Microstructure and microhardness of aluminium alloy friction stir welds with heat treatment

In this study, the Al–Mg–Si aluminium alloy with a T6 temper is self-support friction stir welded, and the effect of post-weld heat treatment on joints is investigated by detailed microstructural investigations and microhardness measurements. The hardness decreases from average 59 to 52 HV with holding time ranging from 10 to 60 minutes at temperature 723 K. With increasing holding temperature, the microhardness becomes more uniform and increases. Microhardness profiles are explained by precipitate distributions and precipitation sequences. The fine grain structure undergoes abnormal grain growth, and the degree of which depends on holding temperature and intense plastic deformation, and the biggest size is more than 1 mm.