Advanced Materials Research Vols. 26-28

Abstract: The effect of post weld heat treatment on mechanical properties of friction welded Alloy 718 and SNCRW was investigated. Friction welding tests were carried out at a constant rotation speed and pressure. Optimum friction condition was found to be the friction pressure of 25kg/cm2, friction time of 40sec, upset pressure of 80 kg/cm2, and dwell time of 5sec. After friction welding tests, post weld heat treatments were performed in the temperature range of 500-900°C for 8hrs in order to investigate the microstructure and mechanical properties of weld joint. Specimens with the post weld heat treatment at 720°C for 8hrs show optimal mechanical properties. Residual stress of post weld heat treated specimens was measured to weld joint in the same temperature range. After friction welding tests on samples with a diameter of 80mm, tensile properties of post-weld-heat-treated and non-heat-treated samples were compared.

Abstract: Zinc oxide and tin oxide (ZnSnOx) films on PET (Polyethylene Terephthalate) substrate were prepared by the electron cyclotron resonance-metal organic chemical vapor deposition (ECR-MOCVD) under an hydrogen, oxygen and argon atmosphere. The used tin and zinc precursor are TMT (tetramethyltin) and DEZn(diethylzinc), respectively. The metal (Zinc and Tin) oxidation content plays an important role to control the optical and electrical characteristics of the films. Therefore the optimum DEZn/TMT content can be determined by the counter stability effect between oxidation and zinc-tin deposition. The obtained ZnSnOx (or SnOx-ZnO) of high mobility films exhibited c.a. 7.0×10-3 ohm ·cm of electrical resistivity.

Abstract: The alpha-case formation mechanism was elucidated for the economic titanium casting. The α-case formation reaction between Ti and Al2O3 mold was examined in a plasma arc melting furnace. The reaction products were characterized by electron probe micro-analyzer and transmission electron microscopy. The α-case generation between Ti and Al2O3 mold was not able to be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, from our experimental results and thermodynamic calculations and Calphad modeling (Thermo-calc®), it was confirmed that the α-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials. Our newly established α-case formation mechanism will surely lead to a variety of significant applications of the α-case controlled Ti casting.

Abstract: The typical morphology of metal lead (Pb) in ancient bronzes made in the Spring-Autumn Warring States Period (770 B.C. –221 B.C.), China, was examined by using electron microscopic techniques. We propose that the main purposes for adding metallic Pb in a large quantity into ancient bronzes such as arrows and ritual vessel-Ding were for increasing weight and economical reasons. SEM observations from the fracture surface of the bronzes revealed that the Pb particle possessed a “hollow–cored bubble” structure, which was formed due to absorbing casting gases during solidification and therefore indirectly eliminated the casting porosity and loosen.

Abstract: Ti-Al intermetallic compounds are regarded as promising materials for the hightemperature structural and coating applications. We focused on the joining of Al casting alloy with Ti-Al intermetallic compounds by in-situ combustion synthesis to improve the surface properties of Al casting components. Microstructures and phase formation behavior of Ti-Al based intermetallic compounds synthesized by combustion reaction were analyzed using scanning electron microscope(SEM) equipped with energy dispersive x-ray spectroscopy (EDS) and x-ray diffractometer(XRD) in Ti-Al intermetallic compounds. Three kinds of titanium aluminides of Ti3Al, TiAl and TiAl3 were synthesized by the heat from the Al molten metal and a coating layer of intermetallic phase were formed simultaneously on solidifed Al alloy surface. The shapes and microstructures of reacted compacts were varied by mixing ratio of elemental powders. The TiAl3 intermetallic compound was observed in the compacts regardless of the mixing ratio of elemental powders. And the unreacted Ti powders were remained in the reacted compacts due to the big size of Ti powder and low exothermic heat of reaction between Ti and Al powders. The zone that poured Al alloy diffused into the reacted Ti-25at.%Al compact of about 200 μm thickness was formed at the interface by the reaction between liquid molten Al alloy and solid Ti powders in green compact.

Abstract: For thin-walled casting development of austempered ductile iron (ADI), permanent mold casting and accompanied heat treatment practice were systematically investigated to suppress and/or remove chill defects of ductile cast iron (DCI) with various thickness of 2 to 9 mm and to ensure mechanical properties of the final ADI casting. Si content was increased up to 3.8% to reduce the chill formation tendency under a high cooling rate. The residual Mg content remarkably affected the nodule count, while the nodule size and spherodization were proven to have weak relationships. Austenitizing process followed by austempering was very sensitive to chemical compositions (Si and Sn) and heat treatment temperature. As a practical application, the steel bar coupler for a structural frame was tried to produce without subsequent machining.

Abstract: This study investigated the characterization of the additive powder on microstructural evolution during the heating of the powder mixture of additive and filler metal powder by in-situ high temperature environmental scanning electron microscopy (HT-ESEM) up to 1200°C. The IN738 powder (additive) and BNi-3 powder (filler metal) were used for wide-gap brazing process. A field emission gun environmental scanning electron microscope (XL 30 ESEM-FEG, FEI) equipped with a 1500°C hot stage was used for in-situ gaseous secondary electron imaging at high temperature (HT-ESEM image). The melting of filler metal powder initiated at 1224K and was spread on the IN738 additive powder with increasing temperature. After cooling, the IN738 additive powder was increased from 75μm to 100μm. It was found that the additive powder added to the wide-gap brazed region avoided the possibility of directional solidification.

Abstract: Laser welding process is widely used in the industrial field due to its numerous advantages: a small heat affected zone(HAZ), deep penetration, high welding speed, ease of automation, single-pass thick section capability, enhanced design flexibility, and small distortion after welding. In this paper, the laser weldability of Austenite stainless steel and INCONEL600 at dissimilar metal welds using a continuous wave Nd:YAG laser are experimentally investigated. INCONEL600 is being used in a steam generator tubing of pressurized water reactor(PWR) exposed to some corrosion. Therefore stress corrosion cracking can occur on this material. A research work is conducted to determine effects of welding parameters, on eliminating or reducing the extent welding zone formation at dissimilar metal welds and to optimize those parameters that have the most influence parameters such as focus length, laser power and welding speed were tested.

Abstract: In order to rheocast using Al-Si foundry alloy with higher concentration of iron, semi-solid slurries were tried to manufacture by two different processes. The former process made the slurry with granular solid phase and iron containing intermetallic compound β-phases of average length of 60μm. In the latter process, morphology of the β-phases became thinner, although no difference in their lengths. These slurries were rheocast to a disk shape with diameter of 80mm and thickness of 5mm at slurry temperature of 873K. Ductility of the disk rheocast using the latter slurry was slightly improved in comparison with that of the former slurry. However these levels of ductility were still much lower than that of the slurry without β-phases.

Abstract: In continuous casting, slab weight is normally calculated just after torch cutting of slab using the process parameters and measured length. Because the weight is dependent on the slab temperature as well as slab width, the conventional method has the possibility of inaccuracy in calculated slab weight where contraction ratio of hot-slab was used in weight calculation. In this study, the slab width and surface temperature were measured using pyrometer simultaneously and used for determining the slab dimension and high temperature density. Slab density was formulated as a function of slab surface temperature and slab dimension. The accuracy of slab weight could be improved using this new measured method, and the slab could be cut with good matching weight with ordered-weight.