Study On Process And High Temperature Performance Of 24CrNiMo Alloy Steel Fabricated By Selective Laser Melting

The 24 CrNiMo alloy steel has good strength and toughness matching as well as thermal stability,which is widely used as raw material for brake disc.The traditional process of preparing alloy steel for brake disc is complex and cannot meet the increasingly complex design needs.Meanwhile,the brake disc is subjected to high temperature thermal fatigue and friction as well as wear during the long-term service,resulting in cracks and wear.Selective Laser Melting(SLM)can freely produce various parts with complex shapes.It has high forming accuracy and good quality,and can greatly reduce the research and development cycle of products.Therefore,the SLM process is widely used in aerospace,automotive,and national defense industries.In the research,the SLM technology was utilized to fabricate the brake disc for high-speed trains.Taking 24 CrNiMo alloy steel as the research object,the forming process of SLM was studied.The forming characteristics of 24 CrNiMo alloy steel forming parts under different process parameters were explored,and the effects of different scanning strategies on the microstructure and mechanical properties of the formed parts were expounded.According to the service conditions of the brake disc,the high temperature performances of thermal fatigue and friction as well as wear were studied,which provided basic research for the manufacture of 24 CrNiMo alloy steel brake disc employing the SLM technology.The effects of different process parameters(laser power,scanning speed,scanning distance)on the quality of 24 CrNiMo alloy steel were investigated.Under the parameters of laser power(360 W),scanning speed(600 mm/s),scanning interval(0.11 mm)and thickness(70?m),good forming quality can be obtained with fewer pores and unfused defects.The microstructure,stress and mechanical properties of the specimens with different scanning strategies were analyzed.The microstructure of SLM specimens of 24 CrNiMo alloy steel consists of granular bainite,martensite and residual austenite.The phase compositions of the formed parts with different scanning strategies are ?-Fe and a small amount of ?-Fe.Under the 0° linear scanning strategy,the sample has a strong preferred orientation,and the rotating scanning strategy shows the characteristics of weak texture.The 0° linear scanning strategy has the maximum mean tensile stress of 927 MPa along the direction of the welding pass.The rotating scanning strategy can homogenize the stress distribution of the specimen in the X direction and Y direction,and the specimen has the minimum residual stress distribution under the 67° rotation scanning strategy.The 0° linear scanning specimen has the maximum tensile strength of 1294 MPa along the direction of the weld.The tensile strength of the specimens under 45°,67° and 90° rotation strategies are 1204 MPa,1217 MPa and 1201 MPa,respectively.The rotation angle has little effect on the tensile strength.The thermal fatigue and friction properties of the specimens with different scanning strategies were studied.The specimens were oxidized obviously at high temperature and mainly formed iron oxides.Oxidation and thermal stress are important reasons for thermal fatigue crack propagation.The 0° linear scanning direction has the longest crack propagation morphology along the notch direction.The rotating scanning strategy can slow down the propagation of thermal fatigue cracks.The samples with different scanning strategies were oxidized at 600 °C,showing the mixed wear mechanism of abrasive wear and adhesive wear.0° linear scanning strategy has the maximum abrasion loss,and 90° rotating scanning strategy has the minimum abrasion loss.