Papers by Author: Ali Shokuhfar

Abstract: Air Spindles have been used in ultra precision machines for several years due to their advantages such as high speed rotation, low friction, and low vibration, [1]. Air spindles are widely used in these machines for producing precise work pieces. Although, spindles function on a very complicated theoretical basis, [2, 3], their structure is very simple and consists of mainly a rotor and a stator. The rotor/stator could be made of different shapes. A cylindrical shape is the one commonly in use. The spindle designed in this work has a spherical configuration. It has been designed so that it could be moved without application of electric motor and only by a wind turbine system, [4]. The spindle studied in this research uses compressed air for rotor suspension, and has an air turbine for rotating its shaft. A thin air film acts as bearing layer between rotor and stator. In design procedure, operation parameters such as air inlet pressure for turbine, air inlet pressure for bearing, diameter of turbine nuzzles, diameter of bearing nuzzles, clearance between rotor and stator and etc. have been considered, [5]. A prototype spindle has been manufactured using design criteria. The influence of above mentioned parameters have been recognized through experiments.

Abstract: Technological requirements urges high technology industries using materials with advanced properties. Although it is compulsory using these materials due to their advantages, some of them should be tolerated because of their weak machining and mechanical properties. Attempts for qualitative machining of complex curves on many work pieces may result in the advent of new finishing techniques. One of the most newly introduced techniques is Magnetic Abrasive Finishing, MAF. In this paper a study on the effects of tool and work piece gap, and lubricant volume, on the roughness and material removal rate have been studied. The results show that setting the working gap to 3 mm and the lubricant volume to 0.7 ml, yields a change in surface roughness (Ra), and the material removal (MR) will gain its best result.

Abstract: The present paper deals with different effects of homogenization time and cooling environment on Ni-42.5wt%Ti-7.5wt%Cu alloy. The alloy was prepared by vacuum arc melting. Afterwards, three homogenization times (half, one and two hour) and three cooling environments (water, air and furnace) at 1373 K were selected. Optical and Scanning Electron Microscopic methods, EDX, DSC and hardness tests have been used to evaluate the microstructure, transformation temperatures and hardness. Results indicate that specimens that were cooled in air are super-saturated. Also, the microstructure from furnace cooling has many disparities with the other cooling environments’ microstructure and two types of precipitates exhibit in the matrix, but in other cooling environments, only one phase can be seen. Particles of the Ti2(Ni,Cu) phase are distributed in the matrix in all of the microstructures irrespective of cooling rate. Observations show that increasing the time of homogenization results in finer precipitates and uniform distribution in the matrix. In addition, alteration of cooling rate and time of homogenization affect the martensitic transformation temperatures. On the other hand, the hardness varies slightly for different homogenization times but declines extremely with decreasing cooling rate. Moreover homogenization time and the cooling environment affect the transformation temperatures on furnace cooled samples.

Abstract: NiTi alloys containing more than 55%wt nickel undergo precipitation of Ni4Ti3, Ni3Ti2, and Ni3Ti phases during various heat treatments which could have a great effect on the chemical composition of the matrix and behavior of alloy. In this investigation, a NiTi alloy with Ti-57.5%wt nickel content, produced by vacuum induction melting in a graphite crucible, were subjected to the homogenization heat treatments in 1100oC and for various time periods (0.5, 1, 2, and 4 hours). The subsequent cooling was conducted in different cooling media (furnace and air) in order to examine the effect of cooling rate. Microstructural investigations show Ni4Ti3 particles with bimodal size distribution in furnace cooling. Differential scanning calorimetry demonstrates the correlation between homogenization time and transformation temperatures of the alloy.

Abstract: In this investigation, the dynamic recrystallization phenomenon (DRP) of an Al-Li-Cu-Mg-Zr alloy was studied at two temperatures, 350°C and 400°C. Wedge samples were subjected to hot rolling deformation in both temperature and one passes. For wedge specimens, reduction up to 70% was considered. Results showed that the grain size of the specimens after hot rolling decreases from 100 μm to 30 μm because of DRP. Furthermore, it is observed that critical reduction for starting DR at 400°C is 40% and at 350°C reduction must increase to 50%.

Abstract: Among various biocompatible materials, hydroxyapatite (HA) is widely used in medical applications. Hydroxyapatite can be used as temporary substitute material for the human bone. Despite of the risk of contamination during milling, the mechanochemical method shows higher reproducibility and low processing cost. In this investigation, the mechanochemical method has been carried out to produce nanocrystalline powders of hydroxyapatite using two experimental procedures (HA1: CaHPO4 + Ca (OH) 2; HA2: CaCO3 + CaHPO4) in polymeric and metallic vials at different milling time. The Effects of milling time, milling media and also chemical composition of initial materials on the crystallinity and morphological properties of obtained materials using X-ray diffraction (XRD) and transmission electron microscopy (TEM) were studied. Appropriate equation and graphs for determining crystallinity degree were used. The obtained results show that the crystallites sizes are within the nanometer range and also indicated that nanocrystalline hydroxyapatite with spherical morphology and high crystallinity degree can be produced much better in polymeric vials; therefore using polymeric vials with high wear resistance can have better performance during the mechanochemical process for the production of high quality nanocrystalline hydroxyapatite. Further work is needed to expand the idea for mass production.

Abstract: The effects of volume fraction at different milling times and impact forces, defined as the ball-to-powder weight ratio (BPR), on the elemental diffusion during mechanical alloying process of Al-4.5wt%Cu/SiC composite were evaluated and compared with the SiC free samples (Al-4.5wt%Cu alloy) in the current work. X-ray diffraction patterns of the monolithic and composite samples imply the fact that a higher level of mutual diffusion of constituents, Al and Cu, happened in the matrix in the presence of SiC particles. This effect of the reinforcing particles can be attributed to the increased densities of dislocation and vacancy caused by the presence of SiC particles within the matrix giving rise to increasing the micro-strain, lattice parameter and decreasing the crystallite size.

Abstract: The study of mechanical alloying (MA) process on the immiscible Al–Cu systems having positive heats of mixing has been investigated by the earlier researchers. However, a comprehensive understanding of the diffusion phenomenon during the mechanical alloying process is still far from complete. The effects of milling time and impact force, defined as the ball-to-powder weight ratio (BPR), on the elemental diffusion during mechanical alloying process of Al-4.5wt%Cu were evaluated in the current work. X-ray diffraction results showed that increasing the milling time and impact force led to increasing the dislocation as because of increasing the micro-strain, lattice parameter and decreasing the crystallite size. As a result of this, the diffusion rate was enhanced. The interpretation of data resulted have been discussed in details.

Abstract: Ultra precision machine tools are used in nano machining technology. Two main assemblies creating rotational and linear motion, called air spindle and linear air table are used in these machines. The linear air table has been simulated experimentally like a linear air bearing. This bearing moves in the main direction, X, while it has straightness error motions in Y and Z directions. The error values vary due to different parameters. This investigation deals with the influential parameters and their proportion in the error value. As far as the two mating surfaces are separated by a thin layer of pressurized air, the air pressure, location and amount of external load, are some of the parameters which have been studied. Results show that the more air pressure, the more stability and stiffer table. There will be less error motions as well. The error amount is not the same in X and Y directions. Finally, in order to have linear stiffness, the optimum air pressure, external load, and load location have been decided.

Abstract: Air spindles in ultra precision machines produce rotational movements for the cutting tool or work piece. The common combination of a simple cylindrical rotor and stator is the design for most spindles. If the length of such a spindle is longer than usual, it will deviate from its stable situation and start vibrating during operation, especially in high rotational speeds. In order to overcome the vibration problem, one of possible solutions is the application of a spherical rotor and stator. The manufacturing and assembly limitations do not allow obeying the spherical shape exactly. Thus, the design has been committed according to a quasi-sphere. This form of the rotor will be more stable. The subsequent result of stability improvement will be less air pressure and power consumption. There are some specific characterizations of the spindle which must be calculated for the spherical case. For this purpose a computer model of the object was made. Then, the model was put under finite element study to find the best air pressure and air flow velocity condition.