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Lightweight Design worldwide

, Volume 10, Issue 2, pp 12–15 | Cite as

Ultra-High-Strength Aluminium Alloys — Vehicle Production’s Next Big Thing

  • Andreas Afseth
Cover Story Lightweight Metals
  • 335 Downloads

Keywords

Aluminium Alloy Automotive Industry Solution Heat Treatment Press Hardening Conventional Steel 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Ultra-high-strength aluminium alloys could threaten steel’s leading position for strength constrained vehicle parts. With the same strength, they allow significant weight savings compared to Ultra High Strength Steel (UHSS). for material processing companies such as Constellium the material poses a formability challenge — which specific processing steps can address.

As a measure to combat climate change, the European Union has committed itself to reduce carbon dioxide emissions by a total of 20 % by 2020. This objective also covers the transport sector, which causes about 26 % of the EU‘s total emissions. According to the regulation, the CO2 emissions of the European car fleet should be reduced — stepwise to 95 g CO2/km by 2020 [1].

Reducing carbon dioxide emissions is not only on the policital agenda in the European Union. The governments of many other countries had enacted laws to reduce emissions including limit values as well. The US and Canada, which want to reach a limit of 97 g CO2/km driven by 2025, are similarly ambitious as the European Union [2]. Contrary to political efforts, however, consumers are more likely to buy bigger and heavier vehicles with a higher fuel consumption. In Europe, the average vehicle weight, also due to new safety and communication facilities, has therefore been increasing for years [3].

These two developments pose enormous challenges to the automotive manufacturers. The automotive industry has already expressed public concerns that the limit values are too strict. The European CO2 targets cannot be achieved only by optimising conventional engines [4]. Therefore, the automotive industry must try to reduce the overall weight of vehicles while meeting the high safety and aesthetic requirements of customers and legislators. In order to make vehicles more fuel efficient, new engine systems such as the electric motor and the reduction of weight are indispensable steps. Thereby, lightweight materials, in particular aluminium, are of crucial importance, Table 1.
Tabelle 1

Comparison between different lightweight materials (© Constellium)

Material

Level of maturity

Costs

Potential weight saving versus steel

Ease of adoption for OEMs

Ultra-high-strength steels

High

0.8 to 1.5€/kg

10 to 20%

++++

Aluminium

High

3 to 5€/kg

30 to 50%

+++

Magnesium

Medium

10 to 20€/kg

40 to 60%

+

Fibre-reinforced plastics

Low

40 to 80€/kg

60 to 70%

+

Current statistics and forecasts underline that conventional steel has become less important as a material for vehicle construction in the last years.

The importance of lightweight construction in the automotive industry is therefore no longer a question. The vehicle weight is one of the topics that suppliers and car manufacturers alike are concerned about. The emissions of a vehicle can be reduced by about 0.08 g CO2/km per saved kilogram of weight. In addition to he environmental aspect, lightweight construction also increases road safety — both inside and outside a vehicle. This is because lighter vehicles can be driven more easily and require shorter braking distances up to a standstill.

Growth Potential for Light-Weight Materials and Particularly Aluminium

Several lightweight construction technologies and materials can be distinguished, each of them with a different maturity level in terms of industrial automotive application. The most frequently used materials in automotive engineering are high-strength and ultra-high-strength steels, aluminium, magnesium and fibre-reinforced plastics. Aluminium is a light metal, which weighs only 2.7 g/cm3. Despite of its high strength, the material can be relatively easily deformed. Of particular importance, the corrosion resistance required for outdoor use is a critical success factor for many other materials — not so much for aluminium. Thus aluminium similarly to steel has a high maturity compared to other materials. The industrial application of magnesium or fibre-reinforced plastics is by no means as mature as aluminium, which is why the materials are not yet equally suitable for industrial production [3, 4].

Although fibre-reinforced plastics and magnesium allow potentially high weight reductions, they are not as profitable as aluminium. As a substitute for steel, aluminium enables weight savings of up to 50 % for certain components — with a comparatively low cost of three to five euros per kg. Compared to other materials, aluminium is the best compromise between weight-saving potential and costs. The substitution of steel by aluminium is also relatively simple because both materials can be handeled similarly. Automotive manufacturers can adopt these materials without major modifications of their production lines. The cost for switching the production to other materials, such as fibre-reinforced plastics, are significantly higher.

For these reasons, the average quantity of aluminium used in a passenger car has increased continuously. In 1990, it was still 50 kg, and today it is 140 kg [4]. This share is expected to rise further in the next years, at a level of 196 kg by 2025 [5], Figure 1. With the optimum use of aluminium, the overall weight of vehicles could be reduced by a total of 36 %, which significantly reduces CO2 emissions during the car usage phase [3]. Another environmental aspect is the recyclability of the materials. Here, too, metals such as aluminium outclass the new types of plastics, because they are endlessly recyclable.
Figure 1

Average aluminium share in vehicles in kg (© Ducker)

Aluminium — Response to the Industrial and Political Requirements

Current statistics and forecasts underline that conventional steel has become less important as a material for vehicle construction in the last years. Mainly, this is because of two developments: first the use of aluminium in the automotive industry has expanded significantly. Aluminium solutions, which offer significant weight savings compared to steel, are no longer used only in premium vehicles, but also in the mid-range segment. Additionally, on average more and more parts of a vehicle are made of aluminium. Second, conventional steel has replaced by new high-strength and ultra-high-strength steels, which offer more weight-saving potential than standard ones.

Ultra-high-strength steels meet the needs of the automotive industry with regard to technical criteria. These ultra-high-strength steels are setting up new targets for the aluminium industry. Already aluminium high-strength alloys from the 6000 family can easily reach a yield strength of 400 MPa. However, ultra-high-strength steels offer a higher yield strength of 1100 MPa. But Aluminium is much lighter triggering increased demand from OEMs. In order to offer both strength and lightweighting to replace steel, the aluminium industry is heavily investing in research and development — with concrete results. A new generation of ultra-high-strength aluminium alloys offers automotive manufacturers a real alternative to ultra-high-strength steels. The alloys of the 7000 series, which are primarily mixed with zinc and contain smaller quantities of magnesium and copper, are hardenable and very strong . The most popular alloys from this group, 7050 and 7075, are already widely used in aviation and aerospace, often for example for aircraft wings. The use in the automotive industry is still beginning.

Ultralex for Heavily Stressed Vehicle Parts

One example of such an ultra-high-strength aluminium alloy is the Ultralex product line, developed by the aluminium producer Constellium in close cooperation with automotive manufacturers and research institutes such as the University of Toulouse in France and the University of Manchester in the UK, Figure 2. Ultralex is part of the 7000 series and is a mix of aluminium, zinc, magnesium and copper. While an aluminium-zinc-magnesium mixture already offers a good combination of low density and high strength, Ultralex’ strength is further increased by the addition of copper [6]. The use of ultra-high-strength aluminium alloys has various advantages: compared to ultra-high-strength steel, significant weight savings can be achieved. The use of aluminium for particularly stressed components such as the B-pillar strenghening or the vehicle roof simplifies the connection with the remaining parts. For example, components made from the 7000 series can be integrated much more easily into an aluminium structure than ultra-high-strength steels, Figure 3.
Figure 2

Exploring of microstructures at Constellium C-TEC in Voreppe, France (© Constellium)

Figure 3

Safety cage of a vehicle made from ultra-high-strength aluminium (© Constellium)

Processing of Ultra-High-Strength Aluminium Alloys Similar to ultra-high-strength steels, there are certain challenges in processing ultra-high-strength aluminium alloys. In contrast to alloys of the 6000 series, which contain magnesium and silicon, these alloys are more difficult to form. One possibility, which is also used in the steel sector, is the method of hot stamping, which is also called press hardening. During press hardening, the strength of the material is increased by heating the metal to about 500 °C. Then shaping takes place before quenching the metal in a cooled forming tool. As a result of this process, the components are of high strength and resilience and also have the desired mechanical properties. At the same time, the use of a shaping tool in the hardening process prevents the component from going out of shape during cooling.

In addition to press hardening, ultra-high-strength aluminium alloys can be processed by solution heat treatment with subsequent hot forming. During the solution heat treatment, the aluminium mass is heated to around 500 °C, so that the elements contained shift away and distribute homogeneously in the base mass. After cooling to room temperature, the metal is then heated again to 200 to 250 °C for hot forming in an oven, then shaped and cooled. This is not possible with ultra-high-strength steels and conventional boron steels. Thus, automobile manufacturers have an alternative, depending on the complexity of the component, for using different shaping possibilities, Figure 4.
Figure 4

Feasibility study of vehicle parts using numerical simulation (© Constellium)

Weight-Reduction Compared to UHSS

The main criterion for the comparison of ultra-high-strength steels and ultra-high-strength aluminium alloys is the so-called yield strength. The yield point is a parameter that describes the maximum load, which can be exerted on a material before an irreversible deformation occurs. The final characteristics of an aluminium component depend on the type of heat treatment added to the aluminium alloy before. The 7075 aluminium alloy can be obtained in two tempers, T6 and T7. At 7075-T6, a yield point of about 500 MPa is reached, while with 7075-T7 it is about 450 MPa. A yield point between 450 and 500 MPa is already a very high value for an aluminium alloy. Nevertheless, if we consider the same product thickness, ultra-high-strength steels still enjoy a yield strength of 1100 MPa which is significantly stronger . The advantage of ultra-high-strength aluminium alloys lies in its density: At around 2.7 g/cm3, aluminium is roughly two thirds lighter than steel which has a density of around 7.8 g/cm3. For this reason, with increased thickness, ultra-high-strength aluminium alloys such as Ultralex achieve significant weight savings of approximately 17 % and an identical strength as ultra-high-strength steels.

Safety in Accident Situations

In terms of strength and energy absorption, vehicle parts of both materials offer similar characteristics in the event of a collision. An important value is the so-called elongation. This indicates the permanent extension of a measured length in relation to the initially measured length of a material sample after the fracture when stress is exerted. The value of ultra-high-strength aluminium alloy Ultralex is 15 %, which is more than twice as high than that of ultra-high-strength steels (6 %). A low elongation shows that a material is rather brittle. The breaking elongation of vehicle parts, which are expected to have a high strength, must not be too low nor too high. The B-pillar should neither break in the event of an accident, nor be deformed too far. Otherwise it could enter the passenger cabin and injure the occupants. For example, the alloys of the 6000 series with a breaking elongation of about 25 % are not suitable for heavily stressed parts. Ultra-high-strength aluminium alloys, however, enable good energy absorption and an optimum protection for drivers and passengers.

Material with a Future

Up to now, ultra-high-strength steels have played a leading role in vehicle production for heavily stressed components. But with new, innovative ultra-high-strength aluminium alloys, car makers have a real alternative that offers many advantages, Figure 5. The key advantage is the outstanding density-to-strength ratio, which allows significant weight savings with the same mechanical properties as ultra-high-strength steels. Aluminium producers such as Constellium will continue to invest in research and development over the coming decades in order to push ultra-high-strength alloys and provide automotive manufacturers with additional options to make their vehicles lighter without sacrificing safety.
Figure 5

Corrosion testing with a 7xxx alloy (© Constellium)

References

  1. [1]
    German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB): Die EU-Verordnung zur Verminderung der CO2-Emissionen von PersonenkraftwagenGoogle Scholar
  2. [2]
    International Council on Clean Transportation: Policy Update — EU CO2 Emission Standards for Passenger Cars and Light-Commercial VehiclesGoogle Scholar
  3. [3]
    European Aluminium Association: Aluminium in Cars. Unlocking the Light-Weighting PotentialGoogle Scholar
  4. [4]
    Verband der Automobilindustrie: CO2-Regulierung bis 2020Google Scholar
  5. [5]
    Ducker Worldwide: Aluminium Content in CarsGoogle Scholar
  6. [6]
    E.A. Starke Jr.: Heat-treatable aluminium alloys. In A.K. Vasudevan, R.D. Doherty (Eds.): Aluminium Alloys — Contemporary Research and Applications, Vol. 31, Academic Press, Inc., San Diego, USAGoogle Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Andreas Afseth
    • 1
  1. 1.Constellium’s Automotive DivisionVoreppeFrance

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