| As the main material of aerospace engines,nickel-based superalloys should be capable of withstanding high temperature,high pressure and high dynamic load.However,when processed by traditional methods,it faces problems such as segregation,difficult-to-form and difficult-to-cut as a result of high degree of alloying.Hot isostatic pressing(HIP),combined with mold control technology,has the ability to fabricate fully dense parts directly from pre-alloyed powder under high temperature and high pressure conditions.However,the process is not yet mature,and the evolution mechanism of macro and micro properties is not completely clear.The heat treatment mechanism,high temperature performance and its failure mechanism need to be explored.To this end,in view of the problems during HIP process,two nickel-based powdered superalloys are selected as research materials.The effects of HIP temperature,cooling rate and three kinds of heat treatment on the microstructure and properties of the superalloy are studied.The main contents are as follows:(1)The influence of HIP temperature on the microstructure and properties of FGH4097superalloy was studied,and the microstructure evolution and failure mechanism were revealed.When the HIP temperature is lower than?'solidus temperature,the superalloy is consisting of dendritic structure,and the density of primary particle boundaries(PPBs)in the matrix is high.Besides,cracks are easy to initiate and propagate at PPBs,and the fracture mode is transgranular fracture.When the HIP temperature is higher than?'solidus temperature,the structure is consisting of equiaxed grains and fine?'phase distributed homogeniously in the matrix.The number of PPBs in the matrix decreases,and the fracture mode is intergranular fracture.Under the experimental conditions,increasing the HIP temperature,the tensile strength first increased and then decreased,reaching a maximum at 1200?(room temperature:1410 MPa,650?:1236 MPa),which is higher than the FGH4097 technical standard(room temperature:1275 MPa,650?:1220 MPa).This study revealed the relationship between hot isostatic pressing process parameters,microstructure and tensile properties,which provided a theoretical basis for subsequent process optimization and post-treatment.(2)The effects of HIP cooling rate on the microstructure and properties of FGH4097 and Inconel 718 superalloys were studied.With the increase of cooling rate,the average size of?'phase in FGH4097 superalloy decreased from 0.61?m to 0.15?m.The cooling rate has little effect on the tensile strength.After aging at 850?,the yield strength increases.When the cooling rate is 3.33 K/s,the FGH4097 superalloy has the highest yield strength(room temperature:1183 MPa,650?:1125 MPa).Increasing the cooling rate is beneficial to the performance regulation of FGH4097 superalloy For Inconel 718 superalloy,as a result of PPBs,when increasing the cooling rate,the average grain size increases from 21.9?m to 51.8?m,and the tensile strength decreases.After 720?/8 h+620?/8 h two-stage aging treatment,the strength of the superalloy increased,but it was still lower than the furnace cooling state.Increasing HIP cooling rate is not benificial to the performance control of Inconel 718superalloy.The rapid cooling process controls the performance of the sample,thereby shortening the entire HIP process.(3)The effects of three kinds of heat treatment methods including solution treatment,direct aging heat treatment and cryogenic treatment on microstructure structure and properties of FGH4097 superalloy were studied.After solution treatment and direct aging treatment,the?'phase at PPBs is coarsened and the teritary?'phase is precipitated in the matrix,and the microhardness is improved.After solution treatment,the high temperature tensile strength was reduced from 1173 MPa to 1155 MPa,but the yield strength was significantly increased,from872 MPa to 990 MPa.After cryogenic treatment,the wear resistance of the superalloy was significantly improved,the friction coefficient was reduced from 0.59 to 0.18,and the wear rate was reduced from 1.1×10-4mm3/N·m to 1.3×10-5 mm3/N·m.By studying the evolution of the main strengthening phase and properties of FGH4097 superalloy under different heat treatment conditions,it provides guidance for heat treatment process regulation.(4)The high-temperature wear resistance of FGH4097 superalloy prepared by HIP was studied,and the high-temperature wear mechanism was revealed.Below 400?,as the temperature increases,the thickness of the wear layer decreases.The main wear mechanisms are abrasive wear and adhesive wear.When the temperature is higher than 600?,the oxidation rate of the surface increases,and the wear layer transforms into a continuous and dense(Ni,Co)O oxide layer,which plays a role of high temperature lubrication and reduces the friction coefficient and wear rate,and the main wear mechanisms is oxidation wear.Due to the formation of a dense oxide layer at high temperature,the wear resistance of FGH4097superalloy at high temperature is better than that of at room temperature.This study reveals the evolution law of FGH4097 superalloy's wear performance,which provides guidance for the application of high temperature wear-resistant components.In this paper,nickel-based powdered superalloys are selected as the research objects,and the influence rules of HIP process and heat treatment on the microstructure and properties of the superalloys are studied,which provides a useful guidance for the complex forming of high-performance superalloy parts in HIP. |
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