A New Paint-Bake-Hardenable Aluminum Alloy for Auto Body Sheet Applications

A new 5xxx-series aluminum alloy, which was designed to have an excellent paint-bake-hardenability with practically no room-temperature aging, has been developed for automobile body sheets. The alloy contains 2∼3%Mg with additional amounts of Cu and Si. Yield strength in the as-shipped condition is very low, 80∼100MPa, but it can be increased by as much as about 60MPa through 2% stretch and paint-bake at 170°C. Such an excellent response to paint-bake was revealed to result from Guinier-Preston-Bagaryatskii(GPB) zone formation and enhanced by the optimum addition of Si. Density of vacancies in the alloy was reduced through a special annealing process to suppress room-temperature aging.

Press-forming for fender outer and engine hood inner panels was done, and the alloy was demonstrated to have formability superior to 6xxx-series alloys and Al-4.5%Mg alloys, due to the reduced initial yield strength which also suppressed susceptibility to spring back. High bake-hardenability successfully provided sufficient dent-resistance in the paint-baked panels. Spot weldability was proved much better than Al-4.5%Mg and Al-5.5%Mg alloys due to lower Mg content of the alloy. Zinc phosphating performance was almost equal to other aluminum alloys. The cost advantage was also obtained mainly through better workability and higher material yield against other aluminum alloys with higher Mg content.