Cold spray (CS) is a rapidly developing solid-state repair and coating process, wherein metal deposition is produced without significant heating or melting of metal powder. Solid state bonding of powder particles is produced by impact of high-velocity powder particles on a substrate. In CS process, metal powder particles typically of Aluminum or Copper are suspended in light weight carrier gas medium. Here high pressure and high temperature carrier gas is expanded through a converging-diverging nozzle to generate supersonic gas velocity at nozzle exit.
The CS process typically uses Helium as the carrier gas due to its low molecular weight, but Helium gas is quite expensive. This warrants a need to explore alternate carrier gases to make the CS repair process more economical. Researchers are exploring another viable option of using pure Nitrogen as a carrier gas due to its significant cost benefits over Helium. However, it shows challenges in achieving desired powder particle velocities and hence metal deposition efficiency. The work presented in this paper explores a carrier gas mixture of H2 and N2 as an alternate to Helium, to reduce the carrier gas cost.
This paper describes a CFD methodology developed for predicting particle impact velocities in CS repair process at the exit of converging diverging nozzle. The key challenge in developing CFD methodology was about coupling gas & powder particle flow dynamics and application of appropriate drag laws. This modeling methodology was able to predict particle velocity at nozzle exit with more than 95% accuracy. Subsequently CFD methodology was deployed to recommend suitable process parameters such as carrier gas composition, pressure, and temperature. The process parameters have been studied using a design of experiments (DoE) study with the aim of arriving at a carrier gas composition that can help in achieving critical deposition velocity at a reduced cost.