Powder Metallurgy of Titanium II

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Authors: Matt Dunstan, Z. Zak Fang, Mark Koopman, James Paramore
Abstract: Hydrogen sintering phase transformation (HSPT) is a low-cost, blended elemental, press and sinter powder metallurgy process. During HSPT, compacts of TiH2 powder are sintered in dynamically controlled partial pressures of hydrogen followed by a vacuum anneal (dehydrogenation). The use of hydrogen in the sintering atmosphere allows phase transformations in the Ti – H system to create an ultra-fine lamellar microstructure in the as-sintered state with mechanical properties that exceed ASTM standards. Additionally, the fine lamellar structure allows for secondary heat treatments to produce wrought-like microstructures. The removal of hydrogen in the dehydrogenation step is critical to prevent hydrogen embrittlement. The kinetics of dehydrogenation are discussed, in which a model for the concentration profile and an empirical equation for maximum hydrogen concentration as a function of time and size are developed.
Authors: Sergey V. Konovalov, Irina Komissarova, Dmitry Kosinov, Yurii F. Ivanov, Victor Gromov, Olga Semina
Abstract: The processing of commercially pure titanium VT1-0 with high intensity pulsed electron beam was carried out and the mode of irradiation allowing the increase in fatigue life of the material was revealed. Structure investigations of modified layer and fracture surface of commercially pure titanium samples subjected to fatigue multicycle tests were implemented. By methods of scanning electron microscopy the structural transformations responsible for increase in fatigue life of titanium irradiated with high intensity pulsed electron beam were revealed. It was shown that irradiation of commercially pure titanium of grade VT1-0 with high intensity electron beam (25 J/cm2, 150 μs, 3 pulses) resulted in the refinement of grain structure, formation of multilayer structure. Irradiation facilitated the formation of additional structural levels of micro and nanosize range in surface layer.
Authors: Wolfgang Limberg, Thomas Ebel
Abstract: For highly loaded parts, such as medical implants as well as for engineering application, high strength and good ductility are indispensable, but also the fatigue resistance plays an important role. For both, quasi static and fatigue properties, a fine microstructure is essential. Furthermore, especially for titanium alloys the control of interstitial impurities like oxygen and nitrogen is very important. In a former study additions of several oxides of rare earth elements were tested with respect to their effect on the microstructure of MIM processed Ti-6Al-4V. Yttrium oxide has shown the strongest effect on the colony size of Ti-6Al-4V.In this study elementary yttrium powder was added to Ti-6Al-4V. During sintering, the yttrium scavenges oxygen out of the titanium matrix, increasing the ductility. The yttrium oxide, which is formed, leads to a colony refinement and therefore to a higher strength.For preliminary tests cylindrical samples consisting of Ti-6Al-4V powder blended with coarse yttrium powder and a wax and polyethylene-based binder were manufactured by uniaxial pressing and sintering. The addition of yttrium led to a slight decrease of the colony size: specimens sintered at a high temperature of 1400 °C show a stronger dependency of yttrium content on the colony size than those sintered at 1300 °C.Tensile test specimens were produced by MIM using gas atomized Ti-6Al-4V-powder with additions of yttrium powder between 0.1 and 0.5 wt.-%. Sintering took place at temperatures between 1300 °C and 1400 °C with two different dwell times (2 and 4 hours). Tensile tests were conducted in air at room temperature. The microstructures were observed by SEM and light optical microscope. Oxygen and nitrogen contents were analysed by a melt extraction technique.
Authors: Elena Gordo, Roberto G. das Neves, Begoña Ferrari, Antonia Jiménez-Morales, Andreia Lima, Alexandra C. Alves, Ana Maria Pires Pinto, Fatih Toptan
Abstract: This work focuses in the corrosion and wear properties of titanium reinforced with 1% wt. alumina particles, produced by a combination of colloidal techniques and powder metallurgy. The alumina particles were added to control the grain growth of titanium during sintering, and simultaneously to increase hardness and wear resistance. Colloidal techniques permitted a homogeneous dispersion of alumina particles on the surface of fine Ti particles by the formulation of stable aqueous suspensions that were further processed by spray-dry to obtain spherical granules with improved compressibility. Ti-alumina samples were produced by uniaxial pressing of granules and vacuum sintering leading to materials with homogeneous microstructure, a reduction of grain size higher than 50 % with respect to pure titanium, and a sensible increase in hardness. But the addition of ceramic particles can also have an influence on the corrosion behavior that is one of the most interesting properties of titanium alloys, and on wear resistance, that is one of the drawbacks of Ti. Moreover, the study of simultaneous action of wear and corrosion (tribocorrosion) is an area of highest interest in applications like biomedical or automotive. The corrosion behavior was evaluated by Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PP) in NaCl at two concentrations (0.9 % and 3.5 %) and temperatures (37 oC, and room temperature). Tribocorrosion tests were performed using a reciprocating ball-on-plate tribometer where a 10 mm diameter alumina ball was used as counter material, and 10 N normal load was applied during 30 min in the same concentrations and temperatures of NaCl as in the static corrosion tests. The results showed a clear improvement of wear resistance on the composite without reducing the corrosion behavior in both conditions.
Authors: Erich Neubauer, Lilla Vály, Michael Kitzmantel, David Grech, Ana Rovira Ortega, Isabel Montealegre-Meléndez, Cristina Arévalo
Abstract: Materials with specific stiffness value above 100 [GPa/(g/cm3)] are typically fiber‑reinforced materials. These materials suffer from the fact that they have anisotropic behavior which means high specific stiffness values can only be obtained in the direction parallel to the fiber. In order to obtain materials with a specific stiffness >60 in all directions, several Titanium based composites have been screened. Fillers based on B4C particles have been identified as most promising to reinforce a Titanium or Titanium alloy matrix for this purpose. For the manufacturing of the composites a rapid hot pressing technique (similar to Spark Plasma Sintering) was used. Besides the assessment and characterization of the Young´s Modulus and the hardness the impact of processing parameters on the microstructure was also investigated.
Authors: Alexandra Amherd Hidalgo, Thomas Ebel, Wolfgang Limberg, Florian Pyczak
Abstract: One of the challenges in PM Ti alloys is to control the impurities level. Oxygen affects the microstructure and the mechanical properties of titanium alloys. Ti-6Al-7Nb is a promising alloy to use in PM due to its outstanding biocompatibility and mechanical properties required for load bearing medical implants. In this work, the influence of the impurities content on the ductility, fatigue resistance and microstructure of Ti-6Al-7Nb alloy processed by metal injection moulding was examined. Tensile and fatigue specimens were manufactured using Ti-6Al-7Nb gas atomized powder. Depending on the thermal treatment time, various oxygen contents were introduced into the specimens. The resulting oxygen content was determined by melt extraction technique. Tensile tests and high cycle four-point bending fatigue tests at room temperature were performed. First studies about the effect of oxygen content on crack initiation and propagation were done by the observation of microstructures and fractured surfaces using light and electron microscopy (SEM).
Authors: Yi Feng Zheng, Xun Yao, Yong Jun Su, De Liang Zhang
Abstract: In this work, in-situ TiC reinforced Ti matrix composites (TMCs) have been fabricated via blending TiH2 powder and multi-walled carbon nanotubes (CNTs) followed by thermomechanical consolidation of the TiH2/CNTs powder mixture. The dehydrogenation, in situ reaction and consolidation occurred simultaneously and took less than 15 minutes in total. The effect of CNTs content (1 and 3 vol.% (0.56 and 1.69 wt.%)) on the evolution of microstructures and mechanical performances of the extruded samples has been investigated. The results showed that the extruded TMCs had a duplex microstructure consisting of coarse alpha titanium grains and ultrafine grained (UFG) regions, and the in-situ formed TiC particles had a near-spherical shape. The extruded sample with 1 vol.% (0.56 wt.%) CNTs reinforced exhibited a yield strength of 807.3 MPa, ultimate tensile strength of 1085.9 MPa and elongation to fracture of 3.3% at room temperature. The mechanism of microstructural evolution and material failure are discussed.e are discussed.
Authors: Ming Tu Jia, Brian Gabbitas
Abstract: Powder compact forging in combination with induction sintering, a field assisted sintering technique (FAST), was used to produce commercially pure (CP) Ti and Ti-13V-11Cr-3Al parts. Green powder compacts with high relative density were manufactured by cold compaction and warm compaction, respectively. During the powder compact forging process, CP titanium powder was consolidated completely to produce a near net shaped top cover for a diving helmet with full density and good mechanical properties. Also, a Ti-13V-11Cr-3Al alloy was fully consolidated into a cylinder using blended elemental powders. As a comparison, raw titanium powder with different oxygen contents was used to make a Ti-13V-11Cr-3Al powder compact forging. Using a starting powder with low oxygen content, a forged cylinder with good mechanical properties was produced.
Authors: Fei Yang, Brian Gabbitas, Ajit Pal Singh, Stiliana Raynova, Hui Yang Lu, Barry Robinson
Abstract: Blended Elemental Powder Metallurgy (BE-PM) is a very attractive method for producing titanium alloys, which can be near-net shape formed with compositional freedom. However, a minimization of oxygen pick-up during processing into manufactured parts is a big challenge for powder metallurgy of titanium alloys. In this paper, different approaches for preparing titanium alloy parts by powder compact extrusion with 0.05-0.1wt.% of oxygen pick-up during manufacturing are discussed. The starting materials were a powder mixture of HDH titanium powder, other elemental powders and a master alloy powder. Different titanium alloys and composites, such as Ti-6Al-4V, Ti-4Al-4Sn-4Mo-0.5Si, Ti-5Al-5V-5Mo-3Cr, and Ti-5Al-5V-5Mo-3Cr-5vol%TiB, with different profiles such as round and rectangular bars, a wedge profile, wire and tubes have been successfully manufactured on a laboratory and pilot-plant scale. Furthermore, a possible route for scaling up the titanium processing capabilities in the University of Waikato has also been discussed.

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