https://doi.org/10.3365/KJMM.2026.64.1.1

임하윤(Ha-Yoon Lim) ; 김병주(Byung-Joo Kim) ; 천현석(Hyun-Seok Cheon) ; 김수현(Su-Hyeon Kim) ; 이욱진(Wook-Jin Lee)

This study examined the influence of Sr addition on the microstructure and mechanical properties of Al?Si/Cu bimetals fabricated by compound casting. Particular emphasis was placed on the multi-phase layer that formed beyond the intermetallic compound layer at the Al?Si/Cu interface. While most previous research has primarily highlighted how the thickness of the intermetallic compound layer affected mechanical performance, the present work focused on modification of eutectic phases in the multiphase region and their impact on overall material behavior. The addition of Sr to the Al-Si alloy did not produce major changes in the intermetallic compound layer itself. However, it significantly refined and homogenized the eutectic phases in the multi-phase region, leading to a finer and more uniformly distributed microstructure. Compression testing demonstrated that the bimetals containing Sr exhibited both higher strength and greater elongation compared to those without Sr. Furthermore, a clear transition in fracture behavior was observed: cracks that used to propagate rapidly along brittle intermetallic compound phases in the Sr-free bimetals were delayed and gradually deflected through the interfaces in the Sr-modified bimetals. Overall, the results demonstrate that controlling eutectic phases in the multiphase layer, combined with compound casting, provides an effective strategy to optimize microstructure and improve the mechanical reliability of the Al?Si/Cu bimetals.

https://doi.org/10.3365/KJMM.2026.64.1.14

유채은(Chae Eun You) ; 윤성민(Seong Min Yun) ; 강남현(Namhyun Kang) ; 김용(Yong Kim) ; 이제인(Je In Lee)

Substantial efforts have been devoted to laser welding techniques to join Al and Cu in battery packs because the formation of intermetallic compounds during fusion welding degrades both the electrical and mechanical properties of the welds. In this study, a dual-beam laser with a fixed core beam power (700 W) was employed for the dissimilar welding of Al and Cu thin sheets, and the ring beam power was adjusted to optimize the microstructure and mechanical properties of the lap joints. The Cu base metal was partially penetrated at ring beam powers below 500 W, while the Cu sheet was fully penetrated at ring beam powers above 700 W. At the ring beam power of 300 W, the formation of intermetallic compounds was suppressed in the Al fusion zone due to the lower recoil pressure of the melt pool. However, a ring beam power above 700 W resulted in the formation of intermetallic compound layers at the Cu fusion line. Resistance to crack propagation along the interface between the Cu fusion zone and the heat-affected zone was revealed to be crucial to improving the mechanical properties of the Al-Cu lap joints. The highest tensile shear strength and improved fatigue life was obtained at the ring beam power of 300 W. Results demonstrated that the dualbeam laser welding could effectively control the formation of intermetallic compounds in the fusion zone and enhance the mechanical performance of the welds.

https://doi.org/10.3365/KJMM.2026.64.1.27

하정우(Jung-Woo Ha) ; 김성웅(Sung-woong Kim) ; 정민찬(Min-Chan Jung) ; 손호상(Ho-Sang Sohn)

This study investigates a methodology for quantitatively assessing the welding phenomenon during High-Frequency Electric Resistance Welding (HF-ERW) to achieve stable resistance against Hydrogen Embrittlement (HE). This assessment is performed by analyzing variations in oscillation frequency. The occurrence of Hydrogen-Induced Cracking (HIC) was observed to depend on variations in heat input, with cracks primarily initiating and propagating from welding inclusions (penetrator) distributed along the ND(Normal Direction) within the bondline. The residual presence of these inclusions is fundamentally determined by the formation of the narrow gap and migration of the bridge, which are themselves dictated by changes in the welding phenomenon. Since the dynamic evolution of the narrow gap and bridge directly causes fluctuations in the current path, these path changes were quantitatively detected by monitoring variations in the oscillation frequency. From these frequency variations, a key metric termed ‘‘Strength of Heat Input’’?representing the heat input applied to the material during one cycle of bridge movement?was extracted. The stability of the welding process can be reliably assessed by monitoring the deviation of the Strength of Heat Input over a unit of time. A large deviation under the constant welding conditions signifies irregular formation and migration of the bridge. This irregularity subsequently increases the probability of residual welding inclusions (penetrator), that severely reduce resistance to hydrogen embrittlement. This research contributes to enhancing the hydrogen embrittlement resistance of HF-ERW by establishing a quantitative framework for assessing and maintaining stable welding phenomena.

https://doi.org/10.3365/KJMM.2026.64.1.39

조대현(Dae-Hyun Jo) ; 도정현(Jung-Hyun Do) ; 서성문(Seong-Moon Seo) ; 권석환(Suk-Hwan Kwon) ; 안성철(Seung-Cheol Ahn) ; 구지호(Ji-Ho Gu) ; 이재현(Jehyun Lee)

Nickel-based single-crystal superalloys are widely used in turbine blades for aerospace and power generation applications due to their excellent high-temperature properties including superior creep resistance and microstructural stability. However, complex compositions increase susceptibility to freckle defects that severely degrade mechanical performance by disrupting single-crystal structure and forming detrimental grain boundaries. This study compares conventional CMSX-4 SLS with newly developed NASX (New Alloy Single Crystal superalloy) under identical directional solidification conditions. CMSX-4 SLS exhibits freckles in thick sections due to density inversions from the segregation of refractory elements (Re, W) to solid phases and light elements (Al) to liquid phases, causing buoyancy-driven convection that disrupts dendritic structure and promotes freckle chain formation. NASX was strategically designed with reduced Re and Al contents and increased Ta and Mo to minimize liquid density gradients during solidification. High-density Ta concentrates in the interdendritic liquid to compensate for the density reduction caused by Al segregation and Re/W depletion. Mo mitigates Re segregation through favorable elemental interactions. Thermo-Calc calculations confirmed NASX exhibits a significant reduction in density variations between interdendritic and bulk liquids compared to CMSX-4 SLS. Rayleigh number analysis established critical thresholds, with NASX remaining below this limit. Experimental results confirmed complete freckle suppression in NASX castings under conditions where CMSX-4 SLS developed extensive freckles, demonstrating effective composition-controlled alloy design for improved castability and manufacturing yield.

https://doi.org/10.3365/KJMM.2026.64.1.49

허우로(Uro Heo) ; 이재원(Jaewon Lee) ; 배영훈(Young Hoon Bae) ; 김경훈(Kyeonghun Kim) ; 양해웅(Haewoong Yang)

This study investigates the effects of minor lanthanum (La) addition and heat treatment on the microstructure and mechanical properties of Al?10%Si?2%Cu alloys produced by high-pressure die casting. Alloys containing 0 wt.% and 0.5 wt.% La were solution-treated at 500 oC and aged at 160 oC for 0 and 6 hours. Microstructural observations showed that the combined application of La addition and aging led to significant grain refinement of α-Al and a more homogeneous distribution of eutectic Si phases. SEM/EDS and XRD analyses revealed the formation of polygonal and needle-like La?Cu intermetallic compounds, including the thermally stable LaCu2Al4Si1 phase, which effectively inhibited grain coarsening during aging. DSC analysis showed that the contact angle between α-Al and Si phases decreased from 21.3o to 14.4o after aging, indicating improved interfacial wettability and reduced nucleation energy. These changes promoted the formation of fine and uniform microstructures. As a result, the alloy containing 0.5 wt.% La and aged for 6 hours exhibited the highest average microhardness of 124 HV. The improved mechanical performance is attributed to the combined effects of grain refinement, suppression of interfacial energy, and uniform precipitation behavior. These findings emphasize the critical role of La in stabilizing the microstructure and enhancing hardness through controlled phase distribution and interfacial modification in diecast aluminum alloys.

https://doi.org/10.3365/KJMM.2026.64.1.57

배민아(Min A Bae) ; 김양도(Yang do Kim) ; 백재호(Jae Ho Baek)

In recent years, the steel industry has been actively pursuing the development of cold briquetting technology for iron ore as a response to both resource depletion and the urgent demand for carbon dioxide (CO2) emission reduction. Compared with conventional sintering and pelletizing processes, cold briquetting at ambient temperature can significantly reduce energy consumption by eliminating the high-temperature firing step. Moreover, by directly charging briquettes made from natural iron ore into the blast furnace, the reaction efficiency inside the furnace can be improved, which in turn reduces coal consumption of coal and contributes to the overall decarbonization of the ironmaking process. However, to enable their direct use in the blast furnace, the briquettes must exhibit sufficient strength under both ambient and elevated temperature conditions. In this study, a composite binder system was synthesized using sodium silicate was employed as the main component due to its excellent thermal stability. To further enhance the binding performance, controlled amounts of sodium hydroxide and hexametaphosphate were incorporated into the binder formulation. The chemical bonding structure of the synthesized binder was systematically analyzed to clarify the mechanism of strength development in the binder. Using the optimized binder, cold briquettes were fabricated using the optimized binder and their mechanical properties were evaluated under both room and high-temperature conditions. The experimental results enabled the determination of the optimum binder composition and provided useful insights into the practical applicability of sodium silicate-based binders for cold briquette production in blast furnace operations.

https://doi.org/10.3365/KJMM.2026.64.1.64

이초현(Cho-Hyeon Lee) ; 이민영(Min-young Lee) ; 조원희(Won-Hui Jo) ; 김장중(Jang-Jung Kim) ; 천재은(Jae-Eun Cheon) ; 한주연(Ju-Yeon Han) ; 최현주(Hyun-Joo Choi) ; 조기섭(Ki-Sub Cho) ; 이현정(Hyun-Jung Lee) ; 이영국(Young-Kook Lee) ; 설재복(Jae-Bok Seol)

Recent research on medium manganese steels has highlighted their potential for achieving an exceptional strength-to-ductility balance, primarily through the synergistic activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) mechanisms. Medium manganese steels, generally containing 3?12 wt.% Mn, offer a unique combination of high strength and elongation with lower alloying costs than high-Mn steels. The TRIP mechanism enhances strength by promoting the stress-induced transformation of metastable austenite into martensite, while the TWIP mechanism increases ductility through twin formation and the associated high work-hardening rate. These mechanisms are strongly influenced by stacking fault energy (SFE). Medium mn steels with SFE values in the range of 18?25 mJ/m2 can simultaneously activate both, thereby overcoming the conventional strength?ductility trade-off. TRIP tends to dominate during the early stages of deformation, providing rapid strain hardening, while TWIP becomes more active at later stages, sustaining the hardening rate and prolonging the balance between strength and ductility. This paper reviews the microstructural basis of TRIP and TWIP, focusing on analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), as well as recent in-situ TEM observations that directly capture phase transformation and twinning. Insights into the material's potential applications and future research directions are provided to highlight medium manganese steels as a promising material for next-generation high-performance applications.