Welding Parameters Optimization And Joints Properties Investigation Of Dissimilar Resistance Spot Welds Between DP980 And SPCC

As an important direction for the future development of automobile industry,lightweight needs to consider factors such as performance,technology and cost.Therefore,the mixed use of multiple materials is the inevitable trend of lightweight development of automobiles.DP980 high strength steel has the properties of high plasticity,high tensile strength,high initial work-hardening ratio,low yield ratio and other more excellent characteristics,As a result,it is the main material for automotive safety cage components.Due to its low cost,good formability and good weldability,SPCC mild steel is widely used in the manufacture of non-load bearing parts,connectors and interior and exterior parts of automobiles.Therefore,the combined use of DP980 high-strength steel and SPCC mild steel on the body of the vehicle not only enhances the overall strength of the body but also effectively controls the increase of the cost.Resistance spot welding has many advantages,such as high automation,high efficiency,low cost and so on.At present,it is mainly used for the main assembly of car body connection.However,DP980 high-strength steel in the smelting process by adding more Si,Mn and other alloying elements deteriorate the weldability.Especially,the welding of DP980 high strength steel and SPCC mild steel is difficult because of the difference of the physical properties and chemical composition of the heat conductivity,resistivity and thermal expansion coefficient of the two steels.Therefore,it is very important to study the resistance spot welding technology and its joint performance of DP980 high strength steel and SPCC low carbon steel to improve the environmental protection,safety and economy of automobile.Appropriate welding parameters can improve the performance of joints,so the better welding parameters are the basic guarantee of joint strength.According to the quality requirements for resistance spot welding,the DP980/SPCC resistance spot welding weldability lobe is established on the basis of the weld diameters,welding splash and the Damage of the electrode tip.It is an energy absorbing process during the automobile happened impact,so according the signal to noise ratio of failure energy of DP980/SPCC resistance spot welding process parameters are optimized by Taguchi method in the weldability lobe.The macrostructure,microstructure and hardness profile of the joint under the optimum welding parameters of DP980/SPCC were investigated.The results show that the spot welding joint mainly consists of three zones: nugget zone,heat affected zone and base metal zone.The microstructure of nugget zone is mainly martensite,the heat affected zone of DP980 high strength steel are mainly martensite and ferrite,the heat affected zone on the side of SPCC mild steel is mainly martensite,ferrite and pearlite.The hardness of heat affected zone of DP980 high strength steel is the highest,followed by nugget zone and SPCC heat affected zone,and the hardness of SPCC base metal is lowest.The mechanical properties and fracture failure behavior of DP980/SPCC spot welded joint were investigated.The results show that fracture failure of DP980/SPCC is mainly composed of two mode: interfacial failure and pullout failure mode.And the fracture mode is mainly affected by nugget size.The interfacial failure is brittle fracture,and the pullout failure is ductile fracture.Failure is initiated from the nugget circumference,located at the SPCC base metal.With the increase of heat input,the nugget size of DP980/SPCC spot welding joint increases continuously,and the tensile shear strength and failure energy also increase.However,the mechanical properties of the welded joint decrease after splash.