1,720,980 research outputs found
Conventional and Innovative Titanium Alloys Produced Using Different Additive Manufacturing Technologies
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A study on the microstructure and mechanical properties of the Ti-6Al-2Sn-4Zr-6Mo alloy produced via Laser Powder Bed Fusion
Full text linkLaser Powder Bed Fusion (LPBF) is an additive manufacturing technology which has been the subject of thorough research and successfully adopted in several industrial sectors. Among all the processable classes of materials, titanium alloys are especially interesting due to their favourable combination of mechanical properties and corrosion resistance. Most of the literature focuses on Ti-6Al-4V, although there are other alloys which are widely applied in fields that can benefit from the advantages of LPBF techniques, such as Ti-6Al-2Sn-4Zr-6Mo, thus far not investigated for this technology. This alloy is generally preferred to Ti-6Al-4V for the production of some components in the aerospace industry, mostly due to its superior strength. In this work, the most suitable process window for this alloy was investigated. Samples produced with two different combinations of process parameters, located in the selected process window, were then thoroughly studied in order to assess the effect of building conditions on the microstructure, phases and mechanical properties of the as-built and heat-treated material. To do so, an X-ray diffraction analysis was conducted with the aim of determining the phase composition and lattice parameters. Moreover, microstructural features, such as α” needles and α lath widths, were analysed in order to correlate the thermal history of the process to the final microstructure of the specimens. Furthermore, the hardness and the tensile properties of the alloy processed by LPBF were quantified and compared with the data available in the literature relative to conventional manufacturing technologies
Towards customized heat treatments and mechanical properties in the LPBF-processed Ti-6Al-2Sn-4Zr-6Mo alloy
Full text linkThe Ti-6Al-2Sn-4Zr-6Mo alloy was recently successfully processed using the laser powder bed fusion technology. This material is characterized by a very low strength in the as-built state, due to the presence of α” martensite. In this work, the transformations taking place at different temperatures in this alloy were investigated with the aim of achieving the necessary knowledge to customize the mechanical properties via heat treatments. Martensite decomposition, residual stress relaxation and the β-transus temperature were identified at progressively increasing temperatures. Then, different annealing treatments, corresponding to the temperatures identified, were conducted. For each condition, dissimilar α + β microstructures were obtained, varying in terms of morphology (lamellar, bi-lamellar) and size of the α laths (0.5–2.2 μm). The corresponding tensile properties were also significantly different. In particular, outstanding strength values were achieved when the residual stress was retained. For higher temperatures, a slight drop in strength was coupled with an outstanding improvement in terms of ductility. Overall, the most balanced combination of tensile properties was obtained for an annealing temperature of 875 °C. In this condition, the bi-lamellar microstructure grants a reduction in the slip length due to the presence of secondary α laths
Single Scans of Ti-6Al-4V by Directed Energy Deposition: A Cost and Time Effective Methodology to Assess the Proper Process Window
Full text linkAbstract: Directed energy deposition is an additive manufacturing technology which usually relies on prototype machines or hybrid systems, assembled with parts from different producers. Because of this lack of standardization, the optimization of the process parameters is often a mandatory step in order to develop an efficient building process. Although, this preliminary phase is usually expensive both in terms of time and cost. The single scan approach allows to drastically reduce deposition time and material usage, as in fact only a stripe per parameters combination is deposited. These specimens can then be investigated, for example in terms of geometrical features (e.g. growth, width) and microstructure to assess the most suitable process window. In this work, Ti-6Al-4V single scans, produced by means of directed energy deposition, corresponding to a total of 50 different parameters combinations, were analyzed, focusing on several geometrical features and relative parameters correlations. Moreover, considering the susceptibility of the material to oxygen pick-up, the necessity of an additional shielding gas system was also evaluated, by comparing the specimens obtained with and without using a supplementary argon flow. A process window, which varies according to the user needs, was found together with a relationship between microstructure and process parameters, in both shielding scenarios
Advanced powder characterization for laser powder-bed fusion of AlSi10Mg
No full textLaser Powder-Bed Fusion is an additive manufacturing technique that allows to produce complex parts. Apart from process parameters, a key aspect of this system is powder quality, which strongly affects processability. In fact, a low powder quality, in terms of flowability, usually results in undesired phenomena, such as inadequate parts produced or machine clogging. It is then necessary to accurately determine whether a powder will flow properly or not. Currently, the most common flow test used is the Hall flow, which lacks accuracy for powders with intermediate properties. Hence the necessity to find new powder characterisation techniques. Varying tests have been performed, ranging from flowability, particle size distribution, powder permeability to compressibility. Some correlations between the flowing behaviour and other powder properties, such as permeability, have been found. Ranking samples according to their flowability was also possible, granting a deeper accuracy level with respect to the simple Hall flow test
Productivity Enhancement in Directed Energy Deposition: The Oscillating Scanning Strategy Approach
Full text linkDirected Energy Deposition (DED) is an additive manufacturing process that enables the production of large metal components by melting the feedstock material while being deposited. An improvement of the production speed of this process would further increase its applicability in many industrial fields. The DED building rate is strictly related to the building parameters adopted, in particular to the laser spot diameter, which also affects the build accuracy and the surface quality of the components. The possibility of using a variable laser spot would result in a significant increase in the production rate in bulky zones, while also providing a good surface quality where needed. In the present work, an oscillating scanning strategy was used to create a large apparent laser spot (+ 170% of the nominal value) to produce 316L stainless steel samples via DED. The optimisation of the DED parameters with the oscillating strategy was performed using the single scan tracks (SSTs) approach. The morphologies of the SSTs obtained with different process parameters were assessed and the geometrical features related to the melt pools were analysed in order to select the most suitable X and Z displacements for the production of the cubic samples. The analyses of the cubes revealed that, if the correct overlap among nearby scans is selected, it is possible to obtain dense samples with all the oscillating diameters tested. Finally, comparing the building rate and powder efficiency values confirmed that this method can accelerate the building process and improve its overall performance
Effect of Aging and Cooling Path on the Super β-Transus Heat-Treated Ti-6Al-4V Alloy Produced via Electron Beam Melting (EBM)
Full text linkThis work focuses on the effect of different heat treatments on the Ti-6Al-4V alloy processed by means of electron beam melting (EBM). Super β-transus annealing was conducted at 1050 °C for 1 h on Ti-6Al-4V samples, considering two different cooling paths (furnace cooling and water quenching). This heat treatment induces microstructural recrystallization, thus reducing the anisotropy generated by the EBM process (columnar prior-β grains). Subsequently, the annealed furnace-cooled and water-quenched samples were aged at 540 °C for 4 h. The results showed the influence of the aging treatment on the microstructure and the mechanical properties of the annealed EBM-produced Ti-6Al-4V. A comparison with the traditional processed heat-treated material was also conducted. In the furnace-cooled specimens consisting of lamellar α+β, the aging treatment improved ductility and strength by inducing microstructural thickening of the α laths and reducing the β fraction. The effect of the aging treatment was also more marked in the water-quenched samples, characterized by high tensile strengths but limited ductility due to the presence of martensite. In fact, the aging treatment was effective in the recovery of the ductility loss, maintaining high tensile strength properties due to the variation in the relative number of α/α’ interfaces resulting from α’ decomposition. This study, therefore, offers an in-depth investigation of the potential beneficial effects of the aging treatment on the microstructure and mechanical properties of the EBM-processed super β-transus heat-treated Ti-6Al-4V alloy under different cooling conditions
La strada verso la qualificazione delle tecnologie di manifattura additiva nel campo della corrosione: un caso studio della lega 625
No full textLe tecnologie di manifattura additiva (additive manufacturing - AM) stanno costantemente guadagnando popolarità in
diversi settori industriali strategici come quelli dell’Oil&Gas, aerospaziale e chimico. Questa famiglia di tecnologie,
alcune con principi molto diversi tra loro, genera componenti caratterizzati talvolta da microstrutture non in equilibrio
termodinamico capaci di conferire al materiale proprietà uniche. Tuttavia, risulta sempre più forte l’esigenza di appro-
fondire la relazione tra la microstruttura e le proprietà dei materiali, non solo dal punto di vista meccanico ma anche
rispetto alla corrosione che, in settori fortemente normati come l’Oil&Gas, è forse l’elemento più rilevante. Questo
lavoro affronta lo studio del comportamento a corrosione di una superlega di nichel, nota come lega 625, al fine di de-
finire la relazione tra la tecnologia produttiva e il comportamento in ambiente aggressivo. Tutte le valutazioni effettuate
hanno permesso di stabilire una connessione tra le diverse tecnologie AM adottate, la microstruttura e il comporta-
mento a corrosione del materiale. Il lavoro pone le basi per ulteriori studi, volti a stabilire quale tecnologia AM possa
essere più adatta per applicazioni specifiche
Improving the Corrosion Performance of LPBF- and EBM-Processed Ti-6Al-4V by Chemical Pickling
No full textTi-6Al-4V is a popular material in the biomedical industry for orthopedic prosthetics production. Moreover, this alloy is well-processable via additive manufacturing (AM) technologies, allowing to tailor the design of the implant according to the specific needs of each individual patient. Nevertheless, AM technologies deploy metal powders, resulting in very rough topologies due to partially melted/adhered residual particles on the surfaces generated. Although this promotes osseointegration, corrosion-induced particle dropping can result in a severe inflammatory response in the patient. To overcome this, a pickling treatment was specifically developed and optimized to decrease the concentration of residual particles, without compromising surface roughness. Specimens produced via laser- and electron beam-powder bed fusion (PBF) were investigated. Three different surface finishing conditions (AM-generated, polished and pickled) were also compared via potentiostatic polarization tests. The specimens that underwent the pickling process proved to achieve lower current densities for long term exposures in simulated body fluid (SBF). Another critical phenomenon that occurs in prosthetics is the release of metal ions over time. To assess this issue, multiple electrochemical tests (potentiostatic polarization, electrochemical impedance spectroscopy) were deployed to assess the effect of the different PBF technologies and heat treatments on the ions release rate of Ti-6Al-4V in SBF
An investigation on the effect of different multi-step heat treatments on the microstructure, texture and mechanical properties of the DED-produced Ti-6Al-4V alloy
Full text linkThis work deals with the effect of different heat treatments on directed energy deposition (DED)-produced Ti-6Al-4V samples. Annealing treatments at 1050 °C followed by different cooling rates were conducted to allow a complete recrystallization of the microstructure and remove the columnar prior-β grains, thus increasing the overall isotropy of the material. An agine treatment at 540 °C was also performed for further microstructural stabilization. The microstructures, textures and mechanical properties were then assessed. Due to the heat treatments, greatly differing microstructures were achieved in an equiaxed grain morphology. However, a “grain memory” effect was detected which resulted in the grains size increasing along the height of the samples. This effect was correlated to the intrinsic prior-β grain width variation along Z on the as-printed specimens, typical of the DED technology. Electron backscatter diffraction analyses proved that the intensity of the preferential directions increased after the heat treatments, likely due to the crystallographic variant selection mechanisms taking place when the samples cool down from the annealing temperature. This effect is also influenced by the significant difference in terms of prior-β grains sizes between the heat-treated and the as-printed specimens. To sum up, a complete homogenization of the material via heat treatment above the β-transus temperature proved to be challenging. In fact, the data suggest that the intrinsic texture-related anisotropy granted by the manufacturing process is very difficult to be eliminated
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