Papers by Author: Masahiro Okumiya

Abstract: N-Quench, which is a new surface heat treatment to infiltrate nitrogen into steel parts followed by quenching to achieve hardening, is gathering attention in the nitriding field as it affords low distortion while maintaining a higher effective case depth (ECD) compared with conventional nitriding. N-Quench is conducted mainly between 680°C and 800°C, where the two-phase region of ferrite and austenite exists in the Fe-N phase diagram. However, a few studies have reported on nitriding at temperatures higher than 800°C due to decomposition of NH₃, which is a key source of nitrogen infiltration. Our results revealed that in a conventional furnace such as resistance heating furnace, no nitrogen infiltrated the specimen at 930°C, which is the general carburizing temperature. On the other hand, in the infrared heating furnace, nitrogen infiltrated the specimen at 930°C successfully with lesser NH₃ introduction than that required by the conventional furnace. Therefore, in this study, the limit of NH₃ decomposition is assessed and possibility of extending the applicability of N-Quench, especially increasing the ECD while maintaining a low distortion, is examined.

Abstract: In this study, aluminum nitride films were formed on aluminum substrates by gas nitriding in order to improve their low emissivities. To accomplish this, aluminum alloys were subjected to nitriding conditions at 773 and 823 K for 0–5 h, using alumina and magnesium powders. The resulting aluminum nitride films were several micrometers thick and the films were dark brown or black. The surface structures of the aluminum nitride films were investigated using a scanning electron microscope, which showed fine acicular aluminum nitride nodules with diameters on the order of several micrometers. Emissivities were evaluated at 298 K using Fourier transform infrared spectroscopy, in a wavelength range of 2–14 μm. Total emissivities at temperatures between 323 and 383 K were estimated from emissivity results obtained at 298 K. It was subsequently found that emissivity decreases with increasing wavelength and an emissivity of 0.80 was observed at a wavelength of 2 μm. Total emissivity was 0.49 % at 298 K and was in excess of 0.50 between 323 and 383 K.

Abstract: Very high plastic strain zones with equivalent plastic strain above 0.2, PZ_0.2 and above 0.5, PZ_0.5 in 304 stainless steel small punch specimens loaded at RT to various level were observed and measured by martensite formation and recrystallization technique, respectively. It is found that both the very high plastic zones are formed ,at middle stage of the small punch test, at first near the outer surface region of the specimen where the loading ball is contacted to the specimen. The zones extend with increasing load toward the inner surface. Thus the contact area part of the specimen with the ball causes a significant strain gradient through thickness. This will be due to the constraint of the plastic deformation near the contact region by the friction force.

Abstract: The authors developed a new nitriding process to form an aluminum nitride film on a pure aluminum surface. In this study, a rotary barrel polishing method was applied to gas nitriding in an attempt to form an aluminum nitride film on the surface of Al-Mg alloys. Nitriding was carried out at 823 K, 853 K, and 873 K for 3.6 ks to 25.2 ks. Alumina- and aluminum - 50mass% magnesium powders were used as the media that flowed in the barrel tank. An aluminum nitride film was formed on the aluminum alloy surfaces within a relatively short time. The effect of the treatment temperature on the formation and growth of the aluminum nitride was investigated. It was found that the formation and growth of the aluminum nitride film were greatly affected by differences in the Mg content in the aluminum substrate and by the treatment temperature.

Abstract: Ultrasonic vibration has been applied to various molten metal processes owing to the functions of (a) improvement in wettability, (b) liquid adhesion at a vibrating end surface and (c) sono-solidification such as grain refinement. The present study is focused on the sono-solidification with acoustic cavitaion in hypereutectic Al-18mass%Si alloy. There appears an equilibrium microstructure composed of primary silicon and coupled eutectic -Al/Si phases in Al-18mass%Si alloy, however, non-equilibrium -Al grains develop along with the equilibrium phases through the sono-solidification. During the sono-solidification of Al-18mass%Si alloy, non-equilibrium -Al grains are recognized in the molten metal close to the ultrasonic radiator just before reaching the eutectic temperature of 577 oC in addition to the refined primary silicon particles. The appearance of -Al grains is understood through acoustic cavitation: ultrasound in molten Al-Si alloys exhibits two outstanding behaviors of cavitation bubbling and acoustic streaming. Firstly the de-coupled eutectic reaction, which is recognized in the solidified eutectic Al-Si alloy with severe stirring, causes divorced -Al grains by the acoustic streaming with cavitation. Secondly it is expected that high pressure of over 1 GPa generated by the collapse of cavitaion babbles leads to not only an increase in the eutectic temperature, but also higher silicon content at the eutectic point in Al-Si alloy. Consequently, non-equilibrium -Al grains are nucleated at collapsed cavitaion bubble sites, and they are characterized by higher silicon content compared with that of primary -Al grains in hypoeutectic Al-7masst%Si alloy.

Abstract: The ultrasonic melt treatment is highly efficient in controlling the size and morphology of the alpha-aluminum phase in the Al-Si cast alloys. However, the influence of this treatment on the other phases, namely: the eutectic Si and the Fe-intermetallic phases, has not been thoroughly investigated. This study was undertaken to investigate the effect of ultrasonic melt treatment on the morphology and size of these phases. Four Al-Si cast alloys, 384, 380, 356 and 356 (with 0.8% Fe), were considered in this study. The treatment temperatures were varied from about 100oC above the liquidus temperatures down to the Al-Si eutectic temperature, for different treatment times (4 to 54 s). The results showed that the Si particles are only affected by the ultrasonic vibrations when the eutectic reaction takes place under the ultrasonic field, where more compacted and large Si particles are formed. This limited effect was observed only near to the ultrasonic horn. For other investigated conditions, including treatment in the liquid and semisolid states, the Si particles were obviously unaffected by the ultrasonic treatment. On the other hand, the iron intermetallic phases changed their morphology from large plate-like particles to a more compacted globular form, by the application of ultrasonic vibrations at temperatures up to 10oC above liquidus. Treatments at higher temperatures have limited effect on the morphology of the Fe-intermetallic phases. These observations are general and apply to all the studied alloys.

Abstract: Ultrasonic irradiation during the solidification of molten metals has an effect on grain refinement. However, the mechanism of grain refinement by ultrasonic vibration has not been fully understood yet, so that there exist difficulties to apply the ultrasonic grain refinement to industrial casting processes. In the present study, we propose the mechanism of ultrasonic grain refinement: the nucleation is based on the extremely high pressure generated by the collapse of acoustic cavitation in molten Al-Si alloys. The effect of ultrasonic irradiation into molten Al-Si alloy on the microstructure was firstly studied, that is, molten Al-12.6wt%Si alloy was rapidly cooled down from just above the eutectic temperature after the ultrasonic irradiation. The detailed microstructure observation exhibits that ultrasonic irradiation above the eutectic temperature causes crystalline -Al and silicon to nucleate. Through the measurement of silicon content in -Al nodules solidified with ultrasonic irradiation, the silicon content is higher than that in non-irradiated -Al nodules. It is known that the collapse of acoustic cavitation generates extremely high pressure. At the highly pressurized sites, the eutectic temperature rises and the crystallized -Al nodules contain higher amount of silicon compared with those solidified at ambient pressure. According to the fact that the -Al nodules crystallized above the eutectic temperature contain higher amount of silicon, the irradiated microstructure of -Al nodules is developed at the highly pressurized sites, that is, the collapse of acoustic cavitation induces nucleation and causes grain refinement.

Abstract: The effect of reheating to the semisolid state (soaking treatment) on the microstructure evolution of the A356 aluminum alloy prepared by ultrasonic melt treatment was studied in this paper. The results showed that in general the longer the soaking process the larger and the more round the grains obtained. Higher roundness occurs at shorter soaking times in the fine-grained ascast samples, and at longer times in the inhomogeneous or the coarser-grained as-cast structures. The optimum thixotropic condition (high roundness, 0.72, and small globule sizes < 90 μm) are achieved after 5 min. soaking in the samples treated by UST at 623 and 620oC, which is the typical soaking time dictated by the industrial practice in SSM. The amount of entrapped eutectic as observed after soaking treatments is uniquely very small, suggesting that the UST-treated ingots will have better formability in the semisolid state. The growth rate constants are substantially low: in the order of 479-748 μm3/s. These growth rate constants are much lower than those reported for MHDcast A356 ingots. The growth rates of the samples produced by UST in the liquid state (i.e., 626, 623 and 620oC. Note that liquidus temperature is 619oC) are lower than those of the samples treated in the semi-solid temperatures, i.e., 617 and 614oC. The Ostwald ripening is most likely the dominant growth mechanism in the UST-treated samples during the soaking treatments. These results reveal the feasibility and competence of UST as a potential route for thixotropic feedstock production.

Abstract: In a previous study, an aluminum nitride (AlN) layer was formed below the melting point of aluminum (Al) on the surface of an Al substrate (JIS-A1050) in a barrel with alumina/aluminum-magnesium alloy powder which activates the substrate in the nitrogen atmosphere. In this study, the mechanism of formation of AlN in the barrel was examined. AlN formation requires an incubation period. During the incubation period, a white region on the surface is observed by the optical microscopy. The Electron Probe Micro Analyzer (EPMA) show that this region contains magnesium (Mg). It seems that Mg penetrates and diffuses from the filled Al-Mg powder in the barrel. After that, the nitride is partially generated at the surface of the Al substrate, and it grows with the substrate surface. The AlN layer thickness increases proportionally with the square root of time, and it has good adhesion.

Abstract: Acetylene and ethylene are frequently used in vacuum carburizing in Japan. In this study the natural gas which is available from the lifeline is applied to vacuum carburizing. The gas composition inside the furnace was analyzed by the gas chromatography in order to examine the carbon infiltration mechanism. Unsaturated hydrocarbon gases (such as acetylene and ethylene) are generated from the natural gas. The effect of acetylene concentration in the furnace on the carbon infiltration rate was investigated. The carbon amount which infiltrates into the steel increases, as acetylene concentration in furnace increases. It is possible that carbon concentration of specimen surface increases to the cementite precipitation concentration in the short term, when natural gas flow rate increases in the initial carburizing stage. After that, carbon concentration of specimen surface does not decrease, even if the natural gas flow decreases, because carbon atoms which are consumed for diffusion to inside are sufficiently supplied. By using this method, inhibition of soot generation, reduction of process gas and shortening of the carburizing period are possible. The carbon concentration profile of the vacuum carburized specimen was compared with the simulation.