Mechanical and metallurgical characterisation of electroformed nickel for helicopter erosion shield applications

The influence heat treatment has on the tensile, notch toughness, microstructure and fatigue properties of electroformed nickel have been studied. From the test data it is clear that by exposing electroformed nickel to temperatures in excess of 250 °C a significant loss of tensile strength, ductility and notch toughness occurs. In addition stiffness has been shown to gradually increase when exposed to temperatures from 200 °C onwards. The change in properties appears to be associated with microstructural changes, i.e. the crystallite structure is thermally unstable and gradually recrystallizes leading to breakdown of the fine crystallites and subsequent grain growth. In the case of ductility the severe reduction is considered to be primarily due to embrittlement caused by sulphur within the electroform precipitating out around the grain and sub-grain boundaries. Consequently, for electroformed nickel to be considered an engineering material for erosion shield applications it should not be subjected to temperatures above 200 °C. on erosion shield cut ups, electroformed nickel appears to be essentially isotropic in nature due in part to the extremely fine crystalline microstructure. As mechanical property variability is greater with electroformed nickel when compared to wrought products it is essential that integral mechanical property test pieces are employed in order to be able to fully characterise the properties within the electroform. Certain electroforming defects have been shown to lower high cycle fatigue life; hence it is vitally important that in case of critical components, robust Non-Destructive Inspection (NDI) techniques are employed to ensure they are eliminated from production components.