Effect of Nb micro-alloying on microstructure and properties of A7204-T4 aluminum alloy joints with fiber laser-VPTIG hybrid welding

In this study, A7204-T4 aluminum alloy sheet with a thickness of 6 mm was welded using fiber laser variable polarity tungsten inert gas (VPTIG) hybrid welding with ER5356 filler wire, during which Nb (niobium) foils with different thicknesses (10 μm, 25 μm, 35 μm, and 50 μm) were preset between the butt joint faces. The joint microstructure and properties with different Nb content were investigated in detail. According to the results of this study, the welds after Nb micro-alloying exhibited significantly reduced pores, while Nb-concentrating phases and the NbAl3 precipitated phase gradually appeared and acted as heterogeneous nucleation particles, which can refine the grains. Therefore, columnar crystals and secondary dendritic crystals at the weld center disappeared and fine equiaxed crystals appeared. After the addition of 10 μm Nb foil in the hybrid welding, the micro-hardness of weld metals was significantly enhanced, heat-affected zone in the weld joint became narrower, and the softening phenomenon reduced significantly. However, as the added Nb content increased, the micro-hardness was enhanced less significantly. After Nb micro-alloying, the tensile strength, yield strength, and elongation rate were enhanced significantly. The mechanical performance of the weld joint reached the optimum after 10 μm Nb was added in the welding process; specifically, the mean tensile strength, mean yield strength, and mean elongation rate reached 344 MPa, 269 MPa, and 6.2%, respectively, which were 91.5%, 89.4%, and 61.4% of those of base metal, respectively. The significant enhancement of weld micro-structure and mechanical properties can be attributed to the formation of effective nucleation particles during the solid solution of Nb in the Al matrix, thereby leading to grain refinement and a change in the corresponding precipitated phase.