دکتر علیرضا فلاحی آرزودار

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.

The present study investigates the influence of three different microstructure features including volume fraction of α phase (A), thickness of α phase (B), and aspect ratio of primary α (C) on tensile properties of Ti-6Al-4V alloy, by response surface methodology with central composite design (CCD). The experimental data required for the design of experiment (DOE) and analysis of variance (ANOVA) is predicted using the artificial neural network (ANN). First using the experimental data of other researchers, the ANN with two hidden layers by the error propagation algorithm was trained. The main objective of this study is to compare the two feedforward and feedback neural networks in as well as examine the influence of microstructure on the mechanical properties of the Ti-6Al-4V alloy. The results showed that the feedback neural network has higher accuracy than the feedforward neural network to predict the values of yield strength and elongation. Besides, according to ANOVA and response surface method, C, B2, AB2, and A2C factors and A, C, B2, BC, and A2B factors have more significant effects on yield strength and elongation in Ti-6Al-4V alloy, respectively.

The use of two-layer sheets to improve mechanical properties such as ductility and strength and to improve chemical properties such as corrosion resistance has led to an increasing number of such materials in the industry. In this study, the formability of aluminum-copper two-layer sheets at a high strain rates is investigated by electromagnetic forming method. The simulation of electromagnetic forming of the two-layer sheet was performed at high strain rate using Maxwell and Abaqus software. By making coil and die and using sheets with different geometries and grids on the sheets, the forming limit diagrams (FLD) was also extracted experimentally. The simulation results showed that the electromagnetic pressure applied on the sheet in CA lay-up was 19% higher than in AC lay-up. Using the second derivative of strain criterion, the FLD of aluminum-copper two-layer sheet was derived. The FLD of aluminum-copper two-layer sheet with an initial thickness of 0.5mm is 30% higher in the AC lay-up than in CA lay-up. The reason for this improvement is that in the AC lay-up the sheet with more ductility (copper) is in the outer layer and has greater resistance to tensile stress and necking. The outer layer with better ductility can improve the ductility of the two-layer sheet. The FLD of aluminum-copper two-layer sheets has improved 120% in right-hand side and 55% in left-hand side at high strain rates compared to static conditions. There is about a 6% differences between the simulation and experimental results for forming limit diagram.

Severe Plastic Deformation (SPD) processes are widely used in order to decrease the grain size and improve mechanical properties of engineering alloys. Equal- Channel Angular Pressing (ECAP) is one of such processes that can be performed on bulk materials. In this paper effect of ECAP and heattreatment coupling on 7075 Aluminum alloy was investigated. ECAP was performed on samples after solution treatment. Considerable grain size decrease was observed. Sample’s hardness was increased after ECAP. In order to investigate the effect of solution treatment time on hardness, samples were solution treated at 500° degrees for 5, 10 and 15 hours. It was shown that increasing the solution treatment time from 5 to 15 hours decreases the hardness in solid solution state from 45 to 39 HRB due to increase in the solution of the initial precipitates. Aging treatment at 80 and 120 degrees was performed to further harden the alloy. It was observed that aging treatment after ECAP increases the hardness of all samples. Increasing the solution treatment time from 5 to 15 hours increases the maximum hardness in aging state at 120 degrees from 92 to 98 HRB.

The present research is aimed to study on effect of friction stir welding process parameters on aluminum alloy 1100-H18 to pure copper joints. The effect of welding parameters¡ including rotational speed¡ feed rate and pin offset on the tensile strength of samples investigated and were optimized by response surface method. In order to verify the results of tensile tests¡ metallographic evaluation¡ microhardness and X-ray diffraction tests were conducted. Microstructural evaluation of the weld samples revealed that the interfacial regions are characterized by mixture layers of aluminum and copper. High Vickers microhardness values were measured at the joint interfaces¡ which corresponded with the intermetallic compounds. The diffractograms of the X-Ray Diffraction analysis showed small peaks for intermetallics in the welds. Welds produced with low heat input did not have intermetallics formed at the joint interface¡ whereas welds produced by highest heat input¡ have the highest content of intermetallics. As well as to determine the maximum temperature in weld nugget¡ FEM analysis have been conducted and formation temperature of intermetallic components was determined. To evaluate the effect of welding parameters¡ including rotational¡ linear speed and tool offset¡ on weld properties¡ optimization using Response Surface method is performed and highest tensile strength is achieved. By increasing the rotational speed and tool offset to aluminum side and reducing the linear speed¡ the generated heat input during welding was increased and this caused to increasing UTS to a maximum and then decreased. By varying the process parameters and increasing the heat input until optimize strength¡ mixing operations are performed as appropriate and increases the tensile strength¡ with a further increasing in heat input¡ grains are being coarse and intermetallic compounds increases and then tensile strength has been decreased.