Evolution of Cold-Sprayed Copper Deposit Mechanical Properties as Function of Substrate Geometry and Heat Treatment Parameters

Abstract

As cold spray transitions from a coating technology to an additive manufacturing process, the variety of part shapes and sizes is significantly increasing while the drive to optimize the mechanical properties of cold-sprayed deposits has emerged. This study investigates the effect of substrate characteristics, a variable typically neglected, on copper cold-sprayed deposit hardness and tensile properties before and after deposit heat treatment. Substrate material, shape and size were varied, resulting in a range of substrate process temperatures as evaluated using IR camera measurements. Deposits sprayed on the smallest parts, i.e., those displaying the highest temperatures, exhibited lower hardness due to spontaneous in situ annealing caused by the thermal energy rise intrinsic to the cold spray process. Smallest parts also presented the best ductility from a combination of spontaneous in situ annealing and improved interparticle cohesion. The as-sprayed deposit strength was rather optimized at an intermediate substrate size and temperature where partial spontaneous in situ annealing was combined with improved interparticle cohesion. After deposit heat treatment, differences among tested substrate sizes vanish. Deposit heat treatment at 350 °C causes softening, a decrease in strength and an increase in ductility due to the annealing effect. The sintering effect becomes predominant for deposit heat treatment at 600 °C, leading to an increase in ultimate tensile strength and further improvement in deposit ductility. This work has important implications for cold spray process scale-up, as one cannot assume the properties achieved after process optimization on small laboratory-size coupons will be maintained on larger parts.