| Summary: | The microstructure and tensile failure evolution of AA5754O aluminum alloy CMT joints were investigated in this study. First, the microstructure and properties of aluminum alloy were observed using a hardness test and metallographic test. The microstructure and tensile failure evolution of AA5754O aluminum alloy CMT joints were studied using in situ CT tests. The defects in the heat-affected zone were mainly composed of pores with large sphericity. The softening failure was mainly due to the decrease in the effective bearing area due to the increase in the number of defects. There were a large number of shrinkage pores with sphericity less than 0.6 in the fusion zone defects. The softening failure was mainly due to the continuous growth and combination of shrinkage pores, which led to a decrease in the effective bearing area. Meanwhile, the variation process of the mean radii of the meso-defects in the heat-affected zone and fusion zone were analyzed. The material constants α<sub>RT</sub> and α<sub>RTm</sub> were 1.87 and 6.20 in the heat-affected zone and 7.21 and 5.31 in the fusion zone, respectively, which were found using the Rich and Tracey model and the improved Rich and Tracey model.
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