Microstructure and mechanical properties of robot cold metal transfer Al5.5Zn2.5Mg2.2Cu aluminium alloy joints

Al5.5Zn2.5Mg2.2Cu aluminium alloys were butt welded using the robot cold metal transfer method. Optimal welding parameters were defined based on experiments to ensure penetration welding of low heat input. The morphology and evolution of the microstructure, mechanical properties and fracture behaviour of the welded joint after 360 h of natural ageing were investigated. The experimental results indicate that a Mg2Si phase existed in the weld zone, a large amount of MgZn2 in the solid solution zone near the weld dissolved into the Al matrix and MgZn2 precipitated in the overageing softening zone. Compared with the base metal, the matrix of the solid solution zone mainly consisted of coarse precipitates, η precipitates were continuously distributed in the grain boundaries, and a precipitate-free zone was not obvious. Coarse and discontinuous precipitates were distributed in the grain boundary of the overageing softening zone, and the width of the precipitate-free zone became larger with coarsening of the grain boundary precipitates in the overageing softening zone. The mechanical properties were measured by using local samples extracted from the base metal, heat affected zone and welded zone. The base metal had the highest tensile strength and yield strength, in contrast with the welded zone. The tensile strength of the heat affected zone was 92% that of the base metal, and its elongation was increased by 16.7% due to the coarsening of the grain boundary precipitates in the heat affected zone.