Amir Pakdel; Masoud Emami; Hassan Farhangi; Mohammad Habibi Parsa
Abstract
Al-SiC composites are among the most demanding metal matrix composites due to their excellent strength, good ductility, good corrosion resistance, low coefficient of thermal expansion and reasonable price. Manufacturing of cast metal matrix composites usually involves some problems such as inhomogeneous ...
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Al-SiC composites are among the most demanding metal matrix composites due to their excellent strength, good ductility, good corrosion resistance, low coefficient of thermal expansion and reasonable price. Manufacturing of cast metal matrix composites usually involves some problems such as inhomogeneous distribution of the particles due to poor wetability of ceramics to molten alloys, porosity and formation of particle-void clusters. Thus it seems necessary to utilize secondary processes for these materials in order to obtain suitable properties. In this research Al6061 composites reinforced with 10 volume percent SiC particles of 48?m average size were produced by the stir casting method and effect of extrusion process on the microstructure and strength of these materials was investigated. Results showed that the average particle size and porosity of the composite samples decreased after extrusion. Moreover, tensile strength of the composite increased by increasing the extrusion temperature and/or the extrusion ratio since the pores in the microstructure diminished and the interface bonding was improved.
Majid Parvizi; Hamid Reza Ghasemi Monfared Rad; Mohammad Habibi Parsa
Abstract
Wire-brushing is used for different purposes such as removing metal surfaces oxides, increasing surface brightness, improving fatigue properties and surface activation before coating. The wire-brushing severely scratches and stirs the metals in the surface layers, so that complicated metal flow gives ...
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Wire-brushing is used for different purposes such as removing metal surfaces oxides, increasing surface brightness, improving fatigue properties and surface activation before coating. The wire-brushing severely scratches and stirs the metals in the surface layers, so that complicated metal flow gives rise to very large amount of plastic strain and increases dislocation density of the surface layer. After recovery, subdivided grains and a nanocrystalline structure are formed on the surface. Formation of nanocrystalline structure on the surface of Al 6061 alloy by wire brushing at various brushing speeds was investigated by using atomic force microscopy and X-ray diffraction. The results showed that the severe plastic deformation decreased the surface grain size to 30-70 nm.
Maryam Haj Fath'alian; Mahmoud Nili Ahmadabadi; Mohammad Habibi Parsa; Tahereh Shah Hosseini; Hannaneh Ghadirian; Tahereh Hosseinzadeh Nik
Abstract
Equi-atomic Nickel-Titanium (NiTi or Nitinol) has the ability to return to a former shape when subjected to an appropriate thermo-mechanical regime. Pseudoelastic and shape memory effects are some of the behaviors presented by these alloys. The unique properties concerning these alloys have encouraged ...
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Equi-atomic Nickel-Titanium (NiTi or Nitinol) has the ability to return to a former shape when subjected to an appropriate thermo-mechanical regime. Pseudoelastic and shape memory effects are some of the behaviors presented by these alloys. The unique properties concerning these alloys have encouraged many investigators to look for applications of NiTi for biomedical applications. One of the most successful applications of Nitinol is orthodontic archwire. The best features of these wires are super-elasticity, the phenomena that causes easy engagement (loading conditions). Superelastic nitinol wires deliver clinically desired light continuous force during deactivation (unloading conditions), enabling effective tooth movement with minimal damage for periodontal tissues. Superelasticity is characterized by a load-deflection plot with a horizontal region [plateau] during unloading, implying that a constant force may be exerted over that particular range of tooth movement. It is known that the NiTi alloy wire undergoes a phase transformation from an austenitic to a martensitic phase as the load increases during the loading process. Metallurgical studies have attributed these characteristics to a reversible phase transformation from the body centered cubic structure to the monoclinic structure of nickel-titanium when the stress reaches a certain level during deformation.The increasing amount of energy stored inside the NiTi wire during this process is consumed during the unloading process as the transformation is reversed, and the martensite structure reverts to austenite . Superelasticity is only exhibited by wires showing high endothermic energy in the reverse transformation from the martensitic phase to the parent phase and with low Load/deflection ratios. These wires show nearly constant forces in the unloading process, a desirable physiological property for orthodontic tooth movement. NiTi archwires have gained acceptable by orthodontists as initial alignment wires. Most of the information about the behavior of orthodontic wires is based on mechanical laboratory three point bending tests to study load-deflection characteristics. A three-point bending test allowed load-deflection curves offer reproducibility. Variations in model design have been shown to affect load- deflection plots. The load deflection performance of NiTi wires depends on the design of the test model. Modified three point bending test which simulates wire force on the teeth in the oral conditions has more correct results than ordinary three point bending test. The purpose of this study is to investigate the load-deflection characteristics of superelastic nickel titanium wires with a new model design trough modified bending tests. In this research a new three point bending fixture was invented and designed to determine the superelastic property in clinical conditions, and the wire samples were held in the fixture similar to oral cavity. By means of this instrument the three point bending test simulates wire force on the teeth in the oral configuration. The lower section of fixture is a rail fabricated from steel, a special movable base with a curved canal assembled over the rail. The upper section designed to simulate the teeth arrangement and curvature: A stainless steel disk (316L, ?80 mm, h10 mm) selected; and twelve rods (316L, ?5 mm, h10 mm) welded to the points representing center of teeth on the disk. The center points of teeth were located on the medium upper standard arc. Distance between the center points of teeth (interbracket distance) was similar to Wilkinson model.To achieve deflections 1, 2 and 4 mm, teeth 5,3 (right) and 2 (left) were selected respectively and rods in these points were movable. A fillet face machined on the rod surface parallel to the standard arc. Brackets fixed on the flat face of rod by superglue and orthodontic wire attached to fixed appliance. The superelastic behavior has been investigated through load-deflection test
