| Laser Direct Metal Deposition(LDMD)technology is a new kind of material-adding manufacturing technology with laser as heat source.It has the characteristics of high flexibility,high processing accuracy,short time,high efficiency and high energy density.With the start of the "Made in China 2025" project,universities and research institutes are actively participating in the laser industry,it has become one of the symbolic technologies in advanced manufacturing field.Based on the background of manufacturing axle-mounted brake discs for high-speed trains by laser addition,this paper tries to provide theoretical basis for 24CrNiMo alloy powder laser direct metal deposition technology,which can produce parts with low cost and good performance.This paper will explore the influence of technological parameters on the structure and properties during the forming process,and analyze the residual stress distribution of the formed specimens with the help of finite element technology.On the premise of determining the technological window,we compare the microstructures and properties,the variation of temperature field during deposition and the residual stress on the surface of specimens at different laser power(1300W-1500W)and scanning speed(7mm/s-9mm/s)to study the effects of laser power and scanning speed on the properties of 24CrNiMo alloy steel.ABAQUS software is used to simulate the temperature field and residual stress of the process,in order to obtain an optimal process parameters.In the density measurement test,the density of specimens with laser power over 1300W is above 95%.With the decrease of laser power and the increase of scanning speed,the density tends to decrease.The metallographic structure of the specimen is observed,the surface layer is mainly feathery bainite,lath martensite and acicular ferrite,while the near base layer is mainly granular bainite and ferrite.In the tensile test,the tensile strength under each parameter is higher than 980 MPa,and the performance under different parameters is compared.The highest strength is obtained when laser power is 1400W and scanning speed is 8mm/s,the tensile strength is 1131 MPa,which is 15%higher than the national standard.In the test of cross-section hardness of specimens,the hardness decreases gradually from the surface to the near base,with the increase of laser power and the decrease of scanning speed,the hardness increases first and then decreases.The temperature field and residual stress distribution of laser material adding process are simulated by ABAQUS finite element analysis software and the variation rules of maximum temperature,temperature gradient,cooling rate and residual stress of molten pool under different parameters are obtained.Longitudinal stress on specimen surface concentrates on both sides of deposition site and is tensile stress and the edge is mainly compressive stress;The transverse stress on the surface of specimens concentrates at both ends of the deposition site and is tensile stress,while the rest is compressive stress.The stress value of the specimens increases with the increase of laser power and decreases with the increase of scanning speed. |
