Study On Microstructures And Mechanical Properties Dissimilar Thickness High Strength Steel DP780/HC660RSW Joints

It is one of the important means to achieve lightweight and ensure the safety of theautomotive collision to design and manufacture the load-bearing vehicles parts usingdissimilar thickness high-strength steels. Due to the physical properties, chemicalcomposition, mechanical properties and forming process of dissimilar thicknesshigh-strength steels are different, the electricity, heat, power distribution of the weldingpool were complex, and the range of process parameters are narrow. It is difficult tocontrol the welding process. High-strength steel is extremely sensitive to the change ofthermal processes and chemical compositions. During the spot welding process, the fastcooling rate can completely change the original balance of the base metal, reducing thestrength and corrosion resistance properties of the weld joints.In this paper, the research on microstructure and mechanical properties of thedissimilar thickness high strength steel DP780/HC660RSW joints were carried out forthe longitudinal beam structure characteristics. The effect of process parameters (weldingcurrent, welding time and electrode force) on the RSW joints microstructure, tensileproperties and joint tensile fracture modes were analyzed.The experiment results showed that the microstructure on both sides of theDP780/HC660RSW joint nugget zone were coarse gained martensite and ferrite. TheRSW joint HAZ were consisted of fine gained martensite and ferrite. The critical areasmicrostructure were finer than the base metal. With the distance to the nugget centerbecoming closer, the grain on both sides of the HAZ became large and the amount ofmartensite gradually increased. The microhardness distribution of the DP780/HC660RSW joint was as follows: the nugget zone was the highest, followed by the HAZ, andthe base metal was the lowest. The microhardness of HC660side was higher than DP780side. The average microhardness value of the DP780side fusion zone, HAZ and basemetal were322.9HV,273.8HV and240.3HV, respectively. While the average value of the DP780side were353.5HV、352.5HV and269.1HV, respectively. There were three failuremodes in the DP780/HC660RSW joints tensile shear test: interfacial fracture,partial-interfacial fracture and nugget peel fracture. Most or all of the nugget were left onthe DP780side. With the increase of the welding current, the penetration rate, indentationratio and indentation diameter of the joints increased, the grain of HAZ and nugget zonebecame coarse, and the amount of martensite increased. With the increase of the weldingtime, the joint penetration rate increased and then decreased, the indentation rate,indentation diameter and nugget diameter increased, the grain of HAZ and nugget zonebecame coarse. The electrode force had little effect on the macroscopic properties of thejoint, the grain was refined. The best process parameters of the welding is that coolingtime40cycles, tempering current4.0kA and tempering time60cycles with a propertempering pulse. The microstructure of the nugget zone on both sides of RSW joint werecoarse lath martensite and tempered martensite. The both sides microstructure of theRSW HAZ were lath martensite and ferrite. The microhardness of both the HAZ andnugget zone were slightly decreased, the plasticity was improved, and the elongationincreased by17%. The maximum shear force increased by6%.