| Nickel-based superalloys are widely used in aviation,aerospace,energy and other high temperature fields due to their excellent high temperature mechanical properties,corrosion resistance and oxidation resistance.The conventional processing and manufacturing methods of nickel-based superalloys mainly include forging and casting.The corresponding preparation cost is relatively high and the obtained workpiece shape is subject to certain limitations.Meanwhile,nickel-based superalloys are also supposed to be one of the difficult-to-machine materials owing to their high hardness.Selective Laser Melting(SLM)additive manufacturing technology has the advantages of simple process,high processing precision and adaptability to the forming of complex shaped workpieces.It has been used for the preparation and processing of complex nickel-based superalloy components.K447 A nickel-based superalloy has excellent high temperature strength,and its working temperature can reach 1000 °C.Its high temperature strength is mainly due to the high volume fraction of γ’ precipitates,but the high content of γ’precipitates typically triggers high crack sensitivity of the alloy during laser additive manufacturing.In this paper,K447 A superalloy was fabricated by SLM,and the effects of process parameters on the microstructure and properties of the alloy were studied.Then,different proportions of IN718 and K447 A powders were mixed and printed.The printing performance of K447 A superalloy was improved by adding IN718 superalloy,and the microstructure and mechanical properties of the alloy were studied in detail to clarify the micromechanism of the influence of alloy composition,microstructure and heat treatment on mechanical properties.The research results indicate that the microstructure of SLM-K447 A superalloy in cross section direction is mainly composed of cellular crystals and fine dendrites,and the building direction basically consists of columnar crystals growing along the <001> direction.The increase of laser energy density will improve the relative density of the alloy to a certain extent,but due to the presence of cracks,the relative density is up to 97.74%.The cracks formed in the alloy are principally solidification cracks and liquation cracks formed along the high angle grain boundaries.As a result of more cracks,the alloy exhibits low tensile strength and plasticity.The results of K447A/IN718 superalloy powder hybrid printing manifest that with the increase of IN718 superalloy,the defect density of the printed alloy gradually decreases,the tensile mechanical strength and plasticity increase first and then decrease,and the mechanical properties of the alloy gradually change from crack to strengthening effect.In the printed nickelbased superalloy,the optimized mixed powder ratio is KI28 [ K447A(20 %)/ IN718(80 %)],the yield strength of the alloy is 784 MPa,the tensile strength is 1105 MPa,and the elongation is 23.7%.The results of different heat treatments on KI28 superalloy suggest that the mechanical properties of the alloy are significantly improved due to the further precipitation of nanostrengthening phases after heat treatment.For the double aging treatment process,the γ′/γ′′strengthening phase in the alloy is fully precipitated,and the alloy is characterized by high hardness and high strength,but the presence of Laves phase reduces the ductility of the alloy.For the solution and double aging treatment process,with the increase of solution temperature,the harmful η phase in the alloy gradually decreases until it disappears,resulting in the gradual increase of the strength and plasticity of the alloy.When the solution temperature is 1100 °C,the nano γ′/γ′′ phase with finer size is acquired,and the alloy with high strength and good ductility after this treatment.The high temperature tensile test at 650 °C showed that the yield strength,tensile strength and elongation of KI28 superalloy increased by 14%,19.3% and 0.6%respectively compared with IN718 superalloy treated by the same process. |
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