Fracture and fatigue crack growth behaviour of PMMC friction stir welded butt joints

The paper is focused on the evaluation of the fracture and Fatigue Crack Growth (FCG) properties of butt joints of particulate metal–matrix composite (PMMC) obtained by friction stir welding (FSW). The materials considered are two aluminum alloy matrix/alumina particle PMMCs (AA6061/Al2O3/20p and AA7005/Al2O3/10p). Tests were conducted on unwelded and welded PMMCs using CT and Extended CT (ECT) specimen geometries, respectively. The crack growth rate was monitored by means of compliance with a strain gage attached on the back of the specimen. FCG experiments were carried out both at the centre and in the Thermo-Mechanically Altered Zone (TMAZ) at the side of the weld. The comparison between unwelded and welded PMMCs showed that FSW influences fracture toughness and FCG rate in a different fashion depending on the material. In particular, the FSW AA6061/Al2O3/20p butt joint exhibited comparable fracture toughness and higher FCG threshold with respect to the unwelded material, while in the case of AA7005/Al2O3/10p the behaviour is the opposite. The interpretation of this trend has been carried out by optical analysis of the crack path roughness and its correlation with the FCG rate. The dynamic recrystallization of the aluminum matrix and particle shaping operated by the FSW tool are at the ground of the explanation.