Numerical and experimental study on the molten pool dynamics and fusion zone formation in multi-pass narrow gap laser welding

The development and evolution of molten pools provide valuable insight into the complexity of multi-pass narrow gap laser welding (NGLW) process. However, in-situ monitoring of the molten pool characteristics proves to be challenging in NGLW due to the existence of the narrow and deep groove. In this study, a three-dimensional transient model coupled with a self-adaptive Gaussian heat source was employed to help understand the molten pool behaviour and fusion zone formation during multi-pass NGLW process. Three weld bead profiles including convex, flat and concave were investigated by varying the process parameters. With more passes being processed, the shape of the molten pool transformed from teardrop to an elliptical one with increased tailing edge angles; contact angle of the cross-section decreased, demonstrating welds with more concave profiles. In order to obtain the fusion zone of the welds, enthalpy-porosity method was used to overlay liquid fraction of the cross-section over the complete melting and solidification process. Compared with the previous passes, the last filling pass presented a different molten pool morphology, contributing to variant top surface geometries. Final fusion zone of multi-pass NGLW was achieved by combining the fusion zone of each separate pass. The simulated results were compared with experiments.