Extruded aluminum bumper crash cans protect drivers, passengers

  • 16-Jan-2009 10:13 EST
Extruded Structures.jpg
New 7000 aluminum alloys provide high strength, allowing multivoid hollow structures to consistently absorb energy.

A large portion of Kaiser Aluminum’s automotive business is extruded aluminum products. Customers are enthusiastic about the weight reduction delivered by aluminum and its impact on vehicle performance and fuel economy.

“Aluminum extrusions are remarkably efficient structures,” said Doug Richman, Vice President of Engineering and Technology, Kaiser Aluminum Corp. “Our focus is currently on multivoid hollow structures: bumpers, the crash can which is the energy absorber behind the bumper, the engine cradles, and subframes.”

Multivoid hollow structures are an important growth market; so much so that Kaiser is in the process of converting an extrusion plant to all automotive multivoid hollow structures just to meet the market need.

“The objective here is to be very high strength,” Richman said. “We have had to develop some new alloys to be compatible with the extrusion process while delivering higher strength than the normal 6061 alloy but still provide equal corrosion resistance.”

To do this, Kaiser went to the aerospace side of its business and took the much stronger but less corrosion-resistant 7000 series of aluminum alloys and added compounds to remove that drawback. The result is a 7000 alloy series, which can be used in bumper crash cans or other automotive structures, that provides about 60% higher strength than the 6061 alloy but matches 6061 for corrosion resistance. It also extrudes very well.

“One of the costs in extrusion is heavily influenced by the speed at which you can extrude the product,” Richman explained. “Generally, high-strength alloys wind up extruding very slowly and therefore cost a lot more to extrude. Our new lean 7000 alloy extrudes the same as 6061, which is a very free extruding material.”

By combining strength and extrusion capabilities of the materials to get a cost-effective multivoid hollow structural component, Kaiser has come up with a strong and stiff alloy that is exactly what automotive component designers need.

The crash can behind the bumper is engineered to manage the pulse so that any impact is not transferred into the passenger cab. “These are very sophisticated engineering designs,” Richman said. “We don’t manufacture the assemblies; we work with the system designers to help them design crash cans that meet their design requirements and can be cost-effectively produced.  The goal is to dissipate peak collision energy in the crash can instead of allowing it to transfer into the passenger compartment.”

The crash can is a relatively inexpensive, disposable unit. After impact, it is unbolted and a new one is installed. The rest of the vehicle is intact. Typically, in Europe all bumpers are either steel or aluminum, but most crash cans inside are aluminum.

“Aluminum inherently has excellent energy absorption qualities and that’s what designers want in a crash can,” Richman said. “Those absorption characteristics are the key, but we can also create the whole bumper system at about half the weight of a steel version.” 

This can only be done, however, if the aluminum can be extruded with high integrity. These extrusions are extremely difficult to make because of the thin wall and tight tolerances required. A very tight tolerance is needed to ensure predictable safety and corrosion performance.

The new 7000 aluminum alloys provide high strength to consistently absorb energy. Aside from the internal crash can, aluminum bumpers themselves are seeing increasing interest vs. steel and are taking a share of the marketplace.

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