Study On Preparation And Performance Of PM Ni-based Superalloy Using Master Alloy Method

Powder metallurgy(PM)superalloy has excellent high-temperature strength,oxidation and corrosion resistance.The current preparation method of PM superalloy is mainly hot isostatic pressing+isothermal forging(hot extrusion),which has the problems of high cost of raw material powder and difficult preparation for small and complex-shaped components.This paper proposes a method of combining master alloy method and injection molding technology to prepare nickel-based superalloy.The mixed powder of master powder and ultra-fine carbonyl nickel powder was used as raw materials.Through increasing the specific surface area and lattice distortion of the powder,and increasing the chemical composition gradient between the powders to improve the driving potential of atom migration in the sintering process,the purpose of enhancing sintering process was achieved and the low-cost preparation of small and complex-shaped components of PM superalloy was realized.The thermodynamic calculation and diffusion couple method were applied to design the master alloy composition.The wettability and infiltration of the liquid phase,the diffusion and phase evolution process of the solid phase in master alloy were studied.Increasing the concentration gradient of alloying element can reduce the amount of master alloy powder in the mixed powder,but also increases the difference of phase constitution between master alloy and taget alloy,resulting in the forming of NiAl barrier layer and a phase during the solid phase diffusion process,which hinders the homogenization process of alloying elements.The master alloy needs low liquidus temperature,similar phase constitution to the target alloy,good wettability with the matrix,and fast in filtration rate under capillary force in the porous body.MA35Ni master alloy meets the above conditions.It is the most suitable master alloy composition for preparing MIM418 alloy.The MA35Ni master alloy powder was prepared by the gas atomizing method.The GA master alloy powder is spherical and has low oxygen content of only 490 ppm.At the same time,a lower cost powder preparation process has been developed.The master alloy powder was prepared by ingot crushing method.The particle size is smaller,but the oxygen content is slightly higher(?0.2%).The ultimate strength of MIM418 alloy using master alloy powder prepared by the gas atomizing method and the ingot crushing method is 1047 MPa and 930 MPa respectively,which is much higher than that of the cast alloy.The densification mechanism of MIM418 alloy using master alloy powder during sintering process was studied.The diffusion behavior and phase evolution process were characterized.During the sintering process,the alloy element diffusion behavior is not synchronized.Due to the highest diffusion rate in the Ni matrix,A1 element diffuses into the Ni matrix through the solid phase before the appearance of liquid pahse,and precipitates a large amount of fine ?' phases of about 5 nm at the front interface.As a result,the morphology and size of the ?' phase change with the Al concentration during the heating process.However,since the final sintering temperature is higher than the solution temperature of the ?' phase,the ?' phase is uniformly precipitated from the supersaturated solid solution during the cooling process.High melting point elements,such as Cr and Mo,mainly diffuse through the liquid phase.The Nb in the master alloy mainly exists in(Nb,Ti)C.Due to the high dissolution temperature of MC carbides,the diffusion of Nb is difficult.The evolution of the transient liquid phase during the sintering process is verified to isothermal solidification process.Ni particles gradually dissolve into the transient liquid phase,leading to the increase of the melting point of the alloy at the front interface and then solidifying.The main alloying element in the transient liquid phase is Cr,and the Cr-rich liquid phase after solidification evolves into M23C6 carbides on the grain boundaries,which is beneficial to pin the grain boundaries and reduce the grain size.The appearance of the transient liquid phase accelerates the densification process and the homogenization process of alloy elements.After optimizating the sintering process,the relative density of the sintered alloy is up to 98.37%.The mechanical properties of MIM418 alloy using master alloy method were systematically characterized.The tensile strengths at room temperature and elevated temperature of 800? are 1047 MPa and 819 MPa respectively,which are higher than that of cast K418 alloy by 70.7%and 8.5%respectively.After heat exposure for long period,the performance is still stable.After the heat exposure at 900? for 200 h,the tensile strength and elongation are 1246 MPa and 7.8%respectively.The turbine wheel with uniform shrinkage and defect-free appearance was prepared by master alloy method and injection molding technology,achieving the net-shape forming of small and complex-shaped PM superalloy components.This paper is expected to establish the theoretical and technological foundation for the fabrication of highly alloyed materials through master alloy method and have positive effct on the application of PM superalloy with high performance and powder injection molding technique.