Distinct morphology of keyhole wall during high power fibre laser deep penetration welding

The keyhole wall is the interaction interface between the laser and the material during the laser deep penetration welding. Measuring the morphology of the keyhole wall is thus of significance for understanding the high power fibre laser deep penetration welding process. In this paper, the clear keyhole wall was reserved by suddenly closing laser during high power fibre laser welding of copper alloy. The results indicate that the keyhole can be divided into laser action region and metallic vapour pressure maintenance region. The laser action region is on the front wall of keyhole, and a series of concentric elliptical rings are observed in this region. In another region, its diameter is significantly larger than the spot diameter and the keyhole wall is basically smooth. The results are different from those generally accepted viewpoints, which clarify the flowing law of molten fluid in the keyhole and are thus of great guiding significance for optimising the welding technology.