Last year, the company produced almost 700,000 aluminum hoods and 407,000 aluminum liftgates.
The auto maker currently uses the lighter, but substantially more-expensive, material for hoods on nine models and for the liftgates of its large SUVs. It costs about $1,400 per metric ton for aluminum, $600 for steel, says Mark White,Motor Co's senior body structures manager-Jaguar and Land Rover.
That number will drop to six for a 3-year stretch (’05-’07) and increase by one for ’08, says Jody N. Hall, engineering group manager-GM Metal Fabricating Div.
While declining to specify which vehicles will switch to less-costly steel, Hall tells Ward’s some liftgates will make the move as part of a midcycle model change, while the hoods will be part of a full model changeover.
Jody N. Hall
She says this is in keeping with an overall strategy in which GM is more aggressively pursuing advanced high-strength steels, specifically dual-phase and martensitic steel.
Use of these advanced steels will increase tenfold over the next six model years, while conventional “mild” steel is phased out, Hall says. More than 50% of the body structure of these new vehicles by 2010 will be made from advanced steels that are as much as three times stronger than mild steel.
The ’92 Pontiac Grand Am, for example, was predominantly mild steel, with 5% high-strength steel.
GM recognizes it must look past mild steels, but cost – almost double that of steel – is keeping the auto maker from fully embracing aluminum. The material also is more difficult to form and weld than steel, Hall says.
European customers are quicker to pay the premium for an aluminum-intensive vehicle because the payback time for the lighter, more fuel-efficient vehicle is one or two years, compared with five to seven years in North America, says Hall. That is longer than the average person owns their vehicle, she points out.
Nevertheless, additional weight is being piled on the typical vehicle in the quest to improve crashworthiness, reduce emissions and add creature comforts – at the rate of about 22 lbs. (10 kg) per year, by some estimates.
GM has decided to use several different types of advanced high-strength steels to solve the conundrum. While not as light as aluminum, high-strength steels are far less expensive than aluminum and can be fabricated into much lighter and versatile structures than mild steels.
The current Epsilon platform (Malibu, Malibu Maxx, Saab 9-3 and Opel Vectra) saw the amount of mild steel cut in half.
Lambda, a future all and front-wheel-drive platform expected to shoulder future minivans and cross/utility vehicles, only will sport about 11% mild steel, with 14% dual-phase and 8% martensite steel that better resists crushing.
Dual-stage steels crush well and are especially suited for body structures designed for crash-energy management, while martinsitic steels resist crushing and are ideal for use for occupant-protecting safety cages.
“We think steel is the answer for structures for improved performance,” Hall tells attendees of the Advanced Materials and Manufacturing Processes session of the Management Briefing Seminars here.
However, she points out that high-strength steels bring with them their own special set of challenges, including problems with “springback” and “sidewall curl,” that makes it difficult for fabricated components to hold their shape.
The advanced steels also cause metal-forming dies to wear out quickly.
Hall says these issues all are being worked on by the Auto Steel Partnership, a consortium of auto makers and steel suppliers.