• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2006년도 하계학술대회 논문집 Vol.7
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• pp.325-326
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• 2006

In this study, in order to develop piezoelectric device for multilayer piezoelectric ultrasonic motor, low temperature sintering $Pb(Mn_{1/3}Nb_{2/3})_{0.02}(Ni_{1/3}Nb_{2/3})_{0.12}(Zr_{0.48}Ti_{0.52})_{0.86}O_3$ system ceramics were fabricated according to the variations of forming pressure of casting sheet. At the 300[$kgf/cm^2$] forming pressure, the maximum density of 7.8[$g/cm^3$] was obtained. At the 350[$kgf/cm^2$] forming pressure, the maximum values of effective electromechanical coupling factor $k_{cff}\;=\;0.24$ and mechanical quality factor Qm=628 were obtained.

• 한국주조공학회지
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• 제38권5호
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• pp.95-102
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• 2018

The dependence of the tensile properties on variations in the porosity of A356 aluminium alloys was investigated in terms of the quality index of the tensile properties based upon the ultimate tensile strength and elongation as well as the variation of the strength coefficient and strain-hardening exponent with regard to a T6 treatment. The test specimens were prepared by low-pressure die-casting and a subsequent T6 treatment, and the experimental results of a tensile test carried out at room temperature were compared to the theoretical description using a modified constitutive model. The nominal value of the quality index of A356 alloys increases gradually with a lapse of the ageing time upon a T6 treatment, despite the fact that this value is temporarily decreased during the initial stage of ageing from a solutionised condition. Additionally, the quality index depends practically upon the porosity variation with a power law relationship without regard to whether in solutionised or artificial aged conditions. The theoretical description indicates that the strength coefficient directly determines the nominal level of the quality index. Moreover, the overall dependence of the quality index on the porosity variation is remarkably weakened with an increase in the tensile strain, whereas the quality index depends sensitively upon the porosity variation with a low value of the strain-hardening exponent.

• 대한기계학회논문집A
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• 제40권7호
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• pp.653-658
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• 2016

가스터빈엔진의 고온 부품은 초내열 합금 재료를 이용하며, 냉각설계 적용으로 형상이 복잡하여 정밀 주조 과정을 거치게 된다. 터빈 부품에 주로 적용이 되는 일방향 응고 및 단결정 재료는 제조 과정에서 결정립 성장 방향이 설계와 다르게 섭동을 가지게 되며, 이는 각 방향에 대한 재료 상수의 섭동을 유발하여 응력 분포의 변화와 함께 피로 수명에 큰 산포를 야기하게 된다. 본 연구에서는 일방향 응고 재료 노즐에 대하여 결정립 성장 방향의 섭동에 대한 구조 건전성의 영향을 저주기 피로 수명을 통해 확인하여, 향후 제작 허용값에 대한 제안 및 좀 더 정교한 통계적 접근이 필요함을 확인하고자 한다. 이를 위해 복합 열전달 해석을 통해 금속 온도 분포를 계산하고 이를 근거로 구조 해석 및 저주기 피로 수명을 계산하였다.

• 한국산업융합학회 논문집
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• 제21권1호
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• pp.29-36
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• 2018

Low pressure casting process for unidirectional carbon fiber reinforced aluminum (UD-CF/Al) composites which is an infiltration route of molten Al into porous UD-CF preform has been a cost-effective way to obtain metal matrix composites (MMCs) but, easy to cause non-uniform fiber distribution as CF clustering. Such clustered CFs have been a problem to decrease the density and thermal conductivity (TC) of composites, due to the existence of pores in the clustered area. To obtain high thermal performance composites for heat-sink application, the relationship between fiber distribution and porosity has to be clearly investigated. In this study, the CF distribution was evaluated with quantification approach by using two-dimensional spatial distribution method as local number 2-dimension (LN2D) analysis. Note that the CFs distribution in composites sensitively changed by sizes of Cu bridging particles between the CFs added in the UD-CF preform fabrication stage, and influenced on only $LN2D_{var}$ values.

• 한국주조공학회지
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• 제31권4호
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• pp.198-204
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• 2011

The present study was aimed to investigate the dependence of fatigue property on microporosity variation of low-pressure die-cast (LPDC) A356 alloy. The fatigue property of A356 alloy was evaluated through high cycle fatigue test, and the microporosity-terms used were the fractographic porosity measured from SEM observation on fractured surface and the volumetric porosity obtained through the density measurement using Archimedes's principle. The number of cycles to failure of A356 alloys depends obviously upon the variation of fractographic porosity, and can describe in terms of the defect susceptibility which depends on the microporosity variation at a given value of stress amplitude. The modified Basquin's equation was suggested through the combination of microporosity variation and static maximum tensile stress to fatigue strength coefficient. Using modified Basquin's equation, it could suggest that the maximum values of fatigue strength coefficient and exponent achievable in defect-free condition of A356 alloy are 265 MPa, -0.07, respectively.

• 한국주조공학회지
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• 제39권2호
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• pp.26-31
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• 2019

In this study, the mechanical characteristics and microstructure of hypereutectic Al-17Si-5Fe extruded alloys prepared by a rapid solidification process (RSP) were investigated. The hypereutectic Al alloy was fabricated by means of RSP and permanent casting. For RSP, the Al alloy melted at $920^{\circ}C$, cooling the specimens at a rate of $10^6^{\circ}C/s$ when the RSP was used, thus allowing the refining of primary Si particles more than when using permanent casting, at a rate of about 91%. We tested an extrusion RSP billet and a permanent-cast billet. Before the hot-extrusion process, heating to $450^{\circ}C$ took place for one hour. The samples were then hotextruded with a condition of extrusion ratio of 27 and a ram speed of 0.5 mm/s. Microstructural analyses of the extruded RSP method and the permanent casting method were carried out with OM and SEM-EDS mapping. The mechanical properties in both cases were evaluated by Vickers micro-hardness, wear resistance and tensile tests. It was found that when hypereutectic Al-17Si-5Fe alloys were fabricated by a rapid solidification method, it becomes possible to refine Si and intermetallic compounds. During the preparation of the hypereutectic Al-17Si-5Fe alloy by the rapid solidification method, the pressure of the melting crucible was low, and at faster drum speeds, smaller grain alloy flakes could be produced. Hot extrusion of the hypereutectic Al-17Si-5Fe alloy during the rapid solidification method required higher pressure levels than hot extrusion of the permanent mold-casted alloy. However, it was possible to produce an extruded material with a better surface than that of the hot extruded material processed by permanent mold casting.

• 대한심미치과학회지
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• 제3권1호
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• pp.32-43
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• 1995

Ni-Ti alloy has excellent corrosion resistance, biocompatibility, shape memory effect and superelasticity, so it has been used widely in biomedical fields. But it has difficulty in casting due to its high melting temperature and oxygen affinity at high temperature. Recently it has been attempted to cast Ni-Ti alloy using new casting machine and investment. The purpose of this study was to examine the superelastic behavior of cast shape memory Ni-Ti alloy and to compare the mechanical properties of the cast shape memory alloy with those of commercial alloys for removable partial denture framework. Ni-Ti alloy(Ni 50.25%, Ti 49.75% : atomic ratio) was cast with dental argon-arc pressure casting machine and Type IV gold alloy, Co-Cr alloy, Ni-Cr alloy, pure titanium were cast as reference. Experimental cast Ni-Ti alloy was treated with heat($500{\pm}2^{\circ}C$) in muffle furnace for 1 hour. Transformation temperature range of cast Ni-Ti alloy was measured with differential scanning calorimetry. The superelastic behavior and mechanical properties of cat Ni-Ti alloy were observed and evaluated by three point bending test, ultimate tensile test, Vickers microhardness test and scanning electron microscope. The results were as follows : 1. Cast Ni-Ti alloy(Ni 50.25%, Ti 49.75% : atomic ratio) was found to have superelastic behavior. 2. Stiffness of cast Ni-Ti alloy was considerably lesser than that of commercial alloys for removable partial denture. 3. Permanent deformation was observed in commercial alloys for removable partial denture framework at three point bending test over proportional limit(1.5mm deflection), but was not nearly observed in cast Ni-Ti alloy. 4. On the mechanical properties of ultimate tensile strength, elongation and Vickers microhardness number, cast Ni-Ti alloy was similiar to Type IV gold alloy, Co-Cr alloy, Ni-Cr alloy and pure titanium. With these results, cast Ni-Ti alloy had superelastic behavior and low stiffness. Therefore, it is suggested that cast Ni-Ti alloy may be applicated to base metal alloy for removable partial denture framework.

• 한국세라믹학회지
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• 제31권11호
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• pp.1323-1329
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• 1994

Tape casting and lamination were used to produce heterogeneous laminates with alternating layers of different porosity and homogeneous laminates with component layers of the same porosity. The pore structure was investigated for heterogeneous laminates, and bend strength was measured for comparison with that of homogeneous laminates. For a reference, strength measurement was made for the porous body fabricated by sintering samples dry-pressed at low pressure with spray-dried granules. Strength increase, in the range 50~120 MPa, was achieved in the presence of the surface dense layer, while extensive delamination, presumably responsible for enhanced fracture toughness, took place through the internal porous layer.

• 한국표면공학회지
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• 제30권4호
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• pp.248-254
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• 1997

The sulfidation behavior of a sputter-deposited amorphous coating of 69.0%Nb-16.9Ni-11.9%Al-2.2%Si (at.%) has been investigated as a funtion of temperature.(973-1173K) in pure sulfur pressure of 0.01atm. The sulfidation kinetics of the casting obyed the parabolic rate low over the whole temperature ranges studied. The stlfidation rate increased with the temperature, as expected. The sulfide scale, the composition of which was $Al_2S_3,\;NbS_2,\;Ni_{3-x}S_2\;and\;FeCrS_4$, formed on the amorphous coating was primarily bilayered. Both the outer fastgrowing non-protective 4Al_2S_3$scale and the inner slowly-growing protective $NbS_2$,/TEX> scale and the inner slowly-growing protective $NbS_2$ scale had some Fe and Cr dissolution, which evidently came from the base substrate alloy of stainless steel type 304. Belows the coating, Kirkendall void formation was noticed. Nevertheless, a dramatic improvement of sulfidation resistance was achieved by sputter-depositing Nb-2 Ni-Al-Si layer on the stainless steel 304.

• 한국산학기술학회:학술대회논문집
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• 한국산학기술학회 2001년도 춘계학술대회 발표논문집
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• pp.224-228
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• 2001

자동차용 Al Wheel의 반복 금형 주조 공정에 있어서의 적정 냉각조건 확립 및 수축공 등의 주조결함을 제어하기 위하여 3차원 응고, 유동, 응력 해석 프로그램 연동 기술과 자동 금형온도 조절장치를 실제 현장에 적용하여 생산되는 Al Wheel 제품의 최적 주조조건 및 냉각조건을 확립하였다. 주조과정 중의 금형의 온도 변화 및 주물의 응고, 유동 거동은 수치해석 결과와 잘 일치하는 경향을 나타내었다. 또한 금형온도 자동 조절 장치를 사용함으로서 금형 작업온도의 가열 및 유지가 정량적으로 제어 가능하며, 제품 품질의 유지 및 Cycle Time 최적화를 이룰 수 있으며 궁극적으로는 금형 수명 연장 및 생산성 향상 등을 이룰 수 있었다.