Multiple repair welding of 7N01 aluminum alloy with pulsed MIG and DC CMT welding

The 7N01 aluminum alloy for high speed trains is sensitive to hot crack and it is more serious during multiple repair welding. Pulsed MIG and DC CMT welding with low heat input were conducted to repair welding 4 mm thick 7N01 aluminum alloy butt joints for one time, two times and three times. The macroscopic forming, microstructure and microhardness were analyzed for the repaired weld. The experimental results show that the sinking and width of the repaired weld by pulsed MIG welding were larger than those by DC CMT welding. When the joints were repaired by pulsed MIG welding, the interface between the repaired weld and the pre-weld section was distinct, the grains in the pre-weld section grew coarse, and the fusion zone was enlarged. While the microstructure of the joints repaired by DC CMT welding showed that no such obvious interface existed, the grains in the pre-weld section remained fine and the fusion zone didn't change significantly. The softening shown in the microhardness curves by DC CMT welding was weaker than that by pulsed MIG welding. The results show that using DC CMT welding to repair 7N01 aluminum alloy can effectively reduce the hot cracking sensitivity and alleviate the degradation of the joint performance.