Investigation of keyhole and melt pool dynamics during laser butt welding of sheet steel using a high speed camera

An investigation was undertaken using a high power carbon dioxide laser into the butt welding of mild sheet steel, addressing the problem of variable weld quality as a result of keyhole and thus melt pool stability. Using a high speed video camera the temporal behaviour of the keyhole and melt pool are monitored. Experimentation is centred around the effect of various interface geometries; guillotine, milled and angled edges on the weld quality. The range of processing conditions with respect to speed/power used varies from 140 to 260mm/s AT 3.5KW. When the processing speed approaches a critical level, related to the type of interface profile, the onset of high speed weld defects occurs particularly the humping phenomenon and weld porosity. From the pictures obtained using the video equipment the extent to which and at what speed this occurs is largely dependent on the capillary flow conditions within the melt pool. The onset of this defect can be prolonged by carrying out the welding operation on samples with guillotined edges angled perpendicular to the direction of welding with the laser beam slightly offset from the welding line. This process improvement offers increased penetration and more importantly increased reliability for the welding operation at a processing speed of 15.5m/min. with the added advantage of no special edge preparation.