying both the metal matrix and reinforcement phases with overall strategy to enhance the mechanical strength while improving or retaining the electrical conductivity. The fiber-reinforced, aluminum matrix composite wire used for mechanical reinforcement in the manufacture of aluminum conductor composite-reinforced (ACCR) is covered by the ASTM-B976 specification. The carbon fiber thermoset polymer matrix composite core (CFC) for use in overhead electrical conductors is covered by the ASTM-B987 specification.

To understand how the matrix and reinforcements interact in controlling the flow of electrons in the composite structure and predict its electrical conductivity, different models were developed [207]. Although electrically non-conducting particles in a metal matrix enhance the mechanical properties, they can decrease the overall electrical conductivity. Models for composites containing high volume fractions of non-conducting inclusions were verified for the equiaxed or angular alumina particles of various sizes and size distributions embedded in a matrix of pure aluminum [208]. In addition to commercial composite conductors, there are research activities involving experimental compositions and processing, as described below.

Clad metal/metal composites

Clad conductors, understood as combinations of two or more dissimilar metals, are created to optimize the overall properties, in particular conductivity and strength.

Copper-clad aluminum wire

Cladding of electrical conductors was introduced in the 1920s and initially involved coating of copper with aluminum and other alloys. For example, tinning or plating coatings were used to prevent reactions between copper and insulation [209]. In contrast, the copper-clad aluminum wire, consisting of a solid aluminum core covered by a copper skin, was designed to improve the aluminum performance in home installations in the 1970s and it is included in the National Electrical Code since 1971 [210] (Fig. 20a).

Figure 20

Clad composite conductor wires: a copper-clad aluminum wire, reproduced from [270]; b schematic of concentric annular pattern and arrangement of the designed composite prior to the primary extrusion with diameter 50 mm and extruded wire with diameter 12.7 mm and EBSD analysis result showing grain morphology of the annealed MMMC with a 50:50% weight composition, reproduced from [214] with permission from Elsevier; c composite wire produced by extrusion of AA6201 clad and AA1370 core alloys, reproduced from [215] with permission from Elsevier; d composite wire of Al–Zr–Mg clad and Al–Zr core after  thermomechanical treatment consisting of annealing at 400 ^oC, equal-channel angular pressing and cold drawing: TEM of Al–0.4Zr (wt.%) core and Al–1.17Mg–0.35Zr (wt.%) clad, reproduced from [216] under 3.0 license

The copper-clad aluminum composite wire (CCA) consisting of the AA8000 series core, and the exterior layer of high-purity (99.8% O₂-free) copper is manufactured by extrusion and wire drawing. During production, aluminum is extruded and the resulting core, when still warm, is lined with copper by deforming a copper tape around the core, resulting in metallurgical bond [211]. A comparison between the oxygen-free high conductivity (OFHC) copper and composite wire showed that a reduction ratio of 99.6% applied to the composite wire resulted in electrical conductivity of over 66.8% IACS [212]. At the same time, tensile strength increased 37% and yield strength 183%. The strength improvement was attributed to the fine precipitates distributed in the aluminum alloy core and the residual stress, generated during deformation of materials with different elastic properties.

Clad wires of two aluminum alloys

The conductor wire composite of two aluminum alloys covers cladding and core of alloys with different strength and conductivity. The metal–metal composites are manufactured by heavy deformation including rolling, swaging or wire drawing of two-phase, ductile metal mixtures. The reduced thickness and interphase spacing allows simultaneous improvement of strength and ductility. Typically, in this composite a higher-strength core alloy with electrical conductivity less than 50% IACS is combined with at least one alloy cladding having higher electrical conductivity than the core [213].

A design of aluminum– aluminum composite wire for overhead transmission/distribution lines manufactured by extrusion is shown in Fig. 20b [214]. The wire was developed by use of the AA6061 alloy concentric tubes having different diameters with the AA1050 alloy rods sandwiched between the tubes and arranged in a concentric angular pattern. In this concept, the AA1050 alloy provides good electrical conductivity while the AA6061 alloy provides good strength. When changing the contribution of both alloys the strength/conductivity ratio may be modified. In another example, two commercial alloys the AA6201 and AA1370 were combined by co-drawing that was followed by diffusion annealing to generate the composite of graded chemistry (Fig. 20c) [215]. The microstructure with spatial gradients of nanometer size precipitates caused an improved combination of electrical conductivity and mechanical strength in torsion as compared to the predictions based on a classical rule of mixture.

To generate composite conductors, experimental Al alloys were also used. For example, a clad of Al1.17Mg0.35Zr (wt.%) alloy and the core of Al0.4Zr (wt.%) were combined by equal-channel angular pressing followed by annealing at 400 °C (Fig. 20d) [216]. For such a composite wire with ultra-fine grain structure, an ultimate tensile strength of over 360 MPa and a long-term operation temperature of 150 °C were achieved along with an electrical conductivity of about 50% IACS.

Deformation-processed metal–metal composite wires

Deformation-processed Al metal–metal composites are manufactured by deformation using drawing, swaging or rolling of Al (matrix) and different metal, typically body centered cubic (bcc) second phase. Since both the matrix and reinforcing phase are ductile metals, they are heavily deformed during fabrication. Deformation-processed Al with other metal composites have a better combination of strength and electrical conductivity than Al alloys. The composite wires are seen as the economically efficient option for high-voltage power transmission.

In terms of pure metal reinforcement, several systems were tested. For example, the Al–20 vol.% Ti composite produced by powder metallurgy and deformation processing achieved strength of 890 MPa along with electrical resistivity of 43 nΩm [217]. The Al–20 vol % Mg and Al–13 vol % Mg deformation-processed composites achieved the electrical resistivity slightly higher than that of pure Al [218]. The Mg second phase showed a convoluted, ribbon shape filamentary morphology after deformation and the composite strength increased exponentially with reduced spacing of Mg filaments. The Al–Ca conductors are of interest for transmission lines since Ca has several desirable properties including good conductivity of 0.294 (μΩ·cm)⁻¹, very low density and strength like Al.

The Al/Ca (20 vol%) nanofilamentary metal–metal composite produced by powder metallurgy and severe plastic deformation reached the ultimate tensile strength of 476 MPa with electrical resistivity in the 305–518 × 10⁻⁴ μΩ m range, depending on annealing time at 300 °C [219]. In the same study, pure Al deformed and annealed the same way reached the resistivity of 280 × 10⁻⁴ μΩ m [220]. Heavy deformation drawing was found effective to manufacture the Al–Fe composite wire with 0.07, 0.1 and 0.2 volume fractions of Fe filaments and Al–Ca composite wire with 0.03, 0.06 and 0.09 volume fractions of Ca filaments [221]. The tensile strength reached 197 MPa for Al–Fe but was quite low of 90 MPa for Al–Ca with not clear conductivity results. In case of Nb, high strength up to 1030 MPa was achieved for Al composites wire with 20 vol.% Nb [222].

Composite of Al/carbon-containing strengtheners

Carbon in various forms is being considered as a component in Al-based composites to increase conductivity and decrease density. Carbon-reinforced MMCs exhibit a better overall performance than ceramic-reinforced MMCs due to their high electrical and thermal conductivity, excellent vibration damping capabilities and self-lubricating properties [223] 224225226. As examples, nanocarbon-based and metal–nanocarbon conductors are researched, where allotropes of carbon such as graphite, graphene and carbon nanotubes constitute the basis [227] 228. The carbon nanostructures include fullerenes, graphene, carbon nanofibers, carbon filaments, carbon nanotubes and various carbides as shown in Fig. 21a. There is a discussion on conductivity of carbon-containing compounds. The carbon nanotubes can act as conductor or semiconductor, but the graphite is good conductor of electricity. The carbon fibers conduct electricity which is enhanced by adding some reinforcing material. However, compared to aluminum, the electrical conductivity of carbon fibers is quite low to be used efficiently for practical purposes [229]. In addition to above forms of carbon, some interest is recently devoted to the form known as “covetic” [230].

Figure 21

Aluminum–carbon composite conductors: a different allotropes of carbon, reproduced from [223] under CC-BY license; b hardness, electrical conductivity and microstructure of Al–CNT composite, reproduced from [238] with permission from Elsevier.

Al/graphite flakes (GF), graphite nanoparticles (GNPs)

The flake powder metallurgy by vacuum hot pressing was found effective to fabricate the graphite flake-reinforced aluminum matrix composites [231]. The higher thermal conductivity in the in-plane direction of the graphite flakes was measured for the composite containing aluminum flake powder. However, due to the morphology incompatibility between the graphite flakes and the spherical aluminum powder, the damaged inner structure of graphite flakes contributed to the limited enhancement of the composite thermal conductivity.

The graphite nanoparticles were explored to synthesize the AA1100 matrix composite via hot extrusion [232]. An addition of 0.25 wt.% GNPs to the Al substrate improved its electrical conductivity by 2.1%, current density by 7.9%, ultimate tensile strength by 6.1% and yield strength by 30.3% compared to the reference alloy with no GNP additives.

Al/carbon nanotubes (CNT)

Incorporating nanocarbon phases into metal–matrix composites is a promising strategy for simultaneously enhancing electrical conductivity and mechanical properties of metals [233, 234]. The carbon nanotubes have a high aspect ratio and exceptional electrical conductivity as high as 1.7–2.0 × 10⁶ S/m and are considered as an attractive electrically conductive fillers for the fabrication of conductive composites [235, 236]. The nanocarbon-aluminum composites produced by an electro-charge-assisted process exhibited 5.7% increase in electrical conductivity and 8.2% higher hardness compared to the base metal alloy. In another example, composites manufactured using aluminum powders, reinforced with 0.5 wt.% of multi-walled carbon nanotubes and consolidated through spark plasma sintering exhibited electrical conductivity affected by the carbon nanotubes dispersion and the Al particles morphology [237]. The electrical conductivity improved around 40% IACS due to the effective multi-walled carbon nanotube network, formed within the nanocomposite material that promoted electron transport.

The composite of nanocrystalline Al with CNT, prepared by using bare Al nanopowder as a matrix, overcame the trade-off between the hardness and electrical conductivity. The electrical conductivity of the Al–CNT composite achieved 1.52 × 10⁷ S/m being 33 times higher than that of an Al–NC composite of 0.05 × 10⁷ S/m with the same hardness of 1.4 GPa (Fig. 21b) [238].

Al/graphene nanoplatelets (GNPs), nanosheets (GNSs)

Graphene nanoplatelet additions to pure aluminum are often used to manufacture materials with high strength and high conductivity. For example, aluminum matrix composites with 0.2 wt.% GNPs, fabricated by casting, the ball-milled powder followed by rolling, reached the ultimate tensile strength about 36.8% higher than that of pure Al with the same casting and rolling process caused by the lamellar structure and load transfer [239]. At the same time, the conductivity of the composites decreased by approximately 0.7% IACS, indicating that interface scattering of electrons between Al and GNPs was very limited.

In another example, the composite produced by casting with a mechanical electromagnetic stirrer, hot extrusion and annealing of Al with 0.5 wt.% GNPs showed that the tensile strength, elongation and electrical conductivity were altered by + 142%, − 7% and + 2%, respectively, at 20 °C, while at 180 °C the tensile strength and electrical conductivity rose by 67.3% and 5%, respectively [240]. An application of friction stir processing and hot extrusion, used to produce nanocomposites by incorporating graphene as the reinforcement, caused the simultaneously enhanced mechanical properties and electrical conductivity of pure aluminum [241]. The electrical conductivity, tensile strength and elongation of graphene/Al nanocomposite were 2.1%, 17.3% and 35.4% higher than those of pure Al. The observed breakthrough of trade-off tendency between mechanical properties and electrical conductivity was due to the homogeneously dispersed graphene within nanocomposite.

The graphene sheets were successfully produced from graphite in 2004 [242]. Graphene monolayers exhibit extraordinary levels of electrical, mechanical and thermal properties [243]. The aluminum composite with graphene nanosheets reinforced, fabricated from graphite by a novel hot accumulative extrusion bonding (AEB) method with only three cycles showed GNSs with thickness about 10 nm being gradually exfoliated with the process arrested at weak defective areas, such as vacancies and grain boundaries [244]. The elastic modulus of GNSs/Al of 80 GPa was achieved along with the electrical conductivity of GNSs/AA6063 of 1% IACS higher than that of reference Al after the same processing.

Aluminum–carbon covetic composites

Another class of metal–carbon composites termed covetics was first described by Scherer et al. in 2009 [245], following by process patents granted to Shugart and Scherer of Third Millennium Metals LLC (now known as GDC Industries LLC) [246]. The essence of the patent is that carbon can be incorporated into some metals by melting a metal and mixing it with carbon while applying high current so it forms a single-phase metal-carbon material.

The composites cover the metal carbon materials where carbon is infused into metal (Al, Cu, Fe, Ag) with amounts far higher than predicted by phase diagrams [230]. The objective is to create the carbon–metal bond under non-equilibrium conditions. In case of Al carbon covetics, it is claimed that during manufacturing a conversion reaction occurs, with the carbon-forming covalent bonds with aluminum, and that the covetic processing enables higher carbon solubility [247]. Traditionally, covetics were fabricated in an induction furnace with high-power electrical current in the liquid metal–carbon mixture. As a result, composites have increased mechanical properties and can conduct electricity more efficiently than conventional metals.

It should be noted that although there is evidence supporting some features of covetic materials, more experimental verification of claimed benefits is still required. In particular, some earlier research techniques are questioned, manufacturing details are not complete, and the covalent bonding of carbon and metal still requires strong evidence [248].

In situ Al–Al₂O₃ composites formed during consolidation of Al powders

Powder metallurgy (PM) processing provides the fine and homogeneous microstructure and near-net shape capability for aluminum alloys. The above properties are better than those achieved through ingot metallurgy. In fact, the extruded Al–Al₂O₃ composites are formed in situ during consolidation of ultra-fine Al powders. The powder metallurgy processed aluminum develops great mechanical properties and creep resistance at increased temperature due to nanoscale γ-Al₂O₃ dispersoids originated from the native amorphous Al₂O₃ films in the powder (Fig. 22) [249].

Figure 22

Conductor composite Al–γ-Al₂O₃ manufactured by powder metallurgy: a production schematics; EBSD of vacuum sintered state showing b orientation image map of grains, c map of high-angle grain boundaries, d map of low-angle grain boundaries, e map of kernel average misorientation and inverse pole figures; f ultimate tensile strength, yield strength, elongation and electrical conductivity (EC) after: hot extrusion (1) and annealing at 300 °C (2) and 500 °C (3), cold drawing (4) and annealing at 300 °C (5). Reproduced from [250] with permission from Elsevier.

The powder metallurgy followed by cold isostatic pressing, sintering, hot extrusion and cold drawing allowed manufacturing the novel Al conductor stabilized with a low volume fraction of γ-Al₂O₃ nanoparticles [250]. Under optimum processing through cold drawing the wire reached the strength of 172 MPa and electrical conductivity of 60.2% IACS. The fine grain stabilized by γ-Al₂O₃ nanoparticles at high-angle boundaries and texture play a significant role in the conductor performance.

Al composites with other reinforcement particles

A number of other reinforcements were researched for electrical conductor applications. Additions of TiB₂ particles combined with severe plastic deformation improved aluminum yield strength from 38 to 103 MPa and ultimate tensile strength from 73 to 165 MPa while maintaining electrical conductivity of cast aluminum without reinforcing particles of 53.9–54.1% IACS [251]. The alloy subjected to severe plastic deformation only achieved the high electrical conductivity of 63.1% IACS. This effect is explained by the grain refinement and the transfer of impurity elements from the solid solution to the boundaries. Also, 10 vol.% TiB₂ nanoparticles with a size below 10 nm incorporated into the aluminum matrix via flux-assisted liquid processing led to hardness of 130 HV and electrical conductivity of 41% IACS so they could be considered for the electrical and electronic applications [252].

Generating gradients in composition and in grain size

Gradient microstructures, where the grain size increases from nanoscale at the surface to coarse-grained in the core, were originally discovered in copper as an effective approach to simultaneously improve both the strength and ductility [253]. In general, creating the gradient microstructure in a wire requires a multistage process based on plastic deformation by twisting, compression bonding or friction–extrusion combination [254, 255]. The same concept was found effective to overcome the strength–electrical conductivity trade-off; dual gradient microstructure simultaneously improved the strength and electrical conductivity of aluminum wire.

In practical example, improving the combination of electrical conductivity and tensile strength of the AA1070 alloy by rotary swaging deformation was achieved through microstructure modification; elongated grains with a high fraction of 75% of low-angle grain boundaries and the < 1 1 1 > fiber texture [256]. An increase in hardness of 70%  to 109 MPa from 65 MPa was accompanied by a moderate conductivity reduction from 58.6% IACS to 56.7% IACS (1.9% IACS) (Fig. 23a). Since the elongated grain boundaries parallel to the direction of current are beneficial to the electrical conductivity, grain refinement led to substantial increase of strength and rather small conductivity reduction.

Figure 23

Improving the strength versus conductivity trade-off by microstructure modifications: A: graded microstructure of AA1070 alloy: a rotary swage deformation; b microstructure of fine and elongated grains; c strength vs conductivity plot, reproduced from [256] with permission from Elsevier; B: dual grain size microstructure of AA1050 H14 alloy a deformation by clockwise and anticlockwise rotation; b gradient and dual gradient grain size; c strength and conductivity distribution, reproduced from [257] with permission from Elsevier.

Experiments with the AA1050 H14 alloy, having the gradient microstructure and dual gradient microstructure that was achieved through clockwise torsion and subsequent anticlockwise torsion, showed an improvement in wire strength without compromising its electrical conductivity [257]. As shown in Fig. 23b, the dual gradient grain size provided extra strain hardening but less electrical resistivity, contributing to the observed superior mechanical and electrical properties.

Concluding remarks

The development of clean energy technologies requires high-performance conductors where aluminum with its characteristics is of strategic importance. In addition to conventional applications in power transmission and distribution, aluminum conductors are increasingly used in modern, renewable energy solutions including solar systems and transportation infrastructure. The lightweighting features of aluminums make it increasingly important in electric vehicles for wiring harnesses and traction motor windings. The novel market is emerging for cast components of traction motors, rotors or inverters, where cast aluminum alloys deliver the combination of high electrical conductivity and strength required to achieve the machine high-power output and efficiency.

The growing application opportunities result in intensive research activities aiming at development of new generation of aluminum conductors through alloying and processing including the ultra-fast crystallization technologies, thermomechanical treatments and severe plastic deformation aiming at grain reduction to nanometer scale, grain boundary engineering and crystallographic texture control. To develop conductors with high strength and superior thermal stability, transition metals and rare earths are explored as promising alloying candidates with a particular attention paid to immiscible systems, such as Al–Ce, Al–La and Al–Y. In addition to alloying and processing, a direction of aluminum matrix composites is seen as the route to advanced conductors where special attention is paid to the carbon-containing strengtheners. The key approach of all strategies of modern conductor development is to overcome the strength–conductivity trade-off and achieve the optimum required by the application design.