Titanium Alloy Properties
Titanium and its alloys possess tensile strengths from 30,000 psi to 200,000 psi (210-1380 MPa), which are equivalent to those strengths found in most of alloy steels. The density of titanium is only 56 percent that of steel, and its corrosion resistance compares well with that of platinum.
Titanium and titanium alloys are heat treated for the following purposes:
- To reduce residual stresses developed during fabrication
- To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
- To increase strength (solution treating and aging)
- To optimize special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.
- Titanium Physical Properties
Titanium Physical Properties
If all the elements are assembled in order of atomic number, it can he noticed that there is a relationship in properties corresponding to the atomic number. Titanium is found in column four along with chemically similar zirconium, hafnium, and thorium. Therefore, it was not unexpected that titanium would possess some properties similar to those found in these metals.
Titanium has two electrons in the third shell and two electrons in the fourth shell. When this arrangement of electrons, where outer shells are filled before the inner shells are completely occupied, occurs in a metal, it is known as a transition metal. This arrangement of electrons is responsible for the unique physical properties of titanium. To mention a few, chromium, manganese, iron, cobalt, and nickel are found in the transition series.
The atomic weight of titanium is 47.88, while aluminum has an atomic weight of 26.97, and iron 55.84.
A crystal structure may he thought of as a physically homogeneous solid in which the atoms are arranged in a repeating pattern. This arrangement is instrumental in the physical behavior of a metal. Most metals have either a body-centered cubic, face-centered cubic, or a hexagonal-close-packed structure.
Titanium has a high temperature melting point of 3135°F (1725°C). This melting point is approximately 400°F above the melting point of steel and approximately 2000°F above that of aluminum.
Another important characteristic of titanium- base materials is the reversible transformation of the crystal structure from alpha (hexagonal close-packed) structure to beta (body-centered cubic) structure when the temperatures exceed certain level. This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum.
Titanium Thermal Conductivity
The ability of a metal to conduct or transfer heat is called its thermal conductivity. Thus, a material, to be a good insulator, would have a low thermal conductivity, whereas a radiator would have a high rate of conductivity to dissipate the heat. The physicist would define this phenomenon as the time rate of transfer by conduction, through unit thickness, across unit area for unit temperature gradient.
Titanium Linear Coefficient of Expansion
Heating a metal to temperatures below its melting point causes it to expand or increase in length. If a bar or rod is uniformly heated along its length, every unit of length of the bar increases. This increase per unit length per degree rise in temperature is called the coefficient of linear expansion. Where a metal will be alternately subjected to beating and cooling cycles and must maintain a certain tolerance of dimensions, a low coefficient of thermal expansion is desirable. When in contact with a metal of a different coefficient, this consideration assumes greater importance. Titanium has a low coefficient of linear expansion which is equal to 5.0x10-6 inch per inch/°F, whereas that of stainless steel is 7.8x10-6, copper 16.5x10-6, and aluminum 12.9x10-6.
Titanium Electrical Conductivity and Resistivity
The flow of electrons through a metal due to a drop in potential is known as electrical conductivity. The atomic structure of a metal strongly influences its electrical behavior. Titanium is not a good conductor of electricity. If the conductivity of copper is considered to be 100%, titanium would have a conductivity of 3.1%. From this it follows that titanium would not be used where good conductivity is a prime factor. For comparison, stainless steel has a conductivity of 3.5% and aluminum has a conductivity of 30%. Electrical resistance is the opposition a material presents to the flow of electrons. Since titanium is a poor conductor, it follows that it is a fair resistor.
Titanium Magnetic Properties
If a metal is placed in a magnetic field, a force is exerted on it. The intensity of the magnetization, called M, can be measured in terms of the force exerted and its relation to the magnetic field strength, H, depending upon the susceptibility, K, which is a property of the metal.
Metals have a wide variance in susceptibility and can be classified in three groups:
- The diamagnetic substances in which K is small and negative, and thus are feebly repelled by a magnetic field; examples are copper, silver, gold and bismuth.
- The paramagnetic substances in which K is small and positive, and thus are slightly attracted by a magnetic field; the alkali, alkaline and the nonferromagnetic transition metals fall in this group (it can be seen that titanium is slightly paramagnetic).
- The ferromagnetic substances, which have a large K value and are positive; iron, cobalt, nickel, and gallium fall under this heading.
- Titanium Alloy Compositions of Various Titanium Alloys
Because the allotropic behavior of titanium allows diverse changes in microstructures by variations in thermomechanical processing, a broad range of properties and applications can be served with a minimum number of grades. This is especially true of the alloys with a two-phase crystal structure. The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy. This alloy is well understood and is also very tolerant on variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminum. Alloy Ti-6Al-4V, which has limited section size hardenability, is most commonly used in the annealed condition. Other titanium alloys are designed for particular application areas. For example:
- Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and Ti-6Al-2Sn-4Zr-6Mo for high strength in heavy sections at elevated temperatures.
- Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) for creep resistance
- Alloys Ti-6Al-2Nb-ITa-Imo and Ti-6Al-4V-ELI are designed both to resist stress corrosion in aqueous salt solutions and for high fracture toughness
- Alloy Ti-5Al-2,5Sn is designed for weldability, and the ELI grade is used extensively for cryogenic applications
- Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.
When Welding Titanium
Welding has the greatest potential for affecting material properties. In all types of welds, contamination by interstitial impurities such as oxygen and nitrogen must be minimized to maintain useful ductility in the weldment. Alloy composition, welding procedure, and subsequent heat treatment are highly important in determining the final properties of welded joints. Some general principles can be summarized as follows:
- Welding generally increases strength and hardness
- Welding generally decreases tensile and bend ductility
- Welds in unalloyed titanium grades 1, 2 and 3 do not require post-weld treatment unless the material will be highly stressed in a strongly reducing atmosphere
- Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn have a high likelihood of fracturing with little or no plastic straining.
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