Study on microstructure and mechanical properties of new aluminum alloy MIG welded joint

With the development of light weight and fast speed of ships, replacing steel with aluminum has become an inevitable trend of future shipbuilding. A new 10 mm thick aluminum alloy was tested by single-side MIG welding technology, and the effects of two layers and three-pass welding/three layers and four-pass welding on the comprehensive properties of the joint were studied. The results show that the surface and bottom surface of the weld seams have better macroscopic morphology and there are α-Al, Al₆(Fe, Mn) and Mg₂Si phase. In the heat-affected zone near the fusion line, large equiaxed crystals are formed, and columnar crystals are formed along the direction perpendicular to the fusion line at the edge of the weld. The center of the weld zone is characterized by dendrites, and the grain size of each layer is different, the grain size of the front layer is smaller than that of the back layer. The Fe and Mn elements are segregated in the heat affected zone to form the Al₆(Fe, Mn) phase, and Mg tends to precipitate along the weld grain boundary or distributed in the weld microstructure in the form a balanced phase. The average tensile strength of No.1 joint is 294 MPa, reaching 78.4% of the tensile strength of the base material, its elongation is 6.56%; the average tensile strength of No.2 joint is 350 MPa, reaching 93.3% of the tensile strength of the base material, its elongation is 12.3%. The weld zone has the lowest hardness for the joint overall performance, followed by the heat affected zone and the highest in the base metal. For multilayer and multipass welding, the hardness of each layer is related to the welding sequence, and the hardness of the first layer is the highest.