| The scarcity of resources and environmental crisis are two major problems in the world that the manufacturing industry is facing today.As one of the largest and most important industries in the world,the automobile manufacturing industry is already moving towards to the direction of “light manufacturing” and “new energy” for further development.In the process of automobile lightening,the integration of steel and aluminum used in automobile’s white body structure after all,is a preferred solution.Resistance spot welding(RSW)still acts as one of the inevitable technologies for automobile body structure manufacturing,due to its advantages in high efficiency,high degree of automation,good adaptability,and low cost.However,the great discrepancies in thermal physical properties between Al alloy and steel makes it very challenging to join them,as the brittle interetallic compounds(IMC)are the fragile part of the weld.Therefore,it is of great significance to realize the resistance spot welding of the dissimilar metals of steel and aluminum and obtain a stable and reliable spot welding joint,which is critical to promote the process of light-weight vehicles as soon as possible.In this paper,1.2mm thick DP590 high-strength galvanized steel and 1.2mm thick 6061-T6 aluminum alloy sheet were used as research objects.By using self-designed ring-spherical electrode(RSE)and spherical electrode(SE)electrodes,the aluminum sheet was connected to the upper anode whereas steel sheet was connected to the cathode on the bottom.Experiments were carried out under different welding conditions for achieving different results.In addition,the coefficient of variation(C.V)was used as an index,accompanied with results from professional simulation software SORPAS,to judge the stability of dissimilar resistance spot welding joints and the mechanism of difference in spot welding performance and interfacial behavior was comprehensively analyzed.In the study of the effect electrode’s tip morphology,it was found that the spot welding joint obtained by a self-designed RSE electrode had a more significant effect on the nugget diameter,the joint strength and its coefficient of variation,the fracture mode etc.compared with the SE electrode under single pulse mode,and the cracking risk factor(CRF)values of the RSE joints in both current modes were lower than those of the SE joints,which meaned that the ring design of the electrode tip had positive effect on reducing cracking risk of joints.The IMC thickness of RSE joints was less than 2μm and uniform in single pulse.The thickness of IMC of SE joints was more than 2μm,showing a serrated distribution with low strength and stability.Compared with single pulse,multi-pulse mode increased the C.V value of the joint’s tensile shear strength using RSE electrode.The temperature curve showed a “multi-pulse” property,which promoted the formation of thick IMC layer and the thickness at the interface was about 1.9~5.7μm thicker than that under single pulse mode,which was negative for joint strength.The maximum tensile shear load of steel-aluminum spot welded joints increased by 63.8% with additional forging pressure.When the current is small,suitable forging force within a certain range can improve the joint’s tensile shear strength,while over-load forging force could cause adverse effect on the joint by increasing the weld indentation.Therefore,a suitable forging force had a positive effect on the strength of the spot weld joint.In the same nugget diameter range,the order of tensile-shear load of spot welding joints under different states of steel surface was as follows: galvanized> uncoated> shot peening,which was mainly related to IMC.Shot peening had no positive effect on improving the performance of spot welded joints of steel and aluminum because the shot peening resulted in a thicker IMC at the interface.Both the tensile load and the nugget diameter of the spot welded joints under uncoated state decreased.The strength of the spot welding joint obtained under galvanized state was higher with better welding performance.For resistance spot welding 1.2mm thick DP590 galvanized steel to 6061-T6 aluminum alloy,the self-designed alumina-copper RSE type electrode is used in single pulse(welding time 300 ms,welding current 12.0~13.0kA,electrode pressure 3500N),obtaining good tensile shear load and thinner interfacial IMC,and the crack risk factor of the joint is smaller as well.The additional forge force(0.26~0.28MPa)can significantly improve the tensile shear strength of the joint. |
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