Papers by Author: Yeong Maw Hwang

Abstract: Compared with forging and extrusion processes, tube end forming includes various forming methods, such as pipe expansion, bending, bulging, folding, shrinking and profile processing. Therefore, research and development of pipe processing and forming are of great importance in the industry. In this study, the authors used the finite element analysis software DEFORM to analyze the end deformation behavior of the single pass forming. The deformation behavior of the pipe end, load curve and formability are discussed. The single-pass forming die design enables the pipe ends to be formed into two different shapes. By reducing the processing procedures with the geometric design of the dies, the processing advantages of single pass forming are achieved.

Abstract: The paper extends Orowan’s method to the prediction of ductile fracture in plane strain rolling. In general, any uncoupled ductile fracture criterion can be used in conjunction with the solution found. However, the present paper focuses on a ductile fracture criterion based on the workability diagram. It is assumed that the initiation of fracture occurs at the axis of symmetry.

Abstract: Magnesium alloys have been widely used in automotive, bicycle, aerospace industries. Improving the mechanical properties of the materials by various forming processes has been an important issue. Grain refinement is an effective method to improve material properties. In this study, single-pass and multi-pass hot rolling processes are carried out to generate dynamic recrystallization (DRX) and fine grains and to obtain better mechanical properties on the rolled products. A mathematical model linked with FEM software DEFORM-2D is proposed to predict the dynamic recrystallization ratio and the grain size distributions during various multi-pass hot rolling processes. The effects of different thickness reductions on the grain size distributions inside the sheet are discussed. Multi-pass hot rolling experiments of magnesium alloy ZK60 sheets are carried out and the metallographic microstructures are observed. The measured grain sizes are compared with the simulation results to verify the validity of the proposed model. The effects of different thickness reductions on the mechanical properties of the rolled magnesium alloy ZK60 sheets are also investigated.

Abstract: In this study, a design guideline for the die cavity and die bearing length distribution is proposed during an extrusion process of an asymmetrical thin sheet, such as clips of cell-phones. The plastic flow pattern of the billet inside the die cavity is analyzed using a commercial finite element package “DEFORM 3D”. The Module of Die Stress Analysis in the finite element software “DEFORM 3D” is also used to simulate the stress, strain, and the displacement distributions of the die during extrusion of clips of cell-phones. The extrusion load, the stress and strain distributions of the die, the temperature distribution, and thickness distribution of the extruded product are investigated. Furthermore, hot extrusion experiments using A6061 as the specimen are executed. The experimental results of temperature, thickness distribution of the product, extrusion force, etc, are compared with the analytical values to verify the validity of the proposed die design guideline.

Abstract: In this paper, finite element codes LS-DYNA and DYNAFORM are used to analyze the plastic flow pattern of a tube hydroforming into a product with large expansion ratio and eccentric axes. Tube hydroforming with a movable die is proposed to enhance the forming capacity of tube hydroforming technology. The relative speed of the axial feedings to the movable die for obtaining a sound product is determined by a geometric analysis. The whole forming processes are divided into two stages. At the first stage, an internal pressure is applied on the inner surface of the tube and two axial feedings and a movable die move forward simultaneouly. At the second stage, one of the axial feedings keeps moving forward, whereas, the movable die moves backward. With this forming schedule for the axial feedings and movable die, products with more uniform thickness distributions are obtained. Finally, experiments of tube hydroforming with a movable die are conducted. Low-carbon steels are used as the tube specimen in the experiments. The simulation results of the product shape and thickness distributions are compared with experimental results to verify the validity of the finite element modeling and the proposed forming schedules.

Abstract: This paper aims to manufacture magnesium alloy tubes with gradient hardness using hot extrusion process. A two-stage porthole die together with a mandrel is designed to have a straight channel part combined with a conical part. Materials are pushed through this specially-designed die cavity to generate a non-uniform velocity distribution at cross sections between the mandrel and the die and result in different strain and strain rate distributions. Accordingly, a gradient microstructure or hardness product can be obtained. Using the finite element analysis, the effective strain distributions inside the die cavity and at the die exit are firstly discussed for various inclination angles in the conical part of the mandrel. Then, hot extrusion experiments with a two stage porthole die set are conducted to obtain magnesium alloy products with gradient microstructures and hardness. Using a die set of mandrel inclination angle of 10^o and die inclination angle of 25^o, gradient microstructures of grain sizes of 4.30μm, 5.92 μm and 3.67μm at the outer surface, center zone, and inner surface, respectively, are achieved.

Abstract: This study uses FEM simulation based on Deform 3D commercial software to explore the plastic behaviors in the rotating compression of double-layer bounded clad ring under constant shear friction. The effective stress, the effective strain, the velocity field, the compression force under various forming conditions can be determined from FEM simulation. The realistic experiment has been performed to verify the acceptance of FEM simulation. From the comparisons, the compression force and free surfaces of outer and inner diameters shows a good agreement with the experiments, it proves the FEM simulation can be accepted as industry references.

Abstract: This study establishes an analysis model based on the slab method assuming constant shear friction. The rotating compression forming of the double-layers bounded clad ring is derived by considering von Mises’ yield criterion with the shear stress. Effects of the frictional factor, shear yield stress ratio, rotating angular speed of the ring, frictional factor ratio, clad ring height ratio etc on compression characteristics are investigated effectively. Moreover, the compression pressure, radial stress for each layer, radial stress of the whole layer, and compression force are also predicted effectively. Finally, the FEM simulation is carried out to compare with the slab method to realize the variations of both models.

Abstract: The study aims at exploring the effects of rotating compression forming on the material behaviors of cylinder. The experiments have been carried out under various angular velocities to realize the flow stress, the compression force, the rotating torque, the grain size, and the hardness. Furthermore, the FEM simulation is also performed to compare with the experiments. From the experimental results, as the angular velocity increases, the compression force and the grain size decrease, but the rotating torque and the hardness increase. The results can provide the industry as the reference.

Abstract: Stainless steel tubes are widely used in various fields, such as electrode tubes, probe tubes, electronic parts, painless injection needles, and micro-nozzles etc. In this study, a self-developed prototype dieless drawing machine is used to explore the formability of the dieless drawing process of SUS304 stainless steel tubes. The critical process parameters of the forming temperature, drawing velocity, the drawing accelerations and area reduction are investigated. A commercial finite element code of DEFORM 3D is used to simulate the temperature, stress, strain distributions and drawing velocity limit in the dieless drawing process of the stainless steel tubes. The maximum area reduction obtained can reach 50% and the better forming temperatures are between 1000°C and 1100 °C.