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The Solid-State Spray Forming of Low-Oxide Titanium Components

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The Solid-State Spray Forming of Low-Oxide Titanium Components Innovations in Titanium Overview The Solid-State Spray Forming of Low-Oxide Titanium Components Ralph M. Tapphorn and Howard Gabel Editor’s Note: This paper was presented at the 1998 TMS new concept for the fabrication of mi- particles. The metal particles must be Annual Meeting as part of the Innovations in Titanium symposium, which was sponsored by the Office of Industrial crometer-sized, essentially OFTi pow- kept in a gas suspension, because with- Technologies, Energy Efficiency and Renewable Energy, ders. The process can deliver powder out an oxide or nitride coating, they will U.S. Department of Energy, Innovative Concepts program. directly to downstream processes, such readily agglomerate. Oxygen and nitro- This issue of JOM represents the proceedings of that sympo- sium. as SSF, for fabrication into finished parts. gen must be excluded from the process also to reduce the risk of particle com- INTRODUCTION SSF is a proprietary process devel- oped by Innovative Technology for the bustion or explosion. An inert gas (e.g., This paper presents a novel approach near-net-shape fabrication of metallic helium) is used as a purge gas to remove for the production of essentially oxide- components.5 OFTi powder is produced the hydrogen from the dehydride sec- free titanium parts in near-net shapes. in micrometer size within a HDH reac- tion of the reactor and as a process gas The hydride-dehydride (HDH) process tor. Figure 1 contains a schematic repre- for transporting the OFTi powder to is used to produce oxide-free titanium sentation of the system. The reactor uses downstream processes such as SSF. (OFTi) powder; the powder is then used high-temperature hydrogen gas to re- Solid-Spray Forming of Near-Net- as feedstock for the solid-state spray move the oxide layer from coarse tita- Shape Components forming (SSF) process. Older spray form- nium and convert the titanium metal to ing techniques (e.g., thermal, Osprey, titanium-hydride particles. Titanium- SSF is a new concept for fabricating and low-pressure plasma spray)1–4 are hydride powder is sufficiently friable to near-net-shape components using OFTi not particularly suited to the production spontaneously fracture into microme- powders. This process uses the HDH of parts of near-net shape and typically ter-sized particles.6 Next, the hydrogen powder reactor to produce micrometer- introduce supplementary oxidation. is removed by increasing the tempera- sized titanium powders that are ren- The other process most often used for ture, which renders fine oxide-free metal dered oxide-free in passage through the obtaining near-net-shape tita- reactor. As a low-temperature nium parts is powder metallurgy deposition process, SSF does not (P/M). The primary material melt the OFTi feedstock and, benefits of SSF over P/M is im- thus, reduces small grain size. proved strength and increased These features permit the near- ductility due to extremely small net-shape fabrication of titanium grain size and low oxide concen- or titanium-alloy components tration, respectively. SSF and without metallurgically altering P/M processes use similar feed- the properties of the feedstock stock materials, but SSF reduces material. Lower oxide concen- the manufacturing steps from tration will lead to higher ductil- three for P/M (pressing, sinter- ity while smaller grain size will ing, and hot isostatic pressing) to result in increased strength per one for SSF (spray forming the the Hall-Petch relationship.7 OFTi powder). The handling of The SSF of OFTi is performed micrometer-sized powder is cur- with a patented process devel- rently a deterrent to titanium oped by Innovative Technology P/M because of the propensity (Figure 2).8 For this application, for metal dust explosions. The the process employs a modified, SSF process combined with the two-phase, converging-diverg- HDH reactor is expected to miti- Figure 1. A schematic of the HDH process for the production of ing deposition nozzle to acceler- gate these problems because the micrometer-sized titanium powder. ate the micrometer-sized OFTi feedstock powders are more particles entrained in an inert coarse, and inert gases are used gas. The OFTi particles are de- Regulated Hydride-Dehydride s to convey the powder to the s posited onto a substrate or into a nozzle for deposition. Inert Gas Oxide-Free Ti Powder mold to enable the near-net- Supply Reactor shape fabrication of specific com- PRODUCING NEAR- Application Gun ponents. The high-speed colli- NET-SHAPE TITANIUM Inert Gas & sion of the OFTi particles causes COMPONENTS THROUGH Debris s true metallurgical bonds to form HDH AND SSF Recovery upon contact. Metallurgical Hydride/Dehydride System Component bonding is achieved exclusively Production of Titanium through solid-state reaction— Powders bulk melting does not occur.9 The continuous HDH produc- Figure 2. The SSF system for fabricating near-net-shape OFTi Other spray-forming techniques tion of titanium powders is a components. (e.g., thermal, low-pressure 1998 September • JOM 45 the OFTi production by limiting the hydride con- version to a superficial layer on each particle. This cost reduction can- not be accurately esti- mated at this stage be- cause the threshold tem- perature for completely removing the oxide layer 100 µm from the powder is not Figure 5. A polarized light micrograph of SSF yet known. This mode of titanium after HIPing. operation would not per- mit the production of converted to OFTi metal by heating. finer titanium powders During the heat treatment, helium gas because the friable char- was used to purge hydrogen from the acteristics of the tita- sample and prevent oxidation. The OFTi Figure 3. The nozzle translation lathe system. nium-hydride powder sample was cooled to room temperature, are not realized. It is im- then transferred to the sealed cabinet for plasma spray, and Osprey) cannot be portant to note that the above costs are further processing. used to deposit OFTi powders. for near-net-shape components and, Because the laboratory dehydride con- Debris generated during the SSF fab- thus, compare favorably to parts fabri- version process was conducted with rication process is removed with a co- cated with conventional techniques. powder in intimate contact, it was neces- axial suction nozzle or suction cabinet Titanium is used extensively by the sary to grind the agglomerated particles driven by a debris-recovery system. The aerospace, defense, marine, automotive, with a mortar and pestle and sieve debris-recovery system filters contami- chemical, pulp and paper, textile, medi- through a 325 mesh. Spray-formed nants from the recovered effluent and cal, mining, minerals, metal processing, samples 11.5 cm × 1.5 cm × 0.3 cm (Figure enables the recycling of excess powder petroleum, and sports industries. Ex- 4) were deposited successfully onto an and the inert gas. panded use of titanium in lightweight aluminum substrate using the SSF appa- vehicles is anticipated as a replacement ratus shown in Figure 3. Helium was ECONOMICS AND MARKET for steel where superior strength-to- used as the inert process gas for both POTENTIAL weight ratios and corrosion resistance entraining the OFTi powder and operat- The cost of operating the HDH reactor are required. The augmented use of tita- ing the SSF nozzle. to produce 2 kg/h of OFTi is estimated nium by the automotive industry is ex- Future testing of the OFTi sample is to be approximately $1.60–2.75/kg, as- pected to reduce machining and manu- planned to assess the mechanical and suming that the inert carrier gas is re- facturing costs and permit the manufac- metallurgical properties of the SSF depo- cycled after the removal of hydrogen ture of more energy-efficient vehicles. sitions. Visual observations of the and other contaminants. The capital samples indicate that residual stresses KEY EXPERIMENTAL RESULTS equipment cost for the HDH reactor is were induced and post-deposition an- approximately $50,000, exclusive of gas- Initial tests were conducted on a labo- nealing will be required. Hydrogen recovery systems. With a theoretical ratory scale to demonstrate the feasibil- embrittlement10 of the SSF material is deposition rate of 2 kg/h, the cost of ity of the HDH-SSF processes. The most not expected to be a problem because the operating a small SSF nozzle (1.6 mm critical processing step was to demon- hydrogen is driven off at high tempera- throat) using helium in a recycle mode is strate that the OFTi powder could in- tures. Previous SSF deposition work expected to be $10/kg. The combined deed be spray formed using the SSF with commercial (non-OFTi) powders11,12 cost of producing OFTi powder and then system (Figure 3). produced SSF titanium samples that SSF into near-net shape is, thus, approxi- For the HDH laboratory-demonstra- were subsequently annealed at 780 K, mately $12–15/kg. The capital equip- tion tests, TiH2 (–325 mesh) powder from then hot isostatically pressed at 780 K ment cost for a complete SSF unit is Micron Metals was used as feedstock. and 10.3 MPa to achieve 100% density. approximately $250,000, including the First, the published threshold tempera- Figure 5 shows a polarized light micro- cost of the HDH reactor. ture for the dehydride reaction was con- graph of this material. Note that the It may be possible to reduce the cost of firmed. These tests were conducted by equiaxed 25–75 micrometer grain size observing the temperature at which a resembles a wrought, normalized struc- thin coating of TiH2 powder sprayed ture. onto a brass or aluminum sheet was FUTURE DEVELOPMENT converted first to titanium metal and then to titanium oxide. The test results This work successfully demonstrated yielded a threshold reaction tempera- the concept of using the HDH process ture in good agreement with the 750 K for producing micrometer-sized OFTi temperature cited by Greenspan and powder.
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