Nho-Kwang Park

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.

Continuous cooling transformation behavior of Alloy 718

Abstract The transformation behavior of Alloy 718 is affected significantly by the cooling rate. The γ″-phase appears at cooling rates less than 20 °C/min and δ-phase appears at grain boundaries as well as the MC type carbides at cooling rates below 5 °C/min. The δ-phase nucleates and grows preferentially at grain boundaries, and less preferentially at the MC carbides. The size of the γ″ and δ-precipitates increases consistently with decreasing cooling rate for the given conditions. The hardness varies with the transformation behavior. A hardness peak was noticed for a cooling rate of 5 °C/min. The hardness peak corresponded to the maximum volume fraction of γ″ which in turn was strongly affected by the presence of the δ-phase.

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 rate

Effects of various sintering methods on microstructure and mechanical properties of CP-Ti powder consolidations

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MODELING AND SIMULATION OF DYNAMIC RECRYSTALLIZATION AND GRAIN GROWTH DURING HOT WORKING OF INCONEL 783 SUPERALLOY

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.

Effect of Processing Condition on the Hot Extrusion of Al-Zn-Mg-Sc Alloy

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.

Precipitation behavior of hot-extruded alloy 718 during isothermal treatment

Abstract Effects of various sintering methods such as spark plasma sintering (SPS), hot pressing (HP) and electric resistance sintering (ERS) on the microstructure and mechanical properties of commercial pure titanium (CP-Ti) powder consolidations with particle size of

Effect of Processing Conditions on the Deep Drawability of Ti-6Al-4V Sheet at Warm Temperatures

The evolution of sinterability, microstructure and mechanical properties for the spark plasma sintered(SPS) Ti from commercial pure titanium(CP-Ti) was studied. The densification of titanium with 200 mesh and 400 mesh pass powder was achieved by SPS at under 10 MPa pressure and the flowing +Ar mixed gas atmosphere. The microstructure of Ti sintered up to consisted of equiaxed grains. In contrast, the growth of large elongated grains was shown in sintered bodies at with the 400 mesh pass powder and the lamella grains microstructure had been developed by increasing sintering temperature. The Vickers hardness of 240~270 HV and biaxial strength of 320~340 MPa were found for the specimen prepared at .