November/December 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 | N O V E M B E R / D E C E M B E R 2 0 1 8 1 6 In 1976, Alcoa registered such a pair of alloys [4] . The first, 6010, was os- tensibly designed for the outer skins with a T4 yield strength similar to 2036. The second, 6009, was aimed at inner panels, offering better formability and lower yield strength, more in line with 5182-O. Both had a better hardening response in the E-coat oven than 2036, allowing 6010 to achieve strength in ex- cess of 300 MPa after paint, even with a modest pre-strain. These were the first alloys designed specifically to take ad- vantage of the E-coat bake ovens and deliver outer quality surface at a yield strength higher than the mild steels used at that time. It would take al- most a decade before bake-hardenable steels would enter production in signif- icant volume. From a production point of view, these 6xxx alloys were much easier to produce than the first generation 2xxx/5xxx compositions and took bet- ter advantage of Davenport Works’ 86-in. continuous heat treat line. GM switched some hoods to the new com- bination, while keeping others at 2036 and producing another group with 2036 outers and 5182 inners. In prac- tice, 6009 was better than 2036, but it could never equal 5182 for parts re- quiring high formability. In fact, 5182-O retains the formability crown to this day. Ford stood by its uni-alloy approach and because the design and tooling had been tuned to 2036, the company saw no need to change. Ford also stuck to its two-supplier credo, forcing Alcoa to keep producing 2036 sheet. RMC’s response to Alcoa’s new twins arrived in 1980 when they regis- tered alloy 2038. It was a modified ver- sion of 2036, with lower Cu and higher Si to improve the hardening capability. It offered good formability with more robust strength after paint. ONGOING CHALLENGES What happened next further illus- trates the difficulties faced by the alu- minum industry. By 1980, it was clear that no application was secure: Hoods and decklids were closure panels that could be switched back to steel for immediate cost savings. The majori- ty of GM’s aluminum hoods were only used for specialty models or were re- gion-specific, and most of the produc- tion sported more economical steel parts. The situation was slightly bet- ter at Ford. The Lincoln Versailles was discontinued after four years of pro- duction, but at least the Lincoln Town Car was launching with an aluminum hood. The bad news was that its design and tooling were already committed to 2036. The reason was simple: With a sin- gle hood slated for production and a re- quirement that two suppliers compete for the business, Ford had no intention of introducing new alloys unique to one supplier. Alloy 2036 was a proven solu- tion. The supply was secure with two established producers and Ford saw no reason to change—so 2038 went quietly by the wayside. At this stage, it was clear that the rosy predictions of increased alumi- num use in the vast automotive market would not materialize anytime soon, and neither Alcoa nor RMC saw much reason to continue alloy development or invest in new production facilities. They unwittingly passed the baton to a new challenger. Montreal-based Al- can had been watching the situation from a distance and concluded that an exclusive focus on closure panels was flawed. Alcan saw the future in applica- tions that would involve the body struc- ture itself. The company leveraged its two research laboratories, with the U.K. lab working on body structure solutions and the Canadian lab working on a new skin alloy designated X6111 [5] . This al- loy was designed based on a systemat- ic study of other alloys available at the time: 2036, 2038, 6009, 6010, plus two extrusion alloys, 6061 and 6063. Alcan researchers focused on solv- ing key issues with the incumbent 2036 and 6010 alloys—2036 exhibited good formability but had little or no strength- ening for short times at paint tempera- tures (175°−205°C), while 6010 was actually a step back in formability. The cause of the poor paint bake response was the reversion of Cu-Mg clusters formed during room temperature natu- ral aging. These clusters are unstable at TABLE 1 — 6009 AND 6010 DEVELOPMENT Alloy Year Producer Si Fe Cu Mn Mg Cr 2036 1970 RMC 0.50 0.50 2.2 - 3.0 0.10 - 0.40 0.30 - 0.6 0.10 6151 1928 Alcoa 0.6 - 1.2 1.0 0.35 0.20 0.45 - 0.8 0.15 - 0.35 6009 1976 Alcoa 0.6 - 1.0 0.50 0.15 - 0.6 0.20 - 0.8 0.45 - 0.8 0.10 6010 1976 Alcoa 0.8 - 1.2 0.50 0.15 - 0.6 0.20 - 0.8 0.60 - 1.0 0.10 TABLE 2 — SKIN ALLOYS MADE IN USA, 1980 Alloy Year Producer Si Fe Cu Mn Mg Cr 2036 1970 RMC 0.50 0.50 2.2 - 3.0 0.10 - 0.40 0.30 - 0.6 0.10 2038 1980 RMC 0.50 - 1.3 0.6 0.8 - 1.8 0.10 - 0.40 0.45 - 1.0 0.20 6009 1976 Alcoa 0.6 - 1.0 0.50 0.15 - 0.6 0.20 - 0.8 0.45 - 0.8 0.10 6010 1976 Alcoa 0.8 - 1.2 0.50 0.15 - 0.6 0.20 - 0.8 0.60 - 1.0 0.10