The structural evolution of superalloy ingots during hot working

251

Microstructures and a microstructural, columnar architecture as well as mechanical behavior of as-fabricated and processed INCONEL alloy 625 components produced by additive manufacturing using electron beam melting (EBM) of prealloyed precursor powder are examined in this study. As-fabricated and hot-isostatically pressed (''hipped'') [at 1393 K (1120°C)] cylinders examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive (X-ray) spectrometry (EDS), and X-ray diffraction (XRD) exhibited an initial EBM-developed c¢¢ (bct) Ni 3 Nb precipitate platelet columnar architecture within columnar [200] textured c (fcc) Ni-Cr grains aligned in the cylinder axis, parallel to the EBM build direction. Upon annealing at 1393 K (1120°C) (hot-isostatic press (HIP)), these precipitate columns dissolve and the columnar, c, grains recrystallized forming generally equiaxed grains (with coherent {111} annealing twins), containing NbCr 2 laves precipitates. Microindentation hardnesses decreased from~2.7 tõ 2.2 GPa following hot-isostatic pressing (''hipping''), and the corresponding engineering (0.2 pct) offset yield stress decreased from 0.41 to 0.33 GPa, while the UTS increased from 0.75 to 0.77 GPa. However, the corresponding elongation increased from 44 to 69 pct for the hipped components.

Section: Resultsmentioning

confidence: 95%

mentioning

confidence: 99%

Microstructural Architecture, Microstructures, and Mechanical Properties for a Nickel-Base Superalloy Fabricated by Electron Beam Melting

Murr

Martínez

Gaytan

et al. 2011

251

“…Characteristic decreases in the flow stress or softening due to dynamic recrystallization/adiabatic heating were restricted to deformation occurring above both a critical strain ( CRIT ) and strain rate ( CRIT ). Following a critical strain, the rate of dynamic recrystallization ( ) and the volume fraction of recrystallized alloy (V DRX ) for each specified time increment (⌬t) can be expressed as: [4] [5]…”

Section: Coggingmentioning

confidence: 99%

“…Due to the extended high-temperature exposure required for homogenization, significant grain growth occurs during this process. [4] Since the grain size plays a vital role in determining the yield strength and fatigue resistance of hightemperature structural materials, control of these features during processing is critical to the resulting performance of rotating aeroengine components. The process of hot working that converts large cast and homogenized ingots into semifinished billet products is often referred to as cogging.…”

Section: Introductionmentioning

confidence: 99%

Integrated modeling for the manufacture of Ni-based superalloy discs from solidification to final heat treatment

Tin

Lee

Kermanpur

et al. 2005

Process models of the various stages of gas-turbine disc manufacture have been integrated to simulate the physical and microstructural transformations occurring within a nickel-based superalloy throughout the entire manufacturing route. Production of these critical rotating structural components requires several distinct processing stages: vacuum-induction melting (VIM), vacuum-arc remelting (VAR), homogenization heat treatment, cogging, forging, final heat treatment, and machining. During the course of these consecutive manufacturing stages, the various thermal and thermomechanical processes lead to significant changes in both the microstructural characteristics and internal stresses in the alloy. Although separate models have previously been developed to simulate the individual processing stages, this article describes how these models, which are explicitly expressed in terms of the initial and evolving microstructure, can be integrated to simulate the entire manufacturing process from secondary melting through to the final forging and heat treatment. The grain structure predicted for one stage is explicitly transferred as the initial conditions for the model, which simulates the grain evolution during the next step. This information also provides the basis of microstructure-explicit constitutive equations describing the material behavior. Industrial-scale manufacturing trials associated with the production of an INCONEL alloy 718 aeroengine turbine disc were used to validate the integrated model for grain size. The model also allows intrinsic or extrinsic defects entrained within the material during the initial solidification stage to be tracked through the subsequent processes. This provides a basis for calculating the areas of discs likely to be vulnerable to such defects and whether they might be removed during machining. It was shown that the microstructure of the alloy changes significantly throughout the process chain, the final microstructure and defect distribution at each stage being related to those formed in the previous stages. There was good agreement between the model predictions and experimental observations for both the intermediate and final processing stages. The implications of such integrated modeling for quality assurance through process control are discussed.

“…Moreover, many of the compositional changes made to conventional c-c¢ Ni-base superalloys do not appear to be capable of achieving the considerable improvements in properties required by advanced gas turbine engine concepts and tend to impair the formability of the material. [8] As a result, new concepts or alloying approaches are needed that provide transformational improvements in properties and enable the design of advanced gas turbine engine concepts.…”

Section: Introductionmentioning

confidence: 99%

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Detrois

Helmink

Tin

2014

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermomechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic c-c¢-d Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four c-c¢-d Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080°C and 1160°C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.