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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 9 length, and the cube of the wall thick- ness. In a stamped construction, the wall thickness is the sheet thickness. Because the elastic modulus of alumi- num is one third that of steel, one needs to compensate by either increasing the thickness or upgrading the section ge- ometry. Packaging considerations lim- it section growth, and cost and weight argue for minimizing sheet thickness increases. Unfortunately, aluminum’s high electrical conductivity forces spot welds to be set further apart than for steel to avoid shunting. In short, a prac- tical continuous joining method would be essential to minimize material costs and achieve the weight savings prom- ised by aluminum substitution. Complicating matters, stamping was still very much more art than en- gineering in 1970. There was a critical need to understand the design limita- tions imposed by aluminum’s lower formability. The forming limit curve (FLC) and strain analysis were only five years old, the first practical fruit of decades of research. Major and mi- nor forming strains are plotted on the two axes and evaluated against the forming limit diagram, also called the Keeler-Goodwin diagram. Predicting forming severity was still decades away, but equipped with the FLC for a given material, one could at last evalu- ate the forming severity of a stamping operation. Because aluminum’s form- ing capabilities were universally known to be lower than for mild steel, under- standing its limitations was of prime importance. Therefore, it was only nat- ural that in 1971, GM’s Bill Brazier was heading a project to develop FLCs for the existing aluminum alloys proposed for body panel applications. Alert readers may recall that in April of that same year, John DeLorean, then Chevrolet’s general manager, con- tracted RMC to build an all-aluminum version of a mid-engine Corvette steel prototype. The project was a hercu- lean effort focused on understand- ing the weight savings potential of an all-aluminum structure if packaged within steel-derived geometrical con- straints. And, although structural adhe- sives were used to maximize structural efficiency, there were no evaluations of du- rability, safety perfor- mance, or any other aspects of the metal substitution. So these were left to a 1973 thesis by Lawrence J. Dupuis, a General Motors Institute stu- dent, entitled, “The Structural Feasibil- ity of All-Aluminum Body Construction.” The thesis touched on all the major aspects of an alumi- num body structure, including man- ufacturing considerations and some rudimentary formability considerations based on Brazier’s FLCs. Laboratory work included RSW tests and the construction of simple prototype parts and assemblies for structural testing. Results confirmed the importance of developing suitable structural adhesives to achieve optimal weight savings. The thesis is a remark- able snapshot of the knowledge base in 1972, but it was a one-person effort that did not indicate any serious cor- porate interest in pursuing aluminum structures. Adding to that impression is the fact that the SAE paper related to the aluminum XP-895 prototype build (“Contruction Experience on Aluminum Experimental Body,” by K.F. Glaser and G.E. Johnson of RMC) did not include any GM authors. LANGZEITAUTO PROJECT The next visible step toward an all-aluminum body structure took the form of a cooperative effort between Alusuisse and Porsche. Readers may re- member that the Porsche 928 had been the launch vehicle for the first modern 6xxx skin alloy, Ac120. The two compa- nies initiated the “Langzeitauto” proj- ect, which translates to “long-term car.” It involved the design, construction, and testing of an aluminum version of the 928-S, entirely made with Ac120 sheet. The first prototype was displayed at the Alusingen stand at the Frankfurt Auto Show in 1981. It was clearly an ef- fort to promote aluminum in the same vein as the RMC/GM aluminum Cor- vette nine years earlier, but it repre- sented the first modern effort to an- alyze the full impact of an aluminum sheet body structure. Respecting the unibody, stamped sheet construction of the steel version, the study covered special design and manufacturing aspects, including form- ing, welding, and surface treatment for paint, as well as the testing of two pro- totypes. Unlike the XP-895 prototype build, the authors of the resultant pa- per, “The All-Aluminum Auto Body − A Study Based on the Porsche 928,” were all Porsche engineers. The pa- per noted a 47% weight savings while maintaining the required characteris- tics of static and dynamic stiffness. It further noted that other requirements such as operational reliability, passive safety properties, corrosion resistance, noise level, and repairability were also successfully met. The authors did not explore any method capable of bring- ing continuous joining into production. Almost concurrently, another study was quietly underway between Audi and Alcoa. At the time, Audi was viewed more as a gussied-up VW rath- er than a Mercedes or BMW compet- itor and management had decided that only a sustained technological on- slaught would change that perception. The effort started in 1980 with the in- troduction of the Audi quattro all wheel drive, followed by lightweighting and aerodynamic demonstrations. In 1982, Audi and Alcoa signed a development agreement, but unlike Porsche, Audi was working toward a future produc- tion vehicle. They undertook a similar aluminum-for-steel conversion, but the project explored all aspects of design Aluminumprototype of the Porsche 928 S.