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Technical info

Introduction

Titanium is a chemical element with the symbol Ti having atom number 22. It is fair to say the element was discovered twice, in 1791 by William Gregor in the mineral ilmenite and a few years later (1795) by Martin Heinrich Klaproth in rutile ore. The latter named it “Titanium” after the Titans in Greek mythology.

The fact that titanium is highly reactive with oxygen and carbon at high temperatures complicates the production of pure titanium. Anton Eduard van Arkel was the first person to produce pure, metallic titanium in 1922. (It should be noted that about 95% of the total titanium volume produced is being processed into TiO2 (titanium dioxide), which has paint, plastic and paper as end use applications. However, these applications are beyond the scope of this article).

Properties


Applications

The properties mentioned above give titanium a wide range of applications:



Titanium supply chain

Titanium supply chain

The titanium supply chain (figure A) involves three basic steps:

1Mining / basic processing
The mining process of titanium is rather straightforward: the sands (containing a mix of minerals: rutile, ilmenite, leucoxene and zircon) are either strip mined or dredged. Physical methods, like gravity separation spirals and magnets, are employed to separate the different minerals. The titanium dioxide (TiO2) content of rutile is being processed chemically into titanium tetrachloride (TiCl4), a key input for the titanium metal industry. Ilmenite must be upgraded into titanium slag (as being done in Russia, the Ukraine and Kazakhstan), or it must be converted into synthetic rutile (all other countries). The latter can be done using either the Becher or the Benilite process. Synthetic rutile is being produced in seven locations worldwide, with the lion’s share of capacity in located in Western Australia.

2Metal processing
Titanium metal uses the Kroll process to produce titanium as an intermediate product.
Since it takes several days to convert a batch of TiCl4 into titanium sponge (to be physically removed from the reaction vessel) this is a costly process. The titanium sponge is subsequently melted into a titanium ingot.
The ingot is being worked into mill products (such as sheet, bar, wire, tube and forgings). This process tends to produce around 40% scrap, which is being remelted into ingot. Scrap is also obtained from end users, especially in the case of aerospace applications.

Titanium and its alloys can be heat treated. This is done in order to:

(source: www.keytometals.com)

Alloy types and microstructure

Unalloyed titanium is allotropic (exists in more than one different form)



Adding alloying elements may affect the α-β transition temperature:



Titanium alloys are classified as α, near-α, α-β, or β alloys.



Effects of oxygen and iron content

Post heat treatment properties are influenced significantly by oxygen and iron content.



Stress relieving
Stress relieving can be applied without adverse effects on strength and ductility. These treatments decrease undesirable residual stresses resulting from

Removal of residual stresses increases shape stability. Stress relieving may be omitted when machining symmetrical shapes in the annealed condition. This also applies when the manufacturing process can be adjusted using annealing or hardening as the stress relieving technique, for example annealing may relieve forging stresses prior to machining.

Annealing

Annealing can be applied in order to increase

Generally, improvement in one category goes at the expense of another property⁄properties.

Common annealing types:

In addition to annealing, sizing, straightening or flattening may be needed in order to meet dimensional requirements.

Solution treating and aging

α-β and β alloys can be solution treated and aged in order to obtain the desired strength levels. The high-temperature β phase is unstable at lower temperatures, causing the response of titanium alloys to heat treatment. The treatment involves heating an α-β alloy to the solution treatment temperature, producing a higher ratio of β phase. This condition is maintained by quenching; subsequent aging causes decomposition of the unstable β phase which gives high strength.

Solution treating involves heating to temperatures around the β transition point, the selected temperature being dependent upon alloy type and practical considerations.

β alloys generally are supplied in the solution treated condition, needing only to be aged. Complete solutioning must by ensured by applying adequate soak times. Solution treatment of β alloys involves heating above the β transition point; due to the absence of a second phase rapid grain growth may occur.

α-β alloys are being solution heat treated at the temperature suitable for obtaining the desired mechanical properties after aging. As the α⁄β ratio depends on this temperature, consequently the response to aging does vary with this ratio.

Normally, temperatures 25 to 85°C below the β transus are used, giving high strength with adequate ductility. However, if high fracture toughness or increased stress corrosion are required, β annealing or β solution treating may be desirable (at the sacrifice of a loss in ductility). Usually, α-β alloys are heat treated below the β transus, giving the optimum balance of ductility, fracture toughness, creep and stress rupture properties.

Further reading:

Research and development in titanium : Excellent publication of the Australian Natural Resources Data Library
SAE international : Ultimate knowledge source for mobility engineering
ASTM international : American organization for the development of international standards
Key to Metals - Chemical and Mechanical Properties of Titanium and its Alloys : The world’s most comprehensive Metals Database


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