BMW is making a big bet in fielding its new i3 electric city car, and that means taking efficiency to the max. The company came up with an ultra-lightweight platform for the i3 — called LifeDrive — that combines a passenger cell made of carbon-fiber-reinforced plastic with an aluminum Drive module that holds the battery, powertrain and structural crash protection. Even the instrument panel supports are made of magnesium, another very light material. “Every step in the development of the i3 has been shaped by the demands of weight optimization,” BMW says.
As automakers enter a new world constrained by stricter global fuel economy and emission mandates, they’re using every means at their disposal to make vehicles lighter, and that often means doing what the BMW engineers did — joining two lightweight structures together. The Bosch Automotive Handbook looked at a modern hybrid body structure and found 47 percent high-strength low-alloy steel, 42 percent standard steel, 10 percent aluminum and 1 percent plastic. Unfortunately, joining all these parts isn’t easily done. Steel and aluminum, for instance, are what the British call “chalk and cheese,” meaning they’re about as compatible as a rock and a dairy product. And that’s a problem when you want to attach a lightweight aluminum roof panel to a steel body.
The company found one answer in a version of friction stir welding, which joins steel and aluminum through mechanical pressure using a rotating tool. Announced in 2012, it already was in use for the 2013 Accord.
Now Brigham Young University and Oak Ridge National Laboratory researchers have come up with a just-in-time solution called friction bit joining, which yields a tough connection between ultra-high-strength steel and lightweight aluminum. It’s likely to be used in practical applications very soon. Zhili Feng is group leader in the materials joining group at Oak Ridge, which has been working with Honda. He says that there’s “a lot of push” from auto companies to figure out how to get metals to work together. He explains that temperature is the key.
“Welding steel to steel is not a problem because the two pieces being joined have the same melting temperature,” he says. “But carbon steel liquefies at a much higher temperature [2,600-2,800 degrees Fahrenheit] than aluminum [1,220 degrees F], so the two don’t connect well. The quality of the weld is not good — cracking is a big problem, for instance. That’s why we needed a way of putting them together without melting.”
Source: GreenBiz.Com | Jim Motavalli