Papers by Author: Sang Ho Ahn

Abstract: In this study, we investigate the effects of high energy milling under Ar atmosphere on the morphology, size and microstructure of Cu-Cr alloy powders prepared by gas atomization. The attrition milling using stainless steel balls is performed up to 60 hrs. The observation by SEM and TEM shows apparent sequential changes of morphology and size of powders similar to those that are typical for mechanically alloyed elemental powders. Prolonged milling in the steady state regime produces a nano-crystalline structure, consisting of extremely small grains of 20 to 50 nm in size and finely dispersed Cr particles. It is also shown that the uptake of Fe from the stainless steel balls and vessel is appreciable. The microhardness of milled powder increases with the milling time, reaches its peak and then slightly decreases.

Abstract: The formability of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass has been investigated in the present study in relation to the heating rate. A series of extrusion tests after rapid heating has been performed in a laboratory scale together with sheet forming tests after slow heating for comparison purpose. The basic processing map based on dynamic materials model (DMM) and compression test data has been utilized to evaluate feasible forming conditions. The macroscopic formability, classified by fully formed, partially formed or a catastrophic fracture, is found to have a good correspondence with the iso-efficiency contour in the processing map. The region of high power dissipation efficiencies with η>0.8 is found to be broaden by avoiding crystallization events due to reduced exposure time in extrusion process with a faster heating rate.

Abstract: In the present study, powder injection molding (PIM) process, in which the porosity of sintered parts can be easily controlled, has been employed to produce specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel, which is the precipitation hardenable stainless steel containing 4% of Cu and well known to show high strength and the high corrosion resistance at the same time. The 17-4 PH stainless steel powders used in this study were produced by the high-pressure water atomization method. Based on the results of tensile tests on the specimens with the various porosity, a new approach to predict the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established.

Abstract: In this study, optimum processing conditions for strip-cast AZ31 Mg alloy was investigated on the basis of processing map and microstructural analysis. To obtain the processing map, isothermal compression tests were carried out to a strain of 0.5 at temperatures of 200 ∼ 400°C with the strain rates of 0.01 ∼ 10s-1. It was found that maximum efficiency indicating the optimum processing condition occurred at 300°C and 10s-1. The possible deformation mechanisms operating at high temperature was also discussed.

Abstract: The effect of warm rolling under various conditions on the microstructure and mechanical property was investigated using an AZ31 Mg alloy sheet. Several processing parameters such as initial thickness, thickness reduction by a single pass rolling, rolling temperature, roll speed, and roll temperature were varied to elicit an optimum condition for the warm rolling process of AZ31 Mg alloy. Microstructure and mechanical properties were measured for specimens subjected to rolling experiments of various conditions. Warm rolling of 30% thickness reduction per pass was possible without any side-crack at temperatures as low as 200oC under the roll speed of 30 m/min. The initial microstructure before rolling was the mixed one consisting of partially recrystallized and cast structures. Grain refinement was found to occur actively during the warm rolling, producing a very fine grain size of 7 µm after 50% reduction in single pass rolling at 200oC. Yield strength of 204MPa, tensile strength of 330MPa and uniform elongation of 32% have been obtained in warm rolled sheets.

Abstract: The spray forming process was used to produce Al-25Si alloy in this study. The microstructure of the hypereutectic Al-25Si alloy appeared to consist of Al matrix and equiaxed Si particles with average diameter of 5~7 µm. To deduce the extrusion condition, a series of compression tests has been conducted at temperatures ranging from RT to 500oC. The strain rate sensitivity parameter (m) of this alloy has been found to be very low ( ≤0.1) below 300oC and reached maximum value of about 0.13 at 500°C. The extrusion has been successfully conducted at the temperatures of 300°C and above with the ratio of area reduction of 28 and 40. The processing map for hot working has also been constructed.

Abstract: In the present study, PIM process has been employed to produce sintered specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel. Based on the results of tensile tests on PIMed specimens with the various porosity, a new approach to predict the strength and the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established. By applying this new model to not only the results obtained in this study but also to previously published data, its generality and validity were sufficiently verified.