Stability analysis and porosity inhibition mechanism ofoscillating laser-arc hybrid welding process for medium-thick plate TC4 titanium alloy

The influence of oscillating laser frequency and oscillation diameter on the stability and porosity of laser-arc hybrid welding of titanium alloys were investigated. The microstructure and basic mechanical properties of oscillating laser-arc hybrid welded joints were revealed. The results show that the oscillating laser significantly improved the macroscopic morphology of the joints, reducing the porosity from 5.48 % to 0.12 %. In the conventional laser-arc hybrid welding process, the number of keyhole surface closures was more and the time of droplet transition was longer, resulting in poor process stability. When the frequency was 150 Hz and the diameter was 1.5 mm at oscillating laser conditions, the number of keyhole closures within 200 ms was significantly reduced to 5 times compared to the conventional laser-arc hybrid welding (23 times). The droplet transition was stable, which resulted in a more stable hybrid welding process. In addition, after one oscillation cycle, the forward distance of the laser was relatively short, which can reopen the smaller pores generated in the previous cycle, leading to a significant reduction in porosity. When the frequency and diameter of oscillation decreased, the fluctuation for keyhole morphology was greater and the droplet transition was instability, resulting in an increase in porosity; When the oscillation increased, the fluctuation of the keyhole outer diameter was larger and the penetration depth decreased, which was not conducive to the welding of medium-thick plates. In the oscillating laser-arc hybrid welding process, columnar crystals in the middle of the molten pool grew towards the surface, reducing the segregation of impurities in the weld center. As a result, the elongation percentage of the joints was improved by 36.58 % compared to the conventional laser joints.