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Interlayer Material Design Reducing Transient Liquid Phase Bonding Time
Sunghyun Sohn,Byungrok Moon,Junghoon Lee,Namhyun Kang,Young Hoon Moon 대한금속·재료학회 2020 ELECTRONIC MATERIALS LETTERS Vol.16 No.2
Power semiconductors require a large bonding area for die attachment. For this purpose, transient liquid phase bonding(TLPB) was applied using fabricated interlayers, namely Sn–3Cu and Sn–10Cu, to study the intermetallic compound (IMC)formation, and the results were compared with those obtained with a pure-Sn interlayer. The Sn–3Cu and Sn–10Cu interlayersexhibited primary IMC fraction of 0.06 and 0.24, respectively, before the TLPB. For a Cu/interlayer/Cu sandwich structure,the TLPB was applied at 250 °C over various time periods (1–4 h). A reduction in the bonding time was more signifi cant fora Sn–10Cu interlayer with a larger amount of primary IMCs than for the Sn–3Cu interlayer. The time exponent of the IMCfraction with respect to the bonding time was approximately 0.3 for all interlayers. This implies that the IMC growth mechanismis governed by a liquid channel or wet grain boundary diff usion. The nearly constant fraction and increasing size of theprimary IMCs produced during the TLPB indicate that the primary IMCs coalesced during this process. The primary IMCspreferentially coalesced with the interface IMCs produced during the TLPB when they had the same crystalline orientation.
Seonghoon Yoo,Byungrok Moon,Myeonghwan Choi,Xian Guo,Junghoon Lee,Namhyun Kang 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.11
In this study, the effect of the Ti/N ratio on the coarsening behaviour of (Ti, Nb)(C, N) precipitates and prior austenite grainsize (PAGS) in the coarse-grained heat-affected zone (CGHAZ) during the welding process was investigated. Oscillationand tandem electro-gas welding (EGW) were simulated using the finite element method to calculate the thermal history ofthe CGHAZ. The calculated thermal history was applied to the Gleeble specimens to simulate the CGHAZ of tandem EGW. As the Ti/N ratio decreased to ~1.5, and the amount of N increased, finer and more (Ti, Nb)(C, N) precipitates existed inthe as-rolled and CGHAZ specimens (peak temperature ~ 1335 °C), thereby producing a small PAGS. Comparing with theprevious studies regarding to the isothermal kinetics conducted by isothermal heat treatment of Ti(C,N), the athermal kinetics(simulation of tandem EGW) of (Ti,Nb)(C,N) showed the insignificant growth of PAGS. Furthermore, Nb-rich precipitateswere dissolved in the matrix above 1000 °C, and they did not contribute to the PAGS growth of the CGHAZ.
유한요소해석을 통해 모사된 tandem EGW의 CGHAZ에서 Ti / N 비와 Nb 함량이 초기 오스테나이트 결정립 크기에 미치는 영향
유성훈(Seonghoon Yoo),문병록(Byungrok Moon),최명환(Myeonghwan Choi),강남현(Namhyun Kang) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
조선, 해양플랜트 및 초고층 구조물 등에 적용되는 후판 강재의 경우 용접시간 단축을 통해 획기적인 원가 절감 및 생산성 향상이 가능하다. 이러한 후판 강재의 용접 생산성은 350 kJ/cm 이상의 EGW/tandem EGW 초대입열 용접을 통해 확보할 수 있다. 하지만 초대입열 용접의 과도한 용접 입열로 인해 용착금속과 열영향부 인성이 저하되는 문제점이 있기 때문에 초대입열 용접에도 충격 인성을 확보할 수 있는 강재 개발이 필요하다. 최근 Ti, Nb, V, Mo 등을 첨가하여 탄화물, 질화물 그리고 탄질화물을 형성해 초기 오스테나이트 결정립 성장을 억제시켜 충격 인성을 확보하는 방법이 적용 중이다. 하지만 일정한 주기를 가지고 진동하는 2개의 용접봉이 적용된 tandem EGW의 모델링 및 여러 가지 화학 조성이 기계적 특성에 미치는 영향에 관한 연구가 미비하며 유한요소해석을 통해 tandem EGW의 CGHAZ를 모사해 CGHAZ의 충격인성 확보를 위한 화학조성 최적화에 관련된 연구가 필요하다. 따라서 본 연구는 유한요소해석으로 tandem EGW를 모사했고, 결과로 얻은 CGHAZ에서의 열이력을 글리블 실험을 통해 여러 가지 화학 조성의 강재에 적용해 Ti/N 비와 Nb 함량이 CGHAZ의 초기 오스테나이트 결정립 크기에 미치는 영향에 대해 연구하였다. 글리블 실험 후 얻은 TEM 결과를 통해 석출물의 평균 입도 및 분포와 부피 분율을 측정했다. Ti/N 비가 변화함에 따라 초기 오스테나이트 결정립 크기가 변하였고 hypo-stoichiometric ratio의 비를 가질 때 더 우수한 결정립 미세화 효과를 보였으며 Nb 함량이 증가할수록 Nb - rich 상의 heterogeneous nucleation이 증가해 더 많은 양의 석출물이 관찰되었다.
Sourav Kr. Saha,Hyoungjin An(안형진),Byungrok Moon(문병록),Jaeseok Yoo(유재석),Jong Min Park(박종민),Kwang Hee Yun(윤광희),Namhyun Kang(강남현) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
The study is devoted towards the investigation of a hydrogen-induced cold cracks (HICC) in welded joints of structural steels. HICC is a well-known phenomenon in structural metals that limit their uses in various valuable applications such as hydrogen transport, marine application, ship building etc. Normally, hydrogen in the weld metal produces hydrogen embrittlement (HE), therefore suppressing the cold crack and HE should be considered. In the present study, multi-pass butt welds were fabricated with low and high heat input conditions (10 kJ/cm and 35 kJ/cm) using various welding fillers having various strength levels as of American Welding Society standard and two base metals (DH36 and S500). HE effects on various specimens were evaluated using the in-situ slow strain rate test (SSRT) and hydrogen permeation test. The HE sensitivity index (HE index) calculated using the SSRT suggests that HE index increased with the strength of welding fillers in low heat input condition. However, reverse phenomenon occurred in high heat input condition. Hydrogen diffusion coefficient (Deff) and reversible hydrogen trap concentration (Crev) calculated using permeation test reflects that Crev increased with the strength level of welding filler. On the other hand, welding specimen fabricated in high heat input condition showed a lower Crev and higher Deff in comparison to the low heat input specimen and thereby decreasing the HE. Microstructure analysis suggests that in low heat input condition, low transformation bainite phase was dominated and vulnerable for HE with increasing strength level of welding filler. On the other hand, high heat input condition produced larger amount of acicular ferrite with increasing strength level and inhibited HE.