امیر عبدالله

In this article, the numerical and experimental investigation of the effect of ultrasonic waves on the heat transfer rate with an increase of the wave amplitude is discussed. Numerical modeling determines the possibility of the investigation of the ultrasonic wave’s effects on fluid flow distribution and heat transfer. For this purpose, a cylindrical tank is considered inside which a spiral heater is placed at a fixed height in the water. In addition, ultrasonic transducers are considered as circular plates under the bottom of the tank. In order to simulate, the ANSYS Fluent software is used and the modeling is accomplished in two stages before and after ultrasonic excitation. To validate the numerical results, they are compared with those of the experiments. For this purpose, an experimental setup is prepared witch consists two coaxial cylinders, a spiral heater kept at a certain height in the water, and five transducers attached to the bottom of the tank. Both experimental and numerical results show that the convection heat transfer coefficient increases with the use of ultrasonic waves with a discrepancy of nearly 4% between the results. By increasing the heat transfer coefficient, the heater surface temperature decreases. The discrepancy between the measured and calculated temperature is about 5%. The velocity and temperature distributions obtained from the numerical results show that using ultrasonic waves enhance the fluid flow mixing which in turn increases the convection heat transfer. The higher the amplitude of the ultrasonic wave, the higher the heat transfer coefficient will result.

Superimposing high power ultrasonic vibration on metal forming processes changes the mechanical behavior of the material and reduces its flow stress. These phenomena improve the ductility of material and reduce forming forces. To investigate the influence of ultrasonic vibration on various materials, standard compression and tensile tests under ultrasonic vibration could be performed. This research investigates the compression behavior of Ti-6Al-4V alloy under superimposed ultrasonic vibration. For this purpose, a special set-up was designed, fabricated and tested. Ultrasonic vibration is transmitted from the ultrasonic transducer to the booster and then guided by horn (punch) to apply to test sample. Horn geometry, vibrational mode shape and ultrasonic power were selected as input variables. The effect of specimen position in system vibrational mode shape was studied by using different ultrasonic transmitters, which allowed the specimen to place at node and anti-node positions. It is found that ultrasonic vibrations reduced the formability of this alloy. Acoustic hardening phenomena do not observe in this material. Also, flow stress of the material reduced under ultrasonic vibration up to 17.63% which has direct relation to ultrasonic power. In addition, the most efficiency was observed when the specimen placed in the node position.

Ultrasonic welding as a new technology in many industries has replaced the usual methods of joining such as welding, brazing, sewing and etc. In this technology, ultrasonic energy is generated by the ultrasonic transducer and transmitted to the joining area through the booster and horn. Ultrasonic energy is dissipated and absorbed in the joining area and generates heat. Simultaneous application of static force / pressure and heat causes the involved parts to join with each other. In this process, various parameters such as power, frequency, static force / pressure, time, material and shape of parts and etc affect the joining properties and the resulting sample. In this paper, with the aim of acquainting researchers with the principles and applications of this technology, scientific and industrial research in the field of ultrasonic welding has been reviewed. Due to the major application and different effective mechanisms, this paper is divided into two main parts of metal welding and plastic welding. Considering the importance and application of the subject, the principles and methods of applying ultrasonic vibrations, different mechanisms of ultrasonic, the parameters of the ultrasonic welding process and the important applications of this technology have been studied. Finally, a perspective on the application of high power ultrasonic vibrations in new manufacturing and joining technologies in the future of the industry is presented.

Machining of hardened steels, which their hardness is generally higher than 45 Rockwell C, is called hard turning. These components usually work under dynamic loading conditions and require a high level of surface finish (in the order of 0.15 μm Ra) which cannot be achieved by sequential hard turning and burnishing processes. However, there are serious concerns about this complimant grinding operation; grinding process, on one hand increases tensile residual stresses and on the other hand, increases crack nucleation regions. Therefore, these two factors might decrease workpiece fatigue strength. So, in this paper, the effects of adding a grinding operation before ball burnishing process, have been experimentally studied on final surface roughness and burnishing forces; at the same time, in order to consider the possible destructive effects of grinding process, a set of experimental measurements including surface residual stresses and endurance limit measurement, have been done for AISI 4130 fatigue samples. Based on the achieved results, adding a grinding operation before burnishing process, has lead to 91.56% improvement in surface finish and 39.52% reduction in burnishing forces. In addition, surface residual stress are compressive and there is slight difference in the magnitude of compressive residual stresses in comparison to burnished hard turned samples. Due to these positive findings, endurance limit of produced samples show 10.95% improvement in comparison to burnished hard turned samples.

Wire Electrical Discharge Machining (WEDM) is known as an advanced manufacturing process, especially for producing delicate and intricate shapes and cutting difficult-to-cut materials. Machining error on is an important problem associated with this process. The current paper investigates experimentally the machining errors of three-stage WEDM on the small straight and arced paths. To reveal the reason behind these errors and to compensate them, residual materials of each cutting stage on the straight and arced corner paths were separately measured and analyzed. Machining errors of each WEDM stage in both paths were accurately considered and the causes of these errors in the straight and small arced paths were experimentally and theoretically determined and discussed. Experiments showed that the roughing stage has such a serious deteriorating influence the machining errors on the arced paths that it cannot be compensated in the following finishing stages. The spark angle domains of the roughing stage on the arced paths were calculated and the effects of these domains on the machining errors due to wire diversion from the programmed path were analyzed. In addition, this research proposes a novel guide in multi-stage WEDM by defining some machining concepts and developing equations for error calculation of WEDM finishing stages on these paths. The machining errors estimated by equations have consistency with the related experimental ones. of this study can be employed in the accurate WEDM cuttings.

In this paper, fracture behavior of functionally graded material weakened by U-notches under mode I loading has been investigated. Electro slag remelting process has been used to produce functionally graded specimens in a notch arrester configuration. Hardness test has been utilized to define the position of each layer. Mechanical properties, including elastic modulus and poisson's ratio, vary along the width of U-notched specimens. The critical fracture load (Fcr) was achieved by performing three point bending examination and using force-displacement curve.
Then, the process simulation was done by finite element software. Firstly, Jcr of each specimen was calculated by using critical value of strain energy averaged over a well-defined control volume. Then the critical fracture load was evaluated by means of the J Integral criterion. In this research, the effect of the notch root radius, for a fixed notch depth, on the Jcr value as well as the critical fracture load has been studied. To compare fracture behavior of the FG specimens with the corresponding homogeneous ones, have the same properties with the notch root layer in FG specimen, Fcr and Jcr value of each have been investigated. The average difference between the predicted Fcr by J integral criteria and experimental results is 17.84%. Finally, the effect of number of graded area layers on Jcr and Fcr has been investigated. The results shows, the value of Jcr and Fcr do not be affected while the number the number of layers more than 20.

Investigating the effects of process parameters on forming forces and defects formation in tube spinning process of AA6061
˜یÏå [English]
Tube spinning or flow forming process is used for manufacturing of seamless tubes widely put into service in advanced industries. The ideal flow for materials entering the deformation zone in this process is extrusion-type flow in axial direction. Very localized deformation zone which is confined by outer materials and forming tools is very important aspect of this process. Therefore, development of defects during the deformation process with undesirable flow of materials can be easily occurred. The main reason of undesirable flow of materials is choosing inappropriate process parameters which results in arising various geometrical and dimensional defects. In this paper, the effects of process parameters on formation and growth of different defects and their correlations with material flow and forming forces in tube spinning of AA6061 was investigated by using design of experiment (DOE) method. The results of experiments show that by applying the optimized values of reduction and feed rate per revolution, these defects can be controlled. Also, by comparing the experimentally measured and theoretically calculated forming forces it can be shown that the larger the deviation of measured forces from calculated ones gets the more severe formation of defects and undesirable materials flow becomes.

Hot pressing of metal powders is used in production of parts for similar properties to wrought materials. Since residual porosity, inhomogeneous properties, dimensional and geometrical instability and therefore reduction in mechanical strength are the main problems in powder metallurgy components which are due to friction between powder particle interfaces and powder compact and die walls. Because of this, access to parts with high density and homogeneous structure is the great object in powder metallurgy. One of the remedies can be employment of ultrasonic vibrations which is thought to result in increased rates of densification and therefore higher efficiency of the process in increase of part density and strength. To evaluate this solution, this paper deals with the effects of high power longitudinal ultrasonic vibrations on the densification of AA1100 aluminum and Ti-6Al-4V titanium alloy powder under constant applied stress and different temperatures. The effects of powder type and process temperature on the densification behavior and ultrasonic efficiency are discussed. For experimental tests, setup of ultrasonic assisted hot pressing of powders were designed and fabricated. The results show that applying ultrasonic vibrations leads to obtaining higher relative density. In addition, it is found that the effect of ultrasonic vibrations is greater for Ti-6Al-4V powders. However, the temperature has different effects on the ultrasonic vibrations efficiency in two types of powders.

Metal forming is a conventional manufacturing process that a material with simple form is subjected to plastic deformation and emerged to industrial end products. Reduction of forming forces and improving of products quality have been a promising object for investigators and artisans. To accede this purpose, primary methods such as increasing material temperature and modern methods such as use of high power low amplitude ultrasonic vibrations were introduced. In ultrasonic assisted forming, high power ultrasonic transducer produces low amplitude high frequency mechanical vibrations which transmitted to material subjected to deformation and contacting surfaces of tool/workpiece. Results show reduction of forming forces and tool wear as well as improving surface integrity and dimensional stability that lead to increasing production rate and process efficiency. By regarding to importance and capability of ultrasonic assisted metal forming, this paper is concerned with application of ultrasonic vibration on metal forming processes. To this purpose, fundamental and mechanisms of application of high power ultrasonic were introduces and discussed. Also, industrial future of this technology as well as its advantages, range of application and its restriction were mentioned.

The increased material removal rate in ultrasonic-assisted wire electrical discharge turning (by∼2 times)was attributed to compressive and rarefying wave front in dielectric and discharge column sliding into plasma channel and molten crater due to applying lateral ultrasonic vibration on the wire. A single discharge was analytically and experimentally studied and the effective forces and factors on plasma channel including Lorentz force¡ increase of pressure and surrounding vapor ionization of plasma channel¡ elongation of molten crater due to lateral displacement of wire and creation of positive and negative pressures of the ultrasonic wave on molten crater were investigated. Experiments were performed in different machining parameters of voltage and power with and without ultrasonic vibrations. By using ultrasonic vibration¡ the plasma density was increased due to decrease of bubble growth. The current density was increased and pressure drop in the end of discharge was increased. As a result¡ a great amount of the material was exploded out of the crater due to bulk boiling. The plasma channel sliding was elongated the molten crater on workpiece surface¡ which leads to material removal increase.

Wire electrical discharge machining (wire-EDM) is very important in the manufacturing, due to its growing industrial and research applications. Inaccurate geometry in corner and especially sharp-curved corner cutting is one of the major problems in this process. Creation of exact geometry and high dimensional accuracy has been subject of many research works. In the present research, actual cutting slots on straight and curved paths are studied in roughing operation. Experiments are designed using the Full Factorial Method by considering frequency of discharges, wire tension and radius of curvature as variables. By employing statistical techniques in analysis, influences of these parameters on width of the cut of slot, on both straight and curved paths, are evaluated. Practical results for straight side gap and curved corner average gap are achieved, for each set of parameters. Because of concavity of wall on inner arc and convexity of wall on outer arc, measuring errors appear in the measurement of slot width at the curved corner. In addition, depletion of material and accumulation of material are observed at entrance and exit of the curved path, respectively. By calculating variable side gap on a curved corner, practical wire diversion and machining error are obtained.

Reduction of cutting force in a machining process offers several advantages including increase in tool life, and improvement in the quality of the machined surface. One the new techniques for reducing cutting force relates to ultrasonic vibration assisted machining. In the present paper, one-dimensional ultrasonic vibration-assisted side milling process of Al7022 aluminum alloy has been studied. In order to investigate the effect of cutting speed, feed rate, radial depth of cut, and vibration amplitude on three cutting force components and their resultant, a special experimental setup has been designed and established which applies one dimensional ultrasonic vibration to work piece. Applying the ultrasonic vibrations on milling process, affects mostly on feed component of cutting force which is unidirectional with the work piece vibration, and decreases it by 33.5% in average. Decrease in cutting speed and increase in vibration amplitude, results to increase the separation of tool and work piece from each other in a portion of each vibration cycle, and larger decrease of the feed force. The average decrease of the resultant cutting force in ultrasonic-assisted milling process is 10.8%.

Incremental Sheet Metal Forming (ISMF) is based on localized plastic deformation. In this process, a hemispherical-head tool, controlled by a CNC milling machine, shapes a sheet metal according to a defined path. Study of the forming force is one of the most important topics in this process. Increasing of vertical step size, tool diameter, wall angle and sheet thickness together with using of high strength sheet metals and lightweight alloys, leads to an increase in the forming force. In this paper, the performance of a novel forming process, named Ultrasonic Vibration assisted Incremental Sheet Metal Forming (UVaISMF) has been investigated. The procedure of design, manufacture and test of vibratory forming tool, is presented. The occurrence of longitudinal mode and resonance phenomenon has been confirmed by the results of modal analysis and experimental test. Furthermore, the effect of ultrasonic vibration on the vertical component of forming force and spring-back has been studied. Aluminium sheet of grade Al 1050-O is used as a work material. Experimental results obtained from straight groove test, indicate that ultrasonic excitation of forming tool, will reduce the average of vertical component of forming force and spring-back in comparison to conventional process.

Calculation the cutting force in machining processes is of great importance. In this paper، undeformed chip thickness in one-dimensional ultrasonic vibration assisted milling is calculated and then، a model for determining the cutting force in this process is presented. Analytical relations show that in ultrasonic assisted milling (UAM)، the maximum cutting force is greater than in conventional milling (CM)، but the average cutting force is decreased. To verify the proposed relations، with the aid of a particular experimental setup، one-dimensional vibration in feed direction is applied to workpiece and cutting force in CM and UAM is measured experimentally. Greater maximum cutting force in UAM and decrease of average cutting force in UAM compared to CM is observed experimentally as well. Comparison of average values of cutting force shows that the analytical relations for predicting the cutting force have 16% average error in CM and 40% average error in UAM. Given that the analytical calculation of undeformed chip thickness and cutting force in UAM and also comparison of experimental forces with the modeled ones has been done in this paper for the first time، the accuracy of proposed relations are acceptable.

Digital image processing is one of efficient methods that is adopted today for the determination of fuel spray characteristics in internal combustion engines. The imaging method and the images quality are two important factors that influence the accuracy of results. In this paper, an edge finding method was employed for processing of the Schlieren image of the gaseous jet, and sensitivity analysis has been conducted to investigate effects of the edge finding method threshold, the experiments scale factor and the level of the blur and the noise of images on image processing results. A method based on the Otsu method has been proposed also for finding the dynamic threshold for Schlieren jet images. Results showed that the sensitivity of the jet tip velocity to the edge finding method threshold was low (in the valid range of the threshold), but in regions that the jet tip velocity was low, this sensitivity was as high as 14%. The sensitivity of the jet tip penetration to the blur level of images was less than 2%, while the tip velocity was more sensitive to the blur content of images, and the blur with the standard deviation of 4 could led to 28% error in the tip velocity.

Wire electrical discharge machining (wire-EDM) is able to generate delicate and complex shapes on hard materials which are difficult to cut. Inaccuracy in the cutting of small radius curved corners is one of the major problems in this process. In the present paper, corner radii machining errors have been investigated. Experiments are designed using the Full Factorial Method for roughing operations by considering frequency of discharges, wire tension and radius of curvature as variables, and the residual material on straight and curved paths as the output parameters. Practical works are implemented on tool steel 1.2510. By employing statistical techniques for analysis and by calculating variable side gap on curved corners, the machining error is obtained for 150, 300 and 450 µm radius curvatures. Results show that the thickness of residual material left on the workpiece on straight and curved paths can be the same if the cutting path on curvature is corrected according to the findings of this paper.

Wire electrical discharge machining (wire-EDM) has a significant position among production technologies mainly due to its capacity of machining hard materials and intricate shapes. One of the major problems with this process is the error in cutting corners. Processing forces acting on the wire and low rigidity of the wire are responsible for wire deformation، which has a direct influence on the accuracy of the corner cutting. In this research، investigation is focused on the convex corner radii errors and alternative solutions are proposed for the case of successive cuts (one roughing and two finishings). Experiments are carried out for roughing operation by considering frequency of discharges and feed speed. The residual materials on straight and curved paths are the output parameters. Results indicate that optimization of these parameters have a better influence for control of residual material thickness on straight paths than on curved corners. One important conclusion is that roughing is the most influential stage of cutting by WEDM. Then، concave corner radii produced during successive cuts، the effect of corner angle and corner radii are investigated. Errors at radii of different corner angles are identified and related to arc length and residual material thickness in the curved corner. Finally، an effective approach is presented for improving the accuracy of the small-radius concave corner radii of finishing stage. The main conclusion is that to achieve accurate corner radii، one must increase the traversed corner arc length by wire in the small-radius concave corner radii.

Vibration cutting (VC), elliptical vibration cutting (EVC) and ordinary cutting (OC) of In738 work-pieces have been experimentally investigated in the present research. The experiments were carried out by using single crystal diamond tool and ultra-precision CNC lathe. Vibration frequency is 36.16KHz and the amplitude is. The influence of various cutting parameters including cutting speed, feed-rate, vibration amplitude and phase angle on the cutting force, surface roughness and tool life have been studied and the results have been compared. The results indicate that the cutting force in EVC is much less than the two other processes. The surface roughness in both the cutting and feed directions was also less than those in other processes.

Lack of machining accuracy، at corners and corner radii، is one of the major problems in Wire-EDM process. The focus of investigation in this work was on corner radii errors through successive cuts of roughing and finishing operations. Effects of discharge frequency، inter-electrode average gap voltage، and feed-speed were investigated for finishing operations. Residual material left on straight path and over-cut on convex curved path were measured. Results indicated that the influence of the input variable parameters on the thickness of the residual material on straight paths is remarkable in comparison to curved corners. Ultimately، for corner radii، an optimum set of parameters could reduce the errors only by 12%. It was، then، concluded that the roughing operation is the most influential stage of cutting on corner radii

As for spray-guided direct injection gaseous fueled engines, a few time is available for mixture preparation, exact determination of the gaseous jet characteristics emanating from the injector is very important. The main purpose of the present work is investigation of the effects of pressure and density ratios on the multi hole gaseous jet tip axial and radial penetration lengths and its velocity. For this, ultra high speed Schlieren imaging of the transient jet with high spatial and temporal resolution was utilized. jet images were analyzed using digital image processing method and the jet characteristics were extracted using an in house software which worked based on edge detection approach. Results revealed that regarding to the velocity curves, formation of the multi-hole gaseous jet is comprised of three stages of: emergence of the individual plumes, merging and penetration of the main spray. It was also found that changing the chamber gas density wouldn’t alter the maximum and minimum times in velocity curves and decreasing the density ratio of the injection gas to chamber gas would decrease axial penetration and jet tip velocity and increase jet spreading angle.

In this study, the effect of acoustic streaming on heat transfer enhancement of a down-ward-facing horizontal heating surface in a closed cylindrical enclosure filled with water was investigated experimentally. Standing waves were generated between the heating source as a reflector and vibrating lower plate. Acoustic streaming is a steady circular flow induced by this standing waves field. The upper plate was heated with a constant heat flux and side-walls were kept at the constant temperature. Therefore, the gravitational effects were negligible and the heat transfer enhancement was due to ultrasonic vibrations. In order to find out the best range of ultrasonic power, the acoustic pressure was measured. The results show that the enhancement of the heat transfer can be up to 400% by the ultrasonic vibrations. The increase in the transducer power and the decrease in the height of the heater cause the higher heat transfer coefficient in the enclosure. In addition, the increase in cavitation phenomenon severely weakens the increase in heat transfer coefficient.