Developments of Aluminium for the Automotive Industry

All of the world’s main light metal manufacturers offer new alloys for use in mass-produced vehicles.

di Giuseppe Giordano

Innovation in the automotive sector leads to the development of new alloys for all the various parts of the car. Metallurgical knowledge is the basis for the development of the use of aluminium in new car models and it is used to respond to the various requests of manufacturers and not just to the priority requirement of lightweighting. The characteristics of light alloys have for a long time been the most effective choice for lightweighting accompanied by an improvement in the formability, corrosion resistance and capability of responding to current demands such as the request for a guarantee of economically sustainable recyclability. The drive which the automotive sector has always provided to Research and Development of aluminium alloys is today particularly intense for the mandatory requirement on the part of the car industry of reducing fuel consumption and emissions. Every decrease in consumption is linked to the development of some of the special peculiarities of aluminium alloys. We are facing a natural trend which began almost a century ago because the low volume density of light alloys always aroused the interest of manufacturers of any type of vehicle; sustainable lightweighting became during recent years a stringent requirement made possible by using aluminium and by means of innovation in materials, processes, processing techniques and the adoption of design criteria suitable for the properties of aluminium alloys. The main reasons for the central role of light alloys in the evolution of cars are shown in Figure 1 [1].
The development of the use of aluminium in new car models does not only concern car bodies but it is widely present in other parts of the vehicle. In a recent publication focused on Research and Development by the Rio Tinto group regarding the automotive industry [2] car parts are subdivided based on the use of innovative alloys to satisfy particular requirements. Specifically the classification of car parts presented is the following: Body-in-white parts such as doors, bonnets, wheels and such castings as cylinder heads and fluid pumps; Structural parts which must be produced using elements capable of withstanding intense stress while absorbing a high amount of energy in case of a crash; Materials for the main electrical system, from the battery to wiring with the recent launch on the market of electric and/or hybrid car engines; Materials for heat transfer both in the engine cooling circuit and in the air conditioning system.

1 – Body-in-white parts and castings for engine parts, fluids and wheels
Panels making up the body-in-white are the most interesting sector for the expansion of the use of aluminium in car bodies. Specifically these applications may make use of different compositions of alloys belonging to the 5xxx and 6xxx families[3].
Al-Mg-Si alloys in the 6xxx series are known to be medium resistance heat treatment alloys, used in different segments, especially extrusion. Rolled products using 6xxx alloys recently went through a remarkable development in the automotive industry in the making of hot and cold formed car body parts. Tables 1A and 2 show the chemical composition and some typical property of some of the most widespread alloys in the series, also used in engine parts [7].
Inside panels of the body-in-white are generally made out of 5xxx alloys. The chemical composition of some of these alloys is shown in Table 1B. Castings for engines and fluids, but also, increasingly, structural double-walled parts in the chassis and transmission sections, are mainly obtained using die casting techniques. Castings made using this technology show an interesting growth.

2 – Structural parts and/or parts which can absorb energy in case of a crash
For the production of castings with traditional methods [4] or using die casting (even with high pressure), specific alloys have been developed capable of providing different performances with respect to traditional products [2]. High pressure die castings are used as structural details for cars. Alloys must guarantee that high tensile strength values may be reached and above all provide a high elasticity and capability of absorbing energy in case of a crash. All main manufacturers of primary foundry alloys broadened the range developing specific compositions; Alcoa (now Arconic) launched on the market such interesting examples of innovation as SupraCast, EZCast, VersaCast and EverCast, brand names which identify refined versions of alloying agent combinations which provide high mechanical properties and high levels of fatigue resistance. There are manufacturers who only offer special alloys for castings subject to high stress levels: for instance, the German manufacturer, Aluminium Rheinfelden, was one of the first to notice the great importance of details in the chemical composition of new light alloys for castings, from impurity control to the choice of additional elements to influence the granulometric properties, right up to the restricted combination of the main alloying agents to ensure the best metallurgical, technological and mechanical resistance properties. Figure 5 shows a section of theAluminium Rheinfelden [7] catalogue illustrating the final employment field of some of the many hypo-eutectic and eutectic Al-Si alloys. Structural parts in light alloys are not only made up of castings and rolled products but also extrusions, especially as regards the creation of car space frames, the framework of the vehicle which also performs the function of protecting the passenger compartment in case of an accident. Figure 6A shows the mandatory mechanical characteristics which may be obtained using extrusion alloys from the 6xxx family, out of which the main role is played by the alloys shown in Figure 6B [3].

3 – Materials for the electrical system
The development of the electric car, preceded by the partial solution, the hybrid car, affected the choice of materials. The electric car sub-sector is still experimental, but the advantages to be derived in terms of drastic reduction of vehicles’ polluting power are driving car makers and Government Authorities to accelerate test schedules. An important role played by aluminium in electric and/or hybrid cars may be found in the production of engine rotors obtained from castings by means of mechanical machining [2]. Compared to traditional copper rotors, aluminium rotors, which show a decrease in weight of about 50%, prove more reliable in that they are less stressed during the numerous passages between traditional and electric drive which occur especially during urban travel. Even lighting equipment is at the centre of a quantum leap with the introduction of LED technology which involves castings and extrusions as cooling elements. It should be noted that aluminium could play a leading role in the segment of air-metal batteries. Next-generation electric cars will be equipped with more efficient batteries, and from a technological standpoint there are many innovations, but it is necessary to determine which path to follow in order to obtain an adequate reliability. Car makers do not agree upon a single choice: if the Japanese manufacturer, Toyota, and Germany’s BMW consider lithium-air batteries to be the answer, others place their bets on sodium-air batteries and yet others experiment the aluminium-air combination. Aluminium-air batteries have a density which is 100 times greater than that of present-day lithium ion batteries but, even in this case, the problem lies with the working life span. Batteries tested so far run out soon and cannot be recharged, but they will have to be replaced thanks to the development of a network of service stations similar to that of the current gas stations.

4 – Materials for heat transfer
Within a few years, air conditioning systems went from costly optional features of upper-bracket models to standard equipment for all models. This development brought with it the need to reduce the size of heat transfer batteries to a minimum and/or to increase their efficiency. A new type of fin stock was therefore developed, no longer made out of uncoated sheet metal, but coated with paints capable of affecting the wetting properties of the transfer surfaces. By means of extrusion techniques with high size accuracy, innovative methods of heat transfer have been defined, such as, techniques with micro-pipes. Following the trend towards the replacement of copper, which became much more expensive than aluminium several years ago, “all-aluminium” systems have been developed (8) using pipes with micro-canals as shown in Figure 11. The result of the replacement leads to the following advantages: decrease in the cooling fluid recharged into the equipment, equal to roughly 40%; improvement of energy efficiency – performance coefficient by up to 10%
decrease (over 50%) in the consumption of raw materials; considerable cost savings with respect to Al and Cu heat exchangers; better recyclability of all-aluminium plants at the end of their working life.