Papers by Author: Chun Pyo Hong

Abstract: The dynamic process of fine grain evolution of 6061 aluminum alloy during modified strain-induced, melt-activated (SIMA) process was studied. The modified SIMA process employed casting, two stage homogenization, warm multi-forging, and recrystallization and partial melting (RAP). Multi-forging was carried out at a strain rate of 9x10-3 s-1 to accumulate high strains, with decreasing temperature from 250 to 200 °C. The alloy multi-forged with the accumulated strain of about 12 and RAP at 640 °C for 10 min exhibited the uniform equiaxed recrystallized grain structure. Accordingly, it was evident that multi-forging was very effective on grain refinement and grain size uniformity. The present modified SIMA process was discussed as an alternative thermo-mechanical processing for preparing the alloys with fine grained structure for semi solid processing.

Abstract: Effects of ingate velocities and the morphology of α-particles on feeding behavior of thin plate were investigated in rheo-diecasting with H-NCM semisolid. In order to acquire various morphological characteristics, three kinds of semisolid, which had different casting parameters, were prepared. In this study, we found that the range of ingate velocity which could achieve laminar flow in thin plate was from 3.88m/s to 6.47m/s. In this range, however, the laminar flow length and the total flow length showed different results according to the characteristics of semisolid. Although the semisolid had high globularity
 the semisolid which had larger α-particles than 100 showed unstable feeding behavior in the range of stable ingate velocity.

Abstract: The effects of die temperature and plunger velocity on the soundness of semisolid cast products were investigated in regards to the flow behavior of the slurry of A356 alloy in the H-NCM rheo-diecasting process. The sound cast was obtained with the die-temperatures of above 220
. The sudden changes of casting velocity resulted in irregular microstructures with micro-porosity and shrinkage defects due to turbulent filling pattern. The high casting velocity at the end of filling has no effect on the castability and the formation of casting defects in H-NCM(Hong-NanoCasting method). The Sr modification provides much finer and spheriodized eutectic silicon particles.

Abstract: Among semisolid forming processes, increasing attention has now been focused on the commercialization of casting processes based on rheocasting in aluminum automotive casting fields because of its advantages compared to thixoforming, in regards to the efficiency in cost and productivity, and mechanical properties. In this paper, an advanced rheocasting process is introduced, which has been developed recently at Yonsei University and is now on the way to practical uses in Korea and Japan. In this method, the semisolid slurry production is based on the nucleation method, in which the nuclei of primary
phase formed in the melt are prevented to grow into dendrites, instead they grow into fine globular particles to become semisolid slurry having rheological behavior. By increasing the number of nuclei, very fine and uniform globular microstructures can be obtained, resulting in an increase of mechanical properties. Recently, commercial rheo-diecasting machines based on the present method have been developed, and now are being used in manufacturing aluminum automotive casting parts. Furthermore, the present method is now being tested to investigate its possibility of application to various alloy systems.