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ADC12 다이캐스트재 조직 및 기계적성질 개선을 위한 마찰교반용접 적용
김영곤,Fujii Hidetoshi 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
본 연구에서는 소재 내 함유 가스에 기인해서 용접부에 블로홀이 발생하기 쉽고, 취약한 금속간화합물도 조대화하기 쉽기 때문에, 일반적으로 종래의 용융용접이 곤란한 재료인 Al합금 다이캐스트재의 ADC12에 대해서 고상접합인 마찰교반용접(Friction Stir Welding)을 적용했다. 마찰교반용접은 합금의 융점이하에서 행해지는 접합프로세스이므로, 용융용접에 비해서 용접변형이 거의 없다. 또한, 산화막의 혼입에 의한 결함형성과 합금원소의 증발에 의한 강도 저하는 큰 문제가 되지 않는다고 보고되고 있다. 그러나, 지금까지의 관련 연구를 보면, Al합금 전신재에 집중되고 있어서, Al합금 주조재 및 다이캐스트재에 관한 마찰교반접합의 실시는 많지 않았다. 이러한 관점에서, 접합시의 툴 가압력을 제어 가능한 하중제어방식의 마찰교반용접 장치를 이용하여, 두께 4mm의 ADC12에 대해서 Stir-in-plate 및 I형 개선 맞대기접합을 행했다. 특히, 조직개질방법의 마찰교반프로세싱의 적용도 함께 실시하여 개질부의 기계적성질의 개선효과를 검토했다.
Thermodynamic calculation of the stacking fault energy in Fe-Cr-Mn-C-N steels
Lee, Seung-Joon,Fujii, Hidetoshi,Ushioda, Kohsaku Elsevier 2018 Journal of Alloys and Compounds Vol.749 No.-
<P><B>Abstract</B></P> <P>To determine the thermodynamic parameters for the calculation of the accurate stacking fault energy (SFE) in Fe-Cr-Mn-C-N steels using the sublattice model, the comparison between the calculated and experimental SFE values was conducted. It was realized that the relationship between SFE and alloying elements in Fe-Cr-Mn-C-N steels was different from that of the conventional Fe-Cr-Ni stainless steels. The SFE increased with the increasing Cr concentration up to a critical value, then decreased again with further increased Cr concentration. The critical value decreased with the addition of Mn, C and N. In contrast to Cr, the addition of Mn continuously increased the SFE, regardless of the additions of C and N. Regarding the C and N, they also increased the SFE linearly and the impact of N on the SFE was only slightly effective relative to that of C. Accordingly, we realized that the thermodynamic calculation using the suggested combination of thermodynamic parameters should be considered for more accurate SFE calculation in Fe-Cr-Mn-C-N steels.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Stacking fault energy in Fe-Cr-Mn-C-N steels was thermodynamically calculated. </LI> <LI> For accurate stacking fault energy, measured and predicted energies were compared. </LI> <LI> Cr increases stacking fault energy, then decreases it again over a critical value. </LI> <LI> Mn, C and N continuously increase stacking fault energy. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Lee, Seung-Joon,Sun, Yufeng,Fujii, Hidetoshi Elsevier 2018 Acta materialia Vol.148 No.-
<P><B>Abstract</B></P> <P>The effect of friction stir welding (FSW) on the microstructure, stacking-fault energy (SFE) and strain hardening rate (SHR) of Fe-18Mn-0.6C-(0 and 1.5)Al (wt.%) twinning-induced plasticity steels using three welding speeds (50, 100 and 200 mm min<SUP>−1</SUP>) was investigated. The yield strength of the FSWed 0Al and 1.5Al steels improved due to both grain boundary strengthening by grain refinement and dislocation hardening by the introduction of dislocations with an increase in the welding speed. Their SHR with three stages and without the yield drop increased due to the active mechanical twinning and the introduction of dislocations during the FSW when the welding speed was increased. Among the 0Al steels, 0Al-200 steel with a fine grain exhibited more active twinning than the coarse-grained specimen (0Al-50), which is contrast to the 1.5Al steel. Regardless of the specimens, the slight increase in the SFE, which was attributed to both the shear strain energy caused by the introduction of dislocations and the excess free energy by the grain refinement during the FSW, leads to an increase in the critical twinning stress (<I>σ</I> <SUB> <I>tw</I> </SUB>). Despite the fine grain of the 0Al steel, the origin of its active twinning was the highly increased yield strength relative to <I>σ</I> <SUB> <I>tw</I> </SUB>, and the promoted dislocation interactions, giving rise to an increase in the number of sites at which twin nucleation occurred.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>