1,721,018 research outputs found
Development and Characterisation of Aluminium Alloys and Aluminium Matrix Composites Produced via Laser Powder Bed Fusion
No full textNowadays, despite the growing interest about metal additive manufacturing, only a few commercial alloys used for casting are available for these technologies. Therefore, there is the necessity to develop new materials that can exploit the unique opportunities of additive manufacturing processes. The goal of this thesis was to create and characterize new aluminium based materials for powder bed additive manufacturing technologies. At first, the feasibility of the production of aluminium matrix composites by laser powder bed fusion (LPBF) was investigated. Three different composites were produced in order to study the effect of the reinforcement size and properties on the consolidation. Even if the introduction of a ceramic particles alters the consolidation phenomena of the base alloy, the LPBF process allows the production of dense and crack free aluminium matrix composites. The mechanical characterization of these composites revealed that the mechanical properties, and in particular the yield and ultimate strength, are strongly related to the building parameters used during the LPBF process. Regarding the study of existing or new alloys, it was observed that the laser scanning and the phenomena that arise in the melt pool allow the obtainment of an almost homogenous composition even when realizing the alloy in situ, i.e. starting from mixed powders of different composition. A specific alloy that takes advantage from the fast cooling that arises during the laser scanning, and that therefore has excellent mechanical properties, was selected in this study. For both composites and alloys, the laser-powder interaction and the consolidation phenomena were studied and compared by means of single scan tracks analyses which proved to be a promising solution to foresee the behavior of a specific material during the LPBF process. Finally, the effect of a post processing heat treatment on the microstructure on the mechanical properties of LPBF samples was analysed. In most of the cases, the as-built state can be considered as a supersaturated solid solution and therefore a direct-ageing heat treatment allows the achievement of enhanced mechanical properties. This effect was clearly visible on the A357 alloy and on a modified-7075 alloy, in which 15% and 20% respectively higher hardness values were achieved by the selection of the most suitable ageing heat treatment
In-situ alloying in laser-based additive manufacturing processes: A critical review
No full textThe growing need to develop cost-effective and efficient alloys requires enhancing the flexibility of the current production methods. Additive manufacturing (AM), as an emerging technology, has re-shaped the manufacturing strategies and largely influenced the industrial production lines. The feedstock, the main concern with regard to the powder-based AM methods, are mostly in the form of pre-alloyed powders. Pre-alloyed powders have a narrow composition range, suffer from limited availability and are expensive, making additive manufacturing of new alloys inflexible and costly. A growing number of works in the literature are dedicated to AM in-situ alloying, i.e. employing pure elemental blends as opposed to pre-alloyed ones for producing samples using additive manufacturing. This strategy gives added flexibility to the AM methods by benefiting from the laser local energy and paves the way towards an on-demand alloy design framework that is proportionate to the growing needs of the industry. This review is intended to shed light on the key components of AM in-situ alloying process, from initial blend preparation to requirements for the final composition homogeneity
Real-time laser spot modulation in Laser Powder-Directed Energy Deposition via wobbling: A path to superior 316L stainless steel components
Full text linkAAchieving high-quality Laser Powder-Directed Energy Deposition (LP-DED) components require the precise tuning of process parameters such as laser power, scanning speed, and laser spot size. Among these, laser spot size is a particularly critical factor. The use of a large spot size led to high building rate and process efficiency at the expense of surface finish and geometrical accuracy. This investigation aimed to understand how a variable laser spot affects the thermal history of LP-DED AISI 316L stainless steel components and consequently their microstructure and mechanical properties. The study first examined the effects of the laser spot and sample geometry on the microstructural features using optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Secondly, the characterization of mechanical properties was conducted via tensile tests and the effects of the variable laser spot on ductility and on the various strengthening mechanisms were analysed. The results revealed that using a larger laser spot leads to heat accumulation, which becomes especially crucial when depositing geometries characterized by a small cross section. These findings lay the groundwork for enhancing the LP-DED process efficiency in a more viable manner taking advantage of the different solidification conditions
Directed energy deposition of 18NiM300 steel: effect of process and post processing conditions on microstructure and properties
Full text linkThis current study investigates the effect of Direct Energy Deposition (DED) process conditions on the properties and microstructure of M300 maraging steel samples. The investigation centers on two key factors: laser power and deposition environment. The microstructure of this tool steel is analyzed by computing the Primary Cellular Arm Spacing. The findings revealed a significant influence of both inert atmosphere and laser power on cooling conditions. These different cooling rates influence the phase content as demonstrated by X-Ray Diffraction and Electron Backscatter Diffraction measurements. It was demonstrated the presence of different content of residual austenite at cell boundaries. These distinct microstructural features caused variations in the hardness values of the printed samples. Furthermore, a direct aging heat treatment was implemented, that was chosen from Differential Scanning Calorimetry measurements results. This heat treatment proves effective in achieving consistent hardness increases and eliminated the differences among samples built in different process conditions. This outcome suggests the possibility of selecting the most economically viable DED parameters for optimal results
Evaluation of a Laboratory-Scale Gas-Atomized AlSi10Mg Powder and a Commercial-Grade Counterpart for Laser Powder Bed Fusion Processing
Full text linkLaser powder bed fusion (LPBF) is an additive manufacturing technology that implies using metal powder as a raw material. The powders suitable for this kind of technology must respect some specific characteristics. Controlled gas atomization and post-processing operations can strongly affect the final properties of the powders, and, as a consequence, the characteristics of the bulk components. In fact, a complete characterization of the powders is mandatory to fully determine their properties. Beyond the most used tests, such as the volume particle size distribution (PSD) and flowability, the PSD number, the Hausner ratio and the oxidation level can give additional information otherwise not detectable. The present work concerns the complete characterization of two AlSi10Mg powders: a commercial-grade gas atomized powder and a laboratory-scale gas atomized counterpart. The laboratory-scale gas atomization allows to better manage the amount of the fine particles and the oxidation level. As a consequence, a higher particle packing can be reached with an increase in the final density and tensile strength of the LPBF bulk samples
Microstructural and mechanical characterization of AlSi10Mg – TiB2 composites produced by direct metal laser sintering technique
No full textLaser Powder Bed Fusion in-situ alloying of Ti-5%Cu alloy: Process-structure relationships
No full textAdditive manufacturing has emerged as a promising method for developing new Ti-based alloys. However, very few works have been dedicated to the in-situ synthesis of Ti-Cu alloys using Laser powder bed fusion (LPBF) method. The current work investigates the correlations between the volumetric energy density (VED) and some structural features of the in-situ Ti-5 wt.% Cu alloys produced by LPBF of the elemental powders. The results highlighted the importance of selecting the size of constituents in accordance with their thermophysical properties. It was also shown that crystallite size and lattice distortion of the α phase increased with increasing the VED. These were attributed to the higher temperature and lower cooling rate of the melt pools as well as improved particles melting and copper dissolution in the titanium lattice at higher VEDs. Isotropy of the microstructure was also improved at higher VEDs
Powder Reuse Assessment through Advanced Characterization Techniques for Additive Manufacturing Applications
No full textOne of the most promising features provided by additive manufacturing technologies is the possibility to increase significantly the material usage with respect to other conventional techniques. Consequently, powder reuse plays a key role in order to minimize waste. This option is both cost-saving and environmentally friendly. However, the feedstock material might be physically and chemically altered during processing. For example, morphological changes of the particles and consequently variations in the rheological behaviour of the powder might occur. The particles might also be compositionally altered. For instance, the Ti-6Al-4V alloy is significantly susceptible to oxygen enrichment, which results in a severe embrittlement of the material. In this work, the influence of the building process on the chemical and morphological properties of the powders was assessed; conventional (flow and bulk characteristics evaluation) and advanced (rheology, in-depth morphological analyses) techniques were adopted for analysing changes in powders features after their reuse
Tribological Behavior of Aluminum Alloy AlSi10Mg-TiB2 Composites Produced by Direct Metal Laser Sintering (DMLS)
No full textAn 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 di-
rections 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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