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 2 hot rolling and poor productivity (ex- tra passes and edge cracking) during cold rolling. Both alloys were delisted in 1977 by The Aluminum Association at Alcoa’s request. The favorite alloy for difficult inner panels quickly became 5182 because its deep drawability was superior to 2036 and other 6xxx alloys. In addition, with 4.5%Mg and 0.3%Mn, it wasmuchmore amenable to existing rolling equipment than X5085. However, like other 5xxx al- loys, 5182 was prone to Lüders bands during forming, which made it unac- ceptable for outer skins. At the time, Reynolds proposed that this issue could be solved by special processing tech- niques. The 5182 SSF (stretcher-strain free) material exhibited better surfac- es after stretch-forming, but still could not meet the automakers’ surface qual- ity requirements. Finally, 5182’s high Mg content made it susceptible to stress corrosion cracking (SCC) under certain environmental conditions. One of the Big Three learned that the hard way when air filter boxes made out of 5182 started cracking for no apparent rea- son. The root cause of the failure turned out to be SCC, prompted by the high temperature and sometimes highly hu- mid underhood environment and the stresses experienced while being at- tached to a vibrating engine. Kaiser Aluminum did not possess the resources of its two bigger competi- tors and was left behind in these efforts, although the company was far from in- active despite not being able to offer any new alloy of its own. There were talks about licensing the French alloys, but nothing seems to have come of it. They did supply some 2036 for trial pur- poses, but did not enter into production with any automaker. Several aluminum sheet papers were presented at the 1973 SAE Con- gress in Detroit, including one from a visiting delegation from Pechiney, who even brought some stamped parts made with its own AU2G and AG3 al- loys. Papers presented by the auto- makers noted the growing weight of cars: From an average of 3850 lb in 1973, weight was expected to jump to 4200 lb by 1974 and 4500 lb by 1977. All stressed the need to save weight with a target to shed 700 lb by 1977, using aluminum extrusions for the bumpers, plus high strength steels and alumi- num sheet whenever possible. Smell- ing success, the aluminum industry was quietly making preparations for selling between 15,000 to 18,000 tons for the 1975 model year, mostly for GM. Ford was playing catch-up, having only re- cently decided to consider aluminum sheet for body parts. What the aluminum industry in- siders did not realize was that having a potential alloy solution and making prototype parts was one thing; having a complete solution capable of surviving in mass production was still four long years away. There were many challeng- es to be overcome for aluminum sheet to challenge steel. For one, neither in- dustry understood the other. The alu- minum mills, coming from aerospace production, were used to strict specifi- cations, custom solutions, and careful and measured production methods. If something did not turn out as expect- ed, the project stopped until the issue was solved. MOVING TOWARD MASS PRODUCTION Automotive mass production is quite different. Product development follows a strict schedule and delays are normally unthinkable, especially in the new environment of government man- dated changes. In the early 1970s, the system worked because everyone in- volved in the launch of the product had an implicit knowledge of what to do. These included the designers who styled it, engineers who designed it, manufacturing engineers who prepared the production processes, and the tool shop workers who designed, built, and launched the stamping tools. This was especially true for the le- gions of tool and die makers who were critical to crafting the stamping tools to actually produce acceptable parts. De- signing and getting a draw die to work was the world of a few highly prized specialists who used their years of ex- perience, intuition, and guts to move things forward. It took years to gain the knowledge to become a draw die specialist, but all of their accumulat- ed knowledge and expertise was based on mild steel. When aluminum arrived, they were suddenly at sea with a ma- terial that did not behave as expected. Aluminum sheet was very different than mild steel. While it stretched reason- ably well, it had a much lower ability to be drawn into corners, with wrinkles that none of the old tricks could con- quer. The only remedy was to change the part design by simplifying and soft- ening shapes. Design guidelines to help product engineers get closer to a work- able design on the first try were urgent- ly needed, but it would take time. The second and more intractable problem was springback. When a die opens, the formed part is suddenly re- leased of its external constraints. In- ternal residual stresses spring the part away from the tool, springing it back to an unloaded state, hence the term. Springback is directly related to the ra- tio of yield strength to elastic modu- lus, and unfortunately for aluminum, its elastic modulus is one-third that of steel. At near equal yield strength, this represents a threefold increase in springback potential. Springback is not linear—it is a complex 3D deformation. The toolmaker had to guess what dis- tortion to apply to the tool shape so that the part, once released, assumed the design shape. The problem was exacerbated by the fact that few en- gineers truly understood the poten- tial artificial aging enabled by the new E-coat ovens, and still specified alloys with high incoming yield strengths. In 1970, without analytical or computer modeling, correcting springback was a daunting task that could take eight to 10 iterations, each lasting four to six weeks. It added a great deal of uncer- tainty to both cost and timing, a situa- tion despised by program planners. ALUMINUM SHEET CHALLENGES All car bodies and closure parts were spot welded together. Steel spot welding was a well understood process, with capable equipment suppliers. One of the advantages of spot welding was