| With the continuous development of "high speed" and "heavy load" for high-speed train,more stringent requirements are put on train brake discs.Due to the complex structure of high-speed train brake discs,the core requires high strength and toughness and the surface requires high wear resistance and thermal fatigue resistance.The traditional manufacturing technology has many disadvantages,such as long processing cycle,high cost and difficult processing.In recent years,a new material manufacturing method for near net forming of metal parts-Laser Rapid Manufacturing has been successfully applied in aviation and other fields,which can make up for the shortage of traditional manufacturing.Laser rapid manufacturing technology with adjustable alloy composition,is easy to manufacture metal parts with complex shapes and the character of difficult processing.So laser rapid manufacturing technology was adopted to fabricate CrNiMoTi low alloy steel brake disc material,and its microstructure and properties of evolution was studied in this paper.The 24CrNiMoTi material was deposited by FL-Dlight02-3000W semiconductor laser and JK1002 single-mode continuous Yb-doped fiber laser,and some samples were treated by heat treatment.A preliminary study of 24CrNiMoRE low alloy steel samples by selective laser melting and hot isostatic pressing treatment was carried out.The microstructure of the samples were characterized by OM,SEM,TEM and XRD.The influence of laser rapid manufacturing parameters and post-treatment process on the microstructure was studied.The mechanical properties and wear properties were analyzed by microhardness tester,universal material testing machine and friction wear tester.The parameters with the laser power of 2200W,the scanning speed of 5mm/s,the defocusing amount of 4mm,the powder feeding rate of 9.02g/min,the overlapping rate 30%were optimized for semiconductor laser.Laser power of 300W,scanning speed of 5mm/s,defocusing amount of 5mm,feed rate of 4g/min,40%overlapping ratio were optimized under the fiber laser.The bulk of 24CrNiMoTi low alloy steel was successfully prepared under the optimization parameters.The experimental results show that there are no obvious defects such as porosity and crack in 24CrNiMoTi low alloy steel,and the density is up to 97.1～99.4%compared with forged steel brake disc.The microstructure of the top zone consist of martensite and lower bainite,and the middle zone is ferrite and bainite,and the bottom zone is mainly tempered martensite,the carbide with the shape of granular,needle and flake or fibre disperse in matrix,there are also spherical or ellipsoidal sub-micron size TiC.The average hardness of the sample deposited by semiconductor laser is 350HV,the yield strength is 590MPa,the tensile strength is 700MPa,the elongation is 13.2%.The average hardness of the sample deposited by a fiber laser is 420HV,the yield strength is 610MPa,the tensile strength is 1230MPa,the elongation is 10.9%,respectively.The yield strength is 1270MPa,tensile strength is 1555MPa,elongation is 8.7%for the samples fabricated by fiber laser after the heat treatment(880℃/10min,oil quenching,tempering/250℃).It can satisfy high-speed train brake technical indicators,whose tensile strength exceeds the level of forging especially.For laser rapid manufacturing 24CrNiMoTi deposited samples,the volumetric wear loss(1.62×10-4 mm3/N·m)of the as-deposited sample prepared by fiber laser is 51%of the as-deposited sample(3.16×104mm3/N·m)prepared by semiconductor laser.For fiber laser rapid manufacturing+post heat treatment 24CrNiMoTi samples,the volumetric wear loss(1.17×10-4mm3/N·m)of sample(880℃/10min,oil quenching,tempering/250℃)is 72%of the as-depositon sample(1.62×l0-4 mm3/N·m).The wear resistance of the sample(880℃/10min,holding at 200℃)is the best,and the volumetric wear loss(0.55 ×10-4 mm3/N·m)is 34%of the as-deposited sample. |
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