Net shape HIPping of a Ni-superalloy: A study of the influence of an as-leached surface on mechanical properties

Abstract

Hot Isostatic Pressing (HIP) is a net-shape powder metallurgy technique where powders densification is achieved through the application of high temperature and pressure at the same time. Powders are allocated into a hollow steel mold called capsule or canister which gives the final shape to the particles. This technique is particularly useful for manufacturing complex components made of materials which are extremely difficult to process via forging or casting. Thus, HIP is particularly indicated to handle superalloy powders such as Astroloy, which is the object of the following study. One of the most attractive peculiarities of HIP is the low material waste obtained since the overstock is limited to the layers immediately beneath the steel capsule. At the end of the HIP cycle, the canister is typically removed using an acid leaching bath which is responsible for the alteration of the outermost layers of the final product. Only a little number of research papers deal with the optimization of the removal of these layers; Consequently, manufacturers often apply a very conservative approach by eliminating more material than is actually needed with a final machining procedure. This paper aims to optimize this procedure by systematically assessing the total thickness of the altered layer of material deriving from the HIPping and leaching process together. To achieve this goal, a set of samples were prepared by removing progressively thicker layers of material and then they were bend tested. Finally, the recorded mechanical properties were compared with those obtained with the samples machined from the core material. One of the main findings is that the removal of 500 μm of material is enough to recover mechanical properties which are comparable with those observed in samples coming from the core. More specifically, by eliminating the first 100 μm material, all the corroded layer is removed, which results in an overall increase of all the mechanical properties except for ductility. This property strongly depends on the number of prior particle boundaries arising from the HIPping process itself. Thus, the correct amount of overstock material must include both these layers.