Jong-Taek Yeom

Characterization of deformation stability in hot forging of conventional Ti–6Al–4V using processing maps

Abstract The deformation behavior of Ti–6Al–4V alloy under hot compression conditions was characterized in the temperature range 850–1000xa0°C and strain rate range 0.001–10xa0s−1. Processing maps were generated using the dynamic material model (DMM), unifying the relationships between constitutive deformation behavior, hot workability and microstructures evolution. Stable deformation regions were defined by Malas’ stability criteria. The processing maps obtained at different strains were implemented onto the user-subroutine of DEFORM-2D. FE simulation was carried out to evaluate the flow and microstructure stability for Ti–6Al–4V pancake forgings. The prediction of stability and instability in forged workpieces was in good agreement with the microstructures of hot-forged Ti–6Al–4V pancakes.

Hot forging of a nickel-base superalloy

Abstract Compression tests were carried out to investigate the hot-deformation behavior of nickel-based Alloy 718. The tests were conducted in the temperature range 982–1066°C and the strain rate range 5×10 −4 –5.0xa0s −1 . Pancakes were also made by multi-step forging and their microstructures were compared with specimen-base compression tests. Hot-deformation behavior was led mainly by dynamic recrystallization as well as by dynamic recovery. Dynamic recrystallization as well as meta-dynamic recrystallization plays a significant role in determining final microstructures for the given temperature ranges. Once the dynamic recrystallization is finished, no significant stress relaxation attributed to meta-dynamic recrystallization occurs and stress hardening following the intermittent dwell time between the first and the second compression tests occurs even at 1066°C due to the fact that grain growth becomes prominent. In such two-step compression tests, large second strains produce second dynamic recrystallization, which leads to fine grain sizes. To control the grain size through dynamic recrystallization in multi-step forging, one needs to consider the strain partitioning in each forging step for the given temperature and strain rate ranges. FEM simulation was used to evaluate the microstructural variation during pancake forging, taking into account the variation of grain sizes following dynamic recrystallization.

Simulation of microstructures for Alloy 718 blade forging using 3D FEM simulator

Abstract Mechanical properties of Ni–Cr–Fe-based Alloy 718 depend very much on the grain size, as well as the presence of strengthening phases, γ′ and γ″. The grain size of superalloy parts can be controlled by the thermo-mechanical processes, including heating, forging and cooling sequences. In order to predict the evolution of grain size in Alloy 718 blade forging, a series of constitutive equations for dynamic recrystallization, meta-dynamic recrystallization and grain growth were developed and implemented into a 3D FE simulator. The evolution of grain structure in the process of two-step blade forging of Alloy 718 was simulated using the 3D FE simulator combined with a microstructure module. The microstructure prediction tool was validated by comparing the simulated grain structure with that of the experimental blade-type forging.

Creep strain and creep-life prediction for alloy 718 using the omega method

AbstractThe creep behavior of Alloy 718 was investigated in relation to the MPCs omega (Ω) method. To evaluate the creep model and determine material parameters, constant load creep tests were performed at different initial stresses in a temperature range between 550°C and 700°C. The imaginary initial strain raten

Effects of TiFe Intermetallic Compounds on the Tensile Behavior of Ti-4Al-4Fe-0.25Si Alloy

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Evaluation of High Temperature Workability of A350 LF2 Using the Deformation Processing Map

n and omega (Ω), considered to be important variables in the model, were expressed as a function of initial stress and temperature. For these variables, power-law and hyperbolic sine-law equations were used as constitutive equations for the creep of Alloy 718. To consider the effect of γ″ coarsening leading to a radical drop of tensile strength and creep strength at temperatures above 650°C, different material constants at the temperatures above 650°C were applied. The reliability of the models was investigated in relation to the creep curve and creep life.

Characterization of deformation behavior for waspaloy under creep-fatigue interaction

The deformation behavior of Waspaloy during strain-controlled fatigue tests and creep tests is investigated based on the Chaboche viscoplastic model. Material parameters were determined using the least square fit to uniaxial strain-controlled low-cycle fatique and creep test results. A standard viscoplastic model using nonlinear kinematic and isotropic hardening rules gave a good description for the cyclic hardening or softening observed in symmetric low-cycle fatigue tests, but the application of the deformation model to asymmetric strain conditions resulted in a large overestimation of the mean-stress relaxation. Two modified models are presented as solutions for the overestimation of stress relaxation. A combined nonlinear and linear kinematic hardening rule (NLK+LK model) and a nonlinear kinematic hardening rule with a threshold (2-NLK-TH model) are proposed and their applications are discussed.

Aging Temperature Dependence of α ″-Martensite Decomposition Mechanism in Ti-Al-Fe-Si Alloy

The effect of the B2 (ordered BCC) intermetallic compound TiFe on the tensile behavior of the Ti-4Al-4Fe-0.25Si alloy was investigated. The nucleation mechanism of TiFe was dependent on the solution temperature, and the solution treatment and aging temperatures were also important to the final alloy. The presence of intra-granular TiFe, which nucleated at α′ (HCP) sites during aging, resulted in alloy brittleness. Alternatively, the presence of inter-granular TiFe, which nucleated only at nano-sized α (HCP) sites during aging, resulted in an excellent combination of strength and ductility compared to the original microstructure.