Thermocapillary convection in the weld pool for single and dual beam laser welding systems.

Abstract

A study was undertaken in order to understand the complex flow structure and heat transfer during dual beam laser welding. Motion of the liquid metal in the weld pool is dominated by the surface tension gradient during conduction mode laser welding. The first goal of this study is to investigate the feasibility of controlling flow in the weld pool with the dual beam laser welding system. Both numerical simulation and experimental investigation are conducted. For numerical simulations, the analysis is based on a transient two-dimensional simulation of flow in a moving plate for a dual heat source. Each of the two laser beams at the top surface of the weld pool is modeled as a Gaussian source. The heat transfer mechanisms considered include conduction in the solid region, as well as natural and Marangoni convection in the liquid region. Toward the numerical simulations, a complex flow structure with four flow cells occurs in the weld pool. The change in fluid flow structure affects the weld pool geometry and surface deformation. The inter-beam spacing, power distribution between the two beams, and the total power density are found to have direct impact on the flow structure during dual beam laser welding. The counteraction of flow cells in the inter-beam region may produce deeper penetration, depending on the welding conditions applied. The experimental investigations using a PRC/3000 CO$\sb2$ dual beam laser welding system were performed for AISI 304 stainless steel. The results show that the weld bead shape can be changed during dual beam laser welding system. A 50% width-to-depth aspect ratio increase is found when an inter-beam spacing of 1.0 mm is applied to the dual beam system with a 200 W concentrated beam and an 800 W defocused beam. After investigating the control of fluid flow induced by thermocapillary convection, the cold corner phenomenon occurring at the weld pool edges is studied numerically using the domain decomposition method. The local length scale requirement can be resolved when the domain decomposition method is applied. When the fine mesh scheme is applied to the simulation at the weld pool corner region, the flow structure, heat transfer and weld pool geometry all change. An inclined solid-liquid interface is found at the weld pool corner region and this inclined interface does not favor flow separation. The inclined angle of the solid-liquid interface and weld pool shape at the corner region are found to vary with a change in the laser power density.