밀링 조건이 고엔트로피 합금의 미세조직 및 기계적 특성에 미치는 영향

서남혁,전준협,김광훈,박정빈,손승배,이석재,Seo, Namhyuk,Jeon, Junhyub,Kim, Gwanghoon,Park, Jungbin,Son, Seung Bae,Lee, Seok-Jae 한국분말재료학회 (*구 분말야금학회) 2021 한국분말재료학회지 (KPMI) Vol.28 No.2

High-entropy alloys have excellent mechanical properties under extreme environments, rendering them promising candidates for next-generation structural materials. It is desirable to develop non-equiatomic high-entropy alloys that do not require many expensive or heavy elements, contrary to the requirements of typical high-entropy alloys. In this study, a non-equiatomic high-entropy alloy powder Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5 (at.%) is prepared by high energy ball milling and fabricated by spark plasma sintering. By combining different ball milling times and ball-to-powder ratios, we attempt to find a proper mechanical alloying condition to achieve improved mechanical properties. The milled powder and sintered specimens are examined using X-ray diffraction to investigate the progress of mechanical alloying and microstructural changes. A miniature tensile specimen after sintering is used to investigate the mechanical properties. Furthermore, quantitative analysis of the microstructure is performed using electron backscatter diffraction.

다른 크기의 분말 적층을 통해 얻은 Fe-6%Mn합금의 경사 미세조직과 기계적 특성

서남혁,이준호,신우철,전준협,박정빈,손승배,정재길,이석재,Seo, Namhyuk,Lee, Junho,Shin, Woocheol,Jeon, Junhyub,Park, Jungbin,Son, Seung Bae,Jung, Jae-Gil,Lee, Seok-Jae 한국분말재료학회 (*구 분말야금학회) 2022 한국분말재료학회지 (KPMI) Vol.29 No.5

A typical trade-off relationship exists between strength and elongation in face-centered cubic metals. Studies have recently been conducted to enhance strength without ductility reduction through surface-treatment-based ultrasonic nanocrystalline surface modification (UNSM), which creates a gradient microstructure in which grains become smaller from the inside to the surface. The transformation-induced plasticity effect in Fe-Mn alloys results in excellent strength and ductility due to their high work-hardening rate. This rate is achieved through strain-induced martensitic transformation when an alloy is plastically deformed. In this study, Fe-6%Mn powders with different sizes were prepared by high-energy ball milling and sintered through spark plasma sintering to produce Fe-6%Mn samples. A gradient microstructure was obtained by stacking the different-sized powders to achieve similar effects as those derived from UNSM. A compressive test was performed to investigate the mechanical properties, including the yielding behavior. The deformed microstructure was observed through electron backscatter diffraction to determine the effects of gradient plastic deformation.

임계간 온도에서 열처리한 구상흑연주철의 미세조직 및 경도 예측

서남혁,전준협,송수영,김종수,김민수 한국주조공학회 2023 한국주조공학회지 Vol.43 No.6

본 연구에서는 임계간 온도 범위에서 열처리한 구상흑연주철의 열처리 온도에 따른 물성 예측을 위해 , 인장강도 450 MPa 급구상흑연주철을 다양한 온도에서 열처리한 후 공냉하여 물성 예측에 필요한 미세조직을 분석하고 브리넬 경도를 측정하였다 . 임계간 온도 구간에서 열처리 온도가 증가할수록 구상흑연주철 내 페라이트 분율은 감소함과 동시에 펄라이트 분율은 증가하였으나 , 흑연 구상화율 및 구상흑연입수는 주방상태에서 측정된 값과 유사하였다 . 열처리한 구상흑연주철의 브리넬 경도는 열처리 온도가 증가할수록 점점 증가하였다 . 측정된 합금 조성 및 각 안정상의 분율 , 그리고 문헌에 알려진 구상흑연주철의 브리넬 경도 예측 모델을 활용하여 열처리 온도 별 구상흑연주철의 경도 값을 계산해 본 결과 , 측정값과 매우 유사한 값을 얻을 수 있었다 . 또한 열역학 계산을 통해 예측된 상분율을 활용하여 정확한 경도 예측이 가능할지 확인해보기 위해 , 열처리 온도 별로 구상흑연주철 내 흑연, 페라이트 및 오스테나이트의 부피를 계산한 후, 이를 면적으로 변환하여 동일한 구상흑연주철의 경도 예측 모델에 적용하였다 . 이렇게 열역학 계산과 경도 예측 모델을 동시에 활용하여 계산된 구상흑연주철의 경도 값은 실제 측정한 브리넬 경도 대비 최대27의 오차 범위 내에서 유사한 값을 나타내었다. In order to predict the mechanical properties of ductile cast iron heat treated in an intercritical temperature range, samples machined fromcast iron with a tensile strength of 450 MPa were heat-treated at various intercritical temperatures and air-cooled, after which a micro-structural analysis and Brinell hardness test were conducted. As the heat treatment temperature was increased in the intercritical temperaturerange, the ferrite fraction in the ductile cast iron decreased and the pearlite fraction increased, whereas the nodularity and nodule count didnot change considerably from the corresponding values in the as-cast condition. The Brinell hardness values of the heat-treated ductile castiron increased gradually as the heat treatment temperature was increased. Based on the measured alloy composition, the fraction of each sta-ble phase and the hardness model from the literature, the hardness of the ductile cast iron heat treated in the intercritical temperature rangewas calculated, showing values very similar to the measured hardness data. In order to check whether it is possible to predict the hardness ofheat-treated ductile cast iron by using the phase fraction obtained from thermodynamic calculations, the volumes of graphite, ferrite, and aus-tenite in the alloy were calculated for each temperature condition. Those volume fractions were then converted into areas of each phase forhardness prediction of the heat-treated ductile cast iron. The hardness values of the cast iron samples based on thermodynamic calculationsand on the hardness prediction model were similar within an error range up to 27 compared to the measured hardness data.

철계 비동일분율 고엔트로피 합금의 인장 강도에 미치는 소결 조건 영향

서남혁,전준협,김광훈,박정빈,손승배,이석재,Seo, Namhyuk,Jeon, Junhyub,Kim, Gwanghun,Park, Jungbin,Son, Seung Bae,Lee, Seok-Jae 한국분말재료학회 (*구 분말야금학회) 2021 한국분말재료학회지 (KPMI) Vol.28 No.3

We fabricate the non-equiatomic high-entropy alloy (NE-HEA) Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5 (at.%) using spark plasma sintering under various sintering conditions. Each elemental pure powder is milled by high-energy ball milling to prepare NE-HEA powder. The microstructure and mechanical properties of the sintered samples are investigated using various methods. We use the X-ray diffraction (XRD) method to investigate the microstructural characteristics. Quantitative phase analysis is performed by direct comparison of the XRD results. A tensile test is used to compare the mechanical properties of small samples. Next, electron backscatter diffraction analysis is performed to analyze the phase fraction, and the results are compared to those of XRD analysis. By combining different sintering durations and temperature conditions, we attempt to identify suitable spark plasma sintering conditions that yield mechanical properties comparable with previously reported values. The samples sintered at 900 and 1000℃ with no holding time have a tensile strength of over 1000 MPa.

다이캐스팅으로 제조된 Silafont-36 합금의 기계적 특성에 미치는 T6 열처리 영향

서남혁 ( Namhyuk Seo ),최승규 ( Seunggyu Choi ),전준협 ( Junhyub Jeon ),윤상일 ( Sang-il Yoon ),김동현 ( Dong-hyeon Kim ),김태영 ( Taeyoung Kim ),손승배 ( Seung Bae Son ),이석재 ( Seok-jae Lee ) 조선대학교 공학기술연구원 2020 공학기술논문지 Vol.13 No.4

Silafont-36 alloy is an Al-Si-Mg based alloy widely used in automotive applications. Its mechanical properties can be changed through T5 or T6 heat treatment. In this study, we investigated the influence of T6 heat treatment on the mechanical properties of Silafont-36 alloy prepared by die casting. The standard T6 heat treatment condition is as follows: (1) solution heat treatment for 2 hours in vacuum atmosphere at 510°C and water quenching, and (2) artificial aging treatment for 4 hours at 180°C in vacuum atmosphere and air cooling. Heat treatments were applied based on the above treatment conditions with changes in the solution treatment and artificial aging treatment conditions. Tensile test, hardness test, and microstructure analysis were performed on the heat-treated specimens. As the solution treatment temperature increased, the tensile strength increased. On the other hand, tensile strength decreased with increasing aging temperature and holding time. From a microstructure analysis, it was confirmed that the tensile strength increased with decreasing secondary dendrite arm spacing.

철계 소결합금의 오스테나이트 안정성

최승규,서남혁,전준협,손승배,이석재 한국분말재료학회 2020 한국분말재료학회지 (KPMI) Vol.27 No.5

In the present study, we investigated the austenite stability of a sintered Fe-based nanocrystalline alloy. The volume fraction of austenite was measured based on the X-ray diffraction data of sintered Fe-based nanocrystalline alloys, which were prepared by high-energy ball milling and spark plasma sintering. The sintered alloy samples showed a higher volume fraction of austenite at room temperature as compared to the equilibrium volume fraction of austenite obtained using thermodynamic calculations, which resulted from the nanosized crystalline structure of the sintered alloy. It was proved that the austenite stability of the sintered Fe-based alloy increased with a rise in the amount of austenite stabilizing elements such as Mn, Ni, and C; however, it increased more effectively with a decrease in the actual grain size. Furthermore, we proposed a new equation to predict the martensite starting temperature for sintered Fe-based alloys.

HVOF 용사 코팅 공정 조건에 따른 코팅층의 기공도 예측

전준협,서남혁,이종재,손승배,이석재 한국분말재료학회(구 한국분말야금학회) 2021 한국분말재료학회지 (KPMI) Vol.28 No.6

The effect of the process conditions of high-velocity oxygen fuel (HVOF) thermal spray coating on the porosity of the coating layer is investigated. HVOF coating layers are formed by depositing amorphous FeMoCrBC powder. Oxygen pressure varies from 126 to 146 psi and kerosene pressure from 110 to 130 psi. The Microstructural analysis confirms its porosity. Data analysis is performed using experimental data. The oxygen pressure-kerosene pressure ratio is found to be a key contributor to the porosity. An empirical model is proposed using linear regression analysis. The proposed model is then validated using additional test data. We confirm that the oxygen pressure-kerosene pressure ratio exponentially increases porosity. We present a porosity prediction model relationship for the oxygen pressure-kerosene pressure ratio.

레이저 분말 베드 용융법으로 제조된 AlSi10Mg 합금의 경도 예측을 위한 설명 가능한 인공지능 활용

전준협,서남혁,김민수,손승배,정재길,이석재 한국분말재료학회 2023 한국분말재료학회지 (KPMI) Vol.30 No.3

In this study, machine learning models are proposed to predict the Vickers hardness of AlSi10Mg alloys fabricated by laser powder bed fusion (LPBF). A total of 113 utilizable datasets were collected from the literature. The hyperparameters of the machine-learning models were adjusted to select an accurate predictive model. The random forest regression (RFR) model showed the best performance compared to support vector regression, artificial neural networks, and k-nearest neighbors. The variable importance and prediction mechanisms of the RFR were discussed by Shapley additive explanation (SHAP). Aging time had the greatest influence on the Vickers hardness, followed by solution time, solution temperature, layer thickness, scan speed, power, aging temperature, average particle size, and hatching distance. Detailed prediction mechanisms for RFR are analyzed using SHAP dependence plots.

침탄 공정 대체를 위한 방전 플라즈마 소결 방법

전준협,이준호,서남혁,손승배,정재길,이석재 한국분말재료학회 2022 한국분말재료학회지 (KPMI) Vol.29 No.3

An alternative fabrication method for carburizing steel using spark plasma sintering (SPS) is investigated. The sintered carburized sample, which exhibits surface modification effects such as carburizing, sintered Fe, and sintered Fe–0.8 wt.%C alloys, is fabricated using SPS. X-ray diffraction and micro Vickers tests are employed to confirm the phase and properties. Finite element analysis is performed to evaluate the change in hardness and analyze the carbon content and residual stress of the carburized sample. The change in the hardness of the carburized sample has the same tendency to predict hardness. The difference in hardness between the carburized sample and the predicted value is also discussed. The carburized sample exhibits a compressive residual stress at the surface. These results indicate that the carburized sample experiences a surface modification effect without carburization. Field emission scanning electron microscopy is employed to verify the change in phase. A novel fabrication method for altering the carburization is successfully proposed. We expect this fabrication method to solve the problems associated with carburization.

IN 939 W 합금의 소결 승온 속도에 따른 물리적 특성과 미세조직 분석

전준협,이준호,서남혁,손승배,정재길,이석재,Jeon, Junhyub,Lee, Junho,Seo, Namhyuk,Son, Seung Bae,Jung, Jae-Gil,Lee, Seok-Jae 한국분말재료학회 (*구 분말야금학회) 2022 한국분말재료학회지 (KPMI) Vol.29 No.5

Changes in the mechanical properties and microstructure of an IN 939 W alloy according to the sintering heating rate were evaluated. IN 939 W alloy samples were fabricated by spark plasma sintering. The phase fraction, number density, and mean radius of the IN 939 W alloy were calculated using a thermodynamic calculation. A universal testing machine and micro-Vickers hardness tester were employed to confirm the mechanical properties of the IN 939 W alloy. X-ray diffraction, optical microscopy, field-emission scanning electron microscopy, Cs-corrected-field emission transmission electron microscopy, and energy dispersive X-ray spectrometry were used to evaluate the microstructure of the alloy. The rapid sintering heating rate resulted in a slightly dispersed γ' phase and chromium oxide. It also suppressed the precipitation of the η phase. These helped to reinforce the mechanical properties.