Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding

The microstructure and mechanical properties (strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone (FZ) was lath martensite (LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value (375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone (HAZ) of dissimilar/similar weld joints was as follows: LM (coarse-grained HAZ) →finer LM (fine-grained HAZ) →M-A constituent and ferrite (intercritically HAZ) →tempered martensite (TM) and ferrite (sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ (softening zones) was observed compared to base metal (BM) in dissimilar/similar weld joints. The size of softening zone was 0.2-0.3 mm and reduction in hardness was ∼7.6%-12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity.