Study On Microstructure And Mechanical Properties Of Ultra-high Strength Steel Resistance Spot Welding Joint

In this paper,the macro characteristics,microstructure and mechanical properties of three-layer spot welded joints consisted of dissimilar high strength steels(B1500HS/B250P1/B1500HS)were studied.The effect of process parameters on macro characteristics,microstructure and mechanical properties of the spot joints were analyzed,and then the fracture mode and failure mechanism of spot joints were also discussed based on shear and cross tension tests.The results show that nugget formed in the centre of the three-layer structure with ignorable offset.The nugget diameter increases with the welding current,and has a non monotonic relationship with the welding time.Meanwhile,the effect of electrode force on the nugget diameter is not obvious.Nugget diameter reaches its maximum value 8.4 mm on the condition of 3.5 kN electrode force,9 kA welding current and 300 ms welding time.The spot joint consists of nugget zone(FZ),heat affected zone(HAZ)and base metal(BM).The FZ consists of parallel lath martensite and little retained austenite.The HAZ on B1500 HS side can be divided into forth zones according the distance far from nugget: subcritical zone,intercritical zone,fine grain and coarse grain zones.The subcritical zone consists of tempered martensite and ferrite;the intercritical zone contains a large number of island tempered martensite and ferrite;the fine grain is mainly composed of fine needle like martensite and ferrite or bainite;the coarse grain zone contains abundant coarse lath martensite.The maximum shear and cross tensile force of the spot joints are proportional to welding current,but inversely proportional to electrode force firstly,and then keep the constant vaule.Besides,the relationship between shear force and welding time was non monotonic.However,the cross tensile force keeps constant value with increasing welding time,and have no obvious change in this study.Two fracture modes,interfacial fracture(IF)and pullout fracture(PF),are found during shear test of resistance spot welding(RSW)joints in this study.RSW joints fractures from the tip between 1.6 mm thick B1500 HS and 1.0 mm thick B250P1,and presents brittle characteristic in IF mode.RSW joints fractures on the coarse zone of HAZ at the 1.6mm thick B1500 HS side,and presents quasi cleavage fracture feature in PF mode.Only PF mode is found during cross tension of RSW joints in this study.RSW joints fractures on the coarse zone of HAZ at the 1.6mm thick B1500 HS side in PF mode,which is very similar to the PF in shear test.However,on the same welding condition,the maximum cross tensile force of RSW joint is smaller than shear force’s.Meanwhile,fracture work of RSW joints in cross tension test is higher than that in shear test.The quality of RSW joint is comprehensively evaluated in terms of macrostructure,mechanical properties and fracture mode.In this study,the optimum parameter is 3.5kN electrode force,300 ms welding time and 8k A welding current.On the optimum condition,nugget diameter reaches 7.8mm and the maximum shear force and cross tensile force of joint are 26.47 kN and 9.2 kN respectively.In addition,the joint fracture in PF mode in both shear and cross tensile test.