Royal Belgian Institute of Marine Engineers Titanium Powder Metallurgy: A Review – Part 2

F.H. (Sam) Froes, FASM*

Titanium and its alloys are the materials Prealloyed approach of choice for many applications, but high This method uses prealloyed powder (generally cost often negates their use. Powder spherical in shape) produced by melting, either by plasma rotating electrode processing (PREP), plasma metallurgy offers a cost‐effective (either atomization or spheroidization), or gas fabrication approach. atomization (GA), followed by hot consolidation typically by hot isostatic pressing[2]. Mechanical Titanium alloys offer improved performance in properties are superior to ingot material (because of aerospace and terrestrial systems due to their the refined microstructure and lack of directionality). excellent combinations of specific mechanical Powders contained a metal can (with metal inserts) properties and outstanding corrosion behavior. or ceramic mold (sealed in a can filled with a However, the high cost of Ti alloys compared to secondary pressing media) are compacted by hot competing materials limits their widespread use. isostatic processing. The advantage of using the near‐ Powder metallurgy (PM) techniques offer the net shape PM approach for difficult to process al‐ possibility of lowering manufacturing costs. Part 1 of loys such as intermetallic Ti‐Al type compositions has this article as well as this second part review some of been recognized[3‐5]. the developments in powder metallurgy as a cost No parts are currently being produced using the effective approach to fabrication of titanium ceramic mold process partly because of concerns that components under various aspects of the technology ceramic particles could get into the fabricated including the blended elemental (BE) approach, titanium parts. However, Figs. 2 and 3 show parts prealloyed (PA) methods, additive layer produced using a haped metal can and removable manufacturing (ALM), powder injection molding mild steel inserts (removed by chemical dissolution) (PIM), and spray deposition (SD) processing. are production ready[6]. Despite the 30‐35% volume shrinkage typical for Metal‐matrix composites HIPed PA powders, advanced process modeling Both continuously and discontinuously reinforced allows “net surfaces” to be achieved with minimal titanium components have been produced using PM machining stock on the near‐net surfaces. Near‐net‐ approaches. Figure 1 shows a finished titanium shape titanium parts can be made up to the size of metal‐matrix composite (MMC) ring for spin pit existing HIP furnaces (up to 2 m), considerably larger testing fabricated by IMT Bodycote (Andover, Mass.) than the capabilities of other technologies discussed from HIP‐densified plasma spray tapes. Dynamet in this article. Parts exhibit mechanical properties Technologies Inc. (Burlington, Mass.) uses the superior to those of conventional cast and wrought blended elemental technique to fabricate MMCs (ingot metallurgy) components (Fig. 4). Minimum using particulate and combined cold and hot isostatic values (used in design) for PM material are greater pressing (CHIP) combination, or forging, extrusion, Fig. 1 — and rolling of the CHIP perform[1]. The CermeTi Finished titanium family of titanium alloy matrix composites MMC ring incorporates TiC and TiB2 particulate ceramic TiAl for spin pit intermetallic as reinforcements with minimal testing. Courtesy of particle/matrix interaction. Table 1 compares the IMT‐ mechanical properties of CermeTi material with PM Bodycote, Ti‐6Al‐4V[1]. Dynamet made seven‐layer armor and Andover, dual‐hardness gears of CermeTi material. Mass.

Fig. 2 — Near‐net‐shape Ti‐6Al‐4V engine casing fabricated using the prealloyed metal method. Courtesy of Synertech PM Inc., Garden Grove, Calif.

than those for conventionally fabricated material[4]. Fracture toughness (KIC) of the PM product is superior to cast and wrought material (92.5‐96.5 versus 55 MNm3/2); data courtesy of Dr. Wayne Voice, Rolls‐Royce, UK). Fig. 3 — Selectively net‐shape ELI Ti‐6Al‐4V impeller for a rocket ‐engine turbopump fabricated Additive layer manufacturing using the prealloyed metal method. In this approach, powder is laid down in Courtesy of ynertech PM Inc./P&W Rocketdyne, Canoga Park, Calif. successive layers and melted under the control of a computer to produce virtually any shape (Fig. 5). ALM Ti‐6Al‐4V has a tensile strength of 160 ksi (1104 MPa) with 5 to 6% elongation “as‐formed,” and 140 ksi (965 MPa) after HIP with 15% elongation, properties equivalent to cast and wrought levels (data from B. Dutta, the POM Group Inc., Auburn Hills, Mich.). S‐N fatigue performance of parts made via ALM is equal to or slightly above that of conventional material levels. However, the main issue influencing growth in deployment of ALM for titanium alloys relates to raw material supply. Material Fig. 4 — Comparison of mechanical properties of prealloyed HIP, wrought, and cast material. cost is a major issue, with material cost typically 40 to 50% of total manufacturing cost for ALM titanium. Material supply chain is an issue for both powder and wire; sustainable sources are not always available, and supplies of certain alloys have limited availability[7]. ALM can be used to attach additional part features and can be used in part repair[7].

Powder injection molding Metal powder‐injection molding is based on injection molding of plastics; the process was developed for long Fig. 5 — Sandwich structure production runs of small (normally <400 g) aerofoil demonstrator made using complex shaped titanium parts in a cost‐ additive layer manufacturing. effective manner. Produced at the Centre for Additive Layer Manufacturing The technique involves melting and (University of Exeter, UK) using an pelletization of a titanium powder‐binder electron beam chamber and mixture that is injected into a die. The powder feed. binder is chemically/thermally removed, and the part is sintered (Fig. 6)[8‐9]. The method enables fabricating components with good mechanical properties pro‐ vided the chemistry (particularly oxygen) is Fig. 6 — Schematic of steps controlled[10‐11]. Typical shapes are involved in powder injection shown in Fig. 7. Incorporating a porous molding (PIM), in which a layer on the surface of body implant parts polymer binder and metal powder are mixed to form the cause bone in growth and improved feedstock which is molded bonding between the implant and bone debound, and sintered. [12]. Currently, titanium PIM parts are up

Spray forming This technique can involve either molten metal [14] or solid powder. The challenges associated with molten metal spraying of titanium are considerable because of its very high reactivity. However, both spray forming in an inert environment and under reactive conditions were achieved using appropriately designed equipment [15]. A segmented cold‐wall crucible combined with induction heating and an induction‐heated graphite nozzle was used to produce a stream of molten metal suitable for either atomization, to produce powder, or spray forming. Fig. 7 — Representative titanium PIM components. Courtesy of Praxis There is increased interest in cold spray forming Technology Inc., Queensbury, N.Y. involving solid powder particles[16]. Cold spray (<500°C) can produce both monolithic “chunky” to 1 ft long, but parts over 3 to 4 in. (about 50 g) are shapes and coated components. In the process, solid not common. Limiting factors are dimensional powder is introduced into a de Laval‐type nozzle and reproducibility and chemistry. expanded to achieve supersonic flow. The density of Worldwide titanium PIM part production is the sprayed region is less than full density, but can be estimated at about 3‐5 tons/month[10]. Market increased to 100% density by a subsequent HIP. expansion requires low cost (<$20/lb, or $44/kg) Examples of cold sprayed parts are shown in Fig. 8. powder of the right size (<40) mm and good purity The technique is also very useful to bond together (which is maintained throughout the fabrication normally difficult‐to‐bond metals such as titanium process). For nonaerospace applications, Ti‐6Al‐ 4V and steel. alloy purity level can be less stringent; for example, the oxygen level can be up to 0.3 wt% while still Future considerations exhibiting acceptable ductility levels (aerospace Over the past 30 years, a great deal of money (much requires a maximum oxygen level of 0.2 wt%[13]. of it from federal funding), was spent in attempting Oxygen levels can be even higher for CP grades (up to circumvent the high cost of titanium components to at least 0.4 wt%). Grade 4 CP titanium has a for aero‐space and terrestrial applications. However, specification limit of 0.4 wt%[13]. While the ultimate despite successes with lower integrity BE parts and tensile strength of Grade 4 CP titanium (80 ksi, or 550 recent advances in PIM, the overall market is small; MPa) is lower than that of regular Ti‐6AL‐4V (135 ksi, perhaps 20,000 lb total per year worldwide. or 930 MPa), it may well be a better choice for many However, a variety of high‐quality, low‐cost potential PIM parts where cost is of great concern. powders should be available soon. A number of Grade 4 enables using a lower cost starting stock and developments should lead to reasonable growth of a higher oxygen content in the final part. In the products produced using PM. Three areas where future, beta alloys with their inherent good ductility significant growth can occur are small parts (<1 lb) by (bcc structure) and intermetallics with attractive PIM, larger parts using BE and PA techniques, and elevated temperature capability are potential larger parts using additive candidates for fabrication via PIM. The science, layer manufacturing. technology, and cost of Ti PIM appear to be ready for Blended elemental applications are likely to grow significant growth. especially with innovative approaches such as use of hydrogenated powder to produce uniform high densities[17,18] including large armored vehicle parts (Fig. 9). Barriers to overcome using the PA approach are competition with critical components produced by “tried and true” cast‐and‐wrought or direct casting approaches. Recent developments suggest that immediate applications for PA titanium is in complex parts, very difficult to fabricate TiAl intermetallic alloys, and in metal matrix composites[3,4]. Additive layer manufacturing should also see significant growth in competition with the BE and PA techniques. Fig. 8 — Components produced at CSIRO (Commonwealth Scientific and Industrial Research Organisation, Melbourne, Australia) by cold spraying commercially pure powder.

Acknowledgement: The author recognizes useful discussions with Joe Fravel, Serge Grenier, Mitch Godfrey, G.N. Hayek Jr., Andy Hanson, Larry LaVoie, John (Qiang) Li, Tim McCabe, Colin McCracken, Steve Miller, John Mol, Vladimir Moxson, Vladimir Duz , Takashi Nishimura , Hiroaki Shiraishi, Tessa Stillman, Yoshy Takada, Bruno Unternährer, Jim Withers, and Fred Yolton; as well as Mrs. Marlane Martonick for assistance in formatting and typing the text.

References 1. S.M. Abkowitz et al., Affordable P/M titanium – Microstructure, Properties and Products, MPIF Conference, 2011. 2. F.H. Froes and D. Eylon, Powder Metallurgy of Titanium Alloys, Fig. 9 — Cold pressed, sintered, and hot rolled preform (weighing 95 kg, Intl. Matls. Reviews, 35, 162, 1990. or 209 lb) for commanders hatch forging for the Bradley Fighting 3. J.H. Moll and B.J. McTiernan, MPR, p 18‐26, Jan. 2000. Vehicle. Courtesy of ADMA Products, Hudson, Ohio. 4. J.H. Moll, JOM, Vol 52, No. 5, p 32, May 2003. 5. H. Clemens, et al., Structural Intermetallics, (eds. R. Darolia et 15. F.H. Froes and C. Suryanarayana, Powder Processing of al.), TMS, Warrendale, Pa. 205, 2003. Titanium Alloys, Reviews in Particulate Materials, MPIF, 6. V. Samarov, private communication, Mar. 17, 2011. Princeton, N.J., 1, 223, 1993. 7. J. Sears, PowderMet Conf., San Francisco, May 18‐21, 2011. 8. 16. E. Segall, et al., JOM, Vol. 50, No. 9, p 52, 2000. R.M. German, Powder Metallurgy Science, 2nd Ed., MPIF, 17. G.I. Abakumov, V.A. Duz, and V.S. Moxson, Titanium alloy Princeton, N.J., p 192, 1994. manufactured by low cost solid state P/M processes for military, 9. F.H. Froes and R.M. German, Metal Powder Report, Titanium aerospace and other critical applications, ITA Conference, 2010. Powder Injection Molding (PIM), Vol 55, No. 6, 12, 2000. 18. F.H. Froes, et al., Cost Effective Synthesis of Ti‐6Al‐4V 10. R.M. German, Powder Injection Molding Intl. 3, No. 4, p 21‐ Alloy Components Produced via the P/M Approach, Proc. TMS 37, Dec. 2009. Symposium on High Performance Metallic Materials for Cost 11. E. Baril, Ibid, 4, No. 4, p 22‐32, Dec. 2010 . Sensitive Applications, Seattle, Wash., TMS, Warrendale, Pa., 12. T. MacNeal, private communication, May 2, 2011. 2002. 13. R.R. Boyer, G. Welsch, E.W. Collings (editors), Materials Properties Handbook: Titanium Alloys, ASM Intl., Materials Park, For more information: Dr. F.H. (Sam) Froes, FASM, is a consultant, Ohio, 1994. 5208 Ridge Dr. NE, Tacoma, WA 98422; 14. F.H. Froes, Fourth Intl. Conf. on Spray Forming, LMA, Vol. 58, email: [email protected]. p 72, Feb. 2000. Source: ADVANCED MATERIALS & PROCESSES • OCTOBER 2012