فصلنامه مکانیک هوافضا
سال یازدهم شماره 2 (پیاپی 40، تابستان 1394)

Surface compressive residual stresses have significant effects on the fatigue behavior of components. In this paper a procedure for process parameters selection for obtaining of certain residual stresses distribution on cylindrical aluminum were proposed. The surface cold rolling process was simulated by to study effects of cold rolling process parameters on surface compressive residual stresses. By developing a parametric FEM method the effects of process parameters like roller and specimen diameter and interface magnitude between them were considered. By using a neural network for achieving every distribution and magnitude of residual stress on the surface, process parameter will be selected. Some experimental residual stresses by HDSG method were done and results of FEM analysis and neural network prediction were compared to experimental data and good agreement were found.

Due to the increasing use of multilayer sheets in various industries, the study of widely used blanking operation on these sheets is necessary. In this paper a study on the process of fine blanking of a multilayer sheet of Al / Cu / Al is done. In order to get accurate numerical results, finite element simulations of multilayer plate are conducted by using Johnson–Cook model and ALE formulation in ABAQUS/ Explicit commercial finite element code. The results are in good agreement with experimental results. The results show that with increasing counter punch force the surface quality is improved and the shear force is increased. Microscopic examination of the lateral edges show that below the aluminum layer edge, relatively large amount of plastic deformation is observed.

In this study a new group of metal composite foam was demonstrated as absorption material. Closed cell metal foam was produced by casting around ceramic hollow spheres and mechanical properties were verified by uni-axial compressive test.The effect of random and ordered arrangement, density variation and sample size on mechanical properties were studied andindicated that arrangement and density significantly affect on mechanical properties and by increase density, plateau stress increased and sample size affects on plateau stress. Results of yield stresses have relatively good match by experimental formula.

The cooling slope (CS) method is one of the forming processes in semi-solid state that the molten alloy with a suitable pouring temperature is poured on a cooling slope for achieving fine and non-dendritic structure. In this method, the parameters such as cooling slope length and angle, pouring temperature and rate, mold temperature were affected on microstructure and morphology of primary phases. In this paper, at first discusses the effect of these parameters on the final microstructure of A356 aluminum alloy and the effective equations in cooling slope process and their influence on the grain size diameter and hardness were examined. It was found that both cooling slope length and pouring rate on grain size diameter and hardness are more effective than other factors. The advantages of using the protective atmosphere with tests such as XRD and ultrasonic test are studied. The results shown that with using the protective atmosphere in cooling slope method don’t have a significant effect on the grain size diameter. However, the strength and ductility of the alloy is increased 17.8 and 28.1% respectively, due to the reduction in the porosities and inclusions.

Due to the properties such as high strength at elevated temperatures and good corrosion and fatigue resistance, superalloys are widely used in industrial applications where such properties are required. These applications include aerospace, steam turbine power plants and petrochemical and nuclear industries. On the other hand superalloys are classified as difficult-tomachine materials owing to their retention of high strength at elevated temperatures, rapid work hardening during machining, presence of hard particles, low thermal conductivity and tendency to make built-up edge on the cutting tool. There is hardly any published paper on machining of iron-nickel-base superalloys despite their industrial importance. In this work, the effects of cutting speed, feed rate and depth of cut on surface roughness in turning of N-155 iron-nickel-base superalloy, were evaluated. Experiments were conducted according to L9 Taguchi standard orthogonal array by using coated carbide cutting tools. By the help of signal-to-noise (S/N) quality characteristic and statistical analysis of variance (ANOVA), optimal cutting parameters and statistical significant level of each parameter were determined. Results showed that feed rate and cutting speed were the most effective parameters on Ra and Rmax, respectively. Additionally, optimum cutting conditions for minimum surface roughness were predicted. Finally, the obtained optimum cutting parameters were verified by a confirmation test.

ECAE is defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal. In this research, the plastic deformation behavior andthe grain size of polycrystalline aluminum, in ECAE process, is predicted by using the Bezier method and the upper bound theory. Also, the effects of inner corner radius, the die angle and the angle of outer curved corner on strain, strain rate and grain size is studied and then is simulated. It is found that deformation rate decreases with increasing the die angle, the outer curved corner angle and the inner corner radius A good agreement is found between the simulation results with various experimental data.

Multi-Point forming technology has gained a great attention in recent years, in order to reduce die design time andmanufacturing costs of sheet metal parts. In this process an elastic cushion is used in order to distribute the loads applied onthe sheet by the punch elements. Mechanical properties and thickness of the elastic cushion can affect sheet metalformability during the forming process. In this study, the effects of these parameters on spring-back and dimensionalaccuracy of formed parts have been investigated. The forming process has been examined by using different elasticcushions with different thickness. The obtained results indicated that mechanical properties and thickness of elastic cushionhave considerable effect on the spring-back and in order to produce parts with good quality and high dimensional accuracythe appropriate selection of the elastic cushion is very important.

In this paper chemical machining of an aerofoil of a forged Ti-6Al-4V titanium alloy turbine blade has been investigated. First, the blade has been hot forged and then the metallurgical structure has been investigated. Using the microstructure and hardness measuring, surface alpha case has been investigated. Next, the chemical machining has been studied for removing of the alpha case. Hydrofluoric-nitric acid solution has been used for the chemical machining. Results show that the alpha case thickness on the forged aerofoil is considerable and the used chemical solution can remove it perfectly. Machining from the aerofoil surface is uniform and material removal rate increases as the temperature increases. During the chemical machining, first because of exothermic reaction, material removal rate increases, then it decreases because of decreasing the density of the acids.