Impact behaviour of dissimilar AA2024-T351/7075-T651 FSWed butt-joints: effects of Al2O3-SiC particles addition

Dissimilar friction stir welding joints are widely employed in the industrial field due to the excellent microstructural and mechanical properties of the resulting joints. Nevertheless, to further enhance the weld properties, the addition of reinforcement particles on the joint-line during the process has been proven effective for increasing its mechanical performance. In the present investigation, the microstructure and the impact behaviour of FSWed joints between AA2024-T351 and AA7075-T651 aluminium plates were investigated, considering the effect of different process parameters selected through a full factorial 2k design of experiments: both the rotational and translational speed of the tool, as well as the addition of Al2O3-SiC microparticles, were considered as input parameters. Unnotched 10 x 5 x 55 mm impact specimens were tested through an instrumented 50 J Charpy pendulum: total impact energy, the two complementary initiation and propagation energies as well as the peak force were correlated to the adopted process parameters. From the performed analyses, it was found that joints with reinforcing particles are prone to form wormhole defects across the stir zone that not only affect the microstructural development, but also the impact behaviour since they require less energy at break in comparison with joints fabricated without particles addition

On the improved adhesion of NiTi wires embedded in polyester and vinylester resins

This paper discusses the effect of different surface treatments on shape memory alloy wires embedded in PolyEster (PE) and VinylEster (VE) polymeric matrices. In particular, two types of chemical etching and a chemical bonding with a silane coupling agent have been performed on the surfaces of the wires. Pull-out tests have been carried out on samples made from a specifically designed Teflon mould. Considering the best results of the pull-out tests obtained with PE resin, the debonding induced by strain recovery of 4%, 5% and 6% pre-strained NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and then being embedded in the PE matrix. The results prove that the wires functionalised and embedded in the PE resin show the maximum pull-out forces and the highest interfacial adhesion. Finally, it has been found that debonding induced by strain recovery is strongly related to the propagation towards the radial direction of sharp cracks at the debonding region

Impact behavior of gravity cast AlSi10Mg alloy: Effect of hot isostatic pressing and innovative high pressure T6 heat treatment

In the present study, the impact behavior of gravity casting AlSi10Mg alloy was evaluated with an instrumented Charpy pendulum. The effect of hot isostatic pressing, also followed by a T6 treatment, was analyzed in comparison with samples in the as-cast, annealed and T6 conditions. Furthermore, the effect of the innovative high-pressure T6 was investigated. It was found that the hot isostatic pressing is able to ensure densification of the alloy with an increase in both hardness and energy absorbed during impact. The T6 treatment performed at atmospheric pressure after the hot isostatic pressing is able to increase hardness and peak force. At the same time, the innovative high-pressure T6 is able to ensure similar results than those of hot isostatic pressing followed by T6, leading to a significant decrease in the treatment duration and costs and reducing the carbon footprint of the manufacturing process

On the improved adhesion of NiTi wires embedded in polyester and vinylester resins

This paper discusses the effect of different surface treatments on shape memory alloy wires embedded in PolyEster (PE) and VinylEster (VE) polymeric matrices. In particular, two types of chemical etching and a chemical bonding with a silane coupling agent have been performed on the surfaces of the wires. Pull-out tests have been carried out on samples made from a specifically designed Teflon mould. Considering the best results of the pull-out tests obtained with PE resin, the debonding induced by strain recovery of 4%, 5% and 6% pre-strained NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and then being embedded in the PE matrix. The results prove that the wires functionalised and embedded in the PE resin show the maximum pull-out forces and the highest interfacial adhesion. Finally, it has been found that debonding induced by strain recovery is strongly related to the propagation towards the radial direction of sharp cracks at the debonding region

Pensareal futuro. Percorsi di orientamento alle scelte post-diploma

La scelta del futuro, che sia di un’occupazione o di un percorso di studi uni-versitario, è il momento che per un giovane segna l’inizio del passaggio all’età adulta. Per uno studente che si affaccia al diploma rappresenta spesso una fase critica della vita, che il più delle volte lo coglie impreparato e smarrito. Per questo l’orientamento diviene una parte integrante del processo educati-vo e formativo che supporta i giovani a prendere coscienza di sé e a far fronte in modo consapevole alle scelte che si trovano ad affrontare in particolari fasi di transizione della loro vita. Ma non è esclusivamente una questione che riguarda il singolo. Accompagnare lo studente nel suo progetto personale e professionale signifi ca soprattutto agire su un piano individuale, ma in una prospettiva sociale; signifi ca immaginare una società futura capace di con-trastare l’abbandono scolastico e la dispersione universitaria, di limitare la distanza tra scuola e realtà socio-economiche e il mismatch tra formazione e lavoro e, quindi, di ridurre il fenomeno dei Not in Education, Employment or Training. Ma perché sia effi cace è necessario che si confi guri come un per-corso di cooperazione tra i diversi attori e i diversi mondi che concorrono alla formazione e all’educazione del giovane – scuola, università, famiglia, mondo del lavoro, territorio, istituzioni, aziende; in altre parole, un sistema integrato di orientamento che pone al centro il giovane e le sue attitudini e lo accom-pagna nell’elaborazione del suo progetto di vita.Il volume parte da questa convinzione. Alla luce delle direttive europee e della conseguente riforma dell’orientamento contenuta nel Piano Nazionale di Ripresa e Resilienza, si è sentita la necessità di raccogliere in un unico testo esperienze e punti di vista di studiosi che, con uno sguardo sull’attuale con-testo sociale, culturale ed economico, si raccontano da differenti angolazioni, avanzando ipotesi sui possibili percorsi per favorire e agevolare le scelte dei giovani. Esperienze a confronto, dunque, che messe in connessione fornisco-no un quadro complesso ma coerente della centralità dell’orientamento nella costruzione dei possibili futuri individuali e sociali e offrono indicazioni su procedure di specifi che proposte innovative

Functional characterization of thermally activated shape memory alloys for innovative adaptive structures

Il continuo impiego dei materiali a memoria di forma come elementi attivi deriva dall’effetto a memoria di forma e dalle peculiari proprietà meccaniche di questi materiali, tra le quali la capacità di recuperare elevate deformazioni (fino al 10 %), l’elevato rapporto potenza su peso e la capacità di generare elevati sforzi (fino a 800 MPa). Le leghe a memoria di forma sono spesso utilizzate all’interno di materiali polimerici, eventualmente rinforzati con fibre, per realizzare strutture attive deformabili le cui proprietà posso essere modificate in risposta a stimoli esterni. La possibilità di integrare materiali intelligenti all’interno di sistemi attuativi rappresenta una interessante scelta tecnologica ed una soluzione alternativa all’impiego di sistemi meccanici convenzionali, nello sviluppo di meccanismi con migliore semplicità costruttiva e fattibilità. La possibilità di utilizzare elementi in lega a memoria di forma per la realizzazione di strutture deformabili è di particolare interesse nelle applicazioni aerodinamiche dove tale soluzione costruttiva permette di limitare l’introduzione di elementi di controllo che alterano il flusso. Il concetto di pale a geometria variabile, che possono modificare la loro forma in modo attivo, riducendo (o eliminando) così l’impiego di sistemi di controllo, rappresenta una soluzione estremamente interessante. Sebbene vi siano numerose pubblicazioni e brevetti riguardanti l’impiego di elementi in lega a memoria di forma per il miglioramento dell’efficienza aerodinamica e strutturale dei dispositivi nel settore automotive, studi specifici relativi al comportamento di pale a geometria variabile ottenute con leghe a memoria di forma non sono presenti in letteratura. Scopo del presente lavoro è lo sviluppo e la caratterizzazione sperimentale di un innovativo concetto di pala a geometria variabile, ideata per l’impiego all’interno di ventole di raffreddamento di motori a combustione interna di macchine movimento terra. La pala in oggetto prevede una struttura composita formata da una matrice polimerica, che fornisce l’elasticità al corpo palare, al cui interno sono alloggiate lamine in lega a memoria di forma che rappresentano gli elementi attivi. Al fine di studiare la fattibilità del sistema in termini di adeguato comportamento flessionale delle lamine all’interno della struttura composita della pala, la prima parte del lavoro ha riguardato la caratterizzazione sperimentale del materiale in lega NiTi. La caratterizzazione termica ha permesso l’ottimizzazione dei parametri di trattamento termomeccanico di memorizzazione della forma (temperatura, tempo e grado di deformazione), i quali sono stati determinati sperimentalmente al fine di massimizzare l’effetto a memoria di forma della lamina. Il recupero della forma da parte del materiale, come risultato delle trasformazioni allo stato solido termicamente attivate, è stato valutato prendendo in considerazione l’evoluzione del valore percentuale di recupero della forma all’aumentare Acknoledgments del numero di cicli di attivazione eseguiti. I risultati sperimentali hanno permesso di stabilire il trattamento termomeccanico più adatto in relazione al recupero a flessione da parte della lamina. Partendo quindi dall’esigenza di valutare la capacità di attivazione della lamina in lega NiTi quando inserita all’interno di una matrice polimerica, è stato progettato e realizzato un primo prototipo di struttura composita costituito da una singola lamina collocata all’interno di una matrice polimerica. La capacità di deformazione a flessione è stata studiata attraverso l’esecuzione di cicli di attivazione ripetuti. Le attivazioni termiche hanno previsto l’impiego sia di un sistema a fluido sia di un sistema a flusso d’aria. Lo studio sperimentale condotto sul prototipo di struttura composita con una singola lamina è stato propedeutico al successivo sviluppo della pala a geometria variabile. L’analisi della sua capacità di deformazione è stata condotta attraverso il confronto tra quattro diverse matrici polimeriche al fine di stabilire il miglior compromesso tra rigidezza e deformazione flessionale per la specifica applicazione in esame. Le modifiche di forma della pala, derivanti dall’effetto a memoria di forma delle lamine, sono state studiate sperimentalmente mediante l’impiego di una galleria del vento appositamente progettata e realizzata, al fine di riprodurre le reali rampe termiche degli scambiatori di calore. I risultati sperimentali hanno evidenziato la possibilità di realizzare un sistema di controllo passivo per ventilatori assiali applicati nel settore automotive sfruttando un sistema di attuazione con leghe a memoria di forma in grado produrre modifiche della geometria palare che ne migliorano il comportamento aerodinamico.The ongoing employment of Shape Memory Alloys (SMAs) as solid-state actuators results from the Shape Memory Effect (SME) and the unique mechanical characteristics of these materials, including high reversible strain (up to 10 %), high-power to weight ratio and the ability to generate high recoverable stresses (up to 800 MPa). SMAs are frequently combined within polymeric materials, also reinforced with embedded fibres, to produce adaptive structures whose properties could be tuned in response to external stimuli. The integration of smart materials in actuation systems represents an excellent technological opportunity and an alternative solution to conventional mechanical systems for the development of mechanisms with improved simplicity and reliability. The possibility to use SMA elements for morphing structures is particularly attractive for aerodynamic applications where this actuation solution prevents the introduction of flow-disturbing control elements. The notion of smart advanced blades, which can control themselves and reduce (or eliminate) the need for an active control system, is a highly attractive solution in blade technology. Despite a large number of publications and patents relating to the employment of SMAs for enhancing aerodynamic and structural efficiency of automotive applications, specific studies on the behaviour of SMA-based morphing blades are not available in literature. The aim of this work is the development and the experimental characterisation of a novel concept of morphing blade, intended for cooling fan of internal combustion engines for earth moving machines. The variable-geometry blade comprises a composite structure made up of a polymeric compound, for the elastically deformable body of the blade, with embedded SMA strips as active elements. To assess the reliability of the SMA strip to provide the proper bending behaviour in the composite structure of the blade, the first phase of the work concerned the experimental characterisation of the NiTi material. The thermal characterisation allows to study the best shape setting parameters (temperature, time and strain) which were experimentally tuned to maximise the SME in the NiTi strip. The recovery behaviour of the SMA material, as a result of the thermally activated solid to solid phase transformations, was assessed by considering the percentage of shape recovery with the increasing number of thermal cycles. The experimental findings enabled to set the proper thermomechanical treatment related to the bending recovery performance of the strip. Starting from the need to evaluate the actuation ability of the NiTi strip when it is indirectly embedded into a polymeric matrix, a proof of concept of composite structure, comprising a single strip and a polymeric compound was design on purpose. The bending ability of the structure was evaluated by means of repeated recovery cycles. Thermal activations were achieved by means of a fluid flow and an airstream flow heating/cooling systems. Acknoledgments The experiments performed on the concept of the single strip composite structure were useful for the development of the morphing blade. The study of its morphing capability was conducted by comparing the behaviour of four different polymeric compounds in order to establish the best compromise between stiffness and deflection behaviour for the intended application. The shape changes of the blade, arising from the SME behaviour of the embedded thermomechanically treated NiTi strips, were experimentally evaluated through a purpose-built wind tunnel, developed to reproduce the actual automotive thermal ramps of the heat exchanger. Experimental findings highlighted the opportunity to generate an innovative passive control system applied to an automotive cooling axial fan wherein the SMA driven activation enables the production of favourable aerodynamic shape changes

Recent Advances in Cast Irons

Cast irons are widely used in industry due to their excellent castability, allowing for the production of near-net shape components with complex geometries without the need for additional forging or machining processes. They also are a cost-effective material with good machinability, corrosion resistance, and vibration damping properties, as well as relatively high wear resistance, thanks to graphite’s self-effect. However, achieving the optimal combination of microstructure and mechanical properties requires optimizing the process parameters, solidification conditions, and heat treatment for a specific chemical composition. Hence, continuous research efforts in cast iron are crucial for the improvement of in-service performance. In this frame, this Special Issue includes original research papers and a review that cover the most recent research and development aimed at improving the chemical, physical and metallurgical characteristics of cast irons that, in turn, affect their mechanical, tribological and corrosion performance

Effects of Heat Treatment and Erosion Particle Size on Erosion Resistance of a Hypereutectic High-Chromium Cast Iron

This research addresses the erosive resistance of a hypereutectic high-chromium cast iron subjected to solid particle erosion. The study stems from a specific application of high-chromium cast iron, i.e., the critical surfaces of large industrial fans operating in a cement clinker grinding plant where such damage is a limiting factor for the components’ lifespan. A dedicated experimental investigation on the impact of substrate microstructure and erodent particle size on erosion resistance was set. The experimental campaign, conducted on a dedicated test bench per the ASTM G76 standard, comprised the analysis of the as-received, tempered, and destabilized conditions for the cast iron. From a preliminary image analysis of the microstructural features, two diameters of the erodent powder for the erosion tests were defined. The observed erosion rate decreased with the increase in the mean particle diameter of the erodent, indicating more severe erosive conditions for smaller particles. From the analysis of the worn surfaces, it was possible to highlight the involved mechanisms concerning the considered test combinations. For the as-received condition, the erosion rate with the larger mean particle diameter of the erodent decreased three times compared to the smaller one. For the heat-treated conditions, the erosion rate was halved with the larger mean particle diameter of the erodent. The proposed analysis, intended to acquire more insight into the limiting factor for the components’ lifespan for erosive wear damage, proved that erosion resistance is not dependent on the material’s hardness. The contribution of the mean particle diameter of the erodent is predominant compared to the substrate conditions

Comprehensive Evaluation of Modification Level Assessment in Sr-Modified Aluminium Alloys

One way to improve the mechanical properties of Al–Si alloys is to add chemical modifiers that affect the microstructure of the cast components. Small amounts of Sr cause the modification of the eutectic Si particles from a coarse plate-like morphology into a fine fibrous one. This study sets out to analyse the effects of chemical modification on an Al–7%Si–0.3%Mg foundry alloy treated with Sr, ranging from 62 to 820 ppm. Assessment of modification level was evaluated by both thermal analysis technique and quantitative microstructural investigations. Up to now, little attention has been paid to the role played by microstructural characteristics in evaluating the modification level, which is commonly performed by comparison with standard images. In the present investigation, numerical methods proposed in the literature have been considered in an attempt to understand their effectiveness. The correlation between thermal analysis results and geometrical parameters of eutectic Si particles highlighted the impact of microstructural homogeneity on modification-level evaluation. In addition, the presence of Ca in Sr-based intermetallics has been further investigated by means of scanning electron microscopy combined with energy-dispersive X-ray spectroscopy

Some results on the number of coincidences under exchangeability

One of the most renowned probability problems is the birthday problem: if n balls are randomly dropped into k boxes, what is the chance of a match, that is, that two or more balls fall in the same box? The classical answer is given under the assumption that balls are dropped independently and uniformly into each box. If k=365, the answer gives the probability of two or more coincident birthdays, in a group of n individuals. Some results on the birthday problem with non-uniform occurrence probabilities can be found in the literature. Recently, the birthday problem has been considered in a Bayesian framework. Exact calculations, for the chance of a match, have been presented under uniform prior and symmetric Dirichlet prior. Moreover a Poisson approximation for the law of the number of coincidences has been proved, under general Dirichlet priors. This last result relies on the Chen-Stein approximation methods, applied to negatively associated random variables. In the present paper, the approximation problem for the number of matches is faced, under a general prior, that is only assuming exchangeability. We provide necessary and sufficient conditions for the law of the number of matches to be well approximated by a mixture of Poisson distributions. Moreover, we characterize the prior distribution and give sufficient conditions involving observable quantities alone. This enables, in a Bayesian setting, to decide whether to assume a Poisson model, for the number of matches, on the base of prior information