Hydroformed tubes are seeing slow but steady growth in automotive applications, and aluminum is getting in on some of that action as well.
The increased use of aluminum tubes likely follows a more general trend toward lightweight structures, said Robert Perkins, director of supply chain and technology forecasts at CSM Worldwide, Northville, Mich.
Jay Baron, director of the manufacturing, engineering and technology group at the Center for Automotive Research in Ann Arbor, Mich., agrees that there has been slow but steady growth in hydroformed tubular products, whether they're steel or aluminum. But manufacturers have been slow to adopt hydroformed tubes not so much because tubes cost more but because they take longer to make than stamped parts. The slower forming process and the integration of the tube into the overall structure (e.g. joining) drives up their total cost.
"You can stamp out a part in six seconds; to hydroform a tube takes more like a minute. It's almost a factor of 10," Baron said. Even in optimized processes, it still takes 30 seconds to hydroform a tube, five times slower than a stamped part.
As a result, hydroformed tubes are most common on high-end, low-volume vehicles like the Chevrolet Corvette. And aluminum tubes are used only in more rarified luxury vehicles such as the Audi A8 and certain Jaguar models, which use far more aluminum than most cars.
"All-aluminum cars are not something that you're going to see anytime soon, except in high-end niche vehicles," Baron said. "You won't start seeing aluminum in the body structure until you see major subassemblies or the entire substructure in aluminum."
The use of aluminum tubes is further complicated by joining problems associated with welding steel and aluminum together, he said. As a result, aluminum tubes are used in aluminum subassemblies and aren't directly integrated into a vehicle's steel body. BMW AG, for example, made an entire rear-axle subassembly from aluminum, including hydroformed aluminum tubes and struts.
In general, aluminum use is confined to parts such as hoods and doors that can be isolated from steel in the rest of the vehicle, Baron said.
But hydroformed tubes—both steel and aluminum—should continue to gain ground, particularly as a result of higher European safety standards, which are beginning to be adopted in North America as well. That's because hydroformed tubes are good not only at reducing vehicle weight but also at transferring impact forces away from passengers during a crash.
"Hydroformed tubes are much more prevalent than they used to be," Baron said. "You would expect the percentage of tubular products to increase, but not at an exponential rate." As this point, most of the technology has been developed and it is simply a matter of optimizing the production process, he said.
Many analysts don't follow tubular products in the automotive sector because most of the companies that make such products are either private or have been bought out by other companies, Charles Bradford, analyst at Bradford Research/Soleil Securities Inc., New York, said. "We tend to focus on things where there is stock you can buy and sell."