Documenting weld pool behavior differences in variable-gap laser self-melting and wire-filling welding of titanium alloys

Abstract

In large-scale welding processes involving complex geometries, variations in gap dimensions and the transition of filler wire form distinctive molten pool flow behaviors, significantly affecting weld formation and quality. This paper investigates the differences in the laser self-melting and wire-filling welding pool behavior of titanium alloy with variable gaps through both simulation and experimentation. An innovative three-dimensional transient heat-flow coupling model is developed to simulate laser welding with variable gap structures, incorporating the dynamics of the welding wire-filling process. The different laser welding modes and the filling process under the variable gap structure were numerically simulated. The results indicate that the maximum flow rate of the laser self-melting pool remains stable in proximity to the keyhole, reaching a maximum of approximately 2.491 m/s, with molten metal entering through the area surrounding the keyhole. As the gap size gradually increases to 0.2 mm during the laser self-fusion welding stage, keyhole instability becomes more pronounced. In transitioning from laser self-fusion to wire-filling welding, the welding wire behavior can be divided into three stages. The molten metal at the end of the welding wire is transferred to the liquid melt pool through a liquid metal bridge. The flow rate of this bridge initially rises before gradually decreasing, reaching a maximum flow rate of 1.606 m/s. Notably, the welding wire lags approximately 0.6 mm behind the laser's focal point, with the narrowest width of the liquid bridge measuring about 0.89 mm. Based on the simulation results and considering the melting transition time of the welding wire, the initiation time for the welding wire has been further adjusted to the first 10 ms when the gap threshold reaches 0.2 mm. Experimental verification confirms the successful laser welding of a 2 mm variable gap structure thin plate. This study elucidates the melt pool flow, keyhole fluctuations, and metal transition behaviors during the laser welding process, contributing to a deeper understanding of the complex heat and mass transfer dynamics inherent in variable gap structure laser self-melting and wire-filling welding. Ultimately, these insights aim to enhance the application of laser welding in adaptive welding for large structural components.