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용탕단조법에 의한 AC8A / Al2O3 복합재료의 기계적 성질에 관한 연구
김기배,김경민,조순형,윤의박 ( Ki Bae Kim,Kyoung Min Kim,Soon Hyung Cho,Eui Park Yoon ) 한국주조공학회 1991 한국주조공학회지 Vol.11 No.6
N/A In this study the fabrication technology and mechanical properties of AC8A/Al₂O₃ Composites by squeeze casting process were investigated to develope for application as the piston materials that require good friction, wear resistance, and thermal stability. AC8A/Al₂O₃ composistes without a porosity and the break of preform were fabricated at the melt temperature of 740℃, the preform temperature of 500℃, and mold temperature of 400℃ under the applied pressure of 1200㎏/㎠ as the results of the observation of microstructures. As the results of this study, the tensile strength of AC8A /Al₂O₃composites was not increased linearly with Al₂O₃volume fraction and so it seemed not to agree with the rule of mixture, which had been used often in metal matrix composite. Also the tensile strength after thermal fatigue test was little different from that before the test. Consequently it was thought that AC8A /Al₂O₃composites fabricated under our experimental conditions had a good thermal stability and subsequently a good interface bonding. Wear rate(i.e., volume loss per unit sliding distance) of AC8A/Al₂O₃composites was decreased with Al₂O₃volume fraction and the sliding speed at both room temperature and 250℃ and so there was a good correlation between wear rate and hardness. Also the wear rate of AC /8A20% Al₂O₃composities was obtained the value of 1.65 ㎤/㎝ at sliding speed of 1.14m/sec as compared with about 3.0×10^(-8)㎤/㎝hyereutectie Al-Si alloy(Al-16%Si-2%Cu-1%Fe-1%Ni), which applied presently for piston materials. The wear behavior of Al₂O₃composites was observed to a type of abrasive wear by the SEM view of wear surface.
김기배,김경민,이재철,이호인,윤의박,조남돈 ( Ki Bae Kim,Kyoung Min Kim,Jae Chul Lee,Ho In Lee,Eui Pak Yoon,Nam Don Cho ) 한국주조공학회 1996 한국주조공학회지 Vol.16 No.3
N/A The effect of forced convection on the solute redistribution of the Al ingot is studied in an effort to fabricate high purity aluminum using a fractional solidification processing. Experimental setup is built in order for the Al melt to be solidified from the surface of the graphite cooling tube, which is rotated at a constant revolution rate to generate the forced convection environmental ahead of the solidification front. Based on the experimental results, Fe and Si concentrations, added as impurities within the Al ingot, decrease as the distance measured from the graphite tube and growth rate decreases and as revolution speed increases. In the case of Al melt with high initial solute concentration, spiral columnar grains tend to form along a direction of revolution. In the case of Al melt with low solute concentrations, however, grains tend to grow with planar geometry. In the fractional solidification process, the solute concentration within the solid phase tends to decrease at the early stage of solidification, reaches minimum concentration, and then increases gradually as solidification proceeds. The solute redistribution obtained based on the BPS model incorporated with the tangential flow component as well as the axial flow component within the melt, agree well with the measurements under sufficiently high revolution speed and low solidification rate.
임정호,김기배,윤우영,윤의박 ( Jeong Ho Lim,Ki Bae Kim,Woo Yung Yoon,Eui Pak Yoon ) 한국주조공학회 1998 한국주조공학회지 Vol.18 No.5
N/A Formation of oxide inclusions in the molten aluminium alloys during solidification is investigated. The oxidation tendency of both Al-4.5wt%Cu and Al-7wt%Si alloys is increased with melt temperature, particularly over 700˚C. However, an Al-5wt%Mg alloy exhibits a decreasing mode over 800˚C. The oxidation behavior with holding time shows the S curve shape for all of the alloys. It is shown that the mechanism of oxidation of Al-5wt%Mg alloy has a two step process different from that of Al-4.5wt%Cu and Al-7wt%Si alloys. The species and morphology of oxide inclusions in each alloy is also shown. The microstructure was more coarsened during solidification when the melt contains a large amount of oxide inclusion than when it doesn`t. This result can be explained in terms of both the hindrance of heat extraction by oxide film formed on the aluminium melt and the difference of heat capacity between the aluminium melt and oxide inclusion during solidification. (Received March 2, 1998)
전자교반에 의해 제조된 Al-4.5wt%Cu 합금의 재가열 시 조직변화에 관한 연구
김정호,윤의박,김기배,김경민,조강래 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.10
Electromagnetically stirred Al-4.5 wt%Cu alloy was reheated into semi-solid state by precisely temperature-controllable gold image furnace. Evolution of microstructures in the semi-solid state was investigated with fixed reheating rate, various holding time and holding temperature. The interface of cells consisting of primary particle formed by electro-magnetic stirring disappeared due to particle regrowth during reheating the alloy. Particle with entrapped eutectic appeared by the coarsening of dendrite which was preciously formed by rapid cooling of residual liquid after stirring stopped and entrapped eutectic changed into K phase by diffusion mechanism as holding time increased. Particles were coarsened, showing polygonal shape and the solute content of solid particle(K phase) increased by redistribution of solute between solid particle and liquid phase, as holding time increased more. Liquid fraction was 11.6% at 637℃ when the alloy was held for 420 minutes, although it is expected 50% by Scheil equation. It was thought that particle coarsening resulted from simple particle growth, coalescence, Ostwald ripening during reheating the electromagnetically stirred Al-4.5 wt%Cu alloy.
전자교반에 의한 반응고 Al-4.5wt % Cu 합금의 미세구조 및 수소가스량의 변화
김정호,윤의박,김기배,임정호 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.10
The quantitative analysis of hydrogen content together with the observation of the morphological changes in the microstructures of Al-4.5wt%Cu alloy induced by electromagnetic stirring was carried out. Hydrogen content was measured in two states. In the liquid state above the liquidus line hydrogen content was increased with increasing melt temperature and isothermal holding time. In semi-solid state using electromagnetic stirring hydrogen content was measured by high vacuum extraction method, according to the conditions of stirring such as cooling rate, input voltage and solid fraction. As the cooling rate was increased under the influence of stirring, the amount of hydrogen gas was increased. It is resulted from the fact that the diffusion of hydrogen due to the difference of hydrogen solubility between in liquid state and in semi-solid state is different with cooling rate. As input voltage was increased, hydrogen content was slightly decreased. This result was different from that of conventional cast, and it is explained in terms of the change of microstructures induced by electromagnetic stirring. However, hydrogen content with the solid fraction was not changed significantly. From above the results, it is also shown that both the changes of the microstructure and hydrogen content by electromagnetic stirring have influence on the type of porosity.