Microstructure and fatigue behavior of MIG-welded joints of 6005A aluminum alloy with trace amounts of scandium

Extruded plates of 6005A aluminum alloy with 0.07 wt% Sc (referred to as M6005A alloy) was metal-inert-gas-welded (MIG-welded) with ER5087 filler wire. The microstructure, mechanical properties, and fatigue behavior of the joints were studied. Compared with the base material (BM), recrystallization occurred and the formation of the coarse β' phase was evident in the heat-affected zone close to the weld zone (HAZ 2), which resulted in a significant decrease in the mechanical properties of HAZ 2. However, trace Sc inhibited the growth of the recrystallized grains in the HAZ 2, strengthening the α-AlFeSi and β'′ phases in the BM, which drastically improved the mechanical properties of the BM. In the weld zone (WZ), grains were significantly refined since the presence of Al3(Sc, Zr) particles promoted the nucleation of such grains in the WZ, and the particles also promoted the non-uniform nucleation of the α-AlFeSi phase, which significantly improved the mechanical and fatigue properties of the WZ. The hardness distribution of the MIG-welded joint shows a double-W shape. The lowest hardness is 55 HV, observed in the HAZ 2. The ultimate tensile strength (UTS) and conditional fatigue strength (107 cycles) of the MIG-welded joint of M6005A-T6 are 238.5 MPa and 108.2 MPa, respectively, 38 MPa and 13 MPa higher than the those of the MIG-welded joint of 6005A-T6 alloy, respectively.