Superalloy Materials Research Articles

Boride Zone Formation in Transient Liquid Phase Bonding of Pairings of Parent Superalloy Materials with Different Compositions and Grain Structures

Two nickel-base superalloys are joined via transient liquid phase (TLP) bonding with boron as the MPD. Boride formation is observed in the parent materials at some distance from the solid/liquid interface. The boron concentration profile over the joint is measured with glow discharge optical emission spectroscopy (GDOES). Boron concentration peaks are observed corresponding to the boride formation. Boron distribution is discussed on the basis of theoretical predictions in the literature. It is concluded that diffusion of another element is necessary to explain the results with the second element influencing the solubility of boron.

Residual stress measurements in a ferritic steel/In625 superalloy dissimilar metal weldment using neutron diffraction and deep-hole drilling

This paper reports the use of non-invasive and semi-invasive techniques to measure the residual stresses in a large dissimilar weldment. This took the form of a butt weld between two sections of a P92 steel pipe, joined using an In625 welding consumable. Residual stress measurements have been carried out on the 30 mm thick welded pipe using the deep-hole drilling technique to characterise the through wall section residual stress distribution for the weld metal, HAZ and parent material. In addition, neutron diffraction measurements have been carried out within the weld zone. Diffraction patterns presented a high intensity and sharp peaks for the base P92 steel material. However measurements in the weld superalloy material were proven problematic as very weak diffraction patterns were observed. A thorough examination of the weld material suggested that the likely cause of this phenomenon was texture in the weld material created during the solidification phase of the welding procedure. This paper discusses the challenges in the execution and interpretation of the neutron diffraction results and demonstrates that realistic measurements of residual stresses can be achieved, in complex dissimilar metal weldments.

Machining characteristics of Inconel 718 under several cutting conditions based on Taguchi method

Extensive researchers have conducted several experiments in the past for selecting the optimum parameters in machining nickel based alloy – Inconel 718. These experiments conducted so far are dealt with dry machining and flooded coolant machining of nickel alloy Inconel 718. In this research study, the usage of refrigerated coolant is also dealt with and it is compared with dry machining and flooded coolant machining. Cutting speed, feed and depth of cut are considered as the machining parameters. The effectiveness of the refrigerated coolant in machining the heat resistant super alloy material Inconel 718 with respect to these machining parameters are described in this article. The machinability studies parameters were generated with surface roughness and flank wear. The performance of uncoated carbide cutting tool was investigated at various cutting condition under dry, flooded coolant and refrigerated coolant machining. The relationship between the machining parameters and the performance measures were established and using analysis of variance significant machining parameters determined. This article made an attempt to Taguchi optimization technique to study the machinability performances of Inconel 718. Taguchi approach is an efficient and effective experimental method in which a response variable can be optimized, given various control and noise factors, using fewer experiments than a factorial design. Taguchi’s optimization analysis indicates that the factors level, its significance to influence the surface roughness and flank wear for the machining processes. Confirmation tests were conducted at an optimal condition to make a comparison between the experimental results foreseen from the mentioned correlations.

Fatigue behavior of a thin gage Ni alloy after exposure to elevated temperature

Thin gage Ni-base superalloy materials are being targeted for hypersonic applications up to 1100°C. To achieve an optimized system design, standard mechanical behavior data on these materials are needed under a range of loading conditions such as tensile, creep and fatigue at representative temperatures. In order to meet this need, a unique test system developed in-house for determining materials properties of very thin materials was used to determine the fatigue response of a high temperature Ni alloy, both with and without prior exposure to elevated temperatures. This paper examines the effects of elevated temperature exposure on the ductility and fatigue resistance of Haynes 230 with a thickness of 0.127mm.

Metal Injection Molding of Super Elastic Ti Alloy Materials

Ti-10V-2Fe-3Al is one of super elastic Ti alloys, which is the b type Ti alloy and shows the excellent properties such as high specific strength and corrosion resistance. By the combination of Cold working and heat treatment, high strength as same as the steels are obtained. Therefore, Ti-10V-2Fe-3Al is hoped to be a next generation’s spring material. In this study, Metal Injection Molding of super elastic Ti alloy materials are investigated by changing the process parameters such as powder type (mixed elemental or alloy), debinding and sintering conditions, and heat treatment conditions to obtain the excellent super elastic properties. Tensile strength of injection molded Ti-10V-2Fe-3Al compacts sintered at 1250 with mixed elemental powders showed approximately 85% of wrought materials. On the other hand, alloy powder compacts showed about 80% strength of wrought materials. Their super elastic behavior depended strongly on the heat treatment conditions.

A Study of the Weldability of Gamma Prime Hardened Superalloys

Fusion repair processes such as gas tungsten arc welding (GTAW) and laser welding have been introduced for repairing turbine parts made from Ni-based superalloy materials. The weld-repair of turbine parts is well established, however, the inherent susceptibility of ’ hardened superalloys to weld cracking remains an issue and has resulted in repair limitations for highly loaded areas of turbine parts. This study presents a view of the weldability of superalloys taking both, the impact of the weld process and the weld filler selection into consideration. This comprises the interpretation of specific process parameters into physical parameters controlling the weldability and cracking sensitivity such as thermal gradient in the weld pool and solidification speed. Alloy specific parameters of the weld filler material, such as melting point and solidification interval are studied and set in correlation with the solidification parameters during welding.

Transient Liquid Phase Bonding of Pairings of Parent Superalloy Material with Different Composition and Grain Structure

Joining of different nickel-base superalloys could simplify the manufacturing of turbine blades. The used technique of choice is transient liquid phase bonding, which is an established repair technology for high temperature components. Two nickel-base superalloys with distinct composition and grain structure are bonded and the joints are analysed regarding the microstructure. To quantify the mechanical properties of these joints, tensile and short term creep rupture tests were performed at room and elevated temperatures.

Laser Repair Cladding of K418 Ni-Base Superalloy by CoCrNiW Powder

Laser cladding has been taken into consideration for repairing K418 Ni-based superalloy material with CoCrNiW powder.Composition of cladding materials was revealed by energy dispersive X-ray spectrum(EDS). The characteristic microstructure of the cladding layer and interface were investigated by using an optical microscope and scanning electron microscope(SEM).Cladding coatings were obtained for different process parameters, and a detailed study of the effects of these parameters has been carried out by SEM. Metallurgical bonding between the cladding layer and substrate materials was obtained.Effect of heat input on cladding cracking susceptibility has been studied to obtain optimum condition for crack-free welds. Variations in cracking susceptibility as a function of heat input is discussed with reference to metallurgical characteristics of the clads. The corresponding microstructure induced by different heat input was discussed in this paper too.

Intelligent optimization and selection of machining parameters in finish turning and facing of Inconel 718

The heat-resistant super alloy material like Inconel 718 machining is an inevitable and challenging task even in modern manufacturing processes. This paper describes the genetic algorithm coupled with artificial neural network (ANN) as an intelligent optimization technique for machining parameters optimization of Inconel 718. The machining experiments were conducted based on the design of experiments full-factorial type by varying the cutting speed, feed, and depth of cut as machining parameters against the responses of flank wear and surface roughness. The combined effects of cutting speed, feed, and depth of cut on the performance measures of surface roughness and flank wear were investigated by the analysis of variance. Using these experimental data, the mathematical model and ANN model were developed for constraints and fitness function evaluation in the intelligent optimization process. The optimization results were plotted as Pareto optimal front. Optimal machining parameters were obtained from the Pareto front graph. The confirmation experiments were conducted for the optimal machining parameters, and the betterment has been proved.

Test System for Elevated Temperature Characterization of Thin Metallic Sheets

Thin gage Ni-based superalloy materials are being targeted for hypersonic applications up to 1100 °C. To achieve an optimized system design, standard mechanical behavior data on these materials are needed under a range of loading conditions such as tensile, creep, and fatigue at representative temperatures. These tests require direct measurements of displacements on specimens. In order to meet this need, a test system has been developed as part of a comprehensive in-house program to advance state-of-the-art testing capabilities for thin foils and very thin sheets. The test system was developed using a conventional hydraulic load frame outfitted with specialized capabilities, and was designed for determining material properties on a macro-scale. Specimen thicknesses used in this study range from 127-508 μm, with specimen lengths on the order of 150 mm. This paper outlines the developmental process, including unique challenges, along with the system validation and some preliminary data.

High-Quality High-Material-Usage Multiple-Layer Laser Deposition of Nickel Alloys Using Sonic or Ultrasonic Vibration Powder Feeding

Laser-based metal deposition has been considered and applied as one of the most promising techniques for repairing high-value aerospace components such as turbines and vanes. Low component distortion and minimum heat input are the main advantages of laser-based alloy deposition techniques. The currently used laser deposition techniques are based on the gas delivery of metallic powders to the laser-generated melt pool. Despite efforts in improving delivery nozzle designs, the powder usage efficiency is still not 100 per cent, with some powders ejected from the deposition points, and powder feeding cannot be rapidly switched on and off to synchronize with laser-firing actions; this causes wastage of high-cost superalloy materials and contamination of the work environment. To mitigate this process deficiency, a gas-free vibration powder delivery system has been developed. The system uses sonic or ultrasonic vibration to exert a distributed driving force on the powder and to assist its delivery to the laser-generated melt pool. Three different configurations (off-axial, coaxial, and multiple-stream) were designed and evaluated. The initial problems encountered were instability of the powder flowrate owing to jamming and mass variations. Through various stages of design and optimization, the powder flowrate from these nozzles was found both to be highly stable and to have fast dynamic responses to the electrical control signals. Experiments on the deposition of various alloy materials including Inconel 718 were carried out with a 1.5 kW diode laser, a 1 kW single-mode fibre laser, and a 7 kW multimode fibre laser. They showed that 100 per cent deposition efficiency can be achieved by using the developed vibration delivery system. The deposition quality in terms of the surface roughness, microstructure, and porosity was also much improved in comparison with gas delivery laser deposition techniques. In addition, a high-volume material deposition rate at 3.31 kg/h has been demonstrated.

An Evaluation of the Use of X-ray Residual Stress Determination as a Means of Characterizing Oxidation Damage of Nickel-Based, Cr2O3-Forming Superalloys Subjected to Various Oxidizing Conditions

The use of X-ray residual stress determination as a technique for evaluating the damage incurred by nickel-based, Cr2O3-forming superalloy materials under various service conditions (isothermal heating, thermal cycling, applied stress, stressed and cycled) was investigated. Large and small compressive residual stresses were observed for the oxides and the near surface substrates, respectively. It was expected that the applied stresses and thermal cycling would cause an enhanced degree of oxidation damage that would translate into appreciable differences in residual stress values. Differences in the magnitude of residual stress values were not appreciable condition-to-condition, however. An increase in the severity of the oxidizing conditions in the form of longer oxidation times, higher oxidizing temperatures, and a much greater frequency of thermal cycling is suggested for future studies.

Structure and mechanical properties of Ni-based porous superalloy

The porous superalloy materials with hollow spherical and oriented linear pores were fabricated by means of powder metallurgy using Ni-based superalloy powders. Structure observation revealed that the pores of the porous material exhibited uniform distribution and the pores were of the same size in principle. The sintering necks created between adjacent particles on metal skeleton after sintering. The porosity of the porous material increased with increasing the addition of pore forming agent, and the hollow spherical porous material with a porosity of 81.62% were obtained when the addition of pore forming agent was 40%. The compression test indicated that this kind of materials possesses excellent, energy absorption capability and the compression resistance decreased with the increasing of porosity and pore size.

Preparation, Structure and Mechanical Properties of Nickel Based Porous Spherical Superalloy

The porous superalloy materials with hollow spherical pores were fabricated by using metal powder sintering process. The scanning electron microscope (SEM) observation was applied to the test samples and it revealed that the pores of the porous material exhibited a uniform distribution and the apertures were of same size in principle. The sintering necks appeared between adjacent particles on metal skeleton after sintering. The mechanical properties of the test samples were analyzed and the result showed that this kind of materials possessed excellent energy absorption capability, and the compression resistance decreased with increasing the porosity and aperture.

Recast layer removal after electrical discharge machining via Taguchi analysis: A feasibility study

This study explores the feasibility of removing the recast layer (RCL) using etching and mechanical grinding for Ni-based superalloy materials by means of electrical discharge machining (EDM). The EDM process is widely used for machining hard metals and performing specific tasks that cannot be achieved using conventional techniques. The sparks produced during the EDM process melt the metal's surface, which then undergo ultra rapid quenching. A layer forms on the workpiece surface defined as a recast layer after solidification. Molds and dies desire to remove the RCL even though it is hard and has good matrix adherence. This experiment is divided into three stages. The first stage acquires a thick recast layer by using EDM with a larger discharging energy. A thick recast layer is essential for verification of the EDM technique for observing the recast process. Thus, this work applies the Taguchi L 18 analytical method to acquire the thick recast layer. The second stage optimizes the recast layer removal technique. Therefore, the thick recast layer is intentionally made in the first stage. This work determines the second stage setting using Taguchi's recommendation. Thus, the L 9 orthogonal array sets up the etching and mechanical grinding parameters and observes the recast layer removal quantity analysis. Finally, an experiment studies the surface characteristics of Ni-based superalloys, such as composition and micro-hardness after removing the recast layer.

Optimal closed die finish forgings for nimonic80-A alloy using FEM method

Conventional closed die forging, is widely used for manufacture of superalloy parts. Despite recent technological advancements in the area of forging design, empirical and trial-and-error methods are still widely used in forging super alloy materials. This paper investigates the effect of flash thickness and various geometrical forging design parameters including internal and external drafts and internal and external corner radii, on the die cavity filling, forging load and raw material cost when forging Nimonic80-A a superalloy material. A new design methodology based on DOE-designated fractional factorial and FE simulations is proposed to establish optimal closed die finish forging and to find the optimum values of the design parameters. Also a response-surface-methodology (RSM) based method for forging load prediction is presented.

Key experimental parameters in steam oxidation testing and their impact on interpretation of experimental results

The acceptance of materials for extended duration, safety critical power generation applications usually requires several stages of testing and data generation. Simple, short term exposures under nominally constant atmospheres and temperatures can eliminate materials that are grossly unsuitable, but do not differentiate between materials that have broadly acceptable properties. To better differentiate between candidate materials it is desirable to tailor laboratory tests such that they more closely replicate in service conditions. In terms of components that are exposed to steam oxidation degradation mechanisms, this means replicating the steam conditions with an aim of producing oxide scale morphologies similar to that seen in service. Key experimental parameters have been identified, including water chemistry, pressure, steam delivery and flowrate, and a series of steam exposure tests on ferritic (P92), austenitic (Esshete 1250) and superalloy (IN740) material conducted to evaluate their effect on degradation rate and oxide scale morphology. The oxidation rate of the austenitic, and to a lesser extent the ferritic, material was found to be sensitive to the level of dissolved oxygen in the feed water, low (10 ppb) dissolved oxygen levels producing an increase in oxidation rate. The propensity to spall was also found to be reduced at low dissolved oxygen concentrations. In addition, the steam pressure and steam delivery method were shown to affect the oxidation rate and scale morphology for these materials.

Electrochemical Grinding for Unclosed Internal Cylinder Surface

The electro discharge machining (EDM) process is widely applied to produces surfaces of difficult-to-machine materials that require some form of grinding or finishing operation. This is conducted so as to improve the surface texture and appearance of the component’s surface. However, it is also desirable to remove the white uppermost recast layer or damaged surface layer (produced by the EDM process) in order to improve the functional performance of the surface. The finishing of the surface by traditional manual methods, especially in the operation of small and long internal cylinder surface, is both tedious and time-consuming. A novel electrically conductive diamond mounted point electrochemical grinding (DMP-ECG) process is being developed for hard passive alloys unclosed internal cylinder surface grinding. The process mechanism of DMP-ECG is introduced; the influence parameters of the surface roughness, machining accuracy, and diamond tool wear are investigated experimentally for nickel-based super alloys materials. An application of the DMP-ECG to aircraft engine component is verified in term of the optimized process parameters.

Life Prediction of Thermo-Mechanical Fatigue for Nickel Based Superalloy IN738LC

An experimental program has been carried out to address the thermo-mechanical fatigue life of the uncoated IN738LC nickel-base superalloy. High temperature isothermal Fatigue and out-of-phase(OP), in-phase(IP) TMF experiments in strain control were performed on superalloy materials. Temperature interval of 450-850 was applied to thermo-mechanical fatigue tests. The stress-strain response and the life cycle of the material were measured during the test. The plastic strain energy based life pediction models were applied to the stress-strain history effect on the thermo-mechanical fatigue lives.

Thermo-Mechanical Fatigue of the Nickel Base Superalloy IN738LC for Gas Turbine Blades

A more accurate life prediction for gas turbine blade takes into account the material behavior under the complex thermo-mechanical fatigue (TMF) cycles normally encountered in turbine operation. An experimental program has been carried out to address the thermo-mechanical fatigue life of the IN738LC nickel-base superalloy. High temperature out-of-phase and in-phase TMF experiments in strain control were performed on superalloy materials. Temperature interval of 450-850 was applied to thermo-mechanical fatigue tests. The stress-strain response and the life cycle of the material were measured during the test. The mechanisms of TMF damage is discussed based on the microstructural evolution during TMF. The plastic strain energy based life pediction models were applied to the stress-strain history effect on the thermo-mechanical fatigue lives.