Microstructural evolution and mechanical properties of differently heat-treated binder jet printed samples from gas- and water-atomized alloy 625 powders

An advanced powder metallurgy (P/M) nickel-base superalloy envisaged for aerospace applications is realized through hot isostatic pressing (HIPing) of inert gas atomized powder. Extensive characterization of the microstructural features at different scales by employing optical microscopy, SEM-EBSD and transmission electron microscopy (TEM) divulged multi-modal distribution of γʹ precipitates within austenitic γ-FCC matrix phase in the as-HIPed condition. Investigation of prior particle boundary precipitates (PPBs) through TEM and Electron Probe Micro Analysis (EPMA) techniques revealed the dominance of metallic oxides and MC type carbides. The suitability of the advanced P/M superalloy for realizing near-net shape components by Direct-HIPing for short duration aeroengine rotating components is assessed by evaluating the room temperature and elevated temperature tensile behaviour as well as stress-rupture life in the service temperature regimes. The yield strength and the ultimate tensile strength of the present alloy at 650 °C are found to be 850 MPa and 1275 MPa respectively combined with 28% of ductility which is comparable to those of the similar class of superalloys in service. On the other hand, the as-HIPed superalloy exhibited minimum 1000 h of stress rupture life around service temperatures and bettered the stress rupture life of wrought superalloy IN718 alloy showing its potential for fabrication of near-net shape components for aerospace applications.

Microstructure and mechanical behaviour of an advanced powder metallurgy nickel base superalloy processed through hot isostatic pressing route for aerospace applications

The microstructure with homogeneously distributed grains and less prior particle boundary (PPB) precipitates is always desired for powder metallurgy superalloys after hot isostatic pressing (HIPping). In this work, we studied the effects of HIPping parameters, temperature and pressure on the grain structure in PM superalloy FGH96, by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and Time-of-flight secondary ion spectrometry (ToF-SIMS). It was found that temperature and pressure played different roles in controlling PPB precipitation and grain structure during HIPping, the tendency of grain coarsening under high temperature could be inhibited by increasing HIPping pressure which facilitates the recrystallization. In general, relatively high temperature and pressure of HIPping were preferred to obtain an as-HIPped superalloy FGH96 with diminished PPB precipitation and homogeneously refined grains.

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Effects of Temperature and Pressure of Hot Isostatic Pressing on the Grain Structure of Powder Metallurgy Superalloy

The aim of this study was to gain a deep understanding of the microstructure-mechanical relationship between solid-state sintering and full-melting processes. The IN718 superalloy was fabricated by hot isostatic pressing (HIP) and selective laser melting (SLM). Continuous precipitates were clearly localized along the prior particle boundary (PPB) in the HIP materials, while SLM materials showed a microstructure free of PPB. The mechanical properties of specimens that underwent SLM + solution treatment and aging were comparable to those of conventional wrought specimens both at room temperature and 650 °C. However, a drop was observed in the ductility of HIP material at 650 °C. The brittle particles along the PPB were found to affect the HIP materials’ creep life and ductility during solid-state sintering.

Influence of Powder Surface Contamination in the Ni-Based Superalloy Alloy718 Fabricated by Selective Laser Melting and Hot Isostatic Pressing

Microstructure evolutions of a powder metallurgy superalloy during high-strain-rate deformation

Powder metallurgy of nickel-based super alloys have been developed and used for a wide range of products, owing to their excellent high temperature rupture life, creep strength and fatigue crack growth resistance. Typical processes for high performance PM super alloys include hot isostatic pressing (HIPing), hot extrusion and hot isothermal forging. Hot isostatic pressing is normally conducted at a high temperature by using metal forming standards at low pressure for a long time. However an intrinsic problem, i. e. powder prior boundary (PPB) precipitate networks, can occur in the HIPing process. In this paper, a comprehensive review of PPB formation in the powder metallurgy of super alloys is presented. Research on PPB formation process from room temperature to high temperature and during HIPing, has been reviewed and PPB formation mechanisms are discussed. Methods to reduce PPB effects on the mechanical properties of nickel-based super alloys, including heat treatment, changing the proportions of composition elements, and plastic deformation to break up the PPB networks, are reviewed.

Lin J

Papers

Review and Analysis of Powder Prior Boundary (PPB) Formation in Powder Metallurgy Processes for Nickel-based Super Alloys