Papers by Author: P.K. Seo

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: 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: Many rheocasting processes had been proposed because of the difficulty of recycling, the limit of material, and the high cost of raw material in thixocasting. But, these rheocasting processes also had disadvantages such as the high initial investment cost and the lower mechanical properties than thixocasting. In this study, a continuous fabrication of rheological material with pressure rotation equipment was newly devised to overcome the disadvantages of rheocasting process. In order to investigate the thixoformability, reheating experiments were carried out with the material fabricated by the newly devised equipment.

Abstract: Die design rule for semi-solid die casting (SSDC) with A356 electromagnetic stirring (EMS) aluminum alloy, was proposed. The die design rule included inspection of machine, part requirements, parting line determination, sleeve, plunger, gating system, overflow, air vent, ejector pin, and heating line design. The specification of gating system, overflow, air vent, plunger tip, and sleeve suitable for respective part were regulated. Two steps die system of lower-positioned gate and three steps die system of center-positioned gate were manufactured for 4 automobile suspension parts, based on the die design rule. For the sound filling pattern and solidification behavior, injection speeds of 4 parts were summarized to the interval (from V1 to V4). As a result of observing the microstructure of 4 parts after T6 heat treatment, primary Al-α phase was globularized and fine Si particles were distributed around the grain boundary. The mechanical properties of 4 parts with T6 heat treatment were investigated and showed ultimate tensile strength (UTS) of 330 MPa, yield strength (YS) of 250 MPa, and elongation of 7.5% as average.

Abstract: The new rheology fabrication process has been developed to rheo die casting and rheo forming process. The barrel type equipment, which could continuously fabricate the rheology material, was specially designed to have a function to control cooling rate, shear rate and temperature. During the continuous rotation of barrel with a constant temperature, the shear rate is controlled with the rotation speed. The barrel surface has both the induction heating system and the cooling system to control the temperature of molten metal. By using this system, the effect of the rotation speed and the rotation time on the microstructure was widely examined. The possibility for the rheoforming process was investigated with microstructural characteristic.

Abstract: The parts manufactured by die casting process usually contain liquid segregation and porosities. To solve these problems, the semi-solid forming process has been applied. The process enables material in the semi-solid state to be completely filled, and parts with the complicated shape to be fabricated by applying relatively low pressure. This process is necessary in order to control the microstructure of the billet as well as to achieve the desired semi-solid billet state. In this study, a horizontal high-frequency induction heating device which can be fabricated by semi-solid forming irrespective of a billet's size was developed. A globular structure of the reheated billet and a billet's temperature distribution during the reheating process for A356 were investigated.