Research On Resistance Spot Welding Of Medium-Mn TRIP Steel/DP590 For Lightweight Vehicle Body Structure

In the context of global participation in energy conservation and emission reduction,lightweight has become one of the important trends in automobile development.Advanced high-strength steel has the advantages of high strength,good forming performance,high energy absorption rate and good anti-collision performance,which can achieve the best lightweight effect while ensuring performance.Medium-Mn TRIP Steel has become a research hotspot in recent years for it's excellent mechanical properties which can well meet the requirements of automotive lightweight and safety.Resistance spot welding has become one of the main connection technologies in automobile manufacturing due to its advantages of high production efficiency,low cost,easy welding quality and easy realization of automation.At present,there are few researches on the spot welding process of dissimilar advanced high-strength steel for Medium-Mn TRIP Steel.Therefore,the research on the resistance spot welding process of Medium-Mn TRIP Steel/DP590 has important theoretical significance and engineering application value.In this paper,spot welding process tests were carried out for 1.6 mm 7Mn Steel and 1.2 mm DP590.The effect of welding parameters and tempering current on the quality characteristics and mechanical properties of joints was systematically studied,and the welding parameters during the spot welding process,as well as the microstructure and hardness distribution characteristics of different areas of the joint,and the characteristics of the joint were analyzed.Research indicates:(1)The process parameters are optimized by the controlled variable method,and the best welding parameters under single pulse are determined as: welding current 9k A,welding time 300 ms,electrode pressure 4.5 k N,preloading time 300 ms,holding pressure time 40 ms;The best adding tempering process parameters are: tempering current 7.5 k A,cooling time 1500 ms.The welding parameters have a great influence on the nugget diameter and indentation rate,but the tensile shear force remains stable after the welding heat input is greater than a certain value,and the tensile shear force of the joint with tempering current is increased by 17.13%.(2)From the fusion line to the center of the nugget,the 7Mn Steel/DP590 resistance spot welded joints have crystal morphologies in order of planar crystals,cellular crystals,dendrites and columnar crystals.The tempering line on the DP590 side is shorter than that on the 7Mn Steel side.The FZ structure is composed of lath martensite and a small amount of retained austenite.The CGHAZ and FGHAZ on the DP590 side are coarse and fine massive martensite,respectively.ICHAZ is a two-phase structure of ferrite and martensite.SCHAZ is composed of island-shaped martensite tempered and ferrite,and there are fine quasi-spherical and plate-like interlayer cementite.The CGHAZ on the 7Mn Steel side is martensite,while a small amount of spherical retained austenite is distributed on the matrix;FGHAZ and ICHAZ are two-phase structure of ferrite and island tempered martensite.The tempered martensite content is lower when it is closer to the center of the nugget.SCHAZ is composed of ultra-fine grained retained austenite and ferrite.(3)The failure mode of solder joints under the best process is PF-TT failure.When the IF fails,the HAZ,the centerline of the FZ and the internal fracture of the nugget are respectively brittle fracture,mixed fracture and ductile fracture.When the IF of the solder joint fails,cracks have appeared in the HAZ on the 7Mn Steel side,and there are segregation of C and Al on the crack;When the PF-TT of the joint,the nugget on the side of the 7Mn Steel is peeled off.The fracture at the nugget line of the joint and the internal fracture of the 7Mn Steel HAZ are brittle fracture and ductile fracture.The addition of tempering current did not change the fracture mode and fracture position of the joint,but the tempering area generated by the tempering current in the HAZ on the DP590 side produces a necking,which is the softening zone and the low point of the hardness of the joint.(4)Numerical simulation of resistance spot welding joints of 7Mn Steel/DP590 based on typical welding parameters:The thermophysical properties of 7Mn Steel and DP590 are quite different in low temperature environment,but not much difference in high temperature environment.At the beginning of welding,stress concentration appeared in the welding plate at the edge of the electrode.As the welding progressed,stress concentration appeared in the heat-affected zone of 7Mn Steel,which may also be the reason why the joint failed here.The current density is concentrated at the contact surface of the electrode and the welding plate.The error of the nugget diameter measured by simulation and experiment is within 10%,which verifies the accuracy and reliability of the model.