امیر حسین جباری مستحسن

Magnesium and its alloys have found a wide range of applications in medical industry and manufacturing of absorbable bio-implants because of having a modulus of elasticity similar to bone as well as having biocompatibility and biodegradability characteristics. Moreover, employing magnesium implants with a porous structure can lead to an increase in the rate of bone replacement instead of the initial absorbable implant. On the other hand, by adding hydroxyapatite reinforcing particles to magnesium and its alloy, their mechanical properties and corrosion resistance may be improved. In this paper, the effect of magnesium particle size on compressive strength of Mg/hydroxyapatite porous bio-composites has been studied. The magnesium and composite specimens with non-porous and porous (with a volume porosity of 24% and 58%) structures were produced using powder metallurgy method. For this purpose, two types of magnesium powder with particle sizes of 63 µm and 250 µm, hydroxyapatite particles as reinforcement, and ammonium bicarbonate particles as space holder, were used. The results of the uniaxial compressive tests revealed that the strength of all the pure and composite specimens fabricated by 63 µm powder has improved up to 103%, compared to those of the specimen fabricated by 250 µm powder. In addition, for both amounts of volume porosity, the composites offer a higher compressive strength than that of the pure specimens.

Nowadays, functionally graded materials have different applications in various industries, such as Aerospace industry, turbomachinery, coating industry, etc., due to their unique ability in providing multiple and sometimes opposite properties in a material volume. The purpose of this paper is to fabricate functionally graded material sheet of aluminum based composite with SiC reinforcing particles, using powder metallurgy and hot rolling methods. In this regard, the amount of reinforcement in the direction of thickness has been changed from the value of 0 to 4 weight percent. The samples were prepared in four steps including ball-milling, degassing, cold pressing and sintering, and then were hot-rolled up to three passes. The distribution of the reinforcing particles in the matrix phase was evaluated using optical microscope. Furthermore, the mechanical properties of the FGM samples including their hardness, tensile strength and flexural strength were measured and reported. Finally, the fracture surfaces in the tensile and flexural tests were observed using scanning electron microscope (SEM). According to the images obtained from the microstructure of the samples, the reinforcing particles have an acceptable distribution in the matrix phase. Also, the results indicate that the hardness and strength are enhanced by increasing reinforcing particles and the number of rolling passes. In addition, the main fracture mechanism in pure aluminum layer is the initiation and propagation of cracks between initial aluminum powder particles, while separation of two phases in the matrix-reinforcement interface and small SiC particle agglomerations are responsible for crack initiation in the composite layers.

Nowadays, there is an increasing demand for magnesium and its alloys as the lightest commercial metal with a high strength to density ratio. Nevertheles, afew undesirable properties, such as low hardness and poor wear resistance, have limited the applications of this exclusive metal. Fabrication of magnesium matrix composite could improve hardness and wear resistance in addition to strength increasing. Since converting all of a metal piece to composite could make it more brittle and increase the costs, fabrication of surface composite could be a solution. In this paper, a magnesium sheet with a surface composite has been fabricated by applying warm rolling process. Indeed, a layer of magnesium matrix composite (which has been fabricated by stir-casting) has been conjoined to a magnesium substrate layer using a zinc interlayer. This method could increase the production speed and decrease the costs. In addition to the connection of two layers together, the zinc interlayer would preserve the surfaces of the layers from oxidation without using any controlled atmosphere. The results show a proper connection between the surface composite and the substrate. According to the microhardness results, the hardness of surface composite increased about 23% and 52% in the cross-section and the surface, respectively. Moreover, wear resistance of surface composite improved about 43% in comparison to magnesium substrate. Also, wear rate decreased in the surface composite.

Multi-layer thermal insulations are fabricated by locating consequtive porous insulation and radition shields, which could be used at high temperature and cryogenic applications. In this type of insulations, different heat transfer methods such as conduction, convection and radiation would be occurred, although by using high density insulation (more than 20 kg/m^3 ), convection could be neglected. In this paper, radiation shiels parameters such as thickness, emissivity, distance and number of screens are studied and optimized. For investigating the effect of these parameters on effective thermal conductivity of multi-layer thermal insulation, a mathematical code has been improved in EES software. Then, the obtained results have been validated by another study. Moreover, Powell method has been applied in order to optimize the parameters. The results show that the amount of shield emssivity and shields arrangement have the most impact on the effective thermal conductivity of multi-layer thermal insulations. Also, the optimized distances between radiation shields indicate that this distance increased in the direction of heat transfer.

Increase of strength to weight ratio is one of the main aims in pressure vessel reinforcement. Wire-winding is a safe technique which can be used to reinforce pressure vessels by introducing compression prestress. The purpose of this paper is analytical and numerical investigation of reinforcement effect on CNG vessels by using wire-winding technique. This process increases working pressure to weight ratio in aluminum vessels wound by steel wire in comparison to full-steel vessels. In this work, first, the wire-winding method was explained. Next, by considering synchronic yield in the CNG vessel and number of wire layers, maximum internal pressure is calculated. For this purpose, an analytical solution based on Tresca yield criteria is used. Then, the problem is simulated by finite element software. Finally, the obtained results are compared with the analytical results. It is shown that in reinforcement of aluminum vessel by winding five layers, the maximum ratio of working pressure to weight can increase 55% in comparison with full-steel vessels. Also, maximum error between analytical and numerical results was about 3%.