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The aim of this investigation was to evaluate the microstructural change after laser welding and its effect on the tensile properties and strain hardening behavior of DP600 and DP980 dual-phase steels. Laser welding led to the formation of martensite and significant hardness rise in the fusion zone because of the fast cooling, but the presence of a soft zone in the heat-affected zone was caused by partial vanishing and tempering of the pre-existing martensite. The extent of softening was much larger in the DP980-welded joints than in the DP600-welded joints. Despite the reduction in ductility, the ultimate tensile strength (UTS) remained almost unchanged, and the yield strength (YS) indeed increased stemming from the appearance of yield point phenomena after welding in the DP600 steel. The DP980-welded joints showed lower YS and UTS than the base metal owing to the appearance of severe soft zone. The YS, UTS, and strain hardening exponent increased slightly with increasing strain rate. While the base metals had multi-stage strain hardening, the welded joints showed only stage III hardening. All the welded joints failed in the soft zone, and the fracture surfaces exhibited characteristic dimple fracture.

The application of dual phase (DP) steels in the automobile industry unavoidably involves welding operation. The objective of this thesis was to study the microstructure and mechanical properties of laser welded DP steel joints. The laser welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat affected zone (HAZ). While the soft zone influenced the tensile properties of the joints considerably, the fatigue properties of the welds showed dependence on both the softening and the applied stress amplitudes. Fatigue crack was observed to initiate from the specimen surface and crack propagation was basically characterized by striation-like features. Post-weld heat treatment was found to eradicate the negative effect of the soft zone and improve the mechanical properties of welds. However, the heat treatment resulted in a brittle fracture mode from the dominating ductile mode of fracture of the welded joints.