Mechanical properties and fracture behaviors in remote laser spot welding of quenching and partitioning 980 steel

Quenching and partitioning (Q&P) steel is one of most critical third generation steels in the automotive industry, which provides excellent mechanical properties and have a huge potential to significantly reduce vehicle weight and improve fuel efficiency. One of the bottlenecks for their applications in the automotive industry is the poor weldability. In this paper, a newly-developed remote laser spot welding (R-LSW) process was used to weld Q&P steel with laser beam moving along a spiral path. The welds with full penetration were obtained with laser power ranged between 2400 W and 3600 W, the weld morphology and mechanical properties of the joints were consistent, except for a little drop in case of 3600 W with the crater size of 0.65 mm. Under the shear tensile force, all the welds failed in partial thickness-partial pullout (PT-PP) mode, which was attributed to the formation of a large weld diameter. The weld failure process can be divided into four stages: (I) crack initiation; (II) crack propagation along the thickness and circumference of the joint; (III) crack propagation along the circumference of the joint; and (IV) instantaneous fracture. Furthermore, it was found that the coarse grain heat affected zone (CG-HAZ) has a lowest fraction of high-angle grain boundaries (HAGBs), resulting in the crack propagated along the CG-HAZ during stage II. During crack propagation, the fracture took place as a ductile fracture, but the instantaneous fracture occurred in quasi-cleavage characteristic.

Automated summary

Quenching and partitioning (Q&P) steel is a critical material in the automotive industry due to its excellent mechanical properties and potential for weight reduction. However, poor weldability has been a bottleneck for its application. This study successfully welded Q&P steel using a newly-developed remote laser spot welding (R-LSW) process, revealing key stages and failure mechanisms during the welding process.