Enhancement of diffusivity and plastic deformation in ultrasound-assisted cold spray of tungsten: a molecular dynamics study

Abstract: Tungsten ($W$) is widely valued for its exceptional thermal stability, mechanical strength, and corrosion resistance, making it an ideal candidate for high-performance military and aerospace applications. However, its high melting point and inherent brittleness pose significant challenges for processing $W$ using additive manufacturing (AM). Cold spray (CS), a solid-state AM process that relies on high-velocity particle impact and plastic deformation, offers a promising alternative. In this study, we employ atomistic simulations to investigate the feasibility of CS for tungsten. We show that ultrasound perturbation can significantly enhance the self-diffusivity and plastic deformation of $W$ compared to the negligible diffusion and plastic deformation observed in non-ultrasound-assisted CS of $W$. For different impact velocities, particle sizes, and ultrasound parameters, we demonstrate that ultrasound-assisted viscoplasticity enhances self-diffusivity by inhibiting grain boundaries and incorporating softening in $W$. Moreover, we found that this enhanced diffusion in ultrasound-assisted $W$ can be exploited to promote interdiffusion at the particle-substrate interface, enabling in situ alloy formation. Through the formation of an equimolar $V$-$W$ alloy on a $W$ substrate using ultrasound-assisted CS simulations, we observed distinct mechanical properties and a reduced dislocation density in the deposited coating compared to a pure tungsten substrate. These results highlight the potential of ultrasound-assisted CS as a viable approach for fabricating uniform coatings and engineered alloys, addressing key limitations in the AM of refractory metals.