Post weld heat treatment (PWHT) of Friction Stir Welds (FSW) was carried out at solutioninsing temperatures of 520, 540, and 560degreesC followed by ageing at 175degreesC or 200degreesC. It was found that the weld (stir) region exhibited very coarse grains after the PWHT. The hardness was found to be uniform across the weldment after the PWHT. The samples failed after PWHT during root bend test. Heat treatments to reduce the grain size did not reduce the brittleness of the welds. The brittleness was attributed to the presence of precipitate free zones adjacent to the grain boundaries and the equiaxed structure of the weldment microstructure and the failure was due to a ductile intergranular fracture mechanism.