The Role of Substrate Preheating on the Adhesion Strength of Cold-Sprayed Soft Particles on Hard Substrates

Induction heating cold spray has recently been proposed as a new hybrid additive manufacturing technique and a possible solution to avoid the delamination of soft material coatings from hard substrates. This process relies on the combined contributions of substrate preheating and in situ coating heating as a result of the alternating electromagnetic field applied that generates eddy currents. However, fundamental studies at the micron-scale explaining the role of substrate preheating on the adhesion strength of the soft/hard (particle/substrate) material combination are scarce. Therefore, this study focuses on investigating the role of substrate preheating in induction heating cold spray on bond formation and adhesion strength at the individual particle level by removing sprayed single soft particles (pure aluminum) from hard substrates (Ti-6Al-4V) using the splat adhesion test. Substrate preheating improved the splat adhesion strength by 30% and the bonded area sixfold. In addition, due to an increase in the splat adhesion energy, the failure mode changed from brittle to a more ductile type of failure when the initial substrate surface temperature was increased from 25 to 600 °C. These results are attributed to a combined effect of a more effective oxide removal on the substrate surface due to enhanced localized deformation, and a local heat treatment process in the vicinity of the particle/substrate interface that induces recovery, and, possibly, static recrystallization.