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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 | S E P T E M B E R 2 0 1 8 1 8 for the company’s next model, the Land Rover Discovery. PORSCHE MAKES PROGRESS It would be left to Porsche and Alusuisse to take the next step in auto body sheet alloy development. In the early 1970s, Porsche was looking to re- place the 911 with a more sophisticated Grand Touring vehicle, the 928. It was wider, longer, and had a front engine/ rear drive layout with a V8 engine. To keep weight down, the design specified aluminum for the hood, fenders, and doors; the parts would be produced by Läpple using aluminum sheet supplied by Alusingen, part of Alusuisse. Porsche and Läpple quickly realized that 5xxx alloy sheet would be unsuitable for the skins. The result of their development work with Alusingen was an alloy that would be later marketed as Ac-120 and registered as alloy 6016. Anticorodal 120 or Ac-120 (1.2% Si, 0.4% Mg) was originally developed as a food container sheet alloy to replace higher Mg 5xxx alloys considered hard to roll on their existing equipment. An- ticorodal was the trade name given to Alusuisse 6xxx alloys due to their good corrosion resistance compared to exist- ing 2xxx heat-treatable alloys. The orig- inal concept was to use a continuous heat treatment to increase strength pri- or to cold working to the required can stock temper. For automotive use, they decided to stamp the parts in the more formable T4 temper and take advan- tage of the new E-coat ovens as a no- cost-added heat treat process. Ac-120 was different from other 6xxx alloys of the time because it was lower in Mg and high in Si. The high Si/Mg ratio result- ed in several advantages in mechanical properties and formability. Compared to 2xxx alloys such as A-U2G and 2036, 6016 offered similar strength after paint, but with lower in- coming strength and therefore much more manageable springback (Table 2). The 6016 alloy also had a low de- gree of natural age hardening at room temperature but offered an increase in strength after forming and bake hard- ening comparable to other 6xxx al- loys. Due to the introduction of E-coat, Porsche had a ready-made hardening oven in its paint system. This allowed an alloy like 6016 to be delivered as a relatively low strength alloy in T4 tem- per, offering good formability and man- ageable springback. After the E-coat bake and resultant hardening, the alloy achieved comparable strength levels to the mild steel sheet used at the time. Another special feature of 6016 is its low Cu level. While Cu is known to increase post-paint bake strength, it is generally not beneficial for corrosion resistance. Structural alloys in Europe ranging from 6060 to 6082 had always been Cu-free, so there was no intention- al addition to alloy 6016. The combina- tion of good stretch formability and hemming capability combined with ad- equate strength after paint bake would make 6016 the favorite body skin al- loy in Europe for the next 25 years and spawn numerous variants by multiple suppliers worldwide since then. The use of Ac-120 was not the only first for sheet aluminum on the Porsche 928. The door inners were the first Eu- ropean automotive use of yet anoth- er food container alloy, 5182 (4.5% Mg, 0.3% Mn). This alloy was registered by TABLE 2 – MECHANICAL PROPERTIES OF EARLY ALUMINUM AUTO BODY SHEET ALLOYS Alloy Yield strength, MPa (as received) Tensile strength, MPa Work hardening exponent, n Total elongation, % A-U2G-T4 179 276 0.25 27 2036-T4 186 325 0.22 25 6009-T4 131 234 0.23 24 6010-T4 186 296 0.22 23 6016-T4 127 235 0.26 28 Chemical composition of various 6xxx alloys. Ferry Porsche and the 928 at the 1977 Geneva Motor Show.