1,720,988 research outputs found
Comportamento a fatica di materiali compositi a matrice di titanio
No full textIl principale settore d’impiego dei compositi a matrice di titanio (TMC) è senz’altro quello delle strutture destinate ad operare a temperature di esercizio medio-alte (~600°C), cioè in condizioni in cui i componenti sono sottoposti a sollecitazioni termiche e termo-meccaniche per gran parte della loro vita utile. Tali sollecitazioni provocano modificazioni nel materiale e queste ultime, a loro volta, ne condizionano il comportamento. In particolare, i fenomeni di ossidazione superficiale (dovuti all’esposizione ad elevata temperatura in ambienti ossidanti) e la formazione di prodotti di reazione all’interfaccia fibra-matrice, possono portare, se non debitamente controllati, ad un rapido deterioramento delle proprietà resistenziali del materiale. Di qui l’esigenza di investigare approfonditamente le proprietà dei TMC e la loro risposta in condizioni quanto più possibile prossime a quelle di impiego.
Lo studio qui riportato è volto alla caratterizzazione meccanica e a fatica di TMC prodotti tramite processo foil-fibre-foil. Le prove sono state condotte sia sul materiale tal quale che dopo esposizione ad elevata temperatura; i trattamenti termici sono stati eseguiti sotto vuoto allo scopo di mettere in risalto gli effetti dell’evoluzione microstrutturale (crescita dello spessore di reazione all’interfaccia fibra-matrice) sulle proprietà meccaniche del materiale, eliminando invece quelli dovuti ai fenomeni ossidativi
Preparazione del composito Ti6Al4V + SiC fibre e sua evoluzione strutturale dopo trattamenti termici
No full textBeta Forging of a Ti6Al4V Component for Aeronautic Applications: Microstructure Evolution
No full textTi–6Al–4V is an alloy increasingly used for structural applications in aeronautics due to its characteristics of high mechanical properties, lightness, and corrosion resistance. This alloy is conventionally forged below the beta transus temperature in order to control the microstructure evolution, to obtain a component with the desired properties. In this paper, some experiences of an innovative beta forging process of the Ti–6Al–4V alloy are reported. A preliminary campaign of forging tests in the beta field on cylindrical coupons was carried out in order to study the microstructural evolution in different forging conditions, in terms of both temperature and strain rate. Moreover, in order to study the microstructural evolution due to the beta forging in a complex shaped component, a case study is presented. The forged component showed a microstructure coherent with the forging process experienced; moreover, the hardness values measured were similar to the ones of the Ti–6Al–4V alloy in mill-annealed conditions
Comportamento meccanico in prove di fatica a caldo di compositi metallici a matrice di lega SP-700 rinforzata con fibre di carburo di silicio tipo SCS-6
No full textTension-tension fatigue behavior of a unidirectional titanium-matrix composite (SCS-6/SP-700) at elevated temperature [Comportamento meccanico in prove di fatica a caldo di compositi metallici a matrice di lega SP-700 rinforzata con fibre di Carburo di silicio tipo SCS-6]
No full textTitanium based metal matrix composites (MMC) reinforced by unidirectional continuous SiC fibers are attracting considerable attention for potential use as structural materials for advanced engine and hypersonic aircraft applications because of their higher specific strength and stiffness at medium and elevated temperature with respect to monolithic materials such as superalloys. Before these MMCs can be used in real applications, the deformation and failure mechanisms in the environment under which they will operate must be fully characterized. For aircraft applications, this routinely includes fatigue loading at elevated temperature. This experimental work describes the manufacture and the mechanical characterization of a titanium matrix composite fabricated by a two step process sequence in which foils (also called monotapes), consisting of uniformly spaced continuous fibers in a porous alloy matrix, are produced using plasma spray methods and lay-ups of these monotapes are then consolidated to form near net shape composite using a process known as Hot Isostatic Pressing (HIP). The material investigated in this study was a unidirectional composite consisting of a titanium alloy reinforced with SCS-6 silicon carbide fiber. The matrix alloy was of composition Ti4.5Al-3V-2Mo-2Fe, commonly designated as SP-700. The monotapes were produced by Inert Plasma Spraying (IPS). The preforms in the as sprayed condition have been investigated by means of optical microscopy in order to evaluate their characteristics (closed internal porosity, presence of non completely fused particles). The same investigation was performed on the composite specimens in the as Hiped condition. The results of such tests showed that was achieved a high degree of consolidation, no intra-ply de laminations, negligible porosity and fiber swimming as well as a good interfacial bonding without an extended fiber -matrix reaction zone, thus confirming the quality of the process used. In order to assess the mechanical properties of the composite, tensile tests were conducted both at room temperature and at elevated temperature (300, 450, 600°C). It was possible to underline the improvement of mechanical strength of this composite system if compared with the properties of the unreinforced SP-700 at the same temperatures. The fatigue behavior of this titanium matrix composite was also investigated. Fatigue tests were performed in air at a range of temperatures relevant to gas-turbine compressor operation, namely 450°C and 600°C and at different maximum stresses (as obtained from tensile tests). Tests were performed under load control, at a frequency of 5Hz and R ratio of 0.1, with loading under a sinusoidal waveform. Post-test microscopy was conducted on the fracture surfaces and on the sectioned samples of the tested specimens. Fractographic analysis was performed on all test specimens using a scanning electron microscope (SEM). The fractured specimen was further polished parallel to the loading direction in order to reveal the damage near the fracture surface. It was possible to observe the different damage mechanisms as well as the importance of fiber bridging. The improvements observed in comparison with the unreinforced matrix alloy allow this composite system to be considered as a suitable material for future industrial applications
Titanium matrix reinforced composites produced by hot pressing of plasma-sprayed preforms
No full textThe present work is focused on the fabrication of long fibre reinforced titanium composites by hot pressing of plasma-sprayed preforms. Thermal spraying of two different metallic matrices [Ti-6Al-2Sn-4Zr-2Mo-0.08Si (Ti6242S) and Ti-4.5Al-3V-2Mo-2Fe (SP700)] onto prearranged silicon carbide long fibres was performed using controlled-atmosphere plasma spraying equipment (CAPS), allowing deposition processes also in the high-pressure range. After optimization of the plasma spraying procedure, composite multilayers were fabricated by hot pressing at 1200 bar with different combinations of process parameters (temperature in the range 850-940 °C, and application time from 30 to 45 min). Consolidated samples were characterized by microstructural investigation and tensile testing at room and high temperature (up to 600 °C). Push-out and push-back tests were performed to investigate interfacial properties
Titanium matrix reinforced composites produced by H.I.P. of plasma sprayed preforms
No full textThe present work has been focussed on the fabrication of long fibre reinforced titanium composites by hot isostatic pressing (HIP) of plasma sprayed preforms. Silicon carbide long fibers were used together with two different metallic matrices, the first being a Ti-6Al-2Sn-4Zr-2Mo alloy and the second a Ti-4.5Al-3V-2Mo-2Fe alloy also named SP700. All the spraying trials were performed by using a Controlled Atmosphere Plasma Spraying equipment (CAPS), allowing deposition processes also in the high pressure range. After optimisation of the plasma spraying procedure, on the basis of results obtained with preliminary spraying trials carried out according to a DOE methodology composite multilayers were fabricated by HIP with different comibinations for the process parameters temperature/time/applied pressure, in the range 850-940°C, 30-45 min, 1200 bar They were then qualified by microstructural investigations and tensile testing at room and high temperature (up to 600°C). Obtained results are reported and discussed with particular reference to differences evidenced for the two titanium matrices
Fibre-matrix interactions in an intermetallic matrix composite, base Ni3Al, reinforced with SiC fibre
No full text- …
