Copper and copper alloys such as Aluminum Bronze (Al-Bronze) and Nickel Aluminum Bronze (NAB) are used in components subjected to harsh environments. Some applications include driving bands of projectiles, bearings requiring wear resistance, and ship propellers requiring high corrosion and cavitation resistance. These materials have high resistance against plastic deformation and have high fatigue strength. Ship rudders of low alloy steels, exposed to harsh environments, are predominantly affected by cavitation erosion, which can be mitigated by coating with copper-based alloys. Currently, weld cladding is employed to repair damaged components such as propellers. Thermal spray offers an alternative method to repair damages brought in by cavitation and erosion on ship rudders and propellers.

Thermal spray processes can deposit thick coatings to protect against harsh environments. Cold spray (CS) process, which does not involve the melting of particles and only accelerates the powder particle, has the potential to preserve the microstructure of the powder and deposit very thick coatings for repair applications. However, the gas-atomized powders are hard due to the presence of the martensite phase, and the optimization of the cold spray parameters and heat treatment of the powder is required for a successful deposition. The High-Velocity Oxygen-fuel (HVOF) process involves the spraying of molten/semi-molten particles at supersonic velocities, resulting in very dense coatings. However, there are problems associated with in-flight oxidation and defects due to solidification. The present study aims to compare the microstructure and properties of Cu and Cu-based alloy coatings prepared by HVOF and cold spray.

Preliminary work was carried out on cold spraying of copper. Cold-sprayed copper coatings are known to have low ductility as the microstructure is composed of dynamically recrystallized refined grains and larger crystals with high dislocation density. Cold sprayed copper coatings were subjected to annealing treatment at 200-400 °C temperatures for 1 h. Recrystallization behaviour was studied using Differential Scanning Calorimetry (DSC). The influence of annealing on the microstructure was explored in detail using Electron Back-Scatter Diffraction (EBSD). It was observed that the static recrystallization started at around 200 °C at the particle interfaces. The recrystallized microstructure was found in the 300 °C heat-treated sample. The recrystallization had an effect on the mechanical properties of the coating, which was characterized by hardness and tensile testing. A significant hardness drop was found, and an increase in tensile strength and yield was also observed.

Keywords: Cold Spray, HVOF, Copper, Recrystallization, Aluminum Bronze, Nickel Aluminum Bronze.