Hydro-mechanical Deep Drawing of Light-Alloy Sheets at Elevated Temperatures

نوع: Type: thesis

مقطع: Segment: PH.D

عنوان: Title: Hydro-mechanical Deep Drawing of Light-Alloy Sheets at Elevated Temperatures

ارائه دهنده: Provider: Saeed Yaghoubi

اساتید راهنما: Supervisors: Faramarz Fereshteh-Saniee (Ph.D)

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees:

زمان و تاریخ ارائه: Time and date of presentation: Winter, 2020

مکان ارائه: Place of presentation:

چکیده: Abstract: In the present research work, a sheet metal forming process, namely HMDD method, has been investigated experimentally and numerically. Some light alloys possess low ductility at the room temperature. That is why the forming process of these alloys is usually performed at elevated temperatures. With this regard, the HMDD process of 2024 Al and AZ31B magnesium alloys was at first performed. In the second stage of this study, HMDD operation was applied to produce bimetallic products consisting of St13-1200 Al alloys; and separately AZ31B magnesium-1200 Al alloys. Among different factors affecting this forming process, in the current research work, the influences of process temperature, fluid pressure, pre-bulge pressure, initial sheet diameter, drawing depth, frictional conditions of the punch and the matrix, the gap between the matrix and the blank holder, punch geometry, die entrance radius, and the clearance between the punch and matrix were investigated. The effects of these variables were studied on several important parameters such as microstructural evolution, hardness distribution in the produced cup, texture of the component, plastic strain distribution and contact pressure on the workpiece. In order to extract the relationship between the parameters obtained from the numerical results, artificial neural network training has been applied and, then, the optimization of the outcomes gained from the neural network was performed via two individual methods, namely genetic and bees algorithms. The maximum thickness reduction is one of the most important indices for evaluation of quality of the product obtained from a sheet metal forming process. In the present study, in addition to this parameter, another criterion called thickness variation has also been introduced and employed to investigate the uniformity of the final product. The effects of the above-mentioned variables on these two quality parameters have extensively been studied in the present research. The numerical results showed that increasing the coefficient of friction at the punch-sheet interface, reducing the coefficient of friction at the matrix-sheet interface, as well as reducing the distance between the matrix and the blank holder have caused less thinning and more uniformity for the final product. The geometrical parameters of the die set are among the most important factors for producing sound cups. With this regard, the optimum values of the punch tip radius, the die entrance radius and the clearance between the punch and the matrix for 2024 aluminum alloy were calculated using GMDH neural network training and the bees optimization algorithm. The values of improvement in the objective function, a combination of the maximum thickness reduction and the thickness variation of the workpiece, for the optimal values of the geometrical parameters, compared with the best and worst cases of 27 FE simulations were found to be about 6% and 24%, respectively. These values indicate the remarkable effect of optimization of the die set geometry on the quality of the produced cups. The maximum fluid pressure inside the die cavity during an HMDD process acts as a uniform support for the sheet and improves the uniformity of the product. Since in the corner region of the product, the material is simultaneously bent and stretched around the punch edge and relatively large strains occur in this area, excessive increase in the fluid pressure would just increase the forming force. After examining different values of maximum fluid pressure, the suitable range of changes in this variable was considered between 5 MPa and 15 MPa. The greatest influence of the pre-bulge pressure was observed at the cup wall area, where there was no support from the punch and the matrix. Applying this initial pressure could prevent the product from tearing. The best pre-bulge pressure for the forming process in the present research work was obtained to be 1 Mpa for 2024 Al and AZ31B Mg alloys. As the temperature of the forming process increases, the materials become softer and the formability of the workpiece improves. On the other hand, excessive increase in the process temperature has caused greater increases in the maximum thickness reduction of the produced cup and even could lead to its rupture. Suitable temperatures for the HMDD process of 2024 Al alloy and AZ31B Mg alloy were respectively gained to be 250 °C and 200 °C. The region of the sheet located between the container wall and the punch during the forming operation was the most plastically stretched area. Therefore, it was reasonable that the grain size in this area was smaller than the bottom of the cup. Consequently, the value of hardness in the cup wall was measured to be higher than the other regions. The results gained from the crystallographic texture analyses of 2024 Al alloy were in contrast with the Hall-Petch relationship. The images received from the energy dispersive spectroscopy for different areas of the cross section of the 2024 Al sample demonstrated that the Al2Cu precipitations in the product wall were finer than the other areas and mainly dispersed at the grain boundaries. This type of precipitation distribution was a main origin for increasing the hardness of the cup wall in comparison with the cup corner and bottom. However, for the AZ31B magnesium alloy, the experimental outcomes demonstrated an acceptable agreement between the Hall-Petch relationship and texture analyses. Regarding the bimetallic HMDD process, it can be stated that by applying a pre-bulge pressure, the interlayer separation at the area of the blank which was in contact with the punch tip and the flange area was reduced and this initial pressure could improve the maximum thickness strain and thickness variation parameters up to 57% and 68%, respectively

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