April AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 1 9 1 8 *Member of ASM International F rom the day Edward Budd first in- troduced the unibody concept in 1928, all high-volume automotive structures have beenmadewith stamped parts joined by resistance spot welding. Over time, assembling stamped steel parts into a body structure proved to be the only successful concept for auto body mass production. As a result, the indus- try invested staggering amounts of cap- ital in every step of its production, from the steel mills, blanking suppliers, and stamping plants to the body shops in as- sembly plants the world over. By 1970, small imports were cap- turing an ever-growing share of the U.S. market, forcing the Big Three to intro- duce their first unibody vehicles pow- ered by small four-cylinder engines. At the same time, tighter emission reg- ulations had strangled power outputs and engine muscle could no longer mit- igate weight excesses. Weight saving considerations started to appear in de- sign briefs. As we chronicled in our second article, the late 1960s saw the alumi- num producers waking up to the op- portunities in the auto body sheet (ABS) market. France’s Pechiney had been actively promoting AU2-G in the U.S., achieving some success in the over-the- road truck market. 1970 was also the year Reynolds Metal Company (RMC) in- troduced 2036-T4, marketed as the first spot-weldable aluminum alloy for the U.S. market. Alcoa soon responded with X5020-T4 and X5085-H111. It was not unreasonable to imag- ine that the alloys of the time could be used for the body structure. However, while substituting aluminum for steel in hang-on parts like hoods and trunk lids was challenging, targeting the body structure raised the bar con- siderably higher. Suddenly there were questions about joining methods and joint performance, fatigue, and dura- bility—on top of the usual design con- siderations regarding flexural and tor- sional limits. It would take more than two decades for the industry to formu- late solutions and develop the required production systems. SPOT WELDING AND STAMPING As its name indicates, resistance spot welding (RSW) creates a spot joint and spot-joined sections are inherent- ly less stiff than their ideal equivalents, which assume a continuous joint. But steel’s properties made it an ideal ma- terial for RSW assemblies: A high elastic modulus and relatively low conduc- tivity allow spot welds to be grouped closely enough to approximate con- tinuous joints. Therefore, 1970s auto- motive bodies were literally stitched together by several thousand individu- al spot joints. At the same time, engineers were starting to recognize the importance of body structure stiffness to the vehicle’s overall road handling. The switch to uni- body structures and the need to save weight only amplified that realization. Stiffness of a simple beam is a func- tion of the elastic modulus of the ma- terial, its cross-section geometry and AUTOMOTIVE ALUMINUM—PART VI In Pursuit of Lightweight Body Structures, 1972—1987 The largest and heaviest component in a car is the body structure, so it was only natural to make this the focus of advanced R&D. Laurent Chappuis,* Light Metal Consultants LLC, Grosse Ile, Michigan Robert Sanders,* Novelis Inc., Atlanta 1968 Toyota Corolla.