Lightweighting is not a new topic for the automotive industry and significant advances have already been made in the quest to tackle fuel emissions. But many issues surrounding which material should be used for which component, and by which processes components should be formed and assembled, remain unresolved. There is no single answer, but everyone is working on it, from OEMs through to research institutes.
What is becoming clear is that the car-buying public is not willing to pay a premium for lightweight and efficiency on their own; they pay for features and benefits. The industry is therefore faced with a significant problem: matching weight and performance of materials with the economics of the overall result – the ‘lightweight materials challenge’.
The materials industry is working hard to step up to the job of lightweighting, in order to comply with legislation around CO2 emissions. This has forced the industry to seek alternative materials to replace their heavier steel predecessor. Everyone seems to agree that no single material can meet these demands, so the result is to opt for a multi-materials approach where each component is designed to the optimum for performance and cost for its role in the overall structure. Thus new manufacturing challenges arise, as do the costs associated with them. The economics isn’t driven by the cost of the raw material alone, but also by converting that material into a useful component and then integrating it into the vehicle.
Unlike cars of old, where the main structure of the car was simply various grades of steel, these new materials leave the OEMs and Tier 1s with a bigger issue – multi-material components and assembly. Materials with radically different properties that need to be joined together lead to growth in demand for performance adhesives and bonding solutions.
Existing production and assembly lines simply won’t meet future demands, and therefore new approaches must be developed to manage issues such as thermal expansion, electrical conductivity and corrosion protection of multi-material bonded components.
As we move further towards multi-material design, new problems are arising, such as recyclability. This seems to have been largely ignored due to the low volumes of truly multi- and advanced material cars in production. For example, most carbon fibre reinforced plastic (CFRP) is thermoset based and, as such, very hard to recycle, with only the fibre itself being recoverable with current techniques.
The additional effort in dismantling a multi-material chassis into constituent components is likely to be far more costly and difficult than with cars made from traditional materials. We are simply not geared up to ‘unstick’ complex vehicle structures. Smart design and careful choice of material that consider end of product life must come to the fore; we must take a step back to move forwards.
Another key issue is noise abatement. High density (more traditional) materials tend to provide better sound dampening than low density (more lightweight) ones. The noise is accentuated in e-vehicles as, without the masking engine noise, background noises become more noticeable. Careful design, along with new effective dampening solutions, need to be considered, although anything that adds weight to manage the noise is clearly counter-intuitive to the lightweight design concept.
Lightweight materials meeting the current performance requirements do seem to suffer from failure in the economics department. A few high-level numbers were mentioned at the conference relating to the cost available per material type per kilo removed. Steel runs at about €3/kg, aluminium at €8/kg and CFRP runs at about €10/kg. The challenge here is that the switch to current CFRP is about €40/kg removed. This certainly puts a dampener on increased adoption of CFRP into everyday vehicles and, despite its popularity in top-end cars, a paradigm shift in costs of raw materials, conversion and assembly are required if they are to be more widely used.
The CFRP industry is working to address this specifically for the automotive market by balancing performance (cost-effective fibre and resins) with new production techniques. The introduction of robot fibre layup, snap cure resins and resin transfer moulding (RTM) could be the answer, as together they significantly reduce cycle times (minutes vs hours), compared to traditional hand layup of prepregs and curing by autoclave. Strategic reinforcement by carbon fibre tape placement also seems to be promising. Rather than reinforcing a whole component, you simply reinforce the areas under most stress. This reduces the overall cost of the component, compared to a fully composite equivalent.
Bio-composites are seen as providing a sustainable material and are getting commercial traction, although this is mostly in non-visual internal components where performance, rather than aesthetics, is key. Another major lightweighting trend is foaming; this technique introduces air into the material to reduce the overall weight of the component. Nature uses this approach in bones to provide a maximised balance of weight and strength. Many foaming processes (either physical or chemical) are available on the market and there is a lot of research currently into the processing of components to achieve this balance.
Sandwich panels are becoming more and more prevalent, and can be produced from steel, aluminium and plastic/composite materials. They comprise a honeycomb or foam core between two solid sheets and by mass are far stronger under load than a solid sheet of the same material. The economics of this approach seem increasingly appealing, as inventive manufacturers develop continuous processes for recyclable thermoplastic panels. A further benefit of thermoplastic honeycomb sandwiches is that, post-manufacture, they can be hot pressed into some fairly complex shapes that significantly open the market opportunity.
Additive manufacturing is probably worthy of an article in itself, but this nascent process opens up opportunities, such as faster prototyping through to the ability to mass manufacture highly complex components from both plastics and metals with virtually no waste. Multi-material additive manufacturing is already a reality and the question now is: “What are the possibilities for single process multi-material components?”
Ultimately, production, processing and integration techniques will need to change – perhaps revolution, rather than evolution is needed. The materials required for the cars of the future, along with how cars are used, are changing so much that nothing else may suffice. It will certainly be interesting to see what new or underused materials will step up to the modern manufacturing challenges of the automotive industry over the coming years.
What is probably lacking is an appetite for significant step-changes in design and manufacture. BMW have taken a bold step with cars such as the i3 and the i8, but these are probably more brand ambassadors, rather than a window on the car of the future. Economics are vital and this calls for lower cost materials, as well as efficient and effective processing and assembly. The use of adhesive bonding will certainly increase, as this method offers real opportunities in resolving the challenges of multi-materials. Effective bonding typically needs good surface treatments, an area that Oxford Advanced Surfaces (OAS) is certainly helping to improve.
Thoughtful design that incorporates a cradle-to-grave vision and challenges the status quo is surely the key to success. What is evidently clear is that, by going back to the drawing board and looking at design, the impact of integrating differing materials with differing properties can be significantly reduced. Cars of the future are going to look different on the inside and out – and that’s without taking into account the impact of future cities and autonomous vehicles.