Titanium and Titanium Alloys: Key Engineering Materials for Aerospace Applications

By Dr Thoguluva Raghavan Vijayaram PhD

Senior Lecturer Department of Manufacturing Process and System Faculty of Manufacturing Engineering, UTeM Universiti Teknikal Malaysia Melaka Ayer Keroh, 75450 Melaka Malaysia Email: vijayaram1@gmail.com

Titanium was first discovered in an impure form by Rev William Gregor in England, 1971. It was later given the name titanium (after the titans, in Greek Mythology, the sons of the sky and earth gods) by a German chemist, Martin Kloproth, when he found a dioxide of the metal in rutile, ilmenite, and in many other widely dispersed ores. In 1910, pure titanium was manufactured by M.A.Hunter, an American Chemist. Hunter was able to extract the metal from the ores and developed the process of mixing rutile ore, Titanium oxide with chlorine and coke, then applying extreme heat, producing titanium tetrachloride, which was further reduced with sodium to form titanium. The hunter process successfully produced high quality titanium. Dr Wilhelm Kroll, in 1946, developed the process currently used for producing titanium commercially. The Kroll process reduces titanium tetrachloride with magnesium. This multi-batch, high temperature process proves to be inefficient. It drives the price of titanium to the point where its applications are restricted to the high-priced, niche markets.

The Armstrong process, developed by International Titanium Powder, LLC is a method of making high purity, fine Titanium powder in a continuous process. This process operates at low temperature, in low pressure, and in a small volume equipment. So, capital cost and labor cost is greatly reduced. The product does not require the additional purification needed by sponge produced from the hunter or kroll process. The powder is suitable for various applications such as powder metallurgy, spray forming, and other near net shape processes. Small diameter, high purity powder is produced directly with now waste stream.

Eventhough titanium is in abundance in nature, it was not found until the 18th century that it was discovered. This can be explained based on the fact that titanium does not exist by itself but it is found in conjunction with other elements. It is found in the minerals ilmenite and rutile at quantities that it has proven economically profitable to produce them in large quantities while it is also extracted from minerals such as leucoxene, perovskite, brookite, sphene, and anatase.

FACT SHEETS OF TITANIUM

Fact sheet number: 1

GENERAL FACTS

1. Lustrous, silver metal.

2. Superior strength, yet light weight

3. Corrosion resistant.

4. It can withstand extreme temperatures.

5. Capable of being fabricated into a variety of parts.

6. Biocompatible: medical implants used in the human body.

Fact sheet number: 2

APPLICATIONS

1. Airplanes.

2. Nuclear disposal.

3. International space station.

4. All type of space crafts.

5. Computers.

6. Automobiles.

7. Buildings.

8. Desalination plants.

9. Oil rigs / offshore platforms.

10. Vessels.

Fact sheet number: 3

PHYSICAL, MECHANICAL AND THERMAL PROPERTIES
Tensile strength	234 MPa
Yield strength	138 MPa
Density of solid	4509 Kg/m3
Molar volume	10.64 cm3
Velocity of sound	4140 m/sec
Modulus of Elasticity	115 GPa
Modulus of Rigidity	44 GPa
Bulk Modulus	108 GPa
Poisson’s ratio	0.33
Percentage Elongation	54%
Mineral hardness	6.0
Brinell hardness	716
Vickers hardness	970
Electrical resistivity	0.0000004 micrometre
Thermal conductivity	22 W/m/K
Thermal expansion	0.00086 / K
Enthalpy of fusion	18.70 KJ / mol
Enthalpy of vaporization	425 KJ / mol
Enthalpy of atomization	471 KJ / mol
Melting Point	1668 degree C
Boiling Point	3287 degree C
Super conduction temperature	-272 degree C

Titanium and its alloys are attractive engineering materials for structural applications in the aerospace industries. They have a high strength to weight ratio, high elevated temperature properties upto 550 degree centigrade, and excellent corrosion resistance, particularly in most natural environments. These alloys are more expensive than the common metals. These alloys do compete effectively in many areas, where their special properties can be used to advantage. For example, high strength to weight ratio and high elevated temperature properties of titanium alloys are of prime importance in the aerospace industry.

Titanium is a light weight metal having a density of 4.54 gm/cc, which is intermediate between that of aluminium and iron density. It has a melting point of 1668 degree C, which is higher than that of iron, and a modulus of elasticity of 16800000 lb/square-inch, which is intermediate between the values for aluminium and iron. The crystal structure of titanium is HCP at room temperature. Pure titanium can be cold rolled at room temperature to above 90 % reduction in thickness without serious cracking. Such extensive deformability is unusual for HCP metals like titanium and it is mostly related to the low c/a ratio of titanium. The relatively high ductility of HCP titanium is attributed to the many operative slip systems and available twinning planes in the titanium crystal lattice. Plastic deformation in titanium HCP is dominated by twinning planes. The type of slip in titanium is also very dependent on the concentration of interstitial impurity atoms such as oxygen and nitrogen.

Most of the titanium alloys are ternary and quaternary and are not binary alloys. Titanium alloys are classified according to the phases present in their microstructure. Alloys that consist mainly of the alpha phase are called alpha titanium alloys, whereas those that contain principally the alpha phase along with small amounts of beta-stabilizing elements like aluminium, gallium, and germanium. Alloys that consist of mixtures of alpha and beta phases are classified as alpha-beta alloys. Finally, titanium alloys in which the beta phase is stabilized at room temperature after cooling from a solution heat treatment are classified as beta alloys.

Alpha titanium alloys are non-heat treatable and weldable. They have medium strength, good notch toughness, and good creep resistance at elevated temperatures. Alpha-beta titanium alloys are heat treatable to attain a moderate increase in creep strength. They also have good forming properties, but do not have good creep resistance at elevated temperatures as the alpha titanium alloys or nearly alpha titanium alloys. Beta alloys are heat treatable to achieve very high strengths and are readily formable. These alloys have relatively high density and in the high strength condition have low ductility.

Commercially pure titanium is an unalloyed one in which the purity ranges from 99.5% to 99% titanium. The main elements in unalloyed titanium are iron and the interstitial elements like carbon, oxygen, nitrogen, and hydrogen. It is considered as an alpha phase alloy in which the oxygen content determines the grade and strength. It is lower in strength but more corrosion-resistant and less expensive than titanium alloys. It is used primarily when strength is not the main requirement. It has an excellent to many chemical environments. It is finding increasing use in the petroleum processing industry, especially for heat exchangers. It is used in refineries, since it is resistant to sulphides, chlorides, and many other chemicals encountered in petroleum refining. The addition of 0.2% palladium to commercially pure titanium improves its corrosion resistance in reducing media. Unalloyed titanium are used to design and process air frames, desalination equipments, marine chemical parts, plate type heat exchangers, cold spun or pressed parts, platinized anodes, aircraft engines, condenser and evaporator tubes, surgical implants, high speed fans, and gas compressors.

One important and commercial alpha titanium alloy, which we use today ha the nominal composition of Ti-5%, Al-2.5% and 2.5 Sn. It is an all alpha alloy because aluminium and tin both stabilize the alpha phase in titanium. This alloy is weldable and has good stability and oxidation resistance at elevated temperatures, and its strength is moderate. All alpha titanium alloys have the HCP crystal structure of titanium. Alpha titanium alloy is a weldable alloy for forgings and sheet metal parts such as aircraft engine compressor blades and ducting, and used to produce steam turbine blades. Besides, it is applied as a special grade material for high pressure cryogenic vessels operating down to -423 degree F. Hence for applications requiring good ductility at low temperatures, a low oxygen type Ti-5%, Al-2.5% Sn alloy is produced.

Near alpha titanium alloys are those which contain some beta phase dispersed in an otherwise all alpha structure. Small amounts of Mo and V, about 1% to 2%, which are beta stabilizing elements, are added to these alloys to retain some beta phase at room temperature. The Ti-8% Al-1% Mo-1%V alloy was originally developed for moderately high temperature applications in the compressor section or jet engines and has been used for aircraft skin components. It has desirable properties such as good weldability, good creep resistance, and toughness, high strength, low ductility, and high modulus. This alloy is normally used in the annealed condition, after performing mill annealing and duplex annealing. Near alpha titanium alloys are applied to produce airframe and jet engine parts requiring high strength upto 455 degree C, parts and cases for jet engine compressors, airframe skin components, and jet engine parts.

Alpha-beta titanium alloys contain one or more beta stabilizing elements in sufficient quantity to permit the retention of appreciable amounts of beta phase at room temperature, resulting in an alpha-beta structure. Ti-6% Al-4% V is the most important and widely used titanium alloy, accounting for 60% of the titanium market in 1989. It can be readily welded, forged, and machined, and it is available in a wide variety of mill product forms such as sheets, extrusions, wire, and rod. It is also used extensively for ordnance forgings. For special applications, requiring strengths at elevated temperatures, such as components for advanced jet engines, the Ti-6% Al-2% Sn-4% Zr-6% Mo and Ti-6% Al-2% Sn-2% Zr-2% Mo-2% Cr-0.25% Silicon alloys have been developed. They are more hardenable and cab be used in heavier sections and as well as at higher temperatures.

Alpha-beta titanium alloys are used to manufacture rocket motor cases, blades, and disks for aircraft turbines and compressors, structural forgings and fasteners, pressure vessels, gas, and chemical pumps, cryogenic parts, ordnance equipments, marine components, steam turbine blades, structural aircraft parts, and landing gears, airframes and jet engines, missile forgings, aircraft sheet components, aircraft hydraulic tubing, foils, and components for advanced jet engines.

If sufficient amounts of beta stabilizing alloying elements are added to titanium, a structure consisting of all metastable beta phase can be obtained at room temperature by quenching or even in some cases by air cooling. These alloys are usually used in the solution treated and aged condition in order to obtain their high strengths and they have the highest strengths of all titanium alloys, reaching up to 210 ksi.

More than 100 titanium alloys have been offered commercially since the titanium industry first formed. Over the past years, many new titanium alloys have been developed. Examples of relatively new titanium alloys are Ti-1100, Beta-C, and Beta-21S.

Ti-1100 alloy is an advanced titanium alloy for elevated temperature applications. It was designed to replace the jet engine compressor alloy T-6242-Si, which has a maximum use temperature of about 540 degree C. The chemical composition of Ti-1100 alloy is: Ti-6 Al-2.75 Sn-4.0 Zr-0.40 Mo-0.45 Si-0.07 Oxygen-0.02 Fe. It has improved steady state creep rates at elevated temperatures as compared to the Ti-6242-Si alloy.

Beta-C titanium alloy has the nominal composition as: Ti-3Al-8V-6Cr-4Mo-4.0Zr (Ti-3-8-6-4-4). The 8V, 6Cr, and 4Mo contribute to stabilize the beta phase to lower temperatures in this alloy. Beta-C has attractive properties for some non-aerospace applications that include high strength by solutionizing and aging and excellent corrosion resistance. This alloy has good resistance to mildly reducing chloride environments which allows it to be used in sour gas and high temperature brines.

The new Beta-21S titanium alloy has the nominal composition as: Ti-15Mo-2.7Nb-3Al-0.2Si and has excellent oxidation resistance and elevated tensile properties for a metastable beta alloy. In addition, Beta-21S has excellent corrosion and hydrogen resistance. Proposed use of this alloy is for applications involving extended exposure at elevated temperatures. The high molybdenum content of this alloy provides excellent high temperature stability and the niobium content is responsible for its excellent oxidation resistance. Beta-21S has superior oxidation resistance compared to commercially pure titanium and has roughly 20 times better oxidation resistance than the Ti-15-3 alloy (Ti-15V-3Cr-3Sn-3Al) after exposure at 650 degree C for 24 hours. Titanium has been one of the key materials used in all space launchers, space crafts, and the space station. The ubiquitous existence of titanium on the moon could one day prove to be of pivotal importance for humanity’s endeavors in outer space.

About the author:
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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