To meet federally mandated fuel economy standards, automakers must dramatically reduce vehicle weight. As a result, engineers are using more and more aluminum for body panels, engine components and structural parts.Technology Trickles DownTraditionally, the auto industry has reserved use of aluminum for high-end vehicles, such as the Audi A8, the Jaguar XJR and the Tesla Model S. But, the technology is starting to trickle down to more conventional vehicles, such as sedans and trucks.One of the stars of last month’s North American International Auto Show in Detroit was Ford’s new F-150 pickup. It boasts an all-aluminum body, including the doors, hood, side panels, load bed and tailgate, that shaves 700 pounds off the truck.The F-150 is the first high-volume vehicle to use large amounts of aluminum. Ford assembles as many units of its popular pickup in a month as all other aluminum-bodied vehicles combined sell annually. The aluminum body is joined with structural adhesive and 4,000 rivets vs. 7,000 spot welds on a traditional steel pickup.While aluminum continues to make inroads into the auto industry, steel is not about to fade away. It will continue to be an important material in Detroit for decades. In fact, Ford engineers increased their use of high-strength steel in the new F-150 frame from 23 percent to 77 percent to improve stiffness and durability, while reducing weight.Traditionally, aluminum vehicles have been assembled with structural adhesives and rivets. Because higher production volumes hinder adhesive use, automakers have relied on self-piercing rivets to join aluminum body parts.However, rivets add cost, and riveting guns have a limited range of joint configurations. And, while this approach can be used to join lower-strength steels with aluminum, it isn’t suitable for joining aluminum to ultra-high-strength steel.New Joining MethodsAutomakers and part suppliers are focusing on new ways to join aluminum with tools and technology other than traditional resistance spot welding, which is prone to variability. Engineers at Honda Motor Co. recently developed a new, continuous process for joining aluminum and steel using friction stir welding.The technology generates a stable metallic bond between steel and aluminum sheets by moving a rotating tool on the top of the aluminum, which is lapped over the steel with high pressure. As a result, the welding strength is superior to traditional metal inert gas (MIG) welding.“This new technology contributes to an improvement in fuel economy by reducing body weight by 25 percent compared to a conventional steel subframe,” claims Ron Lietzke, a Honda spokesman. “In addition, electricity consumption during the welding process is reduced by approximately 50 percent.”
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