Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.

AFFRONTIAMO LA CARATTERIZZAZIONE DI UN LEGA DI ALLUMINIO FINALIZZATA ALLA CREAZIONE DI UN MODELLO NUMERICO DI UN PROCESSO DI FORMATURA SUPERPLASTICA. L’INDIVIDUAZIONE DEI PARAMETRI DEFORMATIVI OTTIMALI E DELL’EQUAZIONE COSTITUTIVA DEL MATERIALE HANNO PERMESSO DI ANALIZZARE LE CRITICITÀ DI PROCESSO NELLA REALIZZAZIONE DI UN COMPONENTE DI FORMA COMPLESSA NON REALIZZABILE CON TECNICHE CONVENZIONALI.

In this work the Warm Hydroforming (WHF) process for the production of a 6xxx series Al alloy component has been investigated using a numerical/experimental approach: both experimental and numerical hydroforming tests were carried out using the alloy AC170PX, a pre aged (T4 condition) Al alloy often adopted for automotive applications. In order to evaluate both the mechanical and strain behaviour of the material, tensile tests were carried out at different temperature and strain rate levels using the Gleeble system 3180, keeping also into account the ageing effect; in addition, formability (Nakazima) tests in warm conditions were performed by means of a specific equipment and the Forming Limit Curves at different temperature levels were evaluated according to the ISO standard 12004-2. Hydroforming experiments were carried out using a prototypal press machine specifically designed for WHF and SuperPlastic Forming tests. Such tests, scheduled by a DoE approach, were aimed at investigating the suitability of using the investigated Al alloy in the WHF process: attention was thus focused on those parameters mainly affecting the aging phenomenon (temperature, heating time and cycle time). In order to overcome the actual physical limitation of the hydroforming facilities, a Finite Element (FE) model of the WHF process was also created implementing experimental data (flow stress curves and FLCs) and tuned using data from preliminary WHF tests. In particular, after setting the Coefficient Of Friction (COF) according to temperature and verifying the robustness of numerical simulations, the FE model was used for investigating: (i) the influence of the Blank Holder Force (neglected in the experimental campaign); (ii) the adoption of quite smaller values of the parameter cycle time (being the aim to determine higher strain rates in the material). Through the definition of proper response variables (Flatness, Bursting Pressure and Thickness Ratio) both experimental and numerical results were analyzed by means of polynomial Response Surfaces in order to evaluate the optimal process conditions.

In this work, the mechanical and technological behaviour of AZ31 magnesium alloy laser welded joints is investigated. The forming behaviour of the joints is analysed by both tensile and biaxial stretch tests. Each test is monitored using a digital image correlation system in order to acquire the complete strain field during the whole test. Both in tensile and biaxial stretching tests, the strain maps reveal that the weld bead makes the strain path experienced by the welded specimen more critical than the one experienced by the base material, and this can be related to morphological defects of the weld bead.

The present work is aimed to determine the mechanical behaviour in hot condition (range 600-1200°C) of a super duplex stainless steel (SAF 2507) for applications in the Oil&Gas field (highly corrosive environments). A wide experimental activity (both tensile and creep tests) was carried out using the Gleeble system, using experimental settings able to make the test robust and replicable. In order to evaluate the constant parameters able to model the material behaviour according to the Norton equation, experimental conditions (in terms of temperature and applied stress) were designed: the Response Surface Methodology (RSM) and a subsequent double multi objective optimization were implemented within an integration platform. Finally, using Visual Basic routines model constants were evaluated and/or refined, thus being able to optimally fit real strain –time curves, also in the primary creep stage.

The effect of laser cutting parameters on the mechanical behavior of laser butt welded joints whose edges were obtained by laser cutting was investigated. The paper aims to demonstrate that new high power solid-state fiber lasers not only represent a valid and reliable alternative to the most established CO2 and Nd:YAG laser sources, but also allow to obtain cuts having edges well suited for subsequent direct laser welding. First Ti6Al4V 1 mm thick sheets having edges machined by milling were laser welded. Once the optimal welding condition was determined, the mechanical characterization of sheets cut by fiber laser and then laser welded was performed. Comparative strain analysis performed by a digital image correlation technique highlighted the effect of the gap between the sheets resulting from the different cut edge quality. Experimental results showed that the correct selection of laser cutting parameters allows to obtain butt joints characterised by mechanical properties comparable with the ones obtained by milling. Cutting edge quality in the optimal range of gap values allows to obtain the best mechanical performances of the joint.

The present work is focused on the Deep Drawing process in warm conditions and in particular on the evaluation of optimal working conditions and/or enhancement of the process limits using metamodels. Experimental tests were carried out with the aim of investigating the effect of the most important process parameters affecting the DD process: the blank holder pressure, the temperature of the Blank holder and the punch speed. The output variable “Progress” (defined as the ratio between the effective and the maximum punch stroke) was fitted using a separate parallel approach based on two different Response Surface construction algorithms. Further experimental tests were thus designed using an optimization approach based on the above mentioned RS, being the aim to enhance the process limits in terms of maximum achievable drawing ratio. The experimental results coming from the second campaign could be also used as a validation set for the results obtained from the initial optimization, thus allowing to identify which of the two initial RS was the most reliable in evaluating the process window.

INVESTIGHIAMO SULL’EFFICACIA DELL’UTILIZZO DI UN RISCALDAMENTO/ RAFFREDDAMENTO LOCALIZZATO E QUALE SIA LA MODALITÀ MIGLIORE DI REALIZZARLO PER UNA APPLICAZIONE PRATICA.

Surface hardening with discrete laser spot treatment is an interesting solution since the adoption of a single pulse allows the treatment of different surface geometries avoiding the effect of back tempering. The aim of this work is to find a suitable process window in which operate to get best results in terms of hardness, diameter and depth of the treated region. A single pulse out of a fiber laser source impinging on a bearing hypereutectoid steel was used using different power values, pulse energy and defocussing distances, in order to get the optimal process parameters. The dimensions of the hardened zone and its hardness were then acquired and related to the laser process parameters, to the prior microstructure of the steel (spheroidized and tempered after oil quenching) and to the roughness on the specimen before the laser treatment. Experimental results highlighted that both the surface condition (in terms of roughness) and the initial steel microstructure have a great influence on the achieved hardness values and on the dimension of the laser hardened layer. The pulse energy and power strongly affected the dimension of the hardened layer, too.

A numerical/experimental procedure is proposed for calculating the residual stress state during the cooling phase of the casting process of a superduplex stainless steel (ASTM A890 Gr. 5A). The experimental activity consisted of casting, tensile and creep tests. Casting tests were used to set (by acquiring temperature data at different points) and validate (by measuring displacements after releasing residual stresses by cutting) the Finite Element model. Tensile and creep tests were used to determine the material properties from room temperature to 1200 °C. A fully coupled thermo-mechanical analysis was conducted by neglecting the presence of the sand mould; instead, attention was focused on the material by modelling the creep behaviour using the Bailey-Norton formulation. Measurements of the of the displacements due to the stress release after EDM wire cuts revealed to be in good agreement with the numerical model and confirmed the key role played by viscosity during the cooling phase. Neglecting the viscous strain led to an overestimation (more than twofold) of the stress level in the cast part after the cooling phase in the sand mould. The good matching between experimental and numerical data indicated that a numerical model that incorporates the creep behaviour is able to accurately capture the investigated phenomenon, despite the simplification in the modelling of the casting process (sand mould replaced with virtual convection) which does not substantially affect its accuracy. The robustness of the methodology, which is characterized by small computational cost and good quality of results, was further proved simulating and comparing numerical and experimental results concerning a second casting geometry.

Determination of the interface heat transfer coefficients in casting processes represents a fundamental step in the creation of a reliable numerical model that can predict some of the most common defects (for example hot tear or residual stress) which may affect the process. This work focuses on the most appropriate methodology to determine the heat transfer coefficients for the numerical modelling of the casting process of a super duplex stainless steel (ASTM A890 Gr. 5A) using a silica sand mould. Experimental instrumented castings were used to acquire (both in the casting and in the sand) temperature changes for a number of points of interest by means of thermocouples; in addition, the entire process was simulated using the finite difference method commercial software package MAGMASOFT® (v. 5.2) in order to calculate the temperature at the same points at any step of the process. Firstly, the most influential input parameters were chosen in order to determine the factors to be investigated using a reduced factorial scheme. Further simulations, in which the value of the chosen factors was changed, made it possible to create response surfaces using as a response variable the value of the mismatch between the experimental and numerical temperature changes in the same points. Finally, the optimisation procedure using a multi-objective genetic algorithm was performed, with the goal of finding optimal values for the input parameters, i.e. those with which the mismatch between experimental and numerical temperature changes is minimised. As confirmed by the numerical simulations using the results of the optimisation procedure, the methodology proposed enabled us to determine the correct values of the input variables for modelling the casting process of the ASTM A890 Gr. 5A.

Numerical methods are widespread in forming applications since a deeper understanding and a finer calibration of the process can be reached without most of the assumptions used in analytical approaches. In this calibration procedure the characterization of the material behaviour is an important preliminary step that cannot be avoided. Experimental tests can be numerically modelled and material constants can be found by inverse methods making numerical results as close as possible to experimental ones. In this work material parameters of a superplastic aluminium alloy have been found by experimental forming tests and an inverse analysis. Constant pressure free inflation tests were firstly performed to find the optimal range for temperature and strain rate values. Material constants were then calculated, on the basis of these tests, minimizing errors between experimental and numerical data through a gradient based optimization iterative procedure. Constant strain rate experimental tests were finally used to refine material parameters and to gain a better agreement between experiments and numerical simulations.

Il lavoro analizza gli effetti di un singolo impulso laser di una sorgente in fibra, eseguito nel regime di parametri laser tale da permettere l’indurimento e la rifusione della superficie di un acciaio ipereutettoidico. I parametri laser investigati sono stati la potenza laser, l’energia dell’impulso e la distanza di focalizzazione, il cui effetto è stato determinato valutando la forma, l’ampiezza e la durezza della zona trattata. È stato formulato un modello agli elementi finiti del trattamento termico, calibrato con prove sperimentali, che ha permesso di definire i parametri di processo che garantiscono l’indurimento senza fusione, o la fusione del substrato senza l’insorgere di cricche. Questi risultati sono utili per progettare pattern per la strutturazione via laser di superfici in acciaio.

Magnesium alloys have been used increasingly in various areas of industry, such as the automotive sector, electronic components. A reliable method for joining components produced with these materials will definitely allow for a wider use of these alloys. In this article, the laser welding process with an Nd:YAG laser using a maximum power of 2 kW was studied and reported. The effect of welding parameters, such as laser and welding, was analysed. Metal sheets of 1mm thickness in AZ31B magnesium alloy were butt-welded with helium and argon as shielding gas in the absence of filler material. The mechanical characteristics were measured during tensile tests, using digital image correlation (DIC) as a deformation measurement technique. After a preliminary experimental plan aimed at exploring the range of process parameters, the welding process was optimized using an approach based on the design of experiments (DoE). Consecutive experimental plans led to optimized butt joints, which, following a tensile test, showed a sound weld bead with fracturing in the base material. Local strain analysis by DIC highlighted different tensile behaviours, in terms of deformation of the weld bead, in the optimized butt joints with respect to those that were not optimized.

In this work, the effect of the laser-material interaction on the formability of a superplastic aluminum alloy was investigated. In applications such as Tailor-Welded Blanks and in the manufacturing of very large components with a complex shape, laser welding combined with superplastic forming may be a very fitting industrial tool. Bead on plate tests were carried out in order to simulate the laser-welding process and then, free inflation tests were performed to evaluate the compatibility of these two processing techniques. The Al-Mg alloy used in this work has a very small grain size which ensures the superplastic behavior. With the aim of preserving this peculiarity, the following aspects on the formability were investigated: (i) the surface condition of the bead before the forming test (with and without the removal of the excess of metal); (ii) the effect of the travel speed of the laser source on the mean grain size; (iii) the introduction of a refiner, commonly used in aluminum casts, in the molten pool in order to further reduce the mean grain size.

In this work, the feasibility of reducing the cycle time in superplastic forming through a selective approach in the algorithm that calculates the forming pressure profile was investigated. First, a 3D numerical model of the blow forming process is created. Then, the blank was partitioned in different characteristic areas according to their strain and strain rate histories. Thus, different pressure profiles were numerically calculated choosing different combinations of those partitions of the blank. Experimental trials were finally carried out in order to explore the potential reduction of the forming time that can be achieved through the described approach without affecting the post-forming properties of the formed specimens. Post-forming properties were measured in terms of thickness distribution, mean grain size, and cavitation effects along the formed sheet. In particular, experiments were performed both with the conventional approach (with the whole sheet being monitored) and considering only the area of the sheet that experiences the highest strain values at the end of the forming process. Results highlighted that this latter approach can efficiently reduce the cycle time.

L’articolo riporta i risultati di uno studio sul processo di saldatura ibrida laser-arco di lamiere di spessore 3.0mm e 5.0mm, in lega di titanio Ti6Al4V, nelle configurazioni di giunto di testa e a T. La saldatura ibrida è stata studiata su molti materiali, ma pochi studi sono stati condotti sul titanio e le sue leghe, mentre tale processo può risultare di grande interesse nei settori aeronautico/aerospaziale e navale. Sono stati valutati gli effetti dei parametri del processo di saldatura ibrida quali: potenza del fascio laser, frequenza degli impulsi dell’arco, lunghezza dell’arco, corrente dell’arco, velocità del filo, posizione relativa laser-arco, velocità di saldatura. E’ stata studiata la microstruttura ed è stata eseguita l’analisi morfologica delle sezioni trasversali del cordone di saldatura. Sono riportati i risultati delle analisi energetiche e morfologiche, le caratteristiche meccaniche (durezza Vickers e trazione statica), con un confronto tra giunti saldati con fascio laser senza materiale d’apporto e mediante saldatura ibrida laser CO2-MIG. Il comportamento deformativo del giunto è stato analizzato tramite un sistema ottico di misura delle deformazioni basato sull’acquisizione stereoscopica delle immagini (ARAMIS 3D). E’ stata analizzata l’influenza del gap tra le lamiere.