How Much Do You Know About The Titanium Production Process?(two)

Continuing from the previous article:

Titanium is machined into useful shapes, in the case of EBCHR, this is mostly forging.

Forged and cast titanium

Titanium can then be cast or forged to produce a metal with desired properties. Casting requires heating the metal to melt and is often used for non-critical applications where cost is a major concern.

When the titanium is in a liquid state, it is poured into a mold to form the desired shape. It costs less than forged titanium and can create near net shapes for related applications. The casting process grows dendrites—a tree-like structure that can weaken the metal, limiting its use in certain applications.

Forging is the application of thermal and mechanical energy to a titanium billet or ingot to cause the material to change shape in its solid state. Due to the reactivity of the metal and the high temperature and pressure involved, the ingot is coated with a protective glaze/glass. This prevents it from reacting with the atmosphere, while also allowing it to deform. The forging process can effectively develop the desired metal microstructure.

Heat Treatment of Titanium

Heat treatment allows manipulation of phases in alpha beta alloys. The variables that changed were composition, size and distribution.

Titanium alloy annealing

Annealing is a metallurgical heat treatment process that alters the chemical and physical properties of titanium. It causes atoms to migrate within the metal lattice, changing the properties of the alloy. These improvements include: ductility at ambient temperature, fracture toughness, creep resistance and thermal stability. Many of these properties are mutually exclusive, so the cycle chosen will reflect the end use of the metal. There are four primary annealing treatments.

Alpha and near-alpha alloys are not significantly altered by these processes, they are more likely to undergo stress relief and annealing. This is because they undergo a very limited phase transition, which cannot be reoriented due to the limited presence of the beta phase. Solution treatment and aging will increase the strength of alpha alloys.

a. Mill annealing is the most common type of annealing and produces a finer grain size, which is useful in cases where yield strength is better than creep strength. Usually performed as a unique manufacturing step.

b. Dual phase annealing improves creep resistance and fracture toughness by changing the shape, size and spatial distribution of the metallic phase.

c. Recrystallization annealing is a process that can improve metal ductility. The deformed grains are replaced by defective grains. The primary beta regions formed initially are too large and the gaps between them form potential lines of weakness, not suitable for high stress applications. Recrystallization causes these regions to break up, forming smaller, less uniform crystals that are stronger.

d. Beta annealing is suitable for metastable beta alloys. Not only can they be stress relieved and annealed, they can also be solution treated and aged.

Stress relieved titanium alloy

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 and alpha beta and metastable beta alloys. The purpose is to reduce the residual stress generated during the manufacturing process.

Solution Treatment and Ageing of Titanium Alloys

Solution annealing, quenching and then aging produce the highest strength titanium alloys. The beta phase of titanium alloys begins to decompose at temperatures below the beta transformation, exceeding it in some alpha beta alloys reduces the ductility of the metal.

After the alloy has been heat treated, it can be made into usable basic products, including plates, sheets, tubes, bars and wires, ready for machining.