Aluminium in powertrain applications has not always followed a logical evolution. The 1961 F-85 Oldsmobile all-aluminium V8 was quickly replaced by a cast-iron V8 in 1965 as the company perfected thin-wall iron casting techniques, while the BMW S54B32 engine used in the company’s performance-orientated M Cars had a block made from grey cast iron rather than the aluminium of the standard engines. Then there is the influence of constantly fluctuating aluminium prices versus iron, the latter serving to push OEMs towards other alloys.

In recent times, the move towards reduced emissions and improved fuel consumption through lighter weight, with stricter crash worthiness legislation, has seen aluminium use rise in powertrain applications.

Of particular interest in regards to weight saving in powertrain and its consequent influence on emissions has been the use of light alloy wheels. Lightweight wheels not only have a linear effect on reducing emissions and fuel consumption but can also improve driving performance and efficiency due to a lower rotary inertia. As the energy used to accelerate and decelerate the wheel is reduced, less mass is required in adjacent, unsprung components such as brakes, steering and suspension, with lowered unsprung weight compounding the benefits of fuel economy and reduced emissions.

Supercar weight saving

An example of this strategy is the move by Lamborghini to break the spiral of ever-larger and more powerful engines needing a proportionally heavier chassis and suspension – which in turn demands more power from each new model. For the new Murciélago LP 670-4 SuperVeloce (SV), the carmaker chose Alcoa Auto Wheels to supply forged aluminium wheels that reduce unsprung weight and contribute to the SV’s 100kg (220lbs) mass reduction over the standard Murciélago.

The Murciélago SV is the product of a substantial cross-car weight reduction programme, with the forged aluminium wheels being part of this strategy. The engineering behind Alcoa’s forged wheels returns a 20% weight saving over comparable cast aluminium models, the 18” x 8.5” front wheel weighs just 19.6lbs, while the 18” x 13” rear wheel weighs 27.2lbs, benefiting both vehicle handling and ride.

To get the supplier’s view on this application, AMS spoke to Brian Thomas, Marketing Manager for Alcoa Wheels and Transportation Products. Thomas started by giving some background on GM’s use of forged wheels on a vehicle that although at the other end of the performance spectrum, has possible gathered more headlines than the Lamborghini.

AMS: You are promoting the Alcoa wheel solution not only as a cosmetic enhancement but also to offer a CO2 reduction solution through reduced mass. How are you getting these weight reductions?

BT: Our business is solely forged aluminium; we can design in thinner spokes and back-mill the material to give a thinner wheel. As an example, on the Chevrolet Volt wheel, which will be going into production in the next two months, GM realised that a lighter wheel would not only maximise fuel economy but also extend battery cycle time. We went through about 15 iterations of design to deliver a 17” wheel that weighs about 15lbs. In aluminium, the best solution was a forged wheel, it could be done in magnesium or carbon fibre but these were too expensive for this application.

As a reference point example, a cast aluminium Toyota Prius wheel is 15 inches and weighs the same as our forged 17-inch wheel.

AMS: How does the changing raw aluminium price impact on your customers?

BT: Price fluctuations over the last 18-24 months have been really drastic; a year and a half ago aluminium was about twice today’s price – we could not make enough of it fast enough! Today there is a lot of inventory going unused. In the wheels group we buy and quote at market value; the OEM pays for the wheels (and other products) at the market value, which we average out over the month of order to delivery time.

AMS: Can you tell us about the particular alloys you are using in forged wheels?

BT: We use 6061-T6 aerospace alloy that has been used for wheels for many years. Alcoa has massive experience in the aluminium industry and we can access information on many formulations of alloys. We have come up with a new alloy for wheel applications; this can save 15 to 20% in weight over the 6061-T6. This new alloy, which is still under test, is stronger in the grain structure, allowing us to remove more metal after forging.

AMS: Heat treatment is a very energy-intensive process, how are you keeping the cost and the environmental impact down?

BT: We run our heat treatment facilities on natural gas and we scrub the outgoing air to clean it and reclaim the maximum amount of heat. Where we score environmentally is in the volume of material and keeping the ovens at about 430oC all the time, so there is no heating and cooling energy wastage.

AMS: Corrosion protection – how are you helping OEMs meet the demands of keeping customers happy with their wheels over the extended warranty periods now being offered by many carmakers?

BT: We launched a finish called Durabright about 10 years ago in North America, on heavy truck applications. We proved the finish in cars with the Audi 240 ‘Cass’ test, a 240 hour salt spray test that they use to prove many parts fitted to their cars. We have matched Audi’s warranty with the use of our wheels on the A6 model. Durabright gives the advantage of not requiring dangerous, environmentallydamaging chemicals to keep the wheels clean.

AMS: How do you manage the enormous amount of waste generated by machining forgings?

BT: There is a lot of machining on a forged wheel, they are produced from billet blanks. Every shaving from the wheel-making process is recycled. Alcoa has a large recycling action group that constantly looks at better ways of re-using material.

An environmentally-friendly solution

While aluminium may give eco benefits in reduced weight, production processes can be very polluting. To counter this, from 2010, BMW’s Landshut plant will boast a new sand core production method for gravity die-casting, making it the world’s first light-alloy foundry to use emission-free sand core production. Conventional organic binders will be replaced by eco-friendly inorganic binders, generating virtually no pollutant emissions. The introduction of this innovative production method will allow the light-alloy foundry to reduce its emissions of combustion residues by 98%, allowing the decommissioning of its current waste air treatment systems by the end of that year.

The Landshut light-alloy foundry’s 1,300 employees currently produce around 1.8 million aluminium and magnesium castings a year, with a total weight of 45,000 tonnes. The product range includes engine components such as cylinder heads and crankcases, together with structural components and chassis parts.

Approximately half the castings produced are gravity diecast using sand cores. The light-alloy foundry uses some 120 tonnes of sand daily in sand core production with up to 90% of this volume recycled. Following an initial pilot operation phase, the BMW Group is now poised to become the world’s first manufacturer to use inorganic sand cores in volume production of all engine core components.

“Inorganic sand core production positions us at the forefront of the foundry industry,” says Dr Wolfgang Blümlhuber, head of the light-alloy foundry. “We see inorganic sand core production as key to competitive operation, particularly in highly-industrialised countries with stringent environmental regulations, where manufacturing costs are correspondingly high.”

The light-alloy foundry first introduced this reduce demission production process for use in the manufacturing of aluminium crankcases and cylinder heads for six-cylinder diesel engines. Now inorganic sand core production is gradually being extended to the foundry’s entire product range.

In addition to the environmental aspect, the process also has economic and ergonomic benefits. The strength of the resulting light-alloy components is enhanced by the improved, faster solidification of the liquid aluminium during the casting process, as it cools from a temperature of approximately 750oC. BMW Group is using this light, yet strong design potential as a way of producing energysaving, fuel-efficient engines capable of higher peak cylinder pressures and increased power density.

To accompany the introduction of inorganic sand core production, the light-alloy foundry developed new core shooting tools and equipment. The casting equipment has become less complex, since the previously required venting systems can also be removed. At the same time, the cooling intensity during the casting process can be increased, further reducing manufacturing cycle times by approximately 10%.

The simulation technology for process and tool development was also developed at the BMW Landshut plant.

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