Influence of microstructural evolution on tensile properties of friction stir welded joint of rolled SiCp/AA2009-T351 sheet

To elucidate the intrinsic factors which decreased the strength and ductility of friction stir welded (FSW) joints of precipitation-strengthened aluminium matrix composites, 3 mm thick rolled 17 vol.% SiCp/AA2009 composite sheet in T351 condition was subjected to FSW and detailed microstructure examinations. The microstructure of the base metal was characterized by high dislocation density and few precipitates. After FSW, low dislocation density and a few coarse θ (Al2Cu) precipitates were detected in the nugget zone, whereas a great number of coarse θ and S (Al2CuMg) precipitates and greatly decreased dislocation density were observed in the heat-affected zone (HAZ). Therefore, the HAZ showed the lowest hardness throughout the whole joint and became the weakest region, where fracture occurred during tensile testing. The FSW joint exhibited a good combination of strength and ductility with the ultimate tensile strength, elongation and joint efficiency reaching 443 MPa, 4.7% and 77%, respectively.