Microstructural aspects of the friction-stir welding of 6061-T6 aluminum

The friction-stir weld zone in 6061-T6 aluminum is characterized by what appears to be a dynamic continuous recrystallization microstructure. Second-phase particles in the workpiece are essentially "stirred" into the weld zone where the residual hardness varies from 55 VHN near the top of the weld to 65 VHN near the bottom; in contrast to a workpiece hardness which varies between approx85 and 100 VHN. The weld zone grain size averaged 10 mu m in contrast to 100 mu m for the workpiece. However, complex precipitation phenomena associated with the transition zone extending between the workpiece and the actual weld zone illustrate the need to study this process in considerably more detail.