Papers by Author: H.S. Koo

Abstract: This paper is concerned with the pressure distribution along the die-powder interface in long parts. The pressure exerted on the interface at various points on the moving and stationary punch, and also on the sidewall of container was investigated by the finite element method. A plasticity theory describing asymmetric behavior of powdered metals in tension and compression was briefly summarized. The yield criterion applied to the sintered powdered metals had been modified for describing this asymmetric behavior. The material properties of copper powders under compaction were also briefly described for the completeness of the paper. The copper powders were selected as a model material in the present study. The main purpose of this study is to investigate the pressure distribution along the interface of tooling quantitatively by the finite element method so that the results could be applied usefully to the design of tooling, especially container design for powdered metal compaction. Geometrical condition for analysis was confined to the Class II components which is very long parts without steps. It was concluded from the simulation results that the pressure exerted on the moving punch increases sharply near the outer circumference of punch and the pressure on the sidewall decreases at a distance from moving punch to fixed punch. It was also seen from the simulation that the pressure on the stationary punch is not significantly built up and decreases toward outer periphery. These trends were seen amplified with severe frictional conditions imposed on the tooling and powder interface.

Abstract: Combined extrusion processes generally have advantages of forming in terms of the minimum deformation power since the material is pressed through two or more orifices simultaneously. This paper is concerned with the analysis of forming load characteristics of a forward-backward can extrusion process using thick-walled pipe as an initial billet. The combined tube extrusion process was analyzed by using a commercial finite element code. A thick-walled pipe was selected as an initial billet and the punch geometry has been chosen on the basis of ICFG recommendation. Several tool and process parameters were employed in this analysis and they are punch nose radius, backward tube thickness, punch face angle, and frictional conditions, respectively. The main purpose of this study is to investigate the effect of process parameters on the force requirements in combined extrusion process. The possible extrusion process to form a forward-backward tube parts in different process sequences were also simulated to investigate the force requirements in sequential operations, i.e. separate operations. It was easily concluded from the simulation results that lower forming load was predicted for the combined extrusion, compared to those for separate sequential operations. It was also revealed that the punch nose radius and the punch face angle have little effect on the force requirements and the forming load increases significantly as the frictional condition along tool-workpiece interface becomes severe. The simulation results in this study suggest that the combined extrusion process has strong advantage in terms of force requirements as long as the simultaneous material flow into multiple orifices could be closely controlled.

Abstract: In this paper, the forming limit of flange in radial extrusion process was analyzed by the rigid-plastic finite element method. The selected model material for simulation and experiments was AA 3105 aluminum alloy. The predictions from simulation were made in terms of axial and circumferential strains. Experiments also have been conducted to compare with the simulation results with regards to deformation pattern. Furthermore, the deformation pattern in forming of flange section was closely investigated and categorized in three cases such as sticking, separating and cracking. The analysis in this paper is focused on the transient extrusion process of material flow into the gap in radial direction for different gap heights and die corner radii. The results of present study were summarized in terms of evolution of surface strains in axial and circumferential directions measured from the finite element meshes located in the region where surface cracking occurred in experiments. The forming limit line was drawn in the relationship of circumferential and axial strain. It was concluded from this study that the forming limit line is influenced mainly by circumferential strain on free surface of flange. It was also predicted that ductile fracture on flange surface is likely to occur in the middle of flange gap under the condition of sticking deformation and near bottom of flange gap under the condition of separating deformation, respectively. The forming limit of flange in terms of flange diameter was expected about 2.5do, which is 2.5 times the diameter of original billet.

Abstract: This paper is concerned with the long-term performance of geo-textile (GT) composites in terms of creep deformation and frictional properties. Composites of PVA GT and HDPE GM were made to investigate the advanced properties of long-term performance related to waste landfill applications. The same experiments were also performed for typical polypropylene and polyester GT and compared to PVA GT/HDPE GM composites. The main purpose of this study is to develop high performance GT composites with GM by using PVA GT which is capable of improving frictional property and thus enhances long-term performance of GT composites. In the present experiments, GT composites of PVA GT/HDPE GM, PVA GT of 600, 1000, 1500, 2000g/m2 and HDPE GM were prepared in thermal bonding process. Polyester and polypropylene GT were also made in needle punching process. The creep deformation of GT composites was measured and evaluated in accordance with ASTM D5262. Frictional characteristics of GT composites tested in this study were conducted with compact direct shear apparatus in accordance with ASTM D5321. It was concluded from the present experimental study that friction coefficient of GT composites is relatively large compared with those of polyester and polypropylene non-woven GT as long as the friction media has similar size to the particles of domestic standard earth. In the event that 20% of the maximum tensile strength was added to polypropylene and polyester non-woven geo-textiles, creep deformation reached to 10% or higher, making it even impossible to find reduction factor.

Abstract: This paper is concerned with the performance of geo-textile (GT) against chemical condition. GT is generally adopted for the upper part of geo-membrane (GM) for waste landfills and thus it is very important to consider the performance of GT against certain chemical environments until landfill is completed. In this study, PVA geo-textile/HDPE geo-membrane was prepared to investigate the waste landfill related properties in terms of long-term performance against chemical conditions imposed. GT composites of PVA GT/HDPE GM, PVA GT and HDPE GM were produced in thermal bonding process. Polyester and polypropylene GT were also manufactured in needle punching process. The experiments have been conducted under a modified version of EPA 9090 test method which is very similar to the method of evaluating chemical resistance of flexible membrane liner by the US Environmental Protection Agency (EPA). In this testing method, samples immersed in chemical of different solutions up to 150 days at 30 day interval were obtained to find tensile strength holding rate and chemical resistance. The analysis in this paper is focused to evaluate the effect of different pH conditions and temperature environments on geo-synthetics weights strength retention. It was concluded from the experiments that tensile strength of GT composites against leachate were reduced by 10 to 20% in both polypropylene and polyester non-woven GT. The reduction was more significant at temperatures of 50 °C than that at 25 °C. The experiments conducted in this study demonstrated that PVA GT is excellent in terms of chemical resistance.

Abstract: This paper is concerned with the analysis of the forming load characteristics of a forward-backward can extrusion in both combined and sequence operation. A commercially available finite element program, which is coded in the rigid-plastic finite element method, has been employed to investigate the forming load characteristics. AA 2024 aluminum alloy is selected as a model material. The analysis in the present study is extended to the selection of press frame capacity for producing efficiently final product at low cost. The possible extrusion processes to shape a forward-backward can component with different outer diameters are categorized to estimate quantitatively the force requirement for forming forward-backward can part, forming energy, and maximum pressure exerted on the die-material interfaces, respectively. The categorized processes are composed of combined and/or some basic extrusion processes such as sequence operation. Based on the simulation results about forming load characteristics, the frame capacity of a mechanical press of crank-drive type suitable for a selected process could be determined along with securing the load capacity and with considering productivity. In addition, it is suggested that different load capacities be selected for different dimensions of a part such as wall thickness in forward direction and etc. It is concluded quantitatively from the simulation results that the combined operation is superior to sequence operation in terms of relatively low forming load and thus it leads to low cost for forming equipments. However, it is also known from the simulation results that the precise control of dimensional accuracy is not so easy in combined operation. The results in this paper could be a good reference for analysis of forming process for complex parts and selection of proper frame capacity of a mechanical press to achieve low production cost and thus high productivity.

Abstract: The rigid-plastic finite element method has been applied to three variants of radial extrusion processes to investigate the influence of die geometry on the material flow into the flange gap. Case I involves forcing a cylindrical billet against a flat die, which is a single action pressing process. In case II, another single action pressing process, the upper punch forces a billet against a stationary punch recessed in the lower die. Both the upper and lower punches move together in Case III toward the center of billet at the same speed with a double action tool. Major process parameters are identified as the relative gap height and the die corner radius in constant relative deformation. The relative gap height is defined as the ratio of gap height to billet diameter. Extensive simulation work for various combinations of process parameter value has been performed and then the main characteristics of the deformation patterns of each case are observed to define the terms which represent the forming characteristics of the flange in radial extrusion processes in terms of separation height, asymmetric ratio of height, and asymmetric ratio of angle, respectively. The effect of major process parameters on the material flow into the flange gap has been also analyzed in terms of flange radius and flange angle. The effect of frictional condition on the separation height has been also analyzed to investigate the edge separation of flange from the flat die. AA 6063 aluminum alloy is selected as a model material throughout the analysis. Simple comparison between AA 6063 and AISI 1006 steel has been also made to investigate the effect of material selection on the deformation pattern, especially in terms of separation height in Case I and asymmetry in Case II, respectively.

Abstract: In this paper, the forming process of a central hub by radial-forward extrusion has been analyzed by the rigid-plastic finite element method. In this process, the material flows in radial direction and then deflects 90 degrees into the same direction as that of punch movement. Radial extrusion is used to produce parts that generally feature a central hub with radial protrusions. Design factors such as mandrel diameter, punch nose radius, deflection corner radius, gap width in annular direction, and frictional conditions are applied to the present study by simulation. AA 6063 aluminum alloy is selected as a model material for analysis in the present study. The influence of these design factors on the force requirement during the forming operation and the pressure exerted on the tooling such as the punch and mandrel is investigated and the simulation results are quantitatively summarized in terms of pressure distribution, force-stroke relationships, and maximum force requirement, respectively. The main goal of this study is to investigate the effect of those process parameters on the deformation pattern in radial-forward extrusion process, especially the effect of deflection corner radius. It has been concluded from the simulation results that a) the frictional condition between workpiece and tool does not affect the punch load very much, but the load supported by mandrel is more or less significantly influenced by the frictional condition compared to that of punch, b) the deflection corner radius turns out to be a major process parameter in terms of maximum force requirement, and c) a similar trend is found in the punch and mandrel forces during the radial extrusion process.

Abstract: This paper is concerned with forward rod extrusion combined simultaneously with backward tube extrusion process in both steady and transient states. The analysis has been conducted in numerical manner by employing a rigid-plastic finite element method. AA 2024 aluminum alloy was selected as a model material for analysis. Among many process parameters, major design factors chosen for analysis include frictional condition, thickness of tube in backward direction, punch corner radius, and die corner radius. The main goal of this study is to investigate the material flow characteristics in combined extrusion process, i.e. forward rod extrusion combined simultaneously with backward tube extrusion process. Simulation results have been summarized in term of relationships between process parameters and extruded length and volume ratios, and between process parameters and force requirements, respectively. The extruded length ratio is defined as the ratio of tube length extruded in backward direction to rod length extruded in forward direction, and the volume ratio as that of extruded volume in backward direction to that in forward direction, respectively. It has been revealed from the simulation results that material flow into both backward and forward directions are mostly influenced by the backward tube thickness, and other process parameters such as die corner radius etc. have little influence on the volume ratio particularly in steady state of combined extrusion process. The pressure distributions along the tool-workpiece interface have been also analyzed such that the pressure exerted on die is not so significant in this particular process such as combined operation process. Comparisons between multi-stage forming process in sequence operation and one stage combined operation have been also made in terms of forming load and pressure exerted on die. The simulation results shows that the combined extrusion process has the greatest advantage of lower forming load comparing to that in sequence operation.

Abstract: Conventional multi-step extrusion processes with solid billet are examined by the rigid-plastic finite element method in order to provide criteria for new process sequence for hollow parts. Two examples are taken for the analyses such as the current three-stage cold extrusion process for a hollow flange part and five-stage process for manufacturing an axle housing. Based on the results of simulation of the current three-stage and five-stage manufacturing processes, new design strategy for improving the process sequences is developed simply by replacing the initial billet from solid to hollow one. The developed new process sequences are applied for simulation by FEM and they are compared with the existing processes to confirm the usefulness of new process sequences with hollow initial billets. The results of simulation show that the newly proposed process sequences with hollow billet instead of solid one are more economical way to manufacture required parts, respectively.