January 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 A N U A R Y 2 0 1 9 2 4 *Member of ASM International AUTOMOTIVE ALUMINUM—PART V Process Improvements Leading to Mass Production, 1977—1997 When the first aluminum hood launched in 1977, the mills were delivering rectangular blanks separated by paper to protect the delicate outer surfaces. Laurent Chappuis,* Light Metal Consultants LLC, Grosse Ile, Mich. Robert Sanders,* Novelis Inc., Atlanta U nderstandably, the automakers demanded that the visible outer surfaces of their product were per- fectly uniform and free of any scratches or marks. Due to the growth in can and aerospace sheet, suitable ingot and roll- ingmills were in place, but delivering thin 2xxx or 6xxx alloy sheet meeting the outer quality specifications of the automakers proved to be a challenge for the alumi- nummills. It was not that the mills were un- accustomed to the concept of surface quality, but auto body sheet (ABS) rep- resented the first time surface criti- cal products had been mass-produced through continuous solution heat treat furnaces. The existing furnaces had been developed for much thicker aero- space sheet and were 90 or even 180 meters long, creating numerous op- portunities for defects. In such contin- uous furnaces, aluminum sheet floats on a cushion of air; any contact with the air nozzles or wall results in scratch- es or marks on the sheet (Fig. 1). At the oven’s exit point, a strong water quench could create sheet buckles and localized features called “crow’s feet” or “turkey tracks.” Simply put, outer quality ABS was nearly unique in its dimensions (up to 2000 mm wide and 0.85-1.1 mm thick), and there were no other paral- lel markets of significant volume in the aluminum industry to help justify the investment in process know-how and equipment. And so, the installation of solution heat treat lines optimized for aluminum ABS would have to wait until justified by the demand. HEAT TREATING ALUMINUM ABS Moving thin sheet through a long heat treat oven without causing sur- face damage and quenching without distortion were vexing issues for mills that usually performed simple anneal- ing operations or heat treatment of much thicker aerospace sheet. The best the mills could do was to optimize their existing process and rely on visual in- spection to remove portions of the coils that failed outer quality specifications. Delivering rectangles from their own cut-to-length lines created an effec- tive quality feedback loop and reject- ed blanks could be efficiently recycled. This approach had the added benefit of hiding the mill’s internal yield issues from the automakers. Concerns about surface quality were so acute that Ford would continue to receive rectangular, paper-separat- ed, outer quality blanks directly from Davenport until the mid-1980s, while blanking their own inner quality coils in-house. However, being limited to re- ceiving rectangular blanks was not ac- ceptable in an industry where material represents 70% or more of the finished part cost. The fact that aluminum sheet was more than twice the price of the in- cumbent steel made the situation that much worse. While steel mills often slit coils to the final specified width for large cus- tomers, they generally do not deliver finished blanks to a stamping plant. For large body panels, a coil is first slit to the exact width specified for the intend- ed part, then cut into flat blanks that are stacked onto pallets in high-speed blanking lines, either in the stamping plant itself or by a nearby supplier. The blanks are rarely the basic rectangles that were produced by the simple cut- to-length lines of the aluminummills. As ABS usage grew, the industry realized that the existing slitting and blanking equipment was ill-suited to the tender aluminum surface. Major investments would be required to up- grade or even replace this equipment. Again, the issue was the return on in- vestment: A single blanking line can support the needs of several stamping lines, and all of the new aluminum ap- plications combined could not fill even a single stamping line. In 1979, the solu- tion came from Marvin Hole, founder of the AluminumBlanking Company (ABC) in Pontiac, Michigan, devoted to pro- cessing aluminum coils for the automo- tive and transportation industry. Fig. 1 — In a flotation furnace, an aluminum sheet is heat treated while floating on an air cushion.