Aluminium gets tough

The advantages to be gained from using aluminium to build cars – whether as body panels, chassis solutions or powertrains – are now widely recognised. The material is significantly lighter than sheet steel, there is no shortage of supply and it’s highly recyclable.

Some facts and figures were set out in a report ‘Aluminium in Cars’, published by the European Aluminium Association (EAA) based in Brussels, Belgium. The details serve as a useful benchmark for what aluminium can achieve, with the most fundamental covering CO2 emissions. In the report, the EAA states that CO2 output can be reduced by 9g/km for every 100kg reduction in total vehicle weight.

With a density just one third that of steel, the value of aluminium as an alternative to steel is self-evident if vehicle lightweighting is the priority. Yet given that the material is also currently around four times the price of steel, there must be stringent economies applied to its usage – effectively the ‘lightweighting of lightweighting’. Significantly, the report notes that established practice means that when aluminium is substituted for steel, the thickness of the lighter material is usually 1.5 times greater. Although this still provides a weight saving of 50%, it can also have less desirable implications for cost and space requirements.

Research into steel substitution

The extent to which aluminium is currently capable of being used as a substitute for steel was the subject of a further study published by the EAA in 2010. Based on research carried out by the University of Aachen in Germany, the core argument of the report ‘Stiffness Relevance and Crash Relevance in Crash of Car Body Components’ is that lightweighting through the replacement of lower-strength mild steels by higher-strength counterparts can highlight an inherent inhibiting factor - that the elastic modulus of mildand high-strength steels is approximately the same.

As a consequence, reducing the weight of automobiles through the use of thinner layers of higher-strength steels will also reduce the stiffness of the components involved, with possibly negative effects in terms of performance. As such, the report states: “If the overall structural performance shall not be reduced, significant weight savings by the substitution of mild material with high-strength material can only be achieved for components for which strength is a major requirement.”

More pertinently, the research team then subjected a generic vehicle, the ‘SuperLightCar’ (SLC) used in previous European-funded research and described as ‘similar’ to the fifth-generation Volkswagen Golf, to a detailed comparison between usage of steel and aluminium components. In fact, they went further than the previous research by analysing not just the 22 components identified as constituting the body-in-white, but by also scrutinising the weight reduction potential of the front doors of the vehicle, including their crash management and hinge reinforcement systems. The bonnet and boot lid were excluded from the analysis.

The result was a global estimate that “the achievable weight reduction when substituting aluminium for steel in a state-of-the-art car body - including closures - is assessed to be 40%.” Admittedly, the report cautions that in practice this figure is a theoretical upper limit that would be subject to inhibiting factors, such as the limitations of joining technologies. Nevertheless, the figure can still be regarded as credible. According to Bernard Gilmont, Building and Transport Director for the EAA, the findings of the report were reviewed and approved as realistic by a panel of industry experts that he says was drawn from major vehicle OEMs, though their identities remain confidential.

Recycling aluminium for automotive usage

The use of aluminium for both bodies and engines is, of course, already a fact of life in the automotive industry, but it is still obstinately confined for the most part to premium vehicles. An obvious example is Jaguar Land Rover (JLR), whose Jaguar XJ Series is generally acknowledged to have set an industry benchmark for the use of the material in that sector. But intriguingly, JLR is also the lead industrial partner in three collaborative research projects funded in part by the UK’s Technology Strategy Board (TSB) that could push the boundaries for aluminium usage in ways that could have significant implications for its availability, easeof- use and applicability.

The first of these, called REALCAR, should be formally concluded at the end of September this year after being conducted over almost four years. Its basic aim, as explained by project leader Adrian Tautscher, is to establish the feasibility of two new sources of aluminium for automotive use – material recovered from end-of-life vehicles and ‘postconsumer’ scrap, effectively recycled drinks containers.

Tautscher confirms that JLR already operates a successful ‘closed loop’ recycling system for aluminium scrap generated by manufacturing operations at its Castle Bromwich plant. Around 50% of all the aluminium that enters the plant, most of it 5XXX series material used for structural applications, with some 6XXX series material used for external skins, is turned into scrap during manufacturing, a high proportion of which is already returned to its source of supply after appropriate collection and separation procedures within the plant. Tautscher says that part of the project has involved sharing these practices with external component manufacturers also using the material.

But establishing the feasibility of these two supply streams involving previously used aluminium is obviously where the project could have the most impact for the future. Tautscher says that in practice using such sources is likely to mean accepting a slight “change in chemistry” of the aluminium used by the industry. The trick will be to enable that to happen without degradation in the associated performance qualities of the material.

In the case of end-of-life material, Tautscher says that the project has gone so far as to “shred and melt” 42 JLR vehicles in order to recover one particular type of aluminium – the commonly used 5754 alloy – and incorporate it into coils of new metal that have been provided by one of the other partner materials supplier, Novelis. None of the material has been used for actual production, but Tautscher confirms that it has been used to make various sample pieces, including stamped panels, that have been subjected to extensive testing. Though he will not pre-empt the formal project report, due to be published before the end of the year, Tautscher does say that the prospects for the use of material recovered from end-of-life vehicles are “looking very good”. Meanwhile, the project has gone on to work with actual coils of material, though in this case no recovered aluminium has been used. Instead, the test coils have been specially made to simulate the likely composition of coils incorporating post-consumer metal. Again Tautscher indicates positive results, although he refuses to estimate likely timescales for future use of the new materials in real production vehicles. He does, however, say that he regards the post-consumer variant as offering the “bigger opportunity”. Some 6.6 billion aluminium drinks cans were used in the UK in 2009, for instance, and around 45,000 tonnes of the material goes to landfill each year.

Stadco predicts increased aluminium usage

Apart from JLR and Novelis, the other partners in the REALCAR project are Brunel University, Innoval, Norton Aluminium, Zyomax and sheet parts provider Stadco.

Working with aluminium as an automotive material is already a day-to-day experience for Stadco and hence for Paul Meeson, the company’s Manufacturing Engineering Director, who says he believes that the company is the biggest processor of the material in the UK automotive sector, aside from actual OEMs. It is an experience that he confidently expects to increase quite dramatically in scale over the next half decade. “I would say that at the moment only about 15% of our work involves aluminium, with the rest in steel,” he states. “But in five years time, I would expect those proportions to be reversed.”

Meeson, who is based at Stadco’s headquarters in Shrewsbury, UK, says that the general trend towards lightweighting, which is driving the increasing take-up of aluminium, means that a large percentage of the company’s customers would be classified as ‘premium segment’ manufacturers. As such he states that keeping abreast of developments in the applicability of the material to ensure that its capabilities are state-of-the-art is a “business critical” requirement for the company. One of the main ways is not just though participation in then REALCAR project, but also in another JLR-led project called Warm Aluminium Forming Technology (WAFT). The name of the project is self-explanatory. All stamping and pressing of aluminium for automotive usage currently takes place at ambient temperatures. But, as Meeson confirms, if those operations could take place at higher temperatures in heated moulds then a number of benefits could be realized. He explains that the crucial material properties governing metal pressing operations are those concerned with the ease and controllability of ‘elongation’.

But, put simply, “aluminium stretches less than mild steel.”

The consequence of this is not so much that operations involving aluminium are more difficult than those involving mild steel, but when comparing aluminium to steel, properties of the former mean that it is effectively impossible to produce extreme shapes. “[Using aluminium] you cannot have depths that are as deep or radii that are as tight,” he states. The practical implication of this is that substituting aluminium for mild steel for use in automotive production may in certain cases require multiple parts to be made to replace a single steel component. Clearly this will increase costs, so if aluminium can be ‘hot stamped’ to produce single parts with more complex geometries than is achievable at present through cold forming, then one of the cost barriers to its adoption may be significantly mitigated.

Testing to continue

But if this is the underlying economic driver of the WAFT project, then the technical issues involved, says Meeson, revolve in large part around the temperature at which aluminium may be most effectively hot formed. Quite simply even that basic parameter cannot be identified with any certainty and Meeson indicates that one of the goals of the project is to carry out experiments across a range of temperatures to establish just what the ideal temperature or narrow temperature range will be most appropriate.

Meeson will not say what that temperature might be – organisational details of the project remain under wraps for the moment - but the nature of the material means that it will certainly be lower than the 900oC at which hot stamping of steel takes place. But this seeming simplification of the process actually introduces a complication. As Meeson explains, if aluminium only needs to be heated to a lower temperature then it will cool more quickly once it is introduced to the mould, meaning that unless a technique can be devised to maintain the initial temperature of the material, the period of time in which it can be processed will be correspondingly shorter. “The key problem is the rapid cooling of the metal in the mould,” he confirms.

Interestingly, though, Meeson adds that another trend in aluminium processing which could also help facilitate the production of more complex shapes has been quietly building for several years. This is the increasing use of ‘servomechanical’ pressing machines rather than the hydraulic or purely mechanical counterparts that have previously held sway. Such machines have only been available for little more than five years, he observes, but they offer two great advantages.

The first and most obvious is much greater speed of operation; perhaps “twice as fast”, in Meeson’s estimation.

The second, less immediately evident but more profound point is in the process potential, the fact that servomechanical machines allow for “full control of the press ram throughout its stroke”.

Exactly how this capability can be exploited remains to be seen. In the case of Stadco, Meeson indicates that servo-mechanical machines continue to be used to produce what he terms “simpler parts”, while large panels are still produced entirely on hydraulic machines. But he clearly expects this situation to change and that as the sheer volume of aluminium parts increases – whether in the case of Stadco in particular or throughout the automotive industry – servomechanical technology will have an increasingly important role to play in making this possible.
 

AMC takes shape

This could mean that not too far in the future there are very exciting prospects for the use of aluminium that, amongst other benefits, hold out the promise of further ‘lightweighting’ potential in comparison to how the material is currently used. The technique in this case is ‘aluminium matrix composite’ (AMC) technology, which as the name indicates involves the use of aluminium in a composite material. In fact, it involves the use of the material twice over, as both the reinforcement fibre and as the matrix that constitutes the major part of the resulting composite. But the potential of the technique is not limited to weight saving. In various cases, it could also make appreciable space savings through the use of components smaller than those used today, with equal or better performance.

The possibilities are outlined by David Price, Commercial Director for Composite Metal Technology (CMT), located in Basingstoke, UK. As he explains, the company, which is still a relatively small operation, was set up in 2003 as a spin-off from military-related research work carried out in the UK, for which the technique was first devised. Since then the company has been developing the basic technology in order to establish a commercially-viable production process, which Price says has now been achieved and patented.

Specifically the technique uses fibres made from alumina – or aluminium oxide – as the reinforcement, with the matrix formed from an aluminium alloy. The fundamental problem that had to be overcome was how to effectively combine the materials, which Price says was achieved by ensuring appropriate ‘wetting’ of the reinforcement in the matrix – in other words ensuring that the manufacturing process allowed them to bond together to ultimately produce the combined benefits of strength and light weight that are associated with composites.

Finding the right combination of temperature, pressure and – crucially – timing for the production process has been extremely difficult, confirms Price. But that knowledge now constitutes the company’s essential ‘know-how’ and is embodied in working production facilities at the company’s site.

Price indicates that, superficially at least, the technique is similar to conventional composites production processes.

Fibre preforms are laid into mould dies and then molten aluminium is injected at a temperature of around 700oC. But one of the unusual features of the method is that either completed AMC pre-casts or raw fibres can be used in the moulds to provide selective strengthening of final parts at key points, with the rest of the part being made of only alloy material. Either way, there is no superfluous use of expensive alumina fibres in areas of completed parts where they are not necessary. “An aerospace part might have fibres throughout, but it is unlikely that an automotive part will do so,” he explains.

This flexibility of application, observes Price, means that the technique is of greatest potential for parts where the twin objectives of weight saving and solving a technical problem are involved. In practice this is likely to include parts subjected to fluctuating or intermittent loads and stresses or dynamic applications where low inertia is beneficial. In addition, that capability means that if and when the technique is transferred through to actual industrial use, pre-cast inserts could be supplied by specialist manufacturers to other companies that might then be able to use existing production equipment to incorporate them into final parts.

Breakthrough weight savings

As yet, though, the technique has still to be used commercially for the production of usable parts. But CMT has figures for specific, projected applications that give an idea what it could be used to achieve. The replacement of a cast iron brake calliper, for instance, using an aluminium/ AMC equivalent of the same overall dimensions could achieve a mass saving of an astonishing 64%; even in comparison to a pure aluminium part, the savings achieved could reach 43%. Further, a second example involving a motorsport suspension strut illustrates the potential of AMC to achieve space savings. Not only could the aluminium/ AMC part have a mass some 30% less than its aluminium counterpart, it could also provide a volume reduction of the same proportion. While both instances are theoretical, Price is emphatic that they represent real, achievable targets, using the technology as it now stands.

The AMC approach is currently undergoing an intense prove-out in another project involving JLR as lead company. But, as Price explains, the 30-month project is intent on taking the technology a step further by exploring the potential of a new type of woven ‘3D’ fibre configuration.

A key benefit, he says, will be that AMC material made using the technique will have considerable transverse as well as longitudinal strength, in contrast to existing epoxybased composites in which transverse strength is minimal compared to longitudinal properties. Moreover in this approach, the percentage of fibre used is actually less than in 1D and 2D arrangements.

Price’s hopes for the initiative are shared by JLR project leader Mike Shergold, who is based at the Warwick Manufacturing Group (WMG) site at the University of Warwick. He says that the ultimate aim is to achieve the “strength and stiffness of steel with aluminium” and is prepared to quantify that objective quite precisely. The Young’s Modulus for aluminium – the basic measure of the material’s stiffness - is, he says, 70GPa (gigapascals). The corresponding figure for steel is nearly three times as much at 205GPa. But the intention is to produce an aluminium material with a rating of 200-250GPa.

The innovative nature of the AMC material does not mean it will be confined to niche applications. “We have identified applications all round the vehicle,” confirms Shergold, including the powertrain, chassis and body structure. Even though the project has only been underway for a year, Shergold reports that real progress has been made. “We have made test parts, and though there have been a lot of issues to explore, we have so far overcome them. We are getting results we quite like.” Clearly the shape of things to come in aluminium could, quite literally, be much more varied than it has been previously.