Titanium alloys parts are ideally suited for advanced aerospace systems because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent corrosion resistance. Despite these features, use of titanium alloys in engines and airframes is limited by cost. The alloys processing by powder metallurgy eases the obtainment of parts with complex geometry.
The metallurgy of titanium and titanium-base alloys has been intensely investigated in the last 50 years. Titanium has unique properties like its high strength-to-weight ratio, good resistance to many corrosive environments and can be used over a wide range of temperatures. Typical engineering applications of titanium alloys include the manufacture of cryogenic devices and aerospace components.
The high buy-to-fly ratio associated with many titanium components, combined with forging and machining difficulties and recent availability problems, has led to a strong drive for near-net titanium fabrication. A very promising method of attaining this goal is powder metallurgy (P/M).
The primary justifications for using titanium in the aerospace industry are:
- weight savings (primarily as a steel replacement);
- space limitation (replace Al alloys);
- operating temperature (Al, Ni, steel alloys replacement);
- corrosion resistance (replace Al and low alloy steels); and
- composite compatibility (replace Al alloys).
Weight savings is due to the high strength-to-weight ratio. The lower density of titanium compared with steel permits weight savings, replacing steels even though they may be higher strength. As the strength of titanium alloys is significantly higher than Al alloys, weight savings can be achieved in their replacement in spite of the 60% higher density, assuming that the component is not gage limited.
Titanium could also replace aluminum when the operating temperature exceeds about 130°C, which is the normal maximum operating temperature for conventional aluminum. These conditions exist, for example, in the nacelle and auxiliary power unit (APU) areas and wing anti-icing system for airframe structures. Steel and nickel-base alloys are obvious alternative, but they have a density about 1.7 times that of titanium.
Over the last decade, the focus of titanium alloy development has shifted from aerospace to industrial applications. However, the titanium industry will still dependent on the aerospace market and this sector will constitute a significant percentage of total consumption for years to come.
As aircraft engine manufactures seek to use cast titanium at higher operating temperatures, Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo are being specified more frequently. Other advanced high-temperature titanium alloys are used for service up to 595°C, such as Ti-1100 and IMI-834 are being developed as castings. The wrought products are the most readily available product form of titanium-base materials, although cast and P/M products are also available for applications that require complex shapes or the use of P/M techniques.
However, negating widespread use is the high cost of titanium alloys compared to competing materials. This has led to numerous investigations of various potentially lower cost processes, including P/M techniques. Recently there has been renewed interest in titanium powder metallurgy P/M as a cost-effective way of fabricating components from this expensive metal.
Titanium powder metallurgy can produce high performance and low cost titanium parts. Compared with those by conventional processes, high performance P/M titanium parts have many advantages: excellent mechanical properties at least comparable to the level of wrought titanium material, near-net-shape and low cost, being easy to fabricate complex shape parts, full dense material, no inner defect, fine and uniform microstructure, no texture, no segregation, low internal stress, excellent stability of dimension and being easy to fabricate titanium based composite parts. Powder metallurgy technology of titanium alloys has been commercially used in developed countries and further research of possible utilization of P/M titanium alloys is performed to meet the increasing need of high performance-to-cost parts.
Powder metallurgy of titanium is mainly restricted to space and missile applications. Titanium-base products have the combination of low density [4.5 g/cm3] and high strength. The strengths vary from 480 MPa for some grades of commercial titanium to about 1100 MPa for structural titanium alloy products and over 1725 MPa for special forms such as wires and springs. Another important characteristic of titanium-base materials is the reversible transformation of the crystal structure from alpha [a, hexagonal close-packed] structure to beta [b, body-centered cubic] structure when the temperatures exceed certain level. Pure titanium wrought products, which have minimum titanium contents ranging from about 98,635 to 99, 5 wt% and are used primarily for corrosion resistance.
In general terms, powder metallurgy involves production, processing and consolidation of fine particles to produce a solid article. The small, homogeneous powder particles result in a uniform microstructure in the final product. If the final product is made net-shape by application of hot isostatic pressing (HIP), a lack of texture can result, thus giving equal properties in all directions.
Titanium powder metallurgy is generally divided into two "approaches", the "elemental approach" and the "pre-alloyed approach". With the "elemental approach", the small (-100 mesh) regular grains of titanium normally rejected during the conversion of ore to ingot (commonly called "sponge fines"), are used as starting stock.
Alloy additions, normally in the form of a powdered master alloy, are added to these fines, so that the desired bulk chemistry is achieved. The blended mixture is then compacted cold, under pressures up to 420 MPa (60 ksi), to a density of 85-90%. This operation can be carried out either isostatically or with a relatively simple mechanical press. The "green" compact is then sintered to increase density to 95-99.8% theoretical density and to homogenize the chemistry.
The cold isostatic pressing is often referred to as CIP. A further increase in density can be achieved by hot isostatic pressing the article, which also generally improves the mechanical properties of the article. The combined cold/hot isostatic pressing process is referred to as CHIP.
The CHIP process using elastomeric molds can produce extremely complex shares, which are very difficult to achieve by forging processes. Caution must be used in applying parts made by this technique in critical components, such as rotating parts, where fatigue behavior is usually very important. Available data indicate that parts made from elemental material are inferior to wrought material in fatigue performance.
The starting stock for production of net shape articles by powder metallurgy contains the desired alloy components. Suitable powders include, for example, Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-2.5Sn, Ti-2.5Al-13V-7Sn-2Zr,Ti-10V-2Fe-3Al, Ti-11.5Mo-6Zr-4.5Sn, Ti-5Al-6Sn-2Zr-1Mo-0.2Sn, Ti-6Al-2Sn-4Zr-6Mo, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-8Mo-8V-2Fe-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-13V-11Cr-3Al and the like.
Consolidation of the pre-alloyed powder may be accomplished using any procedure known in the art. Following consolidation, the formed article may optionally be subjected to an annealing heat treatment. Such treatment is typically carried out at temperature about 20 to 30% below the beta-transus temperature (in °C.) of the alloy for about 2 to 36 hours in a vacuum or inert environment to protect the surface of the article from oxidation. For example, heat treatment of Ti-6Al-4V alloy is typically carried out between 700°C and 800°C. for about 2 to 8 hours.
Following consolidation, and optionally, the annealing step, the article is hydrogenated. Titanium and its alloys have an affinity for hydrogen, being able to dissolve up to about 3 weight percent (60 atomic %) hydrogen at 590°C. While it may be possible to hydrogenate the article to the maximum quantity, it is presently preferred to hydrogenate the article to a level of about 0.5 to 1.5 weight percent hydrogen to prevent cracking during the subsequent cooling step.
Titanium powders can be divided into categories shown in shown in Table 1.
Table 1: Categories of Titanium P/M
||Powder feed melted with a laser
|Powder Injection Molding
||Use of a binder to produce complex small parts
||Solid or potentially liquid
|Near Net Shapes
||Pre-alloyed and blended elemental
|Far from Equilibrium Processes
||Rapid solidification, mechanical alloying and vapor deposition
Titanium P/M can be divided into the categories of laser forming, powder injection molding, spraying, near net shapes (blended elemental and pre-alloyed), metal matrix composites, and far from equilibrium processing (rapid solidification, mechanical alloying and vapor deposition). It is proposed that with the availability of cost-affordable powders, and advances in fabrication techniques, the powder injection molding and blended elemental approaches in particular should see significant growth.
The atomized powders are generally pre-alloyed and spherical, while the hydride-dehydride powders which are generally also pre-alloyed are angular in nature, and sponge fines (a by-product of sponge production) are “sponge-like” in nature and contain remnant salt (which prevents achievement of full density and adversely affects weldability). There is also a new type of powder produced by a reverse electrolysis process.
Metal powder injection molding (PIM) is based upon the injection molding of plastics, a process developed for long production runs of small (normally below 400 gm.) complex shaped parts in a cost-effective manner. By increasing the metal (or ceramic) particle content, the process evolved into a process for production of high density metal, inter-metallic or ceramic components.
Spray forming can involve either molten metal or solid powder. Because of its very high reactivity the challenges associated with molten metal spraying of titanium are quite considerable. However, both spray forming in an inert environment and under reactive conditions have been achieved with appropriately designed equipment. 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.
Recently there has been increased interest in cold spray forming involving solid powder particles.
Near Net Shapes
Many of the techniques generally available for production of near net shapes (NNS) are amenable for use with various types of titanium powders; these include conventional press-and-sinter, elastomeric bag cold isostatic pressing (CIP’ing), and ceramic mold or metal can hot isostatic pressing (HIP’ing). For convenience, NNS will be divided into those produced using blended elemental (BE) powders and those produced from pre-alloyed (PA) powders.