Experimental and numerical study on local mechanical properties and failure analysis of laser welded DP980 steels

DP980 steel was welded using Nd:YAG laser with low heat input (4 m/min) and high heat input (1 m/min). The ultimate tensile strength (UTS) of the low heat input welded joint reached 99.7% of base metal, while the joint with high heat input reached 95.6%. The sub-critical heat affected zone (HAZ) was softened with obvious yielding platform, and higher heat input caused lower strength of local HAZs. The low heat input welded joint fractured at base metal, while the joint with high heat input failed at sub-critical HAZ with significantly lower fracture strain. The simulation results revealed that UTS of joint decreased with the increase of HAZ width and HAZ softening degree. Moreover, there were critical values of HAZ width and softening degree to predict the failure location of the joint. With the increase of fusion zone width, the UTS of joint increased gradually closing to that of base metal, and the joint tended to fail at base metal due to biaxial stress intensification effect.