Microstructure and fracture behavior of laser welded joints of DP steels with different heat inputs

800 MPa strength cold rolled annealed DP steels was welded by different heat inputs (6-66 J/mm) using 6 kW fiber laser, to study the microstructure and fracture behavior of laser welded joints with sub-micron scale soften zone. The microstructure and fracture morphology of welded joints were observed by scanning electron microscope (SEM) and optical microscope (OM). Hardness, tensile properties and formability of welded joints were tested using microhardness tester, universal material testing equipment and Erichsen test machine. The results showed that the microstructure of welded joints was similar, but decomposition of martensite and precipitation of carbides was significant gradually with heat input increased such that the hardness was decreased. For the complete penetration of DP780 laser welded joints, the soften zone was impossible to eliminate by adjusting the heat input. When the size of soften zone was controlled within 539 µm, the tensile properties and formability of welded joints were more than 95% of BM, due to the double effect of the size and microstructure of soften zone.