Jonghun Yoon

Strain hardening model of pure titanium considering effects of deformation twinning

This paper is concerned with the effect of deformation twinning on the strain hardening behavior of commercially pure titanium during the compressive loading. In accordance with many studies on titanium, the strain hardening behavior of titanium during compression has different characteristics from those of general metallic materials. It has been reported that the strain hardening rate of titanium during compression can be divided into three stages. In the first stage, the strain hardening rate decreases as the strain increases due to dynamic recovery. Following the first stage, however, a sudden increase in the strain hardening rate is observed in the second stage. It is well known that the occurrence of the second stage is due to the generation of deformation twinning. After the second stage, the strain hardening rate decreases again as the strain increases in the third stage. In this paper, a strain hardening model that can represent the three stages of strain hardening is proposed based on the investigated effect of deformation twinning on the strain hardening behavior of titanium. The electron backscatter diffraction (EBSD) analyses are conducted to quantify the twin volume fraction with increase of compressive plastic strain, which provide fundamental frame of the hardening model for titanium.

Warm forging of magnesium AZ80 alloy for the control arm in an automobile

With the growing environmental concerns surrounding transportation machines, magnesium alloy, which has an excellent specific strength, good vibration damping and a high electromagnetic shielding capability, is increasingly being used in automobile body parts in order to reduce the carbon emissions and to increase the energy efficiency through a reduction in the mass. However, forming techniques with magnesium alloy at room temperature pose considerable difficulty owing to its strong plastic anisotropy of tension and compression. In this study, a control arm, which is a complex-shaped part in the suspension system of a vehicle, was warm forged with Mg–8 mass % Al–0.5 mass % Zn (AZ80) alloy to reduce the mass. To validate the applicability of the magnesium alloy control arm in the automobile, its microstructure and its mechanical properties were analysed.

Enhanced Piezoelectricity in a Robust and Harmonious Multilayer Assembly of Electrospun Nanofiber Mats and Microbead-Based Electrodes

Here, we present a simple yet highly efficient method to enhance the output performance of a piezoelectric device containing electrospun nanofiber mats. Multiple nanofiber mats were assembled together to harness larger piezoelectric sources in the as-spun fibers, thereby providing enhanced voltage and current outputs compared to those of a single-mat device. In addition to the multilayer assembly, microbead-based electrodes were integrated with the nanofiber mats to deliver a complexed compression and tension force excitation to the piezoelectric layers. A vacuum-packing process was performed to attain a tight and well-organized assembly of the device components even though the total thickness was several millimeters. The integrated piezoelectric device exhibited a maximum voltage and current of 10.4 V and 2.3 μA, respectively. Furthermore, the robust integrity of the device components could provide high-precision sensitivity to perceive small pressures down to approximately 100 Pa while retaining a linear input-output relationship.

Warm forging of an aluminium alloy for the differential case in an automobile transmission

This paper presents a warm-forging process for the manufacture of an aluminium alloy differential case in an automobile, which has usually been produced from ductile cast iron. In order to replace the conventional ductile cast iron with a lightweight material in the differential case, aluminium alloy 6082 is utilized in the proposed warm-forging process, which is then analysed by numerical simulations using DEFORM-3D. Experimental warm forging using aluminium alloy 6082 was conducted in a 1600 tonf press shop to evaluate the proposed design in the die and the punch as well as the forgeability of the aluminium alloy differential case. The as-forged aluminium alloy differential case not only provides a net shape but also has a crack-free volume internally, while reducing the total weight by about 40% based on the proposed design.

Effect of tensile twins on the subsequent plastic deformation in rolled Mg-3Al-1Zn alloy

The {101¯2} tensile twins influence plastic flow of magnesium alloys for the subsequent plastic deformation since it contributes to grain refinement and texture hardening between the twinned and untwined regions. This paper investigates the variation of plastic flow of the rolled Mg-3Al-1Zn alloy which is compressed with a small plastic strain at the room temperature to induce the twins in the initial specimen. Subsequent tension and compression along the rolling and transverse direction are conducted with the twin induced specimens in order to examine the effect of the initial tensile twins.