Microstructure and fatigue damage of welded joint of aluminum alloy medium plate

7N01-T5 aluminum alloy plates with thickness of 12 mm were welded by MIG welding. The microstructure of the welded joints were analyzed, and the tensile and fatigue tests were carried out. The secondary phase between fatigue strips was determined to be Fe-containing by energy spectrum analysis. A fatigue micro-crack initiation model caused by secondary phase was proposed, and the causes of micro-crack initiation at the second phase between fatigue strips were explained. The results show that the tensile strength of the welded joint was 259 MPa and the elongation was 11.4%. The tensile fracture form of the joint was a mixture of ductile fracture and brittle fracture. The micro-cracks caused by secondary phase would accelerate the propagation rate of fatigue cracks.