MICROSTRUCTURAL CONTROL OF ALUMINUM SHEET USED IN AUTOMOTIVE APPLICATIONS

The microstructure of both 5000 and 6000 series alloy sheet can be controlled to provide the properties required for particular automotive applications. The work-hardening 5000 series alloys, with between 3 and 6% Mg as the major alloying addition, are supplied to the automotive companies in the annealed temper and are characterized by a recrystallized grain structure which is influenced by the insoluble Fe-based intermetallics, dispersoids, and the work-hardening rate. The annealed strength and formability is strongly dependent upon grain size, Mg content, and, to a lesser extent, on crystallographic texture. The 6000 series alloys containing Cu, Mg, and Si are somewhat more complicated to control, because of precipitation of the age-hardening phases during fabrication. It is necessary to control the processes of dispersoid and precipitate formation so as to obtain the desired strength, grain size, and crystallographic texture in the final sheet. These alloys also offer a low solution-treated strength for high formability, combined with rapid age hardening to a relatively high strength in the formed component during the paint bake cycle.