In-flight temperature of solid micrometric powders during cold spray additive manufacturing

During cold dynamic gas spray additive manufacturing, the thermal field within particles is generally simplified by a state of an instantaneous uniform distribution over the particle's media at any position inside and outside the De-Laval nozzle. This paper addresses critical discussions using analytical and computational analysis of the transient heat transfer within the solid particles due to the convective exchange with the flowing gas. An analytical criterion depicts conditions of instantaneous uniform temperature over the range of typical cold spray data including various particle thermal conductivity, particle size range, dragging velocities, gas nature and gas setting conditions. The analytical depiction draws the conclusion that, during cold spraying, the temperature field within particles is mostly instantaneously uniform. The notion of instantaneousness is further investigated via computational analysis of the transient thermal gradient regimes within particles. The phenomenological computations enable to characterize the duration of the transient stage prior to the state of uniform temperature. Comparisons with analytical particle's residence time using various scales of travelled distance give a more relevant notion of instantaneousness. The particle temperature is not strictly instantaneously uniform. However, such instantaneousness prevails for a covered distance unit of 1 mm which is widely enacted in the literature to compute the particles temperature during cold spraying. As conclusive remarks, issues due to this instantaneousness of the particle temperature are reviewed and suitable alternatives for efficiently heating the particles during cold spraying are reported.