Iranian Journal of Materials Science and Engineering

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A new process for recovering scheelite ores comprises producing a concentrate from the ore, then leaching the concentrate with H_2SO_4 in the presence of H_3PO_4 and Na Cl at atmosphericpressure are discussed. Finely purification of the product will be described. The amounts of dissolution of tungsten in acid depend on the parameters such as time, temperature, type and concentration of acid and stilt as well as solid-liquid reaction. These factors were optimized for the result and described in details.

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Effects of temperature on properties and behavior of a 20 vol % particulate SiC reinforced 6061 aluminum alloy and 6061 unreinforced Al alloy were investigated. Yield strength and elongation to failure were measured as a function of test temperatures up to 180^oC. In addition, the effects of holding time at 180^ oC on tensile properties and fracture mechanisms of the materials at this temperature were studied. The behaviors of the materials were characterized by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS), X-ray diffraction (XRD), atomic absorption (AA), hardness measurement and image analyzing (IA). The results show that an increase in temperature leads to a decrease in the yield strength and increase in the elongation to failure of the materials. On the other hand, while increasing holding time at 180^oC produces an increase in the elongation to failure of the unreinforced alloy, it reduces the elongation to failure of the composite. It was also observed that reduction in yield strength with increasing holding time at 180^oC was faster for the composite material compared to the unreinforced alloy. The results from SEM, XRD, EDS, IA and hardness tests indicated that some chemical reactions had taken place at the interface between the reinforcement and the matrix alloy during holding the specimens at elevated temperature. Therefore, different trend in elongation to failure of the unreinforced alloy and the composite material with holding time at elevated temperature could be attributed to development of chemical reactions between the reinforcement and the matrix alloy at the interface.

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A two dimensional mathematical model has been developed for describing the temperature, flow, and electric fields in the are column of the Gas Metal Arc Welding (GMAW) of aluminum in argon shielding gas using axisymmetric Navier-Stokes, Maxwell, and differential thermal energy equations. The predicted results are most sensitive to the cathode spot radius and an optimum cathode spot radius exists on the basis of the minimum arc power consumption. The consumable electrode shape change due to droplet detachment is simulated in a quasi-steady manner using different electrode diameters. The change in electric field profile with different electrodes gives rise to large changes in flow conditions. This flow perturbation plus the non-uniform and transient J*B force field are suspected to enhance the recoil experience by the droplet on detachment, thus leading to violent and chaotic metal transfer in GMAW.

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In this paper redox reaction processes and phase, formation on ternary Pb-2Sn-0.08Ca alloy utilized as insoluble lead anodes in copper electro winning cells, were investigated in 2M H__2SO__4 electrolytes, using cyclic voltammetry technique (CVA). A potential range between - 1.3V to 2.6V was chosen at various scan rates in order to study the anodic behavior and phase composition of the oxide layers on Pb-2Sn-0.08Ca alloy. Potential measurements were carried out with respect to a standard calomel electrode (SCE). The surface examination and phase composition of the lead alloy were determined by electron microscopy analysis (SEM) and x-ray diffraction technique, after- exposure to the cyclic voltammetry tests. The results indicated that the protective oxide layer (Pb0__2) formed at a much slower rate when exposed to 2M H__2SO__4 solutions, whilst its degradation due to a reduction in the applied potential occurred at a much faster rate. Hence, Pb0__2 did not remain stable, when the externally applied potential dropped below 1.SV and as a result, pitting initiated sporadically on the surface of the electrode.

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The continuity and thickness of the coating layer, are the most important factors in wetting properties and strength of carbon fibers. These factors are crucial in the quality of metal matrix composites made with carbon fibers. In this research the Polyacrylonitrail base carbon fibers have been nickel coated with 0.2, 0.5, 0.8 and 11 ,u in thickness, by the electroless method. The effect of the thickness of nickel coating on surface condition and also the tensile strength of the carbon fibers has been investigated. The study of surface condition of the coated carbon fibers by SEM showed that the nickel coating at the thickness of about 0.5 pin has the best continuity oil the carbon fibers. The results of tensile tests of carbon fibers coated with different thickness of nickel showed that increasing the thickness of coating layer decreases the overall strength of fibers.

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In principal, a proper combination of strength and ductility is achieved through micro component refinement in steels. This is particularly empowered with ferrite refinement down to micron sizes in ferrite pearlite engineering steels. The latter is achieved through various well-defined methods in which strain induced transformation (SIT) has shown spectacular capabilities. In the present study, to address the effect of thermo mechanical processing parameters on the (SIT) behavior, two plain carbon steels were studied through single pass rolling. This was carried out at the corresponding Ar_3 + 20°C temperature of the steels. The results indicated that the transformation behavior and ferrite morphology would be .strongly influenced by both the chemical composition (i.e., carbon content) and the amount of applied strain. Furthermore, a high volume fraction of very fine ferrite with mean grain size of less than 2 µm was obtained. This was attributed to the ferrite nucleation at deformation bands and serrated austenite grain boundaries.

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In this research, effects of changes in aluminizing conditions on microstructure of Pt - aluminide coating applied oil a Ni - base superalloy GTD -111, has been studied. A thin layer (i.e.68,#mm ) of Pt was electroplated onto the surface of the .samples, and then they were aluminized by pack cementation technique under various conditions of time, temperature, rate of heating and pack powder compositions In addition, by application of a thin Ni layer on the substrate before applying Pt, the source of nickel available for diffusing into Pt layer during aluminizing process enhanced and the need for an extra heat treatment cycle before aluminizing process was practically omitted. Addition of a nickel layer, also prevented scaling of Pt layer during its electroplating and aluminizing processes that helps enhancement of cohesiveness of the coated layers.The general microstructure of the coating consisted of four layers, which are PtA12layer internal diffusion layer external diffusion layer and interdiffusion zone. The structure of each layer has been studied by optical and scanning electron microscopes as well as XRD technique.The results show the presence of the original Pt - electroplated layer had no effect on the thicknesses of the coating layers, but higher aluminizing time and temperature had increased the thicknesses of interdiffusion and internally diffused layers. In addition, at high temperature, aluminizing with a lower heating rate caused an increase in the thickness of internal diffusion layer. Aluminizing with a lower heating rate at high temperature (more than 900°C) had increased the thickness of interdiffusion laver Attempt has been made to justify , the changes occurred in microstructures and thick nesses of various coating layers as they exposed to different aluminizing conditions.

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The effects of gating system and pattern geometry on the metal flow in the lost foam casting (LFC) process have been investigated using glass covered mold and video recording system. Unlike convectional casting process, the type of the gating system showed little effect on fillability in lost foam, but pattern thickness had large effect on mold filling. The mold filling behavior seems to be controlled by the combined influences of heat and mass transfer. The flow rate increased with increasing pattern thickness.

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In this investigation, α 2-D Finite Volume Method (FVM) with unstructured triangular mesh is developed to simulate the mould filling process. The simulation of fluid flow and track of free surface is based on the Marker And Cell (MAC) technique. This technique has capability ofhandling the arbitrary curved solid boundaries in the casting processes. In order to verify the computational results of the simulation, a thin disk plate with transparent mould was tested. The mould filling process was recorded using a 16mm high-speed camera. Images were analyzed frame by frame, in order to tracking of free surface and filling rate during mould filling. Comparison between the experimental method and the simulation results has shown a good agreement.

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The new in situ method for AI-TiC composite fabrication has been carried out. In this method, fabrication of AI-TiC composite by simultaneous introduction of titanium oxide and carbon into aluminum melt was investigated.. Under the process conditions, titanium and carbon reaction results in titanium carbide whiskers. The salt containing keriolite (Na3AIF6), titanium oxide (TiO2) and graphite used for this purpose. Using Scanning Electron Microscopy (SEM) and X-Ray Diffraction analysis (XRD) the resulted composite was characterized. It was shown that it contains Al as matrix and TiC as the reinforcement. Then, mechanical properties of fabricated composite were examined.

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In the current research, the optimum conditions for the electrolytic deposition of TiO2 coatings on titanium pieces were experimentally investigated. Flat pieces of commercially available titanium with dimensions of 50 x20 x3 mm were used as the anode and cathode electrodes. The coatings were applied on the cathode in an electrolyte solution essentially from water and methanol, containing different amounts of TiCI4, and H202. Coatings of sufficient thickness and adequate adhesion to the substrate were obtained at the optimum conditions of theELD process. The latter conditions were electrode gap distance of 3 cm, TiCl4 concentration of 0.005M, H202 concentration of 0.1 M, current density of 35 mA/cm2, methanol/water volume ratio of 9, and pH of the electrolyte in the vicinity of 1.40. Results of XRD analysis revealed the presence of anatase crystals of titanium oxide in the coated layers, where the deposited coating was treated at some temperatures in the range of 400 to 600°C for a period of at least 2 hours. Scanning electron microscopy (SEM) pictures also confirmed the formation of a uniform coating layer with cracked suiface area. At the optimum conditions of the process coatings with thicknesses of up to 10 flm were easily obtained through the application of one to three deposited layers.

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In this investigation a new model was developed to calculate gas pressure at the melt/foam interface (Gap) resulting from foam degradation during mould filling in the Lost Foam Casting (LFC) process. Different aspects of the process, such as foam degradation, gas elimination, transient mass, heat transfer, and permeability of the refractory coating were incorporated into this model. A Computational Fluid Dynamic (CFD) code was developed based on the numerical technique of the SOLution Algorithm- Volume Of Fluid (SOLA- VOF) utilizing model, for the simulation and prediction of the fluid flow in the LFC process. In order to verify the computational results of the simulation, a thin plate of gray iron was poured into a transparentfoam mould. The mould filling process was recorded using a 16mm high-speed camera. Images were analysed frame by frame, in order to measuring foam depolymerization rate and the gap volume during mould filling. Comparison between the experimental method and the simulation results, for the LFC filling sequence, has shown a good agreement.

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In the present research influences of different combinations of five types of inoculants and four types of nodularizers on eutectic nucleation in ductile cast iron were studied. The alloys were kept at 1320°C for 0, 5, 10, 15 and 20 min under nitrogen atmosphere. Cooling curves and image analysis were used to characterize the influence of holding time and different combinations of inoculants and nodularizers on the graphite morphologies and eutectic nucleation. When dominant morphology of graphite is spheroidal or flake, by decreasing the different combinations of nodularizers and inoculants fading resistance, temperature of eutectic undercooling decreases. Nevertheless, when there is combination of spheroidal, vermicular and flake graphites temperature of eutectic undercooling increases by reduction of material fading resistance.

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The surface condition and microstructure of near stoichiometric (Fe823Ndll.8B5.9) Nd-Fe-B alloy ribbons and the effect of melt spinning parameters were investigated using optical, scanning and transmission electron microscopes (SEM, TEM). The formation of gas pockets on the roll surface of the ribbons during melt spinning can prevent heat transform and result in local coarse grains. The local thickness would also be less in these places and thus perforates preferentially during ion beam milling. Therefore different areas of the sample should be carefully observed in the TEM. Reducing inert gas pressure in the chamber will eliminate the gas pockets. As a general trend, decrease in the ribbon thickness and mean Nd2 Fe14B grain size were observed on increasing the roll speed. By careful adjustment of the melt spinning parameters, the nanostructure will develop. An orientation relationship was found between Nd2 Fe14B and α-Fe precipitates for coarse grain samples melt spun at low roll speed. Dark field image of such grain also shows that some of these α-Fe precipitates have the same orientation. X-ray diffraction evident the development of texture by decreasing the roll speed.

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ZnS : Cu phosphors were prepared by using laboratory grade chemicals through coprecipitating Cu along with ZnS using H2S and thiourea. Photo- and electroluminescence studies indicate that these phosphors have better emission characteristics compared to the phosphors in which activator is externally added. Phosphors with luminescence at ~530nrn were prepared. The difference between the characteristic properties of the samples seems to be due to formation of nanoparticles during the preparation of the samples by different methods.

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Two commercial methods are used for the production of strontium carbonate:1) Direct conversion of Celsetite to strontium carbonate by hot sodium carbonate,2) Carbothermic reduction of celestite with coal followed by water leaching of strontium sulfide(SrS) and its conversion to strontium carbonate.The present study has been made on the carbothermic reduction of celestite ores of Varamin (Iran) mines. Effects of temperature, time, pellet size, particle size of celestite ore, pellet compactness and type of reducing agent have been studied. In the range of 800-1100°C, reduction rate increases notably with temperature, which may mean that the reduction is predominantly chemical controlled. Activation energy of around 22.5 kcal/mol supports the idea of chemical control mechanism. Further support for this postulation is provided by the following facts:1) Increasing rate with carbon reactivity (graphite, coal, and charcoal)2) Small dependency of rate on pellet compactness.3) Small dependency of rate on pellet size

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Thin sheets of hydroxyapatite were fabricated by tape casting process. The non-aqueoustape casting formulation was optimized by adjusting the concentration of organic additivesspecially those of binder and plasticizer in the slurry. The optimized slurry was cast on a glasscarrier using a designed laboratory scale doctor blade. Following a binder removal stage, thetapes were sintered at different temperatures in air atmosphere. Heat treatment at 1250 °C led tothe formation of a dense microstructure as was evidenced by the scanning electron microscopy.

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During mold filling, molten metal can only advance as quickly as the air inside thecavity is expelled. In this work an analytical model describing air flow is developed based on aincompressible flow theory. Air pressure has serious effects upon the filling behaviour such assurface profile and filling time. In this work a new mathematical model is proposed for calculationthe air pressure during the mold filling. A single phase computational fluid dynamic code based onthe SOLA-VOF algorithm used for prediction the fluid flow. Air discharged through the vents ismodelled by ideal gas assumption, conservation of mass equation and Bernoulli law. A newalgorithm was developed to interpolates the air pressure on the surface cell. The creation of airback pressure was correlated with sizes of vents and pouring basin height. In order to verify thecomputational results a series of experimental test was conducted. Comparison between theexperimental data and simulation results has shown a good agreement.

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A large number of reinforced concrete structures subjected to chloride ions. Two basicapproaches for preventing corrosion of reinforcing steel embedded in concrete are: Increasing theconsolidation of concrete and using different coating on rebars. In present research steel rebarsare coated in different ways: a) 40 µm of zinc electroplated on steel rebar b) Zinc powder withepoxy paste (zinc rich). The rebars were placed in a macrocell design according to ASTM G109-92. Concrete operations were done with mixture designs of high and normal strength concrete.The results show corrosion decrease of zinc coated rebars.

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AA3004 alloy is widely used in can making. The major concern in the production of canbodies is earing, which develop by high planar anisotropy of rolled sheet. Balance ofrecrystallisation and rolling textures together with a uniform and fine grain microstructure canminimize the earing. The effects of finish rolling temperature (FRT) on planar anisotropy,microstructure, texture development and mechanical properties of sheet have been analyzedIncreasing the FRT resulted in promotion of cube and G texture in hot rolled sheet. Lower |ΔR| ofthe final sheet with higher FRY, i.e. the lower planar anisotropy, is accompained with a balance ofrolling and recrystallisation orientations.