Lost wax Investment Casting - Applied to superalloys

This article deals with the characteristics of Investment Casting for one of its most modern applications, jet engine blades. The description of these processes is limited by the industrial property rights of the companies that developed them.

Here we return to the well-known stages of lost-wax casting, practised since at least the 2nd millennium B.C. in various parts of the world, and which has enabled the casting of many remarkable artistic works in bronze. Donatello (1386 to 1466) was the first since antiquity to cast an equestrian statue in Europe.

Progress made over the last forty years in the formulation of metallic materials with high mechanical properties at high temperatures has led to the development of structurally hardened nickel- and cobalt-based alloys, and their use in the components of turbomachinery subject to the highest temperature loads.

The use of precision molding techniques for their shaping is particularly justified, if not imposed, by the following reasons:

• These alloys, which by definition have low heat deformability, cannot be forged or easily machined;

• In order to extend the service life of parts subjected to the highest operating temperatures, their designers include a ventilation circuit through which air flows to cool them during operation. The core-making technique, an integral part of every foundry process, is used to produce these hollow parts;

• the complex aerodynamic shapes of certain jet engine components are obtained directly from the foundry, with reduced dimensional tolerances and high-quality surface finishes, and at considerably lower manufacturing costs than using any other machining technique, conventional or otherwise;

• control of metallurgical structures by controlling the solidification modes of the metal after casting, and the associated heat treatments, to optimize material properties according to operating conditions;

• finally, the implementation of quality assurance procedures, combined with the performance of non-destructive testing equipment, guarantees the integrity of the material and its level of characteristics.