| As hot-stamped boron steel is the representative of ultra-high strength steel,which can guarantee the strength and stiffness,and reduce body weight effectively,so it has been applied in automotive light-weighting more and more widely and has replaced the traditional low-carbon steel gradually.And this steel has been combined with resistance spot welding which is the most common connection technology applied in manufacturing enterprise due to low cost and high degree of automation.Thus it is significant and important to study the bonding strength for both academic research and automobile manufacturing industry.However,there exist unique material properties for hot-stamped boron steel and "softening" phenomenon of its heat-affected zone,which cause the fracture modes much uncertain,including Interface Fracture(IF)and Partial Plug Fracture(PPF).It has been the focus of both business and academia that how to correctly predict the welding joint fracture mode and optimize welding parameters to improve the joint quality to meet the needs of production.In the actual production process of car body,there are varieties of factors affecting the mechanical behavior and fracture mode including welding current intensity and electrode force and welding time and sheet thickness.It's important to establish the related model and propose solutions to explain the influence of external process parameters and steel material properties on weld quality.To solve the above problems,this paper focuses on the spot-welded joint of hot-stamped boron steel.Firstly by using finite element simulation method,the geometrical / material properties of nugget zone and heat-affected zone of spot-welded joints were considered and made research on stress and strain and fracture modes of lap-shear process.Then the prediction of different parameters which would have influence on fracture modes of lap-shear joints were made and the experiment results were contrasted with simulation results.The spot welding and mechanical test system of ultra-high strength steel were established and the effects of different process parameters on joint fracture modes were studied.On this basis,the response surface model of the welding / material parameters on the nugget diameter was established.The main contents and conclusions of this paper are as follows:(1)Numerical simulation analysis of lap-shear fracture modes of hot-stamped boron steel spot-welded jointA finite element simulation model of ultra-high strength steel lap-shear joint was established,which explained the exact definition of the sub zones of heat-affected zone and its material properties and analyzed mechanical properties and different fracture modes of joints.By changing sheet width and nugget size and sub zones of softening of the model,the effect of different input parameters on fracture modes were investigated.The results showed that fracture locations of Interface Fracture(IF)and Partial Plug Fracture(PPF)were different;the former failed in the nugget zone combined with heat-affected zone and cracked along the thickness direction,the latter failure began at the edge of the nugget zone and cracked along its axis;sheet width and sub zones of softening zone had no effect on fracture modes;the nugget size was an important parameter for evaluation of hot-stamped boron steel spot-welded joint fracture modes.When the nugget size was smaller,IF tended to happen more;when the nugget size was bigger,IF tended to happen more;partitioning the softing zone could effectively guarantee the accuracy of the spot-welded joint fracture mode of the simulation model.(2)Experimental analysis of bonding and material parameters on hot-stamped boron steel joint fracture modesThe ultra-high strength steel welding and lap-shear experiment system was established and influence of different parameters such as welding current and welding time and electrode force and sheet width,and sheet thickness on joint mechanical properties and fracture modes were analyzed,the results showed that the welding current had significant influence on the hot-stamped boron steel spot-welded joint fracture mode.When the welding current increased,fracture mode transited from IF to PPF;when the welding current was large,the degree of PPF increased gradually with the electrode force increased;welding time had an obvious effect on the spot-welded joint tensile shear strength and fracture mode,when the welding time increased,the critical welding current of IF and PPF decreased gradually and spattered welding current reduced;the electrode force had some influence on the fracture mode of ultra high strength steel spot-welded joint;when the welding current was large,the degree of PPF gradually increased with the electrode force increasing;the sheet width did not affect the spot-welded joint tensile shear strength and fracture mode;the sheet thickness had certain effect on spot-welded joint tensile shear strength and fracture mode,when the sheet thickness increased,the fracture mode tended to change from PPF to IF.(3)Response surface model of hot-stamped boron steel spot-welding process parameters and material parameters on nugget diameterBy using response surface modeling method combined with experimental design,welding process parameters and sheet parameters which would influence nugget diameter of hot-stamped boron steel spot-welded joint were analyzed.A regression function based on response surface was established and using SQP to optimize parameters,the results show that the welding current and electrode force and sheet thickness were the main factors affecting nugget diameter of hot-stamped boron steel spot-welded joint.The welding current increased and electrode force decreased,and sheet thickness increased,resulting in the increase of nugget diameter of hot-stamped boron steel spot-welded joint. |
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