Dae-Geun Nam

Effect of solution treatment and artificial aging on microstructure and mechanical properties of Al−Cu alloy

Abstract In order to achieve good mechanical properties of Al-Cu alloys such as high strength and good toughness, precipitation hardening and artificial aging treatment were applied. As defined by the T6 heat treatment, the standard artificial aging treatment for Al-Cu alloy followed heat treatments of solution treatment at 510–530 °C for 2 h, quenching in water at 60 °C and then artificial aging at 160–190 °C for 2–8 h. The effects of solution treatment and artificial aging on the microstructure and mechanical properties of Al-Cu alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and tensile test. The results of solution treatment indicate that the mechanical properties of Al-Cu alloy increase and then decrease with the increase of solution temperature. This is because the residual phases dissolve gradually into the matrix, and the fraction of the precipitation and the size of the re-crystallized grain increased. Compared to the solution temperature, the solution holding time has less effect on the microstructure and the mechanical properties of Al-Cu alloy. The artificial aging treatments were conducted at 160–180 °C for 2–8 h. The results show that the ultimate tensile strength can be obtained at 180 °C for 8 h. Ultimate tensile strength increased with increasing time or temperature. Yield strength was found as the same as the ultimate tensile strength result.

A Hybrid Joining Technology for Aluminum/Zinc Coated Steels in Vehicles

Currently, in the automotive industry, joining of the aluminum alloys with the steel is a crucial problem to be solved. Conventional joining techniques including resistance spot and gas metal arc welding are not acceptable for those applications due to a number of metallurgical problems. The investigation was carried out to develop the hybrid joining process combining the resistance spot welding and brazing. In this study, an attempt was made to apply hybrid process to the joining of dissimilar sheet metals, Al-Mg-Si (6000 series) alloy and low carbon steel sheet. Hybrid process (resistance spot weld/brazing) using filler metal was found to be effective to overcome the incompatibility between aluminum alloy and steel. Although hybrid joining process of Al alloy sheet and steel sheet did not produce acceptable bond strength, it was proved to have reasonable interfacial bond layer if the optimal process condition was applied.

Microstructures of brazing zone between titanium alloy and stainless steel using various filler metals

Abstract Titanium alloy (Ti–Al–V alloy) substrate was brazed with stainless steel (STS304) using filler metal. At an optimized brazing condition, various filler metals were used. Microstructures were observed at each condition. Filler metals were titanium based 40Ti–20Zr–20Cu–20Ni, silver based Ag–5Pd, and nickel based Ni–7Cr–3.1B–4.5Si–3Fe–0.06C (BNi 2 ) and Ni–14Cr–10P–0.06C (BNi 7 ). To select a good filler metal for brazing process, wetting test was performed at 880–1050 °C. It was not brazed using silver based filler metals, but at the conditions using titanium and nickel based filler metals had brazed zone between titanium alloy and stainless steel. However, titanium alloy was eroded during brazing using titanium based filler metals. Nickel based filler metal has a good brazed zone between titanium alloy and stainless steel among the filler metals.

Evaluation of Resistance Spot Weld Interfacial Fractures in Tensile-Shear Tests of TRIP 1180 Steels

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

Electrochemical oxidation of some basic alcohols on multiwalled carbon nanotube–platinum composites

Some composites of multiwalled carbon nanotubes, which were chemically treated in acidic and/or hydrogen peroxide solution, and platinum nanoparticles were prepared by the simple reduction in glycerol solution. Carboxylated and/or hydroxyl MWNTs were structurally analysed using X-ray photoelectron spectroscopy. In addition, the MWNT–Pt composites were characterized by XRD and TEM in detail. The electrochemical oxidation of some basic alcohols, which was catalyzed by the MWNT–Pt composites, was analysed by cyclic voltammetry. Their catalytic activities were studied with cyclic voltammograms of alcohols.

Characterization of Graphite Oxide Reduced by Thermal and/or Chemical Treatments

Copyright ©2015 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. pISSN: 1229-7607 eISSN: 2092-7592 DOI: http://dx.doi.org/10.4313/TEEM.2015.16.5.274 OAK Central: http://central.oak.go.kr

Microstructure and mechanical properties of Ti-B-C-N-Si nanocomposite films deposited by unbalanced magnetron sputtering

Quinary Ti–B–C–N–Si nanocomposite thin films were deposited on AISI 304 stainless steel substrates by d.c. unbalanced magnetron sputtering from a TiB2–TiC compound target and a pure Si target. The relationship between microstructure and mechanical properties of the films was investigated in terms of the nanosized crystallites/amorphous system. The synthesized Ti–B–C–N–Si films were characterized using x-ray diffraction, x-ray photoelectron spectroscopy, atomic force microscopy, and high resolution transmission electron microscopy. The results showed that the Ti–B–C–N–Si films were nanocomposites composed of nanosized TiB2, TiC, and TiSi2 crystallites (2-3 nm in size) embedded in an amorphous matrix. The addition of Si to the Ti–B–C–N film led to precipitation of nanosized crystalline TiSi2 and percolation of amorphous SiC phases. The Ti–B–C–N–Si films with up to 7 at. % Si content presented high hardness (≥35 GPa), H/E (≥0.0095), and We (>50%) with compressive residual stress (∼0.5 GPa). A systematic inve...

Synthesis and Characterization of Methyltriethoxysilyl-Mediated Mesoporous Silicalites

A series of mesoporous silicalites was synthesized using different compositions of tetraethylorthosilicate and methyltriethoxysilane (MTES) as the silica source. Cetyltrimethylammonium bromide was used as the organic template. Their detailed pore structures were investigated by transmission electron microscopy, X-ray diffraction, and N2 adsorption method. The thermal properties of these silicalites were studied by thermogravimetric analysis. The increased amount of MTES destroyed mesoporous channels and reduced pore sizes from 3.4 nm to 2.8 nm in calcined silicalites. The calcined silicalite transformed completely into an amorphous state at 30% MTES loading. Methyl pending groups of MTES hindered the structural ordering of ≡Si-O- frameworks, resulting in an amorphous structure. This was caused by the insufficient formation of supramolecular assembly with the organic template. No capillary condensation step was found in MS 7/3 silicalite. The other capillary condensation steps shifted toward the lower relative pressure with increasing MTES content, indicating the reduction of pore sizes.

Effect of Ti and C Contents on Prior Austenite Grain Size in Ti Added Steels

Prior austenite grain size plays an important role in the production of high strength hot-rolled steel. This study investigated the effect of Ti and C contents on the precipitates and prior austenite grain size. Steel with no Ti solutes had prior austenite grain size of about 620 . The addition of Ti ~ 0.03 wt.% and 0.11 wt.% reduced the prior austenite grain size to 180 and 120 , respectively. The amount of Ti required to significantly decrease the prior austenite grain size was in the range of 0.03 wt.%. However, the amount of carbon required to significantly decrease the prior austenite grain size was not present from 0.04 wt.% to 0.12 wt.%. Oxides of Ti () were observed as the Ti content increased to 0.03 wt.%. The specimen containing 0.11 wt.% of Ti exhibited the complex carbides of (Ti, Nb) C. The formation of Ti precipitates was critical to reduce the prior austenite grain size. Furthermore, the consistency of prior austenite grain size increased as the carbon and Ti contents increased. During the reheating process of hot-rolled steel, the most critical factor for controlling the prior austenite grain size seems to be the presence of Ti precipitates.

Surface Properties of Chromium Nitrided Carbon Steel as Separator for PEMFC

Separator of stack in polymer electrolyte membrane fuel cell (PEMFC) is high cost and heavy. If we make it low cost and lighter, it will have a great ripple. In this study, low carbon steel is used as base metal of separator because the cost of low carbon steel is very cheaper commercial metal material than stainless steels, which is widely used as separator. Low carbon steel has not a good corrosion resistance. In order to improve the corrosion resistance and electrolytic conductivity, low carbon steel needs to be surface treated. We made Chromium electroplated layer of , thickness on the surface of low carbon steel and it was nitrided for 2 hours at in a furnace with 100 torr nitrogen gas pressure. Cross-sectional and surface microstructures of surface treated low carbon steel are investigated using SEM. And crystal structures are investigated by XRD. Interfacial contact resistance and corrosion tests were considered to simulate the internal operating conditions of PEMFC stack. The corrosion test was performed in 0.1 N + 2 ppm solution at . Throughout this research, we try to know that low carbon steel can be replaced stainless steel in separator of PEMFC.