Sharif Digital Repository

Considering that automobile weigth reduction in industry is important, improvement the strength of low carbon steel sheets is important. These sheets can be supported the service condition in lower thicknesses. Bulk nano-structured materials have been mostly attracted in materials science due to their unique physical and mechanical properties. Severe plastic deformation (SPD) is one of the most efficient methods to produce ultra fine grained or nano-structured materials. Materials produced by SPD are of great importance because of (1) non-porous structure, (2) great mechanical properties such as high strength and toughness and (3) proper dimension for mechanical and physical testing. ... 

Wireless sensor networks have many applications, vary in size, and are deployed in a wide variety of areas. They are often deployed in potentially adverse or even hostile environment so security issues are of much concern in these networks. Key management is a fundamental security issue in sensor networks. In this thesis, we are mainly focusing on key establishment and group rekeying schemes aspects of key management. Due to limited resources of sensor networks, key pre-distribution schemes are currently viewed as the most promising solution. Key predistribution in sensor networks refers to the problem of distributing secret keys among sensor nodes prior to deployment. Recently, many key... 

By increasing speed gap between microprocessors and off-chip Last Level Cache, Optimization in Last Level Cache makes improvement in system performance. With development of new generation of multi-core processors and sharing LLC between these cores, the so called issue of Memory Wall has caused an incremental effect of LLC on system performance. There are three approaches to use this memory more efficiently:
1. Increasing cache capacity
2. Making cache hierarchical and adding different layers to hierarchy
3. Improvement of replacement algorithms in cache memory
The first approach has not been used in regard with limitation of technology and growth of access time due to... 

Trahertz gap science and technology is becoming more and more attractive due to the wide range of applications in medical science, communications and security. In addition, this band connect the optical and microwave devices. In this thesis, special optical structures composed of superconducting nanostructure based on the elemental type-II and layered high-Tc superconductors are proposed. The photonic band structure and the optical S-matrix are calculated for these proposed structures for different geometrical values. Such structures produce terahertz (THz) photonic band gap, which is controlled by the temperature and geometrical parameters. In addition, the proposed structures show... 

Today steel tubular systems are used more than before, especially in recent years. A difficulty with this type of systems is making beam to column connections using bolts due to no access to holes of bolts from the inside of tube or hollow circular column. Reverse channel connection is devised and introduced to prevail this problem. Access to holes in this connection is from the inside of channel. In this connection, the web of the channel is connected to steel beam using bolts and the two legs of channel is welded to face of tube column.
This type of connection does include some innovations; not too many researchers work on different versions of this connection. This thesis provides an... 

In this research, Friction stir welding (FSW) of 7075-O and 7075-T6 aluminum alloys was performed to investigate the effects of the base material conditions, effects of process parameters on the FSW characteristics and optimum parameters which lead to best mechanical properties of joints. The experimental results indicated that the base material condition has a significant effect on weld morphologies, weld defects, and mechanical properties of joints. The optimum parameters were 1230 rpm and 60 mm/min for 7075-O and 1230 rpm and 100mm/min for 7075-T6. Also, Double welded friction stir welding of 7075 aluminum alloy was performed to investigate the effects of the second pass of weld on the... 

The aim of this research is to fabricate Mg based nanocomposite by friction stir processing and investigation of the process parameters on the microstructure and mechanical properties of the nanocomposite. In order to this aim, alumina nanoparticles as reinforcement were poured in the groove, with 5mm depth, and due to rotation of friction stir tool, these nanoparticles were mixed with the matrix and nanocomposite was produced. To reach to the best condition in nanocomposite fabrication, the effect of friction stir processing parameters such as, tool rotation speed, tool design, number of friction stir passes, cooling media and reinforcement percent on microstructure, hardness, tensile... 

Flux-cored arc welding has become prevalent in the recent decades worldwide. One of the variants of this welding method is self-shielded welding which requires no external shielding, neither with mineral flux nor with shielding gases. In this study, the variation of microstructure and mechanical properties of flux-cored self-shielded welds was investigated and optimization of this properties was also carried out. For this purpose, a basic combination of powders was chosen from the literature to fabricate the flux-cored wire. These powders contain fluorine, calcium carbonate, aluminum, magnesium and iron. Then, same amount of alloying elements such as molybdenum and nickel was added to this... 

Friction stir soldering (FSS) or brazing (FSB) as an emerging joining process, was first developed to resolve some shortcomings in FSLW and FSSLW such as excessive IMC layer thickness and confined bond area. Although it has advantages over conventional furnace soldering and/or brazing, including localized heating, less detrimental thermal effects on the substrates and capability to reduce the amount of voids at the joint interface. The aim of this study is to investigate the effects of rotational speed, traverse speed and plunge depth of the pinless tool on the shear strength and electrical resistivity of the copper-coated graphite-copper FSSed joints with the aid of Sn-37Pb solder alloy. At... 

Ultra-fine and fine-grained aluminum matrix nano-composites were fabricated by friction stir processing (FSP) of 1050 Al alloy sheets containing 6 vol% volume fractions of pre-inserted MMO nanoparticles. Annealed sheets were processed at different rotational (w=800-2000 rpm) and traverse speeds (v-50-200 mm/min). A special design tool (concave shoulder and cylindrical pin with regular threads) without changing the rotation and traveling directions was utilized. It is shown that the best and highest desired mechanical properties of the base metal is attained at w=1600 rpm and v =100 mm/min through one single pass. The structure of grains are changed due to the applied severe plastic... 

The repair and production of Ni-based and Co-based superalloy parts have high importance in Oil and Gas, and Aerospace industries. Due to this issue, Transient Liquid Phase bonding is one of the most reliable welding methods for repair parts and fabricating new parts.In the present study, creep properties of similar transient liquid phase bonding of FSX-414 Co-based superalloy has been investigated and compared with creep properties of the un-bonded base metal. Firstly, the optimum condition of the bonding process was resulted from microstructural observations from different bonding conditions. Then, cylindrical samples for the aim of creep test specimens were fabricated in two different... 

In this research, aluminum matrix composites (AMCs) reinforced by different contents of zirconium dioxide nanoparticles, graphite microparticles, and a mixture of both with various hybrid ratios, into a cold-rolled Aluminum1050 as base metal, fabricated by 4-passes friction stir processing. Observations show that the processing of composites reinforced by Gr particles is significantly hard due to the low friction coefficient of graphite resulting in the decline in the maximum temperature during the process and the type of base metal. The mixtures of graphite and zirconia resulted in more homogenous dispersion of particles in the matrix due to deterring a drastic drop in friction coefficient... 

Friction Stir Spot Welding (FSSW) with ability of refilling weld’s keyhole, is one of the new solid state joining processes which have gained many applications, especially in automobile industry. Considering newness of the process, first part of this study has been dedicated to design and manufacture of a FSSW machine which is done entirely by author of this study. Second part was survey on effect of parameters such as Tool rotation speed, Weld time, and Forge time for refilling the hole, on properties of an aluminum joint. After welds were made, specimens were subjected to Lap-shear tensile test, after that, studies on macro and microstructure have been carried out. Generally an increase in... 

In present study, a CP-Ti surface was coated with cored wire electrodes filled with graphite powder by means of TIG cladding technique. The process was performed under controlled atmosphere with different composition of Argon and Nitrogen. The parameters of the process were changed in order to investigate the resulting effect on the coating microstructure and the wear properties. The microstructure and the composition in the weld and heat affected zone were carefully investigated by optical microscope, SEM, EDS and XRD. As the result, TiC dendrites were produced in-situ with different morphologies which are dependent on variation of the parameters. Moreover, TiN dendrites were in-situ...