Tamping-induced densification and transient phase transformation in heterogeneous Ni–Al cold spray: An atomic-scale study

Current cold spray research relies predominantly on simplified single-particle impact models, which cannot capture the critical effects of multi-particle interactions and complex thermal histories on coating microstructures, particularly in dissimilar material systems. This study employs molecular dynamics simulations to investigate sequential deposition of Ni–Al heterogeneous particles onto an aluminum substrate. Results reveal three distinct velocity-dependent regimes of coating evolution: (1) At sub-critical velocities (500 m/s), insufficient localized plasticity results in porous deposits with unclosed interfacial voids; (2) within the optimal 750–1000 m/s window, a pronounced tamping effect from subsequent impacts synergizes with strain localization facilitated by rigid Ni particles, driving extensive strain field overlap and layer densification; (3) at hyper-critical velocities (1250 m/s), global thermomechanical instability causes severe substrate erosion. Microstructural analysis reveals a fundamental asymmetric thermomechanical response: the softer Al phase undergoes transient local melting, deforming via viscous flow, while the harder Ni phase predominantly retains its solid crystalline structure, deforming through dislocation-mediated plasticity. Quantitative dislocation analysis shows the total dislocation segment count (Nseg)—a proxy for work hardening capacity—peaks at 1000 m/s, then declines sharply at 1250 m/s due to complete defect annihilation in molten Al and onset of dynamic recovery in solid Ni. This work establishes a physics-based processing window (750–1000 m/s) for Ni–Al/Al systems that balances mechanical densification with thermodynamic stability, highlighting the pivotal roles of cumulative heating and asymmetric phase transformations in cold spray additive manufacturing.