Effect of beam defocusing on porosity formation mechanism in laser-MIG hybrid welded titanium alloy joints

In this paper, laser-MIG hybrid welding was applied to weld TC4 titanium alloy with a thickness of 5 mm. Scanning electron microscopy (SEM) analysis determined that the porosity of the laser-MIG hybrid welded joints was mainly process porosity. The evolution of the keyhole under different defocusing amount conditions was observed using a high-speed camera. The influence of the beam defocusing amount on the keyhole stability was analyzed in detail, and the formation mechanism of the process-type porosity was elucidated. The keyhole stability was poor with the defocusing amounts of + 2, 0, − 2, and − 4 mm. Moreover, the highest porosity in the weld was 4.21