THE LASER-WELDING OF THIN METAL SHEETS - AN INTEGRATED KEYHOLE AND WELD POOL MODEL WITH SUPPORTING EXPERIMENTS

An integrated mathematical model for laser welding of thin metal sheets under a variety of laser material processing conditions haa been developed and tested against the results of experiments. Full account is taken in the model of the interaction of the laser-generated keyhole with the weld pool. Results calculated from the model are found to agree well with experiment for appropriate values of the keyhole radius. The analysis yields values for power absorption in the metal. In a complementary calculation the total absorption of the laser energy is determined from detailed consideration of the inverse Bremsstrahlung absorption in the plasma and Fresnel absorption at the keyhole walls. To test these results, experiments were performed on 1 mm mild steel using a high-speed video camera, which measured the surface dimensions of the melt pool. Processing parameters were varied to study the effect on the melt pool; parameters considered included traverse speed, laser power and shroud gas species. The general shape of the weld pool was found to depend on whether penetration was full, partial or blind; only the results for full penetration were compared with the theory, which is for complete penetration only. The melt pool changed shape as the degree of penetration reduced, from tear-drop shaped to a more parallel-sided oval. Changes in processing parameters were seen to affect the length of the weld pool more than the width or shape. Experimental results on the melt pool dimensions correlate well with the theoretical predictions. To verify the absorption predictions a simple calorimetric technique was used to monitor both absorption and transmission of the laser beam by the weld sample.