Study On Nugget Morphology And Mechanical Properties Of Resistance Spot Welded Joints Of High Strength Steel

With the rapid development of industrialization in China and the increase of automobile production,the problems of energy consumption and environmental pollution have become increasingly prominent.Under the premise of ensuring safety performance,achieving weight reduction of automobiles and improving fuel economy have become the main countermeasures to cope with energy and environmental problems.High strength steel is characterized by high strength,good ductility and formability,which meets the requirements of automotive steel material selection.Resistance spot welding(RSW)has the advantages of high productivity and easy automation,which is widely used in the welding manufacture of automobile bodies.In this paper,aiming at the problem of RSW of high strength steel of automobile bodies,the morphology of the RSW nugget is controlled through a special electrode structure type,in order to effectively control the size of spot welding nugget,improve strength and reduce deformation of RSW joints.In this paper,circular plane electrode and annular electrode with ceramic core were used to perform the RSW process test on 2 mm equal-thickness DP600 high strength steel,and two types of RSW joints with circular nugget and annular nugget were obtained.The finite element(FE)software ABAQUS is used to numerically simulate the welding temperature of high strength steel RSW process,and to study the influence of RSW process parameters such as welding current,welding time and electrode pressure on the nugget morphology and mechanical properties of RSW joints,and to conduct comparative experimental research on the mechanical properties and surface condition of RSW joints under different electrode type process conditions.On this basis,this paper proposes a FE prediction method for the tension-shear mechanical properties of spot weld joints,simulating the impact of the local constitutive properties of RSW joints on the overall mechanical behavior.The results show that the circular RSW joints consists of fusion zone(FZ),heat affected zone(HAZ)and base metal(BM),while the HAZ of the annular RSW joints is divided into inner HAZ and outer HAZ.According to the temperature characteristics,HAZ is subdivided into coarse-grained heat affected zone(CGHAZ),fine-grained heat affected zone(FGHAZ),intercritical heat affected zone(ICHAZ)and subcritical heat affected zone(SCHAZ).Microstructure of FZ consists of slatted martensite,CGHAZ and FGHAZ consists of different sizes of block martensite,ICHAZ and SCHAZ is similar to the BM,consisting of martensite and ferrite.Microhardness test showed that the hardness of FZ was the highest.The tensile-shear test shows that the failure mode is divided into interface fracture(IF)and pull-out fracture(PF),and PF mode has the characteristics of high fracture strength and more energy absorption.RSW process parameters have a significant impact on the nugget morphology and mechanical properties of RSW joints,with the increase in welding current and welding time,circular and annular RSW joints show an increasing trend in melting rate,indentation depth,nugget diameter and tensile-shear strength,which is mainly related to the increase in heat input.Since increasing electrode pressure will cause the current density of the welding zone to decrease and the heat dissipation of the electrode will increase,as the electrode pressure increases,melting rate,nugget diameter and tensileshear strength of RSW joints will decrease,and the indentation depth will first decrease and then increase.The comparative analysis of annular nugget and circular nugget RSW joints shows that annular RSW has a dispersive effect on resistance heat,and annular RSW joints can obtain larger nugget diameters and tensile-shear strength,which are8.1mm and 28.45 k N,respectively.Compared with the traditional circular RSW joints,the peak tensile-shear strength is increased by 8.26%,and the surface indentation is also improved.The results of FE analysis of tensile-shear mechanical properties of RSW joints show that the overall mechanical behavior of RSW joints is affected by local constitutive heterogeneity.Numerical simulation can reproduce the crack initiation,propagation,and failure during the tensile-shear test of RSW joints.The simulated force-displacement curve,deformation behavior and failure mode are in good agreement with the test results.