The technologies of titanium powder metallurgy

Key Engineering Materials, 2013

The Ti-6Al-7Nb alloy was specially developed to replace the well-known Ti-6Al-4V alloy in biomedical applications due to supposed cytotoxicity of vanadium in the human body. This alloy is normally fabricated by conventional ingot metallurgy by forging bulk material. Nevertheless, powder metallurgy techniques could be used to obtain this alloy with specific properties. This is because by changing the processing parameters, such as the sintering temperature, it is possible to vary the porosity level and to tailor the final properties. This work deals with the production of the Ti-6Al-7Nb alloy by means of the master alloy addition variant of the blending elemental approach. The powder is processed by means of different powder metallurgy routes considering diverse processing conditions for each method. The materials are characterised in terms of microstructural features, relative density and hardness. Homogeneous microstructures as well as properties comparable to those of the wrought alloy are generally obtained.

Defence Science Journal, 2015

IntroductIon Titanium is a low-density material having non-magnetic property and stands in the middle of aluminium and steel with good specific strength. Ti-alloy has disadvantage for high temperature applications compared to Ni-base superalloy where creep resistance is the primary requirement. Allowable temperature capability of this alloy if 0.4 T m , whereas for Nibase superalloy it is 0.9 T m. Density of titanium is 60 per cent of Steel and is considered as one of the work horse material for aerospace applications. Pure titanium has excellent resistance to corrosion by forming passive titanium oxide layer. Chemical plants using steel vessels are clad with titanium due to its corrosion resistance. Ti-alloy has wide applications in cold-end rotating and non-rotating components of gas turbine engines like fan disk, blades, casings, nozzle guide vanes, etc 1-3. Usage of Ti-alloy is restricted to automobile industries due to its high processing cost. For structural applications, Ti-alloys are strengthened by suitable alloying additions 4-10. Titanium shows allotropic property at high temperature. At room temperature, it exists as hexagonal close pack (HCP) crystal structure (α-phase), but at temperature beyond 883 °C, it transforms into body centre cubic (BCC) crystal structure and this BCC structure (β-phase) remains stable till it reaches meting temperature. For structural applications this material is further strengthen by suitable alloying addition by exploiting the urge of alloying with large number of elements. All elements within a range of atomic radius of 0.85-1.15 of Titanium, form substitutional solid solution. Elements with atomic radii less than 0.59 have the tendency to form interstitial solid solution. Tendency to form solid solution makes the Ti-alloy system difficult to form precipitation harden alloy. The alloying elements like Al, Ga, O, and N have the tendency to stabilise the α-phase at higher temperature by rising the β transus temperature. Mo, V, W, and Ta are β stabilisers and form solid solutions. Another set of β stabilisers like Cu, Mn, Fe, Ni, Co, H form eutectoid. Alloying elements like Zn, Sn, and Si play neutral role in Tialloy. Based upon the alloy content and the resultant crystal structure, the Ti-alloys are categorised into three groups; alpha (α)-alloy, β (beta)-alloy, and α-β (alpha-beta) alloy systems. The composition of α and β can be adjusted in such a way that optimum combination of creep, fatigue, and yield strength can be achieved based upon the component and its application. This can be achieved by applying suitable thermomechanical treatment to this alloy along with appropriate heat-treatment cycle. Till date, cold-end components of a gas turbine engine are dominated by Ti-alloys. Ti-6Al-4V is widely used α-β

Key Engineering Materials, 2012

Metal injection moulding (MIM) attracts growing interest as an economic net-shape manufacturing technique for the processing of titanium and titanium alloys. Even for titaniumaluminides, intended for high-temperature applications, MIM is seen as a reasonable technique to overcome processing problems with conventional methods. In this paper, basic requirements in terms of raw materials, facilities and processing in order to produce high performance components are presented. Main focus is laid on the well-known Ti-6Al-4V alloy. It is shown that the tensile properties of specimens after MIM processing can exceed the requirements given by ASTM standards even without performing an additional HIP process. For an oxygen content ranging from 0.15 to 0.33 wt% plastic elongation yields excellent 14%. Fatigue measurements performed by means of 4-point-bending tests show that grain size is more important than residual porosity in order to achieve a high endurance limit. This is shown by addition of boron powder which refines the microstructure dramatically. The modified alloy Ti-6Al-4V-0.5B yields an endurance limit of 640 MPa compared to 450 MPa of MIM parts made from standard alloy powder. Sintered components from Ti-45Al-5Nb-0.2B-0.2C (at%) powder made by inert gas atomising (EIGA technique) and processed by MIM exhibit a residual porosity of only 0.2% and tensile properties comparable to cast material.

Materials Research, 2005

Titanium alloys have several advantages over ferrous and non-ferrous metallic materials, such as high strength-toweight ratio and excellent corrosion resistance. A blended elemental titanium powder metallurgy process has been developed to offer low cost commercial products. The process employs hydride-dehydride (HDH) powders as raw material. In this work, results of the Ti-35Nb alloy sintering are presented. This alloy due to its lower modulus of elasticity and high biocompatibility is a promising candidate for aerospace and medical use. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by isochronal sintering between 900 up to 1600 °C, in vacuum. Sintering behavior was studied by means of microscopy and density. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Samples sintered at high temperatures display a fine plate-like α structure and intergranular β. A few remaining pores are still found and density above 90% for specimens sintered in temperatures over 1500 °C is reached.

Materials Chemistry and Physics, 2012

Inductive hot-pressing is a field-assisted sintering process (FAST) in which an electrical current is used to enhance the densification of the powder. Inductive hot-pressing could be employed to enable titanium powder to reach a higher density in less time than the pressing and sintering process. In this study, titanium and titanium alloy powders with different features were processed by means of inductive hotpressing. The influence of processing temperature on density, microstructure, quantity of interstitial elements and hardness was investigated. Generally, practically fully dense materials were obtained without any carbon pick-up, even if the materials were in contact with the graphite matrix during processing. Nevertheless, there was an increment of the nitrogen content and some oxygen pick-up, especially for the powders with smaller particle size. Hardness is not significantly affected by the pressing temperature, but it strongly depends on the amount of interstitials.

Computer Methods in Material Science, 2019

Titanium alloys are mainly used in the automotive, aviation, shipbuilding and military industries. Their main advantages are low specific gravity, resistance to cracking and corrosion, high strength as well as fatigue strength. The most important disadvantages of titanium alloys include low thermal conductivity, difficulties in their machining and high cost of manufacturing. For the latter reason, titanium alloys are primarily used for the manufacturing of highly responsible components, such as implants and aviation structures, while the remaining products are produced in limited series. In the appropriate conditions, many titanium alloys can be formed in hot working processes. At present, in the processes of manufacturing structural elements of titanium alloys, semi-finished products obtained by the casting method are commonly used. However, more and more research is being carried out on the use of powder metallurgy based material in this field. This approach opens up the possibility of decreasing production costs. As initial material, the alloy powders or mixtures of elemental powders can be used. The properties of alloy powder products are usually high and stable, however, the cost of powder production is high. Obtaining a product from titanium alloys based on a powders mixture is relatively simple and significantly cheaper. The disproportion of prices causes, that a great number of research projects realized in recent years in the field of implementation of powder metallurgy for manufacturing titanium-based products is directed towards the use of powder mixtures since this approach gives real chances for the successful implementation of costeffective titanium alloys processing technology.

Materials Science and Engineering: C, 2005

The microstructure of Fe 30 Ni 20 Mn 20 Al 30 in both the as-cast condition and after annealing at 823 K for various times up to 72 h was characterized using transmission electron microscopy, scanning transmission electron microscopy, synchrotron-based X-ray diffraction, and atom probe tomography. The microstructure exhibited a basketweave morphology of (Mn, Fe)-rich B2-ordered (ordered b.c.c.) and (Ni, Al)-rich L2 1 -ordered (Heusler type) phases with a lattice misfit of only 0.85 % and interfaces aligned along h100i. The phase width increased from 5 nm for the as-cast alloy to 25 nm for 72 h annealed material, with no change in the elemental partitioning between the phases, with a time exponent for the coarsening kinetics of 0.19. Surprisingly, it was found that the room temperature hardness was largely independent of the phase width.

Key Engineering Materials, 2012

A novel powder metallurgical technique for the fabrication of titanium alloys has been developed by utilizing a pressure-assisted, resistance-heating sintering technique. In this technique, the high electrical resistance of oxide layers present on the surface of powder particles has been exploited to ensure effective resistance heating of green compacts. Ti-6Al-4V pre-alloyed powders of 100 µm size were compressed while being heated under a variety of conditions of sintering temperature, pressure and time. The outcomes of our experiments have proven that resistance heating can be a very effective means of heating during powder consolidation. The results have indicated that the required sintering time and temperature in the new resistance-heated sintering technique are much reduced in compared to sinter-press and/or hot isostatic pressing techniques, resulting in a refined microstructure with a concomitant improvement in mechanical properties.

2015

This work focuses on optimizing the sintering process of Ti-6Al-4V using Al-V master alloy powder to achieve a cost efficient product with acceptable mechanical properties. Reference was also made to Ti-6Al-4V products produced by Clinning (University of Cape Town 2012) using elemental Al and V instead of the master alloy addition proposed in this work. Commercially pure titanium (CP-Ti, Alfa Aesar) was used in this study as a reference,

Materials, 2013

Metal powder injection molding is a shaping technology that has achieved solid scientific underpinnings. It is from this science base that recent progress has occurred in titanium powder injection molding. Much of the progress awaited development of the required particles with specific characteristics of particle size, particle shape, and purity. The production of titanium components by injection molding is stabilized by a good understanding of how each process variable impacts density and impurity level. As summarized here, recent research has isolated the four critical success factors in titanium metal powder injection molding (Ti-MIM) that must be simultaneously satisfied-density, purity, alloying, and microstructure. The critical role of density and impurities, and the inability to remove impurities with sintering, compels attention to starting Ti-MIM with high quality alloy powders. This article addresses the four critical success factors to rationalize Ti-MIM processing conditions to the requirements for demanding applications in aerospace and medical fields. Based on extensive research, a baseline process is identified and reported here with attention to linking mechanical properties to the four critical success factors.