Why is Inconel hard to machine?
Inconel 718 superalloy is one of the most important alloys in the nickel-based superalloy family. This high-temperature alloy is mainly used in the aerospace industry to manufacture aerospace parts and gas engine components. In gas engines, about 50% of the components are made from nickel-based superalloys. The material is suitable for heat treatment vessels such as gas turbine blades, turbine blades, etc. The use of Inconel 718 in the aerospace industry stems from its unique properties, such as high resistance to oxidation, corrosion resistance in corrosive environments, resistance to creep, thermal fatigue resistance at high temperatures (up to 923 °K) and low temperatures, and its high mechanical strength under these conditions.
Components of modern gas turbine engines, such as turbine blades, operate at high temperatures (in the range 823.15–1373.15 °K). This has led to the production of components using advanced engineering materials, including nickel-based superalloys. Among these superalloys, Inconel 718 is the most widely used. In order to improve the durability of this high-temperature alloy, a considerable number of holes (20,000-40,000) are drilled into the structure of the turbine blades, with a diameter of 0.3-5 mm and an aspect ratio of (40-600): 1 (depth to -diameter ratio). These holes are identified as "cooling holes" because coolant (gas or liquid) is able to flow through them, lowering the temperature of the component material. Dimensional shape accuracy and hole inner surface quality affect the efficiency of the cooling process. Unconventional methods are often chosen to drill cooling holes, such as electrical discharge machining (EDM).
The chemical composition of Inconel 718 ensures its excellent mechanical properties. Alloying elements such as Ni and Cr resist corrosion, oxidation, carburization and other damage mechanisms at high temperatures. Ni and Cr crystallize as a gamma phase (face-centered cubic). In addition, Al, Ti, Nb, Co, Cu and W are added to improve mechanical and corrosion resistance. Nb is added to form the hardened precipitate γ' (Ni3Nb, body-centered tetragonal metastable phase). Ti and Al are added to precipitate the intermetallic compound γ' form (Ni3Ti, Al, simple cubic crystal). These alloying elements are characterized by their ratio The impact of γ" particles is smaller. Two phases (γ' and γ") provide the high strength of Inconel 718 superalloy (ultimate tensile strength of 1.1 GPa). In addition, C is added to precipitate to form MC carbides (M = Ti or Nb). The C content must be sufficient low, so that Nb and Ti precipitate in the form of γ' and γ" particles.
