Beyond Critical Velocity: Unveiling the Dynamics of Impact Bonding for Enhanced Cold Spray Deposition

This study investigates the micromechanics of impact-induced metallic bonding via in situ microparticle impact experiments and site-specific micromechanical measurements. We reveal a significant gradient of bond strength across bonded interfaces, starting with weak bonding at the impact center, a rapid twofold increase, and a plateau towards the periphery. This gradient is associated with a localized surface opening during the early stages of impact. High-resolution TEM observations demonstrate that the form of the native oxide (layers, particles, debris) at the interface dictates bond strength. Stronger bonding correlates with the transition of the oxide from layers to debris.

Our findings indicate that metallurgical bonding requires both sufficient surface exposure via lateral expansion and high local contact pressures, bringing surfaces into atomic proximity. We introduce a predictive framework where bond strength is proportional to effective pressure and surface exposure, constrained by the base metal's flow stress. Finite element simulations validate experimental results, showing that increased impact velocity enhances bond strength. This research provides insights for designing materials and processes that utilize supersonic impact-induced bonding.