Titanium Production Processes

Titanium must undergo a number of processes to get from an ore to a finished product. Depending on the final application, the number of steps undertaken varies. The titanium used in a high-end car exhaust, hip implant or watch does not require the same rigorous management of microstructure as that used in aviation, where the risks and consequences of failure are considerably greater.

The Kroll Process

Regardless of the final usage titanium must first be removed from its ore and turned into pure titanium.

This is done by processing titanium oxide manufactured from either ilmenite or rutile though the Kroll process. The output is a titanium sponge which is purified, melted and alloyed with other metals. It may then undergo further processing into what is called a master alloy before being made into an ingot that can be fabricated into a bar, plate, sheet or wire as a general mill product.

The Kroll Process It is a multi-stage reaction which starts in a fluidized bed reactor. Purified titanium oxide Ti02 is oxidised with chlorine to create titanium tetrachloride TiCl4 known affectionately in the industry as “tickle”. This reaction is done at 1000°C. From there the TiCl4 and other metal chloride impurities are fractionally distilled to produce a pure mixture of TiCl4.

This is then moved into a separate stainless steel reactor where it can be mixed with magnesium in the second half of the process. The reaction takes place in an atmosphere of argon at a preheated temperature of 1000°C. Titanium (lll) and Titanium (ll) chlorides are produced which slowly reduce to form pure titanium and magnesium chloride over a number of days.

The leftover magnesium chloride is then recycled by separating it back into its constituents. Whilst the titanium, now in “sponge” form, is jackhammered out, crushed into smaller pieces, and pressed back together to produce a uniform piece. It is ready to be melted as the electrode in the Vacuum Arc Remelting (VAR) process.

Vacuum Arc Remelting Titanium – Alloying

VAR has been the primary method of titanium alloy manufacture since the 1950s. It is a process used to create high performance titanium products. VAR exercises precise control over the melting and solidification of the melt which allows the reliable creation of high quality finished alloys with excellent purity.

Most Titanium alloys are put through this process at least twice. This is to ensure an acceptable level of consistency throughout the metal. The process is still used for industrial applications. Triple VAR was mandated for aerospace in the second half of the twentieth century in response to deficiencies in the metal. Although in the last ten years it has begun to be superseded by electron cold hearth re-melting for alloys destined for aviation.

The VAR process takes place in a large cylindrical crucible. The electrode is suspended from the top with several kiloamperes of DC current passed through it to cause it to melt and drip to the bottom of the chamber where the alloy reforms. In the case of Ti 6AL-4V aluminium and vanadium are added.

Electron Beam Cold Hearth Remelting

Cold hearth melting provides an effective fix to one of the deficiencies of the VAR process the ability to effectively remove high density and low-density inclusions from the melt (HDI and LDI). It is also used to process waste chips from the machining process. A vacuum is formed inside a water-cooled copper hearth. Then a high-temperature electron beam is concentrated onto the titanium feedstock (a mixture of sponge, VAR metal, and chips) placed in the hearths rear. The molten titanium drips into the melting area, it then flows into a refining channel before pouring into the mould where the metal crystallises. Volatile compounds evaporate namely oxygen and nitrogen inclusions whilst dense tungsten carbide from cutting tools sinks to the bottom. The extremities of the ingot are then machined away leaving the titanium alloy.

Next, the titanium is processed into useful shapes, in the case of EBCHR this is predominantly forging.

Forging and casting titanium

Titanium can then either be cast or forged to produce a metal with the desired properties. Casting requires the metal to be heated until molten and is usually used for non-critical applications where cost is the primary concern.

While in a liquid state the titanium it is poured into a mould to create the desired shape. It is less costly than forging titanium and allows the creation of a near net shape for the application in question. The process of casting can grow dendrite grains – a tree like structure which can make the metal weaker restricting the use in some applications.

Forging is the application of thermal and mechanical energy to titanium billets or ingots to cause the material to change shape while in a solid state. Due to the reactivity of the metal and the high temperatures and pressures involved the ingot is coated with protective glaze/glass. This prevents it reacting with the atmosphere whilst also allowing it to deform. The process of forging allows the desired microstructure of the metal to be developed effectively.

Heat Treatment of Titanium

Heat treatment allows phases to be manipulated in an alpha beta alloy. The variables altered are compositions, sizes, and distributions.

Annealing of titanium alloys

Annealing is a metallurgical heat treating process of titanium that alters its chemical and physical properties. It causes atoms to migrate within the metals lattice allowing alterations to an alloy’s properties to be made. These improvements include: ductility at ambient temperatures, fracture toughness, creep resistance and thermal stability. Many of these properties are mutually exclusive, so the cycle chosen would reflect the metal’s final uses. There are four primary annealing treatments.

Alpha and near alpha alloys are not dramatically altered by these processes, they are more likely to undergo stress relieving and annealing. This is because they undergo a very limited phase change due to the limited presence of beta phase to re-orientate. Solution treatment and aging will improve the strength of alpha alloys.

  • Mill annealing is the most common type of annealing it produces a finer grain size which can be useful where increased yield strength is preferred to creep strength. Is typically undertaken as a distinct manufacturing step.
  • Duplex annealing improves creep resistance and fracture toughness by altering the shape, size and spatial distribution of the metals phases.
  • Recrystallisation annealing is the process by which a metal’s ductility can be improved. Deformed grains are replaced with defect grains. The initial primary beta areas that form are too large, the gaps between them form potential lines of weakness ill-suited to high stress applications. Recrystallisation causes these zones to break up forming smaller less homogenous crystals which are stronger.
  • Beta annealing is for metastable beta alloys. They can not only be stress relieved and annealed but they can also be solution treated and aged.

Stress Relieving Titanium Alloys

This is the most common form of heat treatment. It is used in a wide range of titanium alloys including alpha and near alpha alloys as well as alpha beta and meta-stable beta alloys. The aim is to reduce residual stresses that are developed during fabrication.

Solution treating and aging of Titanium alloys

Solution annealing, quenching and then aging yields the highest strength titanium alloys. A titanium alloy’s beta phase begins to decompose at temperatures below the beta transus, exceeding it in some alpha beta alloys can reduce the metal’s ductility.

After the alloys have been heat treated they can be fabricated into usable basic products including plate, sheet, tube, bar and wire, ready for machining.