Research On Selective Laser Melting Formability Of 30Cr3 Ultra-High Strength Steel

Ultra-high strength steel is the preferred material of main load-bearing mechanical components.It is widely used in aviation,aerospace,marine,energy,vehicles,and dynamic and other high-tech fields for its excellent properties.In aviation industry,components made of ultra-high strength steel have sophisticated structure making conventional casting,forging and machining difficult to meet manufacturing requirements.Selective Laser Melting?SLM?is an advanced Additive Manufacturing?AM?technology.It is particularly suitable for fabricating parts with irregular surface and complex structure,it has the characters of shorter development period and higher material utilization.In order to discover the SLM formability of 30Cr3 ultra-high strength steel,using Optical Microscope?OM?,Scanning Electron Microscope?SEM?and X-ray Diffraction?XRD?characterization analysis methods and tensile property test,hardness test and impact and fracture toughness test,the forming process parameter,the microstructure and mechanical properties of the SLM samples in as-built condition and the effects of heat treatment on the microstructure and the mechanical properties were studied in this paper.Laser scanning speed is the main factor that affects the stability of molten pool.Low scanning speed makes molten pool maintain stable and the sample has a small amount of pore defect.High scanning speed causes the balling phenomenon and the deformation of molten pool leading to the decrease and disappearance of overlapping area.A large quantity of defect with big size appears as a result.The increment of laser power aggravates the deformation of molten pool and give rise to the sample porosity.The variation of laser power has no influence on the stability of molten pool when laser scanning speed is low.The microstructure of SLM of 30Cr3 steel is lath martensite.The microstructure locates at molten pool boundary is coarser than inner region of molten pool because of the thermal effect of adjacent molten pool.The layer by layer deposition of molten pool results in alternative variation of coarse and fine grain size.Columnar crystals were observed.Some columnar crystals grow with the direction of maximum temperature gradient while some extend through multiple molten pool because of the epitaxial growth on the substrate.Crystal growth around molten pool boundary were more random,and formed high-angle grain boundaries about 30 degrees.Crystalline grains were composed of cellular dendrite with arm spacing of 0.75 ?m.The tensile strength,yield strength and elongation of SLM of 30Cr3 steel is 1600 MPa,1400 MPa and 12% respectively.Defects significantly lowers the elongation but has no obvious effects on tensile strength and yield strength.The samples have a certain anisotropy that cross section has higher hardness than top surface.The tensile fracture mode is transgranular fracture.Samples with low porosity had cup-cone shaped facture with fiber area and shear lip area.For samples with high porosity,large defects become crack initiation,the extension and convergence of multiple crack at the same time lead to the eventual fracture,elongation reduces significantly as a result.Heat treatment can improve microstructure and property.When the tempering temperature reached 400°C,lath martensite started shredding,most of martensite decomposed at 500°C,martensite decomposed completely at 600°C while the tensile strength decreased and the elongation increased at the same time.From 200°C,fusion phenomenon of cellular dendrite occurred.The elongation decreased obviously after tempered at 300°C.After the heat treatment process of forging material?normalizing at 930°C for 15 minutes,then oil quenching after maintained at 915°C for 15 minutes,tempering at 200°C for 2 hours with furnace cooling?,the tensile strength and yield strength of SLM of 30Cr3 were enhanced to the level of forging material,while the elongation decreased to 5.73%,indicating that the heat treatment process of forging material cannot be applied to SLM.After tempered at 200 °C for 2 hours,sample of XY direction has the impact energy of 30 J while Z direction has 23 J.The anisotropy still existed after heat treatment.The fracture toughness of XY direction sample is 87 MPa?m^1/m.