Lightweight and Strong – At A Cost
Titanium is an expensive metal. But it’s light, strong at sub-zero, ambient and elevated temperatures, tough and ductile, corrosion resistant and relatively easy to form and machine, although it is generally twice as costly to machine as aluminum. It has enabled us to fly, and at great heights, and sometimes supersonic speeds, mostly in reasonable comfort. Its corrosion resistance has enabled us to render seawater potable. Its strength and wear resistance have made it possible for people with worn out hip joints to get up and comfortably walk again.
Although ‘discovered’ a couple of centuries ago, titanium only showed its face on the industrial scene in the fifties. Extraction of titanium from its ore, ilmenite, is a long, expensive process, thus is the metal itself expensive. Titanium is extracted from its ore by what is called the Kroll process. The Kroll process is the primary method for producing titanium metal. It involves several steps, including carbochlorination of titanium dioxide, purification of titanium tetrachloride, and reduction of titanium tetrachloride with magnesium. The resulting titanium sponge is then purified, crushed, and melted into ingots.
The metal and its alloys find the majority of their applications in the aerospace industry, but they are also widely used in medical equipment and in the aforementioned desalination plants.
The basic metallurgy of titanium and its alloys, put simply, involves two phases, namely alpha and beta. Alpha has what we call a hexagonal close packed (atomic) structure and beta has a body centered cubic structure. There are, further, three types of alloys, namely alpha, alpha-beta and beta. All this is controlled by additions of alloying elements to the basic metal, additions developed over the years. The various alloying constituents also impart different degrees of strengthening. The metallurgy of the metal and its alloys is, in fact, quite complicated, but suffice it to say that it allows the production, fabrication, and heat treatment of a range of alloys that is good for a very broad range of applications. Titanium in this article may refer to titanium or its alloys.
One of the more important applications of titanium is in the aerospace industry. Here, commercially pure titanium finds itself alongside other highly alloyed grades that were developed in the fifties through the eighties. These all play their part in the behemoths that ply the skies today. All titanium forging alloys are produced commercially as cylindrical ingots by double vacuum – arc melting, with some being triple melted to produce larger size ingots, or for the production of very high-quality material. Ingots are first cogged to large billet sizes, to prepare shapes suitable for further processing.
Materials produced are mainly bars for the forging of fan cases, low-pressure compressor blades, low-pressure compressor stator vanes, and the like. Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloys are most commonly used, and effective production technologies of such materials have been put into practice, providing high quality in a wide range of sizes, from 30mm diameter small bars to billets exceeding 300mm. Since foreign substances can be introduced into the material during the ingot manufacturing process, which might then cause fatigue fracture during operations, contamination prevention for aircraft engines is crucial.
Titanium application purposes for aircraft are shown below. Outside temperatures during flight can be −60˚C or lower; however, titanium is resistant to embrittlement at low temperatures. Furthermore, there is no concern about corrosion even when dew condenses after a drop in temperature. Titanium has a low thermal expansion that is close to that of Carbon Fiber Reinforced Plastic (CFRP), a material that is used in conjunction with titanium on the airframe parts. Commercially pure titanium is equipped with all of the characteristics required, namely weight saving, (its density is 60% that of steel), heat resistance, resistance to embrittlement at low temperatures, high corrosion resistance, and low-thermal expansion, and is therefore used for various airframe parts. Examples of such applications are the nacelle at the entry of an engine, parts of the pylon for hanging engines, and hot air piping that prevents freezing.
Other alloys used for the airframe:
Ti-6Al-4V for cockpit window frame, wing box, fastener
Ti-3Al-2.5V for hydraulic pipe
Ti-10V-2Fe-3Al for landing gear, track beam
Ti-6Al-2Sn-4Zr-2Mo for exhaust, tail cone
Ti-15V-3Cr-3Sn-3Al for duct
For engines of medium and small size aircraft, forged solid fan blades are employed, while for large engines with larger fan blades, hollow fan blades devised for saving weight are employed.
Materials having high strength and high toughness are required, and therefore, titanium alloys such as Ti6Al-4V and Ti-17 (Ti-5Al-2Sn-2Zr-4Cr-4Mo) are used. Titanium alloys are mainly used for sections of the engines where the temperature is relatively low (600˚C or lower).
The following titanium materials are used in airframes and engines:
- Commercially pure titanium, of which there are four categories, according to strength. They are used for non-structural applications, such as water supply systems for galleys and sanitary, and for ducts and piping, many of which require corrosion resistance and good formability.
- Ti-6Al-4V alloy is designed for a good balance of characteristics, including: strength, ductility, fracture toughness, high temperature strength, creep characteristics, weldability, workability, and response to heat treatment.
- Ti-6Al-2Sn-4Zr-2Mo alloy has a heat resistance of approximately 450˚C. In the latter half of the 1970s, Ti-6Al-2Sn-4Zr-2Mo-0.1Si was developed to improve oxidation resistance and creep properties with the addition of Si of 0.06~0.2 wt%, and the heat-resistant temperature was improved to approximately 500˚C.
- Ti-5Al-2Sn-2Zr-4Cr-4Mo alloy (occasionally referred to as “Ti17” alloy) was developed in the USA in the 1970s as an alloy having high strength and excellent fracture toughness. Its heat resistant temperature is approximately 350˚C.
- Ti-6Al-2Sn-4Zr-6Mo is a titanium alloy developed around 1966. Its heat resistant temperature is about 450˚C. This alloy has high strength and excellent creep characteristics.
- Ti-10V-2Fe-3Al alloy has excellent hardenability, high strength, and high fatigue strength. It is mainly used for landing gear (part of the main landing gear for take-off and landing).
We are dealing here with very expensive alloys. As such, it is very important that all phases of production, from melting, through forging, heat treatment, and machining, be performed according to the parameters that are special to each grade. It is worthy of note, for example, that alloy Ti-6Al-4V requires one and a half to two times the machine capacity required to forge low-alloy steel into comparable shapes.
Heat treatment of titanium and alloys involves stress relief to eliminate residual stresses that may arise from non-uniform hot or cold working, annealing, and solution treating and aging.
The composition of titanium and alloys, together with the extreme care that must be taken in their processing, bring home the fact that experience in all these aspects is of prime importance. AMFG has this experience, and works with its customers to ensure that the correct grade and condition is supplied to meet its customers’ demanding requirements, paying particular attention to the customers’ end uses.
