فصلنامه مکانیک هوافضا
سال یازدهم شماره 1 (پیاپی 39، بهار 1394)

Bursting is the most important defect in tube hydroforming. Prediction of bursting can help to design the process. Aim of this paper is the application of ductile fracture criteria for prediction of bursting, forming limit curve at neck and forming limit curve at fracture in tube hydroforming process. Different ductile fracture criteria have been calibrated and applied in simulation to identify the best criterion for prediction of fracture zone. A free bulge setup has been manufactured to verify the simulation results. This setup has fixed bulging length and the ability of applying axial feed. To bulge the tube under fixed strain ratio, different loading curves (internal pressure versus axial feed) have been created. Using loading curves and prediction of ductile fracture criteria, FLCN and FLCF have been obtained and compared with experimental ones. Predicted FLCN using ductile fracture criteria have been compared with M-K Model too. Results show that the criteria which consider the effect of stress triaxiality on the prediction of fracture are more appropriate for tube hydroforming. Prediction of Ayada' s criterion and Rice' s criterion have the best conformity with the experimental results.

There are many methods to produce ingot with non-dendritic microstructure. Cooling slope method has attracting more attentions because of its simplicity and high process speed. In this method molten metal is poured on tilted slope that is cooled by water circulation underneath. Due to shear stress exerted to the slurry by flowing on the slope, it is solidified with non-dendritic microstructure. By simulation of semi-solid casting with cooling slope, the effects of different pouring conditions on the microstructure of A356 aluminum alloy are investigated. The simulations are carried out by commercial FLOW3D software. The results of simulation tests are compared to experimental results. In the simulations, the time of slurry presence on the slope, solid fraction of the slurry, strain rate and turbulence were investigated. Comparing the results of simulations with experimental results showed that for having the favorable microstructure, with higher sphericity and lower grain size, the time of slurry flowing on the slope must be enough while the shear stress over 60s-1 and turbulence must be as high as possible.

An advantage of tube spinning process is high accuracy of products. Balancing forces among rollers is one of the most effective parameters on product accuracy in this process. Unbalance forces cause mandrel deflection that directly reduces product thickness accuracy. Unbalance forces also reduce surface quality of the product. In this paper, a procedure is proposed to omit unbalance forces between rollers and improve product accuracy. For this purpose, rollers forces are calculated as functions of rollers radial positions. Then coefficients of these functions are calculated from FEM simulations by ABAQUS software. Using these functions the radial positions of rollers are obtained such that the unbalance force among rollers is omitted. As a result, the mandrel deflection due to rollers unbalance force is reduced and the accuracy of product is increased. In verifying the method, mandrel force before and after applying the method has been compared and it is observed that this force is reduced by 97 percent.

In this paper, the effect of the position of the pin relative to tool shoulder has been investigated in the friction stir welding process on aluminum 5083. Firstly, the preparation of the process is performed that includes tool, work piece fastened to dynamometer by special fixture and milling machine. Test is performed by tool with pin coaxial with the axis of tool shoulder in this process. So, the tool enters the edges of joint location and moves along joint line. Results are heat generation, severely plastic deformation and material displacement in joint location and this leads to complete joint of the edges. Force and temperature in joint region are measured during the process. Then, the parameters of the shape and the position of tool pin change and joint test is performed. The shape of the tool changes by offsetting the axis of tool pin relative to the axis of tool shoulder with different designs and the results of welding forces and tool temperature and mechanical properties are considered.

In this paper, the equations of acoustic wave scattering from hollow or fluid-filled cylindrical cavities embedded in an isotropic elastic solid are derived and the normalized backscattered pressure spectrum (form function) is plotted. When the wave strikes the body of the cylindrical cavity, surface waves are formed on the periphery of the target. These surface waves circumnavigate the body of the cavity and at certian frequencies interfere constructively and build up to the circumfrerencial resonance modes of the cylindrical cavity. The resonance modes appear as sudden pressure drops in the form functoin. We propose a new method for identification of resonance modes of the cylindrical cavity. Measurements conducted on a cylidrical cavity machined in an aluminum block indicates that the resonance modes identified in the analytical model completely agree with those observed in experimental measurements

Spingback is one of the important factors in metal forming process. Study on springback shows that this phenomenon can have great impacts on shape and formation of the part. Therefore, to increase the accuracy and the quality of the produced part, the important factors have to be recognized. In this paper, machining forming for thin part which is a combination of machining and incremental forming is first introduced. Then numerical and experimental results are compared. The effects of forming parameters such as tool diameter, part angle, forming steps, and tool position on springback are presented. The results show that the step depth has a great affect on springback.

In this article, design and manufacturing of a dynamic pressure sensor based on piezoelectric structure has been put on the agenda. Essential sensor elements are consists of a piezoelectric element that screen printed as sensor element and The five basic components consists of diaphragm, housing, the electrodes and the connector. laser welding, Wire Cut, electrical discharge machining, mold pressing, turning, milling and. .. are used in Manufacturing processes. The sensitive element, consisting of a circular piezoelectric layer sandwiched in two conductive layers, is screen printed, it has 0.5 mm thickness and 5 mm diameter. The sensitive element has a structure of a plane capacitor with the piezoelectric film as dielectric and the two conductive layers, based on Ag material, as armatures. Device was encapsulated in a stainless steel housing for the dynamic test. The measuring range covers pressure up to 100 MPa while the reaction time is less than 10 µs. This dynamic pressure transducer is ideal for measuring pressure transients or dynamic pressure pulsations in liquids or gases.

In this research, successfully joint of dissimilar 5050 Al alloy to 304stainless steel by friction Stir Welding (FSW) has been studied. In FSW many parameters on the microstructure and mechanical properties is effective. Therefore, to achieve the highest strength possible, the important process parameters such as tool rotational speed, feedrate tool offset and pin profile have been investigated. Considering the results of the mechanical properties, the best condition to obtain the highest strength in weld nugget was 80mm/min feedrate, 500rpm tool rotational speed, with a conical pin profile and 1.8mm tool offset respectivly. In this case the strength of the 94/203MPa is obtained that this value is equal to 97% strength aluminum alloy base metal.