The microscopic damage initiation characteristic in welded joint greatly determines the subsequent damage evolution and fracture behavior of aluminum alloy tailor-welded blank (TWB) during plastic forming.In this study,the interactive dependence of void nucleation on microstructure and stress state in the welded joint of a 2219 aluminum alloy TWB was quantitatively explored by in-situ SEM testing.Moreover,a micromechanical model based on actual microstructure was adopted to reveal the underlying mechanisms from the perspective of microscopic heterogeneous deformation.The results showed that three void nucleation mechanisms,including particle-cracking,interface-debonding and matrix-cracking,coexisted in the deformation at different micro-structure regions and stress states.The nucleation strain of each mechanism mainly depended on the particle volume fraction,grain size and stress triaxiality.Besides,the proportions of particle-cracking and interface-debonding greatly varied with the grain size and particle volume fraction,and the variation law changed with the stress state.The proportion of matrix-cracking had a weak dependence on the microstructure,while increased with the stress triaxiality decreasing.It made the damage initiation in aluminum alloy welded joint transit from particle-cracking dominance to matrix-cracking dominance with the stress triaxiality decreasing.The micromechanical modeling results suggested that the changes of evolutions and distributions of Mises stress in particle,hydrostatic stress at interface and plastic strain in matrix with microstructure and stress state were responsible for the above effects.