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% when the defocusing amount was − 2 mm. The keyhole only collapsed at the root, and the porosity decreased by > 80% when the defocusing amounts were + 4 and − 6 mm. The keyhole was stable, and no porosity was generated in the weld when the defocusing amount was + 6 mm. By adjusting the defocusing amount, the diameter of the keyhole was increased, and the degree of keyhole opening was increased. Narrow and distorted keyhole morphology was avoided, and collapses and closures of keyholes that occurred during fluctuations were eliminated. Keyhole stability was improved, and process porosity in the weld was eliminated.