Papers by Author: J.W. Bae

Abstract: This study demonstrates the indirect rheoforging of wrought aluminum alloys fabricated by electromagnetic stirring (EMS). EMS was carried out by varying pouring temperature of theological material and subsequently the rheo-material was forged into the sample which consisted of the direct and indirect forging part, by varying the applied forging pressure. The rheo-material completely filled the die cavity at the applied forging pressure of 150 MPa. To transfer the densification pressure to the end of the part, the applied forging pressure of over 170 MPa was necessary. To control liquid segregation, solid and liquid phases distribute uniformly by deriving the laminar flow. When liquid segregation occurred significantly during rheoforging, the strength revealed as low as 268 MPa. Investigating relationship between microstructural features and mechanical properties of the product, the rheoforged material through EMS revealed the fine and globular microstructure. Microstructure with uniform distribution of solid and liquid phase (no segregation) showed good mechanical property of the rheoforged material whose tensile strength was 341 MPa for Al6061.

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: 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.