Microstructure And Mechanical Properties Of Friction Stir Welding High Mn Steel Joints Under Different Process Conditions

The TWIP(twin induced plasticity)steel with high Mn content(Mn>20wt.%)has the advantages of high strength,plasticity,and energy absorption energy due to TWIP effect occurring during the deformation.It can be used as bulletproof steel plate and other impactresistant steel,car frame and other automotive steel.Welding is a very important step in the application of high Mn steel.At present,the fusion welding is a main joining method for high Mn steel.However,because of the high heat input of fusion welding and the high Mn content in high Mn steel,the Mn evaporation,segregation,and porosity easily occurred in the fusion welded joint,which will seriously deteriorate the toughness and plasticity of the joint.Friction stir welding(FSW)is a solid state welding technique,in which the peak temperature is not higher than 0.8 times of the melting point of the material.In this thesis,3 mm thick hot-rolled high-Mn steel plate was subjected to FSW by changing the welding process parameters and preheating temperature,and the microstructure and mechanical properties of the joint were studied.The main conclusions are as follows:(1)Under different heat input,FSW high Mn steel obtained complete and flawless welded joints,in which both nugget zone(NZ)and heat affected zone(HAZ)microstructure were single austenite without phase transition.With the decrease of heat input,the grain of NZ is gradually refined.At the bottom of NZ different heat input,corrosion resistant areas were found,and the grains in these areas were much finer than the surrounding microstructure.(2)Under different heat input,the hardness distribution of welded joints is close.HAZ has the highest hardness and NZ has the lowest.The joint has good transverse tensile properties.The tensile strength of the joint under high and low heat input is 90.5%and 99.9%of the base material(BM),and the elongation is 50.8%and 75.6%of BM,respectively.Under different heat input,NZ impact energy is close to 66.6 and 67J,reaching 89.3%of BM.Therefore,the mechanical properties of welding seams are the best under low heat input.(3)Under the preheating temperature of 160? and 250?,the NZ microstructure was a single austenite structure.With the increase of preheating temperature,the grain size of NZ increased.After preheating,the corrosion resistant area at the bottom of NZ becomes narrower obviously,which is related to the decrease of the flow stress of the high Mn steel and the weakening of the wear of the stirring tool as the preheating temperature increases.(4)Under different preheating temperature,the hardness distribution of the joint is close.NZ has the highest hardness and HAZ has the lowest.The fracture position of the joint is BM in transverse tensile.The strength of the joint is equal to BM.The elongation of the joint at the preheating temperature of 160 and 250? is 75.5%and 84.9%of BM respectively.The joint has excellent mechanical properties.The tensile strength of the local NZ was all greater than BM.At the preheating temperature of 160 and 250?,the tensile strength of the local NZ was 881 MPa and 860MPa,and the elongation was 40%and 42%of BM,respectively.With the increase of the preheating temperature,the impact energy of NZ increased.At the preheating temperature of 250?,the impact energy of NZ reached the maximum of 65.4J,reaching 87.2%of BM.Therefore,the mechanical properties of welding seams are the best at the preheating temperature of 250?.