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 0 The post-WWII North American aluminum industry quickly focused on products to replace war production. In the U.S., sheet and extruded products for buildings became high priorities for the industry. Alcoa-Tennessee pro- duced roofing sheet from the remains of scrapped warplanes. Alloy 6063, reg- istered in 1944, soon became a staple for extruded doors and windows. Key alloys for the marine and ground trans- portation industry were introduced be- tween 1950 and 1958 including 5083, 5454, 5086, and 5456. These alloys, con- taining ~3-5% Mg, made high strength welded structures possible for truck tankers and shipbuilding. The first in- cremental steps in the development of aluminum cans were also taken during this period. Alcoa, in particular, con- tinued to invest in 2xxx and 7xxx alloy aerospace research. By 1951, ingot dimensions had increased so that a 6000-lb ingot was possible at Alcoa’s Davenport Works. This was only the beginning, as the U.S. government undertook a major “large press” program that led to the development of much larger DC ingots. While much of the effort focused on dif- ficulties with casting 7xxx alloys, the improved casting processes enabled larger ingots of all major rolling alloys. By the 1960s, Alcoa, Reynolds, and Kaiser focused on new alloys and larg- er products for airplanes and ground transportation. All three were engaged in the emergence of the modern bev- erage can industry. Modernization of the aluminum rolling industry brought about by rigid container (can) sheet would prove instrumental for automo- tive sheet years later. Both Reynolds and Alcoa had been quietly working on sheet alloys that could replace steel in auto body parts. But they were facing a signifi- cant challenge. By the 1960s, stamped body parts were produced cheaply and efficiently in modern stamping plants. Hoods, doors, and trunk lids were now assemblies made of an outer skin hemmed over a supporting inner panel, with special spot welding techniques to join the inner and outer components without damaging the outer surface. All of the manufacturing infrastructure and the tooling industry supporting it was optimized for steel; any aluminum sheet solution would have to fit in be- cause any significant changes would simply be too expensive. The most immediate challenge was that, at room temperature, alu- minum alloys with enough strength to replace steel could not match the form- ing abilities of good quality sheet steel. 5xxx series alloys with higher magne- sium content such as 5182 seemed to offer the best combination of strength and formability. Their excellent work hardening characteristics brought them to an acceptable strength lev- el after forming, but at the cost of sur- face appearance. Finished parts often exhibited unacceptable surface mi- cro-ridging known as Lüders bands (Fig. 4). French and British carmakers had been using lower Mg 5xxx series al- loys in series production for years, but they were making do with lots of metal finishing. In response, France’s Pechin- ey had introduced AU2G series alloy for the hood of Citroën’s revolutionary 1955 DS model. In T4 temper, this 2.5% Cu 0.6% Mg alloy had acceptable strength with a formability simi- lar to the AG3 alloy it re- placed, but without the Lüders bands. Citroën would produce close to 1,500,000 hoods with that alloy during the 20- year run of the DS model. E-COAT PROCESS DEBUTS In the U.S., another automotive development held hidden promise for aluminum. In late 1960, G.E. Brewer and his Ford Motor Company boss, G.L. Burnside, had filed a patent for an elec- trophoretic coating process now known as E-coat. The intent was to greatly im- prove the corrosion protection of sheet metal. The process involves dipping the entire body-in-white into an electrolyt- ic bath to deposit a coating even in the most hidden cavities of the body, then baking it before conventional primer and top coats are applied. Ford licensed the rights of the invention to PPG, and despite the large costs associated with the infrastructure additions caused by the process, carmakers the world over rapidly adopted it. By 1978, 70% of the world’s car production was E-coated. The addition of this high temperature baking process was a double-edged sword for aluminum: The baking cycle was hot enough and long enough to ini- tiate softening of 5xxx series alloys, but Fig. 4 — Lüders bands, micro-ridges on aluminum sheet (collection of the author). Citroën DS, produced from 1955-1975. Courtesy of Wikimedia Commons/Klugschnacker.