Examinando por Autor "Rams, J."

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Dry sliding wear behavior of in situ Ti6Al4V/TiN composites 3D manufactured by Laser-Directed Energy Deposition
(Elsevier, 2025-06-03) Sánchez de Rojas Candela, C.; Riquelme, A.; Rodrigo, P.; Torres, B.; Rams, J.
In this work, the wear behavior of Ti6Al4V/TiN composites additively manufactured by laser-directed energy deposition has been studied. The substitution of argon for nitrogen created a reactive atmosphere during the fabrication process, triggering a direct reaction between the molten Ti6Al4V and the nitrogen molecules, creating in situ TiN ceramic reinforcement within the Ti6Al4V metal matrix. The effect of fabrication parameters, such as laser power and scanning speed, on the wear resistance of the different samples was evaluated and compared with the unreinforced base material, which was manufactured under argon. Samples were classified according to heat input: 37.5, 75, 95, and 190 J/mm. Wear tests were performed using a pin-on-disc configuration, employing 316L stainless steel as the counterbody material. The observed wear mechanisms were subsequently identified based on the test results. The influence of microstructure and microhardness on the coefficient of friction, mass loss of pins and counterbodies, and wear rate of each material was also evaluated. The percentage and distribution of the reinforcement generated during fabrication depended on the percentage of nitrogen diffusing into the titanium, which was intrinsically linked to the fabrication parameters. The nitrogen uptake increased with higher heat input, thereby affecting the wear rates and wear mechanisms. The measured Ti6Al4V/TiN Vickers microhardness was double that of the unreinforced samples, although this trend decreased at higher laser power. The Ti6Al4V/TiN sample manufactured with a heat input of 75 J/mm exhibited the highest microhardness (1300 HV2). The formation of TiN reinforcement led to reduced pin mass losses and wear rates, as well as increased coefficients of friction. The Ti6Al4V/TiN samples fabricated at 37.5 and 95 J/mm exhibited the lowest wear rates (2,5 x 10–13 m3/Nm). An abrasive wear mechanism was observed in all Ti6Al4V/TiN samples, predominating when the laser scanning speed was lower (75 and 190 J/mm), due to greater nitride phase formation, which enhanced hardness. This mechanism combined with an oxidative component at a heat input of 190 J/mm, where a mechanically mixed layer was formed, while combining with an adhesive wear mechanism at higher scanning speeds (37.5 and 95 J/mm).
Impact of molten salts composition on the corrosion behavior of NiMoCr and CoNiCrAl coatings on L-PBF 316L stainless steel for CSP plants
(Elsevier, 2024-04-30) Abu-warda, Najib; Bedmar, J.; García-Rodríguez, S.; Utrilla, M.V.; Torres, B.; Rams, J.
The high-temperature corrosion performance of NiMoCr and CoNiCrAl coatings produced by high velocity oxy-fuel (HVOF) on laser powder bed fusion (L-PBF) 316L stainless steel substate has been evaluated in presence of three different molten salts used as thermal energy storage (TES) materials for concentrated solar power (CSP) plants. The coatings have an excellent oxidation resistance at 700 °C, showing a percentage of affected thickness lower than 2 %, due to the formation of a protective layer of Cr2O3 and Al2O3 in both NiMoCr and CoNiCrAl coatings, respectively. The carbonates-based molten salt accelerates the corrosion of the NiMoCr coating, affecting around 43 % of the coating thickness, due to the formation of chromates and the high depletion of Mo in the coating. In contrast, the effective protection shown by the CoNiCrAl coating is explained because the grown LiAlO2 after the lithiation process has a high capacity to act as a strong diffusion barrier for metal ions and inhibited the formation of chromates. In the presence of two different ZnCl2 and MgCl2-rich molten salts, the CoNiCrAl coating behaved worse than NiMoCr coating due to the breakage of the (Al,Cr)-rich oxide layer as a consequence of an active oxidation mechanism and due to the formation of voids along the entire thickness, which were identified in 100 % of the coating thickness. The superior corrosion resistance of the NiMoCr coating in presence of the chloride rich molten salts was attributed to the presence of Mo, which fixed the Cr and inhibited the diffusion of oxygen
Microstructure and corrosion behavior of 316L stainless steel lattice and bulk parts manufactured by LPBF using fiber and CO2 lasers
(Elsevier, 2024-08) García-Rodríguez, S.; Bedmar, J.; Abu-warda, N.; Torres, B.; Rams, J.
AISI 316L stainless steel lattices were manufactured using two distinct Laser Powder Bed Fusion (LPBF) techniques: a commercial printer equipped with a fiber laser and a novel system equipped with a CO2 laser. Fiber laser systems produce parts exhibiting superior detail, finer cell size, and lower porosity compared to CO2 laser ones because of the stability of the laser beam and the higher absorptivity of the laser light by the material powder. Electrochemical tests indicated that lattices exhibited inferior corrosion resistance compared to solid counterparts for both manufacturing techniques. This disparity is caused by the distinct corrosion mechanisms exhibited by lattices and solid parts, as observed through Electrochemical Impedance Spectroscopy (EIS). The observed differences are attributed not only to the increased different specific surface areas but also to the distinctive morphologies formed in the material beneath the surface. CO2 LPBF lattices demonstrated higher corrosion sensitivity than the fiber LPBF lattices due to their distinct microstructure and the more prevalent presence of defects
Ti6Al4V/SiC Metal Matrix Composites Additively Manufactured by Direct Laser Deposition
(Springer, 2022) Sánchez de Rojas Candela, C.; Riquelme, A.; Bonache, V.; Rodrigo, P.; Rams, J.
Nowadays, research on additive manufacturing of Ti6Al4V alloy is growing exponentially but there are just a few studies about additive manufacturing of metal matrix composite components. In this work, highly reinforced Ti6Al4V matrix composites with SiC particles have been additively manufactured by direct laser deposition (DLD). Ti6Al4V powder and SiC particles have been deposited layer by layer to form an additive thin wall structure. The geometry, microstructure, and microhardness of the samples are strongly infuenced by the laser scanning speed used during de fabrication process. In addition, the efect of the SiC increment in reinforcement concentrations and the infuence of SiC particle sizes in the microstructure have been evaluated, and the reaction mechanisms have been established. The percentage of reinforcement measured is lower than expected due to the reinforcement-matrix reactivity that results in partially dissolved SiC particles and the formation of a TiC and Si5Ti3 ring around them. The size and number of particles and reaction products depend on the initial size and percentage of reinforcement and the DLD scanning speed. The higher the size and percentage of SiC particles and reaction products in the matrix, the higher the hardening efect of the composite matrix.