| Nickel-based superalloys have important applications in the fields of Aeronautics and Astronautics,and are indispensable materials for high temperature conditions.However,the poor mechanical properties and difficult to control the casting or forging process are the main issues for these materials.Selective Laser Melting(SLM),an additive manufacturing process,is capable of manufacturing metallic parts with complex shapes directly from Computer-Aided Design(CAD)models.SLM parts are created on a layer-by-layer manner,making it more flexible than traditional material processing techniques,especially suitable for the complex nickel base alloy parts.However,SLM is a kind of rapid cooling and stacking method,which is different from the traditional methods in the aspects of microstructure and properties.In this paper,two typical suoperalloy Inconel 625 and Inconel 718 were studied comprehensively.The process windows and the microstructures were investigated to acquire a fully dense parts.The mechanical properties at high and room temperature were tested.At the same time,the hot corrosion properties at 650 ? were also explored.These works provide a theoretical basis and practical experience for SLM process nickel-based superalloy.The main conclusions are as follows:1)The melting characteristics of Inconel 625 were summarized,and the process window of SLM were determined.The highest density of parts exceeds 99%.The hardness value for SLM parts was 350HV5,significantly higher than the level of casting and forging.There is a positive correlation between SLM subgrain and laser power as the following formula:?= 1.54P-1.27.The original phase of SLMed Inconel 625 is the y matrix,however,after the heat treatment,MC carbide precipitated from the matrix and the lattice distortion decreased.The precipitation of MC carbides leads to the bending of grain boundaries and the formation of "zagzig grain boundaries".2)The study found that there were micro-cracks inside the SLM Inconel 625 parts,and the crack length is not more than 100?m.The internal cause for crack was the formation of local micro segregation(?+Laves)eutectic in the process of solidification.The external cause was the high thermal residual stress caused by uneven heating.When the substrate was preheated to 300?,the residual stress was reduced by more than 50%and the number of cracks decreases obviously.3)The mechanical properties of SLMed Inconel 625.showed the characteristics of high strength and low plasticity.The tensile strength of 1135 MPa and yield strength of 702 MPa were exceed 50%of the minimum ASTM standard,However,the elongation is only about 9.65%,which is much lower than the minimum standard of forging(30%).At the same time,the material had obvious anisotropy,and most of the grains grow along the<100>direction.The texture was a typically {100}(001)cubic texture.The tensile strength in the vertical direction is higher than that in the horizontal direction.4)By comparing the laser energy distribution in the SLM forming process,it was found that the ratio of width to depth for uniform laser was much larger than that of the Gauss laser profile pattern.Meanwhile,convection can be observed in cross section of the single channel.A single crystal superalloy was fabricated on a single crystal substrate by a uniform laser spot,and last for 3mm.A new mathematical CET model for SLM was come up,which considering the effect of convection and impurities caused by the laser.This model can well explain the characteristics of SLM epitaxial growth.5)The law of hardness variation with temperature under different laser modes was investigated.In the low temperature region,which is less than 400°,the grain refinement effect caused by the Gauss spot to play a leading role;when the temperature exceeded 400°,the parts built by uniform laser spot showed high hardness due to the less grain boundaries.6)SLMed Inconel 718 matrix was ? austenite and Laves phase which was composited by Fe2Ti,Fe2Mo,Cr2Nb,Fe2Nb and so on,can be abbreviated(Ni,Fe,Cr)2(Nb,Ti,Mo).By TEM analysis,it was found that there was a short rod shape structure Ni3Nb-?" in the SLM fabricated 718 Inconel alloy,which was parallel to each other,with a length of 15-50nm and a diameter of about 5nm.7)Two kinds of SLM parts failure mechanism under high temperature were found.First,the center of the molten pool presented weak performance and easy to crack when subjected to tension.The secondly,the molten pool boundaries migrated under the tension.However,a hole maybe former due to the uncoordination of the boundaries,result in the expansion of cracks along the pool boundary expansion and the formation of X like crack defects.8)After heat treatment,the strength properties of Inconel 718 parts have been greatly improved.At room temperature,the tensile strength of the material was 150MPa higher than the forging standard and yield strength was 140MPa higher than forging standard,meanwhile the elongation rate was still equivalent the forging standard.At 650?,the tensile strength and yield strength are higher than the standard of forging,either Z or XY,but the elongation of Z is lower than that of forging.9)It was found that,after 75h corrosion,the weight loss reached 18mg/cm2,which was 2.5 times of that after aging process and 9 times of the standard heat treatment.The main products on the surface of the alloy were oxides and sulfides,including Cr203,NiO,Fe304 and Ni2S,in which the S elements are mainly from the addition of corrosive salts Na2S04.After heat treatment,the composition of surface oxidation products did not change.10)The corrosion resistance of the samples treated by standard heat treatment and uniform heat treatment is equivalent to that of the forgings.For the aging samples,at the beginning stage was the same as SLM parts.However,after the diffusion and dissolution of 25h,the molten pool boundary disappeared,channel for S and O elements was shut down.The anti corrosion properties of materials has been greatly enhanced. |
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