Flow characteristics of the molten pool in pulsed laser welding of thin sheet with filler wire

A 3D transient model has been developed o analysis the molten pool flow characteristic in pulsed laser welding thin sheet with filler wire. The free surface is tracked by VOF method, and the filler metal is simplified as droplets. The flow characteristics in welding of Hastelloy C-276 sheets with 0.5 mm in thickness are analyzed with this model. The calculated weld appearance fits well with the experimental one. Before the droplet contacts with the molten pool, the liquid metal driven by surface tension flows towards the center and downward at the upper surface. The max velocity at the center is about 0.97 m/s. At the trailing portion of lower surface, two eddies with similar spin direction are generated due to the outward surface flow and the downward flow. The filler metal has significant influence on the convection pattern in the molten pool, and results in the disappearance of flow characteristics dominated by surface tension. The filler metal flows downward and outward rapidly after the droplet contact with molten pool. The maximum velocity is about 1.74 m/s and appears at the location where the droplet enters into the molten pool. The droplet has an impact effect on the molten pool and the convection pattern is changed. The molten pool shows oscillation characteristic due to the combined effect of droplet impact and surface tension. The volume of the molten pool dose not decrease right after the laser pulse ends, the lower part of the molten pool expands for about 3-4 milliseconds. And this phenomenon is related to the pulse effect of the laser beam, the flow pattern of the molten pool and the thermal properties of the material. Due to high pulse frequency of the laser beam and low thermal conductivity of the material, the molten pool always exists during welding process.