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The Al-4.5Zn-1.5Mg-0.5Mn aluminum alloy was welded by MIG welding in test, and the microstructure and mechanical property of joint were investigated. The results of experimentation show that the microstructure of welding metal is as-cast microstructure, and the microstructure of base metal is isometric crystal. There is a narrow fusion zone near the edge of weld seam, which consists of fine columnar grains. In the base metal and heat affected zone, the precipitation was diffuse in the intragranular, while strengthening phase η mainly focused on grain boundaries. The precipitation phases in the heat-affected zone distributed unevenly with large size, intermittent and segregation. In the welding metal, there is no precipitation phase. The microhardness on center of welding metal is lowest in joint. The tensile strength of welding joint is 272MPa, and the percentage elongation after fracture is 6.6%. The zone of fracture is located in the welding metal.

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The fatigue fractography and surface of A7075 aluminum alloy was investigated by means of optical microscope, scanning electron microscope and technology of metallurgic replica. The results show that the fatigue crack generally initiated in the surface or near the surface of the sample area, such as hole and inclusion. The crack propagation regions under high stress amplitude was characterized by furrow and hollow, and obviously presented fine ripple vein. While lots of fatigue striations and sidesteps with the holes and secondary cracks generated in the crack propagation regions of low stress amplitude. Fatigue transient breaking area is a mixed fracture of brittle and ductile fracture. In the fatigue crack propagation stage, with the increasing of cycle times, secondary cracks gradually merged into primary crack on the path of crack propagation. The dislocation of crack tip,grain boundaries and different grain orientations have great influence on the path of crack growth, which can make cracks bending.

In this paper, 6013 aluminum alloy with the thickness of 2.5mm was overlap welded by fiber laser. The microstructure, mechanical properties and fracture morphology of the joint was tested and observed by Optical Microscope, material testing machines and Scanning Electron Microscope, thus the failure and fracture mechanism of the welded joint are analyzed. The results showed that good shape of weld was achieved under the optimal welding parameters. Equiaxial as-cast microstructures exist in the welding center and the columnar grains are formed near the fusion line in the WZ. The hardness of weld zone is the lowest in the joint, which is about 72 HV, about 57% of that of BM. The tensile shear strength of the joint is 96Mpa, about 25% of tensile strength of BM. The fracture is happened in WZ and the brittle fracture mode is dominated with shear dimples and shear planes.