Localized boiling-induced spatters in the high-power laser welding of stainless steel: three-dimensional visualization and physical understanding

Serious spatter defects occur with the intensive interaction between a high-power density laser beam (>10(7) W/cm(2)) and the material used in high-power (HP) laser welding. In this study, we developed a three-dimensional comprehensive model to physically simulate the keyhole and weld pool dynamics in HP laser welding. Our recent proposed adaptive mesh-refinement method was adopted to solve the model. An interesting phenomenon was directly observed: the intense localized boiling on the mesoscale keyhole wall induced many small spatters (LBiS), sized hundreds of microns, during 10kW HP laser welding of 304 stainless steel. Experimental studies of parameters were also conducted to validate the model and characterize the spatter behaviors outside the keyhole wall during welding. The theoretical predictions of LBiS behaviors agreed well with the experimental observations. Moreover, the flying directions of LBiS generally approached vertical and were relatively insensitive to welding speed, even when the speed was increased to a very high value of 15m/min. This finding shows that LBiS are very different from the commonly observed spatter defects produced by the intense friction effect of a vapor plume toward the keyhole, and this finding demonstrates the variety of the spatters. Furthermore, an energy conservation theory was proposed to characterize LBiS, and a novel dimensionless spatter number was formulated to calculate their formation threshold. The spatter number relates to the Reynold and Weber numbers of the molten flow of the weld pool and shows the relative importance of an inertia flow driven by the recoil pressure and the resistance from viscous dissipation, surface tension and gravity. The spatter number was less than 1 when LBiS was formed. The proposed theory was validated against the numerical predictions, and good agreements were obtained.