Microstructure and mechanical properties of friction stir lap AA6061-Ti6Al4V welds

The AA6061 and Ti6Al4V plates were joined by atomic diffusion and interfacial reactions. Layers of solid solution and intermetallic compound were both formed at the interface, and the interface thickness increased as the heat input increased. The intermetallic compound layer was found to be mainly composed of TiAl3. The shear strength of the friction stir lap AA6061-Ti6Al4V welds was found determined by the interfacial microstructure. The shear performance was promoted initially with the increase of interface thickness due to the formation of intermetallic compound. The maximum shear load of 4500 N was reached with a 7.5-μm-thick interface. When the thickness of interface further increased, the maximum shear load was found to decrease again because of the exorbitant residual stresses induced by the excessive intermetallic compounds. The fracture of the joints started with the micro-cracks formed at the intermetallic compounds and then leads to break during tensile shear test.