Carbon nanotubes (CNTs) are promising reinforcements for light weight and high strength composites due to their exceptional properties. However, until now, the main obstacle is to obtain a homogenous ...dispersion of the CNTs in the desired material matrix. Quite a few methods have been studied to help improving the dispersion of CNTs in a polymer matrix. But not much research has been conducted on how to disperse CNTs in metal matrices. In this study, a two-step process was applied. In the first stage, a block copolymer was used as a dispersion agent to pre-disperse multiwall carbon nanotubes (MWNTs) on Mg alloy chips. Then the chips with the well dispersed MWNTs on their surface were melted and at the same time vigorously stirred. The molten MWNT Mg alloy composites were poured into a cylindrical mould to solidify quickly. For the pre-dispersion step, the microstructures of the Mg alloy chips were studied under SEM. MWNTs were quite successfully dispersed on the surfaces of the Mg alloy chips. The mechanical properties of the MWNT/Mg composites were measured by compression testing. The compression at failure, the compressive yield strength and ultimate compressive strength have all been improved significantly up to 36% by only adding 0.1
wt% MWNTs to the Mg alloy. In order to predict the potential yield strengths of the MWNT reinforced Mg alloy composites, the contributions by load transfer, Orowan strengthening and thermal mismatch were added up.
In the present study, an optimized low-cost numerical model has been developed to simulate melt stirring and mass transfer in aluminum melting furnace. To validate the numerical model, scaled-down ...water model experiments were also conducted. The present numerical results revealed good agreement with the experimental observations. Faster speeds of impeller rotation could improve the stirring efficiency; however, a strong surface vortex was formed at the free surface under these conditions. The perfect mixing time was the longest at middle immersion depth of impeller and the strong surface vortex was generated at the largest immersion depth. The mechanism of surface vortex generation was explained in terms of pressure distribution around the blades, which is created by the impeller rotation.
Full 3D numerical simulation and experimental characterization of the topology and sensitivity of a magnetohydrodynamic flow pattern driven by a travelling magnetic field generated by a specifically ...designed cylindrical Bitter coil have been carried out. This Bitter coil has six windings composed of multiple circular copper plates and insulator separators disposed in a specific way for an optimum round circulation of electric currents, which are three-phase currents at a network frequency. For experimental characterization, experimental measurement data of radial components of the induced magnetic field (Br) at different z-positions were used without any liquid metal for different applied electric currents, and ultrasound Doppler velocimetry was employed to acquire the vertical velocity component along the beam axis in the centre and at different azimuthal positions for a fixed radial coordinate in the liquid GaInSn alloy. On the other hand, for numerical characterization, a numerical model has been developed using the Comsol MultiphysicsTM software to resolve the set of magnetohydrodynamic equations, fundamentally based on low frequency and low magnetic induction, and on an averaged expression of the Lorentz force. The results clearly show that the choice of 3D modelling and the Bitter coil design, which generates instabilities, can change significantly the usually expected axisymmetric torus-type magnetohydrodynamic flow topology and result in a distinct 3D flow configuration. Figs 8, Refs 16.
The effects of in-situ AlP formation on the solidification process of an Al-20 %Si alloy were studied by examining the change law of the solidification microstructures after the addition of Cu–P ...alloy to the melt and stirring. During the early stage of stirring, the eutectic Si was clearly refined and Si mainly existed in a point-like shape. During the middle stage of stirring, the typical eutectic microstructure disappeared, and the microstructure consisted of fine primary Si uniformly distributed in the α matrix. Many AlP precipitates with sizes near the critical nucleation radius of primary Si were formed after the Cu–P alloy was dissolved in the Al-20 %Si alloy and stirred. These AlP precipitates became the nuclei of the primary and eutectic Si. The Si-poor layers around the primary Si provided the conditions necessary for the precipitation of the primary α phases when the primary Si grew. For finer primary Si, the distance between the primary Si was reduced, and lower temperature facilitated the formation of a complete α loop or continuous α phases around the primary Si. During the eutectic reaction, the ultrafine AlP caused the eutectic Si phases to nucleate alone and grow; the eutectic α phases grew based on the primary α phases, which resulted in the formation of divorced eutectic microstructures.
Melt Stirring of Nodular Cast Iron Khan, Fareed Ashraf; Fredriksson, Hasse
11th International Symposium on the Science and Processing of Cast Iron, (SPCI-XI), Jönköping, 2017,
06/2018, Letnik:
925
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
In this work the effects of stirring/vigorous shearing on matrix and graphite phase in nodular cast iron melt during solidification were studied. Several experiments were conducted for different ...cooling and stirring times. Samples were prepared and examinations of the microstructure were conducted and compared using Leica Optical Microscope and Scanning Electron Microscope (SEM). In addition, a chemical analysis of the graphite precipitate was performed using EDX equipment mounted on SEM. Oxidation of the melt and formation of oxide nuclei during stirring were observed and analysed. The influence of melt shearing on the structure, nodule count, distribution, area fraction and on overall graphite precipitation was observed and examined with the help of respective computer programs and soft wares. Stirring resulted in increasing the nucleation sites for graphite precipitation thereby increasing the nodule count and area fraction of carbide and transforming the structure from pearlitic matrix to ferritic matrix.
Abstract
Silicon is the most widely used raw material in photovoltaic industry; however, the quality of the silicon photovoltaic solar cells depends on the quality of the raw material (i.e. ...metallurgical silicon or poly-silicon) and the solidification methods used for the production of the silicon ingot from which the solar cells are produced. This study is related to how improve the quality of the final ingot in the directional solidification process; it is necessary to control the impurity segregation of silicon raw material during the processing. This control can be accomplished by adding an electromagnetic Bitter coil which can generate an external traveling magnetic field (TMF) stirring to control the hydrodynamic flow of silicon melt during the solidification process without contaminating it. To carry out this study, we used a Bridgman vertical directional solidification furnace, equipped with a cylindrical Bitter coil stirrer used in order to have the control of the silicon melt convection on the principal parameters of the solidification process, such as the growth rate, the thermal gradient and the natural convection of silicon melt. For the electromagnetic, heat exchange and silicon melt flow modelling, we used 3D numerical Multiphysics coupled models. Parallel to the numerical results we carried out experimental investigations relating to the characterization of the electromagnetic parameters. This study shows a promising effect of the applied traveling magnetic field on the final ingot quality; indeed, we have the ability to control the silicon melt flow which can affect the thermal configuration, the solidification interface shape and the segregation of impurities by changing the electric current input configuration of the Bitter coil stirrer.
Carbon nanofiber (CNF) reinforced A7xxx nanocomposites were fabricated by a liquid process, and the effect of CNF dispersion on the mechanical properties of the CNF/A7xxx nanocomposites was ...investigated. To enhance the dispersion of CNFs in the A7xxx matrix, we reduced their aspect ratios by vibration milling, and used surfactants and ultrasonication. A Cu layer was coated on the surface of CNFs to enhance the wettability of Al on CNF, to reduce the difference in specific gravity between CNFs and the Al melt, and to suppress the formation of brittle Al4C3 phase. Also, a button-shaped feedstock was fabricated with a mixture of Cu coated CNFs and Al powers to overcome the difficulty in direct injection of CNFs into the Al melt. As a result, CNF/A7xxx nanocomposites were successfully fabricated by a liquid process. The variations in the mechanical properties of the CNF/A7xxx nanocomposites according to the concentration of CNFs were analyzed. The yield strength, ultimate tensile strength, and elastic modulus of the nanocomposites for CNF of 0.76wt% increased by 33%, 55%, and 17%, respectively, in comparison to those of the A7xxx matrix. The CNF dispersion increased the strength of the CNF/A7xxx nanocomposites by about 38%.
Metal matrix composites are considered as advanced materials in the field of automotive, marine and several industrial applications. In the present work investigations have been made on effect of ...nano B4C particulates addition on the tensile behavior of Al2219 alloy. Nano B4C particulates of 500nm were used as the reinforcements in the Al alloy matrix. Nano composites were synthesized by using liquid melt method in steps of 2 and 4 wt. % in the Al2219 alloy. Samples were tested for micro structural characterization by using scanning electron microscope and energy dispersive spectroscope. Tensile properties like ultimate tensile strength (UTS), yield strength (YS) and percentage elongation were evaluated as per ASTM standards. Scanning electron micro photographs revealed the uniform distribution of Nano B4C particulates in the Al2219 alloy and confirmed by EDS analysis. Further, ultimate tensile and yield strength of base matrix Al2219 alloy was enhanced with the addition of Nano B4C reinforcement. There was slight reduction in ductility of composites.
The effects of reinforcement volume ratios (RVR) on the composite structure and thermal conductivity of Al-MgO reinforced metal matrix composites (MMCs) of 5%, 10% and 15% RVR produced by melt ...stirring were examined. EN AW 1050A aluminium alloy was used as the matrix and MgO powders with particle size of -105 micron were used as the reinforcement. The composites were produced at 500 rpm stirring speed, with 750 C liquid matrix temperature and 4 min stirring time. Composite samples were cooled under normal atmosphere. Microstructures of the samples were then determined and evaluated by SEM and EDS. In general, it was observed that the reinforcement exhibited a homogeneous distribution. It was also determined that the increase in the RVR increased porosity. A thermal Ansys model was generated to determine the effective thermal conductivity from SEM images. The effective thermal conductivity of the Al-MgO composites increased with decreasing reinforcement volume ratio.
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