Metallurgical assembly in jet engine construction

The turbojet engines powering today's civil and military aircraft call on a very broad and unique range of materials, due to their wide operating temperature range (-50 ˚C to 1,250 ˚C). Moreover, these materials have often found their source of development from the needs of this industry (titanium alloys for high temperature or high strength, nickel superalloys from powder metallurgy or monocrystalline...).

The constant progress demanded of designers, to increase the thrust/mass ratio of these turbojet engines and drastically reduce ownership costs, has led to the implementation of metallurgical assembly techniques (welding, brazing and derived processes), available or specific, to reduce the mass of the assemblies and to be able to repair certain high value-added assemblies, previously discarded.

A specific feature of assemblies in the aeronautics industry is the high level of quality that must be demonstrated, given the risks associated with transporting people.

The main families of turbojet engine parts are :

• rotating parts: "low-pressure" and "high-pressure" compressor disk assemblies, using different titanium alloys and high-strength wrought nickel superalloys respectively. These parts are designed for low-cycle fatigue and contract life. The processes used are essentially electron beam welding and inertial friction welding;

• structural casings, made of wrought or cast nickel and cobalt alloys. They are dimensioned for vibration fatigue. The processes used are arc welding (TIG: "tungsten inert gas", plasma) and laser welding;

• fixed and mobile turbine parts work from 700 ˚C to over 1,100 ˚C. Most of them are made from lost-wax-cast nickel superalloys with equiaxed, columnar or single-crystal structures. Due to the very poor weldability of these alloys, the main processes used are diffusion brazing and its derivatives.

For these three families of parts, we describe in turn :

• the operating fields of the parts, the mechanical constraints and any constraints linked to the operating environment, and the alloys selected by the designers and used;

• general data on the weldability (or solderability) of the main alloys used. This general data covers many aspects: mainly the control of weld seam non-cracking, but also the notions of pre-weld preparation, protection against oxidation, as well as those of residual stress relaxation and the mechanical characteristics of the resulting bonds;

• experience acquired and published, illustrating the process for producing assembled parts of specified...