With the aim to explore the formation mechanisms of surface underfill, full penetration laser welding of thick stainless steel was conducted, with the use of a 10 kW fiber laser. A modified “sandwich” specimen was used, so as to directly observe the dynamic behaviors of the keyhole, vapor plume, and melt pool with the formation of underfills. On the basis of the experimental investigations, the formation mechanisms of the underfills at the top surface and bottom surface were analyzed. The results show that the downward flow of the molten meta...
