Microstructural evolution of 6063 aluminum during friction-stir welding

The microstructural distribution associated with a hardness profile in a fr iction-stir-welded, age-hardenable 6063 aluminum alloy has been characteriz ed by transmission electron microscopy (TEM) and orientation imaging micros copy (OIM). The friction-stir process produces a softened region in the 606 3 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The s oftened region is characterized by dissolution and growth of the precipitat es during the welding. Simulated weld thermal cycles with different peak te mperatures have shown that the precipitates are dissolved at temperatures h igher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the therma l cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.