A comparison of 1G and 2G position laser welding of thick section D6A ultra-high strength steel

Abstract

Laser welding of thick section ultra-high strength (>1000 MPa) steels often results in bad weld appearance, cracking and porosity. This investigation aims to understand phenomena of laser welding of 15 mm thick D6A ultra-high strength steel. Both 1G (flat or downhand, i.e. laser beam is directed downward to-wards the earth) and 2G (horizontal, i.e. laser beam is horizontal to the earth surface) position welding were carried using an IPG fibre laser at a maximum power of 13 kW. Weld bead geometry, microstructure, morphology, integrity and residual stresses were characterised. The results show that laser welding at 1G position always lead to serious defects including root sagging, while 2G position can overcome these problems and has a much wider operating parameters window, which is caused by the combination of the effect of force status of gravity and surface tension, and the gradual or sudden change of direction of metal plume overflow with the increase in power density. The longitudinal residual stresses of welded joint of D6A steel at 2G position show a nearly symmetric distribution not only in the weld centre but also across the weld thickness which indicates that the residual stresses using 2G position is more uniform than using 1G position.