Papers by Author: Rui Vilar

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Authors: Lino Costa, Imre Felde, Tamás Réti, Zoltán Kálazi, Rogerio Colaço, Rui Vilar, Balázs Verő
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Authors: M.A. McMahon, Andrew Green, Ken G. Watkins, W.M. Steen, Mário G.S. Ferreira, Rui Vilar
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Authors: M. Sivakumar, Vitor Oliveira, Rui Vilar, A.M. Botelho do Rego
Abstract: Laser treatment is a promising technique for dental applications such as caries prevention, dental hypersensitivity reduction and improvement of bond strength of restoration materials. In this study the morphological, structural and chemical changes of enamel surface due to treatment with KrF excimer laser radiation were evaluated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. For radiation fluences near 1 J/cm², laser processing originates a relatively porous surface due to preferential removal of material in the enamel prism sheaths. Increasing the fluence leads to a relatively flat surface with clear evidence of surface melting. The X-ray diffractograms of both treated and untreated enamel are similar and correspond to hydroxyapatite. The only modification due to the laser treatment is a slight shift of the peaks, probably, due to a loss of the structural water of hydroxyapatite. X-ray photoelectron spectroscopy confirmed that organic matter is removed from the irradiated surface but no significant changes in the mineral phase occur.
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Authors: Rui Vilar
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Authors: Amélia Almeida, Patrícia Almeida Carvalho, Rui Vilar
Abstract: Surface alloys with composition ranging from 10 to 20% Cr were produced by laser surface alloying. Their microstructure consists of faceted plate-like Al4Cr intermetallic compound particles dispersed in a matrix of α-Al solid solution. During remelting, heterogeneous nucleation of eutectic Al7Cr/α-Al occurred in the undercooled liquid ahead of the columnar solid-liquid interface, followed by equiaxial solidification, resulting in a microstructure formed of equiaxed cells. Al-Cr alloys present Young’s modulus and hardness values that increase with increasing volume fraction of intermetallic compounds. Wear resistance, measured in dry sliding conditions, increases with increasing load due to the protective effect of a stable mechanically mixed layer that forms at the surface of the samples and the steel counterbody. Alloys formed of equiaxed eutectic cells provide better wear resistance than those formed of large plate-like particles since a thinner, more stable and harder mechanically mixed layer is formed, which offers best protection against wear.
490
Authors: Lino Costa, Rui Vilar, Tamás Réti
Abstract: The microstructure and properties of tool steel parts built by laser powder deposition (LPD) depend considerably on the build-up strategy and on the processing parameters used. This dependence can lead to inconsistent results which may limit the widespread acceptance of LPD. There is, thus, a need for efficient process optimisation tools that take into consideration the complex phase transformations that may occur during the part build-up process and their effect on final properties. A model coupling finite element heat transfer calculations with transformation kinetic theory has been developed, which allows the microstructure and property distributions in parts produced by LPD to be predicted. Application of this model to the deposition of tool steels not only explains the origin of the heterogeneous distribution of properties usually mentioned in the literature but also allows designing build-up strategies that consistently lead to homogeneous, high quality parts. Its application to the study of the influence of substrate pre-heating and idle time between the deposition of consecutive layers is illustrated in the present paper.
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Authors: M. Otasevic, Rogerio Colaço, Elisa Maria Ruiz-Navas, Elena Gordo, Rui Vilar
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Authors: R. Anandkumar, Rogerio Colaço, Václav Ocelík, Jeff T.M. de Hosson, Rui Vilar
Abstract: Surface coatings of Al-Si/SiC metal-matrix composites were deposited on Al-7 wt. % Si alloy substrates by laser cladding. The microstructure of the coatings was characterized by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microstructure of the coating material is profoundly influenced by the processing parameters used, in particular by the particle injection velocity and by the specific energy. When the injection velocity is low or specific energy is high excessive dissolution of SiC in the melt pool occurs. The microstructure of the coatings presents partially dissolved SiC particles, and considerable proportions of Al4SiC4 plates and faceted Si equiaxed crystals dispersed in a α-Al+Si eutectic matrix. On the contrary for high injection velocity or low specific energy dissolution of SiC is very limited and the microstructure of the coatings consists essentially of undissolved SiC particles in a matrix consisting of primary α-Al dendrites and α-Al+Si eutectic. Abrasive wear tests were performed on the coatings using a ball cratering device and a 35 wt. % suspension of 4.25 μm average diameter SiC particles in water as abrasive. Coatings prepared with a high specific energy present an average hardness of 248 HV and an average abrasive wear rate of 17.4x10-5 mm3/m. Coatings deposited with a low specific energy exhibit an average hardness of 117 HV and an average abrasive wear rate of 4.3 x10-5 mm3/m.
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Authors: Can Huang, Yong Zhong Zhang, Rui Vilar
Abstract: Due to the promising properties of high entropy alloy, TiVCrAlSi high entropy alloy coating was prepared on Ti-6Al-4V alloy surface by laser cladding. The microstructure and phase analysis of as-cast TiVCrAlSi alloy and laser clad coating were characterized by scanning electron microscope and X-ray diffraction. The results indicate that as-cast TiVCrAlSi alloy is mainly composed of (Ti,V)5Si3 and bcc phase. The laser clad coating is metallurgically bonded to the substrate with few pores and cracks. Laser clad TiVCrAlSi coating is also composed of (Ti,V)5Si3 and bcc phases.
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