Superalloy Forgings Research Articles

Investigation on Mechanism of Microstructure Evolution during Multi-Process Hot Forming of GH4169 Superalloy Forging.

Typically, in the manufacturing of GH4169 superalloy forgings, the multi-process hot forming that consists of pre-deformation, heat treatment and final deformation is required. This study focuses on the microstructural evolution throughout hot working processes. Considering that δ phase can promote nucleation and limit the growth of grains, a process route was designed, including pre-deformation, aging treatment (AT) to precipitate sufficient δ phases, high temperature holding (HTH) to uniformly heat the forging, and final deformation. The results show that the uneven strain distribution after pre-deformation has a significant impact on the subsequent refinement of the grain microstructure due to the complex coupling relationship between the evolution of the δ phase and recrystallization behavior. After the final deformation, the fine-grain microstructure with short rod-like δ phases as boundaries is easy to form in the region with a large strain of the pre-forging. However, necklace-like mixed grain microstructure is formed in the region with a small strain of the pre-forging. In addition, when the microstructure before final deformation consists of mixed grains, dynamic recrystallization (DRX) nucleation behavior preferentially depends on kernel average misorientation (KAM) values. A large KAM can promote the formation of DRX nuclei. When the KAM values are close, a smaller average grain size of mixed-grain microstructure is more conductive to promote the DRX nucleation. Finally, the interaction mechanisms between δ phase and DRX nucleation are revealed.

A novel recrystallization annealing method to cooperatively control the grain size and δ phase content for initial aged GH4169 superalloy forging

For initial aged GH4169 superalloy forging, it is important and difficult to find a suitable heat treatment method which can refine the deformed mixed grain and control δ phase content cooperatively. In this study, a novel heat treatment method is proposed, which includes an aging treatment to make many δ phase precipitates come out and a subsequent three-stage recrystallization annealing treatment (TSRT). TSRT consists of a high-temperature (990–1010 °C) annealing, a continuous cooling annealing (CRT) and a low-temperature annealing (930–980 °C). TSRT retains the advantages of CRT which is also proposed by authors' team. Moreover, the highlight of TSRT is that an ideal microstructure can be obtained by adjusting the parameters of low-temperature stage. Compared with CRT, TSRT is easier to control the microstructure. The results show that it is beneficial to obtain an ideal microstructure when the temperature of low-temperature stage is 980 °C and 950 °C. Eventually, two feasible annealing processes are obtained. One is “1000 °C×3 min+1000°C-5 min-980 °C+980 °C×20 min”. Through this annealing process, the deformed mixed grain microstructure can be uniformly refined to an average grain size of 7.02 μm, and the δ phase is distributed in spherical shape with a content of 3.28 vol%. The other is “1000 °C×3 min+1000°C-5 min-980 °C+950 °C×30 min”. Using this process, a grain microstructure with an average size of 4.92 μm can be obtained and the δ phase is distributed in rod-like and spherical shape with a content of 5.69 vol%. Besides, the formation mechanism of recrystallized mixed grains is revealed.

Discontinuous dynamic recrystallization and nucleation mechanisms associated with 2-, 3- and 4-grain junctions of polycrystalline nickel-based superalloys

Despite the important role of grain junctions to the dynamic recrystallization (DRX) of nickel-based superalloys, the nucleation mechanisms operating there and the influence factors besides deformation condition on nucleation priority are still unclear. The microstructure evolution considering grain junction effects has not been explored in depth yet. In this paper, DRX nucleation at the 2-, 3- and 4-grain junctions of a nickel-based superalloy was investigated. A proposed passive grain boundary bulging (PGBB) mechanism operated well at 2-grain junctions, and accounted for the continuity of necklace structure during the early stage of hot deformation. For the 3-grain junctions, a two-step strain-induced bulging of grain boundary fragments, which closely adjoined the junctions, was found to dominate the DRX nucleation. In addition, an increasing nucleation priority from 2-, 3- to 4-grain junctions was confirmed by the established thermodynamic model, while the nucleation priority differences of the same kind grain junctions were quantitatively analyzed by using the Taylor factors of their component grains. Finally, the DRX microstructure evolution of a 4-grain stacking unit during hot deformation was described. The understanding of DRX associated with 2-, 3- to 4-grain junctions made it more effective to tailor the microstructure of nickel-based superalloy forgings.

Computer and physical modeling of multiple isothermal forging of EK61 superalloy

Studies on the process of multiple isothermal forging of high-temperature nickel-based EK61 superalloy using computer simulation in the DEFORM-3D software package in a three-dimensional formulation and a comparison with experimental data were carried out. Based on the simulation results, it is shown that with each subsequent stage of forging, the maximum strain values become higher, and the strain differences in the central and peripheral regions become smaller. Such a strain distribution leads to the formation of a homogeneous ultrafine-grained (UFG) microstructure. The initial coarse-grained microstructure is gradually transformed into a fine-grained microduplex type microstructure at 0.77Tmelt and with a further decrease in the processing temperature to 0.73Tmelt, it is transformed into a submicroduplex type (γ + δ) UFG microstructure.

In Situ Study of the Stress Relaxation During Aging of Nickel-Base Superalloy Forgings

Nickel-base superalloys are conventionally aged in order to develop a favorable microstructure as well as to relieve residual stresses. Here neutron diffraction was used to follow the evolution of residual stress during aging in Inconel 718 and Udimet 720LI for the first time by a combination of in situ and ex situ measurements. First, the quench-induced stress profiles were determined for different geometries of Inconel 718 forgings confirming that the amplitude of stress is not significantly affected by the thickness of the component. It was followed by in situ residual stress analysis using neutron diffraction during aging/annealing treatments at 650 °C, 720 °C, and 750 °C. Almost 90 pct of stress relaxation was found to occur primarily during heating to the aging temperature as a result of a combination of plasticity and early-stage creep relaxation. Creep-like stress relaxation was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720 °C and 750 °C, while no further stress relaxation occurred at 650 °C. This means that a change in hold temperature might have a more immediate impact on stress relaxation than a change in duration of heat treatment. The post-aging ex situ measurements showed that a heat treatment at 750 °C for 8 hours reduced the stresses by approximately 70 pct in Inconel 718. By comparison, when heat treating Udimet 720LI in the same way only a 20 pct stress reduction was observed, which is explained by the higher creep resistance of this alloy.

Forging Optimization and Microstructural Simulation of an Inconel 718 Alloy Used in Turbine Discs

Simulating microstructure evolution during forging of superalloys is of great interest for manufacturer of critical components, particularly in aerospace applications. In this research, a phenomenological approach using a mathematical model was employed into a commercial finite element code, i.e. Abaqus, to study recrystallization and grain growth of IN718 supperalloys during forging. Model validation was performed by compression testing. Results show that the measured recrystallized volume fraction and the grain size were in good agreement with the model predictions. Subsequently, the model was used to simulate the evolution of microstructure in a turbine disc. Finally, a technique based on Tagushi method was used to determine the influence of forging parameters such as forging temperature, ingot height to diameter ratio, and top die velocity on the resultant microstructure. Uniform and refined microstructure of final forging was considered into the objective function. Optimum as well as the poorest parameters combination was attained through analysis. As well the contribution of each parameter on microstructure development was determined through variance analysis.

Development of Precise Load Prediction System for Free Forging of Ni-based Superalloy having Softening

Ni-based superalloys such as Alloy 706 that have high heat resistance are applied to gas turbine disks in electric power plant because. Free forging is one of the effective forging processes to form superalloy products, and forging simulation is quite useful method to optimize forging schedule. Softening behaviour at high temperature was not considered and implemented in conventional simulation, that induced the accuracy of load prediction. In this study, we investigated softening behaviour to improve the accuracy of forging simulation.

Modelling Texturised Cast Structures in the Forging of Superalloys

The main objective of remelting processes commonly used in the production of super¬alloys is to obtain a columnar dendritic solidification structure throughout the whole ingot. Besides reduced microsegregation, this cast structure features a preferred orientation, which is depending on the primary dendrites’ growth direction and therefore closely related to the ingot’s pool shape. As a result, non-isotropic material behaviour can be observed during initial forging operations. Since the correct prediction of material flow is a prerequisite for the further analysis of forging processes by means of numerical simulation, the solidification texture’s influence on plastic flow was accounted for by the application of an anisotropic material model. The model according to Barlat was used to scale the flow stress with respect to the crystal orientations observed in the examination of vacuum arc remelted alloy 718, thereby considering the flow stress’ dependency on strain, strain rate and temperature. The parameters defining the material's anisotropy could be determined by the upsetting of cylindrical specimen from a remelted ingot.

Hot shape forging of gas turbine disk using microstructure prediction and finite element analysis

The Waspaloy, a Ni-based superalloy, has been widely used for forging material of gas turbine disk since it requires the high tensile strength at high temperature and good resistance to low cycle fatigue. The purpose of this study is to develop a forging process of turbine disk that satisfies the hot deformation characteristics of Waspaloy. Generally, the hot forging of superalloy has been subjected to isothermal forging since the available temperature range of forging is narrow. However, the non-isothermal forging was used to make a turbine disk in this study. Therefore, the analyses of temperature variation and deformation behavior of the material were important to obtain the sound forging products. The hot compression test was carried out to know formability at high temperature and microstructure evolution during hot deformation. In order to define the optimum forging conditions including material temperature, strain rate, strain, microstructure evolution and forging load, the commercial finite element analysis code was used to simulate the forging procedure of turbine disk. The hot forged turbine disk was heat-treated for obtaining the high temperature properties. The cut-off tests on the heat-treated forged disks were carried out. Experimental results were compared with the simulation results by FE analysis. Test results were in good agreement with the simulations. This study shows that the superalloy turbine disk can be manufactured by the semi-closed die forging.

A novel method for predicting the grain size of superalloy forgings based on the fuzzy method and the FEM

Grain size is one of the important indexes for characterizing the microstructures and properties of superalloy forgings. Under the condition that the quality of the raw material is specified, the grain size of superalloy forgings is the result of synthetic inter action of thermomechanical parameters. Thus, the relationship between grain size and thermomechanical parameters belongs to the fuzzy concept for which the extension is uncertain. Based on the results of isothermal constant-strain-rate compression tests, the membership function for characterizing the grain size of superalloy forgings was first established in terms of the thermomechanical forging parameters. Combining the proposed membership function with thermoviscoplastic coupled numerical simulation software based on the finite element method, a new method was formulated for the prediction of the grain size of superalloy forgings. The results predicted by using the method presented in the paper are in close agreement with the experimental data and measured grain size of an IN718 alloy disk forging. The proposed method can serve as the guide for optimizing the forging process and controlling the forging quality of difficult-to-deformation materials.

A model describing microstructural evolution for Ni-base superalloy forgings during the cogging process

In this paper we consider the development of a computer modelling tool for the thermomechanical cogging of alloy Inconel® 718. The cogging process is used widely by suppliers of aerospace materials, to break down the ascast ingot microstructure to an acceptable level of refinement prior to disc forging. The commercially available viscoplastic finite element software Forge3® has been employed to simulate the cogging process and to calculate the variables such as temperature, strain-rate and effective strain. In addition, a microstructural model has been incorporated into the code, which relates the volume fraction of recrystallised material and the associated grain size, to the state variables. Since dynamic and static recrystallisation phenomena govern microstructural evolution in this alloy, the logic relating the recrystallisation regimes has also been incorporated into the code. In order to validate the process model, results are compared to maps of observed microstructure in the as-cogged billet.

Predicting residual stresses in gas turbine components

This article describes work carried out by a major aircraft-engine builder and one of its suppliers to validate the numerical prediction of heat-treatment-induced residual stresses. For verification, the project used a large-scale, nickel-based super alloy forging-, calculated stresses are compared to those measured using a hole-drilling technique. Even though the analyses were conducted by both companies using different software, the results tie closely with the measurements.

Nitrogen in Spray Formed Superalloys

AbstractSpray forming is of considerable interest for potential commercial production of superalloy forging and rolling preforms. The low degree of macro segregation and the relatively small grain size (25–40 μm) make this process look attractive for compositionally complex modern day nickel based superalloys. Both argon and nitrogen have been used successfully as the atomising gas. Argon atomisation has been used when little or no oxygen or nitrogen pickup is desired and when a small amount of argon filled porosity (∼1 vol.-%) can be tolerated. Nitrogen atomization has been used when a decrease in porosity is desired and when a small amount of nitrogen pickup can be tolerated. In this case, rather than staying in the pores, the nitrogen gas dissolves into the alloy and precipitates as submicrometre titanium and niobium carbonitrides. These precipitates have been observed to have little effect on key mechanical properties (tensile, creep, and fatigue). PM/0649

Modeling of the flow distribution in an oil quench tank

This article reports the results of an investigation of the fluid flow in an agitated quench tank used during heat treatment of superalloy forgings. The flow patterns in a scale model of the system were visualized using a laser light sheet and quantitative velocity measurements were undertaken using laser Doppler anemometry (LDA). The experimental data are compared with numerical predictions obtained by means of a computational fluid dynamics (CFD) computer code. Conclusions are drawn as to the suitability of the CFD code for predicting flows in a complex system of this type.

Effect of titanium content on mechanical properties of an ‘A’ segregation region in large A286 iron-base superalloy forgings: Kohno, M., Honjo, T. and Tsuchiyama, T.Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) July 1990, 76, (7), 1139–1146 (in Japanese)

Effect of titanium content on mechanical properties of an ‘A’ segregation region in large A286 iron-base superalloy forgings: Kohno, M., Honjo, T. and Tsuchiyama, T.Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) July 1990, 76, (7), 1139–1146 (in Japanese)

Fe基耐熱合金A286大形鍛造品の逆V偏析部の機械的性質におよぼすTi量の影響

Fe基耐熱合金A286大形鍛造品の逆V偏析部の機械的性質におよぼすTi量の影響

Modeling the heat treatment of superalloy forgings

In the quest for higher operating temperatures, aircraft engine designers seek to improve properties of existing superalloys through a variety of different means. The heat treatment of superalloy forgings, particularly the cooling rate from solutioning temperature, is a key factor in their final properties. Modeling the cooling rate can be used to optimize the properties of a forging, and can help designers avoid quench cracks and residual stresses as well.

塑性加工プロセスと材料 ＩＩ Ｔｉ合金およびＮｉ基超合金の超塑性鍛造

塑性加工プロセスと材料 ＩＩ Ｔｉ合金およびＮｉ基超合金の超塑性鍛造

Development of a Die Material for Isothermal Forging of Superalloys in Air

In isothermal forging, since the die is heated as well as the workpiece, high-temperature strength is essentially required for the die material. In particular, isothermal forging of superalloys is carried out at high temperature ranging 1000-1100°C, thus a molybdenum-base alloy, TZM, is generally used as a die material. Although TZM has good high-temperature strength, its oxidation resistance is very poor at above 600°C, therefore, forging must be done in vacuum or inert-gas atmosphere.If there is a die material having both enough strength and oxidation resistance at around 1000-1100°C, isothermal forging of superalloys in air will be possible, and the forging apparatus will be simpler.To develop such a die material, the authors directed their attention to chromium free nickel-base cast alloys, because such alloys are able to contain a large amount of heavy metals such as molybdenum and tungsten, as being soluble in γ and γ' instead of chromium, and are expected to have higher strength at above 1000°C.Several conventional and experimental alloys were investigated. From the results of compression, compressive creep and oxidation resistance tests, “Nimowal” (Ni-10Mo-12W-6Al-0.01Y) showed the best properties as a whole. Its short-time strength is equivalent to that of Mar-M200 at 950-1000°C, higher than that of Mar-M200 and equivalent to that of TZM at 1050-1100°C. Its compressive creep strength is higher than that of Mar-M200 at 1000-1100°C. Oxidation resistance of Nimowal is equivalent to that of Mar-M200 and enough to be used in air. Moreover, JVimowal has better machinability than Mar-M200.Simulation tests of isothermal forging of superalloys in air were made with Nimowal being used as the die material. One-tenth model disk of In-100 was successfully forged from the extruded billet without any damage or significant oxidation of the die.

Press forging of superalloys

AbstractThe characteristics of press forging techniques for superalloys are reviewed in the context of the available equipment. The use of thermomechanical processing to meet specific microstructural and ultrasonic requirements is discussed in some detail, and examples are given which cover a wide range of superalloy compositions and structural requirements. Data are presented which demonstrate the all-important factors of repeatability and consistency in manufacture which can be obtained by press forging. The influence of die temperature on forging surface characteristics is outlined.MST/257