Sharif Digital Repository

The purpose of this research is to investigate the residual stresses generates during solidification of a casting part and the effect of casting geometry and thermal treatment conditions on these stresses. The pattern was designed with to arms to ensure tensile tension. CO2 process was used for modeling and due to the high hardness of the mold, it also acts as a constraint and exacerbates the tension. To measure tensions in parts, two methods consist of Hole Drilling, and an innovative method based on heat treatment and dimension change have been used. The results of the two methods show the same values of stress. The Hole Drilling method is a verified and high-precision method, which is... 

Today, Mg and Al as the widely used metals, make a special contribution to automotive, electronics and aerospace industries. Increasing the application of alloys of these metals, has extended the demand for reliable joint between alloys of these two metals. The main problem with the dissimilar joining of A356 alloy and AZ91 alloy is hard and brittle intermetallic compounds formation such as Mg17Al12 and Mg2Si which brings about welding strength reduction. By far, a large number of researches about dissimilar joining of AZ91 alloy and A356 alloy in semi-solid condition have not accomplished. In this method two fragments, one from AZ91 alloy and another from A356 alloy among which was a 2... 

Diverse methods of producing semi-solid metal have been the subject of extensive study for the past 40 years. Among all these methods, electromagnetic stirring (EMS) and gas induced semi solid (GISS) have been used on a commercial scale. Considering the fact that in previous studies the intensity of electromagnetic field in the central part of the stirring chamber was negligible, a hollow cylinder with an almost pure iron core was utilized in the central part of the chamber in order to intensify the electromagnetic field in the center and to introduce gas bubbles into the melt. Aluminum A356 alloy was stirred by a three phase electromagnetic stirrer and introduction of gas bubbles under... 

Liquefaction of gases is one of the applications of cryogenic science. Among the gases, Helium has the lowest boiling point, which at a pressure of 1atm is approximately 4.2K. Liquid helium can be used to cool superconductors in transportation equipment, military radars, and medical imaging equipment. For helium liquefaction, various cycles are used; these cycles have different parameters in their structure such as of input mass fraction to expanders, heat exchanger effectiveness, expander arrangement and ... which their analysis for improving the performance of the cycle is important. In this research, the aim is to study 2, 3 and 4 expander cycle with various parameters and structure. For... 

Aluminum is the lightest industrial element after magnesium. As a result, aluminum alloys have many industrial usages because of their high specific strength. Semisolid casting is one of the processes that is capable of producing aluminum alloys with high mechanical properties. In this process, there are a wide variety of methods to prepare semisolid slurry, one of the best ways of which to produce globular structure is electromagnetic stirrer. In this research, the semisolid slurry of A356 aluminum alloy is fabricated by electromagnetic stirrer under different conditions of stirrer intensity current, stirring time and pouring temperature. Then the fabricated semisolid slurry is injected... 

Estimation of demand, for example, for transportation is an important issue for the planning and programming of various systems. Thus, enhancing the predictability power of such models is of high value. Demand stems from choices of individuals, and choice is subject to uncertainty and fuzziness. Uncertainty comes from many factors including incomplete information of decision-maker, and fuzziness comes from many other factors including personal perception regarding the value of the variables affecting choice. This study tries to introduce a new approach in building choice models, in which variables are transformed into another variable by the help of a global wisdom. This wisdom shows the... 

As the crowded cities require public transportation and because of lack of space on the ground, Subway and boring underground tunnels encountered. In this thesis the interaction of boring the line 7 of Tehran metro with the existing station of line 1 of Tehran metro (Molavi station) is studied. By changing the soil specification, The surface settlement is significant so the soil specification is one of the important parameters of the interaction. For comparinf the effect of these parameters, the soil settlement and bending moments is investigated in different parts of station while the lower tunnel is passing beneath it. The 3D Abaqus software is used for modeling this interaction. In... 

In Persian language, there are some special uncertainties and complexities regarding the extraction of phonemes due to its flexible structure. In this work, we try to solve two problems caused by this issue, homographs disambiguation and Ezafe recognition in Persian text to speech systems.
In this work, two methods are presented for solving the two problems. The first one is a pattern recognition method based on Aho-Corasick algorithm that has the best performance according to the execution time. The second method is a hidden Markov model tagger based on Viterbi algorithm that uses bigram and trigram statistics. These methods are run on 10 million word corpus and the results are compared... 

2024 Aluminum alloy has an abundant application in the aerospace and automobile industries. One of the most important problems of the alloy is hot cracking during the fusion welding process. It is stated that the solidified grains can be refined by a static magnetic field and an alternative electric field applied simultaneously during solidification. To make the alternative electric field, an alternative current was passed through the specimen. In order to prevent hot cracking, electromagnetic vibration was used. Then the effect of intensity of the static magnetic field and alternative electric current on joint was examined. Joining was carried out at electric currents of 40, 60, and 80A and... 

The purpose of this investigation was to study the microstructure of the Inconel718 alloy wich was produced by VAR and suction casting method, and study the effect of cooling rate on the solidification structure of Inconel718 alloy. In the first a labratoary scale VAR furnace was made. After producing some sample's with this furnace, a suction casting sample was produced using a 3mm diameter copper mold. Metallography, SEM, EDS, and microhardness analysis was carried away. Cooling rate evaluation was done by using a equation between secondary dendrite arm spacing and solidification time. Results showed that by increasing cooling rate from 63 c/s to around 2000 c/s, columnar and equiaxed... 

A356 Aluminium alloy has an abundant application in the aerospace and automobile industries. One of the most important problems of the alloy is in it’s microstructure after solidification with long dendrite grain of Aluminium.It is stated that solidied grains can be refined by electromagnetic vibration and an alternative electric feild applied simultaneously. To make electric field, electric current passed through the specimen. In order to change in microstructure, electromagnetic vibration was used. Then the effect of intensity of electromagnetic field and alternative electric current on microstructure was examined.Grain refining carried out at electric current 80A and magnetic flux of 13,... 

Airplanes usually experience minor position change and height loss during cruise flight, but under normal circumstances the aircraft total energy is adequately acceptable to maintain the trajectory and the desired performance without severe oscillations. On the other hand, if the airplane encounters a wind-shear or microburst during take-off or landing phases, it would be a dangerous situation, as the aircraft kinetic and potential energy levels are not as high.
In this research, a model predictive controller is designed and investigated to allow a transport category aircraft to either escape or penetrate the microburst. In this regard, initially a DMC controller is designed for 6-DoF... 

Hypereutectic A390 alloy has some exclusive specification among aluminum alloys because of its' excellent properties such as high wear resistance and low thermal expansion. In this research powder-thixoforming method, a compilation of powder and thixoforming methods is used for production of parts. In this method gas atomized powder of pre-fabricated alloy was consolidated by hot pressing under argon atmosphere and converted to an acceptable feedstock for thixoforming stage. The feedstock is reheated to semi solid region and rapidly transferred to a injection sleeve with central runner. The semi-solid feedstock is injected in a step die by a hydraulic press of 1 m/s ram speed. According to... 

Due to the increasing spread of metal industries and the need for raw material as feed for electric furnaces in order to produce cast iron and steel, as well as scrap deficiency and rising prices, the use of sponge iron as a substitute for scrap was considered in this project Therefore, in order to achieve the chemical analysis of the sorel ingot that is special for spherical graphite cast iron production with low manganese and chromium content, the direct reduction process was carried out using tunnel kiln method on low-manganese iron ore concentrate.After the investigation on direct reduction parameters including the saggers and coal grading,the produced sponge iron has melted in... 

Increasing emissions of greenhouse gases from one hand and decline in the fossile resources of energy on the other hand has raised the motivation of searching for renewable sources. Biodiesel, because of having perfect feature for replacing with petroleum diesel, has been considered as candidate for this purpose. Using enzymatic reaction is one of the methods for biodiesel production. Understanding the kinetics of this reaction is important to achieve higher production rates. In present work, the kinetic of transesterification of castor oil with use of Novozyme 435 (as enzyme) was investigated. Enzymatic biodiesel production is a bi-substrate (oil and alcohol) reaction. Genetic algorithm was... 

Semisolid joining is a relatively new method that is started since 2002. So far, Aluminum alloys semisolid joining is not studied in details. In this research, by obtaining local semisolid region in the vicinity of the joining seam, two A356 Aluminum part were welded. Hot Nitrogen gas was used to prepare local semisolid region. Since the hot gas was not capable to produce the region by itself, samples were preheated. In experimental procedure, preheating plane type, hot gas temperature, gas flow, sample dimensions, and welding speed were investigated. Effects of each parameter were determined by microstructure studies and measuring the mechanical properties of welding seam. Results showed... 

The purpose of this research is to provide a model to explain the dependence of the galaxy distribution within voids on redshift and the density of the environment. The study of the formation of large-scale structures in the universe, as one of the most important observables in cosmology, is of great importance. Investigating the formation of these structures is possible by going beyond the linear regime and using theories such as the Press-Schechter and the excursion set. Also, by developing this type of approach, theories have been made possible to study the statistics of halos in environments such as voids, which can be useful for a better understanding of the formation and evolution of... 

Conventional defects in GTAW welding of Aluminum 5052 alloy are the decrease in weld zone strength because of grain growth, formation of columnar grains in the fusion line, solidification cracks and porosities. In this work, effect of strong field electromagnetic stirring on solidification structure of 5052 alloy and elimination of GTAW defects have been investigated. For arc welding processes (especially tungsten inert gas welding), a process called magnetic stirring was proposed, patented, and investigated in the 1970s. The essential aim was to improve weld quality and they indicated that the the Lurentz force is cause of weld stirring. In the earlier researches, alternating magnetic field...