Spread and fracture patterns in forging superalloy fibre-reinforced composites

Abstract Fibre-reinforced superalloy-matrix composite materials combine superior properties of oxidation resistance, high strength coupled with ductility and toughness at elevated temperatures. However, they are extremely difficult to machine or form by conventional metal working processes due to the strongly anisotropic nature of their properties. Isothermal forging, where the billet and dies are maintained at the forging temperature during deformation, is one of the most important processes evolved in recent years and by combining this process with powder fabrication, highly alloyed casting-type materials can be processed into homogeneous products which are extensively used in the aerospace industry. In the present investigation, the open-die isothermal forging of square-section billets of fibre-reinforced superalloy composites (hot isostatically pressed nickel-base superalloy compacts reinforced with 40% volume fraction of tungsten wires) at constant strain-rates was considered, with the aim of providing useful information concerning the formability of the composite material and, therefore, its industrial applicability, i.e. in the shaping of complex forms. The mode of deformation, the macroscopic fracture behaviour of the composite material and the spread encountered for the various forging parameters has been given. It is concluded that deformation in composites proceeds mainly under conditions of plane-strain, that formability is limited at low strains and that a forging limit criterion of geometric nature is to be expected.