Research On Analysis Of Spot Welding Process For Ultra-High Strength Steel For B-Pillar Base On Collision Safety

With the rapid development of the automotive industry,environmental protection,energy and safety cannot be ignored,and lightweight has become an important development trend in the automotive industry.Quenching & Partitioning(Q&P)steel has excellent comprehensive mechanical properties and moderate cost.It is suitable for body structural parts and internal reinforcing plates,and enhances the body's resistance to deformation.It has broad application prospects.Resistance Spot Welding(RSW)is widely used in the automotive industry because of its advantages of fast operation,low cost,high reliability and automation.In this paper,1mm thick Q&P980 steel plate is resistance spot welded,The influence of different welding process parameters on the cross-section,microstructure and mechanical properties of the resistance spot welded joints are studied.The optimization of welding parameters was realized by response surface method.Based on the above research,the crashworthiness analysis of the Q&P 980 B-pillar was carried out.The number of joints and layout optimization of B-pillars are optimized by topology optimization and interpolation function.The influence of solder joint layout optimization on static stiffness and crashworthiness of B-pillar are studied,which provided a theoretical basis for the use of Q&P980 on the vehicle body.When welding current is 8.5 kA-14.5 kA,welding time is 10 cycle-16 cycle,and electrode pressure is 1.6 k N-2.4 k N,Q&P980 steel achieves a better connection.The joints can be divided into Fusion Zone(FZ),Coarse Grained(CG)HAZ,Fine Grained(FG)HAZ,and Inter Critical(IC)HAZ,Base Material(BM).As the welding heat increases,the penetration rate and joint diameter increase first and then decrease.The FZ undergoes recrystallization,and the HAZ undergoes a phase change due to thermal cycling and plastic deformation.ICHAZ generates tempered martensite,and the tempered martensite content decreases as the heat source strength increases and the welding time decreases.The fusion zone,coarse grain zone and fine grain zone of the joint are lath martensite,the hardness is obviously higher than that of the parent metal,and the ICHAZ hardness increases as the martensite content decreases..The tempered martensite is formed in the inter critical zone and softens.With the increase of welding heat,the microstructure coarsens and the hardness decreases.With the increase of electrode pressure,the dislocation and the hardness both increases.With the decrease of welding current,the welding time and electrode pressure increase,the critical zone get more and more soft.When the nugget diameter and penetration ratio are large,the effective bonding area increases,and symmetrical button fracture occurs in the joint.When the nugget diameter and penetration ratio are small,the effective bonding area decreases,and the button fracture occurs in the joint.With the increase of welding parameters,the tensile and shear loads of joints increase firstly and then decrease.The optimum welding parameters are 14 cycle welding time,12 kA welding current and 2.16 k N electrode pressure.Its tensile and shear loads are 21.341 k N,and the residual of the predicted value is 1.23% of the true value.Through Hyperworks and Ls-dyna,the side impact simulation of the vehicle model with Q&P980 class B column is carried out.The middle part of B-pillar is the main part to bear the impact load,which corresponds to the occupant's chest and abdomen.The invasive displacement and the speed are large and the change trend is basically the same.After the optimization of the B-pillar weld solder joint layout,the number of solder joints was reduced by 23.61%,the structural static stiffness and the first-order torsional frequency were improved,the collision performance remained basically unchanged,and the body assembly cost was reduced.