Nickel-based Superalloy Powder Research Articles

Simultaneous enhancement of elevated temperature strength and ductility in additive-manufactured nickel-based superalloy via doping Y2O3 nanoparticles

In this work, we coated a layer of Y2O3 particles in Hastelloy X (HX) nickel-based superalloy powder by in situ chemical method and combined with laser powder bed fusion (LPBF) technology to develop a high-performance Y2O3-doping alloy, designated as Y-HX. The results show that the doping of Y2O3 particles prevents crack formation during the printing process and reduces solute segregation at cell and grain boundaries by increasing the viscosity of the molten pool. The doping of Y2O3 particles to the printed Y-HX alloy enhances grain boundary characteristics, transforming coarse sheet-like carbides into finely dispersed granular carbides at the boundaries during subsequent heat treatment. Additionally, doping with Y2O3 particles increases the recrystallization activation energy of the Y-HX alloy from 149.4 to 278.8 kJ mol–1. At 750 °C, the Y-HX alloy exhibits an ultimate tensile strength of 619 ± 2 MPa and an elongation of 52 % ± 2 %, along with an ultimate tensile strength of 325 ± 3 MPa and an elongation of 47 % ± 2 % at 900 °C. Our work provides a promising way to develop additive-manufactured superalloys with exceptional thermal stability and remarkable high-temperature mechanical properties.

Partial transient liquid phase bonding of misoriented single crystal superalloys via a mixture of nickel-chromium-tantalum and nickel-based superalloy powders

In order to realize high-performance joining of misoriented single crystal superalloys, a rapid partial transient liquid phase (PTLP) bonding process was proposed in this work. By using a mixture of nickel-chromium-tantalum (Ni-Cr-Ta) and nickel-based superalloy (FGH4096) powders as filler material, high-performance PTLP joints of misoriented single crystal superalloys (DD5) were obtained. The joint bonded at 1260 °C consisted of an epitaxial growth zone (EGZ) containing γ/γ' derived from DD5 and a polycrystalline zone (PCZ) located at the center of the joining layer, in which tantalum dichromide (TaCr2) and tantalum carbide (TaC) existed at the grain boundaries. As the Ni-Cr-Ta content increased, the voids in the joints decreased while the Ta-rich compounds increased and the grain size decreased. With the holding time increasing, the Ta-rich compounds decreased while the grain size increasing. The optimum process was 60 min at 1260℃ with the Ni-Cr-Ta content of 60 wt%. Due to the specially designed filler metal without boron (B) and silicon (Si) elements, there was no low-melting eutectic or compound in the PTLP joints, which ensure the joint with high-performance at elevated temperatures. Compared with transient liquid phase (TLP) bonding process, the PTLP bonding process achieved rapid isothermal solidification with the help of the short-distance and large-area diffusion mechanism between the Ni-Cr-Ta (liquid phase) and FGH4096/DD5. The presence of PCZ prevented the formation of penetrating grain boundary, and thus eliminated the effect of misorientation between two base metals on the performance of the joints. The tensile strengths at room temperature and 980 °C of the PTLP joints which obtained with the optimum process were 926.05 MPa and 681.96 MPa, respectively, up to 90.32% and 92.25% of the base metal. The PTLP bonding process proposed in the present work may provide a high efficiency approach to achieve high-performance single crystal superalloy joint, which is also gifted a major advantage of B/Si-free.

Effect of humid atmosphere on the microstructure and mechanical properties of a PM Ni-based superalloy: From powders to bulk alloys

Understanding the effects of powder storage condition is helpful to improve the mechanical properties of powder metallurgy (PM) and additive manufacturing (AM) superalloys. In this work, nickel-based FGH96 superalloy powders were stored under ambient air (relatively dry) and humid (25 °C/95%RH and 90 °C/95%RH) atmospheres and then consolidated by hot isostatic pressing (HIP). The powders and corresponding bulk alloys were characterized by XPS, OM, FESEM, EPMA, EBSD, and TEM. The results indicate that the humid atmosphere mainly affects the gas content, chemical state, and oxide/hydroxide layer thickness of powders, leading to highest values of oxygen content (~450 ppm), hydrogen content (~25 ppm), and NiO/Ni(OH)2 thickness (~13.9 nm) for 90 °C/95%RH powders, and further forms much severe prior particle boundaries (PPBs) defects in HIPed alloy. The detrimental PPBs are composed of large-sized γ', MC carbides-(Ti, Nb)C, Al2O3 and ZrO2 oxides. In comparison with ambient air powders compact, the strength of 90 °C/95%RH powders compact is unexpectedly increased, which is due to the internal grains pinned inside the original particle boundaries by higher severity degree of PPBs, and leads to the increase of grain boundary strength. Meanwhile, the ductility is significantly decreased, because the fracture mode transited from ductile transgranular fracture to brittle inter-particle debonding. The study reveals the relationship between humid atmosphere and properties of both powders and bulk alloys, and intends to obtain good combination of strength and ductility for PM Ni-based superalloys.

Comparative Study of Nickel-Based Super-Alloy Powders Atomized by Argon and Nitrogen

Comparative Study of Nickel-Based Super-Alloy Powders Atomized by Argon and Nitrogen

Research on Geometrical Features of FGH95 Nickel-based Superalloy by Coaxial Single Track Laser Cladding

FGH95 nickel-base superalloy powders were coated on GH4169 nickel-base superalloy by coaxial single-track laser cladding. The effects of process parameters on cladding width and cladding height were studied based on the three-factor and three-level orthogonal experiment. The experimental results are nonlinearly fitted by MATLAB software, and the mathematical model between the geometrical features and the process parameters is set up. The results show that the effect factor on the clad width is in the order of first power, followed by feeding rate, and scanning speed. However, the influence on the clad height is in the order of first scanning speed, followed by laser power, and feeding rate. The average prediction error of cladding width and height is less than 7%. The model can accurately forecast the geometrical features of the cladding coating and can offer some references for the optimization of laser cladding.

Oxidation Characteristics of Nickel-Based Superalloy Powders Exposed at Ambient Condition

The existence of adsorbed oxygen and oxides on the surface of initial powders has serious effects on the microstructure and mechanical properties of the powder metallurgy alloys. However, the powder surface is inevitably oxidized immediately after the powder preparation. In this work, the oxidation characteristics for the argon atomized powders of a Ni-based superalloy containing Cr, Co, W, Mo, Nb, Ti and Al after exposure at ambient condition for various time were investigated in detail. It is found that various gases can be absorbed on the powder surface, but most of them can be removed by low temperature (<151.5 °C) outgassing procedure. The thermodynamic calculation shows that the oxidation reaction occurs firstly with the alloying elements rather than Ni matrix, whether at room temperature or elevated temperature. The kinetic measurement indicates that the oxygen content on the powder surface approaches a saturation value after 24 h exposure and remains almost stable after 720 h. The oxygen content increases with the decrease of particle size after exposure. X-ray photoelectron spectroscopy characterized that, except the formed oxides, adsorbed oxygen also exists on the powder surface of the as-atomized initial fine powders with particle size <30 μm and the powders with size >18.7 μm after exposure, which may be caused by the internal stress and surface energy of the initial atomized powder. All alloying elements except Ti can form stable oxides directly on the powder surface. For the element of Ti, the metastable TiO forms on the initial powder surface after preparation and it transforms into stable TiO2 or Ti2O3 during exposure. The results provide a deep understanding of absorbed gases and oxide on the surface of powders under treatment and possible desorption approach.

Impact of Atomization Pressure on the Particle Size of Nickel-Based Superalloy Powders by Numerical Simulation.

Vacuum induction melting gas atomization (VIGA) has evolved as an important production technique of superalloy powders used in additive manufacturing. However, the development of powder preparation techniques is limited because the crushing process of gas-atomized metal melt is difficult to characterize by conventional experimental methods. Herein, we report the application of computational fluid dynamics to simulate the breaking behavior of droplets in the process of preparing nickel-based superalloy powders by VIGA, as well as the results on the effect of gas pressure on the atomization process and powder particle size distribution of metal melt. In the process of primary atomization, the crushing morphology of superalloy melt shows an alternate transformation of umbrella shapes and inverted mushroom cloud shapes, and with the increase in atomization pressure, the disorder of the two-phase flow field increases, which is conducive to sufficient breakage of the melt. Most importantly, in the process of secondary atomization and with the increasing atomization pressure, the particle size distribution becomes narrower, the median particle diameter and average particle size decrease, and the decreasing trend of the particle size increases gradually. The simulation results are compliant with the performed nickel-based superalloy powder preparation tests. This study provides insight into the production and process optimization of superalloy powder prepared by the VIGA method.

Influence of standoff distance and laser defocusing distance on direct laser metal deposition of a nickel-based superalloy

The direct laser metal deposition (DLMD) is an additive manufacturing technology, based on laser cladding, which focuses mainly on 3D manufacturing applications. DLMD allows the production of thin-walled components by overlaying single-track depositions. Several issues can affect the deposition process and compromise the flatness of the surface on which subsequent tracks will be deposited. This work focused on deposition troubles simulated by means of a designed variation of the standoff distance and the laser defocusing distance. The effects of these two important process parameters on the deposition process were investigated. The experimental tests were performed by depositing a nickel-based superalloy powder on AISI 304 stainless steel plates through a coaxial nozzle. The work was carried out using an ytterbium fiber laser source and a deposition head equipped with an advanced and innovative motorized optics system. This allows the decoupled variation of the laser defocusing distance and consequently the laser spot size on the substrate surface with respect to the standoff distance. Results showed an influence of standoff distance and laser defocusing distance on the geometrical characteristics of the clad, such as clad width, clad height, penetration depth, and dilution. An experimental setup consisting of a light coaxial to the powder flow and a laterally positioned camera was designed to investigate the spatial powder distribution. Moreover, an analytical model for the powder distribution and clad width were proposed and validated. The analysis of variance (ANOVA) with a general linear model was also employed to describe the results.

Laser powder bed fusion and post processing of alloy 22

A nickel-based superalloy (Alloy 22) powder was gas atomized to be used as a feedstock in the Laser Powder Bed Fusion (L-PBF) process. In this study, laser power and scanning speed were optimized to manufacture parts with the highest density. Samples manufactured with power of 150 W and scanning speed of 200 mm/s showed the highest relative density of 99.6%. Additionally, for a higher production rate, samples were manufactured with power of 225 W and scanning speed of 1200 mm/s. Additively manufactured parts were post processed by conducting solution heat treatment, hot isostatic pressing (HIP), and HIP+solution heat treatments. HIP effectively eliminated most of the defects in the as-printed samples, but Mo-rich ultrafine precipitates were observed in HIP processed samples. Solution heat treatment led to full recrystallization and the total dissolution of the Mo-rich second phases into the Ni matrix. The role of solution treatment on enhancing ductility of HIPPed sample was found significant. Sample manufactured at 150 W and 200 mm/s followed by HIP and solution treatment showed similar mechanical properties to wrought Alloy 22.

Characterization of particle shape of nickel-based superalloy powders using image processing techniques

In order to achieve quality control of nickel-based superalloy powders, it is necessary to quantitatively characterize the particle shape. This paper introduces how the image processing techniques were used to quantitatively evaluate the particle shape and its distribution. Firstly, the appropriate edge extraction operator was determined by multiple comparisons of processing effect of six edge extraction operators. It was found that the morphological processing technology, being similar to the watershed algorithm, demonstrated strong applicability for edge extraction. Secondly, eight shape descriptors were calculated and verified by standard graphics to ensure the accuracy of image processing algorithms. By monitoring single particle, it was found that the image processing algorithms can accurately distinguish the spherical particle from the satellite particle, and the shape descriptors change in an obvious manner. Finally, the probability density distributions of eight shape descriptors were demonstrated and compared. The results show that the image processing techniques can effectively characterize the particle shape and its distribution, which provides a methodological basis for the characterization of superalloy powders and theoretical guidance for the adjustment of atomization process. By mathematically fitting, a relationship was found between the mean shape descriptors and the angle of repose (AOR). Therefore, the method can be used to obtain the average value of shape descriptors, and then establish the relationship between the average shape descriptors and powder flowability, which can predict the process performance of powder and the performance during 3D printing.

Deposition of Nickel-Based Superalloy Claddings on Low Alloy Structural Steel by Direct Laser Deposition

In this study, direct laser deposition (DLD) of nickel-based superalloy powders (Inconel 625) on structural steel (42CrMo4) was analysed. Cladding layers were produced by varying the main processing conditions: laser power, scanning speed, feed rate, and preheating. The processing window was established based on conditions that assured deposited layers without significant structural defects and a dilution between 15 and 30%. Scanning electron microscopy, energy dispersive spectroscopy, and electron backscatter diffraction were performed for microstructural characterisation. The Vickers hardness test was used to analyse the mechanical response of the optimised cladding layers. The results highlight the influence of preheating on the microstructure and mechanical responses, particularly in the heat-affected zone. Substrate preheating to 300 °C has a strong effect on the cladding/substrate interface region, affecting the microstructure and the hardness distribution. Preheating also reduced the formation of the deleterious Laves phase in the cladding and altered the martensite microstructure in the heat-affected zone, with a substantial decrease in hardness.

The role of shot peening on liquation cracking in laser cladding of K447A nickel superalloy powders over its non-weldable cast structure

A novel strategy with shot peening (SP) pretreatment was proposed to prevent heat-affected zone liquation cracking during laser cladding of K447A nickel-based superalloy powder over its non-weldable cast structure. High dislocation density accumulated by the SP pretreatment drove the high-temperature zone of the heat-affected zone to recrystallize during the laser cladding process, as a result, the coarse columnar grain with an average dendrite stem and secondary dendrite arm width of 102.67 ± 4.24 μm was transformed into a fine equiaxed grain with an average size of 10 μm. SP could effectively inhibit liquation cracking. With the increase of SP duration, the tendency of the liquation cracking reduced. Compared with no SP BM, the total length of the liquation cracking in the longitudinal section of the laser cladding sample reduced from 1649 μm to 248.8 μm. In-situ experimental observation and process simulation were opted to study the evolution behavior of recrystallization. Recrystallization firstly developed in the region of the intergranular zone at ~1214 °C and gradually the base metal recrystallized between 1232 and 1242 °C except for the region with γ/γ′ eutectic structure. As the temperature rose further, a fine liquid film network developed in the range of 1260–1267 °C. Finally, the stress and strain-based criteria were exercised to evaluate the crack susceptibility of the liquid film. The fine liquid film network adequately constrained the liquation cracking by reducing the driving stress applied to the liquid film as well as by lowering the pressure drop of the liquid film caused by driving strain.

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.

In situ monitoring of direct laser metal deposition of a nickel-based superalloy using infrared thermography

Metal additive manufacturing is growing its impact on high-tech industrial sectors so far. The capability to recover and repair worn components, with costs that are gradually more affordable, is giving a boost to the development of these technologies. The direct laser metal deposition (DLMD) technology is taking a leading role in this domain. Manufacturing components, with high mechanical properties, require a careful process design and a continuous control. The monitoring of the thermal field thus assumes a crucial role in processes in which high-power sources are involved. Thermal treatments influence the microstructure, morphology and the grain size of the depositions indeed, which determine high-performance standards. In this work, an ytterbium fibre laser source was used to build single-track depositions of a nickel-based superalloy powder on a substrate of the same material. Temperature field monitoring was performed using a high-frequency (100 Hz) IR thermal camera, allowing an accurate monitoring of peeks temperature, thermal cycles and thermal gradients. Thermal data and process parameters were compared with metallographic analysis to capture the relation between the geometrical and microstructural characteristics of clads. The study focuses on the influence of the powder feed rate and energy density on thermal parameters. An innovative approach to the solidification map method, commonly implemented with numerical simulations of welding and additive manufacturing, has been successfully applied to experimental data giving results consistent with the literature.

Phenomenological modelling of direct laser metal deposition for single tracks

The direct laser metal deposition (DLMD) is an emerging technology in the additive manufacturing field, which is becoming widely adopted in several industrial applications. The optimal selection of process parameters is critical, given the cost of physical tests and the requirement for high-quality manufacturing. Correct forecasting of process parameters using predictive physical models is a viable alternative to shorten the setup phase and improve the sustainability. In this paper, firstly, several models available so far in the literature are reviewed, and then, a general model is derived to evaluate the best combination of process parameters in single track operations. The proposed phenomenological model allows simple forecasting based on three main process parameters: laser power (P), powder feed rate (F) and translation speed (V). An innovative approach is proposed to generalise the model and overcome the “try and error” method used in numerous works to determine the exponents (α, β, γ). The model was designed considering the physical parameters which best explained the variability of the exponents: the thermal diffusivity of the base material and the density of the powder material. The best ranges of the process parameter exponents are suggested to establish the most suitable combined parameters for the prediction of geometric characteristics of the clad. The potentialities of the model are proven based on an experimental case performed over a commercially available Nickel-based super-alloy powder. New formulations are recommended, which are in good agreement with the behaviours defined in the literature, as shown in the test case.

Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling.

Analysis of Porosities in Nickel-Based Superalloys Powders

In this paper, we investigated the porosities and their formation mechanism in the nickel-base superalloy powders, which were prepared by electrode induction melting argon atomization and divided into four particle size ranges: d＜60μm, 60μm≤d＜120μm,120μm≤d＜180μm, and d≥180μm. Firstly, the distribution of porosities in the powder were observed by Zeiss electron microscope. Secondly, the number and size of porosities in different particle size range were counted. Finally, the formation mechanism of the porosity was analyzed in details. The results show that the porosities are mainly distributed at the edge and center of the powders, and the size increases with the increase of the powder size. the porosity varies from a few microns to dozens of microns. the biggest pore size in the powder is approximately 120μm.There are no obviously porosities found in powders when their diameters are less than 60μm. While the porosity appeared as the powder diameter is more than 60μm. In addition, the proportion of the powder with porosities also increases with increasing powder size. When powders size is more than 180μm, the proportion of the powder with porosities reached about 91%. In general, the formation of porosities in powders are mainly ascribed to two reasons. the first one is a portion of argon was enclosed by the metal films during the spheroidization; the second one is the metal droplets uneven shrinkage during the solidification.

Microstructure evolution and formation mechanism of a crack-free nickel-based superalloy fabricated by laser engineered net shaping

In this study, the crack-free parts have been successfully fabricated by Laser Engineered Net Shaping (LENS) based on a Ni-Co-Cr-Al-Ti-Fe-Nb-Mo nickel-based superalloy powder, and its microstructure evolution and formation mechanism were investigated. The results revealed that the microstructure along the deposition direction is stray dendrites with disordered growth direction in the bottom two layers, epitaxially growing elongated columnar dendrites dominating the whole cross-section of the as-deposited, fine equiaxed grains in the upper of the last layer, respectively. Moreover, it can be found that although the as-deposited layer contains many high angle grain boundaries, no cracks can be observed. According to the chemical composition and phase analyses, it can be concluded that crack-free microstructure is ascribed to the homogeneous distribution of elements and the formation of single-phase γ in the as-deposited. The effect of 1.34 Al, 1.37 Ti, 0.41 Nb, and 0.50 Mo (wt%) on the formation of low melting point substances is discussed. The room-temperature ultimate tensile strength is about 807 MPa, while the high-temperature ultimate tensile strength is 654 MPa.

Investigation of the IN-738 superalloy microstructure and mechanical properties for the manufacturing of gas turbine engine nozzle guide vane by selective laser melting

Recently, there has been a significant amount of interest in additive manufacturing of aerospace parts from heat-resistant alloys due to the ability of producing more complex shapes parts in comparison with conventional technologies. This paper describes the research results of selective laser melting (SLM) of nickel-based IN-738 superalloy powder. The main research results of studies are microstructure investigation after SLM, hot isostatic pressing (HIP), and heat treatment on the cylindrical samples. The article presents results of short-term strength tests of cylindrical specimens at room temperature and 900 °C, and long-term strength. The research studies the influence of HIP on stress-strain behavior of specimens, which are built at different angles: 0, 45, 90°. The tensile strength of vertical samples is increasing by 60 MPa, at 45° is decreasing by 75 MPa, and at 0° is increasing by 139 MPa compared with the initial state. The developing IN-738 selective laser melting technology is approving by fabrication of GTE nozzle guide vane.

Status and development of powder metallurgy nickel-based disk superalloys

AbstractNickel-based superalloys are crucial materials for the development of aero-engine components, since their excellent properties can meet the demands of turbine disks. For the formation of such alloys, powder metallurgy is considered as the ideal method, due to the resultant uniform composition and structure, fine grain, high yield strength, and good fatigue performance. This paper provides a critical review of the development of powder metallurgy nickel-based disk superalloys, their composition, as well as the evolution of the nickel-based superalloys' microstructure in the past few decades. Moreover, the influence of various elements on the material properties and the three major defects of powder metallurgy superalloys are reviewed. The analysis indicates that these defects may be directly or indirectly caused by the quality of the powder. Therefore, the innovative powder techniques of electrode induction melting gas atomization and spark plasma discharge spheroidization are presented, in order to prepare ceramic-free and superfine nickel-based superalloy powders. The powders prepared by electrode induction melting gas atomization and spark plasma discharge spheroidization have been found to be beneficial in improving the properties of powder metallurgy nickel-based disk superalloys.