Foundry and casting of titanium and titanium alloys

The first demonstrations of an industrial titanium melting and casting process took place in the USA in 1954, using vacuum melting in a consumable electrode arc furnace and vacuum casting in a mold machined from a block of high-density graphite. In France, in 1968, by vacuum melting using electron bombardment and casting in a mold made from graphite powder compressed onto a metal pattern.

In France, the prospect of the development of the supersonic Concorde aircraft and the SNECMA M53 military engine (from Société Nationale d'Étude et de Construction de Moteurs d'Avions) accelerated the development of this process for complex-shaped parts that have to work at temperatures of around 400°C (thus prohibiting the use of aluminum alloys).

Titanium casting is a cost-effective way of obtaining complex shapes, and in particular hollow shapes that are impossible to obtain by forging or machining.

Using the lost-wax casting process, it is now possible to produce titanium castings in almost all the usual shapes of steel castings produced using the same process.

The parts produced have masses ranging from a few grams to several hundred kilograms. Minimum thicknesses are of the order of a millimeter, and can be further reduced to a few tenths of a millimeter by chemical fluonitric machining.

However, the core-making technique for obtaining long channels in relation to their diameter is still under development and imposes limitations (to be examined on a case-by-case basis by the customer and the foundry).

The static mechanical properties of titanium alloy castings are very similar to those of forgings. For TA6V, the most widely used alloy, minimum tensile strength is around 880 MPa, yield strength around 780 MPa and elongation at break around 8%.

The internal metallurgical health of the parts is considerably improved by the use of hot isostatic compaction, which consists in densifying the parts by subjecting them to an argon pressure of around 1,000 bar (remember that 1 bar = 10 ⁵ Pa) at a temperature of around 920°C for 2 h (for TA6V). The result is an improvement in the fatigue properties of the parts, which then approach those of wrought parts.