Microstructure And Mechanical Properties Of Dissimilar Thickness Dual Phase Steel DP1180/DP590 RSW Joints

Automotive lightweighting as the most effective measure refers to the realization of vehicle weight reduction based on ensuring crash safety,material cost and machinability.Using hybrid-structure design and high strength steels to make body safety structure is an important method of automotive lightweighting.Hybrid-structure design is applied to weld various strength grades of high strength steels with dissimilar thickness,which is a compromise between performance and cost for vehicles.Resistance most widely used in body connection technology with high efficiency,low cost and easy to automate.Dual phase steels most widely used in automotive industry with high initial work hardening rates and low yield ratio.There are major differences in the processing technology,chemical composition,material thickness,microstructure,physical and mechanical properties of base metals.The greater differences in thickness and strength levels,the more complicated distribution of temperature field and stress strain field of the materials during resistance spot welding.As a result,the resistance spot welded joints compared with base metals change,which affects the mechanical properties of joints.In this paper,resistance spot welded joints with different thickness of DP1180 and DP590 were obtained.The temperature field and stress-strain field were simulated by finite element software ANSYS.The joint microstructure was observed by scanning electron microscope and transmission electron microscope.The mechanical properties contained microhardness,tensile shear properties and fatigue properties of the joints were analyzed according to the microstructure characteristics.The influence of single parameters contained welding current(I),welding time(T)and electrode pressure(F)on the cross-section properties,microstructure and mechanical properties of joints were analyzed.The multi-objective optimization problem of joints was studied by gray relational analysis based on the Taguchi orthogonal test.The BP neural network was used to predict the DP1180/DP590 RSW joints quality under different welding parameters.The results show that the different thickness of DP1180/DP590 RSW joints were divided into three zones,namely fusion zone(FZ),heat affected zone(HAZ)and base metal(BM).The HAZ was subdivided into coarse-grained HAZ(CGHAZ),fine-grained HAZ(FGHAZ),inter-critical HAZ(ICHAZ),sub-critical HAZ(SCHAZ)according to the temperature characteristics.Based on the results of temperature field,solidification theory and microstructure,the FZ was coarse lath martensite and a small amount of retained austenite,CGHAZ and FGHAZ were martensite,ICHAZ consisted of martensite and ferrite,and SCHAZ formed tempered martensite(TM)and ferrite.In the microhardness test,a measurable decrease of microhardness in SCHAZ and the degrees of softening on the DP1180 and DP590 side were 12.87% and 5.36% respectively.In the tensile shear test,the crack of pullout fracture initiated in the tongue on the DP1180 side and propagated through the softening region SCHAZ on the tension side,and fracture at the edge of FZ on the pressure side.In the fatigue test,the fracture firstly occurred on the DP1180 side.With the decrease of cyclic loading,the fracture location on the DP1180 side transitioned from the FGHAZ to the edge of the FZ,the fracture location on the DP590 side was always in the CGHAZ,the crack source was smooth area,the fatigue crack growth zone became larger and tear ridges were generated on both left and right sides,and the transient fault zone composed of slip steps decreased.The optimized parameters based on grey correlation analysis were I=9.5kA,T=22cycles and F=8.48 kN.The BP neural network model was obtained based on the orthogonal test,which predicted the quality of DP1180/DP590 RSW joints under different welding parameters.