Metal powder is a raw material used for variety of manufacturing technologies, starting from large-scale manufacturing of precision parts for e.g. automotive industry by conventional powder metallurgy (PM) to the customized manufacturing of high-end product by additive manufacturing (AM) for e.g. aerospace and biomedical applications.
Metal powder is characterised by large surface area that is about 10 000 times larger than the surface of a bulk material of the same mass. This results in high surface reactivity of the powder and hence high sensitivity of the powder to e.g. oxidation, starting from powder manufacturing and following powder handling and processing using variety of powder metallurgy and additive manufacturing processes. This is especially important in case of reactive metals as Ti- and Al-alloys and complex alloys containing elements with high sensitivity to oxygen as Ni-base super-alloys, stainless steels, tool steels, etc. In order to assure necessary powder quality, variety of powder manufacturing methods are used for powder manufacturing, selection of which is based on powder alloy composition, required purity, powder quantity and cost. However, powder surface chemistry will undergo significant changes in terms of oxide transformation and distribution in the consolidated material, that is determined by the powder alloy composition and consolidation process applied. Hence, powder surface chemistry determines requirements to the consolidation process and its efficiency as well as properties of the consolidated material when it comes to the final density and mechanical performance.
Analysis of the powder surface chemistry and the assessment of thermodynamics and kinetics of surface reactions are of prime importance for understanding the changes in surface composition related to the process conditions during powder consolidation. Such understanding is of vital importance for manufacturing of oxide-free high-performance components. The analysis is typically performed by means of advanced surface characterisation and thermal analysis techniques in combination with the modelling of the effect of processing conditions. Such experimental input in combination with thermodynamic simulation allows to establish generic models of oxide reduction/formation/transformation in different alloy systems during powder consolidation using different powder metallurgy processes (conventional press&sinter, hot isostatic pressing, liquid phase sintering, etc.) as well as powder-based additive manufacturing (powder bed fusion, binder jetting, material extrusion, etc.).
Keywords: powder metallurgy, powder-based additive manufacturing, surface oxide, powder consolidation, oxide transformation.