Papers by Author: Jae Keun Hong

Abstract: In this study, direct powder rolling was applied to make thin sheets from HDH (hydrogenation and dehydrogenation) powders. Effects of materials and process parameters on tensile properties of sintered sheets are investigated. In the direct powder rolling with two-horizontal counter-rotating rollers, materials parameters including powder size and process parameters such as roll gap, roll speed, and packing height are controlled. With -100mesh Ti powders, CP titanium sheets of a thickness up to 1.5mm and a width up to 300mm were obtained without any rolling defects. Sintering of the rolled strips was performed between 1000°C and 1250°C at 50°C intervals for 1hour to 4 hours. The relative density of the sintered strips was found to be around 85 to 90 percent. After sintering, the complete densification was made by cold rolling, and microstructures and tensile properties were evaluated after annealing treatment at 750°C for 2 hours.

Abstract: Microstructure evolution during ring rolling process of a large-scale Ti-6Al-4V ring was investigated with the combined approaches of three dimensional finite element method (FEM) simulation and microstructure prediction model. A microstructure prediction model was established by considering the volume fractions and grain size of  and  phases varying with process variables, and grain growth. In order to perform FE simulation for ring rolling process of Ti-6Al-4V alloy, a constitutive equation was generated by utilizing the flow stress data obtained from hot compression tests at different temperature and strain rate conditions. The volume fraction and grain size of  and  phases during ring rolling were calculated by de-coupled approach between FEM analysis and microstructure prediction model. The prediction results were compared with the experimental ones. Our proposed microstructure simulation module was useful for designing hot forming process of Ti-6Al-4V alloy

Abstract: The high temperature deformation behavior and flow instabilities of Ni-Fe-Co base superalloy, INCONEL alloy 783 during hot working process were investigated with process maps consisting of a power dissipation of dynamic materials model (DMM) and various flow instability criteria. In order to establish the processing map of INCONEL alloy 783, hot compression tests were carried out under different temperature and strain rate conditions, with true strain up to 0.7. On the basis of the comparison between processing maps and microstructural analysis, the reliability of various flow instability criteria was estimated. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results and experimental observations of actual ring rolling process of INCONEL alloy 783.

Abstract: Effects of solution treatment on the microstructure and mechanical properties in wrought Alloy 718 were investigated. For the improvement of tensile and fatigue properties of wrought Alloy 718, a modified solution heat treatment(MSHT), heating at 1000
for 3 minutes followed by furnace cooling at the rate of 3
/minute and holding at 985
for 8 minutes, was proposed. This treatment was performed repeatedly 3 times and the samples were subject to the standard aging treatment. For the purpose of comparison, a standard heat treatment(SHT) was also performed. The microstructures of modified solution heat treated specimens showed that small spherical shaped δ- phases were precipitated without grain growth, and the amount of δ-phases was smaller than that of standard heat treated specimens. However, the δ-phases of the standard heat-treated specimen showed needle-like morphologies. Tensile and low cycle fatigue tests were performed on both modified heat-treated and standard heat-treated specimens at room temperature, 350
and 650
. Low cycle fatigue tests on the modified heat treated specimens showed promising results without reduction of strength. However, the tensile properties of modified solution treated specimens was almost the same as those of standard heat treated materials both at room temperature and 650
.

Abstract: The oxidation behavior of carbides on Inconel 718 surface was investigated. Oxidation tests were performed at the temperature range of 300 to 650°C. Two types of carbide, NbC and TiC were detected, and the amount of NbC was significantly higher than that of TiC. The oxidation of NbC started at the carbide surface and the matrix interface, and propagated into the interior of the carbide and eruption subsequently occurred with increasing oxidation time. In case of TiC, the oxidation occurred at the outer surface of the carbide and the matrix interface, but did not propagated into the interior of the carbide. Oxides formed on the outer surface of the TiC contained a relatively large amount of elemental Nb. The cross-section of oxidized specimen was investigated. NbC was oxidized more easily due to the significantly lower Gibbs free energy of oxidation than that of TiC. Eruption occurred mainly at the NbC because the Pilling-Bedworth ratio of Nb is higher than the critical value of 2.3 where eruption typically occurs.