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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 7 former low volume luxury automak- er, they were probably accustomed to heavy metal finishing. The low strength was a problem that Panhard had ac- knowledged and by keeping the outer panels to 1.2-mm thickness, they know- ingly accepted inferior dent resistance in pursuit of maximum weight savings. In fact, Jean Panhard admitted years later that he could dent the outer pan- els with a finger and that customers complained about the problem. Cit- roën, on the other hand, was the lead- ing French car producer at the time and they would accept neither situation. The lack of strength could be countered by increasing the panel thickness to 1.5 mm, but that added cost and weight. After a few acrimonious meet- ings, the automaker prevailed over its metal supplier who scrambled for a suitable alternative. The pressure from Citroën preclu- ded lengthy alloy development, thus Péchiney could only look into their ex- isting portfolio. One could raise the magnesium content of a 5xxx alloy to increase strength, but this also in- creased the potential for Lüders bands, eliminating A-G5 as an option. Péchin- ey had unsuccessfully experimented with some 6xxx sheet alloys in the mid 1930s, so the only remaining avenue was a 2xxx alloy that had been used ex- perimentally with Panhard at the time of the Dyna X. The alloy was A-U2G (2% Cu, 0.3% Mg, 0.1% Si). ALLOY DEVELOPMENT CONTINUES A-U2G was a derivative of 2117 (see Table 1), a common aircraft alloy, and had been used for cargo contain- ers under the name of Avional 21. As an automotive alloy, it was still an exper- imental solution because 2xxx alloys had a history of corrosion susceptibili- ty. Further, as an alloy capable of pre- cipitation hardening, its natural aging response needed to be studied. Péchin- ey had started a development program with Panhard in 1949 with lab corro- sion testing and aging characteriza- tion. They had also made a number of prototype parts that remained subject to static weather exposure on the At- lantic Coast. Despite lab characteriza- tions that revealed no insurmountable difficulties, Panhard elected to contin- ue with A-G3 for the Dyna Z. Thus, more than five years into testing without any significant issues and their backs to the wall, Péchiney offered A-U2G to Cit- roën, who promptly adopted it. A-U2G was almost twice as strong as A-G3, so Citroën was immediately able to down- gauge to 1.2 mm. However, with a yield strength of 180 Mpa, A-U2G must have proven quite a challenge for the die- makers. Nonetheless, the parts made it into production and the hood would re- main for the totality of the production run of over 1.5 million units. These aluminum parts signaled the first departure from 5xxx alloys for a modern aluminumautomotive skin. For Péchiney, they marked the beginning of an active market development program focused on the automotive sector, more than a decade earlier than its American counterparts. A-U2G ended up on the hood and other parts of the Citroën SM, the Rover 2000 and 2600, and all the closures at Rolls-Royce. LAND ROVER SUCCESS Across the Channel, the Land Rover had turned into a major com- mercial success. By 1954, the base model’s wheelbase had increased to 86 in. (2184 mm) and a new 107 in. (2718 mm) wheelbase model was in- troduced. Sales were coming close to 30,000 units per year. No longer a stop- gap measure, it had evolved into its own product line. Series II would enter production in 1958, retaining the 5xxx alloy sheets. The Land Rover success prompted the company to look into a larger, more upscale model. The first attempt took place as early as 1951, but the proj- ect fizzled and the idea would lie dor- mant for 15 years. In 1966, the success of the Jeep Wagoneer in the U.S. rekin- dled interest in a more refined 4x4. By 1969, the Range Rover design was final- ized and 26 development prototypes were built. To keep weight in check, it sported aluminum skins for the front and rear fenders, leaf screen, door out- ers, roof, and upper rear quarter pan- els. Rover had not experienced Lüders bands with the flat panels of the Land Rover and so it carried over the alloy se- lection. The Range Rover launched on June 17, 1970. It would soon become another automotive icon, but the selection of a 5xxx alloy for stamped skins would now reveal its limitations, burdening the company with additional metal fin- ishing for its 25-year production run. These challenges would also trigger the search for different suppliers and alloys TABLE 1 – GENEALOGY OF 2XXX SHEET ALLOYS Alloy Si Fe Cu Mn Mg Year Duralumin 3.5-4.5 0.25-1.0 ~0.5 1911 2017 0.2-0.8 0.7 3.5-4.5 0.4-1.0 0.4-0.8 1916 2117 0.8 0.7 2.2-3.0 0.2 0.2-0.5 1921 A-U2G 0.1-0.2 0.4-0.5 2.2-2.5 0.2 0.3-0.4 1950s 2036 0.5 0.5 2.2-3.0 0.1-0.4 0.3-0.6 1970 Citroën DS makes a splash at the 1955 Paris Motor Show, with 12,000 orders taken on the first day. 1978 Range Rover Classic.