| With the introduction of emission regulations for the automobile industry,steelmakers have been designing and producing lightweight materials to reduce body weight,thereby improving fuel efficiency and strengthening the body to withstand greater impact force,in order to improve the car safety performance.However,as ultra-high strength steel is widely used in automobile industry,the problem of its weldability becomes more and more serious.Resistance spot welding is the best welding method in automobile production because of its high production efficiency,good reliability and low cost.In the resistance spot welding of HSS,spatter,shrinkage hole and internal crack are easy to appear,and the existence of zinc coating used for anti-corrosion makes the HSS prone to appear liquid metal brittle crack during spot welding.The above problems seriously affect the application of galvanized ultrahigh strength steel in practical production.The microstructure and mechanical properties of resistance spot welding joint of PHS1500 galvanized cold-rolled annealed hot pressing steel were tested and the characteristics of brittle crack in liquid metal were analyzed by metallographic observation.The effects of the process parameters(current intensity,welding time,electrode pressure)of double-pulse resistance spot welding on the microstructure,mechanical properties and liquid metal brittle crack of PHS1500 hot-formed steel spot welding joint were studied.The fracture mode of spot welding joint in tension-shear test was analyzed.By optimizing the welding pulse current and combining with the tempering process,the changes of microstructure,mechanical properties and liquid metal brittle crack before and after the optimized current were compared and analyzed.The conclusions are as follows:(1).The microstructure of spot welding joint of PHS1500 galvanized hot-formed steel is divided into three regions: nuclide zone(FZ),heat affected zone(HAZ)and base metal(BM).The central structure of the nugget is mainly the coarse lath martensite.The growth of martensite is directional,the growth direction is the fastest direction of heat dissipation,and finally grows into columnar grain.The heat affected zone(HAZ)is mainly composed of fine martensite and softening zone formed by tempered martensite.(2).Spot welding process parameters(current intensity,welding time,electrode pressure)have a great influence on the microstructure and section morphology of PHS1500 galvanized hot-formed steel spot welding joint.With the increase of current intensity and welding time,martensite microstructure in spot welding joint will be coarsened.With the increase of electrode pressure,the martensite microstructure in the joint will be refined.Excessive current intensity,welding time and electrode pressure will cause indentation diameter,indentation rate or even spatter.(3).The tensile shear fracture of PHS1500 galvanized hot-formed steel spot welded joint is mainly interfacial fracture,partial interfacial fracture and complete pull-out fracture.The test results show that when the current intensity is 13.0KA,the welding time is 300 ms,and the electrode pressure is 3.5KN,the nugget diameter is 6.3mm,and the fracture mode of spot welding joint is complete pull-out fracture.The maximum tension-shear force of the spot welded joint is 21.92 KN and the displacement corresponding to nugget fracture is 4.56 mm in the complete pull-out mode.The fracture is mainly composed of small dimples and a few cleavage surfaces surrounded by dimples.(4).Spot welding process parameters(current strength,welding time,electrode pressure)have a great influence on the crack sensitivity of PHS1500 galvanized hot-formed steel spot welding joint liquid metal embrittlement(LME).The maximum length and number of LME cracks increase with the increase of current intensity or welding time.Appropriate electrode pressure can inhibit the increase of crack length and number in LME to a certain extent.Excessive electrode pressure will become the driving force for the increase of crack length and number.(5).Optimizing the welding pulse current can effectively manage the heat input,thus controlling the LME crack from 120?m to 33?m;Increasing the tempering current can reduce the hardenability of the nugget zone of the spot welding joint.After the optimization of pulse current,the comprehensive mechanical properties of spot welded joints are improved,and the maximum tensile shear load is increased from 21.57 KN to 24.33 KN. |
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