Investigation Of Welding Characteristics For Capacitor Discharge Welding Process Between Welding Nut And 22MnB5 High Strength Steel

Nut projection welding is a key technology in automobile welding process,and its welding quality and the stability directly affects the assembly accuracy,product quality and even vehicle reliability and safety.Thus,the investigation for nut projection welding of automtive steel has an important application value and engineering significance.In this paper,capacitor discharge welding(CDW)process is applied to welding M8 hexagonal flanged welding nut and 22MnB5 high strength steel,and the welding characteristics of nut projection welding are studied according to the present situations and problems of nut projection welding.The purpose of this work is to providing a reliable method and technology in order to solve the welding problems.The effect of welding parameters on the mechanical properties and nugget sizes of nut projection welding joints is investigated.The results show that pullout fracture load and nugget sizes increase first and then decrease with the increase of electrode pressure;pullout fracture load and nugget sizes increase first and then decrease with the increase of welding voltages;pullout fracture load and nugget sizes increase with the increase of capacitance.In addition,the capacitance has a direct impact on the welding time in CDW process,when capacitance is more than 36 mF,the welding time can reach the saturable time of 26 ms.Finally,the recommended welding parameters range from 11 kN to 13 kN for electrode pressure,390 V to 420 V for welding voltage,and 36 mF to 40 mF for capacitance.The typical microstructure of welding joint is analyzed by SEM.The results show that the welding joint can be divided into fusion zone(FZ),heat affected zone(HAZ)and base metal(BM),and what's more,the HAZ on the side of 22MnB5 steel can be subdivided into up critical HAZ(HAZ1)and inter critical HAZ(HAZ2).The microstructure of FZ consisted of large lath martensite,and unmelted Al-Si coated layer is detected in the center of FZ;the HAZ on welding nut consisted of ferrite,fine martensite and perlite.The microstructure of HAZ1 consisted of fine martensite,and HAZ2 consisted of ferrite and bainite,which is weak region of welding joint due to lower toughness,strength and hardness.There are three failure modes in the experiment of nut projection welding,including interfacial fracture(IF),partial interfacial fracture(PIF)and pullout fracture(PF).IF mode shows feeble ductile fracture characteristic and crack propagation extends along the centerline of FZ because of remained unmelted Al-Si coated layer.PIF mode shows obvious transitional features included cleavage fracture,quasi-cleavage fracture and ductile fracture;on the other hand,the crack propagation extends along HAZ2 or the edge of HAZ2 and BM,and then suddenly tears to forming a cavity.PF mode is intergranular brittle fracture and crack progation extends along HAZ2 or the edge of HAZ2 and BM,and then toward to the bottom of 22MnB5 steel until pulling out whole steel.A nonlinear multiple regression model is established based on orthogonal experiment and genetic algorithrm,which is used to describe the relationship between welding parameters and welding quality.Moreover,the model has a high reliability which is tested by F test,correlation test and verifactional experiment.Numerical simulation on the process of nut projection welding based on ANSYS.On the one hand,the maximum and minimum Von Mises contact stress of pressurs stage occur at both ends and middle position of the contact surface.On the other hand,the highest temperation occur at the contact surface position;convex crushing stage,which is the first of welding stage,mainly occurs elastic-plastic deformation;the second of welding stage is nugget formation stage.The predict nugget depth is about 0.93 mm based on numerical simulation,which has a small error with real experiment.Consequently,the result of simulation is acceptable.