Superalloy Components Research Articles

Repairing weld for directly solidified Ni-based superalloy substrates using directed energy deposition of Inconel 625

Although the maintenance, repair, and operation of gas turbine systems are important issues in enhancing the operation of power units, advanced welding repair processes have limitations in depositing directionally solidified Ni-based superalloy components. To overcome this problem, in this study, a directed energy deposition method was applied to repair directionally solidified components using an Inconel 625 alloy. The rapid cooling rate and small laser beam diameter during directed energy deposition minimized the heat-affected zone and duration in the mushy zone, resulting in crack inhibition at the interface. The first laser scan partially melted the substrate and formed a mixing zone with a chemical composition gradient. By combining the chemical compositional gradient and dislocation accumulation due to repetitive solidification, that is, with the melting cycles of directed energy deposition, the thermal stability decreased, resulting in recrystallization and grain growth in the mixing zone. Consequently, the crack-free continuous interface of the partially repaired components did not induce tensile fracture near the interface. Although the mechanical properties of the IN625 deposit layer were degraded by severe heat treatment of the substrate, the results demonstrated that energy deposition with post-heat treatment is an effective approach in repairing Ni-based superalloy components without cracks and inclusion initiation.

Unraveling the mechanics of directional coarsening in single crystal Ni-based superalloys via laser peening: Insights from experimental characterization and microstructural analysis

Surface treatment techniques, like laser peening, enhance the longevity of Ni-based superalloy components by reinforcing their microstructure against surface-initiated damage. Recent findings suggest that these methods may also influence precipitate coarsening behavior at high temperatures, leading to rafting phenomena. This study extensively examined γ’ rafting in single crystal Ni-based superalloy CMSX-4 post laser peening and heat treatment. X-ray diffraction revealed surface compressive stresses (-400 MPa to -800 MPa), transitioning to tensile stresses at greater depths before returning to an unstressed state. Correlation with electron microscopy indicated horizontal coarsening in compressive regions and vertical coarsening in tensile regions due to misfit and residual stresses aiding diffusion. Plastic strain near the LPed surface was measurably increased with lattice misorientation values around 5° before returning to an unstrained state after heat treatment due to rafting-aided recovery and defect reorganization. Secondary γ’ precipitates with a radius of approximately 10 nm occupied γ channels between rafted precipitates, indicating solute element diffusion and supersaturation. Energy dispersive x-ray spectroscopy showed a significant depletion of γ’-forming elements around rafted primary precipitates, highlighting preferential solute diffusion.

Creep-fatigue behavior of thermal barrier coated single-crystal nickel-based superalloys

Thermal barrier coatings (TBCs) reduce the surface temperature of superalloy components but may affect the mechanical properties of the substrate. This study investigates the creep-fatigue behavior of TBC-coated single-crystal superalloys under 1 s, 60 s, and 180 s dwell times. The results indicate that TBCs are beneficial at a 1 s dwell time. The comparison of fracture morphology between coated and uncoated samples showed no change in the failure mechanism due to TBCs under creep-fatigue loading. A creep-fatigue lifetime prediction method based on continuous damage mechanics was also proposed. The predicted results were all within a double dispersion coefficient, showing good agreement with the experimental results. This study provides insights into the durability of TBC-coated single-crystal superalloys under creep-fatigue loading.

Solution temperature influence on CRSS of the secondary γ′ phase in nickel-based superalloy FGH4096

Nickel-based superalloy components are capable of being manufactured by direct hot isostatic pressing (HIP) process with near-net shape and nearly 100 % raw materials utilization. However, they cannot be applied directly, due to the poor mechanical properties of HIPed components. Fortunately, heat treatment can improve the properties via regulating the microstructure. Therefore, the effects of varied solution temperatures on the tensile properties of FGH4096 nickel-based superalloy disk at room and high temperatures were studied. The results revealed that the disk after sub-solution treatment (1130 °C) has higher tensile properties (room temperature yield strength: 1194 MPa, elongation: 15.5 %; high temperature yield strength: 1075 MPa, elongation: 10.5 %). The antiphase boundary (APB) shear stress, Orowan bypass stress, stacking faults (SF) shear stress, and micro-twins (MT) shear stress affected by the size and distribution of the secondary γ′ phase were calculated. The results show that the main strengthening mechanism at room temperature is APB shear, and the APB shear stress of solution treatment at 1190 °C is the highest. The high temperature strengthening mechanism is due to SF shear and MT strengthening effects. The difference in SF shear stress between various solution temperatures is minimal. 1190 °C has a higher MT strengthening effect than 1090 °C. This is consistent with the actual result that the tensile properties at 1190 °C are superior to those at 1090 °C. The conclusion is that the combination of high strength reflected by several critical resolved shear stresses and considerable plasticity reflected by the MT strengthening effect can lead to a comprehensive improvement in alloy tensile properties.

Inconel 740H Prepared by Additive Manufacturing: Microstructure and Mechanical Properties

An Inconel 740H nickel-based alloy was fabricated via wire arc additive manufacturing. The as-welded and heat-treated samples were analyzed to investigate their phase composition, microstructure, crystal structure, and mechanical properties. After heat treatment, the sample exhibited a columnar crystal zone microstructure consisting of a γ matrix + precipitated phase, the remelting zone metallographic structure was a γ matrix + precipitated phase, and the HAZ metallographic structure was a γ matrix + precipitated phase. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to show that the welded sample exhibited many dislocations, a few inclusions, and carbides, nitrides, and γ’ precipitates in its crystal structure. In contrast, the crystal structure of the heat-treated sample exhibited a lower number of dislocations and significantly higher carbide and γ’ precipitate content. Moreover, the mechanical performance of these samples was excellent. This heat-treatment process improved the sample strength by about 200 MPa, leading to better high-temperature mechanical properties. This work is anticipated to offer theoretical and experimental support for using additive manufacturing methods in the manufacturing of nickel-based superalloy components.

High temperature resistant composite adhesive with a remarkable bonding strength in a wide temperature range from 25 °C to 1200 °C

Ensuring a consistently high-strength connection in superalloy components of space shuttles from 25 to 1200 °C is essential yet challenging. To achieve the objective, we developed an adhesive using silicon-boron modified phenolic resin (Si-BPF) as the matrix and inorganic fillers as additives. The continuous Si–O–Si–O–B–O–B skeleton endowed the Si-BPF with excellent thermal stability. In addition, the inorganic fillers inhibited the thermal decomposition of Si-BPF. The generation of low-melting-point glass phases compensated for the defects in the adhesive. Therefore, within the “300–600 °C weak strength interval”, the synergistic effect of Si-BPF and inorganic additives increased the adhesive strength to 17.26 MPa. The formation of intermetallic compounds and the ceramization of the adhesive resulted in a gradual increase in the bonding strength of the composite adhesive with increasing temperature, reaching 34.23 MPa after 1200 °C. The adhesive enabled high-strength bonding for superalloys throughout the wide temperature range, enhancing the application of superalloy components in space shuttles.

Optimizing the SPIF parameters for enhancing microhardness and surface quality in Inconel 625 superalloy components

Inconel 625, a high-strength superalloy with excellent corrosion resistance, discovers wide applications in critical sectors such as aerospace, nuclear, marine, and petrochemical. Single-Point Incremental Forming (SPIF) has appeared as an adequate fabricating process for shaping superalloy components. This work used a systematic approach, implementing Response Surface Methodology (RSM) to explore the influence of SPIF process variables, including tool nose diameter, step size, tool spindle speed, and wall angle, on microhardness (MH) and surface roughness (SR). The experimental results have been analyzed to determine the influencing process variables and their optimal settings, which simultaneously enhance MH and reduce SR of Inconel 625 superalloy truncated cones using analysis of variance and the desirability function analysis approach. The most significant parameters that influence MH and SR were TND and SS, respectively. The development of quadratic models for MH and SR has been completed successfully. The maximum MH of 486.8 HV and minimum SR of 0.432 µm have been obtained at the optimal parametric setting of TND = 15 mm, SS = 0.4 mm, TSS = 900 rpm, and WA = 57.5° with the desirability values for MH, SR and combined are 0.832, 0.933, 0.881 respectively. The F-value of the MH model = 109.18 and SR Model = 78.24 implies that both models are significant and both models have excellent prediction strength. The percentage error between predicted and actual values of MH and SR in confirmation run results is less than 5 %.

Tailoring the microstructure and deformation mechanisms of an additively manufactured nickel-based superalloy by post-aging heat treatment

Laser powder bed fusion (LPBF) is a new method for preparing the nickel-based superalloy hot-end components. This study systematically investigated the microstructural characteristics and mechanical performance of a γ′-strengthened Haynes 230 alloy after solution heat treatment (SHT) and aging heat treatment (AHT). The results indicate that γ′ phases continue to grow as the AHT temperature increases. The volume fraction of γ′ phases increases first and then decreases. The maximum γ′ phase fraction can be achieved at 750 °C. The yield strength (YS) of the AHT samples at 25 °C and 800 °C increases first and then decreases, which is mainly caused by γ′ phase fraction evolution. The SHT+750 sample presents an excellent integration of tensile performance at 25 °C and 800 °C. The primary deformation mode at 25 °C is stacking fault (SF) shearing. The δ precipitation and high-density Lomer-Cottrell (L-C) locks are the primary reasons for the higher work hardening rate of the SHT+850 sample. The transition of the dominant deformation mechanism from SF shearing at 25 °C to Orowan bypassing at 800 °C leads to a dramatic reduction in the YS of the SHT+850 sample.

Temperature dependence tensile behaviors of additively manufactured GH4099 Ni-based superalloy

Additive manufacturing was currently widely used to fabricate complex-shaped superalloy components for extreme environment applications. To replace the components prepared by the traditional approaches, it is crucial to ensure the reliability of AMed superalloys, especially in a thermal-stress environment. In this work, the high-temperature tensile properties of AMed GH4099 nickel-based superalloy are studied to establish the relationships among deformation temperatures, mechanisms, and failure responses. The results show that the deformation mechanisms at 600 °C are mainly cross-slip of the slip bands and arrays, which induced a ductile fracture. Increasing the tensile temperature will cause a shift of the dislocation movement from cross-slip to Orowan looping and stacking fault shearing to dislocation entangles-recovery-recrystallization, and the fracture mode transformed from pure ductile to mixed ductile-brittle. Additionally, the neighboring grain boundaries and the carbide-grain boundary bondings will also change with the temperature, which is further linked to the mediate-temperature brittleness and high-temperature abnormal plasticity of this superalloy.

Enhanced strength and ductility of laser-directed energy deposition repaired IN718 superalloy via a novel tailored heat treatment

High-quality repair of damaged Inconel 718 (IN718) superalloy components can achieve great economic benefits. However, the directly double aging (DA) treatment by industrial standards, yields an inferior ductility on the repaired component than that of the wrought base metal. In this work, wrought IN718 components were repaired by laser-directed energy deposition (LDED), a novel tailored heat treatment (THT) schedule consisting of a short-term low-temperature homogenization, and subsequent DA was subsequently conducted to strengthen the repaired IN718 alloys. The microstructure evolution and mechanical properties of the DA and THT-treated repaired alloys were comparatively investigated. The results indicated that the THT effectively dissolved most of the hard brittle Laves precipitates in the deposition region with only slight coarsening of the grains in the substrate. As compared to the DA sample, the elongation of the THT sample increased remarkably by 88% with only a slight reduction of 19.2 MPa in yield stress. Moreover, the strain distribution of the THT sample was overall more even but then destabilized in a narrow abnormal coarsened grain region caused by the static recrystallization. In general, this study breaks through the limitation of the low ductility of the DA-treated repaired IN718 alloys and provides a promising way to further improve the mechanical properties.

Multiphysics modeling and experimental validation of heat and mass transfer for the vacuum induction melting process

Vacuum induction melting is crucial in casting nickel-based superalloy components, ensuring excellent properties for aero-engine applications. Precise melting temperatures are vital for achieving optimal metallurgical quality before casting. Hence, a multiphysics numerical model is developed to simulate the induction melting process for the Inconel 718 superalloy. The proposed holistic model integrates magnetic fields, induced currents, and heat and momentum transfer phenomena in a single coupled model. A moving mesh approach reproduces the magneto-hydrodynamic behavior of the free surface, simulating the oscillations of the melt. The stabilized deformed surface profile is correlated with experimental measurements, reporting a proper correlation. Then, the flow field and recirculation effect are modeled with a Low Reynolds Number turbulence approach and coupled with the melt convective heat transfer, developing a complete magneto-thermo-hydrodynamic model. In a laboratory-scale vacuum induction melting furnace, transient melting operation variables are characterized and introduced to the numerical model to compute the temperature evolution. An accurate reproduction of the transient melt temperature variations is reported with a relative error of less than 5%. The influence of the crucible thermal insulating capacity is assessed, emissivity dependence evaluated, and melt homogenization is reported at different process stages. This comprehensive numerical and experimental approach offers valuable insights for enhancing vacuum induction melting for Ni-based superalloys.

Freckle prediction model incorporating geometrical effects for Ni-based single-crystal superalloy components

A freckle is a grain defect frequently found in Ni-based single-crystal (SC) superalloy castings resulting in a high scrap rate. A precise prediction of the defect before production is of great importance, especially for the complex-shaped blades and vanes. Without reckoning geometrical effects, the state-of-the-art Rayleigh number (Ra) model fails to accurately forecast the freckle formation positions. In this study, a novel Ra-based mathematical model was proposed. It not only shortened the calculation period and eliminated the physically meaningless at the singularity of the existing model, but also quantified the geometrical effects. Using the model, the Ra at every position of SC castings was calculated, which can be used to evaluate the freckling tendency of the whole casting and locate the specific positions of the freckling risk zone. A freckle-free approximate critical Rayleigh number about 1.5 was determined. A highly consistent and accurate prediction of this model was validated by comparing it with the experiments using complex-shaped SC castings, which suggests its great prospects for industrial application.

A novel prediction model of the freckle defects for single-crystal superalloy blades

The freckle is a typical surface defect formed during the directional solidification of SC (single crystal) components of Ni-based superalloys. It generally appears as a long and narrow trail of equiaxed grains aligned roughly parallel to the direction of gravity, which breaks the integrity. Once appears, the freckle can never be avoided by further treatment. With the turbine blade requirements increasing, the state-of-the-art methods include adding more refractory elements into Ni-based SC superalloys, and the design of blades with a more complex geometric shape make the freckle defects grow easier at the special surface zones. This leads to a significant challenge in controlling grain defect formation in SC blades and vanes for freckles. The current consensus to freckle formation is described as the Rayleigh-Taylor instability (RTI) flow that occurred in the interdendritic zone. During solidification, the compositional segregation (CS) occurred as soon as the solid interface forwarding led to the density changes between the interdendritic melting phases and the residual liquids. For nickel-based superalloys, the enrichment of low-density solutes like Al, and Ti at the interdendritic zone will make the liquid lighter. However, the widely used Ra model can’t correctly match the freckle tendency of the components with complex shapes (called geometrical effects of freckles), especially for blades. Recent reports indicate that the freckles occur at the preferred positions in the casting. In this work, a novel model is designed to quantitatively discuss the geometrical effects on freckle formation and to combine existing Ra number models with solidification models to enable freckle predictions at a smaller scale. The proceeding of this work can make the design of complex blades easier.

Influence of laser shock peening on the joint surface strength of laser cladding repair of GH4169 superalloy component

Laser cladding is an effective method to repair damaged aeroengine blades. However, the heat affected zone of laser cladding reduces the strength of the repaired component. A solution method that strengthening the joint surface by laser shock peening (LSP) before laser cladding was proposed. The process parameters of LSP, such as energy, spot size, overlap ratio, spot path, etc., were determined based on the theoretical analysis; and the specimens were prepared by laser cladding after LSP. The influence of LSP on the strength, hardness and microstructure of the repaired specimens of GH4169 superalloy were researched based on the axial tensile test, metallographic test and hardness test. The results show that the tensile fracture of the specimens which only treated by laser cladding appears on the joint surface; however, the tensile fracture of the specimens treated by both of LSP and laser cladding does not necessarily appear on the joint surface, which indicates that the LSP can effectively improve the strength of the joint surface. The tensile strength of the specimen treated by both of LSP and laser cladding increased by 52.3%, and the elongation of the specimen increased from 0.8% to 9.2%. The metallographic observation showed that the grains of the specimens treated by LSP were refined obviously, and the grain size decreases by 36.5%, which is beneficial improve the strength of the joint surface. The microhardness of the specimen was more uniform and the maximum difference between the two sides of the joint surface decreased by 19.4% when the LSP was added in the repairing process.

Thermal cycle induced solid-state phase evolution in IN718 during additive manufacturing: A physical simulation study

Additive manufacturing (AM) is well-known for its capability to produce and repair intricate parts e.g., Ni-based superalloy components used in aero-engine applications. However, a comprehensive understanding of the microstructure evolution and its link to properties remains elusive, with gaps in the through-process evolution of γ grains, γ', γ'', δ, and other phases in response to thermal cycling. The outstanding challenge here is to observe these transformations dynamically during the 3D printing process, as this usually requires complex in-situ measurements. Here we apply a different approach to systematically reveal the impact of thermal cycling on the solid-state microstructure evolution of IN718, via a combination of physical simulations and thermo-kinetic modelling.We replicate the thermal cycles IN718 experiences during laser directed energy deposition. Solid-state thermal cycles are shown to have minor effects on the grain morphology due to short holding times at critical temperatures, however, they do induce plastic strain accumulation. Significantly, we show how thermal cycling influences the evolution of γ', γ", and δ phases. While high initial peak temperatures inhibit the precipitation of γ', γ", and δ phases, prolonged thermal cycling to gradually decreasing peak temperatures promotes their precipitation. The δ phase forms along grain and twin boundaries, while both γ' and γ" predominantly precipitate in Nb-enriched regions, causing heterogeneities in hardness. Our results underpin the suitability of physical simulations for replicating superalloy AM, highlighting its potential as a tool to advance the current limited understanding of the microstructure and property evolution.

Cracking mechanism of GH5188 alloy during laser powder bed fusion additive manufacturing

GH5188 cobalt-based superalloy components fabricated using laser powder bed fusion (LPBF) additive manufacturing technique possess promising application prospects in aviation industry, but high cracking susceptibility of the alloy remains an important concern. In this paper, cracking mechanism of the LPBFed GH5188 alloy was investigated by analyzing microstructure, fracture surfaces, composition distribution and grain boundary characteristics of the prepared sample. It was found that cracks consistently propagate intergranular at grain boundaries with high misorientation angle. The cracks were confirmed to be solidification cracks, due to the existence of fracture surface with dendritic morphology, as well as the absence of eutectic structure, carbide and precipitated phase. Lanthanum oxide particles were observed in the as-built alloy, which may hamper feeding of molten material at interdendritic region, promoting tearing of liquid films. Hence, it could be suggested that low O and La contents help in avoiding crack formation. This work could provide guidance for fabrication of crack-free LPBFed GH5188 alloy components.

Effect of post-weld heat treatment on heat affected zone microstructure of additively manufactured Inconel 718

Laser powder bed fusion for metals is a method of producing end use components for industrial use. Powder bed fusion machines are relatively small, and are usually used to create only the critical part of the larger assembly. Therefore, L-PBF manufactured parts must be attached to each other for example by welding. The industrial world needs to be able to join the printed superalloy components to the traditionally manufactured components to reach better corrosion, wear and/or heat resistance in selected parts in an assembly. The problem is that there is limited amount of information about the suitable welding parameter values for these applications. This study examines how the standard heat treatment cycles affect to the quality of the weld, and if the post-heat treatment is reducing undesired phases in the heat affected zone. Test has shown that post heat treatment highlights cuboidal shaped niobium rich carbides throughout the material to the heat affected zone grain boundaries.

Effect of heat treatment in dissimilar welds of additively manufactured laser welded 316L-Inconel 718 joints

Laser powder bed fusion for metals is a method of producing end use components for industrial use. Powder bed fusion machines are relatively small and are usually used to create only the critical part of the larger assembly. Therefore, L-PBF manufactured parts must be attached to each other for example by welding. The industrial world needs to be able to join the printed superalloy components to the traditionally manufactured components to reach better corrosion, wear and/or heat resistance. The problem is that there is limited amount of information about the suitable welding parameter values for these applications. This study examines how the standard heat treatment cycles affect to the quality of the weld, and if the post-heat treatment is reducing undesired phases in the heat affected zone. It is also examined how post weld heat treatment for Inconel 718 affects the base material in IN718-316L dissimilar welds. Tests show that post-weld heat treatment has a positive effect on weld hardness on IN718 side. After welding, the hardness drops to 50% of the nominal hardness. When the welds are heat treated according to AMS5662, the hardness values are restored. Tests also show that the heat treatment does not have significant effect on tensile or hardness properties of 316L.

Integrating State of the Art Zirconia Thermal Barriers with Ytterbium Silicate Environmental Barriers for Silicon-Based Ceramic Turbine Components

AbstractAs gas turbine firing temperatures continue to increase for the sake of improved operating efficiency, the material's transition from Ni-based superalloy components toward ceramic matrix composites (CMCs) is concurrently in progress. Due to the complex nature of the turbine operating environment (envisaged ultrahigh temperatures, presence of water vapor, etc.), coating solutions for these CMCs are still on the forefront of design optimizations. Typically, rare-earth (RE) silicate environmental barrier coatings (EBCs) have been utilized to protect the CMCs from impinging water vapor; however, they lack the thermal insulation properties to enable continued use of simple and/or easily accessible bond coat materials (i.e., silicon). Combined thermal-environmental barrier coatings (T-EBCs) are such a multifaceted surface solution. T-EBCs have been considered in the past, but to this point have not been demonstrated to be technologically robust either due to high implementation costs or complex processing. This study utilizes and combines straightforward and well-established processes—such as plasma-sprayed 7 wt.% yttria-stabilized zirconia—to demonstrate the feasibility of MultiLayered T-EBCs comprised of zirconia-based oxides and RE silicate EBCs in a single coating. The results show that despite high thermal mismatch strains, the structures cannot only be deposited, but also in certain circumstances sustain cyclic thermomechanical loading.

Degradation and anisotropy characteristics of fracture toughness in Ni-based superalloys after phase coarsening

The effect of phase coarsening on the crack propagation characteristics and the corresponding fracture toughness in Ni-based superalloys is investigated by integrating with the finite element method simulations and related mechanism analysis, and the relationship between the macroscopic mechanical properties of alloys and the evolution in their microstructures are discussed in detail. Related analysis shows that the fracture toughness of alloys degraded considerably with phase coarsening. Moreover, the fracture toughness also demonstrates an anisotropic characteristic. Specifically, when the volume fraction of precipitate is low or the degree of phase coarsening is relatively low, the fracture toughness of alloys increases first and then decreases gradually along with increasing the loading angle, showing an inverted "V" shape. In comparison, in the case of a higher volume fraction of precipitate or higher degree of phase coarsening, the fracture toughness displays an opposite variation trend, and reaches the minimum value at the loading direction of 45°. The corresponding variation characteristics of fracture toughness are correlated with the evolutions of precipitate shape, size, and distribution within the alloys. The change in the microstructure or that in the loading direction affects remarkably the initiation and growth of micro-voids within the alloys, and it further affects the crack propagation characteristics, resulting in the degradation and anisotropy characteristics of macroscopic fracture toughness in the alloys. Related work is of great significance for the structure design of Ni-based superalloys components, the mechanical property evaluation and the life prediction during their service process.