Keyhole dynamics and morphology visualized by in-situ X-ray imaging in laser melting of austenitic stainless steel

Abstract

In-situ X-ray observation was used to investigate the effect of process conditions on keyhole behavior in laser melting with austenitic stainless steel. It was confirmed that the keyhole depth and width increased, and the fluctuation in the keyhole depth decreased as laser power increased. With no shielding gas, there was a large fluctuation in keyhole depth. This is presumed due to the attenuation of laser power and change in laser beam quality by the interaction between laser and metal vapor. We found that when the laser traverse speed increased relative to the keyhole-forming speed, the inclination of the keyhole decreased, and the width and opening of the keyhole expanded. As the defocus distance increased, the inclination of the keyhole decreased and the fluctuation in the keyhole depth increased. When reducing the laser power for a predetermined time, the penetration depth of the laser melting portion did not change and the keyhole depth was maintained at a laser power reduction time of 1 ms. These results will contribute to the improvement in quality and efficiency of the laser-melting process regarding keyhole formation such as additive manufacturing and laser welding and cutting.