The Influence of Feedstock Powder

Abstract

Additive manufacturing is based on the concept of freeform structures built up using a consecutive layer-by-layer material deposition approach, enabling the production of complex and functional components in a single manufacturing step. It allows the creation of high complexity components with minimal time and cost, as opposed to traditional subtractive manufacturing techniques. Current metallic AM technologies include selective laser melting, directed energy deposition, laser engineered net shaping, and plasma spraying. Although used commercially, these processes all suffer from the detrimental effects of high temperature processing, generally resulting in component distortion, uncontrolled phase transformations, undesirable residual stresses, and non-uniform mechanical properties. The cold spray process has recently gained attention in the additive manufacturing field as it may mitigate the undesirable thermal effects of current freeform manufacturing techniques, as well as drastically increase the available deposition rates. In cold spray, feedstock particles are injected in a supersonic gas flow and accelerated to velocities as high as 1200 m/s prior to impact. This high impact velocity is responsible for the material consolidation in the cold spray process. The particle impact velocity is dictated by the particle/gas flow interaction, which can be altered through the modification of the gas stagnation properties and spray nozzle geometry. While the effect of the gas/particle interaction is typically the focus of most cold spray research, it has become apparent that the size, shape, microstructure and quality of the feedstock powder have a large influence on the process efficiency. Hence, the effect of powder properties needs to be properly explored, understood, and considered in the powder selection process. This chapter aims to provide a complete reference on the effect of the feedstock particles on deposition quality in an additive manufacturing framework. It should provide the reader with a comprehensive resource for powder selection, pre-treatment, and storage. The effect of powder morphology will be presented. A descriptive analysis of the manufacturing processes used to produce particles will be included. The broad effect of powder size and shape on the particle velocity and resulting deposition will be discussed. Furthermore, the influence of the powder grain structure on particle distortion, dislocation generation, and recrystallization will be described on the basis of high strain rate deformation processes. Beyond the expected properties of the feedstock materials, it is also apparent that the “quality” of the powder is of great importance. The powder quality is thoroughly described by oxygen content and oxide layer type and thickness. This quality has been shown to greatly influence the process efficiency for some materials, and best practices for handling and storage of the powders is discussed. Finally, the status of powder recycling methods in cold spray will be considered along with the advantages of reprocessing in the field of additive manufacturing.