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Resistance spot welding is used extensively in auto industry. Every commercial vehicle has 4000-5000 spot welds. The weld ability, performance, and reliability are therefore important issues in design. In this paper, we studied weld nugget formation and failure behavior of three-sheet 5052 aluminum alloy resistance spot welds. The Peltier effect between the Cu-Al (the electrode and the Al worksheet) to the nugget formation was noticed. The mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were studied using tensile shear specimen configuration. Three failure modes were identified, namely, interfacial, mixed, and pull-out. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Flow phenomenon in spot welding nugget of dissimilar materials was studied. Metallographs in the course of welding nugget formation were observed and analyzed, through which it was indicated that: in the early stage of welding nugget formation, spot welding of dissimilar materials made liquid metals in the nugget produce a violent stirring action because the electrodynamic force overcame the medium resistance, causing unevenness of elements and compositions in the welding nugget; and then the unevenness was represented by vortex flow by means of different resistances to corrosion. It was proposed that the formation law of welding nugget could be recognized according to the formation law of vortex flow, and the vortex flow often moved from later-molten metal to former-molten metal. The property of welded joint with vortex flow was poorer than that without vortex flow.

In this paper, nugget growth stages during resistance spot welding between aluminum alloy and steel, such as surface breakdown, asperity softening, temperature rising, molten nugget formation, nugget growth and mechanical collapse, was analyzed according to resistance dynamic variation during welding. The effects of work pieces surface condition and electrode force on the occurrence of inter-expulsion and surface expulsion was discussed during resistance spot welding.

Single sided resistance spot welding (SSRSW) developed from RSW is a feasible plan to join vehicle structure to closed-form tube and is increasingly used in automobile manufacturing. During the process of SSSW, large deformation and complex contact status of the workpieces occur because there is no inside support. The complex contact status has directly influence on the size of the nugget which is critical to the quality of welding. In this study, the contact statuses during the current stage of sheet-to-tube spot welding were researched by numerical method. It was found that the widths of the contact regions of electrode to sheet and sheet and sheet to tube have dynamic change during welding process which decided the size and speed of the nugget formation. The results have direct guiding significance to the study on ring nugget formation during sheet-to-tube welding.

It has been proved that the initial gap has obvious influence on nugget formation, but little works focused on the effect of initial gap on the tensile strength of resistance spot welded (RSW) joints. In this paper, a 3D FE model was built for solving this question. The results show that, even though there are some fluctuations of weld diameter and tensile strength of RSW joints with initial gap, the tensile strength and weld diameter of welded joints with initial gap are still larger than that of welded joints without gap, which confirm that the influence of initial gap on tensile shear strength is little significant. The computation results agree well with experiment.

Resistance spot welding process is strongly related to interfacial contact behaviors. The effects of thermal contact is rarely investigated so far and generally ignored in numerical models. In this work, a parametric FE model, which considers the variation of the surface roughness of the electrodes and workpieces, has been developed to investigate the effects of thermal contact on weld nugget formation. With the parametric model, four cases, e.g. ideal smooth surface, minimal roughness surface and maximum roughness surface for steel sheets and electrodes of as-received condition, and highly rough electrode surface, are investigated. Researches show that when the surface roughness of the electrodes exceeds some limit, the thermal contact conductance will substantially affect the weld nugget formation, therefore, must be considered in numerical models to precisely predict welding process.

Projection welding is a variation of electric resistance welding with the dynamic changes of the flow paths for heat and electrical properties with changing temperature caused by the large plastic deformation with collapse of projection. The common standards of projection welding are AWS, IIW and Harris& Riley which have been established and in use for some time. All these standards have the slots located under the projection. However, the standard proposed in this paper researches on is a special one with the slots designed on the two sides of the projection. A comprehensive thermal-electrical-mechanical analysis was performed to quantitatively simulate the projection welding of the special standard by a coupled finite element method. The electrical, thermal and mechanical aspects of the projection welding processes such as the projection collapse mechanism, the nugget formation process and the effects of different electrode forces were discussed in this paper.