The Capacitor Discharging Welding Process Analysis Of Projection Welding Of Fastener Nut To AHSS Usibor1500 Hot-stamped Steel

Projection welding is one of the four major welding processes in automobile manufacture.The projection welding quality of nuts plays an important role in the vehicle assembly quality,safety and comfort.However,the quality and mechanical property of resistance projection welding of nuts to AHSS hot-stamped steel is unstable and poor with traditional alternating current welding?ACW?or direct current welding?DCW?.The application of nut clinch process to hot-stamped steel is limited.In order to make sure the projection welding quality,the coatings on hot-stamped steel should be removed or the CO₂ shielding welding is added to strengthen weld joint after resistance projection welding.Thus,solving the welding problem between hot-stamped steel and welded nut is important.The advantages of capacitor discharging welding process are short welding time,fast welding current build-up and high peak welding current.The CDW process is usually applied to the weld of difficult welding materials and structures.This dissertation focuses on the projection welding of nuts to hot-stamped steel with capacitor discharging welding?CDW?process.?1?The characteristics of weld joint with CDW and DCW were analyzed.The mechanism of weld nugget formation,the microstructure evolution and hardness distribution of weldment were researched.The results show that the projection weld joint with CDW process and DCW process are composed of three zones:the fusion zone?FZ?,the heat affected zone?HAZ?and the bulk material zone?BM?.The FZ under DCW is obviously larger than the FZ under CDW,but there are several shrinkage voids.The FZ is dissymmetric and forms in hot-stamped steel side.The FZ originates from hot-stamped steel.Prominent material softening phenomenon occurs in heat affect zones?HAZ?of hot-stamped steel.While,softening phenomenon don't happen in the HAZ of nut.?2?With Single factor control method,the effects of CDW parameters on welded joint attributes,mechanical properties and dynamic resistance were investigated.When the welding voltages 450⁵10 V,the electrode pressure 0.30⁰.35 MPa,ratio 60:1,and capacitor 0.03 F,the resistance projection welding qualities of nut to hot-stamping steel are better.The weld joint with small shrinkage void can be subjected to high pullout force,and no retained coating can be found.The dynamic resistance of weldment with CDW firstly decreases rapidly in 0¹1 ms,and then is stable at a certain value.As the weld voltage increasing,the dynamic resistance of weld joint in 0¹1 ms reduces.When the electrode pressure increasing,the resistance curve of weld joint in stable stage descends.Comparing to weld joint under CDW,the weld joint under DCW contains retained coating from weld joint boundary to FZ and several large shrinkage voids,which lowers the pullout force of welded nut prominently.?3?Combining finite element analysis and experiment observation,the failure mechanism of weldment was researched.There are three basic failure modes of nut projection welded joint:the interfacial failure?IF?mode,the pullout failure?PF?mode and projection failure mode.As for IF mode,the welded joint fails in the form of ductile and brittle fracture.Small equixed dimples are founded in the outside part of IF fracture,and typical cleavage surface and cleavage river pattern are observed in the inside part.When failed in PF mode,the crack firstly occurs around the boundary of weld joint in the outside part,then the crack expands around the weld joint and across the thickness of steel sheet.The fracture morphology of PF mode includes elongate dimples,shear slip planes and cleavage surfaces.The FEM analysis results are consistent with experimental results.When failed in projection failure mode,the crack produces in brittle way from the sharp notch to the HAZ of nut.The fracture morphology includes large cleavage surfaces and cleavage river pattern.?4?With CDW process,the pullout force of welded nut can increase as the strength of steel advancing.Comparing to the pullout forces of weld nuts to TRIP780 steel which are almost the same about 9800 N,the pullout forces of weld nuts to hot-stamped steel are distinctly larger.