فصلنامه مکانیک هوافضا
سال هشتم شماره 4 (پیاپی 30، زمستان 1391)

In this paper electro discharge machining (EDM) by finite element method (FEM) has been modeled. First temperature distribution in work piece, resulted of EDM has been calculated. In second section, thermo- mechanical analysis has been used for estimation of thermal and residual stress. Afterwards, for validating results of simulation, comparison with experimental results of the other researchers has been done. Then it has been shown that, there is good agreement between experimental and simulation results. Also it has been shown that, the depth where the maximum stress value is observed increases with discharge energy. So the probability of deep cracks increases.

A new velocity field with variable axial velocity has been proposed in this paper. By using this new formulation the velocity field and upper bound on extrusion pressure have been computed. Incorporating the B Spline curve into the upper bound formulation, 3D surfaces of velocity discontinuity at entry and exit have been assumed. Theoretical prediction of grid deformation pattern was also computed using the present theory. Experiments were carried out and extrusion pressure as well as grid deformation were obtained. The present theoretical results were in good agreement with experimental ones and compared to previous work improvements were observed.

Hot forging of the aerofoil blades has complicated mechanical and metallurgical characteristics therefore; it needs numerical and experimental investigations. In this research, hot forging process of a titanium alloy aerofoil blade has been analyzed by finite element method. Then experimental tests have executed. For the numerical simulation, first material behavior of the titanium alloy has been investigated then friction factor has been obtained by ring test. Using the process simulation, material flow, die cavity filling, strain and temperature distribution, forging load and lateral forces applied the dies during the forging process have been investigated. In the next stage, trial tests have been executed. At last, numerical results have been compared with the experimental results. Results show that the forging process has been done successfully and simulation results have a good agreement with the experimental results.

The sheet bending on elastic foundation is a relatively new method in recent years. In this work, the plastic deformation of sheet is studied. The governing differential equations for the sheet deformation zones were obtained. Then, the equations were analytically-numerically solved for a given problem and the effect of indentation depth on the sheet bending radius was studied. A relationship was obtained to correlate the indentation depth to the sheet bending radius. This relationship can be used in controlling the process and also in process planning. The results were verified by a set of experiments.

Metal matrix composites (MMCs) have received considerable attention due to their excellent engineering properties, however, poor machinability has been the main deterrent to their substitution for metal parts. The hardness and abrasive nature of reinforcement phase causes rapid tool wear during machining and consequently, high machining costs. This paper discusses the experimental investigation on the machinability of Al-15%SiC particulate metal matrix composite produced by powder metallurgical process. Continuous dry turning of round composite bars using titanium carbide inserts has been selected as the test method. The influence of machining parameters, e.g. cutting speed, feed rate and depth of cut on the tool wear, cutting forces and finished surface criteria were investigated during the experiments and compared with similar works. The results show that the cutting speed and depth of cut are more dominant factors on machinability than feed rate. Tool wear, surface roughness & machining force, in machining of MMCs produced by powder extrusion are higher than those in machining of similar \ MMCs produced by casting process. The optimum condition for the tool wear, cutting force and surface finish criteria can be achieved by low values of feed rate and depth of cut and moderate values of cutting speed.

The main objective of this study is experimental investigation on the effect of welding parameters on friction stir butt welds of copper plates. The defect-free welds were obtained on 4-mm thick copper plates at a wide range of rotational and traverse speeds using a special tool that has non-threaded pin. The tensile tests showed the welded copper joints have high efficiency and some welds gained 90% weld efficiency as compared to the parent copper.The hardness tests and micro-structural analysis showed that heat-affected zone (HAZ) is formed around the stir zone. This zone has lowest hardness and all of the transvers tensile test specimens were fractured in this zone. Furtheremore a novel surface hardness mapping technique was applied in the present work. This experiment showed that different points on the stir zone have very different hardness values, and weld strength is proportional to the lowest hardness in weld zone.

Multi-Point forming process is a flexible technique for manufacturing of various sheet metal parts. In this paper, Multi-Point forming technique is studied using FE simulation and experiments. In order to estimate proper design parameters, the forming process was initially simulated dynamically, and then the Multi-Point forming die set was built for conducting experimental tests. By considering the widespread usage of aluminum alloys in the field of aerospace, the AA2024 aluminum alloy is used in this research and forming of sheet parts under various forming conditions is examined. The failure modes are predicted and the effect of process parameters such as pin diameter and thickness of elastic cushion on the geometry accuracy and thickness distribution of formed parts are investigated.The results obtained show that this process can be used as an effective and economic method for producing small batch production in different industrial sectors particularly aerospace industry. By proper adjustment of process parameters, the production of parts with different geometry will be possible just by using one same die set.

In recent years, several research works have been performed on the application of reconfigurable discrete dies for forming sheet metals. Due to the rapid changing of these dies, they attracted the attention of flexible manufacturing systems. Meanwhile, only flat sheet metals were examined and no research work has been reported on the application of the mentioned dies for tubular parts. In this paper, a new reconfigurable discrete die for spinning of tubular parts were designed and manufactured. The results obtained in this research illustrated that hollow axisymmetric parts could be produced using this type of die, and that this technique leads to the production of tubular parts with desired accuracy and quality. The strain distribution diagrams of formed samples showed that the length of samples decreases and the wall-thickness of them increases after forming. Increasing of wall thickness in the middle of tube is more than sides and decreasing of length in the sides is more than the middle. The twisting deformation of tube increases with the increment of intended amount. The surface quality of tube increases after forming.