Lightweight materials help automotive sector meet tough targets

The automotive sector is under extreme pressure to reduce tailpipe emissions with decisive and tough 'must meet' targets looming ever closer.

By 2020 and 2025 European OEMs must reduce the average fleet tailpipe emissions below 95g of CO2/km and then 75g of CO2/km. They face a phenomenal challenge. Failing to meet these targets could be catastrophic, leading to hefty fines based on each car produced, which could deal a deathblow to those which don't comply on time.

With internal combustion engines and transmissions nearing the peak of their development curve the likely improvement is limited. It will certainly not, on its own, be able to reach the targets on the table. The only real room for manoeuvre is to go lighter, and take weight out of vehicle structures.

The Global Automotive Lightweight Materials conference next month brings together many of Europe's OEMs, technology companies and material suppliers to discuss just how these tough targets are going to become a reality.

Speaking at the conference will be Max Brandt, head of central technology and innovation management, at European steel giant ThyssenKrupp. He says: "We are trying to change the mindset and apply materials in a systemic way to allow lightweight structures and a cost benefit. This is the challenge. Anyone can do lightweight, but at the moment it comes with a price."

There has been some moves by the likes of Jaguar Land Rover to go to a full aluminium body. The new Ford F150, though still an extremely heavy vehicle by European standards, is completely changing its cabin structure from steel to aluminium. Volkswagen has also made hints that it will move toward aluminium with its next Golf. But using a lighter material is by no means a silver bullet.

"Aluminium, as such, is not the solution," says Brandt. "It is expensive, we have a problem with availability and it does not have the same properties as steel. You simply can't swap one for the other. Aluminium can help but it must be engineered."

Aluminium supplier and technologist, Novelis, is increasingly focussing on providing 'engineered aluminium solutions' to the automotive industry as well as producing new material concepts like its fusion alloy, where two plates of 6000 series alloys embrace an inner core of 4000 alloy to give maximum weldability and formability.

This is the target for material suppliers; develop engineered solutions that bring an overall cost benefit. The World Steel Association last year ran a project to show just what was possible with steel and produced a new all steel body that satisfied both the weight and cost requirements of the OEMs. However, while the project was a success it is not the solution.

The demonstrator used 47% tailored blanks to give a lightweight and cheaper chassis, but at the same time was limited as it did not answer the multi-material question. The same goes for Land Rover. All aluminium bodies do not answer the challenge of the multi-material mix, it is intermediate step.

"You need to use the right material, in the right place, at the right thickness, in the right shape and geometry to get the right weight," says Brandt. "Steel is still a very competitive material compared to aluminium. Especially if you look at the wider scope and range of mechanical properties. The use of aluminium, plastics and composites will grow, of course, but it will not replace metals, it will be in addition to. Expectations are very high about the potential weight reductions possible by combining different materials, different grades of alloys, different coatings, and so on."

Steel is likely to remain on most of the main OEMs bill of materials, but it is likely to be part of a much wider multi-material approach. ThyssenKrupp says that its steel related business is likely to stay in much the same place as it is currently, at about 50 – 55%. It cannot standstill, however, it must further develop lightweight steel materials that have higher tensile strength with higher ductility and formability.

The need for innovation
New high strength steel is likely to offer more down gauging potential and there are important developments continuing to happen in steel metallurgy. However, there is no getting away from the fact that an increasing amount of light metals such as aluminium, magnesium and titanium are going to be used on mainstream consumer vehicles.

"We need to expand the current alloys as they don't give us enough freedom of design at the moment," says Brandt. "There is a lot of development for these new alloys to generate better draw depths for example. You have a draw depth in steel of up to 250mm, but a draw depth in aluminium is limited to a 120mm, so it must come closer.

"There seems to be a lot of competition between materials, but if you look at a car and its future architecture, there is no competition. There is the urgent need of co-operation between the steel and aluminium industry, between magnesium and aluminium industry, in bringing that together. The big industries must co-operate."

Case for carbon
As well as big industries co-operating, the demand for lightweight structures could see the emergence of some new big players. Composite materials, particularly carbon fibre, is commonly touted as a lightweight material that does not compromise strength or stiffness. What it does compromise, however, is price and manufacturability. The current cost of carbon fibre as a primary structure is simply more than most mainstream OEMs can pass on to customers.

Examples of a composite structure being used can often be found in the upper echelons of the sports car market. Perhaps a good example of doing low to mid volume production of composite structures is McLaren with its 12C. Its carbon monocoque, known as the 'tub', is made by an resin transfer moulding (RTM) process which is essentially epoxy resin infused in to woven carbon fabric. It is unsurprising that McLaren is also looking at how to manufacture more efficiently.

Also speaking at the conference is Andy Smith, a principal engineer for composite research at McLaren Automotive. He says: "As we are McLaren, it will always be a composites structure. We have done that since the 80's with the F1; this is what we do.

"We know we can meet the volumes that we require and it is structurally very efficient, so mechanically we are happy. But, how do we make it lighter, and for less? You have a choice to stick with RTM, and use a thermosetting resin or you can go the thermoplastic route."

The need to cure thermosetting resin systems like epoxy in large autoclaves creates a bottle neck in production facility. Boeing and Airbus have found it in the aerospace industry, and the likes of McLaren and Prodrive have found it in the motorsport industry.

A move to 'out of autoclave' composite systems is highly desirable, particularly if the material is going to make it to mainstream automotive. There is also the potential of using a thermoplastic resin with a variation of Nylon and CBT resin systems being touted. These have the potential to be injected as they have very low viscosity and can snap cure with catalysts.

Thermoplastic also has increasing possibility to be used as structural components. Though inherently more expensive than thermosets, and with inferior properties to a carbon fibre epoxy system, they do offer the opportunity of faster processing as there is no cure involved. They can be heated up and cooled down, opening up the possibility of a stamping process, much like steel and aluminium.

"Thermoplastics have quite an attraction to the automotive industry as long as you can get the cost of the raw material down," says Smith. "They don't necessarily need the performance we are looking for in carbon fibre structures, so you can start looking at Nylons and lower end thermoplastic materials, rather than PES, PPS, PEEK type performance materials."

To an extent the multi-material approach is already happening. Audi has been a great proponent of using high strength steels and aluminium in the places where they make the most sense. McLaren too. Though the 12C is built around the composite tub, it is assembled with many other materials. The crash tubes, for example, are aluminium. Though it is technically feasible to use composites, aluminium is just as capable and less costly. It also, more surprisingly, uses steel in the windscreen surround.

"We don't use composite materials because we can, it's got to be there for a purpose to earn its place on the car," says Smith. "There has to be a business case and practical reason why we use a material."

The other issue is that of the supply chain. If the automotive industry took a game changing shift towards composites tomorrow, there is not yet enough carbon fibre manufacturing capability in the world. In automotive the use of carbon fibre is still fairly limited to niche products using prepreg.

The aluminium industry, also, does not have the production capacity to fulfil the demands of the prospective market if all the OEMs go to that material by 2015. If Volkswagen do produce in aluminium it is likely that Toyota, Hyundai and increasingly Ford will follow. If they increase the amount of aluminium in their cars to 20 or 30%, it will put the global supply chain under extreme pressure. And that is not to mention magnesium which has been rumoured could be as high as 7% by 2020. That would put tremendous strain on the global supply chain, and of course, impact price.

The multi-material approach does pose another serious question that is often swept under the carpet, in an effort to meet upcoming regulations. Integrating many different materials is difficult. But, disassembling them again is potentially much harder. Adequate and cost effective economic disassembly is becoming essential. And this is largely unanswered if the use of composites or any reinforced plastics reach high volumes.

High strength steels, increased use of lighter metals and the increasing use of plastic – and perhaps composite - impose a very high redesign of future automotive architecture. Not only how to assemble them and bring them together in the first pace, but how to disassemble them again.

Then there will be issues in-use. For example, if you drive in Dubai in the summer, the car will behave differently than if it driven in Oslo in the winter because of the different thermal expansions.

All of these questions need to be answered and it seems that industry has accepted this needs to be done largely in co-operation, and less so in competition.

The Global Automotive Lightweight Materials Conference takes place on 24th - 25th April 2013 in London. For more information visit www.global-automotive-lightweight-materials.com

Author
Justin Cunningham

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