Research On Microstructures And Properties Of Selective Laser Melting 18Ni300 And Heat Treatment

This paper adopts selective laser melting to form 18Ni300 maraging steel,in view of the low material utilization rate,long processing cycle and low production efficiency of18Ni300 maraging steel in the field of aerospace and precision forging die.The causes of common defects in SLM forming were analyzed,and the forming process was optimized.In order to give full play to the advantages of high strength of the material,solid solution,solid solution+aging heat treatment was selected to study the structure,mechanical properties,and friction and wear properties of the sample in the as-deposited state and different heat treatment conditions.the specific research results are as follows:Firstly,the three factors of laser power,scanning speed and scanning pitch were changed respectively,and the influence law on the density,hardness and surface roughness of the sample was studied.The combination of laser power 400 w,scanning speed 1800 mm/s,scanning pitch 0.07 mm,and paving layer thickness 0.03 mm has the highest density,reaching 99.61%,surface roughness 4.81?m,microhardness 375.6 HV.Defects such as spheroidization,cracks,and pores in the SLM forming affect the quality of the sample.The S-orthogonal scanning strategy was used to investigate the causes of spheroidization,crack and porosity defects in SLM forming 18Ni300 process.It shows that in addition to the good combination of process parameters,the abnormal powder characteristics of the repeatedly used powder are more likely to cause defects in the sample.It is recommended to strictly control the proportion during the forming process to ensure the pass rate of the sample.The final prepared sample has a density of more than 99.6%,uniform hardness distribution and excellent surface roughness,indicating that the SLM forming 18Ni300 process is effective and feasible.The formation of a submicron cell structure at the center of the melt track is due to the Gaussian distribution of the laser energy and the rapid cooling effect.The G/V_s and the preferred growth orientation resulted in the cell crystals in the[001]direction gradually evolved into the cellular dendrites in the middle of the molten pool,while the top and both sides of the cell crystals grow along the[100]and[010]directions,respectively.While the Bernard convection caused by the strong temperature gradient leads to the unbounded transition of sub-grained microstructure occurs in a localized area of the molten pool.Compared with the forged samples,SLMed samples has finer microstructure,the tensile strength is 1159 MPa,yield strength is 1060 MPa,micro-hardness is 375.6 HV,better performance than forgings.After solution treatment,the microstructure is single-phase martensite.The martensite structure is refined first with the increase of solution temperature,then decomposed and recrystallized,and the direction gradually changes from parallel base growth at 800? to Vertical basal growth at 920?.The undissolved Laves phase?Fe₂Mo?also gradually dissolved with the increase of solution temperature.At 840? solution temperature,the tensile strength of the sample reached a maximum of 990 MPa,yield strength is 883 MPa,elongation is 13%,micro-hardness is 313.6 HV and the impact toughness is 30.3 J.Compared with the as-deposited sample,although the hardness is reduced,the plastic toughness is obviously improved,and the tensile fracture morphology shows ductile fracture.The meta-stable martensite transforms into steady-state austenite under aging treatment.The diffraction peak of the?111?crystal plane?phase is significantly enhanced by XRD analysis at 550? compared with other aging temperatures,indicating the intensity of recovery became more violent with the increased aging temperature.At an aging temperature of 450?,the Ni₃X-type precipitate phase is finer and uniformly distributed on the matrix.At this time,compared with the deposited sample the tensile strength is increased by 37.7%,the yield strength is increased by 37.6%,and the elongation is decreased by 6.8%,the hardness increased by36.2%,the impact toughness decreased by 58.2%,and the tensile fracture morphology showed cleavage fracture and a small amount of plastic fracture.Compared to the aging forgings under the same conditions,only the hardness reaches the standard,and other properties need to be improved.At the same time,the corrosion resistance of the material is weakened after aging treatment,and the friction coefficient of the sample with different aging temperature decreases firstly and then increases,the amount of wear has a similar law to the friction coefficient.When the fixed friction parameter is 5 N/5 Hz,the aging temperature increases,and the change of the sample wear mechanism generally conforms to the change rule of hardness.At 450? optimal aging temperature,the sample wear mechanism is abrasive wear+adhesion.Wear,the sample at the optimum aging temperature has better wear resistance than the severe oxidative fatigue wear caused by the low hardness value of the as-deposited sample.