Study On Influence Of Nb Alloying On Mechanical Property Of 22MnB5 Spotweld And Prediction Of Spotweld Failure

The resistance spot welding(RSW)quality of high strength steel plays an important role in the realization of lightweight and high strength of automobile structure.In recent years,a lot of researches applied Nb alloying to refine the grain size and improve the toughness of 22 MnB5 press hardened steel(PHS).However,the effect of Nb alloying on the properties of 22 MnB5 spotweld is still insufficient in research.Therefore,this paper mainly focuses on the influence of Nb alloying on properties of22 MnB5 spotweld and finite element analysis(FEA)technology of spotweld failure,aiming to improve the RSW quality and analytical precision.Firstly,22 MnB5 PHS and22 MnB5Nb PHS were taken as the research object.The spotwelding specimens and equivalent heat-treated specimens were prepared.Secondly,a lot of related experimens including the tensile test,metallographic observation and hardness measurement were carried out,which helped to determine the material parameters of various zones in a spotweld.Thirdly,numerical simulations were conducted on tensile test of spotwelding specimens and crushing test of multi-spotweld combined blank with detailed solid spotweld models and simplified spotweld models.Then the differences of simulation accuracy with various spotweld models were investigated.The main conclusions and innovative outcomes are as follows:(1)A method for measuring the material parameters of spotweld was proposed.The microstructure and hardness of the equivalent heat-treated specimens were calibrated with ones of spotweld zones,to determine that their material parameters are approximately the same.Then,the material parameters of spotweld were determined indirectly by the tensile test results of the equivalent specimens.(2)On one hand,adding Nb into 22MnB5 steel can refine the microstructure of equivalent heat-treated specimens,increase the tensile strength by about 7% and reduce the elongation by about 18%.On the other hand,adding Nb into 22 MnB5 steel can refine the microstructure of varous spotweld zones,reduce the peak tensile load of spotwelding specimens by less than 7% and tighten the spotweld hardness range.But the failure modes of spot-welding specimens and the microstructure types of various spotweld zones were not affected.(3)The detailed HAZ-layered solid spotweld models were proposed,including5I9 S model,3I3 S model,1I1 S model,0I0 S model and FZ/HAZ-integrated model.Among the simulation results of TS,CT and CP tests,the models are ranked in descending accuracy order: 5I9 S model,3I3 S model,16 HEX model,12 HEX model,1I1 S model,0I0 S model,FZ/HAZ-integrated model and Beam model.The 5I9 S,3I3S models are recommended in detailed solid spotweld models for excellent match of loaddisplacement curves and |Er| within 4%.The 16 HEX model is recommended in simplified spotweld models for load-displacement curve matching degree only less than3I3 S model and |Er| about 6%.(4)The mixed simplified spotweld models were proposed.Among the crushing simulation results of multi-spotweld combined blank,the models are ranked in descending accuracy order: 16 HEX model,12HEX(L)?16HEX(R)model,Beam(L)?16HEX(R)model,12 HEX model,Beam(L)?12HEX(R)model and Beam model.When possessing the same spotweld model in the right position,each model shows little difference in simulation accuracy.Therefore,Beam(L)?16HEX(R)mixed simplified spotweld model is recommended for its high simulation precision and efficiency.