Papers by Author: Livan Fratini

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Authors: Gianluca Buffa, Livan Fratini, Fabrizio Micari
Abstract: Friction Stir Welding (FSW) is an energy efficient and environmentally "friendly" welding process. The parts are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. This technology has been successfully used to join materials that are difficult-to-weld or ‘unweldable’ by fusion welding methods. In the paper a neural network was set up and trained in order to predict the final grain size in the transverse section of a FSW butt joint of aluminum alloys. What is more, due to the relationship between the extension of the “material zones” and the joint resistance, the AI tool was able to furnish indications for the design of the welding process. Experimental tests and subsequent microstructure observations were developed in order to verify the numerical predictions.
Authors: Elisabetta Ceretti, Luigino Filice, Livan Fratini, Francesco Gagliardi, Claudio Giardini, Dario La Spisa
Abstract: Porthole die extrusion is an always more important process for industrial applications. It is, however, characterized by a considerable complexity; in fact, different parameters have to be carefully set for improving the final part. A critical zone that strongly influences the goodness of the extruded component is the so called “welding plane”. It is the junction area where material flows converge inside the welding chamber. The variables that have to be controlled for improving the material characteristics in this zone are the effective stress, the pressure and the time that the material takes to cross the welding chamber. Moreover, material temperature is another fundamental issue that influences both the quality and the typology of the final joint. However, especially for complex parts, the material can follow diverse directions to get out from the die; this means that the deformation history can be different, thus influencing the quality of the final jont. In the study here proposed, the property of the welding plane was highlighted for an industrial component that was cut through a profile cross section carrying out both metallurgical and mechanical investigations. More in detail, specimens, derived from the extruded part, were mounted, polished and etched with Keller reagent and observed by a light microscopy. Then, macro and micro observation were developed highlighting the welding line position. Moreover, local values of the average grain size of the material were measured showing the microstructural evolutions undergone by the material due to the extrusion process. As far as mechanical tests are regarded, micro-hardness tests were executed nearby the welding line; in this way, correlations between material metallurgical evolutions and subsequent local mechanical performances were highlighted. Furthermore, a 3D numerical study was developed in order to point out the numerical ability to predict the welding line position for complex parts; finally, a welding criterion was used in order to locally validate the experimental observations. All these aspects are accurately analyzed and discussed in the paper.
Authors: Gianluca Buffa, Livan Fratini
Abstract: During the last years welded titanium components have been extensively applied in aeronautical and aerospace industries because of their high specific strength and corrosion resistance properties. Friction Stir Welding (FSW) is a solid state welding process, currently industrially utilized for difficult to be welded or “unweldable” aluminum and magnesium alloys, able to overcome the drawbacks of traditional fusion welding techniques. When titanium alloys are concerned, additional problems arise as the need for very high strength and high temperature resistant tools, gas shield protection and high stiffness machines. Additionally, the process is characterized by an elevated sensitivity to temperature variations, which, in turn, depends on the main operative parameters. Numerical simulation represents the optimal solution in order to perform an effective process optimization with affordable costs. In this paper, a fully 3D FEM model for the FSW process is proposed, that is thermo-mechanically coupled and with rigid-viscoplastic material behavior. Experimental clamping parts are modeled and the thermal loads are calculated at the varying of the cooling strategy. Finally, the effectiveness of the cooling systems is evaluated through experimental tests.
Authors: A. Barcellona, Livan Fratini, D. Palmeri
Abstract: The success of the thixoforming process depends on the possibility to confer to material, when it is found in the semisolid state, a microstructure characterized by globular particles of solid phase surrounded by a continuous film of liquid phase; such microstructure is obtainable through particular thermo-mechanical treatments. In the present research, in order to optimize the influence of process parameters in the step in which the thixotropic properties are conferred to the AA7075 aluminum alloy, the statistic technique of the Design Of Experiments (DOE) has been used. The advantages in the application of such technique are expressible in terms of reduction the times of development of process and more efficient use of resources. A Central Composite Design factorial plan with two levels has been realized to allow the evaluation of experimental error and to check the adequacy of the model. The experimental tests foreseen from the same plan have been therefore performed. Using the method of the response surfaces (RSM), the function of response has been formulated, and the analysis of the experimental data has been realized by the linear regression method. Finally, the analysis of variance (ANOVA) allowed to value the causes of variability of the results. Statistic test has been performed on the significance of single factors and their interactions.
Authors: Livan Fratini, Fabrizio Micari, Antonio Squillace, G. Giorleo
Abstract: Welding is playing a growing role in transport industry due to relevant advantages it allows. Friction Stir Welding is considered one of the most promising joining technologies, especially when it is applied to light alloys. Focusing attention on FSW of T-joints, several parameters have to be considered, and due to thermo-mechanical features of process, T joints need a dedicated approach. A set of previously developed experiments has shown that the tilt angle plays a relevant role in the joint strength. Furthermore it should be observed that T-joints are very often utilized in aerospace industries since the produced structures are composed of joined skins and stingers. Numerous data are reported in literature about FSW of butt joints, very few data, to authors’ knowledge, exists on T joints. In this paper a micro structural and mechanical analysis has been developed on FSW T-joints of AA 6082 T6 rolled plates, realized setting welding direction both parallel and perpendicular to rolling direction. The obtained results can be considered as a further acquired knowledge in the comprehension and the design of FSW processes.
Authors: Gianluca Buffa, Livan Fratini, Fabrizio Micari, Giuseppe Previte
Abstract: Solid state bonding recurs in several manufacturing processes, as extrusion of hollow profiles and solid welding processes. Among the latter, Friction Stir Welding (FSW) is nowadays of particular industrial interest because of the specific advantages with respect to the classic welding technologies. Proper conditions of pressure, temperature, strain and strain rate are needed in order to get the final effective bonding. In the paper the authors compare different solid state bonding conditions obtained at the varying of the main process parameters in FSW of butt joints of AA5754 aluminum alloys. The experimental results are compared with the numerical ones from a FEM model previously developed by the authors, in order to investigate an effective bonding criterion to be implemented into the FSW numerical model.
Authors: Archimede Forcellese, Livan Fratini, Filippo Gabrielli, M. Simoncini
Abstract: The formability of friction stir welded AZ31 magnesium alloy sheets was investigated by means of uniaxial tensile and hemispherical punch tests performed under different process conditions. The results of the tensile tests were analysed in terms of flow stress and ductility at different temperature and strain rate; the hemispherical punch tests, carried out at different temperatures, provided the limiting dome height. The formability of FSW-ed blanks was compared to the one exhibited by the base material in order to evaluate the quality of the welded joints.
Authors: Carlo Bruni, Gianluca Buffa, Livan Fratini, M. Simoncini
Abstract: Experimental and numerical investigations have been performed in order to study the effect of welding parameters on properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength and numerical field variables distributions, allow to understand the behaviour of such material when FSW-ed using different rotational and welding speeds for a given tool geometry.
Authors: A. Governale, A. Lo Franco, A. Panzeca, Livan Fratini, Fabrizio Micari
Abstract: In the last decades the scenario of the industrial production is remarkably changed, since new market requirements have to be faced by the industries. The market, actually, more and more, asks for vary models and niches product. The necessity to intercept dynamically and to satisfy the demands for the market, driver of the innovation process, involves the necessity to reduce the Timeto- market introducing to new methodologies of engineering, like the 3D-prototyping, for the qualitative and structural analysis of the final component. For these reasons, at the beginning of the nineties, a new philosophy of sheet metal forming process begins to assert on the industrial scene, whose basic logic is to obtain the shape wished through the progressive action of a tool of simple shape. In this job the application of the simplest process of incremental process on an industrial detail - famous in international field like SPIF (Single Point Incremental Forming) - will be described. The process is intrinsically flexible, and therefore is adapted to the rapid prototyping. The cases are still least, notice in the scientific literature, in which the details of industrial interest have been developed by Incremental Forming process; for this reason, the subject of this job is focused on the evaluation of the possibility to obtain real components of the automotive industry through the SPIF process. The job has been carried out in the R&D laboratory of "Fontana Pietro S.p.A.”, leader in the field of die manufacturing and stamping of component of the automotive industry. In particular, two parts of automotive auto body of aluminium sheets have been considered. It has been lead an analysis of technological and process feasibility, optimizing tool path considering experiences to obtain a product/process for the production of auto body prototypes.
Authors: Gianluca Buffa, Livan Fratini, Marion Merklein, Detlev Staud
Abstract: Tight competition characterizing automotive industries in the last decades has determined a strong research effort aimed to improve utilized processes and materials in sheet stamping. As far as the latter are regarded light weight alloys, high strength steels and tailored blanks have been increasingly utilized with the aim to reduce parts weight and fuel consumptions. In the paper the mechanical properties and formability of tailored welded blanks made of a precipitation hardenable aluminum alloy but with different sheet thicknesses, have been investigated: both laser welding and friction stir welding have been developed to obtain the tailored blanks. For both welding operations a wide range of the thickness ratios has been considered. The formability of the obtained blanks has been characterized through tensile tests and cup deep drawing tests, in order to show the formability in dependency of the stress condition; what is more mechanical and metallurgical investigations have been made on the welded joints.
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