Analysis of heat transfer and melt flow in conduction, transition, and keyhole modes for CW laser welding

Abstract

Melting modes have a profound impact on heat transfer and fluid flow in laser material processing. In order to have a better understanding of the thermo-fluidic behavior of the melt pool for different laser melting modes, experiments were conducted to study temperature behavior, melt pool dynamics, and weld characteristics during the transition from conduction melting mode to keyhole melting mode. Infrared thermography was applied to analyze heat transfer for different melting modes. Melt pool dynamics captured by high-speed camera were used to interpret the driving force acting on the molten flow. In conduction mode, the temperature distribution in the weld pool is regular and smoother. Heat transfer in this mode is governed predominantly by heat conduction. In transition mode, the sideway of molten metal and fluid flow are improved by Marangoni convection, resulting in a wider melt pool. When the keyhole is developed, the keyhole mode is achieved and the recoil pressure generated by violent evaporation acts as the principal driving force to push molten metal flow backwards. An upward melt flow occurs along the rear of melt pool. The results also reveal that the surface ripple and grain growth direction is directly dependent on heat transfer and fluid flow in different melting modes.