Papers by Keyword: Oxide Films

Abstract: Aluminum is widely used in various industries in the form of alloys due to its unique properties - lightness, high electrical conductivity, and corrosion resistance. However, when casting alloys, various defects arise, the main of which are nonconformities of the ingot chemical composition, mechanical properties, and internal structure. The RUSAL Bratsk PJSC (, Irkutsk Region) aluminum alloy samples have been examined for pores, cracks, and oxide films. The causes of their occurrence have been analyzed and ways to eliminate these defects proposed. It has been found that, with increasing hydrogen content in the melt, micro-porosity leads to friability and macro-porosity of the alloy structure. According to the study results, the likelihood of cracks in the ingots could be reduced by evenly decreased metal temperature and casting velocity. The oxide film defects have been eliminated by feeding metal into the mold in a steady turbulence-free manner, increasing the metal settling time in a mixer, and reducing the alloy preparation time in a furnace.

Abstract: The influence of the mold filling rate on the content of oxide films in aluminum alloy castings is considered. The existing methods for estimating the probability of formation and entrainment of oxide films in the bulk of the liquid, which are associated with surface disturbances during the melt flow through the channels of the mold, are analyzed. A criterion for estimating oxide impurities in the melt using numerical process modeling is proposed. A comparative analysis of the results of numerical simulation with the data of practical experiments obtained by M. V. Sharov and N. M. Galdin was performed. This comparative analysis and experience in the development of technological processes for shaped castings showed the possibility of using the criterion in numerical modeling.

Abstract: The samples of the tool steel P6M5 were modified by means of laser irradiation (hereinafter - LO). The chemical composition of the sample surface before and after the LO was studied using the Auger and X-ray photoelectron spectroscopy (hereinafter - AES and XPS respectively). It was found that while the steel is exposed to LO, the thick oxide layer consisting mainly of the Fe₂O₃ oxide is formed. It was established that the modification with LO leads to increasing of wear resistance and durability of the R6M5 steel because of a double reduction of the friction coefficient.

Abstract: Oxidation thermodynamics of silicon carbide (SiC)ceramic was studied by means of HSC Chemistry code, and the weight change, morphology and phase of oxidation products were analyzed by thermogravimetric analysis(TG), scanning electron microscopy(SEM ) and X-ray diffraction (XRD). The results showed that SiC ceramic could be oxidized to silicon dioxide(SiO2) with release of small molecular gases under oxidizing atmosphere at 800°C, and the formed SiO2 film with appropriate fluidity and low oxygen diffusion coefficient could prevent the spread of oxygen with the oxidation temperature increasing up to 1200°C, which favored the anti-oxidation of SiC ceramic matrix composite.

Abstract: In this paper, for Super304H steel, the growth law of oxide films and the heat transfer characteristics between the pipe and the working fluid were investigated by using numerical software ANSYS, which simulated comprehensively the effects of pipe size, flow rates of steam, flue gas temperature, and steam temperature on the formation and thickness of the oxide film. A bigger pipe wall thickness, a smaller steam flow rate or a higher flue gas temperature will lead the faster growth of the oxide film thickness, the heat flux density through the wall being smaller and the wall temperature being higher. With increases in steam temperature and thickness of the oxide film, the heat flux through the wall decreases with a small amplitude, and the average temperature of tube walls increases slightly.

Abstract: In this paper, by means of Scanning Electronic Microscope (SEM), X-ray diffraction analyzer (XRD) and Cross hatch scanning analysis, the Structure and mechanism of cracking and spalling for Super304H steel oxide films in high temperature steam were investigated. With the oxidation proceeding, the surface of Super304H steel specimens is covered by a large amount of tiny holes, which causes the occurrence of an outer oxide layer. The Super304H oxide film generally has a double-layer structure: the outer layer mainly for iron oxide Fe₃O₄, also being covered by a small amount of Fe₂O₃, and the inner layer with the FeCr₂O₄ spinel phase as primary component. Because of the inhomogeneity of oxide distribution, large amounts of smaller gaps appear in the Super304H steel oxide films. Due to the differences among the outer layer, the inner layer and steel matrix in thermal expansion coefficient, when the steam temperature around the specimen changes periodically, the stress variation arise in the Super304H steel and oxide films, which causes the cracking and spalling of oxide films.

Abstract: The mechanical properties of Al castings are reduced by inclusions, particularly double oxide films, or bifilms, which are formed due to surface turbulence of the liquid metal during handling and/or pouring. These defects have been reported not only to decrease the tensile and fatigue properties of Al alloy castings, but also to increase their scatter. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. In order to follow changes in the composition of the internal atmosphere of a double oxide film defect within an Al melt, a series of analogue experiments were carried out to determine the changes in gas composition of an air bubble trapped in a melt of commercial purity Al, subjected to stirring. The bubble contents were analysed using a mass spectrometer to determine their change in composition with time. Also, the solid species inside the bubbles solidified in the melt were analysed. The results suggested that first oxygen and then nitrogen inside the bubble were consumed, with consumption rates of 2.5x10^-6 and 1.3x10^-6 mol m^-2s^-1, respectively. Also, hydrogen diffused into the bubble from the melt at an average rate of 3.4x10^-7 mol m^-2s^-1, although the rate of H diffusion increased significantly after the consumption of most of the oxygen inside the bubble. Based upon these reaction rates the time required for a typical alumina bifilm to lose all its oxygen and nitrogen was determined to be just under 10 minutes.

Abstract: Entrainment defects such as double oxide films and entrapped bubbles occur frequently in aluminium alloy castings during the mould-filling process, and are very detrimental to both mechanical properties, and reproducibility of casting properties. In this study a modelling algorithm was used to predict the formation and distribution of entrainment defects in Al-Si-Mg alloy castings. The tensile strength of cast test bars was compared with either the number of defects, or the defect concentration within the bars obtained from the simulation. A general relationship between the mechanical strength of the cast test bars and the quantity of estimated defects was apparent.

Abstract: Oxide films in Al-0.7Mg and Al-9.4Si-2.3Cu-1.0Zn-0.49Mg (in wt%) alloy melts were characterised using advanced analytical electron microscopy. The oxides were collected by pressurised melt filtration for direct examination by SEM and TEM. The results showed that the oxide films consisted of numerous sub-micron sized oxide particles, rather than continuous solid films. The oxide particles formed in the two Mg-containing alloys were identified as MgAl₂O₄ spinel by selected area electron diffraction (SAED) and high resolution TEM combined with EDS analysis. The low level of Mg in the melt resulted in the change of the oxide from alumina to MgAl₂O₄ spinel. The MgAl₂O₄ crystals were typically faceted with their {1 1 1} crystal planes and were about 0.2-1.2 μm in size. High resolution TEM examination of the MgAl₂O₄ / a-Al interfaces revealed that there was a cube-on-cube orientation relationship between the two crystals. The possibility of MgAl₂O₄ particles to act as nucleation sites for α-Al grains during solidification is discussed in terms of the lattice matching at the MgAl₂O₄ / α-Al interfaces along the specific crystallographic orientation relationship.

Abstract: In this paper, typical defects in aluminium alloys cast by conventional pouring of liquid metal into a cylindrical alumina crucible and an induction melting process are characterized by using macroetching, optical microscopy and scanning electron microscopy. Oxide film defects are generated in both processing methods. Oxide films formed during casting are a major type of defect observed in the microstructures of aluminium alloys products. It has been found that pouring of molten metal into a mould and induction stirring lead to different forms of oxide defects. Under induction stirring, such low-density entrained defects are found to be carried to near the surface of the liquid aluminium alloys and submerged surface oxide films become entangled.