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

Abstract

In this paper, laser-MIG hybrid welding was applied to weld TC4 titanium alloy with a thickness of 5 mm.Based on the scanning electron microscopy (SEM) analysis it was 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 were observed using a high-speed camera. The in uence of the beam defocusing amount on the keyhole stability was analyzed in detail and the formation mechanism of the process-type porosity was elucidated. By adjusting the defocusing amount to stabilize the keyhole morphology, the pores in the weld were eliminated. With the defocusing amounts of + 2, 0, -2, and − 4 mm the keyhole stability was poor. And 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 more than 80% when the defocusing amounts were + 4 and − 6 mm. The keyhole was in a stable state and no porosity was generated in the weld when the defocusing amount was + 6mm.