Powder metallurgy for production of aerospace superalloys

Forging, extrusion, rolling and other working operations are used in the forming and shaping of aluminium, magnesium, titanium and steel structural components for aerospace applications. However, it is difficult to shape nickel-based superalloys for jet engine components using these same working operations. Superalloys are so-named because of their very high alloy content; typically 40–60% of alloying elements (such as iron, chromium and molybdenum) in a nickel matrix. The high concentration of alloying elements is needed to create a large volume fraction of hard intermetallic precipitate particles. These particles provide the superalloy with high mechanical performance at the operating temperatures of jet engines. Owing to these particles, however, superalloys cannot be shaped using conventional working operations without tearing and cracking.

Since the mid-1970s, the aerospace industry has manufactured superalloy components using a powder metallurgy process called hot isostatic pressing (HIP). Powder metallurgy is a fabrication technique that involves three major processing stages: (i) production of metal powder, (ii) compaction and shaping of the powder, and (iii) consolidation and fusing of the powder into a solid metal component under high temperature and pressure. The first process step involves the production of fine spherical superalloy powder using a gas atomisation process. Powder is produced by pouring molten superalloy through a narrow hole to produce a thin liquid stream. High-pressure argon gas is blown into the metal to break up the stream into tiny droplets which rapidly solidify at ~ 10⁶ °C s⁻¹. The fine spherical superalloy powder is then injected into a high pressure container to form a weak compact with a shape close to the final component. Air within the container is removed to avoid any contamination of the powder during the final processing stage of hot isostatic pressing. The powder is consolidated in the HIP process under high temperature (1100–1300 °C) and high pressure (15 000–45 000 psi). Pressure is applied to the compacted powder from all directions, hence the term ‘isostatic pressing’. The high temperature and pressure fuse the superalloy particles into a dense solid by plastic flow and diffusion bonding. The finished component has a fine grain structure that is virtually free of porosity.

One of the main advantages of the HIP process is the ability to fabricate components to the near-net shape that requires little machining. Conventional working operations do not usually form products to the near-net shape, and for many aircraft components anywhere from 70–90% of the material must be removed by machining to achieve the final shape. A component made by HIP typically requires the removal of only 10–20% of the material and, as a result, the machining time and cost are reduced.