Research On Basic Problems Of Nugget Formation And Quality Strength During Resistance Spot Welding

Resistance spot welding (RSW) has extensive application as a joining techniquein the field of aerospace, vehicle, electronics and transportation and so on because ofits concentrated energy, low deformation, high productivity, flexibility and suitabilityfor automation and robotisation. However, the RSW process is a complicatedphenomenon, which involves the electrical, thermal, mechanical, magnetic, fluid flow,and metallurgical factors. At the same time, the process is ephemeral and invisible. Allthese leads to the quality of spot weld unstable. Consequently, to reveal the nuggetformation mechanism and explore the new method to strength spot weld quality is theresearch hotspot in RSW.This paper consists of two parts. The first part focuses on some basic problems inthe heat and mass transfer during the nugget formation in RSW.(1) Simultaneously estimate of contact heat generation and thermal contactconductance on the electrode/workpiece interface during RSWThe contact heat generation (CHG) and thermal contact conductance (TCC) onthe electrode/workpiece (E/W) interface during RSW has important effect on thenugget formation process. However, it did not receive enough attention. This paperestablished a two-dimensional inverse heat transfer model of RSW, and estimates thecontact heat generation and thermal contact conductance on the E/W interfaceaccording to theory of inverse heat transfer. The results showed that the TCC on theE/W interface during aluminum alloy RSW was in the order of106W·m-2·K-1, whilethe TCC on the E/W interface during steel RSW was in the order of105W·m-2·K-1.The distribution of TCC on the E/W interface of copper/steel contact was moreuniform than that on the E/W interface of copper/aluminum contact. The CHG on theE/W interface during aluminum alloy RSW was in the order of1012W·m-3, whilethat on the E/W interface during steel RSW fluctuated in the order of1011W·m-3.Both the CHG during aluminum and steel RSW were higher than its body resistanceheat generation. This indicates that the heat generation on the E/W interface shouldnot be ignored during the research of nugget formation. During the whole RSWprocess, the TCC and CHG on the edge of E/W interface were higher than that in themiddle of E/W interface. With the welding time increasing, the TCC and CHGgradually close to a stable value.（2）The magnetic force on liquid nugget during RSW This paper investigates the effect of workpieces magnetization on the magneticflux density and nugget shape during RSW process by high speed photography andmagnetization theory for the first time. The results pointed out that the magnetisationof ferromagnetic workpiece will increases the thickness of nugget. The magnetic fluxdensity induced by workpiece magnetisation and the magnetic flux density induced bywelding current has the same order of magnitude (10-1T). This indicates that themagnetisation of ferromagnetic workpiece is also an important source of magneticforce acting on liquid nugget, and it should not be ignored during the research ofRSW process, especially, during the research of fluid flow in RSW of ferromagneticmaterials.（3）Experimental study on the fluid flow of liquid nuggetThis paper studies the fluid flow of liquid nugget by experimental methods forthe first time. The titanium (Ti) powder, ferrum (Fe) powder, copper (Cu) foil,alumina (Al2O3) powder, zirconia (ZrO2) powder and tungsten carbide (WC) particleswere added to nugget as medium. The fluid flow pattern and velocity can be inferredby the movement of these medium. The results showed that the velocity of nuggetflow was faster than the velocity of nugget formation. When the mass of addedpowder is less than0.3μg, the medium powder always reached the edge of nugget“for the first time”, which makes the observation the flow pattern of liquid nuggetimpossible. For aluminum alloy RSW, the flow velocity is at least27mm·s-1, whichfar faster than the calculated velocity with only gravity considered. This indicates thatthe magnetic force may play an important role in the nugget formation duringaluminum alloy RSW. For RSW of steel, the flow velocity is at least15mm·s-1.Thefluid flow is very weak in the circumferential direction in both aluminum and steelRSW. The mass and shape of added powder has important influence on theobservation of fluid flow, which need further study.Through the above researches, this paper can provides more accurate originaltheoretical basis for improving the accuracy of numerical simulation of RSW anddeepen the understanding of the mechanism of nucleation.The second part aims at improving the aluminum alloy resistance spot weldquality, applies magnetically assisted resistance spot welding process to the joining ofaluminum alloy for the first time. This paper systematically analysis effect of externalmagnetic field (EMF) on the nugget shape, mechanical properties, microstructure andfailure mode and so on. The result showed that the nugget diameter, tensile-shear strength and energy absorption were increased under the help of EMF. Under differentwelding parameters, the nugget diameter can be increased by10%~25%, and thetensile-shear strength can be enhanced by10~30%. The EMF can reduce the weldingcurrent, shorten the welding time, thereby improves the welding efficiency, reduceenergy consumption and extend the electrode tip life. The electromagnetic stirringinduced by EMF makes the nucleation particles distribute more uniform in the liquidnugget, thus contributing to the formation of equiaxed and columnar crystal in weldnugget. Besides, the study also found that both small and larger welding current andshort and long welding time will reduce the effect of EMF. Accordingly, in order tomaximize the effect of the EMF on resistance spot weld, reasonable weldingparameters should be carried out.