Densification Mechanism Of Inconel 718 Pre-Alloyed Powders Consolidated By Hot Isostatic Pressing

Inconel 718 alloy is widely used in cryogenic and high-temperature applications due to its excellent mechanical properties and microstructure stability in the temperature range of-253℃ to 650℃.Inconel 718 precision casting and forging products are widely used in advanced aerospace engines of China.Some of the products have extremely harsh working conditions,requiring high mechanical properties,surface and internal quality.The large variation of cross-section size and complex structure of engine components made a great challenge to forming methods.Powder metallurgy(PM)route can successfully solve the problem of weakness at the weld joint in the process of split forging followed by welding.The application of PM Inconel 718 alloys has been limited by prior particle boundaries(PPBs)which deteriorate mechanical properties such as elongations at elevated temperatures and stress rupture life.Therefore,Inconel 718 was selected as the research material in this paper.For engineering-application,the effects of powder preparation technology,powder size and capsule structure on hot isostatic pressing(HIPing)densification process were studied.The composition of PPBs and its effect on the mechanical properties of the alloy were systematically characterized.The process of eliminating the PPBs was studied to ensure the microstructure stability of the alloy.The main findings can be summarized as follows:There are two major gas atomization methods for producing Inconel 718 powder,Electrode Induction Melting Gas Atomization(EIGA)and Vacuum Induction melting Gas Atomization(VIGA).The differences in the densification process of pre-alloyed powder prepared by EIGA and VIGA during HIPing and the effect of gas atomization methods on the microstructure and mechanical properties of the alloy were studied.The results showed that compared with VIGA powder,EIGA powder had better sphericity,smoother surface and thinner oxide layer.The oxide layer is more easily broken during the HIPing densification process,which increases the bonding strength between particles.The microstructure of EIGA powder forming alloy is similar to that of forgings with equiaxed crystals,and its mechanical properties are comparable to those of forgings.VIGA powder forming alloys have a large number of PPBs in the microstructure,and its comprehensive performance is lower than that of forgings.Therefore,EIGA is preferred as a powder metallurgy powder preparing process.Four pre-alloyed powders with different particle sizes were selected for the study.With the increase of powder size,the surface of the powder becomes rough.Compared with fine powder(P1:15 μm～53 μm),wide size powder(P4:15 μm-106 μm)has better fluidity,which facilitates powder filling.Compared with coarse powders(P2:53～106μm and P3:106～250 μm),fine powders can be filled into the gap between the lapping of coarse powders,thus ensuring better stacking and facilitating the densification process.Considering powder yield,fluidity and mechanical properties,the optimum particle size of pre-alloy powder for HIPing is 15～106μm.At present,simple cylindrical sleeves are commonly used for HIPing densification research.According to the characteristics of complex structural components,two types of special-shaped cylindrical capsules were designed to study the effects of capsule structures(wall thickness and cavity volume)on the HIPing densification process and mechanical properties of alloys.The results show that the wall thickness of the capsule has a shielding effect on the effective stress acting on the powder,which leads to the formation of PPBs in the alloy and decreases the mechanical properties of the alloy.With the increase of cavity volume,there is a gradient change in the densities,but the influence is small.As the size of the capsule increases,it is necessary to select appropriate vibration frequency and vibration time to avoid powder particle size segregation.The composition and mechanism of PPBs were studied,and the evaluation method of PPBs was improved.The results show that PPBs are mainly composed of MC carbide(TiC or NbC)and δ phase,which are prone to become weak interfaces and preferential cracking when subjected to external forces.When proportion of PPBs area is less than 5%,the impact properties and elongation of the alloy meet the forging standard.When proportion of PPBs area for 5%～15%,the elongation of alloy basically meets the forging standard,but the impact properties of alloy are lower than that of the forging.When proportion of PPBs area is more than 15%,the impact properties and elongation of the alloy decrease sharply,which is much lower than the forging standard.Special high-temperature heat treatment(the heat treatment process ensures the pressure>140 MPa)can break the network carbide,eliminate the PPBs,increase the bonding strength between powder particles,improve the mechanical properties of the alloy,especially the elongation and impact properties.Special high-temperature heat treatment can eliminate potential PPBs and ensure the stability of the component.The large and complex components with mechanical properties comparable to forgings can be prepared by this process,which provides a reference for the engineering application of HIPing.