Behaviors of crack propagation in 7075 aluminum alloy friction stir welded joints during tensile process

The flat butt welding was carried out on 7075 aluminum alloy with the thickness of 6 mm by friction stir welding. The flat tensile test of joints at different strain rates were measured by the MTS. The microstructure of both sides of the fracture path in joints and the fracture morphology were observed by OM and SEM. The precipitated phases at the initiation of crack source were observed by the TEM.The influence of strain rate on fracture behaviors of the joints was studied. The results show that with the increase of strain rate,the yield strength and yield strength ratio of the joint increase slightly. The microcracks nucleate at the bottom of the junction of the base metal（BM） and the heat affected zone（ HAZ）. The precipitated phase of MgZn₂ with rod shape,compared to AlCuMg with elliptical shape and Al₂ CuMg with capsule shape,plays a key role in microcrack nucleation. The crack deflects during the propagation process at the low strain rate. With the increase of the strain rate,the crack of joint tends to be straight and the plasticity of joint decreases,simultaneously,the angle of crack propagation decreases with the loading direction. The fracture mode is mainly changed from dimple polymerization fracture to shear fracture.