Study On Parameters And Mechanical Properties In Resistance Spot Welding Of Q&P 980 Steel

With the rapid development of the automotive industry,the issues of energy consumption,environmental pollution and traffic safety are becoming increasingly severe.The lightweight of automobiles has became the development direction in automotive industry.As the latest generation of advanced high strength steel(AHSS),quenching and partitioning(Q&P)steel is considered as a potential candidate for automotive material,due to its good comprehensive mechanical properties and economic applicability.However,the poor weldability is an important factor limiting its industrial applications.Due to the high content of alloying elements,Q&P steel is prone to form coarse lath martensite during resistance spot welding(RS W),which lead to poor mechanical properties and interfacial fracture under external loads.Although the quality control methods of other types of AHSS spot joints have been fully studied in recent years,the research on the improvement of Q&P steel RSW process has not been reported yet.This article took 1.8 mm thick Q&P 980 steel as the research object.The welding processes were improved to modify the morphology and microstructure of the weldment to get enhanced mechanical properties.First,by using single pulse RSW,the morphology,microstructure and mechanical properties of joints were studied.The key factors controlling the mechanical properties of joints were also clarified.Second,the effects of double pulse welding processes on weldment were elucidated.The improvement mechanism were identified according to the fractured weldment and the microstructure.Finally,a novel multi pulse resistance spot welding process was proposed,which could improve the comprehensive mechanical properties.A weldment quality evaluation model was established in accordance with the force analysis and fracture mechanism.Three main results could be obtained as follows:(1)Through the single pulse welding tests,it was found that the Q&P 980 welded joint consisted of four parts:base metal,heat affected zone,partial melted zone and welding nugget.Due to the high contents of alloying elements,weldments were mainly composed of coarse lath martensite,with serve forming tendency of shrinkage voids and liquation crack.The nugget size,micro-hardness and welding defects were controlling factors for the quality of Q&P 980 spot welds.Experimental results illustrated that the Q&P 980 steel was suitable for hard welding.(2)It was found that the morphology and microstructure of the nugget were greatly changed by using the secondary welding pulse,when compared with the single pulse welded joint.The toughness of the joint was improved by applying a medium secondary pulse,which had a tempering effect to homogenize elements distribution in PMZ.While,a high secondary pulse could significantly widen the nugget size,inhibited the formation of shrinkage defects,thus significantly improved the strength and the failure mode.Finally,the partial thickness-partial pullout failure mode was studied in details.The fracture mechanism was quasi-cleavage fracture during crack propagation,then transitioned to ductile fracture during instantaneous fracture.(3)A triple pulse RSW process was proposed,which could both tempered and extended the nugget.Compared with single pulse weldments,the tensile shear strength was improved by nearly 16%,the cross tensile strength was increased by nearly 130%,and the ductility ratio was increased from 0.2 to 0.42,which confirmed that the major limitation of poor toughness of Q&P 980 welds was successfully solved.In addition,the quality evaluation model suitable for Q&P 980 steel was derived by force analysis,which could predict the failure mode of joints more accurately.