Mesoscale Modeling of Cold Spray Deposition of Metal Powders

Cold spray (CS) is an additive manufacturing/repair process where micron-sized powder particles are impacted at high velocities to adhere onto a substrate. The mechanisms related to particle-substrate bonding in metallic CS systems have raised a “correlation or causation” debate, which has motivated several theoretical and experimental studies. Unfortunately, most experimental explanations are restricted to post-mortem observations, and the modeling reports have faced limitations like low interfacial resolution, unavailability of materials data, and inability to include features like deformation micro-mechanisms and crystallinity. Classical molecular dynamics (MD) can ideally incorporate these missing elements but are currently confronted by the enormous and unattainable computational demands to model the spatial (microns) and temporal (nanoseconds) domain of a typical CS process. This dissertation outlines an approach to model impact induced adhesion and the subsequent microstructural evolution at the length and time scales in a cold spray process. This is achieved by merging MD-like insights to the mesoscales using a method known as Quasi-Coarse-Grained Dynamics (QCGD). First, this requires an extensive MD-QCGD validation approach spanning across length scales to comprehend the size-dependence of impact deformation. This is performed for aluminum particles of sizes ranging from 6 nm to 20 microns for impacts on a rigid substrate, from which statistical models are intuited. Second, using a QCGD method that guarantees reasonable accuracy, single particle impact simulations of aluminum-on-aluminum setups are executed, and the process of interfacial material ejection, or “jetting” is studied for a range of particle temperatures and impact velocities. The controversy behind jet initiation mechanisms is addressed, with specific scrutiny on hydrodynamic pressure-wave interaction and adiabatic shear instability (ASI). Investigations of these mechanisms show that a compressive plastic-wave interaction with the particle-substrate interface periphery is crucial to thrust an incipient jet and is responsible for the localized thermal softening. The simulations also reveal the ability of cold sprayed aluminum splats to undergo dynamic recrystallization, and its variation with process parameters. Third, an extension to scarcely investigated system of spherical bcc tantalum powders shows the competing slip/twinning role at lower impact speeds, additional character of melting/amorphization at high impact rates, and its subsequent influence on grain refinement at the particle-substrate interface. Finally, a continuance to multi-particle impact simulations concludes a holistic modeling methodology, one which is transferable to include the effect of brittle oxide layers and alloy systems in the future.