Research On Process And Property Of 18Ni-300 Alloy Manufactured By Selective Laser Melting

Maraging steel is a kind of ultra-high strength steel,which is mainly used in precision forging die.However,the traditional method used for manufacturing the mold is full of disadvantages such as long production cycle,poor hardenability,cracking during quenching,and scrapping.In order to solve the problems of complex process and high rejection rate in the existing molding technology,18Ni-300(a low-carbon maraging steel)was selected for Selective Laser Melting(SLM)experiment.In this paper,the common problems in the SLM process,including balling behavior,spatter,black smoke and cracks were analyzed.Secondly,18Ni-300 powder was used in SLM and the manufacturing process was investigated.The effects of hatch space,scanning speed and laser power on the roughness and density of maraging steel were considered.The microstructure and mechanical properties of the molded parts before and after solution-aging treated were compared and discussed.Finally,the effects of different scanning strategies on the microstructure and mechanical properties of the molded parts were discussed.The results shows that appearance of smoke in the SLM process is mainly due to the burning of low melting point alloy element in the powder,The effect of oxygen have huge influence on the spatter behavior.When the oxygen content is higher than 3000 ppm in the molding room,the sparks became divergent and the spatter angle is becoming larger,which not only affects the molding process.but also makes the manufacturing process unstable.Due to the unreasonable powder ratio or the existing of satellites powder,it’s easy to appear unmelted powder at the edge of the fusion zone and these junction area,which makes element segregation obvious.The result of 18Ni-300 powder SLM process shows that the scanning distance,the laser power,and the scanning speed are the main factors affecting the surface roughness of the molded parts.Part density are influenced by the scanning spacing,scanning speed and laser power.The effect of bulk energy density on roughness and density is related to the scanning speed and laser power.When the volume energy density is in the range of 125-135J/mm3,the upper surface roughness is the lowest and the density is the largest.SLM parts have smaller grains and can reach sub-micron level.The microhardness can be increased by 65% and the tensile strength can reach 1634 MPA.The fracture mechanism after heat treatment is mainly brittle fracture.The cellular crystal grains of the molded part obtained by the partition scanning were finer and the tensile strength was slightly higher than that of the S-shaped orthogonal scan.The residual stress σx and σy in X direction in two scanning modes are mainly tensile stress at the edge of the "pre-forming zone".The tensile stress value of the S-shaped orthogonal scanning molding part decreases gradually with the edge of the part,and is gradually transformed into a large compressive stress.The stress distribution is not uniform,and the tensile stress of the parts is gradually reduced and converted into compressive stress,however the compressive stress is smaller.