Research On Process Optimization And Forming Properties In Selective Laser Melting Of 18Ni300 Die Steel

Selctive Laser Melting（SLM）is a kind of additive manufacturing technology which can quickly form metal components with complex structure,and it can form a complex shape of cooling water channel,which makes the design of cooling water channel more flexible and improves the cooling effect.In this paper,the SLM forming parameters of 18Ni300 die steel powders are optimized,and the effects of powders with different particle sizes on their properties are studied.The effects of different heat treatment methods on the properties of samples are studied.The thermal expansion properties of samples before and after heat treatment are studied.The SLM forming effect with shape-following cooling channel die inserts is analyzed.The experimental results show that the optimum forming parameters are laser power of175 W,scanning speed of 400 mm/s and scanning distance of 0.07 mm.Under these parameters,the relative density,surface roughness,tensile strength,elongation and microhardness of the formed samples are 99.13%,11.34 um,1168 MPa,6.44%and 348.4 HV,respectively.When the laser power remains unchanged,with the increase of scanning speed and scanning distance,the relative density and comprehensive mechanical properties of the sample first increase and then decrease,and the surface roughness of the sample first decreases and then increases.When the scanning speed and distance remain unchanged,the relative density and comprehensive mechanical energy of the sample increase with the increase of laser power,and the surface roughness of the sample decreases.In order to study the influence of different particle size composition on SLM forming effect,three kinds of powders,No.1,No.2 and No.3,were specially prepared.D₁₀,D₅₀ and D₉₀0 of No.1,No.2 and No.3 powders are 9.0μm、38.1μm、68.9μm,15.8μm、28.8μm、50.0μm and 21.1μm、34.4μm、54.8μm,respectively.Loose packing density and compacting density are 4.26 g/cm³ and 5.18 g/cm³,4.48 g/cm³ and 5.35 g/cm³,4.42 g/cm³ and 5.25 g/cm³,respectively.Compressibility is 17.76%,16.26%and 15.81%,respectively.The fluidity of the three powders is good according to the relationship between compressibility and fluidity.The ISO roundness of No.1 and No.2 powders is mainly between 0.8 and 0.9,and that of No.3powders is mainly about 0.9.The powders with no or only one satellite particle on the surface are more than 1/2.The density and mechanical properties of three kinds of 18Ni300 die steel specimens gradually increase with the increase of laser power,and the surface roughness decreases with the increase of laser power.the powder No.3 has the best particle size distribution.With the increase of aging time and aging temperature,the original austenite grain boundary disappears gradually,the amount of lath martensite increases,and the size of lath martensite increases.The mechanical properties of the specimens gradually increase with aging time extending,But first increase and then decrease with aging temperature increase.When aging time is 9 h,the maximum value is 1585 MPa,0.14%,530 HV.The fracture mode of the specimens is quasi-cleavage fracture.The linear expansion coefficients of 18Ni300 die steel before and after heat treatment are 1.20×10^-5/℃and 1.24×10^-5/℃,respectively.The shape-following cooling die insert has good forming effect and the water channel is continuous and smooth.