Youngmoo Kim

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y2O3 dispersed tungsten alloy were investigated as functions of the Y2O3 content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y2O3 content increases, the sintered density increases at a given temperature. The transition temperature (Ttr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y2O3 particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.

Simulation and Experiment of Injection Molding Process for Superalloy Feedstock

Abstract Powder injection molding is an important manufacturing technology to mass produce superalloy compo-nents with complex shape. Injection molding step is particularly important for realizing a desired shape, which requiresmuch time and efforts finding the optimum process condition. Therefore computer aided engineering can be very usefulto find proper injection molding conditions. In this study, we have conducted a finite element method based simulationfor the spiral mold test of superalloy feedstock and compared the results with experimental ones. Sensitivity analysiswith both of simulation and experiment reveals that the melt temperature of superalloy feedstock is the most importantfactor for the full filling of mold cavity. The FEM based simulation matches well the experimental results. This studycontributes to the optimization of superalloy powder injection molding process. Keywords: Injection molding simulation, Powder Injection molding, Spiral mold, Short shot series test, Superalloy feed-stock

Spark Plasma Sintering Behaviors of M-type Barium Hexaferrite Nano Powders

Abstract A magnetic powder, M-type barium hexaferrite (BaFe12O19), was consolidated with the spark plasma sin-tering process. Three different holding temperatures, 850 o C, 875C and 900C were applied to the spark plasma sinteringprocess with the same holding times, heating rates and compaction pressure of 30 MPa. The relative density was mea-sured simultaneously with spark plasma sintering and the convergent relative density after cooling was found to be pro-portional to the holding temperature. The full relative density was obtained at 900 o C and the total sintering time wasonly 33.3 min, which was much less than the conventional furnace sintering method. The higher holding temperaturealso led to the higher saturation magnetic moment (σ s ) and the higher coercivity (H c ) in the vibrating sample magne-tometer measurement. The saturation magnetic moment (σ s ) and the coercivity (H c ) obtained at 900 o C were 56.3 emu/g and541.5 Oe for each. Keywords: M-type Barium hexaferrite, SPS, Magnetic moment, Coercivity

Microstructural Evolution during Hot Deformation of P/M Copper using Processing Map

P/M coppers are subjected to the isothermal compression tests at the strain rate ranging from 0.01 to 10.0 and the temperature from 200 to . The processing map reveals the dynamic recrystallization (DRX) domain in the following temperature and strain rate ranges: and 0.01-10.0 , respectively. In the domain, the region at temperature of and strain rate of shows peak efficiency. From the kinetic analysis, the apparent activation energy in the DRX domain is 190.67 kJ/mol and it suggests that lattice self-diffusion is the rate controlling mechanism.

Microstructural Evolution during Hot Deformation of Molybdenum using Processing Map Approach

Abstract The hot deformation characteristics of pure molybdenum was investigated in the temperature rangeof 600~1200°C and strain rate range of 0.01~10.0/s using a Gleeble test machine. The power dissipation map forhot working was developed on the basis of the Dynamic Materials Model. According to the map, dynamic recrys-tallization (DRX) occurs in the temperature range of 1000~1100°C and the strain rate range of 0.01~10.0/s, whichare the optimum conditions for hot working of this material. The average grain size after DRX is 5 μm. The mate-rial undergoes flow instabilities at temperatures of 900~1200°C and the strain rates of 0.1~10.0/s, as calculated bythe continuum instability criterion. Keywords : Molybdenum, Hot working, Processing map, Dynamic recrystallization 1. 서론 몰리브데늄은 밀도(10.20 g/cc)가 높고 음속(5.0km/s)이 매우 빠르기 때문에 기존 구리를 대체할 차세대 성형작약 라이너 소재로 연구되고 있다[1]. 현재까지 개발된 몰리브데늄 라이너 제조 방법은 단결정, 고에너지 변형성형(high energy rate forming),단조, 압출 및 열간등압성형(hot isostatic pressing)등이 있다[1, 2]. 이 중에서 열간 단조 공정 후, 열처리를 통해 미세조직을 제어하는 기술이 가장 일반적으로 사용되고 있다. 그러나 몰리브데늄의 경우, 상온취성이 있고 강도가 매우 높기 때문에, 냉간이 아닌열간 성형 방법으로 단조를 수행하여야 한다[3]. 일반적으로 성형성(workability)이란, 단조, 압출, 압연 등과 같은 가공 공정 중에 균열이 발생하지 않고쉽게 변형되는 정도, 즉 소성 변형의 용이성을 말한다[4]. 이러한 성형성은 응력 상태(state-of-stress)와고유(intrinsic) 특성으로 구분된다. 우선 전자의 경우,소재의 내부 특성, 즉 미세조직 등과는 무관하며, 변형 영역 및 금형 형상 등과 관계가 있다. 따라서이는 가공 공정 방법에 따라 크게 차이가 나게 된다.또한 후자의 경우, 특정 변형 조건에서의 미세조직변화와 관계가 있으며, 모재의 초기 미세조직과 큰연관성이 있다. 최근에는 고유 성형성을 이론적으로해석하기 위해 변형지도(processing map)이라는 개념이 도입되어, 현재 많은 소재에 적용되고 있다. 이개념은 Frost와 Ashby[5]에 의해 처음 도입 되었으며, 주로 낮은 변형률 속도에서의 크립 거동을 해석*Corresponding Author : [Tel : +82-42-821-2909; E-mail : ymkim78@yahoo.com]