Papers by Author: Claudio Giardini

Abstract: In Friction Stir Welding (FSW) tool geometry plays a critical role in governing heat generation, material flow, and microstructural evolution within the weld. In this study, the feasibility and performance of FSW tools manufactured by Laser Powder Bed Fusion (L-PBF) are experimentally and numerically investigated. A non-conventional FSW tool produced in AISI 316L by L-PBF was designed and compared with a conventional machined steel tool in the welding of AA6082-T6 sheets performed using already optimized process parameters. This was followed by tensile testing and macro-and micro-hardness measurements, and a punctual microstructural analysis. In addition, a 3D thermo-mechanical finite element model was employed to forecast and analyze the temperature distribution, the effective strain and the overall material flow. The results show that the tool manufactured using L-PBF enables FSW joints to achieve mechanical properties and welding efficiency similar to those of the standard tool. Finite Elements Models (FEM), in good agreement with experimental results, show that the geometry of the additive tool promotes greater plastic deformation and lower peak temperatures, confirming both the validity of the model and the suitability of L-PBF for the advanced design of FSW tools.

Abstract: The stretch-reducing mill is a forming process to manufacture tubes by progressive metal deformation. This process is characterized by high complexity and a high number of variables that are strongly interconnected. To overcome the limitations and substantial simplifications of the traditional modelling and offer higher flexibility and suitability for real-time control, an Artificial Neural Networks approach is employed. By defining three parallel networks, they were predicted the milling status, the tube thickness and the angular speeds of the stands composing the process. With the models results, an optimization algorithm is employed to determine the best configuration of angular speeds of the stands to obtain a defined final tube thickness. The Artificial Neural Networks show extremely low RMSE across training, validation, and test sets, confirming their ability to model complex nonlinear dependencies. The optimisation stage reaches the target thickness with only 0.0079% error while preventing unstable operating conditions. The overall methodology provides a tool for implementing the intelligent and data-driven control of the stretch-reducing mill.

Abstract: Ring rolling (RR) is a widely used process for producing seamless rings, but its complex thermo-mechanical behavior often requires costly experiments or FEM simulations. This study presents a novel analytical method for predicting torque and energy in RR that explicitly accounts for the fishtail effect, a lateral deformation of the ring cross-section. The approach combines a slab-based mechanistic model with a regression linking fishtail deformation to the kinematic ratio between mandrel feed and ring rotation. Validation was performed via FEM simulations on an industrial AISI 1045 steel case, covering thirty conditions with varying feed rates and rotational speeds. Results show that conventional models ignoring fishtail can overestimate errors by over 60% for torque and 50% for energy, whereas the proposed method reduces errors below 15% in most cases. These findings highlight the importance of including fishtail effects, offering a fast, reliable, and efficient tool for early-stage RR process design and optimization.

Abstract: Metal Fused Deposition Modelling is a promising multi-step process able to manufacture metal parts by means of a low-cost additive technique. In this study, a metal-polymer composite filament characterized by homogenous mixture of AISI 316L sinterable metal powders and a multi-component polymeric matrix was used to fabricate samples by means of a FDM printer. A 2⁴ full factorial design of experiments was elaborated to define the possible influence of the relevant printing parameters on dimensional shrinkage, bulk density and overall porosity of printed samples. In addition, the mechanical properties of printed AISI 316L samples were investigated by performing tensile tests, compression tests, Charpy impact tests, Rockwell B and Vickers hardness tests. An X-ray diffraction analysis was conducted to assess the crystallographic structure of the FDM AISI 316L samples.

Abstract: In the manufacturing industry, the problem related to the management of metal waste is of considerable importance, since it is produced in large quantities during mechanical processing.However, its recovery is not always a simple task, especially with regard to the metal cutting processes. In fact, due to the presence of surface oxide and contaminating oily residues, the recovery process of these components is often very expensive and polluting. This problem can be solved with the FSE process, patented in 1993 by The Welding Institute. The FSE can be counted among the main innovative processing techniques developed in Industry 4.0, as it involves only metal scraps coming from the machining processes as starting material, without providing for their preliminary re-melting in a billet form, and it uses only the heat generated by the friction between the tool and the metal. Since FSE is a quite recent process, the development of simulative models is useful for understanding its basic mechanisms. The objective of this research is to analyze if and how the bonding phenomena occour considering both the thermal and the stress conditions involved and generated by the process parameters.As a result, FEM analysis proved to be a valid tool to correctly forecast if bonding phenomena really take place and how process parameters affect the bonding quality. Moreover, it was possible to confirm that the Piwnik and Plata bonding model is a good criterion for predicting the effects of this technology.

Abstract: A study was carried out to evaluate how the Friction Stir Spot Welding (FSSW) process parameters affect the temperature distribution in the welding region, the welding forces and the mechanical properties of the joints. An experimental campaign was performed by means of a CNC machine tool and FSSW lap joints on both AA6060 and AA7050 aluminum alloy plates were obtained. Some thermocouples were inserted into the samples to measure the temperatures during FSSW. A set of tests was carried out by varying the process parameters, namely rotational speed, axial feed rate and plunging depth. Axial welding forces were measured during the execution of the experiments by means of a piezoelectric load cell. The mechanical properties of the joints were assessed by executing shear tests on the specimens. A comparison between the quality of the joints obtained on the two materials and a correlation between process parameters and joints properties was found. A FEM model for the simulation of the process was set up using the commercial code Deform 2D. The peculiarity of this model is a 2D approach used for the simulation of a 3D problem, in order to guarantee a very simple and practical model able to achieve results in a very short time. This solution was achieved, based on a specific external routine for the calculation of the developed thermal energy due to the friction between tool and workpiece. The collected experimental data were finally used to validate the model.

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: Ring Rolling is a complex hot forming process where different rolls are involved in the production of seamless rings characterized by extreme dimensions (i.e. external diameter higher more than 1m). Because each roll can be independently controlled from the other ones different speed laws must be set; usually, in the industrial environment, a milling curve is introduced to monitor the shape of the workpiece during the deformation in order to ensure a correct ring production. In former works the authors focused their attention on the influence of different milling curves for an industrial case and the results underlined that a ring produced with a good quality and lower loads and energy could be obtained imposing a linearly descending trend to the Idle roll speed law. However different approaches could be used in order to evaluate the mentioned speed law.In this work the authors enhanced the knowledge about the optimization of the Idle roll speed law: different Idle roll speed laws were designed and simulated and the results were compared in order to identify the best speed law that guarantees a good quality ring with lower loads and energy required for manufacturing.

Abstract: Ring Rolling is a complex hot forming process used for the production of shaped rings, seamless and axis symmetrical workpieces. The main advantage of workpieces produced by ring rolling, compared to other technological processes, is given by the size and orientation of grains, especially on the worked surface which give to the final product excellent mechanical properties. In this process different rolls (Idle, Axial, Guide and Driver) are involved in generating the desired ring shape. Since each roll is characterized by a speed law that can be set independently by the speed law imposed to the other rolls, an optimization is more critical compared with other deformation processes. Usually, in industrial environment, a milling curve is introduced in order to correlate the Idle and Axial roll displacement, however it must be underlined that different milling curves lead to different loads and energy for ring realization. In this work an industrial case study was modeled by a numerical approach: different milling curves characterized by different Idle and Axial roll speed laws (linearly decreasing, constant, linearly increasing) were designed and simulated. The results were compared in order to identify the best milling curve that guarantees a good quality ring (higher diameter, lower fishtail) with lower loads and energy required for manufacturing.

Abstract: A study was performed to evaluate how the Friction Stir Spot Welding process parameters affect both the thermal distribution in the welding region and the welding forces. An experimental campaign was performed by means of a CNC machine tool and FSSW lap joints on AA6060-T6 aluminum alloy plates having a thickness of 2+2 mm were executed. Five thermocouples were inserted into the samples at a specific distance from the specimen center. A set of tests was carried out by varying the process parameters, namely rotational speed, axial feed rate, plunging depth and dwell time. Axial welding forces were also measured during the execution of the experiments by means of a piezoelectric load cell. The experimental data collected were used to set up and to validate a simulative model of the process. In particular, a 2D FEM model was set up using the commercial code Deform 2D. A 2-dimensional FEM code was preferred in order to guarantee a very simple and practical model able to achieve results in a very short time. Since it is not possible to simulate the rotation of the tool in a 2D configuration, a specific external routine for the calculation of the developed thermal energy due to the friction between tool and workpiece was set up and implemented into the code starting from the local pressure distribution along the contact area.