Melt Flow and Porosity Formation in Pulsed Laser Keyhole Welding

Abstract

Instead of CW (continuous wave) mode, pulsed mode laser welding has been popularly used in industry especially for Nd: YAG lasers. In pulsed mode laser keyhole welding, pores have been frequently observed near the root of the solidified weld. Our previous studies have indicated that the formation of porosity is caused by two competing mechanisms during the keyhole collapse process, and they are 1) the speed of solidification process for the melt surrounding the keyhole and 2) the speed of melt backfilling the keyhole. If the solidification process is too fast and completed before the keyhole is filled, pores will be formed. A technique to control the laser power trailing when it is turned off during the keyhole collapse process has been proposed and experimentally validated to postpone the solidification speed and, as a result, to prevent the porosity formation. However, this method fails for a “deep” keyhole. In this study, an electromagnetic force is used to control the melt backfill flow which is proved to be very effective in preventing porosity from occurring. A mathematical model has been developed to calculate the transient heat transfer and fluid flow during the keyhole formation and collapse processes in pulsed laser welding. The continuum model is used to handle the entire domain including solid phase, liquid phase and mush zone. The enthalpy method is employed to handle the absorption and release of latent heat during melting and solidification. The laser induced plasma inside the keyhole due to the Inverse Bremsstrahlung (IB) absorption is considered and the temperature distribution inside the keyhole is calculated. Both the Fresnel absorption and multiple reflections of laser beam energy at the keyhole walls are also considered. Parametric studies to determine the desired strength of the electromagnetic force and its duration under several laser welding conditions have been conducted. Computer animations showing the keyhole formation and collapse, metal flow, and possible formation of pores will be presented.