A method for testing mechanical properties of powders and fitting constitutive laws for cold spray feed-stock materials

Powders are the starting point of every cold spray coating. During the process, they undergo large plastic deformation at the solid state, in highly dynamic regime. Due to their small size and to the extreme strain rates they endure during cold spray deposition, conventional mechanical tests are not suitable in this context. This work aims at establishing an experimental method to assess the mechanical behavior of powder material at the particle scale, composed by two different tests. First, the material behavior is characterized in the quasi-static regime through single particle compression tests. Secondly, the high strain rate is explored by means of the so-called LASHPOL experiment (Laser Shock Powder Launcher), in which single particles are accelerated at cold spray velocities using a pulsed laser source and a specific set-up. Once data are collected in these two experiments, a numerical framework, combining Finite Elements Analysys and a meta-model approach, is developed. This allows fitting the parameters of the constitutive law chosen directly for materials in powder form. Results on two different aluminum alloys show important differences with respect to the bulk materials found in literature. Furthermore, a nano-indentation study indicates that the powders have a non-homogeneous and non-isotropic behavior.