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A clinch-resistance hybrid spot welding method was presented to join Al and steel. The nugget formation was analysed via microstructure observation and numerical simulation. A cup section nugget wa...

Currently, the use of advanced high strength steels (AHSSs) is the most cost effective means of reducing vehicle body weight and maintaining structural integrity at the same time. However, AHSSs present a big challenge to the traditional resistance spot welding (RSW) widely applied in automotive industries because the rapid heating and cooling procedures during RSW produce hardened weld microstructures, which lower the ductility and fatigue properties of welded joints and raise the probability of interfacial failure under external loads. Changing process parameters or post-weld heat treatment may reduce the weld brittleness, but those traditional quality control methods also increase energy consumption and prolong cycle time. In recent years, a magnetically assisted RSW (MA-RSW) method was proposed, in which an externally applied magnetic field would interact with the conduction current to produce a Lorentz force that would affect weld nugget formation. This paper is a review of an experimental MA-RSW pla...

In this work, the effects of the coating composition and the welding parameters on the heat generation and nugget formation during resistance spot welding of the hot press forming steels have been evaluated. Two types of coated steels were used, termed as Al–Si and Zn coating. Al–Si coated steel showed rapid nugget growth toward the electrode direction, which is the probable reason of the higher heat conduction from the electrode–sheet surface toward the bulk material. In Zn coated steel, heat was generated and localised at the faying interface and uniformly propagated to the steel substrate. High speed camera images showed that the presence of oxide at faying interface provides inhomogeneous current flow and violent heat generation in the Zn coated steel.

This paper investigates the source, magnitude and direction of magnetic force on the liquid nugget during resistance spot welding (RSW). High speed photography was used to observe the nugget formation process during half-sectioned RSW of steel (ferromagnetic substance) and aluminium alloy (paramagnetic substance). The induced magnetic force acting on half-sectioned and regular nugget was afterwards analysed and calculated. The results show that in the case of steel RSW, the magnetised workpieces generated a very strong magnetic field around the spot welding zone, while weak magnetic field appeared in the case of aluminium alloy RSW. This strong magnetic field causes a strong convection in the liquid nugget of steel even when the welding current is low. This strong convection will promote that the dimensions of nugget in the steel RSW become larger and closer to rectangle (observed on the cross-section) than in the aluminium alloy RSW.

One of the most important issues in resistance spot welding of three‐sheet stack joints is the insufficient growth of the weld nugget, which may cause problems in places needing larger weld nuggets (i.e. sheet/sheet interface). In this paper, the effect of sheet thickness on the pattern of weld nugget development during resistance spot welding of three‐steel sheets of equal thickness is studied. Results showed that there is a critical sheet thickness of 1·5 mm at which the fusion zone size at the sheet/sheet interface is nearly equal to the fusion zone size at the geometrical centre of the joint. Increasing the sheet thickness beyond the critical size causes a shift in the location of weld nugget formation from the geometrical centre to the sheet/sheet interfaces. Below the critical sheet thickness, the weld nugget growth in the geometrical centre of the joint is higher than that in the sheet/sheet interface.

The effects of alternating current (ac) and direct current (dc) on cooling rate, solute distribution and nugget shape after solidification, which are responsible for microstructure of the fusion zone, during resistance spot welding, are realistically and extensively investigated. The computer program developed by Wang and Wei is used to predict transport variables in workpieces and electrodes during heating, melting, cooling and freezing periods. The model accounts for electromagnetic force, heat generations at the electrode/workpiece interface and faying surface between workpieces, and dynamic electrical resistance including bulk resistance and contact resistances at the faying surface and electrode/workpiece interfaces, which are functions of hardness, temperature, electrode force and surface condition. The computed results show that in contrast to dc, using ac readily produces the nugget in an ellipse shape. Deficit and excess of solute content occur in a thin layer around the boundary and inte...

The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius Rn and the contact radius Rc between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.

The operation life and degradation mechanism of electrode during resistance spot alternating current welding of zinc coated galvannealed steel sheets were investigated. The electrode life in alternate resistance spot welding of zinc coated and uncoated steel sheets was extremely short in comparison with welding only on zinc coated steel sheets. During alternate resistance spot welding, the ring shaped nuggets were formed easily. This is attributable to the sticking of the alloy layers formed on the electrode top during the previous welding of zinc coated steel, to the uncoated steel surface at the following welding, so that notable wear took place, and the surface profile of electrode tip became flat. Additionally, an accumulation of highly resistant carbonaceous substance layer at the centre of electrode top caused an annular flow of welding current to hinder the nugget formation.

Flow and consolidation of the material under the tool shoulder and subsequent nugget formation are among the least understood aspects of friction stir welding and processing (FSW/P). Welding parameters and tool profile impact the process forces acting on the tool. This work is an observational study of the process forces associated with bead on plate runs on two aluminium alloys, 6061 and F357. Polar plots of the resultant forces acting on the tool spindle are analysed and correlated to the process parameters. The dependence of the nugget's width with various heat indices is evaluated.

An axisymmetric finite element model employing coupled thermal–electrical–mechanical analysis of resistance spot welding is presented. The welding parameters considered include: heat generation at the faying surface and the workpiece–electrode surface; Joule heating at the workpiece and the electrode; and the thermal contact conductance between the electrode and the workpiece. The latent heat of phase change due to melting is accounted for. The effect of friction coefficient on the workpiece interface is also studied. The computed results agree well with the experimental data. Heat generation at the faying surface in the initial stages of welding dominates the nugget development, and Joule heating at long times governs the weld nugget growth. A parametric study is carried out for the nugget growth with specific consideration of resistance spot welding of Al alloys. Process control and modelling of resistance spot welding of Al alloys is more difficult than that for steel because of their high elec...

Resistance spot welding is one of the major Joining methods widely used in automotive body fabrication and assembly. Electrode wear has been a major concern in resistance spot welding of galvanised steel and in this paper advanced finite element techniques were used to simulate the thermomechanical interactions between the electrode and the workpiece during welding. First, the coupled electric–thermal–mechanical process associated with nugget formation was studied. The cumulative effects on electrode face deformation were then simulated over a large number of welds. Electrode face pitting effects were also examined. It was found that welding process parameters, such as holding time and pressure trace, played a key role in electrode face extrusion. Any pitting on the electrode tended to accelerate the face extrusion process.