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Park, Sangwon,Park, Chulho,Na, Youngsang,Kim, Hyoung-Seop,Kang, Namhyun Elsevier 2019 JOURNAL OF ALLOYS AND COMPOUNDS Vol.770 No.-
<P><B>Abstract</B></P> <P>In this study, we investigated the influence of (W, Cr) carbide on the hardness and tensile properties of friction stir welds (FSWs) for CoCrFeMnNi high entropy alloys. FSW without cracks or voids were achieved for rotational speeds of 400–1000 rpm. As the rotational speed increased, the thinning of the weld thickness compared to the thickness of the base metal was increased. Specifically, an abnormal tornado-shaped region was observed in the stir zone at rotation speeds of 600 rpm and above. The tensile strength increased as the rotation speed increased from 400 to 800 rpm and decreased at a rotation speed of 1000 rpm. The abnormal region with a fine (W, Cr) carbide of 0.5 μm or less was dispersed in the grain boundaries. At a rotation speed of 800 rpm, the dispersion of fine (W, Cr) carbide was optimized to produce grain refinement and maximum tensile strength. However, at 1000 rpm, (W, Cr) carbide coarsened due to high heat input, and the number of carbide particles per unit area decreased, thereby decreasing the hardness and tensile strength.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Friction stir welds without crack or void were achieved for CoCrFeMnNi alloy. </LI> <LI> Fine (W, Cr) carbides of 0.5 μm or less were dispersed in the grain boundaries. </LI> <LI> Dispersion of fine carbides produced grain refinement and maximum tensile strength. </LI> <LI> High heat input decreased the number of carbides per unit area. </LI> <LI> High heat input coarsened (W, Cr) carbides, thereby decreasing the tensile strength. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Sangwon Park,Hyunbin Nam,Youngsang Na,Hyoungseop Kim,문영훈,강남현 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.5
This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurredfrom friction stir welding of CoCrFeMnNi high entropy alloys (HEAs). The rolled and cast HEAs had a grain size of 2.8and 308 μm, respectively. After friction stir welding, the cast HEA weld had a grain size of 1.8 μm, which was coarser thanthat of the rolled HEA weld (1.4 μm). Therefore, the dynamic recrystallization ratios were 60.7 and 99.6% for the rolled andcast HEAs, respectively. The cast HEA weld with a large grain size contained a higher density of high-angle boundaries andtwins than the rolled HEA weld with the small grain size. The cast HEA had a larger resistance to plastic deformation owingto the larger fraction of Σ3 twin boundaries than the rolled HEA during friction stirring. This was associated with the highstrain hardening rate during tensile testing and to the significant amount of W dissolved from the stirring tool in the castHEA weld. Thus, the cast HEA weld had a higher tunnel defects ratio than the rolled HEA weld. The total unbonded ratiosof the rolled and cast HEA welds were 0.2 and 7.2%, respectively, indicating that the rolled HEA had better weldability thanthe cast HEA.
Laser weldability of cast and rolled high-entropy alloys for cryogenic applications
Nam, Hyunbin,Park, Chulho,Moon, Jongun,Na, Youngsang,Kim, Hyoungseop,Kang, Namhyun Elsevier 2019 Materials science & engineering. properties, micro Vol.742 No.-
<P><B>Abstract</B></P> <P>Laser similar welding of cast and rolled high-entropy alloys (HEAs) was performed using the cantor system (Co<SUB>0.2</SUB>Cr<SUB>0.2</SUB>Fe<SUB>0.2</SUB>Mn<SUB>0.2</SUB>Ni<SUB>0.2</SUB>). As the welding velocity was increased from 6 to 10 m min<SUP>−1</SUP>, the shrinkage voids, primary dendrite arm spacing, and dendrite packet size decreased, thus improving the mechanical properties of the cast and rolled HEA welds. The cast HEA welds showed tensile properties comparable to those of the base metal (BM). In all the specimens fracture occurred near the heat-affected zone and BM at 298 K. However, the rolled HEA welds showed lower tensile strength than the BM, and fracture occurred in the weld metal (WM). This can be attributed to the larger dendrite packet size of the WM than the grain size of the BM. In addition, the tensile properties of the specimens at the cryogenic temperature were superior to those observed at 298 K, regardless of the cast and rolled HEA welds. This is because the formation of deformation twins and dislocations was predominant at 77 K. Therefore, the laser similar welds of cast and rolled HEAs are suitable for cryogenic applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Laser similar welds of the cast and rolled HEAs were obtained with no defects. </LI> <LI> The cast HEA welds and BM showed similar tensile properties. </LI> <LI> The tensile strength of the rolled HEA welds was ~ 90% of that of the BM. </LI> <LI> The tensile properties of all the specimens at 77 K were superior to those at 298 K. </LI> <LI> The formation of deformation twins and dislocations was predominant at 77 K. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Cu를 포함한 CrCoFeMnNi 용접재료를 적용한 GTA 용접부의 PWHT 온도의 영향
유성훈(Seonghoon Yoo),이유나(Yoona Lee),나영상(Youngsang Na),김형섭(Hyoungseop Kim),강남현(Namhyun Kang) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
최근 가혹한 환경에서 적용되는 구조용 재료의 수요가 증가함에 따라 재료의 고기능화가 요구되고 있다. 현재 극저온 환경에서 적용되는 구조용 재료는 FCC 결정구조를 갖는 오스테나이트계 스테인레스 강을 주로 사용하나, 이를 용접 재료를 사용할 경우 고온균열 및 입계 예민화 현상이 발생하게 된다. 따라서 이러한 대안으로 현재 고엔트로피 합금 (High Entropy Alloy, HEA)이 개발되었으며, 이를 상온 및 극저온용 구조재료로 적용하기 위한 연구가 활발히 진행되고 있다. 고엔트로피 합금이란, 여러 원소를 혼합하여 발생하는 배열 엔트로피 증가로 인한 hign-entropy effect, sluggish diffusion, lattice distortion, cocktail effect의 4가지 core 현상을 기반으로 기계적 특성이 우수한 합금이다. 특히 기존의 HEA 대비 가격 경쟁력 및 기계적 성질이 우수한 Fe-base 중엔트로피 합금(Medium Entropy Alloy, MEA)이 최근 개발되고 있으며, 이를 구조용 재료로 적용하기 위해 용접 기술을 최적화하는 연구가 활발히 진행하고 있다. 따라서 본 연구에서는 Fe-rich MEA에 적합한 HEA 용접재료 개발을 목표로 진행하였으며, 용접재료는 결정립 미세화, 석출강화 및 제 2상 FCC를 형성시키는 Cu를 소량 첨가하기 위해 FCC계 HEA인 Cantor alloy(CoCrFeMnNi)에 Cu를 코팅하여 (CoCrFeMnNi)99Cu1 용접재료를 개발하였으며, 이를 Fe-rich MEA 모재에 적용하였다. 또한 구조용 재료에 필요한 용접후열처리(Post Weld Heat Treatment, PWHT) 효과를 확인하기 위해 PWHT를 진행하였다. 본 연구에 사용된 모재는 1.5mm 두께의 Rolled Fe-rich MEA를 사용하였으며, V-groove에 언급한 (CoCrFeMnNi)99Cu1 용접재료를 적용하여 GTAW를 실시하였다. PWHT는 700, 800, 900℃에서 1시간 진행하였으며, As-welded 조건과 PWHT 온도에 따른 기계적 성질 및 미세조직 차이에 대해 검토하였으며, 인장 파단 위치에 영향을 미치는 인자에 대해 미세조직학적으로 고찰하였다.