Papers by Author: C.G. Kang

Abstract: This study demonstrates fabricating rheological material by EMS system attached vacuum pump, in order to improve mechanical properties of rheoforged products by removing defects such as porosity and oxides arising from rheological forging process. The billet fabricated by EMS in vacuum pressure reduced formation of oxides and porosities of the inner material. The billet fabricated by EMS in vacuum pressure below 56 cm/Hg remarkably reduced porosities, comparing to the EMS in atmospheric pressure.

Abstract: This study demonstrated nanoindentation techniques of investigating the effects of size and feature in a microstructure on the mechanical properties of rheology-forged aluminum alloy. Mechanical properties and tribological characteristics of rheology-forged Al2024 wrought aluminum alloy in terms of T6 heat treatment were investigated by varying the aging time by nanoindentation and nanoscratch techniques. By nanoindentation/nanoscratch tests and atomic force microscopy, it was demonstrated that the 4 hour aged material exhibites the highest hardness because of the intermediate precipitate phase θ″, which was precipitated by T6 heat treatment at 495°C. Moreover, the friction coefficients in the precipitates in the eutectic phase region were lower than those in the primary α phase region.

Abstract: The nano/microstructure and mechanical/tribological properties of the eutectic regions in thixo/rheo-cast A356 alloy parts were investigated using nano/micro-indentation and mechanical scratching, combined with optical microscopy and atomic force microscopy (AFM). Most eutectic silicon crystals in the A356 alloy showed a modified morphology as fine-fibers. The aging responses of the eutectic regions in both the thixo/rheo cast A356 alloys aged at 150 0C for different times (0, 2, 4, 8, 10, 16, 24, 36, and 72h) were investigated. Both Vickers hardness (HV) and indentation hardness (HIT) test results showed almost the same trend of aging curves, the peak was obtained at the same aging time of 10 h, and in tribological properties of thixo/rheo-cast, the friction coefficient of thixo-cast was investigated lower than that of rheo-cast.

Abstract: Rheology forming is a novel processing method of semi-solid processing, which is different from traditional mold forging and conventional casting process. The rheological behavior of metallic alloys containing both solid and liquid phases was investigated with the low and high solid fraction ranges. Its obvious advantages are easier to produce complex work pieces because of excellent forming ability, more flexible to shape, and more compact in the inner quality for its high pressure. This research paper presents the theory of the rheology forming process and the results of the finite element simulation of rheology forming for aluminum alloys. In this proposed theoretical models for the rheology forming process involve simultaneous calculations performed with solid phase deformation and the liquid phase flow analysis. To analyze the rheology process, the new flow stress curves of rheology aluminum alloys and the viscosity for the simulation of two-phase flow phenomena have been proposed with as a function of temperature.

Abstract: This paper focuses on fabricated slurry by electromagnetic stirring (EMS) with A6061 and A7075 wrought aluminum alloys. For this EMS process, it is important to find the optimal electromagnetic stirring conditions such as pouring temperature, stirring current and stirring time. After electromagnetic stirring according to each condition, the billets fabricated by EMS were investigated, as was the microstructure and cooling temperature of molten alloy which was directly cooled from liquid state to the semi-solid state during EMS. In use of cooling time and temperature data, it is possible to determine the forming time and stirring time required, respectively.

Abstract: The algorithm two-phase flow model, developed in this study, gives die filling patterns, velocity, temperature and solid fraction of rheology material during rheology forging process. To calculate the velocity and temperature fields, the respective governing equations corresponding to the liquid and solid region were adapted. Therefore, respective numerical models considering the solid and liquid phase co-existent within the rheology material have been developed to predict the defects of part manufactured by the rheology forging process. This study has focused on the simulation of the rheology forging process and calculation of the velocity profiles and temperature distribution. And, to predict the liquid segregation in the part, the deviation of velocity between liquid and solid region in the two-phase flow model was analysed.

Abstract: Semisolid processing is now becoming of greater interest for the production of various parts than pressure die casting. In the rheo die casting process, the important thing is to control the solid particles behavior during the rheo die casting process. So, in this paper, to find out the effect of stirring current and stirring time on the microstructure and mechanical properties, rheo die casting experiments were performed according to the stirring current such as 0, 20, 40 and 60 A and the stirring time such as 20, 40 and 60 sec. The results to the experimental conditions were analyzed.

Abstract: This paper focuses on a rheo-forming of arm parts fabricated by an electromagnetic stirring system (EMS). This forming process takes place under high pressure of high pressure die casting and thin-walled casting is possible. Also, the productivity is higher than for low pressure die casting because of the shorter cycle time. Rheo-forming is advantageous because forming is performed in the semi-solid state with laminar flow and the gas content is low, which makes welding possible. Therefore, this paper examines arm parts with EMS and has investigated the mechanical properties after T6 and T5 heat-treatment.

Abstract: Metal matrix composites (MMCs) have been used for aviation, automobile and nuclear application due to their highlight properties such as superior strength, high specific stiffness, wear, and creep resistance at elevated temperature. For developing MMCs by liquid infiltration, preform will be required. In this research paper, developed a hybrid fiber preform and investigated their microstructure properties. Graphite nano fiber (GNF) and Alumina fiber were used for fabrication of the preform. The main objective of developing a preform is i) attainment of uniform distribution of reinforcement ii) minimization of mechanical and chemical damage. Since, it is extremely difficult to disperse nano-size fibers uniformly into the preform. An attempt has been made for hybrid preform with alumina micro-fiber and graphite nano fiber. Microscopic investigations revealed good disperse of the nanofibers in the preform.

Abstract: In this paper, the influence of electromagnetic stirring on the grain size of A356 alloy inoculated by Al-5Ti-B was investigated. It is found that when stirring is applied at the same time with Al-5Ti-B, the inoculation effect will be disabled. X-ray determination and SEM observation show that stirring hinders the deposition of TiB2 particles; consequently hinder the formation of TiAl3 particles, which were proved to be the nucleus of primary α aluminum phase. If the stirring time is short, the grain size will be abnormally coarse. This experiment also confirms that the grain refinement of rheocasting is caused by dendrite fracture instead of modified nucleation.