Mitigation of porosity defects in fiber laser welding under low vacuum

Laser welding trials were conducted under low vacuum to reduce porosity defects. The effect of vacuum on the characteristics of porosity was quantitatively evaluated using 3D micro-CT. To explore the mechanisms of porosity formation at atmospheric pressure and porosity defects mitigation under vacuum, the weld pool and fluid flow behaviors were directly observed by placing a transparent glass plate on one side of the weld pool. The results showed that the vacuum condition was favorable for reducing porosity, especially process porosity. It was also found that the weld pool size and fluid flow were totally different at atmospheric pressure and under vacuum. At atmospheric pressure, bubbles were trapped in the vortex induced by two opposing fluid flows, failed to escape from the weld pool and finally formed pores. Under vacuum, suppression of vapor plume and reduction in the boiling point led to a deeper keyhole and a thinner weld pool, resulting in an upward flow in the rear weld pool. In this case, the bubbles could escape easily from the weld pool with the help of the upward flow.