Article
Articles, Issue 48 - Spring 2013

Material Trends in Vehicle Lightweighting

As the automotive industry comes under increasing pressure to raise energy efficiency and lower CO2, materials used in vehicle construction are coming under closer scrutiny. Ian Henry considers developments in materials.

Car companies are under constant and rising pressure to reduce fuel consumption and emissions; cleaning up the tailpipe exhaust and making the different components of the engine more efficient clearly have important roles to play, but the main route to cutting consumption and emissions is through reducing vehicle weight.

Weight reduction can be achieved by a number of ways, ie replacing traditional, heavy-duty steels with lighter weight steels, or alternative metals, plastics or carbon fibre, or by designing parts to use less material. Although plastics and to some extent carbon fibre are replacing metal in some parts of the car, metals of various forms remain the predominant material used in vehicle manufacture, and they will remain so in volume cars at least for the foreseeable future.

Lightweight materials also represent big business for those involved; Frost & Sullivan has recently calculated that lightweight steels and plastics will be worth in the region of USD 95-100 billion per year in revenue terms by 2017, a rise of 250% since 2010.

Aluminium is the main metal alternative to steel

Aluminium is the leading alternative lightweight metal to steel. According to AluminumTransportation.org, a reduction of 10% in vehicle weight can result in a direct fuel efficiency gain of between 5-7%; and with this in mind, Ducker, a US research company, has suggested that aluminium use in vehicles will more than double by 2025. All the vehicle manufacturers (VMs) are increasing their use of aluminium, not just the premium brands such as Audi and Jaguar Land Rover who arguably lead the way in the use of aluminium. For example, although aluminium has a low current penetration at GM, it is seen as integral part of the GM policy to reduce its average vehicle weight by 15%.

The continued growth in aluminium across the automotive industry is quite remarkable, according to aluminium specialist Alcoa. Global aluminium use in 2011 totalled around 11.5 million tonnes and Alcoa expects this to increase to an estimated 24.8 million tonnes by 2025, or put another way this will represent around 250 kg of aluminium per car compared to 155 kg per car at the present time. One of the aluminium parts made by Alcoa are forged wheels; and here it is worth noting how a forged aluminium wheel can also save weight over cast aluminium wheels – it is not just the material per se which allows weight savings, but also the means by which the aluminium is processed. For example, while a cast aluminium wheel typically represents a 40% weight saving on a steel wheel, a forged aluminium wheel represents another 20-30% weight saving over a cast aluminium wheel.

Although most VMs have been using aluminium to a greater or lesser extent in recent times, arguably the leader in terms of the take-up of aluminium has been Audi. Audi first presented its aluminium space frame nearly 20 years ago and has remained at the forefront of aluminium use in vehicle bodies ever since. The benefits of aluminium are well-known in terms of weight saving; as a result of the increased use of aluminium on the current Audi A3, it is at least 80 kg lighter than the previous model.

Audi is now leading the way in terms of sustainable use of aluminium and has joined the ‘Aluminium Stewardship’ initiative. This organisation is in the process of developing a global sustainability standard by the end of 2014. It will embrace the whole value chain aluminium use, from the extraction of the ore through to its processing into finished parts and onto recycling at the end of vehicle life.

Reducing vehicle weight – some examples from the field

Let’s now turn to some recent examples of weight reduction to see how this works in practice, with case studies coming from both the vehicle companies and their suppliers.

Aluminium’s contribution to reducing body weight

The latest version of the Range Rover Sport makes extensive use of aluminium and lightweight technology in general. The aluminium body is around 350 kg lighter than on the preceding model; the lighter body has a direct benefit in terms of allowing the reduced weight of other parts, e.g. lighter suspension and braking components. Together with the body, these reduce the weight of the vehicle by 420 kg. One of the many specific components which has been subject to a major reduction in weight is the rear sub-frame, made by Martinrea Honsel. This part is 15 kg lighter than its steel predecessor.

Lighter suspension and braking components.  Together with the body, reduce the weight of the vehicle by 420 kg

Lighter suspension and braking components. Together with the body, reduce the weight of the vehicle by 420 kg

Bodies made from Advanced High Strength Steel (AHSS)

Here we are concerned with the vehicle companies adopting high strength steels which are thinner and much lighter than conventional steels. As a recent example Nissan has revealed plans to increase its use of advanced high strength steel (AHSS) on new models; the use of AHSS will rise by around 25% from 2017, which will result in a weight saving of 15% in body structures. Nissan has developed this new steel in collaboration with Nippon Steel, Kobe Steel and Sumitomo Metal. The first use of the new steel is on the 2013 Infiniti Q50. The use of this metal is integral to Nissan’s ‘Green Program 2016’ which sets out to achieve a 35% improvement in fuel economy across its entire range when compared to 2005 levels.

Joining steel and aluminium – Honda achieves a major breakthrough

One of the factors preventing the widespread use of aluminium in car bodies at some VMs has been the problem of how to join, i.e. weld, it to steel. Until now, car bodies have tended to be either steel or aluminium, with mixed material use the exception rather than the rule. In early 2013, Honda announced it had developed a new means to join steel and aluminium and said that it expected this technology to result in a far greater use of aluminium in its cars in the years ahead.

Honda has used this technology on a door for the new Acura RLX which would normally be made of steel. Joining different metals like steel and aluminium requires parallel work in a number of areas, not just the act of successfully joining the two metals, but also preventing electrical corrosion and controlling thermal deformation. While steel and aluminium can be joined together through MIG welding, this process is not without its problems. The new processes developed by Honda overcome these problems, and having done so, Honda can also eliminate spot welding which is normally used in steel door manufacture. This technology has reduced the weight of the door panel by around seventeen percent compared to a steel door – and there are other benefits too, not just weight saving; this change improves stability by reducing weight on the outside of the vehicle, helping to concentrate the point of gravity towards the centre.

In another development, Honda has begun mass production and use of front sub-frame with a steel-aluminium hybrid structure using a process called friction stir welding (FSW) on the 2013 version of the north American Accord model. FSW works by producing a metal-to-metal bonding between steel and aluminium. This is achieved by putting a rotating tool on top of the aluminium which is lapped over the steel with high pressure. The resultant welding strength is at least the same as that achieved through conventional MIG welding. According to Honda, this process can reduce part weight by 25% when compared to a conventional steel sub-frame.

GM also achieves breakthrough in aluminium welding …

Although aluminium is not widely used at GM, developments in welding aluminium have not been entirely ignored. In late 2012, GM announced that it had developed a new resistance spot welding technique; this uses a patented process, featuring a multi-ring domed electrode to weld aluminium to aluminium. Smooth electrodes have proven to be unreliable in this process in the past. As a result of perfecting the aluminium-to-aluminium welding process, GM believes it can cut around 1 kg worth of rivets from parts such as bonnets/hoods, tailgates and doors. The process is already used on the bonnet of the Cadillac CTS and the liftgate on the Chevrolet Tahoe/GMC Yukon hybrid pick-up truck. Although other VMs are undoubtedly working on this issue, at least at the time of its announcement, GM believed it had stolen a march on its competitors and expects to be using the process widely in the years ahead.

and also moves into lightweight steel

Last year, GM – having emerged from Chapter 11 not so long ago – authorised its investment arm, General Motors Ventures, to invest in a company, NanoSteel, which specialises in nanostructure lightweight steel. Specifically, NanoSteel has developed a new category of steel which facilitates weight saving through using ever thinner, but higher strength, steels and at the same time maintaining the structural integrity of the vehicle body which is required to meet safety standards. GM has co-invested in this company with seven other financial investors, citing NanoSteel’s technology as a potential “game-changer” in terms of reducing vehicle weight in the long term.

GM also sees significant potential in magnesium …

Moving away from steel, GM is also working on lightweight magnesium sheet metal, having developed an industry first, namely a thermal forming process with integral corrosion resistance treatment for magnesium sheets. According to GM magnesium weighs 33% less than aluminium, 60% less than titanium and 75% less than steel, so its weight benefits are clear, although the cost of magnesium somewhat militates against its widespread use.

However, GM is proceeding with the development of magnesium parts and has developed a production-ready component, namely a magnesium rear decklid inner panel. This has survived rigorous testing, including over 75,000 robotic slams and a 250 kg impact drop. In a series production environment, GM claims it can save 1 kg over an equivalent part made in aluminium. Assuming adequate and cost competitive supplies of magnesium are available, the US Automotive Materials Partnership believes that 350 pounds (160 kg) of magnesium can replace 500 pounds (227 kg) of steel and 130 pounds (59 kg) of aluminium per vehicle, achieving a 15% reduction in weight and a fuel saving of around 9-12% depending on driving cycles and styles.

… and also sees aluminium potential in particular areas …

The 2014 Corvette uses an aluminium body frame which reduces weight by 99 pounds or just under 45 kilos, but it is also at the opposite end, at the level of small individual components that GM is saving weight. For example the new Corvette will be the first to use a lightweight heat-activated shape memory alloy wire (rather than a motorised actuator) to close the hatch vent which is used to release air from the trunk. This component saves just over 0.5 kg in weight.

Shape memory alloys are seen as especially useful tools in the manufacture of moving parts; they can reduce part mass, i.e. weight, leading to a direct improvement in fuel economy, but making individual, small improvements like these take time and involve an awful lot of work. For example, GM took nearly five years to perfect use of the shape memory alloy for this part, albeit as part of a wider programme of R&D in the use of smart materials, for which GM has registered in the region of 250 patents.

The role played by component suppliers should not be forgotten

While the car companies themselves lead the way in look for opportunities to cut weight and implement the ideas they develop, the role played by suppliers should not be overlooked:

Continental – lightweight brake boosters

The latest generation of brake boosters developed by Continental are made entirely of aluminium, the first time this has been achieved; the use of aluminium and various improvements to its design has reduced the weight of the new part by around 50%, i.e. 1.7 kg, compared to the previous generation design. In addition, the new boosters are slightly shorter, with the reduced weight and optimised shape of the part having been achieved through the use of thinner metal and improvements to internal design of the part. The thickness of the metal has been reduced by 50%, from 2.4 mm to 1.2 mm, and this has been done without compromising on service life.

The latest generation of brake boosters developed by Continental are made entirely of aluminium, the first time this has been achieved

The latest generation of brake boosters developed by Continental are made entirely of aluminium, the first time this has been achieved

TRW’s lightweight airbag inflators

Another example of small scale weight reduction comes from the airbag world, in this case the inflator. In 2012, TRW launched the DI10 1G45 inflator, its lightest and smallest inflator yet. This has been developed for use in micro-airbag modules and will go into production from 2014, with a weight saving of 25% compared to the previous version.

RW launched the DI10 1G45 inflator, its lightest and smallest inflator yet. This has been developed for use in micro-airbag modules and will go into production from 2014, with a weight saving of 25%

RW launched the DI10 1G45 inflator, its lightest and smallest inflator yet. This has been developed for use in micro-airbag modules and will go into production from 2014, with a weight saving of 25%

Johnson Controls (JCI): saving weight in door panel trim

While the VMs are using lightweight metals to reduce metal outer door panel weight, suppliers are doing the same with door panel interiors. An example in this regard is the work done by JCI for BMW on door panels. The door panels on the latest 3 Series have been made of a combination of natural fibres and plastics which are said to save 20% weight compared to previous generation components. Seat structure weight has been saved by around 3-4 kg.

The door panels on the latest 3 Series have been made of a combination of natural fibres and plastics which are said to save 20% weight

The door panels on the latest 3 Series have been made of a combination of natural fibres and plastics which are said to save 20% weight

The non-visible door panel elements on the 3 Series is made of wood fibre, with the carrier, made of natural fibres, moulded together with plastic. The combined production process is said to reduce the weight, with additional weight being saved through the novel way of laminating the fabric or leather trim to the door panel. This saves an intermediate component and therefore additional weight.

Beyond metal – will carbon fibre make it as a mainstream vehicle body material?

Carbon fibre is well known for being used in Formula 1 racing cars in particular but until now it has been used rarely in volume vehicles. This is beginning to change however. The strength-to-weight ratio of carbon fibre is well-known (it is five times as strong as steel and twice as stiff), but it achieves this while weighing just one third of steel on a part-for-part basis.

The carbon fibres for the i3 and i8 are made in the US and shipped to Germany for transformation into body parts

The carbon fibres for the i3 and i8 are made in the US and shipped to Germany for transformation into body parts

Road-going high performance sports cars have been the initial users of this material, with McLaren, Ferrari and Lamborghini the main adopters. However, BMW has settled on carbon fibre as a means to achieve a major weight saving on its new “i” range of electric drive vehicles. It has established a joint venture with SGL Automotive Carbon Fibres, based in Washington State in the US. The carbon fibres for the i3 and i8 are made in the US and shipped to Germany for transformation into body parts.

Will legislation be required to reduce weight even further?

The car companies and their major suppliers have become remarkably adept in recent years at finding ways of reducing fuel consumption and emissions from traditional engines. The death of the conventional internal combustion engine has been predicted on many occasions but it survives and is going to be with us for many years to come as weight reduction and mechanical efficiency improvements will mean the car companies can meet the next round of fuel efficiency targets which the regulators throw in their path.

According to a recent review by Frost & Sullivan, if governments really want to make significant further inroads into fuel consumption and emissions levels, then the only way to do this is to set tough weight limits for cars. F&S have suggested that limiting all city cars to a maximum weight of 500 kg would have a dramatic effect on emissions and this is what the authorities should do if they really want to address emissions and congestion in urban environments. If they did this, and taking the figures quoted by F&S at face value, then the resultant changes would indeed be dramatic.

For example a 500 kg petrol-engined car (something around the size of a Renault Twizy for example) would, according to F&S emit less CO2 in its lifecycle than the new, all-electric Renault Zoe which weighs around 1,400 kg. Small, ultra-light vehicles already exist, such as the Aixam or Ligier micro-vehicles sold in France, but persuading governments to mandate their use and indeed forcing consumers to use them on a large scale is unlikely, in the short-term at least.

The Japanese, with their kei-car segment, have managed to create sustained demand for very small cars, so it is possible, but in reality, consumer pressure and the persuasive powers of the major car companies are likely to prevent such a switch in vehicle types on the scale suggested by F&S, at least in the foreseeable future.

Carbon Fibre?

Actual use of carbon fibre will be highly specific…

In terms of the main opportunities and applications for carbon fibre in mass produced cars, Gary White, engineering manager at Prodrive Composites believes that cosmetic enhancements are an obvious opportunity. In these instances, the customer would be willing to spend an additional amount to upgrade the vehicle with clearly differentiated interior and external trim components. Prodrive says that advanced composite components can now be produced in a range of deep-lustre colours to the highest quality standards required for luxury products across a wide range of industries.

“It’s a substantial step beyond conventional carbon trim finishes, offering something very special for exclusive vehicle options and other luxury products.”

He added: “In addition, OEMs are looking at the weight saving advantages of carbon fibre for suspension components and body panels, using press and thermosetting technologies to make the processing cost element cheaper, with higher volume capability.”

Terry Graham, managing director of Zircotec, agrees that cosmetic parts currently show the greatest opportunity. “As a supplier of coatings to enable the use of composites in harsh environments, Zircotec is receiving a growing number of enquiries from OEMs and Tier Ones. A number of these requests revolve around the protection of ‘decorative’ carbon trim parts that are tightly packaged close to heat sources and need to be protected. However, our first application was to coat an under bonnet carbon composite inlet manifold.”

Recycling carbon fibre still a major challenge

It appears that processes to manufacture carbon fibre have moved faster than those for recycling it. “Recyclability is still a potential stumbling block,” says White. “However there are a number of people looking into effective ways of recycling carbon and I am sure that as was the case with tyres an effective solution will be found.”

Indeed, a team of researchers at Nottingham University, led by Professor Steve Pickering, Professor of Mechanical Engineering, are developing processes to recycle carbon fibre composite materials, including developing applications for use of the recycled carbon fibre. The team worked closely with Boeing yet Pickering points out that the prospects for transferring this technology to the automotive industry are good. “The technology that we are working on with Boeing would generally be applicable to recycling carbon fibre from any industry and so could be applied to the automotive industry if viable applications were identified. … Carbon fibre has unique structural properties that mean that it can be used to make the lightest structural materials. So it has the potential to make very lightweight cars that could help to reduce fuel consumption and carbon emissions. There are great opportunities to use it in the vehicle structure.”

Writer: just-auto's Ian Henry

About lotusproactive

Lotus proActive is an e-magazine published quarterly by Lotus Engineering, covering engineering articles, industry news and articles from within Group Lotus (Cars, Engineering, Originals and Racing).

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