Introduction

Car manufacturers are forced to come up with innovative technological solutions that reduce the number of emissions produced as a consequence of strict emission standards. The development of low-emission internal combustion engines is significant, despite the fact that the work is mainly focused on the development of electromobility. The high purchase prices of electrical vehicles, which are linked to the scarcity of precious metals for battery production, and the issue of battery recycling are the main attributes that make electrical vehicles unaffordable for many mainstream users.1,2,3,4

Internal combustion engines have undergone significant changes in recent decades. Manufacturers' efforts to eliminate a great volume of emissions have led to reductions in engine displacement or the number of cylinders while maintaining or increasing engine power. The given technological solution of the emission issues causes the temperatures of the combustion process to rise, which causes the operating temperatures of highly stressed components (e.g., cylinder heads, pistons) to rise above 200 °C.4,5,6

Al-Si-Cu-Mg alloys account for nearly 90 % of the total castings used in the automotive industry. Specific strength, good foundry properties, and an advantageous combination of mechanical and physical properties allow their extensive use in the given industry. They find important to use, for example in the production of cylinder head castings. However, the Al-Si-Cu-Mg-based aluminum alloys that are conventionally used for the production of high-stress castings for the automotive industry are severely limited by an operating temperature of up to 200 °C.7,8 In general, the mechanical and physical properties of Al–Si–Cu–Mg alloys are limited by the thermal stability of the Cu- and Mg-rich phases. As a consequence of the presence of Cu and Mg, the effect of heat treatment by precipitation hardening leads to the precipitation of the strengthening precipitates θ-Al2Cu and β-Mg2Si. The strengthening precipitates θ-Al2Cu and β-Mg2Si have thermal stability up to the temperature of about 200 °C.9,10,11,12,13 Exceeding this temperature results in a decrease in mechanical and physical properties due to the coarsening and subsequent dissolution of the Cu- and Mg-rich reinforcing phases.9,10,11,12,13,14,15,16 One of the possibilities of increasing the thermal stability of Al-Si-Cu-Mg alloys at increased operating temperatures is the use of an alloying element or elements. The selected elements tend to form strengthening intermetallic phases, possess low solubility in solid solution α(Al) at operating temperatures, and have low diffusivity in solid solution α(Al). Based on the above conditions, the most suitable chemical elements are those that form the trialuminide compounds Al3X (X = transition metals, lanthanides, actinides). The high cost of lanthanides and the radioactive nature of actinides, transition metals are used in engineering applications to enhance thermal stability.17,18,19

The current development of Al–Si–Cu–Mg alloys focuses on increasing the thermal stability of Al–Si–Cu–Mg alloys through the use of transition elements (e. g. Zr, Ni, V, etc.). The transition elements form intermetallic phases of the Al3X type (X = Zr, Ni, Mo, V) in Al–Si–Cu–Mg alloys, which are characterized by thermodynamic stability, which is important in terms of maintaining the properties of the alloys at increased operating temperatures. Based on the literature survey, it can be declared that Sc and Zr have the most significant influence on the thermal stability of Al–Si–Cu–Mg alloys. Because of the high acquisition costs of Sc, which, together with the ever-increasing cost of energy and input materials, would be a liquidating factor for automotive component manufacturers. Hence, current developments are focused on the use of Zr as an alloying element in Al-Si-Cu-Mg alloys.18,19,20 In metallurgy, Zr is used as an alloying element of aluminum alloys. Zr crystallizes in aluminum alloys preferably in the form of intermetallic phases of the Al3Zr or AlSiZr type. The intermetallic phase Al3Zr crystallizes preferentially in two crystallographic systems, in the highly symmetric cubic system (L12) and the tetragonal system (D023). Al3Zr(L12) phases are significant for high thermal stability, dissolution, and coarsening resistance. The ability of Zr to form thermally stable dispersoids is currently being exploited to develop new aluminum alloys operating at increased temperatures of above 200 °C.20,21,22,23,24,25,26,27,28 Grain refinement has a significant effect on the mechanical properties and homogeneity of Al–Si–Cu–Mg castings. Due to the changes in the distribution of second-phase materials. Grain refinement reduces the porosity and size of the pores, and as a result, the fatigue strength increases. Zr also has an inoculating effect on Al–Si–Cu–Mg alloys. The Al3Zr intermetallic phases act as nucleation seeds of the primary α(Al) phase.29,30

The Al–Ti–B-based master alloys are economically more beneficial and also more efficient than Zr-based master alloys for the grain refinement of most of Al-Si-Cu-Mg alloys. However, compared to conventionally used Al-Si-Cu-Mg alloys for cylinder head castings, AlSi5Cu2Mg is significant for its low Ti content (0.03 wt% max). The specific chemical composition of AlSi5Cu2Mg alloy is an internal regulation of the supplier company we cooperate with. Considering that, for effective grain refinement of hypoeutectic aluminum alloys, it is necessary to add 0.04 to 0.1 wt% Ti, conventionally available AlTi5B1 inoculants cannot be used. Experimental alloy AlSi5Cu2Mg is not limited by the B content. The Al-B master alloys containing particles of AlB2 added to aluminum alloy lead to a significant grain refinement effect. However, Al-B-based master alloy dissolves rapidly in aluminum, where B reacts with Ti and Sr in the melt. The reaction of B and dissolved Ti in the melt results in the formation of “sludge” in the holding furnaces. Similarly, added B interface with Sr-modification potentially leads to a decrease in the modification effect. Therefore, Al-B-based master alloys cannot be used in the foundry.29,30

The research aimed to analyze the effect of graded wt% of Zr on the microstructure, thermal conductivity, degree of gassing, mechanical and physical properties of a hypoeutectic AlSi5Cu2Mg alloy. Non-normalized AlSi5Cu2Mg alloy is significantly used in the production of, e.g., cylinder head castings. Zr represented the function of both the alloying element and the primary phase α(Al) inoculant. The softening effect, affordability, and the ability of Zr to form thermally stable strengthening phases are decisive factors that enable the development of new Al-Si-Cu-Mg-based aluminum alloys with guaranteed thermal stability under elevated operating conditions. Increasing the thermal stability of Al-Si-Cu-Mg alloys intended for cylinder head castings using transition metals significantly expands their field of application. The reason is to increase the performance and durability of components operating at elevated operating conditions.

Methodology and Implementation of Experiments

The hypoeutectic aluminum alloy AlSi5Cu2Mg was chosen for the experimental work. The alloy is used in the production of highly stressed castings, e.g., cylinder heads for the automotive industry. The chemical composition of AlSi5Cu2Mg alloy is given in Table 1. It can be seen in Table 2 that the AlSi5Cu2Mg alloy was supplied in a pre-modified state (containing 0.01 wt% Sr).

Table 1 Required Chemical Composition of Reference Alloy (wt%)
Full size table
Table 2 Chemical Composition of the Experimental alloys AlSi5Cu2Mg with Zr Addition (wt%)
Full size table

The experimental AlSi5Cu2MgZr alloys were alloyed with graded Zr addition (0.05, 0.10, 0.15, and 0.20 wt%). Zr was introduced into the melt in the form of AlZr20. The chemical composition of AlSi5Cu2Mg alloy is given in Table 2. It can be seen in Table 2 that the real Zr content for AlSi5Cu2Mg alloys with 0.15 and 0.20 wt% Zr was lower due to the limited solubility of Zr in the melt.

The experimental alloys were prepared by melting 9 kg of a hypoeutectic aluminum alloy AlSi5Cu2Mg in an electric resistance furnace. At a temperature of 775 ± 5 °C, Zr was introduced into the melt in the form of AlZr20 master alloy. The experimental samples were cast at 745 ± 5°C by gravity casting technology into a metal mold (Figure 1). The material of the metal mold was steel. Metal mold was coated with a graphite powder. The temperature of the metal mold was maintained between 180 and 200°C by flame.

Figure 1
figure 1

Metal mold.

Full size image

The melt was not intentionally degassed. The absence of degassing had two main reasons. The first reason was that consistent degassing with inert gases (e.g., using a laboratory rotary degasser) is not possible in laboratory experiments with a maximum melt weight of 10 kg (small crucible). In these cases, degassing tablets are used. The tablets are based on chloride and fluoride salts and have a high affinity for oxygen. A violent exothermic reaction is produced at contact with the aluminum melt. In a small melt volume, it is assumed that the residues of these degassing tablets remain in the melt as exogenous inclusions, which may adversely affect the mechanical and physical properties.

For each material variant, 10 samples were prepared. Half of the experimental samples were evaluated in the cast state and the other half were evaluated after T7 over-aging heat treatment. The T7 thermal mode for the AlSi5Cu2Mg alloy was chosen due to the complex geometry of the cylinder head casting. The heat treatment T7 results in a relative stabilization of the microstructure and mechanical properties. The heat treatment of the experimental samples by T7 mode consisted of three steps:

  1. 1.

    Solution treatment at 500 ± 5 ºC for 6.5 hours,

  2. 2.

    Quenching into hot water (80 – 90 °C),

  3. 3.

    Artificial aging at 250 ± 5 ºC for 4 hours, followed by air cooling.

The mechanical properties of the experimental alloys were determined by static tensile testing. The tensile test was carried out with the Inspekt desk 50 kN universal tear tester according to the standard EN ISO 6892-1. For each experimental variant, a set of 10 test round bars with a shank diameter of 8 mm was made. The scheme of the tensile test bar is shown in Figure 2. Half of the test bars were evaluated in cast condition and the other half after T7 heat treatment.

Figure 2
figure 2

The scheme of the tensile test bar.

Full size image

The hardness of the experimental alloys was determined by the Brinell hardness test according to EN ISO 6506-1. The measurements were carried out with a Brinell Innovatest Nexus 3000 hardness tester according to HBW 5/250/10 (indentation body–5 mm diameter carbide ball/load size 250 kp/load time 10 s). For each material variant, 5 measurements were carried out.

The microstructure of the experimental alloys was evaluated with a Neophot 32 optical microscope and a TESCAN LMU II scanning electron microscope with a BRUKER EDX analyzer. The preparation of the samples consisted of wet hand grinding, and polishing on polishing wheels impregnated with diamond paste and moistened with alcohol, and then the samples were polished using Struers Laboforce-3 automatic polisher. The experimental samples were subsequently etched with H2SO4. Microstructural analysis was performed on samples with the optimum combination of mechanical properties. The grain size was assessed by quantitative microstructural electron backscatter diffraction (EBSD) analysis. The fractographic evaluation was performed by a TESCAN LMU II-line electron microscope with a BRUKER EDX analyzer.

The methodology for the determination of the thermal conductivity of the experimental samples was based on the evaluation of the conductivity of the samples under examination using the Sigma Check 2 measuring device. The calculation of the thermal conductivity (λ) of the experimental samples was carried out by substituting the values of the electrical conductivity (σ) into the empirical formula (1):

$$ \lambda = 4,29 . \sigma - 13,321 \left[ {{\text{W}}.{\text{m}}^{ - 1} .{\text{K}}^{ - 1} } \right] $$
(1)

The degree of the gassing of the experimental samples was determined by the double weighing method, which consists in measuring the weight of the sample cooled in air and in a vacuum. By comparing the determined masses, the density of the experimental samples solidifying freely at atmospheric pressure and in a vacuum was calculated. The calculated densities of the experimental samples were then substituted to the empirical formula for calculating the density index (DI), which has the form (2):

$$ DI = \frac{{\rho_{atm} - \rho_{vak} }}{{\rho_{atm} }} . 100 \left[ \% \right] $$
(2)

Samples prepared for gassing rate assessments were subsequently used for assessments of proportion of surface porosity. The proportion of surface porosity was evaluated by Quick Photo Industrial 3.1 graphic software. For each experimental sample, 5 random locations were evaluated.

Results and Discussion

Evaluation of Porosity

The mechanical and physical properties of Al–Si–Cu–Mg alloys are affected by the presence of hydrogen in the melt. The presence of hydrogen in the melt promotes the formation of porosity in the castings, which has a negative impact on the final quality of aluminum castings. The final quality of Al–Si–Cu–Mg-based castings can be predicted by controlling the degree of solubility in the melt. The degree of solubility of hydrogen in the melt is assessed indirectly by the determination of the density index DI. The DI density index of aluminum alloys is evaluated by comparing the density of two identical samples solidified at different solidification conditions. The first aluminum alloy sample was solidified under atmospheric pressure, and the second aluminum alloy sample was solidified in a vacuum chamber under a specified vacuum pressure (80 mbar for 4 minutes). The DI density index of the evaluated alloys was subsequently determined by the double-weighting method (2). It should be noted that the melt was not intentionally degassed to research the real effect of graded wt% of Zr on the degree of the gassing of the experimental AlSi5Cu2Mg alloy.

The values of DI density index versus wt% of Zr were processed into the graphical dependence in Figure 3. The DI density index of all experimental alloys with Zr addition increased compared to the alloy without Zr addition. The largest increase in DI was observed with AlSi5Cu2Mg alloy with 0.20 wt% of Zr, while DI increased by approximately 68 % compared to AlSi5Cu2Mg alloy without Zr addition. The smallest increase in DI of approximately 22 % was observed for the AlSi5Cu2Mg alloy with 0.15 wt% of Zr compared to the alloy without Zr addition. Based on the results, it can be concluded that the alloying of AlSi5Cu2Mg Zr alloys has a negative effect on the degree of gassing. The increase in DI has a negative impact on both the mechanical and physical properties and the resulting quality of castings made of AlSi5Cu2Mg alloy with Zr addition.

Figure 3
figure 3

DI values of the experimental alloys.

Full size image

The metallurgical process of melt preparation was able to significantly influence the DI values. Zr was added to the melt in the form of AlZr20 master alloy. Due to the fact that AlZr20 master alloy is hardly soluble, the increase in DI (hydrogen solubility) under our conditions also occurred by increasing the melting temperature up to 780 °C and increasing the residence time at a given temperature. These factors may have significantly influenced the degree of the gassing of the researched alloys with the addition of Zr.

The effect of increasing wt% of Zr on the porosity of AlSi5Cu2Mg alloy was researched by determining the proportion of surface porosity. The proportion of surface porosity was evaluated by Quick Photo Industrial 3.1 graphic software. The values of the proportion of surface porosity of AlSi5Cu2Mg alloy as a function of wt% of Zr were processed into the graphical dependence in Figure 4. The proportion of surface porosity of the experimental alloy without Zr addition was 1.1 %. The addition of 0.05 and 0.1 wt% of Zr did not significantly change the proportion of surface porosity compared to the alloy without Zr addition. The porosity of the experimental alloys with 0.15 and 0.20 wt% of Zr increased by 90 and 155 %, respectively, compared to the alloy without Zr addition. Porosity adversely affects the resulting mechanical and physical properties of castings. The physical properties of aluminum alloys are related to the scattering of electrons by the environment. The magnitude of electron scattering depends on the static imperfections present such as point defects, dislocations, impurities, or secondary phase particles. The pores present in the casting block the transfer of electrons through the environment, thus decreasing the thermal conductivity of the casting. The decrease in thermal conductivity adversely affects the performance of cylinder head castings.

Figure 4
figure 4

Area fraction of porosity depending on the Zr addition.

Full size image

Each experimental sample was subjected to the proportion of surface porosity measurements at five different locations. The pores were observed in the metallographic cut plane using the “phase analysis” function as yellow areas (Figure 5).

Figure 5
figure 5

Comparison of area fraction porosity by using software Quick Photo Industrial 3.1: (a) AlSi5Cu2Mg alloy without Zr addition (b) AlSi5Cu2Mg alloy with 0.10 wt% Zr addition, (H2SO4 etch.); (c) AlSi5Cu2Mg alloy with 0.20 wt% Zr, (H2SO4 etch).

Full size image

Evaluation of Thermal Conductivity

The thermal conductivity of Al-Si-Cu-Mg alloys is an important material characteristic that greatly affects the functionality and performance of automotive components. Cylinder head castings have complex geometries and can be exposed to temperatures over 200 °C under operating conditions. In general, the properties of Al–Si–Cu–Mg alloys are limited by the thermal stability of the Al2Cu and Mg2Si strengthening precipitates, which are stable up to the temperature of about 200 °C. Exceeding this temperature results in a decrease in both mechanical and physical properties due to the coarsening and dissolution of the Cu- and Mg-rich reinforcing phases. Therefore, the effect of Zr as an alloying element on the thermal conductivity of aluminum alloy was researched.9

The values of thermal conductivity in the cast state as a function of wt% of Zr are processed into the graphical dependence in Figure 6a and represent the average values of 5 measurements. The thermal conductivity of the experimental alloys with Zr addition in the cast state decreased compared to the alloy without Zr addition. The largest decrease of approximately 11 % in thermal conductivity compared to AlSi5Cu2Mg alloy without Zr addition was observed for AlSi5Cu2Mg alloys with 0.1 and 0.2 wt% of Zr. The thermal conductivity of the examined alloys with the addition of Zr decreased due to the action of Zr phases as “impurities” that block the electron transfer through the environment. Thermal energy is transferred in metals through the motion of free electrons and the vibration of atoms. In general, it can be stated that any element that is added to the aluminum alloy negatively affects the thermal conductivity values. The alloying element added to the alloy negatively affects the free path of electrons and the vibration of atoms, leading to a decrease in thermal conductivity.31 The thermal conductivity of the alloys in the cast state was also significantly affected by the degree of hydrogen solubility in the melt. By evaluating the density index in correlation with the thermal conductivity, it was demonstrated that the thermal conductivity of the experimental alloys decreases with increasing DI density index as a function of wt% of Zr. Based on the results obtained, it can be concluded that the pores block the heat transfer through the environment, which leads to a significant decrease in the thermal conductivity of the alloys in the cast state.

Figure 6
figure 6

Thermal conductivity of the experimental alloys: (a) as-cast state; (b) after T7 heat treatment.

Full size image

The thermal conductivity values after heat treatment T7 as a function of wt% of Zr are processed into the graphical dependence in Figure 6b. The thermal conductivity of AlSi5Cu2Mg alloy without Zr addition increased by 18 % due to heat treatment. The thermal conductivity of the experimental alloys after heat treatment increased significantly compared to the cast condition. The largest increase in thermal conductivity of almost 33 % was recorded by AlSi5Cu2Mg alloy with 0.15 wt% of Zr. There were no significant changes in thermal conductivity as a function of graded Zr addition in the T7 heat treatment compared to the Zr-free alloy after T7. Based on the summarized results, it can be concluded that the addition of Zr did not significantly affect the thermal conductivity values of AlSi5Cu2Mg alloy. In this respect, Al-Si-Cu-Mg alloys alloyed by Zr are suitable for the development of aluminum alloys for highly stressed components for the automotive industry.

A significant problem with cylinder heads is heat accumulation due to the complex geometry of the casting. The complex geometry of the casting causes the formation of a non-uniform temperature field, which causes a decrease in the physical properties, performance, and service life of highly stressed cylinder head castings. One of the ways to effectively enhance the physical properties of Al-Si-Cu-Mg alloys is to use an optimum heat treatment regime. The optimum heat treatment regime produces a more uniform temperature field due to the increase in the physical properties of the Al-Si-Cu-Mg alloys, thereby increasing the performance of the cylinder head castings.9

Based on the results, it can be concluded that the increase in thermal conductivity after heat treatment T7 was due to the transformation of eutectic Si. In the cast state, eutectic Si is emitted in the form of plates that block the transfer of electrons through the environment (Figure 7a). The effect of T7 heat treatment results in the transformation of eutectic Si to the more energetically favorable state of rounded grains (Figure 7b). Eutectic Si precipitated in the form of rounded grains provides a more favorable electron transfer through the medium, thus favorably increasing the thermal conductivity of the Al-Si-Cu-Mg alloy.32,33,34,35

Figure 7
figure 7

Transfer of electrons: (a) as-cast state; (b) after T7 heat treatment.36

Full size image

Results of Mechanical Properties

The evaluation of the mechanical properties was based on the comparison of the mechanical properties of the experimental alloys with the graded addition of Zr with the primary alloy AlSi5Cu2Mg. The mechanical properties evaluated in the as-cast condition and after heat treatment T7 are shown in Figure 8. The resulting mechanical property values represent the average of 5 measurements.

Figure 8
figure 8

Mechanical properties of the experimental alloys AlSi5Cu2Mg with addition of Zr: a) as-cast state; b) after T7 heat treatment.

Full size image

The addition of Zr to AlSi5Cu2Mg alloy does not significantly affect the values of mechanical properties in the cast state (Figure 8a). An increase in UTS and YS can be observed due to the heat treatment T7 (Figure 8b). The highest UTS and YS values were observed for the alloy without Zr addition. With increasing wt% of Zr, a decrease in UTS and YS was noted. It can be concluded that Zr does not significantly affect the values of UTS and YS. The hardening effect after heat treatment T7 was due to the elimination of Cu- and Mg-based hardening precipitates. The hardness of the experimental alloys increased slightly by the effect of heat treatment. The highest hardness values were recorded by AlSiCu2Mg alloys with addition of 0.05 and 0.10 wt% of Zr. The hardness of the experimental alloys with 0.05 and 0.10 of wt% Zr increased by 6 % compared to the AlSi5Cu2Mg alloy without Zr addition. The hardness of the evaluated alloys with the addition of Zr increased due to the crystallization of hard intermetallic phases of the Al3Zr type. The ductility of all alloys due to T7 heat treatment decreased by 50 % compared to the cast condition.

Evaluation of Microstructure, EBSD Analysis, EDX Phase Analysis, and Fractographic Evaluation

In Figure 9 (blue frames), the microstructure of AlSi5Cu2Mg alloy with graded addition of Zr in the cast state is composed of α-phase, eutectic Si, and intermetallic phases based on Cu, Mg, Fe, and Zr, can be seen. In the metallographic cut plane, eutectic Si can be observed in the form of imperfectly rounded grains. The morphology of the eutectic Si in the cast state does not have a typical plate-like shape in this case due to the fact that the primary alloy was supplied by the manufacturer in a pre-modified state (containing 0.01 wt% Sr). The Fe-rich intermetallic phases were precipitated in the form of gray plates with cleaved terminations. The Cu-rich intermetallic phases were observed as ternary eutectics in a compact morphology in the metallographic cut plane. The presence of Zr phases in the form of discrete coarser needles was detected for AlSi5u2Mg alloys with 0.15 and 0.20 wt% Zr (Figure 9 d-e). The limited occurrence of Zr-rich phases in the experimental alloys with 0.05 and 0.1 wt% Zr addition was probably due to not exceeding the maximum solubility of Zr in the α-(Al) solid solution.

Figure 9
figure 9

Microstructure evaluation of experimental alloys AlSi5Cu2Mg with varying addition of Zr in as-cast state—blue frames and after heat treatment—red frames with the addition of: (a) 0 wt% Zr; (b) 0.5 wt% Zr; (c) 0.10 wt% Zr; (d )0.15 wt% Zr; (e) 0.20 wt% Zr (H2SO4 etch).

Full size image

The microstructures of the experimental alloys after T7 heat treatment are shown in Figure 9 (red frames). The effect of T7 heat treatment is to spheroidize the eutectic Si. Eutectic Si is precipitated in the form of perfectly rounded grains. In the metallographic cut plane, local coarsening of eutectic Si or clustering of eutectic Si particles can also be observed. The Mg- and Cu-rich intermetallic phases were dissolved by heat treatment and subsequently precipitated again in the form of solidifying precipitates. The transformation of eutectic Si and the presence of Mg- and Cu-based reinforcing precipitates increases mechanical and physical properties. No changes in the morphology of the Zr-rich intermetallic phases were observed due to the heat treatment T7 compared to the cast state. This phenomenon confirms the high thermal stability of Zr-based intermetallic phases. The Zr phases were observed in the metallographic cut plane as single coarser needles or clusters of two needles (Figure 9e-d).

Quantitative microstructural EBSD analysis was performed to analyze the possible treatment effect of graded wt% Zr on AlSi5Cu2Mg alloy in the as-cast condition. It is important to note that the effect of cooling rate on grain refinement of the experimental AlSi5Cu2Mg alloy with varying addition of Zr was not evaluated. The resulting grain size values of the experimental alloys processed into a graphical dependence represent the average of 5 measurements (Figure 10).

Figure 10
figure 10

Grain size of the experimental alloys AlSi5Cu2Mg with varying Zr addition.

Full size image

The treatment effect of Zr on AlSi5Cu2Mg alloy is shown in Figure 11. The average grain size of the experimental AlSi5Cu2Mg alloy in the cast state was 510 μm. Based on the results obtained, it can be concluded that the effect of 0.05 wt% Zr did not show significant grain refinement of the primary phase α (Al). For the experimental alloys with 0.10 to 0.20 wt% Zr, a decrease in grain size of approximately 32 % to 47 % was observed compared to the alloy without Zr addition. The results obtained are compliant with Wang's studies, in which he states that the optimum inoculating effect can be achieved by adding 0.10 to 0.20 wt% Zr.25 Grain size is an important parameter that mainly affects the mechanical properties of Al-Si-Cu-Mg alloys within the wide range.

Figure 11
figure 11

EBSD analysis of grain refinement mechanisms of Zr on AlSi5Cu2Mg: (a) 0 wt% Zr; (b) 0.5 wt% Zr; (c) 0.10 wt% Zr; (d) 0.15 wt% Zr; (e) 0.20 wt% Zr (heat treatment, H2SO4 etch.).

Full size image

Based on EDX analysis, the Zr-based intermetallic phases were identified as Al3Zr and AlSiZr type phases. Increased concentrations of Cu and Fe and their interaction with each other were demonstrated around the Zr-rich phases (Figure 12).

Figure 12
figure 12

Interaction of: 1) Zr with Fe and Cu (spectrum 1); 2) Zr with Cu (spectrum 2) (AlSi5Cu2Mg with 0.20 hm. % Zr, T7, H2SO4 etch.).

Full size image

The matrix of Al-Si-Cu-Mg-based alloys consists of a solid solution of α in which eutectic Si crystals and intermetallic phases are precipitated. The solid solution α characterized by the K12 lattice has very good plastic properties. The hard and brittle intermetallic phases achieve only very low plastic properties. The final appearance of the fracture surface, therefore, depends not only on the shape and size of the eutectic Si but also on the number of intermetallic phases present. The fractographic evaluation was carried out for AlSi5Cu2Mg alloys without Zr addition and with addition of 0.15 and 0.20 wt% Zr. The fractographic evaluation of the researched experimental alloys with 0.05 and 0.10 wt% Zr was not performed due to the absence of Zr-rich intermetallic phases in the metallographic cut plane.

In Figure 13 (blue frames), it can be observed that the relief of the fracture surfaces of the individual experimental alloys in the cast state does not differ significantly. The fracture surface of the AlSi5Cu2Mg alloy before heat treatment is characterized by a transcrystalline ductile matrix fracture with pitting morphology and plastically reshaped α-phase ridges. The experimental AlSi5Cu2Mg alloy was supplied by the manufacturer in a pre-modified state; for this reason, eutectic Si on the fracture surfaces can be observed in the form of a skeleton with a common crystallization center. Eutectic Si, like the intermetallic phases present, can be manifested by the appearance of cleavage facets on the fracture surface. Intermetallic phases have different plastic and strength properties than the matrix. Intermetallic phases vary in shape and size depending on concentration.

Figure 13
figure 13

Fractographic evaluation of experimental alloys AlSi5Cu2Mg with varying addition of Zr in as-cast state—blue frames and after heat treatment—red frames with addition of: (a) 0 wt% Zr; (b) 0.15 wt% Zr; ) 0.20 wt% Zr (H2SO4 etch.).

Full size image

In Figure 13 (red frames), the relief of the fracture surfaces of the studied alloys after heat treatment can be observed. The fracture surface, similar to the cast alloys, is characterized by a transcrystalline ductile matrix failure with pitted morphology and with plastically reshaped α-phase ridges. The heat treatment results in the spheroidization of eutectic Si, which occurs on the fracture surface in the form of isolated particles located at the bottom of the pits. Local cleavage failure occurs as a result of the low plastic and strength properties of the intermetallic phases.

The cleavage facets identified by EDX analysis are of the nature of Fe- and Zr-based intermetallic phases (Figure 14). An increased Cu concentration was demonstrated around the Fe-rich intermetallic phases. Based on this fact, it can be concluded that Fe phases act as potential nucleation seeds for Cu-rich phases. Zr-based intermetallic phases were not identified on the fracture surface.

Figure 14
figure 14

EDX mapping of intermetallic phases based on Fe and Zr of AlSi5Cu2Mg with addition of 0.20 wt% Zr after T7: (a) Fe-rich phases; (b) Zr-rich phases.

Full size image

Conclusion

This paper was focused on the development of an aluminum alloy for high-strength cylinder head castings that would have a favorable combination of mechanical and physical properties. The main objective of the research was to analyze the effect of graded wt% Zr on selected properties of a hypoeutectic non-normalized AlSi5Cu2Mg alloy. Based on the obtained results, the following conclusions can be drawn:

  • The density index (DI) and area proportion of porosity of the experimental alloys with Zr addition increased significantly compared to the experimental variant without Zr addition. The melt gassing rate could have been significantly affected due to the increased melting temperature and increased residence time at a given temperature to completely dissolve the AlZr20 master alloy in the melt.

  • Zr addition negatively affected the thermal conductivity of the experimental AlSi5Cu2Mg alloy in the as-cast condition. Zr precipitated in the form of thicker needles or clusters of two needles blocks the transfer of free electrons through the medium thus decreasing the thermal conductivity. The thermal conductivity also decreased with increasing wt% Zr due to increasing melt gassing rate. The pores, like the Zr phases, block the transfer of heat through the medium. After T7 heat treatment, the experimental AlSi5Cu2Mg alloys with Zr addition showed similar conductivity values as the alloy without Zr addition. Based on this fact, it can be concluded that the addition of Zr does not fundamentally affect the thermal conductivity of Al-Si-Cu-Mg alloys after heat treatment. The increase in thermal conductivity due to T7 heat treatment was probably caused by the transformation of eutectic Si to the more energetically favorable state of rounded grains.

  • For experimental alloys with Zr addition without heat treatment, no increase in mechanical properties (UTS, YS, HBW, A5) was demonstrated compared to AlSi5Cu2Mg alloy without Zr addition (as-cast state). A significant increase in the mechanical properties (UTS, YS, HBW) of the experimental alloys compared to the as-cast condition, due to the presence of Mg- and Cu-based strengthening precipitates, was observed as a result of the T7 heat treatment. The experimental AlSi5Cu2Mg alloys with Zr addition showed a slight decrease in YS and UTS compared to the alloy without Zr addition. The decrease in mechanical properties could be due to the morphology of the Zr-based intermetallic phases. In the metallographic cut plane, the Zr phases were observed as single needles or clusters of two needles with slightly split ends (Figure 7e—red frame), which act as stress concentrators. The ductility of experimental alloys due to T7 heat treatment decreased by 50 % compared to the cast condition.

  • Zr-based intermetallic phases were detected in the cast state for AlSi5Cu2Mg alloys with 0.15 and 0.20 wt% Zr. In the metallographic cut plane, the Zr-based intermetallic phases were precipitated in the form of independent coarser needles (Figure 7e—red frame). The presence of Zr phases in the researched alloys with 0.05 and 0.10 wt% Zr was not demonstrated, probably due to the non-exceeding of the maximum solubility of Zr in the α(Al) solid solution.

  • By adding 0.10 to 0.20 wt% Zr to AlSi5Cu2Mg alloy, a significant refining effect of the primary α(Al) phase was demonstrated by quantitative microstructural EBSD analysis. The grain size decreased by almost 58 % compared to the AlSi5Cu2Mg alloy without Zr. No change in grain size of the primary phase α(Al) was observed due to 0.05 wt% Zr. Based on the results, it can be demonstrated that by adding an optimum amount of Zr, a significant refining effect can be achieved and thus Zr can be an inoculant.

  • No change in the morphology of the Zr-based intermetallic phases was observed due to heat treatment, indicating the high thermal stability of the Zr-rich phases. The interaction between the Zr-rich intermetallic phases with Cu- and Fe-based phases was demonstrated by EDX analysis. From this point of view, it can be declared that Zr acts as a nucleation seed for Cu- and Fe-based phases.

  • Based on the fractographic evaluation, changes in fracture surfaces can be detected due to the T7 heat treatment. Zr- and Fe-based intermetallic phases were observed on the fracture surface in the form of cleavage facies.

The morphology of the intermetallic phases adversely affects the mechanical properties of the experimental AlSi5Cu2Mg alloy. The intermetallic phases thus precipitated crystallize in the D023 crystallographic system due to the low cooling rate. Increasing the cooling rate could lead to the crystallization of Zr in the L012 crystallographic system. In this case, the Zr phases would be precipitated after T7 in the form of precipitates, which would have a more positive effect on the resulting mechanical properties. Alloying of AlSi5Cu2Mg Zr alloys to increase thermal stability at increased casting operating temperatures has proven effective due to the presence of thermally stable Al3Zr-based intermetallic phases. However, attention still needs to be paid to the increase and stabilization of the ductility values. Alloying of Al-Si-Cu-Mg alloys by Zr for cylinder head castings provides an effective means of increasing thermal stability at increased operating temperatures.