Investigation of Metal Mixing in Laser Keyhole Welding of Dissimilar Metals

Abstract

Laser welding of dissimilar metals is important in many industrial applications. However, as different metal elements get mixed in the molten pool, brittle intermetallic compounds are often formed in the welds and can undermine the mechanical performance of the welds. This poses challenges to the widespread utilization of this welding technology. Therefore, the objectives of this study were to reveal the physical mechanisms and evaluate their relative significance to the metal mixing, microstructure, and mechanical performance in laser keyhole welding of dissimilar metals. To achieve the objectives, the following research work was conducted in this study: First, the metal mixing process in line-scan laser keyhole welding of dissimilar metals was investigated with a combination of experimental and simulation approaches. To investigate the underlying physics of the welding process, a computational fluid dynamics model was developed to reveal the chain reaction among laser heat flux, temperature field, driving forces, fluid flow, and metal mixing in the molten pool. A parametric study was further conducted to reveal the effects of laser welding parameters on the metal mixing in the molten pool. Second, compared with line-scan laser welding, oscillating laser welding offers additional processing parameters to change the fluid flow and metal mixing in the molten pool. This matter was, again, investigated with a combination of experimental and simulation approaches. Experiments were first performed to identify the effects of laser oscillating parameters on metal mixing. A computational fluid dynamics model was then used to simulate the metal mixing during the welding process. Combining experiments and simulations, this work revealed the four fluid flows in the molten pool that determined the metal mixing and their dependence on the laser oscillating parameters. Third, the effects of metal mixing on the microstructures and mechanical properties of the laser welds were investigated for immiscible and fully miscible alloy systems. For immiscible alloy systems (i.e., aluminum-copper), experiments were performed to correlate the metal elemental composition to the microstructure (i.e., phase composition and morphology) and mechanical strength of the welds. Furthermore, a computational materials model was developed to predict the evolution of phase compositions during molten pool solidification as a function of the non-uniform thermo-solutal conditions. For fully miscible alloy systems (i.e., nickel-copper), mechanical strength testing and fractographic analysis were performed to characterize the strength of the welds and the fracture mode and their dependence on the concentration at the interfacial region of the welds. The knowledge generated from this study provides insights for both science and industries regarding the fundamental physics of the process-microstructure-property relationship and the laser welding process optimization to achieve sound dissimilar metal welds.