On the search for the origin of the bulge effect in high power laser beam welding

The shape of the weld pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudi-nal section a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging MWIR and VIS cameras. The observations show that the di-mensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop shaped weld pool. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching steady state to obtain the weld pool shape and to understand the for-mation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temper-ature field, the effects of phase transition, thermo-capil-lary convection, natural convection and temperature-de-pendent material properties up to evaporation tempera-ture. It was found that the Marangoni convection and the movement of the laser heat source are the dominant fac-tors for the formation of the bulging-region. Good cor-relation between the numerically calculated and the ex-perimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface were found.