Numerical modelling of laser welding pool thermal-dynamics and dissimilar materials mixing of cast iron, carbon-steel and nickel-alloy wire for weld crack prevention

To understand the prevention mechanism of weld cracks induced in laser welding dissimilar materials, carbon steel and cast iron with a nickel-alloy filler wire, it is necessary to investigate the melting pool thermal dynamics, keyhole mechanism and dissimilar materials mixing. In this study, a novel three-dimensional computational fluid dynamics (CFD) modelling on transient tracking of keyholes, transfer of molten wire droplets, mixing of filler and base metals, welding groove, coupled with conventional model of heat transfer, surface tension and recoil pressure, was developed. The simulated molten pool geometry and temperature field were compared with the experimental results, which confirmed the accuracy of the developed numerical model. The numerical results revealed that the keyhole underwent periodic oscillation and partial collapse, during which entrapped vapor formed transient bubbles that later escaped through buoyancy. The molten wire droplets significantly affected the flow field and local temperature, temporarily weakening Marangoni convection and altering keyhole stability. The nickel-alloy material concentrated primarily on the surface, then flowed into welding pool. After solidification, a large amount (65–75 %) of the nickel-alloy material kept in the upper half of the weld metal and a small amount distributed in the lower half zone. These distribution characteristics supplied a graded transition layer between the two dissimilar base metals benefiting to weld crack prevention. This research provided quantitative evaluation in visualizing the in-situ dynamic phenomena of dissimilar materials for laser welding process optimization.