Influence of microstructure and weld size on the mechanical behaviour of dissimilar AHSS resistance spot welds

Resistance spot welds were produced in dissimilar combinations of advanced high strength steels. A 600 MPa dual phase (DP) steel was welded to a high strength low alloy, a 780 MPa DP, and a 780 MPa transformation induced plasticity steel. The microstructure and mechanical properties were characterised using metallurgical techniques and lap shear and cross-tension testing. The results show that a pullout failure mode with improved mechanical properties is obtained when DP600 is paired with other advanced high strength steels, compared to the DP600 welded to itself, which is prone to interfacial failure and poor mechanical properties, given the same weld size. An in depth comparison of the interfacial to pullout failure transition in similar DP600 and DP780 and dissimilar DP600–DP780 welds was performed. The results show that the interfacial to pullout transition for the DP600–DP780 welds is significantly lower than with DP600 welded to itself. Increased fusion zone strength through dilution with the DP780 promotes button pullout at smaller weld sizes. Furthermore, it was observed that softening in the heat affected zone of DP780 promoted a pullout failure mode in that material.