Superalloy FGH96 Research Articles

An LCF lifetime model for PM superalloy considering equivalent ellipsoidal inclusion

The non-metallic inclusions are one of the main defects in powder metallurgy (PM) superalloy, which may promote crack initiation under the cyclic loading and seriously affect the safety of aero engines. To quantify the effects of inclusions to the low cycle fatigue (LCF) lifetime, the LCF tests on PM FGH96 superalloy are firstly carried out to observe the inclusions on fracture. By equating the inclusion to an ellipsoid, the relationship between the inclusion's feature and the stress field is established based on Eshelby's theory and finite element method (FEM). Then, the ellipse-equated fracture is determined according to the maximum shear stress's location. Subsequently, key inclusion-related parameters influencing the LCF lifetime are introduced to construct a modified SWT model. Finally, the model's accuracy is validated by comparing with the LCF experimental data. It is demonstrated that the modified SWT model has a good accuracy, with an improvement of scattering band from ±15 to ±4 compared with the original SWT model.

Microstructurally short crack growth simulation combining crystal plasticity with extended finite element method

Accurate description of microstructurally short crack growth remains a long-standing challenging task, due to the fluctuation of crack growth rate induced by grain size and orientation, and grain boundary impediment. Combining crystal plasticity with extended finite element method, a novel framework for short crack growth prediction is proposed, where a new criterion for short crack propagation is established in terms of accumulated shear strain on specific slip system. Compared with three classic empirical models, the proposed method exhibits substantial advantage on the prediction of the fluctuation in FGH96 superalloy, serving as a promising tool for precise estimation of short crack growth rate in polycrystalline materials.

Grinding force during profile grinding of powder metallurgy superalloy FGH96 turbine disc slots structure using CBN abrasive wheel

Grinding force during profile grinding of powder metallurgy superalloy FGH96 turbine disc slots structure using CBN abrasive wheel

Evaluation of oblique laser shock peening effect of FGH95 superalloy turbine disk material

There are many influence factors and characterization indexes for the processing effect of laser shock peening (LSP) on metal materials, and the influence mechanism is complex. The FGH95 superalloy used for turbine disk manufacturing was taken as the research object in this paper. Taking the laser energy, beam diameter and the number of impacts as test factors, the three-factor and the three-level orthogonal experiment was carried out by taking 4 J, 6 J, 8 J laser energy, 3 mm, 4 mm, 5 mm beam diameter, one, two and three times of impacts， respectively. The nine characterization indexes data were obtained by test instruments and equipment, such as residual stress, microhardness and surface roughness. The coalitional game method and gray relational improved TOPSIS method were used to establish an evaluation index system of oblique laser shock peening (OLSP) effect to explore the influence of laser processing parameters on material properties. The results show that the evaluation method and index system are highly feasible and can be used to evaluate and compare the strengthening effect of different processing samples efficiently, comprehensively and scientifically. When the laser energy is 8 J, the beam diameter is 3 mm and the number of impacts is 3, the gray correlation relative closeness degree is 0.7869, with the relatively best strengthening effect for the FGH95 superalloy. Under the index weight set in this paper, the comprehensive material properties of the FGH95 superalloy is improved by 3.72 times compared with the unimpacted sample. Based on the sample data evaluated, when the laser energy of 6 ~ 7.5 J, 2 or 3 impacts, the beam diameter of 4 ~ 5 mm, the comprehensive material properties of FGH95 superalloy after LSP are close to the comprehensive material properties obtained by the relatively optimal process parameters. Therefore, under the premise of ensuring the quality of strengthening, from the consideration of cost and timeliness, the process parameters with slightly lower laser energy and fewer impact times can be selected. The evaluation index system has essential reference significance for reducing the experimental workload, saving the experimental cost, and promoting and applying laser impact processing technology.

Interfacial reaction between mullite-based inclusions and PM superalloy FGH96

Interfacial reaction between mullite-based inclusions and PM superalloy FGH96

Oblique laser shock peening effect of the FGH95 superalloy with a PCA and comprehensive index

As a metal surface strengthening technology, oblique laser shock peening uses the force effect of pulsed laser beams to significantly improve the mechanical properties of materials and prolong service life. However, the processing of metal parts still requires a high cost for testing the quality of the finished product. At present, there is no systematic and complete measurement method or evaluation system for the finished products of oblique laser shock peening technology. In this paper, a principal component analysis method and a comprehensive index method are organically combined to construct a method for evaluating the effect of oblique laser shock peening of FGH95 superalloy turbine disks. We measured 10 real data of FGH95 superalloy specimens after oblique laser shock peening by using the orthogonal test method and verified the data. Based on the evaluation results, we put forward a series of suggestions to improve the quality of oblique laser shock peening of FGH95 high-temperature turbine disks, providing a certain reference and guidance for processing FGH95 superalloy turbine disks.

Small fatigue crack propagation rate and behaviours in a powder metallurgy superalloy: Role of stress ratio and local microstructure

The role of stress ratio and local microstructure on small crack propagation (0.065–1.0 mm) behaviours in superalloy FGH4096 were investigated using the Photomicroscopy method. ΔK correlated the growth rates of small cracks with different stress ratios well, which is attributed to the low crack closure level. Statistical and crystallographic analysis revealed that grain boundaries resulted in crack temporary arrests, which is independent of loading conditions and crack length. Temporarily arrested cracks propagated further by connecting to the secondary crack that initiated on the slip plane that parallels to the twin boundary or in the grain with a local highest Taylor factor.

Prediction of low-cycle crack initiation life of powder superalloy FGH96 with inclusions based on damage mechanics

The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 °C, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.

Influence of Storage Conditions on Powder Surface State and Hot Deformation Behavior of a Powder Metallurgy Nickel‐Based Superalloy

Powder metallurgy (PM) FGH96 superalloy powders are stored under vacuum, argon, ambient air, and oxygen atmospheres for as long as 500 days. The surface analysis results demonstrate that the chemical state of Ni, Ti, Cr, Co, O, and C remain unchanged after 90 days storage. However, the oxygen content and NiO/Ni(OH)2 layer thickness increase from initial values (∼120 ppm and ∼3.8 nm) to stabilized values (∼200 ppm and ∼10 nm) after a short time storage (7–15 days), and remain basically unchanged with the extension of storage time. Powders stored in oxygen atmosphere possess the highest oxygen content (maximum 213 ppm) while the lowest in vacuum, the gap can reach to 25 ppm. The hot isostatic‐pressed (HIPed) parts that consolidated from original and stored powders are isothermally compressed at different conditions. The results indicate that HIPed parts with more oxygen will cause higher activation energy and narrower processing window due to a lower degree of dynamic recrystallization (DRX). Discontinuous DRX (DDRX) dominates the DRX nucleation mechanism of HIPed FGH96 superalloys with ∼120–200 ppm oxygen content, while continuous DRX (CDRX) is the auxiliary mechanism. The increased oxygen content and surface NiO/Ni(OH)2 layer thickness of superalloy powders generate higher oxygen content of corresponding HIPed parts, thus decreasing the hot workability.

Hot Deformation Behavior and Microstructure Characterization of Hiped Fgh4096 Superalloy Containing High Strain Rate

Hot Deformation Behavior and Microstructure Characterization of Hiped Fgh4096 Superalloy Containing High Strain Rate

Influence of powder particle size on the microstructure of a hot isostatically pressed superalloy

The influence of powder particle size on the microstructure of the powder metallurgy superalloy FGH97 was investigated. The powder atomized by the plasma rotating electrode process was sieved to three types: fine powder (0–53 μm), medium-sized powder (53–150 μm), and coarse powder (150–212 μm), which were all characterized by their nearly perfect spherical shape and low oxygen content. The powder was filled, degassed, sealed, and then densified by hot isostatic pressing. Then the microstructures of the fully dense compacts were characterized. The results show that fine powder led to limited plastic deformation of particles during densification, resulting in a minimum of Σ3 boundaries and modest PPB (Previous Particle Boundary) decoration. The fine powder compact showed that low impact toughness was caused by fracture along the PPB. However, the addition of Nb and Hf promoted the formation of the dispersed stable MC particles inside the grain, thus preventing the diffusion of C to the surface of the powder particles to form M6C carbides, and resulting in little tendency to present PPB for this alloy. Therefore, the medium-sized powder and coarse powder compact did not present PPB. Thus, the medium-sized powder is optimal for HIP compact preparation, given that fine powder produced PPB while the coarse powder increased the grain size.

Experimental Study on FGH95 Superalloy Turbine Disk Joint Material by Oblique Laser Shock Processing

The FGH95 superalloy used for turbine disk manufacturing was strengthened by the oblique laser shock processing (OLSP). The laser energy, beam diameter, and number of impacts were selected as the test factors, and the three-factor three-level oblique laser shock processing orthogonal test was carried out. Based on the analysis of variance and range of the surface residual stress, microhardness, roughness, tensile strength, and yield strength of FGH95 superalloy after LSP, the factor level combination of relative best comprehensive performance was obtained. The results showed that, within the 10% confidence level, the order of influencing factors with significant difference is: laser energy > number of impacts > beam diameter. Compared with the unimpacted sample, the microhardness of the material surface was increased by about 25% after OLSP. With the increase of laser energy, the dimple distribution of the sample was more uniform, smaller in size, and shallower in depth. However, the thickness of the affected layer was very low, and there was no significant effect on the tensile properties. In general, and under the test conditions selected in this study, the combination of 8 Joule laser energy, 3 mm beam diameter, and three impacts can be selected to obtain the best comprehensive performance. The results of this paper provide a reference for the OLSP of the FGH95 superalloy turbine disk and other aero engine parts.

Normal and abnormal grain growth in a FGH96 superalloy during thermomechanical treatment

In order to investigate the grain growth behavior of a FGH96 superalloy during thermomechanical treatment, isothermal compression experiments of the superalloy were carried out on a Gleeble thermal simulator at the temperatures from 1020 to 1110 °C with strain rates from 10−3 to 1 s−1. Uniform heat treatment was performed on the deformed samples with typical Zener-Hollomon (Z) parameters after hot compression. Optical microscope (OM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and finite element (FE) simulation were employed to explore the relationship between deformation conditions and microstructure evolution during thermomechanical treatment of the superalloy. The results show that with the decrease of Z parameter, the deformed microstructure in the center of the cross-section of the specimen transforms from a mixed state to a fully dynamic recrystallization (DRX) state. Under the conditions of high and medium Z parameters, obvious abnormal grain growth (AGG) emerges in the cross-section of the heat-treated samples, and inclines to form at higher strains with the decreasing Z parameter. Combined with the results of FE simulation, deformation conditions (Z parameter and strain) are suggested to be the main factors that control the normal grain growth (NGG) and AGG behaviors during thermomechanical treatment of the alloy, by influencing the deformation energy storage, the degree of DRX and γ′ precipitates. Stored energy in the special range of 1.25–1.38 J/mol, incomplete DRX and partial dissolution of γ′ precipitates can stimulate the occurrence of AGG. Under the stored energy conditions to ensure the occurrence of AGG, less DRX grains will favor individual formation of abnormally large grains (ALGs) with larger size after heat treatment. The microstructure with local misorientation (LM) between 0.25° and 0.8° plays an important role in the later stage of ALGs growth. A variable β described as β = ε·lnZ was proposed to define the formation condition of ALGs during thermomechanical treatment of the superalloy, and its corresponding mechanisms were also elucidated.

Interface between FGH96 Superalloy and Refractory Slurry with Different Soaking Time

In this paper, the contact interface between FGH96 superalloy melts and refractory slurry with corundum powder and silica sol at 1600°C with different soaking time in 10-240 min range was investigated. The morphology and composition of the contact interface were studied by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the reaction layer formed on the contact interface between the melting superalloy and the refractory slurry, and is mainly consist of Alumina and contains a small amount of other metal elements such as Ti and Cr. The reaction layer was not smooth on the micro level, and there was a peninsula-like structure protruding into the internal part of the melting superalloy on the reaction zone, and even started to fall off at some places to form islands. With the increase of soaking time, the reaction between melt of superalloy and refractory slurry increased gradually and the reaction layer began to combine with the refractory slurry substrate and form obvious interaction layered structure, resulting in the corrosion of refractory slurry substrate. With the soaking time over 120 min, the stable contact interface was destroyed. Thermodynamic calculation shows that the substitution reaction between Al in superalloy and SiO2 in refractory slurry meets the thermodynamic conditions, and the reaction can proceed forward.

Helmholtz Free Energy and the Activation Volume of Steady-State Creep in FGH96 Superalloy

The creep properties of FGH96 superalloy were studied in the temperature range of 650 °C to 750 °C and stress range of 690MPa to 897MPa. The results show that the creep life of the alloy decreased significantly with the increase of stress and temperature. However, the temperature produced more effects than that of stress. The most suitable service temperature and stress were also obtained based on the creep results. A physical model base on crystal-plasticity theory was established, but the simplification of the Helmholtz free energy and the activation volume might reduce the accuracy of strain rate prediction. Based on the results of creep at different stresses and temperatures, the Helmholtz free energy and the activation volume of steady-state creep were obtained, which would play a key role in creep life prediction.

Effect of Particle Size and Shape on Flowability of FGH96 Superalloy Powder

The effect of particle size and shape on flowability of FGH96 superalloy powder was investigated by field emission scanning electron microscopy (FE-SEM), laser particle size analyzer (LPSA) and X-ray photoelectron spectroscopy (XPS). The results showed that the powder flowability basically presented a decreasing trend as the median diameter decreased. The Hall velocity of the five median diameter powders (50=203.9 μm, 106.3 μm, 83.2 μm, 73.8 μm, 19.9 μm) was 27.18 s/50g, 23.25 s/50g, 23.86 s/50g, 23.42 s/50g and none, respectively. The surface oxides/ hydroxide/nitride of the five median diameter powders were mostly the same, mainly including Al2O3, Cr2O3, MoO3, Nb2O5, Ni (OH)2, TiO2 and TiN. The median diameter 50, shape factors (circularity, aspect ratio, roundness, solidity) and fractal dimension were selected to quantitatively characterize particle size and shape. For the same fluctuation value of powder flowability, the roundness and solidity showed lower sensitivity. Compared with the two shape factors, the sensitivity of circularity and aspect ratio was at an intermediate level, while the median diameter and fractal dimension displayed higher sensitivity. The median diameter and fractal dimension can be used to characterize the principal variation of flowability. The circularity and aspect ratio can be utilized to characterize the variation of flowability supplementally.

Grinding of fir tree slots of powder metallurgy superalloy FGH96 using profiled electroplated CBN wheel

Broaching is commonly used for machining fir tree slots on turbine disk, which has outstanding advantages and disadvantages, such as high-quality machined surface, high manufacturing accuracy, and high productivity as well as fast tool wear, extremely high processing costs, long preparation time, and poor process flexibility. Utilizing the electroplated cubic boron nitride (CBN) profiled grinding wheels and single-sided local profiled grinding process, the experiments of FGH96 turbine disk slots are carried out. The results show that the high precision of slot profile can be achieved by the developed process. Using the given experimental parameters n = 48,000 rpm, ap = 0.002 mm, vf = 100 mm/min, and 600# electroplated CBN-profiled wheel, the profile error of FGH96 slots is within ±0.012 mm, and the grinding surface roughness is less than Ra 0.8 μm. After four whole slots are machined completely, the grinding wheel still has grinding capability, which proves that 600# electroplated CBN-profiled wheel can meet the grinding needs of FGH96 turbine disk slots.

Effects of composition and particle size on the surface state and degassing behavior of nickel-based superalloy powders

Surface state and degassing behavior of nickel-based superalloy powders are essential to the melting and sintering properties of additive manufacturing (AM) and powder metallurgy (PM). Superalloy powders with various compositions and particle sizes were investigated by FESEM, XPS, AES, HAADF-STEM, and temperature programmed desorption mass spectrometry (TPD-MS). The surface analysis results demonstrate that IN625, IN718, and FGH96 superalloy powders are covered by homogeneous Ni oxide/hydroxide (NiO/Ni(OH)2) layer decorated with various oxides. NiO/Ni(OH)2 layer thickness changes in the range of 5.7 nm to 9.8 nm with different powder compositions and particle sizes, while the influencing factors of thickness can be arranged in the following order (from highest to lowest) as IN625-IN718-FGH96 and (106–150 μm)-(48–106 μm)-(<48 μm) respectively. The TPD-MS results indicate that the degassing amounts significantly increase with the decrease of NiO/Ni(OH)2 layer thickness, in which the degassing behavior is partially controlled by molecular diffusion. Powders with more active metallic elements, i.e. Al and Ti, and smaller particle size possess relatively thinner oxide layer which result in lower diffusion barrier and higher degassing amounts. The investigation reveals the relationship between the surface state and degassing behavior of superalloy powders considering the differences in the composition and particle size, and gives guidance on nickel-based superalloy powder processing.

An Experimental Study on Low-Cycle Fatigue Crack Initiation Life Prediction of Powder Superalloy FGH96 Based on the Manson-Coffin and Damage Mechanics Methods

The applicability of both prediction methods for low-cycle fatigue life of powder superalloy based on the Manson-Coffin equation and damage mechanics were addressed. Both fatigue life prediction models were evaluated by low-cycle fatigue experimental data of powder superalloy FGH96 with non-destructive standard parts and those with inclusions. Due to the characteristics of high strength and low plasticity of powder superalloy FGH96, errors in calculating the plastic strain amplitude deviate severely the prediction outcomes when using Manson-Coffin method. Meanwhile, by introducing the damage variable which characterizes the material damage, the damage evolution equation can be built by fitting the experimental data of standard parts and also applied to powder superalloy specimens containing inclusion. It is indispensable to accurately calculate the damage characterization parameter through finite element analysis in local stress concentration around the inclusion. The applicability of the prediction model was verified by the test life cycles of experimental specimens with different types and sizes of inclusions subsequently. Testing and simulation work showed much better prediction accuracies globally for the damage mechanics approach.

Effect of interaction of refractories with Ni-based superalloy on inclusions during vacuum induction melting

This study documents laboratory-scale observation of the interactions between the Ni-based superalloy FGH4096 and refractories. Three different crucibles were tested—MgO, Al2O3, and MgO-spinel. We studied the variations in the compositions of the inclusions and the alloy-crucible interface with the reaction time using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The results showed that the MgO and MgO-spinel crucibles form MgO-containing inclusions (Al-Mg oxides and Al-Mg-Ti oxides), whereas the inclusions formed when using the Al2O3 crucible are Al2O3 and Al-Ti oxides. We observed a new MgAl2O4 phase at the inner wall of the MgO crucible, with the alloy melted in the MgO crucible exhibiting fewer inclusions. No new phase occurred at the inner wall of the Al2O3 crucible. We discuss the mechanism of interaction between the refractories and the Ni-based superalloy. Physical erosion was found to predominate in the Al2O3 crucible, whereas dissolution and chemical reactions dominated in the MgO crucible. No reaction was observed between three crucibles and the Ti of the melt although the Ti content (3.8wt%) was higher than that of Al (2.1wt%).