Jong Taek Yeom - Academia.edu (original) (raw)

Papers by Jong Taek Yeom

Journal of Materials Research and Technology, 2021

Ti-6Al-4V alloy has limitations in terms of globularization when using additive manufacturing (AM... more Ti-6Al-4V alloy has limitations in terms of globularization when using additive manufacturing (AM), which requires a final forging step. To optimize this approach, this study investigated the effect of processing variables on the dynamic/static globularization of AM-processed Ti-6Al-4V alloy. Eight double-cone specimens were prepared with different processing variables such as effective strain, solution treatment, hot-forging temperature, and subsequent annealing. Microstructural evolutions were quantitatively characterized to interpret the kinetics of globularization based on the aforementioned variables. In particular, the combination of solution treatment, low-temperature (1073 K) forging, and subsequent annealing significantly accelerated the overall globularization. Such an accelerating effect stemmed from the reduced path for boundary splitting in fine a 0 martensitic laths, which was induced via solution treatment. This accelerating effect disappeared at a high temperature (1223 K), which implies the necessity of optimizing the thermomechanical route to exploit martensitic laths.

Metals and Materials International, 2019

Metals and Materials International, 2018

An accurate processing map for a metal provides a means of attaining a desired microstructure and... more An accurate processing map for a metal provides a means of attaining a desired microstructure and required shape through thermo-mechanical processing. To construct such a map, the isothermal flow stress, σ iso , is required. Conventionally, the non-isothermal flow stress measured by experiment is corrected to σ iso using whole-temperature-range linear interpolation (WRLI) or partial-temperature-range linear interpolation (PRLI). However, these approaches could incur significant errors if the non-isothermal flow stress exhibits a non-linear relationship with the temperature. In this study, an artificial neural network (ANN) model was applied to correct the non-isothermal flow stress in 10 wt% Cr steel, which exhibits a non-linear temperature dependence within a target temperature range of 750-1250 °C. Processing maps were constructed using σ iso corrected by applying the WRLI, PRLI, and ANN approaches, respectively, and were then compared with the actual microstructures. The WRLI approach produced the highest minimum error of σ iso (17.2%) and over-predicted the shear-band formation. The PRLI approach reasonably predicted the microstructural changes, but the minimum error for σ iso (8.9%) was somewhat high. The ANN approach not only realized the lowest minimum error of σ iso (~ 0%), but also effectively predicted the microstructural changes.

Journal of Nanoscience and Nanotechnology, 2016

Metallurgical and Materials Transactions A, 2016

The effect of the B2 (ordered BCC) intermetallic compound TiFe on the tensile behavior of the Ti-... more 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 a¢ (HCP) sites during aging, resulted in alloy brittleness. Alternatively, the presence of inter-granular TiFe, which nucleated only at nano-sized a (HCP) sites during aging, resulted in an excellent combination of strength and ductility compared to the original microstructure.

Korean Journal of Metals and Materials, 2012

The effect of aging heat treatment on microstructure evolution of the Ti-6Al-4Fe-0.25Si alloy wit... more The effect of aging heat treatment on microstructure evolution of the Ti-6Al-4Fe-0.25Si alloy with an initial microstructure of an elongated alpha was investigated. Aging treatments of the samples were carried out at 550℃ for up to 100 hours. The microstructure of the 5 hours heat-treated sample consisted of alpha grains, beta matrix and some TiFe intermetallic compounds that were precipitated from the beta matrix. Increasing the aging time to 10 hours, most of the beta matrix was decomposed to very fine alpha grains (~ 0.5 ㎛) and TiFe, and thus the volume fraction of the beta matrix was significantly decreased. EBSD analysis revealed that newly formed tertiary-alpha-grains in the vicinity of TiFe had high angle boundaries with respect to the primary and secondary alpha grains. As a result of these phase transformations during aging, the fraction of the alpha/alpha grain boundary was increased while that of the alpha/beta phase boundary was decreased.

Metals and Materials International, 2014

In this work, a ring-rolling process to formulate ring-shaped components for a wind turbine is de... more In this work, a ring-rolling process to formulate ring-shaped components for a wind turbine is designed by means of a simulation and in an experimental approach. The target of the ring-rolling design is a yaw ring with an outer diameter of approximately 3,130 mm. The ring-rolling design includes the design of the geometry and the optimization of the process variables. A calculation method was used for the geometry design, in this case for the initial billet and the pre-form (or blank) sizes, and for the final rolled ring shape. Also, a deformation map-based approach was utilized to determine the initial ring-rolling temperature and feed rate of the mandrel. A three-dimensional finite element method was used to predict the formation of rolling defects and the deformed shape in the ring-rolled components. The design criteria are to achieve uniform distributions of the strains and temperatures as well as defect-free ring-shaped components. Finally, an optimum process design to obtain a sound large-scale yaw ring without defects is proposed. It is validated by comparisons between the experimental data and the FE analysis results.

ACTA METALLURGICA SINICA, 2013

Journal of the Korean Welding and Joining Society, 2010

In this work, the effect of welding variables on weldability of gas tungsten arc(GTA) welding was... more In this work, the effect of welding variables on weldability of gas tungsten arc(GTA) welding was investigated with experimental analysis for a commercial pure(CP) titanium (Grade.1). The GTA welding tests on sheet samples with 0.5mm in thick were carried out at different process variables such as arc length, welding speed and electrode shape. In order to search an optimum arc length with full penetration, beadon-plate welding before butt-welding were performed with different arc length conditions. From the beadon-plate welding results, the optimum condition considering arc stability and electrode loss was obtained in the arc length of 0.8mm. Butt-welding tests based on the arc length of 0.8mm were carried out to achieve the optimum conditions of welding speed and electrode shape. Optimum conditions of welding speed and electrode shape were suggested as 10 mm/s and truncated electrode shape, respectively. It was successfully validated by the microstructural observation, tensile tests, micro-hardness tests and formability tests.

Metals, 2020

From the point of view of designing materials, it is important to study the complex correlational... more From the point of view of designing materials, it is important to study the complex correlational research that involves measuring several variables and assessing the relation among them. Hence, the notion of machine-oriented data modeling is explored. Among various machine-learning tools, artificial neural networks (ANN) have been used as a stimulating tool to solve engineering-related issues. In this study, the ANN model is designed and trained to correlate the complex relations among composition, temperature and mechanical properties of 25Cr-20Ni-0.4C austenitic stainless steel. The developed model was exploited to estimate the composition–property and temperature–property correlations. The ANN predictions are well suitable for experimental results. The model was able to correlate the complex nature among input and output variables. The model was used to investigate the effect of service temperature on the mechanical properties of 25Cr-20Ni-0.4C steels over a wide temperature ran...

Scientific reports, Jan 9, 2018

The formation mechanism of [Formula: see text] type twinned α'-martensitic structures was inv... more The formation mechanism of [Formula: see text] type twinned α'-martensitic structures was investigated in titanium alloys, and in-depth characterizations of the microstructures were performed using scanning electron microscopy and transmission electron microscopy. The randomly distributed nano-domains nucleated by water quenching were sheared during the primary martensite transformation. Experimental results revealed that this sheared nano-domain interferes with the primary martensite transformation and induces a secondary martensite transformation. In terms of crystallography, the secondary martensite transformed from the sheared nano-domain has a [Formula: see text] type twin relationship with the primary martensite. The growth of both martensites yielded a more twinned martensitic structure as the applied strain increased.

Metals and Materials International, 2016

The effect of yttrium (< 5.5 at%) on the martensite-austenite phase transformation temperatures, ... more The effect of yttrium (< 5.5 at%) on the martensite-austenite phase transformation temperatures, microstructural evolution, and hot workability of Ti-Ni-Hf high-temperature shape memory alloys is investigated. For these purposes, differential scanning calorimetry, hot compression, and thermo-gravimetric tests are conducted. The phase transformation temperatures are not noticeably influenced by the addition of yttrium up to 4.5 at%. Furthermore, the hot workability is not significantly affected by the yttrium addition up to 1.0 at%. However, when the amount of yttrium addition exceeds 1.0 at%, the hot workability deteriorates significantly. In contrast, remarkable improvement in the high temperature oxidation resistance due to the yttrium addition is demonstrated. The total thickness of the oxide layers is substantially thinner in the Y-added specimen. In particular, the thickness of (Ti,Hf) oxide layer is reduced from ~200 m to ~120 m by the addition of 0.3 at% Y.

Journal of Metallurgy, 2012

Microstructure evolution of Ti-6Al-4V alloy during hot backward extrusion process was simulated w... more Microstructure evolution of Ti-6Al-4V alloy during hot backward extrusion process was simulated with the combined approaches of finite element method (FEM) and microstructure prediction model. From experimental analysis, it can be found that the change of microstructure during hot forming process of titanium alloy has a close relation to α/β phase transformation and grain growth behaviour. A microstructure prediction model was established by considering the change of volume fractions and grain size of both phases varying with process variables and then implemented into the user-defined subroutine of FEM analysis. In order to demonstrate the reliability of the model, the volume fraction and grain size of primary α phase during the hot backward extrusion process of Ti-6Al-4V alloy were simulated. The simulation results were compared with the experimental ones.

Journal of Korean Powder Metallurgy Institute, 2011

Korean Journal of Materials Research, 2011

In this paper, high temperature oxidation behavior of newly developed alloys, Ti-6Al-4Fe and Ti-6... more In this paper, high temperature oxidation behavior of newly developed alloys, Ti-6Al-4Fe and Ti-6Al-1Fe, is examined. To understand the effect of Fe on the air oxidation behavior of the Ti-Al-Fe alloy system, thermal oxidation tests are carried out at 700 o C and 800 o C for 96 hours. Ti-6Al-4V alloy is also prepared and tested under the same conditions for comparison with the developed alloys. The oxidation resistance of the Ti-Al-Fe alloy system is superior to that of Ti-6Al-4V alloy. Ti-6Al-4V shows the worst oxidation resistance for all test conditions. This is not a result of the addition of Fe, but rather it is due to the elimination of V, which has deleterious effects on high temperature oxidation. The oxidation of the Ti-Al-Fe alloy system follows the parabolic rate law. At 700 o C, Fe addition does not have a noticeable influence on the amount of weight gain of all specimens. However, at 800 o C, Ti-6Al-4Fe alloy shows remarkable degradation compared to Ti-6Al-1Fe and Ti-6Al. It is discovered that the formation of Al 2 O 3 , a diffusion resistance layer, is remarkably hindered by a relative decrease of the α volume fraction. This is because Fe addition increases the volume fraction of β phase within the Ti-6Al-xFe alloy system. Activities of Al, Ti, and Fe with respect to the formation of oxide layers are calculated and analyzed to explore the oxidation mechanism.

Journal of Nanoscience and Nanotechnology, 2014

Ingots of Ni-Ti-Hf shape memory alloys were prepared by vacuum arc re-melting. Isothermal hot com... more Ingots of Ni-Ti-Hf shape memory alloys were prepared by vacuum arc re-melting. Isothermal hot compression tests were conducted at temperatures ranging from 700 to 1000 C and at strain rates from 10 −2 s −1 to 1.0 s −1. A decrease in the Ni content below 50.2 at.% significantly deteriorated the hot workability due to the formation of a brittle second phase. Also, the low Ni content alloy showed poor workability when the temperature exceeded 900 C. Additional compression tests were conducted under various conditions to clarify the effects of the chemical composition, solidification anisotropy, and the strain rate.

Ott/Superalloy, 2012

In this study, microstructure evolution during ring rolling process of INCONEL alloy 783 (Alloy 7... more In this study, microstructure evolution during ring rolling process of INCONEL alloy 783 (Alloy 783) was investigated with the combined approaches of 3-D FEM (Finite Element Method) simulation and microstructure prediction model. A microstructure prediction model was developed by considering the effects of process variables on recrystallization and grain growth behavior. The model was established by the analysis of hot compression and isothermal heat treatment test results. From the analysis of isothermal heat treatment tests, it can be found that the rate of grain growth dramatically increases after the dissolution of β phase, which acts as an obstacle to grain boundary movement. In order to consider the effect of β phase on grain growth, an additional equation in the transition temperature region was interposed. Microstructure evolution during ring rolling process of Alloy 783 was calculated by de-coupled approach between FEM analysis and microstructure prediction model. The prediction results were compared with the experimental ones. 297 7th International Symposium on Superalloy 718 and Derivatives

Thermochimica Acta, 2014

The effect of Hf addition on the high temperature oxidation behavior and oxide structure of Ti-49... more The effect of Hf addition on the high temperature oxidation behavior and oxide structure of Ti-49Ni-12Hf high temperature shape memory alloy was investigated. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 • C for 100 h in dry air. The kinetic curves of the oxidation were measured by TGA, and then the microstructure and chemical elements distribution in the different regions of the specimens were analyzed by SEM/EDS and XRD after oxidation. The experimental results showed that the oxidation rate in the primary stage was high, and then the mass-gaining-rate decreased gradually with the oxidation time. The oxidation behavior of Ti-50.5Ni alloy obeys parabolic rate law for whole oxidation period while the Ti-49Ni-12Hf alloy showed initially parabolic rate law followed by linear rate law. The Hf-rich oxide layer that formed beneath the outer oxide layer can impede the diffusion of Ti ions, resulting in a notable improvement of oxidation resistance of Ti-49Ni-12Hf alloy at high temperature.

Surface Review and Letters, 2010

In this study, the modeling of recrystallization and grain growth was investigated to predict the... more In this study, the modeling of recrystallization and grain growth was investigated to predict the microstructure evolution during hot working of INCONEL 783 superalloy (Alloy 783). The recrystallization model was constructed on the basis of the Avrami formation. A least-square optimization technique was used to determine several important parameters within the model from isothermal heat treatment and hot compression test results. High temperature compression tests were carried out under different temperatures, strain rates and strain conditions. The model for describing the recrystallization and grain growth behaviors of Alloy 783 was implemented onto the user-subroutine of a commercial FE code. In order to demonstrate the reliability of the model, the microstructure evolution of Alloy 783 alloy during the hot compression test was simulated and was accorded well with the experimental results.

Metals and Materials International, 2003

In this study, dynamic recrystallization during nonisothermal hot compression test was numericall... more In this study, dynamic recrystallization during nonisothermal hot compression test was numerically simulated by finite element analysis using new grain aggregate model for dynamic recrystallization. This model was developed based on mean field approach by assuming grain aggregate as representative element. For each grain aggregate, changes of state variables were calculated using three sub-models for work hardening, nucleation, and nucleus growth. A conventional single parameter dislocation density model was used to calculate change of dislocation density in grains. For modeling nucleation, constant nucleation rate and nucleation criterion developed by Roberts and Ahlblom were used. It was assumed that the nucleation occurs when the dislocation density of certain grain reaches a critical nucleation criterion. Conventional rate theory was used to model nucleus growth. The developed dynamic recrystallization model was validated by comparing with isothermal hot compression of pure copper. Then, the finite element analysis was conducted to predict the local changes of microstructure and average grain size by using the grain aggregate model. The predicted results were compared with nonisothermal hot compression results. The simulation results were in reasonably good agreement with experimentally obtained microstructures and the calculation time was much shorter than cellular automata-finite element method.