Authors: Bong Kwon Chung, Young Gun Ko, Kang Min Lee, Dong Hyuk Shin
2765
Authors: Young Gun Ko, Y.G. Kim, S. Namgung, Dong Hyuk Shin, Sung Hak Lee
Abstract: In this study, dynamic deformation behavior of submicrocrystalline aluminum alloy was established with respect to equal-channel angular (ECA) pressing routes such as A, B, and C. After 8-pass ECA pressings, the deformed samples, regardless of the routes applied, were consisted of ultrafine grains together with high dislocation density near the boundaries. Microstructural observation revealed that the sample deformed via route B showed more diffused diffraction pattern than those deformed via route A and C, suggesting the fact that route B was most effective for a rapid evolution in the grain boundary orientation from low-angle to high-angle characteristics. In the torsion tests, the shear stress decreased once reaching the maximum point. This maximum was the highest in the sample deformed via route B, and decreased in the order of the route C and route A. The dynamic deformation was explained based on microstructural uniformity associated with ECA pressing routes.
1006
Authors: In Jun Hwang, Kang Min Lee, Bong Young Yoo, Dong Hyuk Shin
Abstract: Characterization of nickel oxide layers on the AZ91 Mg alloys by plasma electrolytic oxidation (PEO) was investigated. For this purpose, PEO coating was carried out on the present sample in the alkaline electrolyte with nickel ion at three different voltage responses, i.e., 150, 250, 350 V. The amount of nickel oxide increased by increasing process time, and changed the color of sample from light grey to moderate olive brown. It was observed from potentio-dynamic polarization analysis that polarization resistance of oxide layer was increased with increasing nickel ions. Also microhardness of the sample processed at 350 V was higher than those of the samples.
775
Authors: A.K. Prasada Rao, K.H. Kim, J.H. Bae, Geun Tae Bae, Dong Hyuk Shin, Nack Kim
Abstract: An attempt has been made to clad Mg alloy with Al by twin-roll casting. This was done by inserting an Al sheet between the roll and the Mg alloy melt during twin-roll casting. Microstructural investigation across the transverse section of the as-cast Al-clad Mg alloy sheet reveals a very good interfacial bonding between Al and the base Mg alloy. Annealing of the Al-clad Mg alloy sheet results in the formation of layers of various intermetallic phases along the Mg/Al interface. Subsequent rolling of the as-annealed sheet significantly improves the formability of the reaction zone, as evidenced by the cracking of the base Mg alloy before the cracking of the reaction zone.
467
Authors: J.H. Cha, H.H. Cho, W.H. Kim, S.I. Kwun, Dong Hyuk Shin
Abstract: The low cycle fatigue(LCF) test was performed to characterize the influences of the equal channel angular pressing(ECAP) and subsequent annealing of Al 5052 alloy. In the present research, one group of Al 5052 alloy specimens was directly subjected to ECAP, while another was subjected to ECAP and subsequent annealing. It was found that the tensile strength of the Al 5052 alloy increased, while its elongation decreased, with increasing number of ECAP passes. The LCF test was conducted at constant total strain amplitudes of 0.5%, 0.7%, 0.9% and 1.1%. Only cyclic hardening was observed as the number of fatigue cycles increased at all strain amplitudes in the specimen without ECAP. However, the ECAPed specimens showed a slight amount of cyclic hardening in the beginning and then saturation until fracture.
725
Authors: W.H. Kim, H.H. Cho, J.H. Cha, S.I. Kwun, Dong Hyuk Shin
Abstract: The effects of equal channel angular pressing (ECAP) and subsequent heat treatment on the
low cycle fatigue properties of 6005 Al alloy were investigated. The ECAPed specimens showed
cyclic softening, whereas the peak aged specimens showed cyclic hardening at all strain amplitudes.
After ECAP, artificial aging was performed at 175°C to observe the change of the low cycle fatigue
properties due to precipitation. The fatigue life and behavior of the unECAPed and ECAPed 6005 Al
alloys were discussed in terms of the microstructural changes and aging conditions.
721
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi, Dong Hyuk Shin
Abstract: It is well known that the utilization of superplastic characteristics in manufacturing process
makes many of aerospace components lighter and stiffer. The weight saving is vitally important
especially for aerospace application and even more weight saving is possible when the superplastic
forming is carried out with diffusion bonding. In this study, the lightweight sandwich structure was
fabricated with superplastic forming(SPF) process from diffusion bonded(DB) Ti-6Al-4V sheets. The
solid state diffusion bonding process was conducted in non-vacuum environment under a pressure of
4MPa for 60 minutes at 875°C and the superplastic forming process was followed for another 40
minutes. Good solid state bonding interface have been observed in microstructure observation and the
sandwich structure was successfully manufactured. It is important to note that the forming conditions
of present study are more practical for application than the previously published conditions, which
require vacuum environment, higher temperature and/or pressure.
359
Authors: Duck Young Hwang, Kion Kwon, Dong Hyuk Shin, Kyung Tae Park, Young Gun Ko, Chong Soo Lee
Abstract: Ultrafine grained (UFG) 5083 Al and 5154 Al alloys were prepared by equal channel
angular pressing (ECAP) with an effective strain of ~ 4 or ~ 8. This investigation was aimed at
examining the effect of the ECAP strain and post-rolling inducing different microstructure in these
alloys on the deformation mechanisms at low temperature superplastic (LTS) and high strain
superplastic (HSRS) regimes. The sample after 4 passes (a strain of ∼ 4) did not exhibit LTS, but
superplastic elongations were obtained in the sample after 8 passes (a strain of ∼ 8). An analysis of
the mechanical data in light of the standard deformation mechanisms revealed that deformation of
the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed
to LTS of the sample after 8 passes. In addition, the 5154 Al alloy processed by ECAP and postrolling
was capable of enhancing HSRS elongation significantly. An analysis revealed that the
deformation mode was changed from dislocation viscous glide to grain boundary sliding by
additional ECAP strain and post-rolling.
597
Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi, Dong Hyuk Shin
Abstract: It is known that Ti-6Al-4V alloy is one of the excellent candidates for aerospace
structure due to their high specific strength.However, its higher cost and low formability relative to
other materials tend to limit the wide usage of the material.The purpose of this study is to characterize
the superplasticity of this alloy so to obtain materials and process parameters for superplastic forming
and diffusion bonding for industrial application. High temperature tensile tests was carried out at the
strain rate range of 10-4 to 10-2 s-1 and temperature range of 1123°C to 1223°C. According to the results of
the experiment, the optimum diffusion bonding condition was obtained at 1148°C, applying pressure
of 4MPa for 1 hour in argon gas environment, which condition is more practical than expensive
vacuum condition. It is shown that at the optimum condition for diffusion bonding with parent metal,
the oxide film becomes unstable and the oxygen is diffused into the bulk. At this condition, the
mechanical and microstructural integrity at the bonding interface was observed in a sandwich
structure and a heavy block of titanium part from massive diffusion bonding process.
549
Authors: Seng Ho Yu, Dong Hyuk Shin, Sun Keun Hwang
Abstract: To study the effect of grain size on texture and tensile properties of equal channel angular
pressed commercially pure Ti, specimens were made to have the grain size ranging from 4 m to 60
m with the severe plastic deformation followed by recrystallization annealing. In this initial
condition all the specimens exhibited a strong bimodal split basal texture. During subsequent repressing
at 350°C, the texture pattern became randomized through crystal rotations, the
phenomenon being more pronounced in coarse-grained specimens. The microstructure of deformed
specimens, as examined by electron back scattered diffraction, showed formation of mechanical
twins and microstructural inhomogeneity in the coarse-grained specimens. The room temperature
tensile properties of the re-pressed specimens showed that the yield strength was remarkably
enhanced regardless of the grain size whereas the elongation was reduced as compared to the initial
condition, particularly in coarse-grained specimen. It was concluded that microstructural refinement
during the severe plastic deformation was the main cause of the improved yield strength while the
twinning and microstructural inhomogeneity were responsible for the texture randomization and the
impaired ductility.
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