In-situ laser heat treatment during laser beam welding of dual-phase steels

During laser beam welding of dual-phase steels with tensile strengths above 780 MPa premature weld failure has been detected. Local embrittlement and the formation of micro-cracks are assumed to be possible reasons for the failure. Local in-situ heat treatment has been applied using laser radiation with the aim to soften fused zone and the fusion line area and, simultaneously to rise hardness in the tempering zone. Trials were undertaken to test the feasibility of laser heat treatment at feed rates typical for laser beam welding of thin sheet metal of DP980 in two gauges and from two manufacturing routes. The power density as well as the stand-off were varied to reveal the effects of utilising the weld heat for the heat treatment. The results indicate the possibility to reduce the hardness in fused zone and coarse-grain zone whereas the tempering zone tends to broaden depending on the power applied for laser heat treatment.