Superalloy Inconel Research Articles

Microstructure and mechanical properties at elevated temperatures of as-built and heat-treated Inconel 718 produced through laser powder bed fusion processes

Purpose This study aims to better understand the influence of various post-treatments on the microstructure and mechanical properties of additively manufactured parts for critical applications. Design/methodology/approach In this study, Laser Powder Bed Fusion (LPBF) fabricated Inconel 718 (IN718) samples were subjected to various heat treatments, namely homogenization, solution heat treatment and double aging, to investigate their influence on the microstructure, mechanical properties and fracture mechanism at an elevated temperature of 650 °C. Homogenization treatment was performed at 1080 °C for durations ranging from 1–8 h. The solution treatment temperature varied from 980 °C to 1140 °C for 1 h, followed by double aging treatment. Findings At 650 °C, the as-built sample showed the minimum strength but demonstrated the maximum elongation to failure compared to the heat-treated samples. The strength of the IN718 superalloy increased by 20.26% to 34.81%, while ductility significantly reduced by 65.26% to 72.89% after various heat treatments compared to the as-built state. This change is attributed to the enhancement in grain boundary strength resulting from the pinning effect of the intergranular δ-phase. Originality/value The study observed that the variations in the fracture mechanism of LPBF fabricated IN718 depend on the duration and temperature of heat treatment. This research provides a thorough overview of the high-temperature mechanical properties of LPBF fabricated IN718 subjected to different homogenization times and solution treatment temperatures, correlating these effects to the corresponding changes in microstructure.

Crack inhibition of non-weldable Inconel 738 alloy in ultrasound-assisted laser directed energy deposition

High crack susceptibility restricts reliable laser directed energy deposition (L-DED) and repair of γ′ precipitation-strengthened Nickel-based superalloy Inconel 738. This study investigates the effects of ultrasonic assistance synergized with reduced heat input on microstructures and mechanical properties of the L-DED Inconel 738 alloy. The mechanisms of inhibiting crack initiation and propagation of are studied. Results demonstrate that grain refinement and suppression of epitaxial growth of L-DED Inconel 738 alloy are achieved by ultrasonic assistance. The microstructures evolve from coarse columnar grains (∼82.6 μm) to a bimodal structure of shorter columnar and finer equiaxed grains (∼40 μm). The morphologies of MC carbides transform from a continuous distribution of larger sizes to a discrete distribution of finer sizes. Cracks are highly susceptible to propagate along the long, straight grain boundaries of the coarse columnar grains in deposit without ultrasonic assistance, while tortuous grain boundaries induced by ultrasonic assistance enhance crack resistance and inhibit crack propagation. As a result, crack-free L-DED Inconel 738 alloys with excellent ultimate tensile strength of 1478–1490 MPa and elongation of 15.3–17.8 % are achieved via ultrasonic assistance and are significantly higher than cast Inconel 738 (945 MPa, 7.5 %), indicating the feasibility of utilizing ultrasonic assistance to inhibit cracking in L-DED Inconel 738 alloy.

Tool wear and surface topography shaping after TPl multi-axis milling of Ni-based superalloy of the torus milling cutter using the strategy of adaptive change of the active cutting edge segment

This paper details an investigation of the process of multi-axis high-speed milling of the surface of Ni-based superalloy Inconel 718 parts used the torus milling cutter. Presents the issues and development of the own models of the multi-axis milling in a kinematic variant of the TPl (Tool Pulling) type in terms of tool wear. Because the cutting speed in finishing high-speed milling is the parameter with the greatest influence on tool life and surface roughness, the dependence of the variation of the cutting speed at the torus part of the cutter was first determined as a function of the distance from the tip point along the axial direction and the geometrical parameters of the torus. A model for multi-axis milling of the TPl type was developed and described in a simplified manner, which forms the basis of an own ACACES strategy for segmenting the cutting edge of a torus milling cutter. The adaptation of a given segment to the active cutting edge during machining was based on tool wear and kinematic roughness criteria. Simulations and experiments of multi-axis milling were carried out, and the necessary analyses were performed. In this paper, in addition to testing an in-house strategy for multi-axis milling used the torus milling cutter, the problem of defining the cutting speed in relation to the nominal diameter and effective diameter of the tool is investigated for the first time. In addition, wear measurement was carried out using an in-house methodology, taking full advantage of the capabilities of the multi-axis machining centre, which significantly reduced measurement time and positively influenced the effectiveness of tool life assessment. Using the own ACACES strategy of the multi-axis surface milling of parts made of Ni-based superalloys, an almost fourfold increase in torus cutter life was achieved compared to the conventional strategy, while maintaining the required machined surface roughness parameters.

Spinel oxide enables high-temperature self-lubrication in superalloys

The ability to lubricate and resist wear at temperatures above 600 °C in an oxidative environment remains a significant challenge for metals due to their high-temperature softening, oxidation, and rapid degradation of traditional solid lubricants. Herein, we demonstrate that high-temperature lubricity can be achieved with coefficients of friction (COF) as low as 0.10-0.32 at 600-900 °C by tailoring surface oxidation in additively-manufactured Inconel superalloy. By integrating high-temperature tribological testing, advanced materials characterization, and computations, we show that the formation of spinel-based oxide layers on superalloy promotes sustained self-lubrication due to their lower shear strength and more negative formation and cohesive energy compared to other surface oxides. A reversible phase transformation between the cubic and tetragonal/monoclinic spinel was driven by stress and temperature during high temperature wear. To span Ni- and Cr-based ternary oxide compositional spaces for which little high-temperature COF data exist, we develop a computational design method to predict the lubricity of oxides, incorporating thermodynamics and density functional theory computations. Our finding demonstrates that spinel oxide can exhibit low COF values at temperatures much higher than conventional solid lubricants with 2D layered or Magnéli structures, suggesting a promising design strategy for self-lubricating high-temperature alloys.

Quantitative characterization and mapping relationship between microstructure, composition, orientation, and mechanical properties of nickel-based superalloy Inconel 718

Abstract This study explores the relationship between the microstructure, composition, orientation, and mechanical properties of the nickel-based superalloy Inconel 718. Employing scanning electron microscopy (SEM), electron microprobe (EMP), nano-indentation, and other techniques, the study observes the structure, confirms the composition, determines orientation, and tests mechanical properties in specific micro-zones. Findings reveal a uniform grain distribution in Inconel 718, with a minor δ-phase presence at grain boundaries. There is a notable enrichment of Nb at the grain boundaries, whereas Fe and Cr levels are lower at these boundaries compared to the grain interiors. The indentation hardness and modulus at the grain boundaries are markedly higher than those within the grains. Moreover, grains with different orientations exhibit diverse microscale mechanical properties, such as hardness and elastic modulus. This research establishes a quantitative mapping relationship between the microstructure, composition, orientation, and mechanical properties of Inconel 718, providing a foundation for future multiscale (micro to macro) mechanical property investigations.

Impact of surface micro-dimples on machinability of Inconel 718 alloy

Used extensively in the aerospace industry, the superalloy Inconel 718 is deemed hard to cut because of its rapid work hardening, low thermal conductivity, and excessive shear strength. Proposed here is patterning Inconel 718 with micro-dimples using electrochemical machining and electrical discharge machining prior to conventional cutting. How this micro-dimple patterning influences the machinability of Inconel 718 is analyzed via orthogonal cutting experiments, and with the optimal parameters, the cutting temperature is lowered by 45.5% and the cutting forces are reduced significantly, i.e., the tangential cutting force and the thrust force are reduced by 61.1% and 47.1%, respectively. The predictions of finite-element simulations agree with experimental observations, and it is concluded that dimpled Inconel 718 offers significantly improved tool wear and surface integrity.

Fabrication and tribological properties of WC-reinforced Inconel X-750 alloy at elevated temperature

The nickel-based superalloy Inconel X-750, valued for its exceptional mechanical properties, is used in gas-lubricated foil bearings. However, prolonged exposure to high temperatures can lead to wear failure, affecting their service life. Here, we propose a novel method named ultrasonic strengthening grinding (USG) that applies a WC coating on Inconel X-750. Friction tests reveal that at 600 °C, the friction coefficient and wear rate of the USG-treated samples were reduced by 30.2 % and 79.7 %, respectively. Detailed experiments and analysis demonstrate that the surface grain refinement and the WC-rich coating cooperate to improve the high-temperature tribological properties. These findings offer new insights into the design of wear-resistant coatings for enhancing the high-temperature tribological performance of Inconel X-750 Alloy.

Study on the effect of multi-walled carbon nanotube (MWCNT) addition on the microstructure, mechanical and cutting performance of silicon nitride ceramic tools

Silicon nitride (Si3N4) ceramics incorporated with 0–1.5 wt% multi-walled carbon nanotubes (MWCNTs) were synthesised by the spark plasma sintering technology. The effects of the low-content MWCNT addition on the microstructure, density, mechanical properties and cutting performance against nickel-based superalloys (Inconel 718) of the Si3N4/MWCNTs ceramic tools were investigated by the X-ray diffraction, field emission scanning electron microscope, Vickers hardness tester, universal tester, ball-on-disk reciprocating tester and cutting experiment. The results showed that the incorporation of MWCNTs enhanced the conversion rate of Si3N4 structure from the polar α form to the polar β form and reduced the friction coefficient of the composite. However, due to the agglomeration of MWCNTs, the hardness and fracture toughness of the Si3N4/MWCNTs ceramics were observed to decrease by 5.03 % and 13.97 %, respectively, compared to pure Si3N4 ceramics. A higher addition rate of the MWCNT would further accelerate tool wear and significantly diminish tool life. The predominant wear mechanism of the Si3N4/MWCNTs ceramic tools when milling Inconel 718 was groove-shape wear and adhesive wear.

Pulsed Unipolar-Polarisation Plasma Electrolytic Polishing of Ni-Based Superalloys: A Proof of Conception

The enhanced performance of aerospace equipment drives parts development towards integration, complexity, and structural optimization. This advancement promotes metal near-net fabrication technologies like wire electrical discharge machining (WEDM) and 3D printing. However, the high initial surface roughness from WEDM or 3D printing poses significant challenges for the high-performance surface finishing required. To effectively reduce the surface roughness of the workpieces with high initial surface roughness, this paper proposes pulsed unipolar-polarisation plasma electrolytic polishing (PUP-PEP). The study examined the material removal mechanisms and surface polishing quality of PUP-PEP. This technique combines the high current density and material removal rate of the electrolytic polishing mode with the superior surface polishing quality of PEP through voltage waveform modulation. For an Inconel-718 superalloy part fabricated by WEDM, PUP-PEP reduced surface roughness from Ra 7.39 μm to Ra 0.27 μm in 6 min under optimal conditions. The roughness decreased from Ra 7.39 μm to Ra 0.78 μm in the first 3 min under pulsed unipolar-polarisation voltage, resulting in a remarkable 233% increase in efficiency compared to that with conventional PEP. Subsequently, the voltage output voltage is transformed into a constant voltage mode, and PEP is continued based on PUP-PEP to finally reduce the workpiece surface roughness value to Ra 0.27 μm. The proposed PUP-PEP technology marks the implementation of ‘polishing’ instead of conventional rough-finish machining processes, presenting a new approach to the surface post-processing of metal near-net fabrication technologies.

A Study on Microstructure Evolution and Mechanical Properties of Inconel 690 Subjected to Milling Process

The metallic materials subjected to machining‐induced severe plastic deformation can experience alterations of microstructures and mechanical properties, affecting the functional performances of the components. This study concentrates on the microstructure evolution and mechanical properties analysis of Ni‐based superalloy Inconel 690 under severe plastic deformation induced by milling process. The obtained results show that the microstructures of the machined surface and subsurface exhibit the characteristic of gradient distribution. Plastic behaviors involving dislocation multiplication, grain deformation and refinement, increment of local misorientation and average intragranular misorientation, and transition from high‐angle grain boundaries to low‐angle grain boundaries occur in the superficial layer of the machined workpiece. Both the intensity and the affected depth of plastic deformation for the superficial layer are strengthened following the cutting speed and feed increase, which is attributed to higher cutting forces. No white layer and phase transformation is found under all cutting conditions. In addition, the nanohardness of the superficial layer shows the reduction tendency of hardening effect from the surface to the bulk of the machined workpiece material. With the increase in cutting speed and feed, the affected zones of residual stresses and work hardening below the machined surface become wider.

Thermal Analysis of Micro-Channel Internal Cooling in Cutting Tools: A Machine Learning Approach

The use of coolants for cutting process in metal cutting operations is customary. Turning causes high cutting heat in nickel base super alloy Inconel 718. Nonetheless, it should be acknowledged that although flooding techniques are commonly used in the machining of super alloys, these flood cooling methods have extremely poor efficiencies. Another alternative to increase the cooling capabilities of fluids would be an internal-cooling approach that would enable to lower machining temperatures significantly. The heat dissipation ability in the tool is also greatly influenced by the micro-channel diameter of tool which further causes a significant effect on the coolant outlet velocity. A design of an internal-cooling single point cutting tool with micro channel structures for enhanced coolant heat transfer capability and reduced machining temperature is used for turning Inconel 718 under dry, flooded cooling and internal cooling to study the effects of cooling conditions on cutting force, cutting temperature and surface quality. A regression model is built using the Random Forest (RF) and Support Vector Regression (SVR) methods in machine learning framework. These models were then used to forecast input parameters, such as channel diameter and inlet pressure, which made it easier to obtain output data, such as pressure and maximum velocities at different notches. Eighty percent of the data in the dataset is used to train the model and with the remaining twenty percent set aside for evaluating the model's functionality. When comparing internal-cooling technology to traditional flood cooling, there are clear benefits including increased heat transfer efficiency, which leads to lower cutting temperatures, less cutting force, and better surface quality. More specifically, in the internal-cooling configuration, a direct relationship is shown between rising coolant inlet pressure and falling cutting force and temperature over time. Further highlighting the advantages of this cooling strategy is the relationship between increased intake pressure and decreased surface roughness.

Influence of alloying elements on stacking fault energy in Ni and Ni-based alloy: A first–principles study

The study of the reliability of Ni-based alloys and their performance under high-temperature radiation environments is crucial for nuclear reactor application. The first-principles study has been utilized to investigate the effect of individual alloying elements (Cr, Fe, Mo, Nb) on the stacking fault energy (SFE) of pure Ni in order to understand their specific contribution in the superalloy Inconel 718 (IN718). The result shows that the addition of an alloying element in pure Ni reduces the SFE value with the most significant reduction seen in IN718 alloy. The role of individual alloying element in the reduction of the SFE in IN718 is conjectured. The DOS at EF of all the elemental substitution configurations were also calculated and significant change is seen in the alloy IN718. Such reduction is due to the interrupted distribution of d-electrons and atomic bonding on the stacking fault planes, reflected in the SFE value.

Nitriding Treatments in Nickel–Chromium-Based Superalloy INCONEL 718: A Review

This literature review focuses on the nitriding treatments of nickel-based superalloy UNS N07718, also called INCONEL 718. This alloy was selected due to the relevance of this alloy in the aerospace and oil and gas industries. The referenced studies reflect a clear trend towards improving superalloys through advanced nitriding processes, highlighting plasma nitriding as one of the most promising techniques to enhance alloy 718 against corrosion and wear. The importance of optimizing nitriding parameters, such as temperature and time, is emphasized to increase wear and corrosion resistance while minimizing adverse effects like delamination or the formation of undesirable phases. Finally, the most appropriate alternatives for future research are recommended.

High Temperature Intergranular Oxidation of Nickel Based Superalloy Inconel 718

AbstractIntergranular oxidation (IGO) of the Ni-based superalloy Inconel 718 was studied at 650 °C, 700 °C and 900 °C. The oxidized samples were characterized by X-ray diffraction and scanning electron microscopy. For all the studied temperatures, the external scale was mainly composed of Cr2O3, while the oxides along the grain boundaries were rich in Al and, to a minor extent, Ti. This was consistent with thermodynamic computations. The time evolution of the maximum depth of IGO was found to be parabolic with an apparent activation energy of 164 kJ/mol. The results of this study confirm with three temperatures that IGO kinetics can be described using an extension of the Wagner’s theory of internal oxidation, as recently suggested in the literature at 850 °C. According to this description, the mechanisms controlling the IGO kinetics of Inconel 718 are the aluminum diffusion in the alloy matrix and the oxygen diffusion along the interface between the alloy matrix and the oxidized grain boundary.

Impact of morphological and mechanical components on inconel 625 grinding using common cylindrical grinding wheels

This study used a grinding technique based on Gas Tungsten Arc Welding (GTAW) to create walls composed of Inconel 625 alloy. Mechanical and microstructural (MM) adjustments are structural and mechanical alterations that take place during the additive manufacturing process of Inconel 625 grinding. A thorough examination of the modifications made to the nickel superalloy Inconel 625's (I-625) microstructure was conducted during the grinding process. A circular weave and a stringer bead design were used to construct the wall. Tensile properties and microstructural analyses were assessed for each wall. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the fracture zones of the tensile specimens were examined. The microstructure is mostly composed of equiaxed dendrites, although a unique combination of discontinuous and continuous cellular dendrites can be observed along the cross-section. In tensile testing, circular woven walls performed better than stringer bead walls. The EDS and AFM results show that Ni and Cr make up the majority of the fracture zone, with traces of Nb and Mo. Because there are no lave phases, the fracture mode is ductile. The elemental mapping, which shows the homogenous dispersion of Ni and Cr inside the fracture zone, provides additional evidence in favor of the ductile failure mode. The UTS of the time-consuming TS samples is somewhat higher and exhibits a steadily rising bias in comparison to the specimens with quick TS. The highest level of the UTS sample is 10 %.

Effects of Heat Input and Intertrack Overlap on the Microstructure and Properties of Inconel 686 Weld Overlays.

The objective of this study was to investigate how weld overlays with nickel superalloys are important for the integrity, due the high temperatures and corrosive environments that can be experienced in mineral processing environments, of mining and processing equipment. The Ni-Cr-Mo superalloy Inconel 686 overlays are fabricated through automatic gas metal arc welding with variations in arc voltage and travel speed (i.e., heat input), and they have overlap between adjacent weld tracks for applications in the mining and minerals sector. The impact of variations in the process parameters and the size of the weld overlapping on the dilution, solidification morphology, microsegregation, and microhardness were investigated. Both geometric and chemical composition definitions were used to quantify the extent of the weld dilution. Subsequently, the weld geometry and dilution were correlated with the solidification microstructure and phase transformations. The maximum dilutions were measured to be 13.63% (1/2 overlap, 5.96 kJ·cm-1) and 15.39% (1/3 overlap, 4.77 kJ·cm-1), which shows that less of an overlap increases the dilution level. Scanning electron microscopy and chemical composition analysis revealed that an increase in weld heat input and dilution level led to higher levels of microsegregation for Mo and Cr, as well as the volume fraction of Mo- and Cr-rich phases in the interdendritic/intercellular regions in the overlay layer. Analysis of the weld overlays in the current study revealed strong and unprecedented connections between the weld overlay process conditions, the resultant metallurgy (i.e., dendrite arm spacing, microsegregation, and phase formation), and the hardness of the overlay. It was concluded that the optimal weld overlays in the processing window studied in this investigation were fabricated at mid-level heat inputs (i.e., 4-5 kJ·cm-1) and a 1/2 track overlap.

Experimental study on the preparation of superalloy Inconel718 heat exchanger channels by electrochemical etching method

The nuclear energy field requires the construction of heat exchangers that can withstand sufficiently high temperatures. In this paper, the etching technique was utilized to fabricate heat exchange channels on Inconel718 plates, and experiments were carried out in NaCl-ethylene glycol solution, three temperature conditions of 35 °C, 40 °C, and 45 °C, and two concentrations of electrolyte solutions, 0.5 mol/L and 1 mol/L, with the addition of 50 kHz ultrasound for comparison. The experimental results show that the etching rate can be as high as 2.894 mm/h, and the inter-electrode current is as high as 908 mA. The roughness characterization of the etching channel Ra and Rq measured by the aspheric surface measuring instrument can be as low as 2.416 μm and 2.647 μm, respectively. In the images generated by SEM scanning, it is found that uneven distribution of pits and bumps exist in the etching channel, but their diameters are all less than 0.1 μm, which means the etched channel is acceptably smooth.

X-Ray Diffraction Measurements for Inconel 718 Alloy Elements Created by Incremental Methods

Abstract The work presented is the result of the implementation of diffraction measurements: phase composition and stresses resulting from additive manufacturing process of nickel superalloy Inconel 718 components print. With the help of diffraction methodologies, the key parameters from the point of view of the quality of prints and their strength were determined. The existence of individual phases in the material after printing was demonstrated, and the surface variation of the stress values was presented, showing its dependence on the geometry of the printed part – measurements were made at various points on the surface of samples with different geometries. In addition, the variation of the stress level was shown depending on the distance of the measurement point from the build platform on which the additive manufacturing process was carried out. Components were printed on the surface of a single build plate in order to study the effect of printing differently oriented samples with respect to the platform geometry, as well as the mutual effect of the temperature of samples printed first on the stress state of elements printed in subsequent steps of the procedure, and the effect of the temperature of elements printed later on the rate of temperature decrease, and consequently on the stress state, of elements printed first.

Optimization of superalloy Inconel 718 micro-milling process by combined Taguchi and multi-criteria decision making method

Optimization of superalloy Inconel 718 micro-milling process by combined Taguchi and multi-criteria decision making method

Grain size sensitive modelling of the nonlinear behaviour and fatigue damage of Inconel 718 superalloy

Fatigue tests conducted on various types of materials often exhibit significant scatter, particularly in High Cycle Fatigue tests, attributed to imperfections located at the scale of the microstructure. In metallic materials, one contributor to this dispersion, but not the only one, is the variation in grain size, which cannot be uniform in a structure that has undergone a complex thermal history during its manufacturing process. This paper introduces a new model for predicting the evolution of the grain size in the nickel-based superalloy Inconel® 718 based on the applied thermal load and explores its impact on cyclic elasto-viscoplastic behaviour and fatigue life predictions. The proposed approach is applied to a complete numerical life analysis of an aircraft turbine disk.