The Study On Microstructure And Mechanical Properties Of Selective Laser Melting Additive Manufactured 24CrNiMo Alloy Steel

Alloy steel plays an important role in many industries of national economy and national defense.Because of its good toughness and wear resistance,it has been the main material used for brake discs of high-speed trains.The traditional alloy steel brake disc manufacturing process is cumbersome,the process and processing cycle are long,and can not meet the increasingly complex design requirements.Selective Laser Melting Additive Manufacturing(SLM)technology solves the requirements of complex structural design,high efficiency and cost saving due to the integrated three-dimensional direct forming process.It is an ideal process for environmentally friendly,resource-saving and automated manufacturing.However,due to the complex composition of the alloy steel,the phase and phase transformation are difficult to control,and the relationship between the process parameters-tissue evolution-performance in the selective laser melting additive manufacturing process is difficult to control and limits its development.Therefore,this paper selects the brake disc material 24CrNiMo alloy steel,reveals the influence of laser additive manufacturing process parameters on defect characteristics and microstructure evolution,and studies the microstructure,mechanical behavior,friction and wear properties of laser additive manufacturing test pieces.The interaction relationship of the post-heat treatment process on the microstructure and properties of 24CrNiMo alloy steel produced by laser additive is explored to further improve its performance.This paper applies laser additive manufacturing to 24CrNiMo alloy steel,aiming at obtaining optimized process parameters,reducing defects and improving performance,and providing necessary theoretical basis and practical guidance for solving the problem of controllability control of high-performance alloy steel components.In the study,optical materials such as optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffractometry(XRD),Vickers hardness tester,electronic universal test machine,and friction and wear tester were used to analyze modern materials.The microstructure,phase composition,mechanical properties and tribological properties of 24CrNiMo alloy steel produced by laser additive were studied before and after three heat treatment processes.The heat treatment process was:980°C for 2 h oil quenching;350? for 2h.Air-cooled;980?insulation 2h oil quenching,350? insulation 2h air cooling modulation.The main conclusions are as follows:(1)Based on the 24CrNiMo alloy steel gas atomized powder used in this study,the bulk sample was fabricated by EOSINT M280 selective laser melting metal forming machine.Around the two indexes of the density and microscopic defects of the molded specimen,the optimized process parameters are:laser power 250W,scanning speed 800mm/s,scanning pitch 0.mm,layer thickness 40?m.(2)Laser Additive Manufacturing The microstructure of 24CrNiMo alloy steel is roughly divided into three types:(a)high energy density,dense residual high-temperature austenitic columnar dendritic structure and interdendritic formation at a lower cooling rate.Lath martensite structure;(b)low energy density,low martensite with residual austenite at high cooling rate;(c)martensite and bainite multiphase structure under optimized process a small amount of retained austenite.At the same time,it can be seen that the microstructure of the molded specimens under different laser process parameters generally have different degrees of orientation.(3)After 980?×2h,the grain is refined after oil quenching,the anisotropy is improved,and the lath martensite transforms into acicular martensite;after 350?×2h tempering,anisotropic a small amount of dendrites exist,mainly low carbon martensite and austenite with a small amount of feather-like upper bainite structure;after 980?×2h,oil quenching and 350?×2h modulation,showing good toughness and good coordination Microstructure of bainite and quasi-bainite.(4)The room temperature microhardness of the 24CrNiMo alloy steel produced by the as-deposited laser additive is 560HVo.3,and the maximum value is 586HVo.3 after the preparation.The tensile strength,yield strength and elongation at roomtemperature were 1368MPa,920MPa,and 13.2%,respectively.After the preparation,the tensile strength,yield strength and elongation after fracture were improved,which were 1405MPa,1020MPa and 13.6%,respectively.It shows good toughness matching.The tensile fracture modes of the as-deposited specimens and the specimens of different heat treatment conditions under optimized process parameters are ductile fractures.(5)Under the same load and wear distance,the average wear-loss weight of the as-deposited sample under the optimized process parameters is the minimum,which is1.57mg.After the preparation treatment,the wear resistance is improved,and the average grinding loss is 0.6mg,which is superior to the test pieces in the as-deposited state and other heat treatment conditions,wear mechanism is mainly abrasive wear.