Papers by Author: Antonio Fiorentino

Abstract: Incremental Sheet Forming is a flexible process characterized by low costs and higher process times with respect to traditional forming technologies. It is therefore suitable for prototypes, small series or custom mass productions. Its flexibility derives from the use of a hemispherical punch that is moved by a CNC machine and gradually deforms the sheet in presence, or not, of a counter die. As a consequence, the sheet clamping is reduced and the part accuracy is lower than traditional sheet forming process as stamping. Therefore, the improvement of the part accuracy in Incremental Sheet Forming is a relevant research topic and solutions for error reduction are required for improving the process quality.The present paper describes the use of an Iterative Learning Control (ILC) algorithm for compensating the ISF part geometrical error. In particular, it iteratively corrects the part geometry on the basis of the error map obtained as the difference between formed and target part geometries. The ILC uses the target geometry to form a first trial part, it measures the obtained geometry and estimates the geometrical error map. Then the error map is used to modify the target geometry and another part is formed. This procedure gets iterated until the desired geometrical tolerance is achieved.The correction algorithm was experimentally tested in forming both axisymmetric and not axisymmetric parts using aluminum sheets. Results showed that in few iteration steps it was possible to significantly improve the part accuracy and to achieve geometrical tolerances comparable with the traditional sheet forming processes.

Abstract: Friction plays an important role in forming processes, in fact it influences the material flow and therefore it affects the process and part characteristics. In particular, friction is a very influencing factor in Tube Hydroforming (THF), where high die-part contact pressure and area make the material sliding very difficult. As a consequence, the material hardly flows to the expansion zones and the part formability can be compromised. To obtain sound parts, FEM models allow to study the process and optimize its parameters, but they require the right definition of the friction at tube-die interface. For these reasons, friction represents a key-point in THF processes and its knowledge and prediction are very important even if, nowadays, a comprehensive friction test for THF is not available in literature. With this paper, the Authors want to propose a novel approach to estimate friction for THF processes. In particular it will be described a numerical inverse method that allows to estimate the Coulombian friction coefficient combining experimental test and FE simulation results. The method is based on the effects of friction on the tube final thickness distribution when it is pressurized and compressed by two punches under different lubrication conditions without expansion. In particular, it will be shown how the use of few and fast FE simulations allows to estimate an analytical function that takes into account the process conditions and that can be used in combination with experimental results in order to estimate the friction coefficient in THF processes.

Abstract: This paper deals with Incremental Sheet Forming (ISF), a sheet metal forming process, that knew a wide development in the last years. It consists of a simple hemispherical tool that, moving along a defined path by means of either a CNC machine or a robot or a self designed device, locally deforms a metal sheet. A lot of experimental and simulative researches have been conducted in this field with different aims: to study the sheet formability and part feasibility as a function of the process parameters; to define models able to forecast the final sheet thickness as a function of the drawing angle and tool path strategy; to understand how the sheet deforms and how formability limits can be defined. Nowadays, a lot of these topics are still open. In this paper, the results obtained from an experimental campaign performed to study sheet formability and final part feasibility are reported. The ISF tests were conducted deforming FeP04 deep drawing steel sheet 0.8 mm thick and analyzing the influence of the tool path strategy and of the adopted ISF technique (Single Point Incremental Forming Vs. Two Points Incremental Forming). The part feasibility and formability were evaluated considering final sheet thickness, geometrical errors of the final part, maximum wall angle and depth at which the sheet breaks. Moreover, process forces measurements were carried out by means of a specific device developed by the Authors, allowing to obtain important information about the load acting on the deforming device and necessary for deforming sheet.

Abstract: THF process is a forming technique that consists of tube deformation by means of hydraulic pressure and punches which guarantee the tube ends feeding and sealing. In the last years, this technique found a large and rapid diffusion thanks to its many advantages with respect to conventional processes: parts weight reduction, tighter dimensional tolerances, lower costs or fewer secondary assembly operations. Besides, many lacks in the process knowledge still represent an obstacle and they are usually bypassed by trial and error methods. Therefore, lot of studies were conducted to better understand the influence of the process parameters, such as pressure path, punch stroke or material behaviour. In this paper, an experimental study of tubular T-joints made of copper manufactured by means of THF process is described. The aim of this work is to analyze the tube-die interface friction condition effects on the final part. In fact, material flow during the process is greatly influenced by the friction conditions between tube and die especially due to the high pressure acting inside the tube. Different lubrication types were considered: dry, oil, Teflon, Teflon with oil, Teflon spray and Graphitic oil. Two different experimental campaigns were performed in this investigation. The first one was carried out in order to estimate the lubricant friction coefficients using a Pin on Disk tribometer. The second one was performed to study the effects of the lubrication on the process and the tests were conducted hydroforming T-joints under the different lubrication conditions considered. The collected data allowed to identify how the different lubrication conditions affect the final workpiece geometry. Moreover, a critical aspect of the process related to the tube wrinkling was identified and a solution was proposed.

Abstract: In the present paper a feasibility study of a funerary vase, made of stainless steel, using the Hydromechanical Deep Drawing process, is presented. The component is currently made of bronze and manufactured by die casting technology in a low volume production environment. To investigate the part feasibility, several FE simulations were implemented using the Aquadraw tool of the explicit FE code Pam Stamp 2G 2005®. The FE simulations showed that HDD process can produce the part in one single step without the requirement of finishing operations such as painting or polishing. Furthermore experimental tests were conducted and the first prototypes were successfully produced.

Abstract: The present paper is the continuation of a research conducted on hemming operations by using rolling tools. Sheet hemming is a joining operation widely used in automotive industry when it is necessary to join two sheet parts (such as the engine hood or the door panels with their internal frame) by plastic deformation of the edge of the outer part. The whole process is characterised by a 90° sheet flanging, a pre-hemming (up to approximately 135°) and the final hemming where the outer sheet edge is bended up to 180° clamping the inner sheet. Hemming processes are normally performed using rigid dies in series production and manually in pre-series and small batch production, due to the high cost of the dies. Nowadays, rollers moved by robots are becoming an interesting alternative to the manual operations especially when flexible productions are required. Even if the process time is higher, this solution can help in minimizing set-up times and costs. The required equipments are a support and a blocking system for the sheets together with the rollers mounted on a CNC machine or on a robot. The production flexibility is guaranteed by changing the 3D tool path using a CAD/CAM system. Authors are dealing with this technique having conducted many experiments studying the influence of the hemming process parameters such as flange geometry (edge height, fillet radius), distance of the inner panel from the flange, tool path sequence, along straight paths on steel sheets. The goal of the present research is to study the material behaviour and the produced parts quality when working on aluminium sheets. In particular, both experimental tests and simulations will be carried out in order to optimize the process.