1,721,092 research outputs found
Trattamenti termo-chimici e meccanici d’indurimento superficiale e loro effetti sulla fatica
No full textLe tecnologie per realizzare indurimenti superficiali si pongono l’obiettivo di migliorare le proprietà tribologiche e la durezza che stanno alla base della resistenza a fatica e ad usura di un materiale metallico. I procedimenti adottati si differenziano per il principio fisico, chimico o meccanico adottato. Le caratteristiche tecniche del risultato finale, cioè lo spessore e l’uniformità del rivestimento, insieme al suo livello di durezza e resistenza, dipendono dai parametri di processo come temperatura, pressione, tempo ecc..
Dal punto di vista industriale, l’adozione di trattamenti adibiti all’indurimento in superficie risulta fondamentale laddove ci si trovi di fronte ad applicazioni caratterizzate da carichi ciclici di qualsiasi livello, molto spesso di carattere puntuale. Ne è un esempio la componentistica meccanica, in cui elementi, di solito in acciaio, che operano in condizioni di rotazione e scorrimento (ingranaggi, alberi motore, anelli dei cuscinetti, ecc) devono avere uno strato superficiale indurito resistente all’usura, al grippaggio, al pitting e contemporaneamente un cuore interno tenace per resistere alle sollecitazioni sia statiche che dinamiche. Un esempio è riportato in Fig. 1.
Fig.1 - Esempio di applicazione in cui risulta necessaria l’adozione di trattamenti superficiali d’indurimento.
In altri campi si deve tener conto anche di sollecitazioni uniformi ed estese, in cui i parametri relativi all’attrito e allo scorrimento reciproco delle superfici dei componenti stanno alla base della qualità del prodotto finale (le matrici da estrusione, gli stampi per le materie plastiche, ecc).
Obiettivo di questo capitolo è quello di descrivere ed approfondire i principali processi di indurimento superficiale, in modo da metterne in luce caratteristiche, funzionalità ed effetto specialmente riguardo alla resistenza a fatica. In campo industriale esistono varie tecniche adibite al trattamento di una superficie metallica, alcune delle quali molte volte partecipano insieme ad un unico processo di indurimento.
In via generale i trattamenti più importanti ed utilizzati, su cui è centrato il capitolo, sono:
- pallinatura
- cementazione
- nitrurazione
- carbo-nitrurazione
- trattamenti a bassa temperatura
- trattamenti al plasma
Verranno descritte sia le tecnologie elencate che la conseguenza che queste hanno riguardo alla resistenza a fatica degli acciai
Aging Response of an A357 Al Alloy Processed by Selective Laser Melting
No full textThe A357 Al-Si-Mg alloy produced by selective laser melting is investigated in order to evaluate its response to thermal aging. Solution annealing followed by water quenching reveals responsible for the transformation of the interconnected Si network, typical of the as built material, into a dispersion of globular, and coarser constituent and for the formation of an equiaxed grain structure. The microstructural changes lead to a reduction of material hardness. DSC and aging curves show that as built condition can be considered as fully appropriate for an effective dispersion strengthening process by artificial aging. Natural aging reduces the amount of available sites for heterogeneous nucleation of phase and it is considered as detrimental for tensile properties
La fatica nei compositi a matrice metallica
No full textNei materiali compositi a matrice metallica (MMC – dall’acronimo inglese metal matrix composites) il fenomeno della fatica è progressivo e si manifesta con la formazione di difetti, in genere alla superficie del provino, che col susseguirsi dei cicli di carico e scarico tendono a propagarsi fino ad interessare tutto il materiale. Normalmente, in condizioni di fatica ad alto numero di cicli, il comportamento di un MMC è migliore di quello della corrispondente lega monolitica poiché il rinforzo ceramico funge da ostacolo alla propagazione delle cricche, ritardandone l’avanzamento. In condizioni di fatica a basso numero di cicli, si possono registrare casi in cui i compositi sono invece penalizzati rispetto alle leghe monolitiche per via del prematuro danneggiamento del rinforzo. In entrambe le condizioni si constata l’importanza fondamentale della omogeneità della struttura e la necessità di controllo durante la sintesi e le lavorazioni del materiale per evitare difettosità che costituiscono poi un facile innesco per le cricche di fatica in esercizio. Va infine considerato che, se da un lato il rinforzo particellare può generalmente ritardare la propagazione delle cricche, per contro, la struttura disomogenea su scala microstrutturale non permette di ottenere condizioni di finitura superficiale ottimali per cui la nucleazione delle cricche che può risultare a volte anticipata. Fra le varie tecniche utilizzate per ovviare a questo aspetto viene descritto a titolo di esempio un metodo basato su rivestimenti sottili di Ti realizzati mediante deposizione per sputtering con lo scopo di ottenere una minore rugosità superficiale
Effect of Cu content on hot-crack resistance of Al-Cu-Mg alloys produced by laser powder bed fusion
No full textThe lack of high-strength Al alloys that can be processed by laser powder bed fusion (LPBF) without formation of cracks is considered as one of the main issues for a wider diffusion of this technology in many fields. In this study, the effect of Cu content on the hot-crack resistance of Al-Cu-Mg alloys produced by LPBF was investigated. Powder batches with different compositions were produced by mixing pre-alloyed gas-atomised 2024 alloy powder with increasing amounts (4, 6, 8, 10, and 12 wt.%) of Cu particles and then processed by LPBF. Thermodynamic simulation and microstructure investigations showed that the addition of Cu promotes the formation of abundant eutectic phase mixture, refines the grain structure and reduces the solidification temperature range, thus decreasing the solidification cracking susceptibility of the material. Samples produced with more than 10 wt.% of Cu featured relative density higher than 99.5% and no solidification cracks
On microstructure development and inclusion generation in continously cast resulphurized steel
No full textInvestigation on two Ti–B-reinforced Al alloys for Laser Powder Bed Fusion
No full textOnly few medium: and high-strength aluminium alloys can be processed by Laser Powder Bed Fusion without forming solidification cracks. This constraint limits the diffusion of this technology in many industrial fields, including aerospace and motorsport sectors. In this study, a novel high-strength aluminium alloy for Laser Powder Bed Fusion was designed and its solidification behavior, microstructure and mechanical performance were investigated. The results were compared with those achieved by processing the high-strength A20X alloy processed with the same technology. The alloy was designed based on the chemical composition of the widely used 2618 Al alloy, a conventional high strength Al–Cu–Mg alloy for high temperature applications. The chemical composition of the 2618 alloy was modified by adding Ti and B, which form TiB2 particles that act as nuclei for the solidification of primary α-Al grains. The resulting microstructure made of equiaxed grains revealed resistant to hot cracking. A20X and 2618-modified alloys produced with optimized parameters featured relative densities higher than 99,7% and crack-free microstructures. The A20X-T6 showed yield strength and ultimate tensile strength of 428 MPa and 485 MPa, respectively, while the modified 2618-T6 revealed upper yield strength and ultimate tensile strength of 370 MPa and 468 MPa, respectively. The two alloys also showed a remarkably high strength at 150 °C and 250 °C, exceeding the typical strength values of the 2618 alloy produced by forging
Insight into the effect of different thermal treatment routes on the microstructure of AlSi7Mg produced by laser powder bed fusion
Full text linkThe present work gives an insight into the microstructure evolution of AlSi7Mg produced by laser powder bed fusion and subjected to different heat treatment routes. Synchrotron powder diffraction and diffraction contrast tomography coupled with scanning and transmission electron microscopy investigations allowed an in-depth understanding of the effect of rapid solidification induced by laser powder bed fusion on material structure. In addition, the effects of solution treatment followed by water quenching and artificial aging and that of direct aging from the as built condition was investigated. The as built material exhibits the most important lattice contraction and the lower amount of Si phase, thus suggesting a higher content of solute Si, which decreases after artificial aging and after solution treatment. The eutectic Si is found in nanometric form, with crystallites of about 10 nm in size, which grow significantly after solution treatment. A similar behavior is also observed for the second phase β-Al5FeSi. Traces of the π-Al8FeMg3Si6 were observed after solution treatment. Direct aging induced the formation of a fine dispersion of β” precipitates in the primary Al solidification cells, which is responsible for material hardness and strength increase. The thermal treatments strongly affect the residual macrostrain, which is higher for the as built material and decreases after aging and solution treatment. This was confirmed by diffraction contrast tomography, which provided a spatial resolved structural information and was used as alternative tool to quantify the distribution of Si and Al phases
Effects of matrix and reinforcement properties on low- and high-cycle fatigue behaviour of particulate-reinforced MMCs
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Design and Characterization of Al–Mg–Si–Zr Alloys with Improved Laser Powder Bed Fusion Processability
No full textAbstract: A key-factor for the industrial implementation of beam-based additive manufacturing technologies is the development of novel Al alloys characterized by enhanced hot-tearing resistance. Indeed, most of the standard Al alloys are susceptible to solidification cracking and can hardly be used to produce structural parts by laser-based additive manufacturing processes. In this study, we investigate the strategies to design high-strength Al alloys for Laser Powder Bed Fusion. The addition of Zr to the chemical composition of an Al–Mg–Si alloy (EN AW 6182) was carried out by following two different routes to promote the formation of equiaxed grains which are able to suppress hot cracking and enhance processability of the material. The first route is based on mechanical mixing of ZrH2 particles and gas-atomized Al alloy powder and on the in-situ reaction of the hydride to form Al3Zr nucleants. The second route relies on the use of pre-alloyed gas-atomized powders that feature Zr among the alloy elements. The specimens produced using pre-alloyed powder showed the best mechanical performance. After direct aging from the as-built condition, the alloy showed yield strength and ultimate tensile strength of 354 and 363 MPa, respectively, and elongation at fracture of 9.0 pct. The achieved properties are comparable to those of wrought 6182 alloy processed by conventional routes. Graphical Abstract: [Figure not available: see fulltext.]
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