Effect of the laser power on the mechanical performance of the laser spot welds in dual phase steels

Fibre laser spot (circular) welds (LSW) in dual phase DP780 steel grade were produced with fusion areas equivalent to conventional resistance spot welds (RSW) using different fibre laser parameters. Shear tensile properties show significantly higher loads in fibre laser spot welds primarily because of the combined strength of spot size, and hard microstructures in the fusion zone and heat-affected zone (HAZ). Greater HAZ softening is observed at high heat inputs. The transition of failure mode from interfacial to pull-out and the corresponding strength value is primarily controlled by the weld size. The crack initiation site and propagation mechanism were observed to be different for interfacial and pull-out failure. The increased HAZ softening with increase heat input was quantified with nano-indentation hardness measurement technique.