Shear punch test was used to study the mechanical properties of Ti–6Al–4V titanium alloy laser welded joint. Meanwhile, microhardness test was used to further prove the results of the shear punch test. In order to get a better understanding of the reason why the highest mechanical properties are found in the coarse welded metal, the in situ method for viewing microvoid nucleation, growth and coalescence was applied. The in situ experiment showed that the crack in weld metal (WM) that was nucleated in the twin boundaries of the inner martensite (M) phase and the intersection of slip line and grain boundary grew rapidly once forming; in the heat affected zone (HAZ), there were three main cracks initiating, and only one propagated; microvoid was nucleated first along the interface between α and β phase in base metal (BM), the bluntness and expansion happening alternately. Except for a small amount of quasi-cleavage fracture in WM, the fracture appearance of the joint was all dimples. With the twin presented due to plastic deformation, the damage along the grain boundary area within HAZ and BM happened much more easily than in WM.
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