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Hydroforming of lightweight alloys using MagnetoRheological Fluids (MRF)
Get PDFOggigiorno lo sviluppo di componenti leggeri è una grande sfida in diversi settori: nel campo automobilistico e aerospaziale, la possibilità di sostituire i materiali metallici convenzionali con leghe più leggere aiuta a ridurre i consumi energetici. Inoltre, in campo biomedico, l'uso di protesi realizzate con materiali caratterizzati da densità, modulo elastico e snervamento simili a quelli dell'osso umano aiuta a minimizzare il rischio di effetti di stress shielding. In questo scenario, le leghe di alluminio e magnesio sono buoni candidati rispettivamente per il settore dei trasporti e quello biomedico. Infatti, le leghe di alluminio (Al) presentano buone proprietà meccaniche combinate con una bassa densità, mostrano una buona resistenza alla corrosione, alcune leghe hanno una buona resistenza ai raggi UV (garantendo così prestazioni ottimali per un lungo periodo di tempo), sono adatte al riciclo e la combinazione delle loro diverse proprietà permette a queste leghe di essere formate attraverso molti processi produttivi fornendo una completa libertà di progettazione. Per quanto riguarda le leghe di magnesio (Mg) come metalli biodegradabili nel settore biomedico, l'interesse per queste leghe è fortemente cresciuto negli ultimi decenni principalmente a causa della loro biocompatibilità, biodegradabilità, alto rapporto resistenza-peso e proprietà meccaniche simili a quelle delle ossa umane. Tuttavia, l'attrattività di queste leghe è parzialmente controbilanciata dalla scarsa formabilità a temperatura ambiente. Inoltre, le leghe di Mg mostrano un tasso di corrosione molto veloce nei fluidi corporei a causa del rilascio di idrogeno, che deve essere preso in considerazione quando si considera il materiale per la realizzazione delle protesi. Quindi, sono necessarie soluzioni tecnologiche innovative e flessibili per fabbricare componenti di forma complessa con i suddetti materiali. I trattamenti termici localizzati, per esempio, sono utilizzati per ovviare al problema della scarsa formabilità, permettendo di personalizzare la distribuzione delle proprietà del materiale in base ai requisiti del processo di formatura. D'altra parte, quando il tasso di degradazione diventa un aspetto chiave, diversi studi riportano che il rivestimento di leghe di Mg con materiali specifici, come i polimeri, può essere considerato una soluzione promettente per controllare il comportamento di corrosione del substrato. Queste soluzioni tecnologiche devono essere combinate con un adeguato processo di formatura. Gli attuali processi di formatura innovativi e flessibili come la Formatura SuperPlastica (Superplastic Forming, SPF), la formatura incrementale (Incremental Forming, IF) e l'idroformatura (Hydroforming, HF) permettono di ottenere parti complesse con alta precisione. Negli ultimi anni, la formatura per mezzo di fluidi magnetoreologici in pressione (MagnetoRheological Pressure Forming, MRPF) ha preso piede dal momento che lo spessore e la distribuzione delle deformazioni sulla parte formata possono essere influenzati cambiando opportunamente le proprietà del mezzo formante. Nel caso dell’MRPF, il mezzo formante è un fluido magnetoreologico (MRF). Questo tipo di fluidi fa parte dei cosiddetti "materiali intelligenti": sono costituiti da una sospensione di particelle magnetiche in un liquido portante, il cui comportamento reologico (ad esempio, la sua viscosità) può essere modificato rapidamente e reversibilmente se sottoposto a un campo magnetico. Attualmente, gli MRF sono ampiamente utilizzati in campo automobilistico per freni e frizioni, in campo biomedico per protesi e per queste applicazioni sono già disponibili modelli costitutivi che sono in grado di descrivere le caratteristiche MRF dipendenti dal campo (shear stress vs. shear rate). Al contrario, quando vengono utilizzati per applicazioni di formatura della lamiera, ci sono ancora pochi studi in letteratura sul modo più efficace per caratterizzare il loro comportamento reologico, che è fondamentale per la progettazione e la simulazione del processo basata sugli elementi finiti (FE).
In questo scenario, gli obiettivi del presente lavoro di tesi sono (i) ampliare la realizzazione di componenti in leghe leggere dalla forma complessa a processi innovativi, come il processo di idroformatura utilizzando MRF; (ii) porre le basi per una metodologia efficace per caratterizzare il comportamento reologico di un MRF per applicazioni di formatura della lamiera, (iii) progettare un'attrezzatura sperimentale basata su MRF per caratterizzare tali fluidi e (iv) studiare possibili applicazioni degli MRF ai processi produttivi.Nowadays the development of lightweight components is a great challenge in several sectors: in the automotive and aerospace fields, the possibility of replacing conventional metallic materials with lighter alloys helps to reduce energy consumptions. Moreover, in the biomedical field, the use of prostheses made of materials characterized by density, fracture toughness, elastic modulus and compressive yield strength similar to those of the human bone helps to minimize the risk of stress shielding effects. In this scenario, aluminum and magnesium alloys are good candidates for the transport and biomedical sectors, respectively. In fact, aluminum (Al) alloys can provide good mechanical properties combined with a low density, exhibit a good resistance to corrosion, some grades have good resistance to UV light (thus ensuring optimal performance over a long period of time), are suitable for recycling and the combination of their different properties allows these alloys to be shaped through any process of industrial transformation providing complete freedom of design. As for magnesium (Mg) alloys as biodegradable metals in the biomedical sector, interest for these alloys has strongly grown over the last decades mainly because of their biocompatibility, biodegradability, good machinability, high weight-to-strength ratio and mechanical properties similar to those of the human bones. However, the attractiveness of those alloys is partially counterbalanced by the poor formability at room temperature. Furthermore, Mg alloys exhibit a very fast corrosion rate in body fluids due to the release of Hydrogen, which must be taken into account when considering the material for the prostheses manufacturing. Thus, innovative and flexible technological solutions are needed to manufacture complex-shaped components made of the aforementioned materials. Local heat treatments, for example, are used to overcome poor formability, allowing to tailor the distribution of material properties according to the requirements of the forming process. On the other hand, when the degradation rate becomes a key aspect, several studies report that coating Mg alloys with specific materials, such as polymers, can be considered a promising solution to control the corrosion behaviour of the substrate. Those technological solutions must be combined with suitable forming process. Current innovative and flexible forming processes such as Superplastic Forming (SPF), Incremental Forming (IF) and Hydroforming (HF) enable obtaining complex parts with high accuracy. In the recent years, MagnetoRheological Pressure Forming (MRPF) has gained interest because the thickness and the strain distribution on the formed part can be affected by properly changing the properties of the forming medium. In the case of MRPF, the forming medium is a Magnetorheological Fluid (MRF). This kind of fluids is included in the so called “smart materials”: they are based on a suspension of magnetically responsive particles in a liquid carrier, whose rheological behaviour (e.g., its viscosity) can be changed quickly and reversibly if subjected to a magnetic field. Currently, MRFs are widely used in the automotive field for brakes and clutches, in the biomedical field for prostheses and for these applications there are already available constitutive models which are able to describe the field-dependent MRF characteristics (shear stress vs. shear rate). On the contrary, when used for sheet metal forming applications, there are still few studies in the literature about the most effective way to characterise their rheological behavior, which is fundamental for the Finite Element (FE) based design and simulation of the process.
In this scenario, the aims of the present thesis are (i) expanding the manufacturing of lightweight complex-shaped components to alternative processes, such as the hydroforming process using MRF; (ii) to put the basis for an effective methodology to characterize the rheological behavior of an MRF for sheet metal forming applications, (iii) to design an experimental MRF-based equipment to characterize MRFs and (iv) to study possible applications of MRFs to manufacturing processes
Evaluation of the effectiveness of natural origin metalworking fluids in reducing the environmental impact and the tool wear
No full textIn the present work, due to the key role played by the environmental compliance and the operators' safety when using Metalworking Fluids (MWFs), a novel Natural Origin (NO) lubricant has been comparatively investigated with a commercial Mineral Based (MB) one in terms of: (i) environmental impact and (ii) lubricating/anti-wear performance. As for the former aspect, the values of Non-Ionic Surfactants (NIS) and Chemical Oxygen Demand (COD) in the aqueous phase obtained after the chemical breaking of the emulsions were evaluated; as for the second aspect, the performance of emulsions with different concentrations and oil types (the mineral and the natural one) were assessed both in laboratory (tests using Falex tribometer) and in real production environment (tool flank wear evaluation during machining). The chemical breaking demonstrated that the aqueous phase extracted from the NO emulsion (fresh or after several months of working) was characterized by very low contents of NIS and COD (3 orders of magnitude and 3 times, respectively, less than the MB emulsion if considering fresh emulsions; 3 orders of magnitude and 4 times, respectively, less than the MB emulsion if considering emulsions after several months of working) thus making possible to re-use it in the production process. On the other hand, from the process point of view, tribological and wear tests revealed that the NO emulsion always led to a tool wear smaller than the one determined by MB: around 30% if using an oil content of 5% for both the emulsions; around 19% if using an oil content of 5% for NO and 9% for MB
Numerical/experimental investigation of the effect of the laser treatment on the thickness distribution of a magnesium superplastically formed part
No full textThe growing need for high-performance components in terms of shape and mechanical properties encourages the adoption of integrated technological solutions. In the present work, a novel methodology for affecting the superplastic behaviour and, in turn, the thickness distribution of magnesium alloy components is proposed. Through heat treatments using a CO2 laser, the grain size was locally changed, thus modifying the superplastic behaviour in a predefined area of the blank. Both the grain coarsening produced by the laser heat treatment and the superplastic forming of the heat treated blank were simulated using a finite element model, which allowed to set the related process parameters for the manufacturing of the investigated case study (a truncated cone). The thermal finite element model of the laser heat treatment, calibrated using the experimental temperature evolutions acquired in specific areas during the heat treatment, was used to evaluate the influence of process parameters on the grain size evolution. The laser heat treatment was able to significantly promote the grain growth, increasing the mean grain size from about 8 mu m to twice (about 17 mu m). The resulting grain size distributions were implemented in the mechanical finite element model of the superplastic forming process and the combination of laser parameters which allowed to obtain the most uniform thickness distribution on the final component was finally experimentally reproduced and measured for validation purposes. Even in the case of the laboratory scale application, characterised by quite small dimensions, the proposed approach revealed to be effective, to improving the thinning factor (tMIN/tAVG) of the formed part from 0.85 to 0.89, and providing an increase in the thickness uniformity of about 4.7%
Post Forming Analysis and In Vitro Biological Characterization of AZ31B Processed by Incremental Forming and Coated With Electrospun Polycaprolactone
No full textMain problems related to the adoption of magnesium alloys for temporary orthopedic prostheses manufacturing are (i) the need of an efficient production process and (ii) the high corrosion rate compared with the bone healing time. In this work, the single-point incremental forming (SPIF) process, an effective and flexible solution for manufacturing very small batches even composed by one piece, was investigated. Tests were conducted on AZ31B-H24 sheets and were aimed at understanding the effect of temperature on the mechanical characteristics (microstructure, hardness, and roughness) of the sheet after the above-mentioned forming process and their correlation with both the corrosion rate and the cytocompatibility. In addition, after the forming process, samples processed by SPIF were coated by electrospun polycaprolactone (PCL) to reduce the corrosion rate and to further improve the cytocompatibility. Grain refinement was achieved thanks to the combined effect of temperature and strain rate during forming and finer grain size resulted to improve the magnesium corrosion resistance. In simulated body fluids, the electrospun PCL-coated samples exhibited a slower pH increase compared with uncoated samples. No indirect cytotoxic effects were detected in vitro for MC3T3-E1 cells for both coated and uncoated samples. However, cells colonization was observed only on electrospun PCL-coated samples, suggesting the importance of the polymeric coating in promoting the adhesion and survival of seeded MC3T3-E1 cells on the implant surface
Non-destructive assessment of crystallographic texture and mechanical behaviour in AZ31 magnesium alloys via pulsed laser thermography
No full textThis paper investigates the use of pulsed laser spot thermography as a non-destructive technique to evaluate mechanical properties and crystallographic texture changes in AZ31 magnesium alloys after thermal treatment. Eight AZ31 specimens were studied in two metallurgical conditions: three as-received (H24 tempered) and five annealed at 450°C for 10,000 seconds. Thermal diffusivity was measured non-destructively using laser spot thermography, while standard destructive methods provided hardness and grain size values for reference. Results highlighted an unexpected reduction in thermal diffusivity (around 18%) after annealing, despite a notable increase in grain size (approximately 75%) and a corresponding decrease in hardness (13%). Statistical analysis showed a clear negative correlation between grain size and thermal diffusivity (R = –0.907, p < 0.01), along with a moderate positive correlation between hardness and diffusivity (R = 0.745, p < 0.05). Unlike in steels, where increased grain size usually corresponds to higher thermal conductivity, the observed diffusivity decrease in AZ31 is likely due to microstructural complexities such as texture evolution, lattice distortions, and secondary phases. The study demonstrates–for the first time–that pulsed laser thermography effectively captures microstructural changes related to mechanical behaviour and crystallographic orientation in magnesium alloys, offering a rapid, non-destructive, and reliable alternative to traditional characterisation methods
Correlation between porosity level and elastic modulus in a foamed hiped Ti alloy
No full textNowadays, in the manufacturing of highly customized prosthetic implants, the need of devices with mechanical properties close to the human bone’s ones plays a key role. In the present work, Ti6Al4V-ELI porous structures obtained by a solid-state foaming process were studied from a microstructural and mechanical point of view, being the aim to control the stiffness of the prostheses in order to be as much as possible close to the human bone’s one, thus reducing the stress shielding effect. Samples with different levels of porosity (average diameter variable between a few microns and about 50 microns) were investigated by means of contact ultrasonic tests in order to evaluate changes in terms of elastic properties. Metallographic observations combined with contact ultrasonic tests revealed that a good correlation exists between the foamed structure (quantity and average size of the pores) and the stiffness
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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