Microstructure And Mechanical Properties Of Friction Stir Welding 5 Mn Steel Joints Under Different Process Conditions

Tempered martensite+ fine retained austenite duplex microstructure can be obtained in medium-Mn steels through dual-phase region reverse phase transformation annealing process.Because of TRIP effect,medium-Mn steel has a excellent matching of strength,toughness and broad application prospects in Marine engineering,automobile and other fields.At present,medium-Mn steel is mainly connected by fusion welding.However,the peak temperature is much higher than the melting point in traditional fusion welding.At the same time,the high manganese content in the steel makes the weld prone to Mn evaporation,segregation,porosity and other problems,resulting in the deterioration of joint performance.Friction Stir Welding(FSW)is a new type of solid state welding process,whose peak temperature is much lower than that of fusion welding.In this paper,FSW is used to weld medium-Mn steel,aiming at controlling peak temperature by adjusting welding speed and cooling conditions,so as to obtain a certain proportion of retained austenite and improve joint toughness.The changes of microstructure and mechanical properties of joints under different process conditions are studied.The main conclusions are as follows:(1)Compared with the joint structure of FSW medium-Mn steel at different welding speeds,the grains in the nugget zone are coarse,but the grains in the heat affected zone are obviously refined.The peak temperature in the heat affected zone is relatively low.In the heat affected zone,the structure consists of martensite and fine retained austenite,the retained austenite exists between martensite lath bundles and Mn element distribution occurs in the retained austenite.The peak temperature in the nugget zone is relatively high,the microstructure is completely austenitized,and the final structure is martensite single-phase structure.No retained austenite structure is found in the weld nugget region at different welding speeds.As the welding speed decreases,the heat input increases,the grain size of the welded joint increases,and the content of retained austenite in the heat affected zone decreases.(2)Compared with the mechanical properties of FSW medium-Mn steel joints at different welding speeds,the hardness values in the nugget zone and heat affected zone are approximately the same,about 350HV;Tensile fracture occurs in the base metal.The impact energy in the nugget zone is 70?74J,and the impact energy in the heat affected zone is 98?122J.With the decrease of welding speed,the tensile strength of the nugget zone decreases,and the impact energy of the welded joint decreases continuously.(3)Compared with the joint structure of FSW medium-Mn steel under different cooling conditions,the structure of heat affected zone is composed of martensite and fine retained austenite,and the nugget zone is coarse martensite single-phase structure.As the temperature of cooling medium decreases,the peak temperature decreases,the grain size of welded joints decreases continuously,and the content of retained austenite in heat affected zone increases continuously.(4)Compared with the mechanical properties of FSW medium-Mn steel joints under different cooling conditions,the hardness values in the nugget zone and the heat affected zone are approximately the same,about 350HV;Tensile fracture also occurs in the base metal.The impact energy in the nugget zone is 73?77 J,and the impact energy in the heat affected zone is 121?133 J.As the temperature of cooling medium decreases,the peak temperature decreases,the tensile strength of nugget zone increases,and the impact energy of welded joint increases continuously.