• 한국주조공학회지
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• 제14권6호
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• pp.530-540
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• 1994

Aluminium-silicon alloy(JIS AC8A) matrix composites reinforced with SiC particles were fabricated by spray-cast forming process, and the microstructure of powders and preforms produced were studied by using an optical and scanning electron microscopy. SiC particles were co-sprayed by mixed phase injection method during the spray casting process. Most of the composite powders formed by this mixed phase injection method exhibit morphology of particle-embedded type, and some exhibits the morphology of particle attached type due to additional attachment of the SiC particles on the surface of the powders in flight. The preforms deposited were resulted in dispersed type microstructure. The pre-solidified droplets and the deposited preform of SiC-reinforced aluminium alloy exhibit finer equiaxed grain size than that of unreinforced aluminium alloy. Eutectic silicons of granular type are crystallized at the corner of the aluminum grains in the preforms deposited, and some SiC particles seem to act as nucleation sites for primary/eutectic silicon during solidification. Such primary/eutectic silicons seem to retard grain growth during the continued spray casting process. It is envisaged from the microstructural observations for the deposited preform that the resultant distribution of SiC injected particles in the Al-Si microsturcture is affected by the amount of liquid phase in the top part of the preform and by the solidification rate of the preform deposited.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 춘계학술대회 논문집
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• pp.273-276
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• 2004

A356 alloy is one of the most popular casting aluminum alloys due to its good castability. It is well known that the mechanical properties of A356 alloy strongly depend on its characteristic microstructure, such as the size of eutectic Si, primary $\alpha$ dendrite and so on. These microstructural features are determined during the casting and solidification process, which implies the strong relationship with mechanical properties with solidification methods. In the present study, the mechanical characteristics of A356 alloy was investigated by using squeeze cast control arm in terms of the microstructural features, such as the size of eutectic Si, primary a dendrite. By doing so, the most favorable microstructure of A356 could be determined for Al control arm that should be one of the most reliable parts in automobile.

• 한국주조공학회지
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• 제16권2호
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• pp.141-148
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• 1996

In high chromium cast iron, the control of matrix microstructure as well as carbide structure is important to the performance as a wear resistant material. In this study, 3.0% C-24.0% Cr white cast irons with various molybdenum contents(residual, 1.0%, 3.0% and 5.0%) were solidified conventionally and unidirectionally for studying their effects on the microstructure and hardness. In the conventional casting, two sets of castings were poured from each melt. One set of the castings consisted of cylindrical bars of 10 and 20mm by 155mm long. The second set of the castings was a cylindrical bar of 30mm by 200mm long. On the other hand, a pep-set mold set on the Cu plate was employed to make the solidification unidirectionally. X-ray diffraction method was used to observe retained austenite and carbides in the high chromium cast iron. The morphology of eutectic $M_7C_3$ carbides changed from needle-like type to nodular type with the increase of Mo content. And, the presence of $M_2C$ carbides was identified in the sample where Mo was added over 3.0 %. Primary and eutectic carbides appeared as rod type and corngrain type, respectively in the unidirectionally solidified samples which were cut to parallel to the solidification direction. In the EDX analysis, Cr concentration was higher in the primary and eutectic $M_7C_3$ carbides, Mo in the $M_2C$ carbides, and Fe in the matrix.

• 한국재료학회지
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• 제4권2호
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• pp.177-186
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• 1994

일방향 응고한 $AI-CuAI_{2}$공정 복합재료의 미세조직에 미치는 응고조건과 진동의 영향에 대해서 조사하였다. 진동하에서 성장속도(R) 변화에 따라 일방향응고 시켰을때 열구배(G)는 $32^{\circ}C$/cm와 35$^{\circ}C$/cm를 유지했다. 공정조직은 성장조건(G/R)에 따라 변화되었다. 조직미세화는 진동의 영향도 컸지만 G/R의 영향이 더 크다는 것을 알 수 있었다. 층상 간격은 $\lambda^{2}$ =상수 라는 관계에 따라 성장속도와 더불어 변화 되었다.

• 한국주조공학회지
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• 제21권6호
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• pp.343-349
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• 2001

The effect of cooling rate on the solidification microstructure and segregation behavior of 7075 and 7050 aluminum alloy was investigated. Samples were solidified with cooling rates from 0.3 to $17^{\circ}K/sec$. Using the cooling curves of each sample, liquidus, eutectic and intermetallic reaction temperatures were estimated. The microstructures were characterized in terms of dendrite arm spacing and eutectic volume fraction. The segregation behavior of each alloying element of these alloys in various cooling rates was discussed.

• 한국주조공학회지
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• 제11권3호
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• pp.208-216
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• 1991

The solidification behavior of the squeeze cast composites of aluminum alloys reinforced with boron fiber($100{\mu}m$) and silicon carbide fibers($140{\mu}m$ and $15{\mu}m$) were investigated. Al-4.5wt%Cu and Al-l0wt%Mg were chosen for the matrix phase of the composites. In the squeeze cast specimen with high thermal difference between fiber and melt, the average secondary dendrite arm spacing(DAS) in reinforced alloy is smaller than that in unreinforced alloy. It was also observed that primary ${\alpha}$ and non-equilibrium eutectic, which seems to be penetrated and solidified at the final stage of the solidification of the matrix, are irregularly distributed around fibers. It is considered that cold fibers serve as heterogeneous nucleation site. While in the remelted and resolidified specimen without temperature difference, the DAS was not changed with reinforcement and microstructure reveals non-equilibrium eutectic with relatively uniform thickness around fibers. It might be evident the nucleation starts at interfiber region. Microsegregation decreases with the decrease in cooling rate and with reinforcement in the as-squeeze cast specimen. Al-10wt% Mg alloy shows less microsegregation than Al-4.5wt%Cu alloy.

• 대한기계학회논문집B
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• 제22권5호
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• pp.698-707
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• 1998

A simplified model for predicting microsegregation during columnar-dendritic solidification of binary alloys is developed, in which back diffusion, dendrite arm coarsening and dendrite tip undercooling are simultaneously incorporated. The inclusion of tip undercooling is accomplished by modifying the initial conditions of the existing solute diffusion model, in such forms that tip undercooling depresses the beginning of solidification below the liquidus temperature, and that the secondary arm spacing evolves in accordance with the minimum undercooling theory. Sample calculations for the well-known benchmark system show that the present predictions not only consist with the extablished limiting cases, but also agree favorably with the available experimental data within a reasonable tolerance. In particular, a typical decreasing trend in the eutectic fraction at high cooling rates is successfully resolved. Comparison of the individual and combined effects of characteristic parameters in reference with the limiting cases reveals the interactions among parameters. Every parameter plays the role of reducing the eutectic fraction, and the degree of influence depends primarily on the cooling rate. Coarsening enhances the effect of tip undercooling, while suppressing that of back diffusion. A vigorous back diffusion seems to restrain the apperance of the undercooling effect. Overall, each contribution of the three parameters to microsegregation is estimated to be of the same order, which suffices to justify the present study.

• 한국주조공학회지
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• 제37권4호
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• pp.108-114
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• 2017

In this study, we investigated the effect of the Si content on the secondary dendrite arm spacing (SDAS) of hypoeutectic Al-Si binary alloys in the range of 4~10 wt% Si. Cooling curves were measured during the solidification of the alloy cast in a step-wise mold. We compared two kinds of solidification time: the first is the total solidification time for both dendritic and eutectic growth, and the second is the solidification time for only dendritic growth. The proportional constant in the relationship between SDAS and cooling rate was estimated, as this constant represents the stability of the cast microstructure. The proportional constant decreased with the Si contents from 4 wt% to 8 wt%, and it remains relatively uniform with up to 10 wt% of Si.

• 한국결정성장학회지
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• 제11권4호
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• pp.170-175
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• 2001

It was possible to grow the $Al_{2}O_{3}$ based $Y_{3}A_{5}O_{12}(YAG),ZrO_{2}$ binary and ternary eutectic fibers using micro-pulling down method with a growing rate of 0.1~15 mm/min. While $Al_{2}O_{3}/ZrO_{2}$ showed cellular-lamellar structure, $Al_{2}O_{3}$/YAG and $Al_{2}O_{3}$/YAG/$ZrO_{2}$ternary eutectic fibers showed homogeneous Chinese script lamellar structures. The microstructures of $Al_{2}O_{3}/ZrO_{2}$ binary eutectic fibers changed with solidification rate from lamellar pattern to cellular structure. The interlamellar spacing agreed with the inverse-square-root dependance on pulling rate according to $\lambda$=$kv_p\;{-1/2}$. $Al_{2}O_{3}/ZrO_{2}$ binary eutectic fibers recorded the highest tensile strength of about 1560MPa at room temperature. $Al_2O_3/YAG/ZrO_2$ternary eutectic fiber showed excellent thermal stability to $1200^{\circ}C$ without significant decrease. The maximum strength of ternary eutectic fibers recorded were 1100MPa at $25^{\circ}C$ and 970MPa at $1200^{\circ}C$, respectively.

• 한국주조공학회지
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• 제32권6호
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• pp.261-268
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• 2012

The effects of thickness, base element and additive to inoculant on the number of eutectic cells and chill depth of thin-section gray cast iron were investigated. Meanwhile the number of eutectic cells increased by inoculation, chill depth decreased. The former decreased and the latter increased by holding the melt at the temperature range between 1,450 and $1,500^{\circ}C$. The former was more for the thinner casting with the thickness of 5 mm than the other. The result of thermal analysis coincided well with the change of macrostructure. The former increased and the latter decreased with the increased contents of carbon, silicon and the silicon content by inoculation. The former decreased and the latter increased with increased manganese content. The number of eutectic cells decreased as the amounts of rare earth and the bismuth added to this inoculant increased. With the addition of sulfur of 0.10 wt% of the weight of this inoculant, the maximum number of eutectic cells was obtained.