Papers by Author: Young Hoon Chung

Abstract: Equal Channel Angular Rolling (ECAR), the severe plastic deformation process, is suitable for shear deforming long and thin sheet continuously. An interesting issue is that thickness of a sheet is not reduced during ECAR. Although shear texture and fine grain structure in Al alloys are easily obtained by ECAR, yet the ECAR process’s difficulties in terms of technical control still remain, such as surface defect, low ductility and low processing speed. The surface defects and processing speed are partially improved by applying a series deformation of rolling and ECAR. A high-speed solution heat-treatment is developed for restoring the ductility of Al 6061 alloy.

Abstract: The effect of nitrogen on creep properties of 22Cr-25Ni austenitic heat-resistant steels with different nitrogen content was studied. The tensile and creep strength increased with increasing nitrogen content. The tensile strength increased due to grain refinement by Nb-rich carbonitride. The creep strength increased with increasing nitrogen content because nitrogen retarded the formation and growth rate of Cr-rich carbide and the growth rate of Nb-rich carbonitride during creep.

Abstract: As the surface friction between feeding rolls and metal sheet generates the feeding power of ECAR, the generated feeding power is low, and the friction between the metal sheet and ECAR die should be minimized. However, for obtaining a large shear deformation by ECAR, the metal sheet should be tightly contacted with the wall of ECAR die. In this condition, the thickness of the metal sheet is continuously increased during ECAR. A new ECAR apparatus is developed for maximizing the shear deformation and obtaining sheet thickness uniformity, and succeeding continuous ECAR with such a limited feeding power. By controlling the outlet gap of the ECAR die with elastic unit, the thickness of the metal sheet is kept uniform. Detailed thickness control mechanism during the new ECAR process is analyzed. A sheet of Al 6063 alloy that is 1-pass deformed with the new ECAR apparatus shows below ±0.037 mm of thickness variation and 0.61 of shear strain.

Abstract: Together with conventional alloys, ultra-fine or nano-structured aluminum alloys were prepared by equal channel angular rolling (ECAR) and pressing (ECAP). Formability of cylindrical bosses was investigated by compression tests of a closed die. Finite element (FE) analysis was also carried out to investigate the effect of die friction on the forming behavior. Cylindrical bosses with the aspect ratio over three were formed in a closed die at elevated temperatures even under a frictional condition, although more uniform deformation was expected under a frictionless condition by the FE simulation. Boss formability increased with increasing temperature and decreasing strain rate, and fine structured aluminum alloys had superior boss formability to the conventional alloys. Near-net shape forming of a simplified cellular phone case was performed at elevated temperatures using a set of closed dies. A nano-structured aluminum alloy showed higher formability in all aspects of bosses, sidewalls and face thickness than conventional alloys.

Abstract: The evolution of annealing textures and microstructures in the aluminum alloy 3103, which was subjected to deformation by either cold rolling or equal channel angular rolling (ECAR), was investigated. Samples of AA 3103 sheets were repeatedly deformed by ECAR up to six passes. In addition, AA 3103 was cold rolled to the same hardness level of the ECARed samples. Upon annealing, the cold rolled sample was recrystallized by the discontinuous recrystallization which gave rise to the formation of the cube texture and large grains bigger than 30 µm. In contrast, the ECARed sample was recrystallized by extended recovery which led to the formation of ultra-fine grains having a size smaller than 3.5 µm.

Abstract: The evolution of texture and microstructure during the equal channel angular rolling (ECAR) and subsequent annealing in aluminum alloy 3003 sheets was investigated. The tools of ECAR were designed to provide a constant shear deformation of the order of 0.5 per passage while preserving the original sheet shape. Samples of the aluminum alloy 3003 sheets were repeatedly deformed by ECAR up to twelve passages. Shear textures developed after the first passage of ECAR. However, the intensity of shear texture components decreased with increasing number of ECAR passages. After a large number of ECAR passages, a random texture developed at the expense of shear texture components. Observations by TEM and EBSD revealed that the degree of misorientations within the deformed grains increased with increasing number of ECAR passages. After recrystallization annealing, samples deformed by ECAR displayed pronounced {111}//ND fiber orientations. The annealed sheets comprising of ultra-fine grains were successfully produced in the samples deformed by a large number of ECAR passages.