July/August_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 | J U L Y / A U G U S T 2 0 1 8 2 1 it might serve as an abbreviated arti- ficial aging cycle for the right alloy. In time, the aluminum companies would take notice. By this time, Reynolds was the second largest U.S. aluminum company and it had a sizable research laborato- ry. Headquartered in Richmond, Vir- ginia, it had become a trusted supplier to the automotive industry. From 1957 onward, it supplied GM with the spe- cialty aluminum alloy used in the Cor- vair aluminum block and several other parts, many of which went to the Cor- vette. Their automotive sheet alloy de- velopment bypassed 5xxx and instead followed a path opened more than 20 years earlier by Pechiney and their AU2G alloy. In 1970, they registered the first successful U.S. alloy developed specifically for automotive sheet, 2036. It was the first alloy to combine form- ability, surface quality, and strength, taking advantage of the recently intro- duced E-coat baking process. Due to the artificial aging capability, its strength could actually increase slightly during the baking cycle and this allowed a 20% reduction in gauge for the outer skins compared to existing 5xxx alloys. It also did not suffer from the Lüders bands during forming as the Al-Mg alloys did, eliminating costly metal finishing re- quirements. Suddenly, aluminum skins seemed plausible. ALLOY DEVELOPMENT In 1972, GM had just completed a steel version of an experimental Cor- vette, the XP-895. The car was gorgeous, but it had a problem: The all-steel body weighed 100 lb more than the produc- tion Corvette. John DeLorean, Chevro- let’s general manager, asked Reynolds to reengineer the body as an all-alumi- num solution using Reynolds’ new 2036 sheet alloy (Fig. 5). They were given ac- cess to the existing prototype tools and jigs used for the steel body and com- pleted the task in three months. Side by side and painted silver, the alumi- num and steel versions were twins, but the aluminum version was 450 lb light- er. The car made its public debut at the 1973 New York Auto Show. The ef- fort was not for the faint of heart and the methods used were not produc- tion-ready because the design could not be modified for the aluminum solu- tion. Even 2036’s improved formability could not match that of steel. That same year, Alcoa announced several new alloys for aluminum sheet use (Table 1). In the introduction, the company pointed to aluminum’s suc- cess in other ground transportation applications. They offered 2036 (al- ready in the market from Reynolds) and 6151 alloys as possible solutions to reduce outer panel weight. Alloy X5020 (2.8%Mg-1.6%Cu-0.3%Mn) was a truly unique, heat treatable compo- sition offering a better combination of after-bake strength and formability compared to 2036. This alloy was ap- parently aimed at an improved recy- cling compatibility with 2036 outers. Alloy X5085 (6.2%Mg-0.2%Mn) was touted for inner panels due to its high n-value and uniform elongation, as shown in Table 1. The engineering data published in the company’s 1972 “Body Sheet Alloys” document and 1974 SAE “Development of Aluminum Alloys for Body Sheet” paper included typical r- and n-values as well as forming limit curves similar to those used today. The need for alloys to strengthen during au- tomotive paint baking—and the con- current impact of pre-strain on baked strength—were clearly known and dis- cussed in this publication. However, these alloys were ex- tremely short-lived. The X5020 alloy seems to have had little or no imple- mentation by major automakers. One reason was probably its high incom- ing yield strength, which would have exacerbated the already considerable springback problems seen in other al- loys. The X5085 alloy was successful- ly used by General Motors but quickly withdrawn by Alcoa after a few years and replaced by 5182. Production problems during rolling of the high-Mg X5085 alloy included high loads during Fig. 5 — All-aluminum 1972 XP-895 Corvette produced by General Motors and Reynolds Metals Corporation. TABLE 1 – PROPERTIES OF ALLOYS OFFERED BY ALCOA FOR ALUMINUM AUTOMOTIVE SHEET, 1972 Alloy Temper Yield Strength Ultimate Tensile Strength Elongation n-value KSI MPa KSI MPa 2-in. gage length X2036 T4 26 179 43 296 22 0.21 X5020 T4 30 207 50 345 24 0.25 X5085 H111 23 159 45 310 27 0.30 6151 T4 24 165 38 262 23 0.19