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 propertie