Effect of subplate on sheet metal weld formation in back reflection induced synergistic laser welding and its mechanism

The back reflection induced synergistic laser welding (BRIS-LW) is a newly developing weld technology which could realize X-shape welding joint with relatively low laser energy when compared with general laser welding. The aim of this study is to understand the effect and mechanism of subplate, which is the core part of BRIS-LW. Therefore, 0.8-mm-thin Ti6Al4V plates in a butt configuration were carried out with different subplate status and same laser parameters. Different subplate status includes roughness of subplate surface, subplate material, and distance between subplate and back of welding sheet. The results show that the distance between subplate and back of welding sheet is considered to be the most important factor in BRIS-LW. As the distance increased from 0.1 to 0.5 mm with a step of 0.1 mm, the appearance of welding joint changed from typical X-shape to V-shape. Experimental results indicate that the obtaining of X-shape welding joint with low laser energy threshold in BRIS-LW is due to the formation of a metal-vapor cloud auxiliary energy field at the backside of welding sheet and the auxiliary energy field is induced by the subplate. Meanwhile, the utilization of auxiliary energy field is gradually reduced with the increase of distance. Furthermore, the auxiliary energy field has a synergistic action of thermal effect, force effect, protective effect, and metal evaporation effect on the back of welding zone.