GM Goes Retro (gression)

A forming method used by other industries and perfected for auto applications by General Motors Corp. may allow the company to eventually begin widespread use of aluminum closure panels — a changeover that could be a crushing blow to the steel industry.

“It's not a silver bullet,” Paul Krajewski, GM staff researcher and lab group manager, tells WAW. “But it can make aluminum as formable as steel under certain conditions, and that's what this heat treatment does. It's a local heat treatment for certain conditions of bending, especially forming little, sharp features.”

Called heat treatment retrogression, the flash heating technique improves the bending of aluminum and will allow GM to apply the material on a broader range of vehicles, not just pricey, low-volume products.

The process currently is being used to make the liftgates for GM's fullsize SUVs. Krajewski sees more applications on the way. “The place where it makes the most sense right now for assembly is closure panels. That's where it would have the biggest impact,” says Krajewski, who was named one of the world's top 100 innovators by the Massachusetts Institute of Technology's Technology Review magazine for his work developing retrogression. “I think it can be used very widely.”

Aluminum's potential is well known; it is lighter and stronger than steel. But it's also more difficult to make complex shapes. So auto makers have been working to develop alternate forming processes for aluminum.

Retrogression has been used for years to make aluminum ladders. Krajewski adapted it for sheet metal and alloys typically used by the auto industry. “We were able to develop heat treatments, like less than 10 seconds, and with those heat treatments we're able to get a significant improvement in formability,” explains Krajewski. “We're essentially tricking the material into being soft for a while so we can form what we want.”

However, retrogression shouldn't be considered a death knell for steel. “The optimum vehicle of the future is going to have some aluminum, some steel, some magnesium, some polymer composite,” predicts Krajewski. “I don't think a one-material solution is going to be the way to go.”

Get Ready for Composite Mufflers

Taking with it the experience and success of fuel tanks, the composite industry's next auto underbody target might be mufflers, judging from the development work ongoing at Owens Corning.

During a recent tour of the company's new automotive center in Novi, MI, Owens Corning officials showed off a composite muffler prototype. “We're getting a lot of interest in mufflers, almost too much,” says Dick McKechnie, business director-Owens Corning Automotive. “We have developmental contracts with OEMs.”

Owens Corning's work is focusing on the muffler housing; the piping on its prototype is stainless steel. Used on maybe a handful of vehicles today, Owens Corning says, composite mufflers could provide weight savings, increased horsepower and parts consolidation.

But as with all new technology, there is apprehension. “Everybody says, ‘You want to do what? It'll melt. Look, the spec sheet says it has to withstand 1,400ÞF,’” says Andy Hopkins, general manager-Owens Corning's Automotive. “Well, that's because you make it out of steel. But it doesn't need to (be).”

Besides cutting the weight of a typical steel muffler from 20 lbs. (9 kg) to 10 lbs. (4.5 kg), a composite muffler also doesn't conduct heat or rust, and it offers more flexible design solutions. “It can be any shape you want it,” says McKechnie. But before parts integration is considered, auto makers want to make sure performance won't suffer. It doesn't, answers McKechnie.

Prototypes have endured 124,000-mile (200,000-km) tests and provide a small amount of additional horsepower because there is less air flow resistance. Also, Owens Corning's Silentex noise control system — used on 5.5 million vehicles — can handle sound absorption issues.
— Brian Corbett