Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing H_WN/H_FL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.