Research on Microstructure and Fatigue Properties of 7050-T6 High-Strength Aluminum Alloy Cold Metal Transfer Welded Joint

In this paper, 7050-T6 high-strength aluminum alloy with 3-mm-thickness is taken as the research object, and it is welded by CMT Advanced 4000R NC welding system. The microstructure, mechanical properties, evolution of joint phase and fatigue properties are studied, and the tensile and fatigue fracture are observed and analyzed. It is found that the fusion zone is dominated by columnar crystal structure and typical epitaxial solidification; the structure of weld zone is mainly equiaxed crystal and dendrite. After 360 h of natural aging, the hardness distribution of the joint is not uniform, and the hardness of the weld zone is the smallest. The hardness of the overaging softening zone is 10 HV lower than that of the solid solution zone, and a softening zone with a width of about 5 mm appears in the joint. The HAZ softens obviously, and the fracture surface shows typical dimple fracture morphology. The results show that the conditional fatigue limits of the specimens are 166.4 and 109.3 MPa respectively under the condition of setting N_f = 10⁷ fatigue cycle. The fatigue initiation area of the joint is mainly welding porosity, and the fatigue striation spacing of the extension area is different. In the process of crack propagation, the striation tends to bypass the second phase particles; The instantaneous fracture area consists of dimples of different sizes. The thermal simulation test of the weld is carried out through the moving Gaussian heat source model in ANSYS software. In addition to the plate-like Mg₂Si phase observed in the weld, FeAl₃ and (FeMn)Al₆ impurity phases are also found, only a weak diffraction peak of MgZn₂ phase is found in the solid solution zone. The microstructure evolution of HAZ is analyzed by HRTEM images. It is found that there are tiny GP zone precipitates in the solution zone.