On laser welding meltpool dynamics

Abstract

A theoretical study of the meltpool and keyhole dynamics during laser welding of thin plate mild steel gave predictions on the meltpool shape, size and laser beam absorption in the sample. The experimental results detailed below were obtained in experiments carried out to verify these theoretical predictions. Using a high speed video camera the surface dimensions of the meltpool were obtained. Processing parameters were varied to study the effect on the meltpool: for bead-on-plate welding, traverse speed, laser power, shroud gas species, gas delivery angle and plate thickness and for butt welding, laser power and traverse speed. The general shape of the meltpool was found to be dependent on the penetration of the weld (full, partial or blind). The meltpool changed shape as the degree of penetration reduced, from tear dropped becoming more oval. For a good weld the melt pool dimensions were very stable once the weld was established. Changes in processing parameters were seen to affect the length of the meltpool more so than the width or shape. If the weld formed was faulty the length of the meltpool was likely to be longer than was anticipated and also unstable. Experimental results on the meltpool dimensions correlate well with the existing theoretical predictions. The theoretical model used requires inputs of processing parameters and meltpool width to calculate the meltpool shape and absorption of the laser beam by the sample. To verify the absorption predictions a simple calorimetric technique was used to monitor both the absorption and transmission of the laser beam by the weld sample. This was done on 1 mm mild steel plate for various laser powers and traverse speeds.