Year:2008 | Volume:41 | Issue:8 (110)|Papers Count:15

In the present work, the effects of changing punch and the die radius on several process parameters involved in deep drawing of AISI 304 steel have been studied. Deep drawing process of a cylindrical cup was simulated and analyzed by numerical modeling. The results were compared with those of obtained from a serie of experimental cupping tests. The comparison shows that: maximum drawing force decreases with increasing the die nose radius. Changing in the thickness of cup wall decreases as the punch nose radius increases and thinning of the material occurs in cup wall region in which work hardening is enhanced. Finally, the comparison shows the best verification of experimental and computer simulation results.

Spring back is responsible for inaccuracy of press formed sheet product. Therefore, prediction of spring back is important for precise forming of stampings. To predict the amount of spring back many successful attempts have been made for simple cases such as single curved and U or V shape sheet metals. However, for more complex shapes containing double curves or double variable curved surfaces there are few research works. In order to propose a method to solve such problem, theoretical researches followed by experimental examinations have been carried out. A new formulation for prediction of spring back in a double curved sheet metal has been proposed by authors. In this formulation an improved yield criterion that have been proposed by Yoshida, has been incorporated. This yield criterion is capable to reveal the influence of transient Baushinger effect and work hardening stagnation on spring back analysis. Using proposed formulation, the effect of thickness on spring back has been calculated and compared with experimental results. Results show that by increasing of sheet thickness, spring back increases. Reveal behaviors have been explained based on the variation of neutral axis position.

In this study, Eulerian elasticity theory based upon the logarithmic stress rate was coded in the finite element formulation for large strain and large rotation cases. Modified Newton algorithm and algorithmic tangent module were used for solving of the nonlinear equations. In order to verify finite element code, numerical and analytical results in simple shear problem were compared. The results show that two and three dimensional finite element implementation are effective. In this study, consistency integrability properties of some hypo-elasticity models such as logarithmic, Jaumann, Green-Naghdi, Eulerian and Lagrangian triad based rates also Truesdell and Cotter-Rivlin rates were discussed by means of finite element determination of dissipation energy at the end of three dimensional elastic closed deformation paths. Results showed that, among all the stress rates studied here, only logarithmic rate has presented no dissipation energy at the end of three dimensional elastic closed deformation paths. This study underlines some results previously obtained by other researchers, i.e., among all considered stress rates, the logarithmic rate manifests the best result in respect of elasticity requirements. In this study, some stress integration algorithms such as Hughes-Wingt and Zhou-Tamma algorithms were compared in the simple shear test and a modified algorithm was presented. Results show that integration time and precision have been improved by using modified algorithm.

Morphology of surface oxide film formed during pouring of molten magnesium alloy has been investigated. Due to surface turbulence during casting, the oxide film necessarily makes folded cause in a dry surface to dry surface mode creating a double oxide film with the volume of air that can be encapsulated between folds of the film and this led to make gas bubbles or shrinkage cavities form. These kinds of oxides called new oxide films that form in a very short time during pouring. It seems to be one of the main reasons for dross-like defects. However, study of characterization and features of oxide film affected on prediction of final mechanical properties. The inner, unwetted surfaces of the doubled film representing an unbounded interface in the liquid and therefore, effectively constitute a crack. Samples for the study were prepared based on a technique in which an oxide metal sandwich was made by the bubble impingement technique, after impingement the contact areas of two adjacent and entrapped bubbles oxide-metal-oxide layer (OMO) were selected for the study. Features such as thickness, size, morphology and chemical composition of the oxide film were studied using a scanning electron microscope (SEM). Energy dispersive X-ray (EDX) microanalysis was performed for detection of the composition of the oxide layers. Results showed that the morphology of the oxide film in molten of magnesium alloys is folded and quite rough included globally phases of magnesium oxide. Recently, researches showed that the morphology of the oxide film in aluminum alloys is different due to composition of base alloy. Magnesium alloys in liquid state due to high oxidation rate is sensitive to formation of oxide film. Thickness of the oxide film folds in magnesium alloys is 2-4 µm that in comparison to aluminum alloys are ten times higher. However, potential of casting defects is higher in Mg alloys. The contacting interface between impinged bubbles (OMO) represents an elegant and powerful means for studying surface films of liquid metals and study for solidification structure

Although the number and the severity of TBC applications on hot section components have dramatically increased in the past decade, premature spallation failure of TBCs, due to mismatch of thermal expansion at the metal/ceramic interface of the two coating layers, during service is still an overriding concern. Therefore, functionally graded materials (FGMs) with a gradual compositional variation have been introduced. In this study, comparison of properties of two different types of thermal barrier coatings (TBCs) was made to improve the surface characteristics on high temperature components. These TBCs consisted of a duplex TBC and a five layered functionally graded TBC. In both coatings, Yittria partially stabilized Zirconia topcoat (YSZ) was deposited by air plasma spraying (APS) and NiCrAlY bond coat was deposited by high velocity oxyfuel spraying (HVOF). In FGM coating, functionally graded layer was sprayed by air plasma process by varying the feeding ratio of YSZ/NiCrAlY powders using two separate powder feeders. Then, isothermal oxidation was carried out at 950oC in atmosphere to obtain the plot of mass change vs. time to study oxidation kinetic. Micro structural and compositional changes of coating, oxides formed during service were examined by optical microscope and SEM with EDS. FGM coating failed after 2100 h and duplex coating failed after 1700 h. Finally, it was found that FGM coating is more qualified than duplex TBC and stands for a longer time.

In the present work, the effect of aluminum content, austenitizing temperature and austempering time and temperature on the mechanical properties and hardness of Fe-C-Al grey cast iron was investigated. For this purpose, three alloys containing 1, 2 and 4 weight percent of aluminum were provided. The tensile samples were austenitized at 850oC and 900oC for 120 min, followed by austempering process at 275oC, 325oC and 375oC for 1-120 min. The ultimate strength and hardness of samples with tensile and hardness tests were investigated. The results showed that the samples without aluminum had the maximum strength. When amount of aluminum increased to 2 wt% ultimate strength decreased and while the amount of aluminum increased to 4 wt% the ultimate strength improved somewhat. The increase of austenitizing temperature from 850oC to 900oC in the samples not containing aluminum and containing 4 wt% aluminum increases ultimate tensile strength and hardness. In the alloys with 1 wt% and 2 wt% aluminum, the increase in the austenitizing temperature leads to decrease in the ultimate tensile strength and hardness. The results also indicate that in the samples not containing aluminum and/or containing 4 wt% aluminum, the first stage of transformation is completed after 30 minutes. Between 30 and 60 minutes, the second stage of transformation starts and the increase of austempering time to over 60 minutes leads to the beginning of the third stage of transformation. In the alloy with 1 wt% of aluminum maximum elongation was seen after 30 minutes but more austempering time caused the decrease of the elongation. In the alloy containing 2 wt% aluminum, using low austempering temperature (275oC, 325oC) and increasing the austempering time leads to the rising of elongation. In the high austempering temperature (375oC), the third stage of transformation begins after 30 minutes and the elongation decreases.

Low water-wettability and oxidation resistance of graphite have limited its application in carbon containing refractory castables. The aims of this study were improvement of water-wettability and the oxidation resistance of natural flaky graphite by applying an oxide coating on its surface. To develop the coating, magnesium aluminate spinel sol was formulated via a citrate-nitrate route and graphite powder was then added to the sol. The mixture was heat treated in appropriate temperature and atmosphere to get the polycrystalline MgAl2O4 coating on graphite particles surface. The microstructure of coating was studied by X-ray diffractometer, SEM and TEM. The water-wettability was evaluated by measuring the water drop contact angle and plotting the zeta potential vs. pH. The results showed the development of a stable nanocrystalline MgAl2O4 spinel coating which improved the water-wettability and oxidation resistance of graphite significantly.

The molybdenum dicilicide is an intermetallic compound with two allotropies tetragonal MoSi2 and hexagonal MoSi2. The MoSi2 phase is established to 1900oC and MoSi2 phase from 1900 to 2050oC. This material has considerable properties that provide different applications in different industries such as air-space industry and gas turbines. The molybdenum synthesis can be down in various ways such as melting and casting, SHS (Self propagating High temperature Synthesis). Recently, the mechanical alloying method because of its considerable advantages such as powder particle compound homogenizing, fragmentation and contact area increasing and also crystal defect increasing that affects ductility increasing of brittle compound such as intermetallic compounds, is used for progressive materials synthesizing. In this research, the synthesis and formation of molybdenum diciliside was investigated by mechanical alloying method from stoichiometric mixing molybdenum and silicon (Mo/2Si) and also its phase transformations. A mixture of elemental molybdenum and silicon powders at the stoichiometric composition of MoSi2 were ball milled to 100 hours using a planetary ball mill. The milling was performed at rotational speeds (vial speeds) of 300 and 400 rpm and stainless steel balls (5 and 10 mm in diameter). Other synthesis conditions such as ball to powder weight ratio and mass of the charged powder were chosen similarly. The results demonstrate that mechanical milling affects the formation of nanocrystalline MoSi2 from elemental powders through solid state reaction. In other words, formation of molybdenum disilicide from its primary elements during mechanical alloying depends on effective parameters in mechanical alloying (collision frequency of ball to powder particles, contact area creation and heat transferred to powder particles view point) according to XRD results. The results indicate that increasing in milling intensity and decreasing in ball diameter cause to fast formation of MoSi2, which is derived of increasing in milling energy. The thermal analysis investigation confirmed affection of mechanical activation on the acceleration of MoSi2 synthesis with reduction of formation temperature of molybdenum disilicide during mechanical alloying and electron microscopic investigation showed that MoSi2 forms on the surface of molybdenum grains during mechanical alloying. It was also demonstrated that formation of MoSi2 (a and/or b) depends on mechanical activation conditions and selection of parameters during mechanical alloying. So that if intensity of mechanical activation be chosen high and/or selection of parameters contribute with high heat production in milling vials, MoSi2 phase alone or with MoSi2 phase (that has low activation energy with respect to MoSi2) will be produced. In lower milling intensity, usually at first MoSi2 is produced. Phase transformation of occurs during mechanical alloying when the crystallite size decreases to approximately 12 nm. The final product of mechanical alloying is MoSi2.

During the last three decades, intermetallic alloys have focused attention because of their high strength to weight ratio and good creep resistance. Titanium–aluminide alloys based on g-TiAl are potential candidates to replace Ni-based super alloys currently used in jet engine components at high temperatures because of their low density, high melting temperature, good elevated-temperature strength and modulus retention, high resistance to oxidation and hydrogen absorption, and excellent creep properties. One of the major concerns in these alloys is their poor ductility at room and intermediate temperatures which has been improved slightly by microstructure modifications through heat treatment. Thus, modification of microstructure during cooling and CCT diagram in these alloys is of vital importance. In this study, Ti-47Al-2Cr intermetallic alloy has been prepared by remelting 4 times with a vacuum arc remelting furnace (VAR). Homogenizing treatment was done at 1125oC for 72 h in a sealed vacuum quartz tube. All heat treatments on the samples were carried out in a vacuum heat treatment furnace under a pressure of 10-1 bar. The atmosphere inside the furnace was changed to that of high purity argon for each heat treatment as an added precaution against oxidation. In this paper, phase transformations in a g-TiAl based intermetallic alloy containing chromium were investigated. Heat treatments on samples of this alloy at temperatures above Ta and subsequent cooling with various cooling rates resulted in variety of microstructures. The schematic CCT diagram for this alloy was drawn from microstructural studies using microscopy routs and X-ray diffraction. Then, cyclic heat treatment with grain refining purpose was conducted on a sample of this alloy having massive gamma microstructure. During cyclic heat treatment, gradual dissociation of the gamma phase resulted in the formation of a widmanstaten type structure. Trend of microstructure evolution and formation of widmanstaten structure has been analyzed considering the crystallographic characteristics of the parent phase. The results show that homogenizing treatment at 1100oC may bring about a microstructure containing, 2 single phase grains and initial and secondary lamellar grains. Also the various cooling rate from phase region may result in formation variety microstructure, as at low cooling rate may cause formation lamellar coarse grains microstructure and by increasing the cooling rate will result in feathery, massive and fully 2 microstructures.

This research concentrates on the effects of low frequency, vertical mechanical vibration during solidification on the microstructure and mechanical properties of A380 aluminum alloy. Two series of Sr-modified and Al-5Ti-B grain refined A380 alloy samples were solidified in both static and dynamic conditions in the frequency and amplitude of 60 Hz and 0.2 m, respectively. Their Physical, micro-structural and mechanical properties such as UTS, Y, elongation, hardness, density and SDAS were compared to those of sample solidified in the dynamic condition without any additives. The results indicated that the gas and shrinkage porosities were reduced by vibration through intensifying mass feeding. As a result, density and hardness increased with applying vibrational energy. Vibration decreased SDAS and refined eutectic silicon in unmodified condition and caused coarsening of eutectic silicon in Sr-modified samples. Mechanical parameters such as UTS, Y, elongation and quality indices were increased in dynamically solidified samples. Vibration influenced elongation more than tensile strength.

Copper–silver bimetallic strips were produced by cold roll welding process and were treated by diffusion annealing in the temperature range 250–800oC. The interface bonding strength was determined by bending and peeling tests, Resistance, conductivity and micro hardness profiles were determined and microstructure in the interface region was observed by optical microscope. Electrical resistance and bonding strength in the interface depend on the diffusion annealing temperature. Diffusion annealing above 600oC produces fine-grained eutectic phases in the interface region and silver matrix. The eutectic phase formation and the movement of interface is a chemical–diffusion process. It is observed that the strength is greatly reduced by increasing the thickness of eutectic compounds. These compounds have detrimental influences on physical and mechanical properties of the interface. The results indicate that there is an optimum annealing temperature at which the sheet exhibits a satisfactory formability together with a high bonding strength.

Effect of concentration parameters on recovery of zinc oxidized minerals from lead flotation tailings of Dandy mineral processing plant in north-western Iran was studied. A sulfidization-flotation method has been used on a laboratory scale to investigate the effect of various reagents such as sodium sulfide as sulfidizing agent, primary amine as collector, dispersants and flocculants with different concentrations to reach an optimum zinc recovery. Among the dispersants tested, hexametaphosphate gave a higher zinc grade 40.4% and 70% recovery. Usually de-sliming is used to increase the recovery of zinc, however, analyses have shown that fine particles in the sample mostly contain zinc, thus de-sliming is not suggested.