Papers by Author: V.A.R. Henriques

Abstract: The alloy design and efficient routes of TiAl processing are important technological challenges for the development of new aerospace systems. Gamma-TiAl alloys are potential replacements for nickel and conventional titanium alloys in hot sections of turbine engines, as well as in sub-structures of orbital platform vehicles. Powder metallurgy (P/M) of Ti-based alloys may lead to the obtainment of components having weak-to-absent textures, uniform grain structure and higher homogeneity compared with conventional wrought products. This paper aims to investigate the microstructural evolution and densification aspects involved in the obtainment of Ti-48Al-2Cr-2Nb (at.%) alloy by three P/M-processing routes. Samples were prepared from elemental and pre-alloyed powders mixed for 2 h, followed by cold uniaxial and isostatic pressing followed by sintering and hot pressing stages between 1100°C up to 1400°C, for 1 h. After metallographic preparation, sintered samples were characterized by means of scanning electron microscopy (SEM) in the backscattered mode (BSE), X-ray diffraction (XRD), and density measurements. The results showed the potential of TiAl pre-alloyed powders to prevent Kirkendall porosity. A full lamellar microstructure was obtained by the pressureless route while a duplex microstructure was observed in samples produced by the hot pressing route.

Abstract: The use of hydrogenated titanium powders combined with traditional PM techniques may lead to a significant reduction in the manufacturing costs of titanium components. In this work, the advantages and limitations of the use of TiH₂ powder consolidated through the conventional press-and-sinter method were investigated. Processing parameters related to the compaction and sintering were studied for a TiH₂ powder in the particle sizes <355 μm, <150 μm and <45 μm. Optimized compaction conditions were achieved by using admixed lubricant and compaction pressure of 800 MPa. The mechanisms involved in the compaction of powders were detailed through the fit of compressibility data to a theoretical model originally developed for titanium powders. Densification of samples was favored by the reduction in particle size and increase in sintering temperature up to 1300 °C. The positive effects of hydrogen release during dehydrogenation were verified through the results of sintered densities and the reduction of oxygen levels. Limitations were observed mainly regarding the flowability of powders and the difficulty to achieve full densification.

Abstract: Titanium nitride (TiN) is a hard material, often used as coating to improve the wear properties of titanium alloys in machining, implant and aerospace applications. Electron Beam Physical Vapor Deposition (EB-PVD) is a technique which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum, producing a thin film in a substrate. In this work, results of TiN films depositions on Ti-13Nb-13Zr substrates by EB-PVD are studied. Titanium targets were obtained by a purified ingot and the substrates produced by powder metallurgy. Sintered samples of Ti-13Nb-13Zr and TiN layers were characterized by X-ray diffraction, scanning electron microscopy, Vickers microhardness and wear tests. The TiN films presented high hardness values, continuity and large thickness. The coatings improved the tribological properties of the substrate due to high adhesion and low wear rate.

Abstract: The use of hydride powders in titanium powder metallurgy (P/M) is a low cost alternative for the manufacture of titanium alloys. However, due to the high reactivity of these powders, parts produced using this technique may contain interstitial impurities such as oxygen, nitrogen and carbon. In this work a factorial design approach was used to evaluate the influence of some stages of P/M upon the levels of these elements in sintered samples of Ti-6Al-4V. Milling time of titanium hydride powders, sintering temperature and holding time were evaluated. The effect of milling time was detected as the most significant for the increase in oxygen levels. The contents of all elements were affected by the increase of sintering temperature from 1200 °C to 1400 °C. Holding time was shown to be significant only for the carbon absorption in the samples sintered at 1400 °C.

Abstract: During the recent years, alloys based on the intermetallic compound TiAl have attracted a considerable interest as potential competitors to steels and superalloys. Gamma-TiAl alloys are potential replacements for nickel and conventional titanium alloys in hot sections of turbine engines, as well as in orbital platform vehicles. The alloy design and efficient routes of TiAl processing are important technological challenges. Powder metallurgy is a near net shape process that allows the parts production with complex geometry at low costs. In this work, samples of Ti-48Al-2Cr-2Nb (at.%) were prepared from elemental and pre-alloyed powders mixed for 2 h, followed by cold uniaxial and isostatic pressing and sintered between 800 up to 1400°C, for 1 h, under vacuum. After metallographic preparation, sintered samples were characterized by SEM (Scanning Electron Microscopy), density analyses and Vickers microhardness measurements. The results indicated the viability of the pre-alloyed route and the tendency of a full lamellar microstructure of alternating gamma and α₂ phases in high sintering temperatures.

Abstract: Multilayer coatings are synthetic structures constituted by alternate layers of different materials. The technological applications of the multilayer coatings can involve optical, electromagnetism and wear areas. The target of this work is the production of multilayer TiN/ZrN coatings by Electron Beam Physical Vapor Deposition (EB-PVD) over titanium alloys produced by powder metallurgy. P/M-Ti-35Nb-7Zr-5Ta cylindrical samples used as substrates were produced by mixing of the elemental powders with subsequent cold pressing steps and sintering at 1400°C, in high vacuum. TiN/ZrN coatings were obtained by evaporation of alternating Ti and Zr cylindrical targets. The multilayers were characterized by optical and scanning electron microscopy (SEM), chemical analysis via energy dispersive spectrometry (EDS) and Vickers indentation. Besides the possibility of obtaining several layers, the results show coatings with coherent columnar structure, low discontinuity, large and homogenous thickness and high adhesion to substrate.

Abstract: Ti-35Nb-7Zr-5Ta alloy is a promising new material for a bone graft substitute with good strength properties and an elastic modulus closer to that of bone than any other metallic material. TNZT samples until 50 vol % porosity were manufactured using ‘‘space holder’’ technique and sintering methods. Irregular ammonium carbonate powders were used as a space holder material. Complete removal carbonate from the green compact was achieved by heating at 200 °C for 5 hours and subsequent sintering at 1600 °C, with heating rate of 10 °C/min. For the alloy microstructural characterization, scanning electron microscopy was used. Density was measured by Archimedes method. The results show that the blended elemental P/M process and the space holder technique are efficient for the obtainment of highly porous samples. Foams with porosities in the range between 10% and 50% could be reached.