The occurrence of an allotropic transformation in pure titanium controls the type of structures that can be produced by the heat treatment of titanium-rich alloys and hence increases the range of mechanical properties that can be obtained from the alloys. Titanium alloys are divided into three classes depending on the predominant phases present: a or near a, a + β, and β.
Common annealing treatments include mill, duplex, recrystallization, and β - annealing.
Titanium is chemically very active at elevated temperatures, and reacts readily with oxygen, nitrogen, carbon, and hydrogen in a furnace atmosphere. Furthermore, all contamination reactions cause degradation of properties and, with the exception of the hydrogen reaction, are irreversible (if a high hydrogen level is found, vacuum annealing is required).
The atmosphere in the furnace should be free from water vapor and should be slightly oxidizing.
Definition according to ISO 3252: Sintering is a ”thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.”
Sintering of titanium and its alloys takes place between 900 and 1000 °C in an inert gas such as argon, which exclude the hydrogenation of titan.
In hot isostatic pressing (HIP), chemically clean components are placed in a heated, argon-filled vessel and subjected to pressures of 70 to 105 MPa.
Brazing and high-temperature brazing
Brazing is a joining process wherein metals are bonded together using a filler metal with a melting temperature greater than 450 °C, but lower than the melting temperature of the base metal.
High-temperature brazing is flux-free brazing under exclusion of air (vacuum, protective gas) with filler metals whose melting temperature is above 900 °C.
Depending on the base material, two different types of gas atmospheres are used in furnace brazing using flux and inert gas and in high-temperature brazing:
Chemically inert atmospheres, which protect the parts being brazed from coming into contact with other gaseous elements, which might react with the metals being joined thereby producing surface films that might inhibit flowing of, and wetting by the molten brazing alloy.
Titanium and its alloys have oxide films of a high stability; also, they tend to absorb both nitrogen and hydrogen from any atmosphere in their surroundings. Both titanium hydride and titanium nitride will embrittle titanium, and it is, therefore, of fundamental importance to ensure that when brazing titanium or its alloys one employs only vacuum brazing, helium or argon which is free of both these gases.