Research On Key Technology Of Mechanical Property Control Of Laser Additive Manufactured 24CrNiMo Steel

Brake disc is one of the key parts of high-speed train.The brake disc has complex structure and requires high mechanical properties.The traditional method of casting and heat treatment is used to prepare the brake disc.It has a long preparation cycle and the defects are difficult to control for the tradition method.Laser additive manufacturing(LAM)technology is based on three-dimensional digital model,and it forms parts track by track and layer by layer.LAM is a rapid developing manufacturing technology,and it has the incomparable advantages of short manufacturing cycle,low cost and complex structure.LAM mainly includes selective laser melting(SLM)and laser melting deposition(LMD).According to the manufacturing characteristics of the two technologies,in order to realize SLM/LMD forming high performance 24CrNiMo steel,the research route of process/composition-defect/microstructure-mechanical property control was adopted,the mechanism of defect formation and the law of microstructure evolution in the forming process of the two technologies were studied.Aiming at controling defects of 24CrNiMo steel produced by selective laser melting,single-track and multi-track samples were prepared respectively.The types and distribution of defects were studied.The influence of process parameters(laser power,scanning speed,lap ratio,powder thickness)on macro defects such as inter-track and inter-layer defects and the control methods of macro defects were analyzed comprehensively.The fluctuated dimention of the molten pool in selective laser melting process was studied.It was found that the heat transfer mode of molten pool changed from conduction mode to keyhole mode due to the fluctuation of molten pool volume,and the mechanism of keyhole mode on the formation of macro defects in molten pool was explained.The results show that the optimum process window of selective laser melting forming 24CrNiMo steel is as follows:the thickness of powder should not exceed 50?m,the lap ratio is 45%-60%,the laser power is 200W-400W,and the scanning speed should not more than 100mm/s.The hardness of the samples prepared in this process window is 330-370HV,and the elongation is 8%-11%.The hardness and elongation of 24CrNiMo steel fluctuated in a wide range due to the inhomogeneous microstructure caused by thermal cycling,heat accumulation and the occasional induced macro defects,ao that the mechanical properties of 24CrNiMo steel may be lower than that of the cast 24CrNiMo steel.Due to the large heat input and the large size of molten pool,the effect of thermal cycling and heat accumulation on microstructure is more significant in LMD formed samples.In order to study the formation and evolution mechanism of inhomogeneous microstructure of 24CrNiMo steel formed by LMD,single track,single-layer and multi-layer samples were prepared to track the evolution process of microstructure.The results indicate that the thermal cycle causes the microstructure to undergo several heating and cooling processes,and experienced several solid-state transformation processes,especially the rapid austenitizing process resulting in grain refinement and texture weakening.The higher the substrate temperature is,the lower the cooling rate is.The heat accumulation changes the cooling process of the thermal cycle,and then changes the change law of the microstructure,resulting in grain refinement and texture weakening.The thermal cycle and heat accumulation lead to the inhomogeneous microstructure and complex phase composition of multi-track samples.The inhomogeneous microstructure leads to the instability of mechanical properties.By controlling the heat accumulation during forming,the samples with gradient structure from surface to interior were prepared.The microstructure of surface is a mixture of martensite and lower bainite,and the internal microstructure is mainly tempered microstructure.From the surface to interior,the tensile strength of the samples decreased from 1218 MPa to 1113 MPa,the hardness decreased from 490HV to 440 HV,and the elongation increased from 11%to 19%.In order to further improve the mechanical properties of 24CrNiMo steel produced by LAM,rare earth Y was added into 24CrNiMo powder.It is found that the addition of 0.02-0.08%(mass fraction)Y in 24CrNiMo steel powder results in grain refinement,solution strengthening and second phase strengthening,which can significantly enhance the mechanical properties of SLM and LMD formed samples.The tensile strength,plasticity and hardness of 24CrNiMo steel powder increased by12%,73%and 6%,respectively.When 0.08-0.2%(mass fraction)Y is added into the powder of 24CrNiMo steel,continuous large-size rare earth oxides and rare earth carbides are formed.In addition,the rare earth Y causes C element to segregate at the grain boundary to form Fe₃C,as the melting point of Fe₃C is lower than that of24CrNiMo steel,the final solidification grain boundary in molten pool forms crack under tensile stress.At the same time,the remelting of Fe₃C at the grain boundary in the HAZ also leads to crack formation under tensile stress.