Advanced Titanium Powder Technologies
One of the major topics at Titanium 2012 was additive manufacturing technologies for building net-shape parts of titanium powder. As discussed in previous articles, these technologies include direct metal laser sintering, laser engineered net shaping, electron beam melting, and ion fusion formation. However, all depend on the quality of the powder to achieve final properties. The following resentations focused on advanced technologies to produce high quality powder while minimizing cost. In general, they are already commercialized or are in the pilot-scale stage of development.
They include variations on the Kroll process, methods of blending elemental powders, processing blocky powders into spherical shapes, and metal injection molding. Presentations summarized here were by ADMA Products; Institute for Metal Physics; Ametek Reading Alloys; CSIRO; and Element 22. M.V. Matviychuk of ADMA Products discussed “Blended Elemental Powder Titanium Alloys Strengthened by Heat Treatment.” He reported on a study showing that high-strength titanium alloys can be successfully sintered by the blended elemental powder metallurgy (BEPM) approach using titanium hydride powder.
In addition, their mechanical properties can be improved with post-sintering processes. The study covered the following alloys: Ti-5Al-5V-5Mo-3Cr, Ti-10V-2Fe-3Al, and Ti-1Al-8V-5Fe. Because of limited space, this summary will consider only the results of the study of Ti-1Al-8V-5Fe, a low-cost/high strength alloy developed in the 1950’s that was discontinued because of the segregation of iron that occurs during melting. In an effort to achieve high properties in this alloy, thermomechanical processing was performed on a billet of Ti-1Al-8V-5Fe that had been produced by a novel low-cost BEPM process in which the base material was TiH2 powder. Hot-working followed by sintering not only eliminated porosity, but also developed microstructures that resulted in high strength.
Tensile testing was conducted on the as-rolled and heat-treated materials to evaluate their mechanical properties, and results showed that these matched well with the high mechanical properties produced by conventional processes. Dr. Matviychuk concluded that the study showed the BEPM approach is especially important in the case of alloys that are difficult to produce via ingot metallurgy.
For these alloys, the controlled formation of fine grains, reduced porosity during alloy sintering, properly selected post-sinter thermomechanical processing, and correct heat treatment, together enable attainment of properties that meet required highstrength specifications. According to Dr. V.A. Duz of ADMA Products, one of the reasons that titanium powder metallurgy is not fully developed is the lack of low-cost, highquality titanium powder. He pointed out in “Transformational Non-Kroll Process: Hydrogenated Titanium Powder Production” that some of the reasons include chemistry issues, meaning the high content of impurities such as chlorine, magnesium, and sodium; and property issues, meaning inferior low-cycle fatigue properties, low fracture toughness, and weldability problems. He explained that after ADMA researchers completed an extensive review of the various routes of titanium powder production, they decided that only melting can remove impurities and make titanium and titanium alloys acceptable for critical applications.
His presentation showed that magnesium reduction of titanium chloride followed by hydrogenation may be the most cost-effective approach to producing high quality titanium powder. Therefore, researchers developed a technology based on breaking up the titanium sponge mass upon its saturation with hydrogen into titanium hydride powder.
To produce the powder, titanium tetrachloride is reduced with magnesium, and titanium sponge is purified by vacuum distillation and hydrogenation. An important aspect of the process is introduction of additional titanium hydride powder together with titanium tetrachloride. This added powder positively affects the kinetics of the magnesium reduction process by causing emission of additional atomic hydrogen, which helps to reduce oxides in the system, cleans the interparticle interfaces of the product, and enhances the diffusion between components in the powder mixture.
The additional hydrogen considerably reduces vacuum distillation time, increases furnace output, reduces electric power consumption, and reduces labor costs. As a result, the cost of ADMA TiH2 powder is 15% lower than the cost of conventional Ti sponge. A pilot scale unit with a capacity of 660 pounds per run for TiH2 powder production is being built and will be installed by the end of 2012. Another approach that takes advantage of low-cost TiH2 powders was presented by Dr. Colin McCracken of Ametek Reading Alloys. He explained in “Plasma Spheroidized Titanium Powders” that these powders are based on TiH2, and are therefore a lower-cost alternative to powders made via PREP and Gas/Plasma atomization.
The plasma spheroidizing process is a high-volume batch process in which blocky hydride-dehydride (HDH)-processed powders are made spherical. The particle size distribution (PSD) range of the powder is predetermined at the HDH process stage, thereby improving powder yield and utilization. The HDH process is based on the fact that titanium has a very high affinity for interstitial elements such as oxygen, hydrogen, nitrogen, and carbon. When heated in a hydrogen atmosphere, a stable but brittle titanium hydride (delta phase) is produced through the following reaction: Ti (s) + H2 (g) <──> TiH2 (s) Titanium hydride can be readily crushed, milled, and screened into titanium hydride powder.
These blocky, angular powders are ideally suited for press/sinter, CIP/sinter, roll compaction, and plasma spray. However, for more advanced technologies, the powder particles must be spherical. After much research and development, Ametek/Reading Alloys has developed a low-cost method for making high-purity spherical powder from blocky TiH2 powder. The powder is fed into an induction-coupled plasma field, where the particles are melted and then solidified in spherical shapes.
The resulting morphology is very similar to the spherical particles produced by the plasma rotating electrode powder (PREP) process, but is lower in cost. It is free from agglomerates and satellites, thus eliminating argon gas entrapment. The plasmaspheroidized powder has significantly higher apparent density and powder flow compared to the feedstock. The PS process can produce the full range of particle size distribution, from coarse to fine, and PS titanium powders are ideally suited for advanced manufacturing technologies such as HIP, additive manufacturing, and metal injection molding.
Another program to produce spherical powders for additive manufacturing was discussed by Dr. Christian Doblin of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO). He reported on “The Ongoing Development of the TiRO Process,” which is based on the chemistry of the Kroll batch process, in which titanium tetrachloride is reduced with magnesium. The goal of the TiRO process is continuous production of high-quality titanium powder via fluidized-bed technology.
Although the chemistry is similar to that of the Kroll process, TiRO operates within a temperature range suitable for fluidized bed technology. It exploits the principle that when suspended in a gas, solid particles behave like a fluid and react more rapidly. Therefore, the Kroll chloride separation step has been redesigned and integrated into the overall TiRO process. The result is a powder production system capable of continuously producing high quality titanium powder suitable for near net-shape manufacturing in a fraction of the time with minimal waste.
Moreover, process conditions can be adjusted to generate particles tailored in shape and size to suit differing downstream applications. This is an advantage for fabrication methods in which the powders can be consolidated directly, thereby avoiding remelting. The powder product can be designed for downstream techniques such as hot isostatic pressing, metal injection molding, cold spray, and laser forming.
Over a two-year timeframe, CSIRO has refined the fluidized-bed reactor and built a pilot-scale system with production capacity of 2.0 kg/hour. This reactor is suitable for scale-up to commercial size, and will support development of a demonstration plant with a capacity of 100 metric tons per year.
The project currently stands poised for scale-up to commercial production. Matthias Scharvogel of Element 22 described how spherical powders could be used in metal injection molding (MIM), which features only one working step: the filling of the injection mold. This step determines the final geometry, which can be so complex that it would be either impossible or much too expensive to produce by conventional methods.
To begin the process, the metal powder is mixed with a binder, forming a feedstock that is pressed by a conventional commercial injection machine into a mold under high pressure at approximately 100°C. The pressed green part has the final geometry, but it is very frangible. In the next step, the binder is removed in a multi-stage chemical and thermal process, resulting in a metal part with high porosity. In the final step, the part is sintered at approximately 1200°C to consolidate the powder and form a strong solid metal component. Up to now, the strong dependence of titanium mechanical characteristics on the content of oxygen, nitrogen, and carbon in particular, have ruled out any commercial use of this technology. However, in co-operation with research-institutes, Element22 has been able to overcome these problems and is now capable of producing components from Ti-6Al-7Nb with mechanical properties equivalent to those made by conventional techniques.
Therefore, titanium parts made via MIM are now being produced for medical devices. Titanium and its alloys are nearly ideal materials for medical technology, especially for implants, because it is neither toxic nor rejected by the human body, nor does it induce allergic reactions. In addition, titanium materials are nonmagnetic and therefore trouble-free for magnetic resonance imaging (MRI). With these properties and the ability to make small complex parts, titanium MIM devices are making inroads into the medical market.