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HYDROFORMED TUBE Tooling, welding concerns are driving caution in Detroit

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Suppliers might be bullish on the prospects for hydroformed tubes in car and truck bodies, but many consumers are more cautious. Hydroformed tubes can help reduce weight and boost safety, they acknowledge, but in most cases they take longer to make and are more expensive than traditional stamped parts.

In general, there aren't many hydroformed tubular structures in Chrysler LLC vehicles, said David Reed, the Auburn Hills, Mich.-based automaker's body-in-white core lead. Part of the problem is that hydroformed tooling is expensive—roughly double the cost of stamped construction. "Your business case has to be really solid as far as piece-cost savings to pay for the tooling," he said.

Tubes also are limited by the vehicle manufacturing process, which typically involves resistance spot welding, Reed said. That involves two electrodes coming together and sending an electric current through two metal panels in order to join them. However, because a tube is hollow the current won't move between the two sides. Instead, single-sided welding or laser welding is required—and those technologies aren't as easy to integrate into the traditional vehicle manufacturing processes.

Nonetheless, tubes are lighter, safer and less expensive for some applications, he said, noting that the Dodge Dakota and Dodge Ram pickups, as well as the Jeep Wrangler, use hydroformed steel tubes for "body-on-frame" applications such as the engine box. In addition, the Chrysler Sebring sedan and the convertible version of the Dodge Avenger use tubular structures to protect against side impact.

While no major increases in tubular content are planned, the calculus could change as automakers drive for safer, more fuel-efficient vehicles.

Ford Motor Co., Dearborn, Mich., for example, has hydroformed tubes lined up for use in the D-pillar box structure of a handful of its vehicles this year. The Ford Super Duty pickup truck, the Expedition SUV and the F-150 pickup will each carry about 65 pounds in their front ends. The F-150 also will use the tubes in the body side, weighing between 24.2 pounds and 37.2 pounds per vehicle, while the Sport Trac SUV will have 29.1 pounds in the box assembly.

Hydroformed tubes have gone through a lot of growth and are more common now than they were a decade ago, John K. Catterall, General Motors Corp.'s representative to the American Iron and Steel Institute's Auto/Steel Partnership, said. But the growth has likely leveled off. "All the places it can be applied have been studied," he said.

GM uses hydroformed tubes in frames and cradles and to a lesser extent in body structures. In cars, cradles are bolted onto the body and carry the engine and front- and rear-end suspensions. In contrast to "unibody" cars, most trucks and sport utility vehicles employ a "body-on-frame" design. Frames form the backbone of such vehicles, provide strength and stiffness and carry most of the load. The body, or exterior, simply sits on top of the frame.

Most body panels are stamped, and it's tougher to integrate them with tubes because of welding issues, Catterall said. Manufacturers use single-sided, or metal inert gas, welding to make chassis, frames and cradles. That makes it easier to integrate tubular components, which generally require the same kind of welding. Spot welding is more commonly used to join the stampings in body structures.

But the competition between stampings and tubing continues, especially in the wake of new fuel-efficiency standards in the United States, Catterall said. "There has been a little bit of backwards and forwards on which one is best, which pretty much tells you that you can make either one better depending on how you go about it. There is no reason for it to be a huge battle. It's whatever one works best at the time."

Tubes do in some cases allow for weight savings because they eliminate the extra mass of flanges inherent in stampings. They also allow for parts consolidation and fewer welds in some instances, he said. "Is it going to be a game changer? I wouldn't say so. But it's another tool in our toolbox."

But steel stampings have one big advantage they are more versatile. "You can shape them into pretty much anything you want. There is a reason they've stuck around for so long," he said.


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