Cold Gas Dynamic Spray or Supersonic Cold Spray, or simply ‘Cold Spray’, is an emerging technology for rapidly building thin films, thick coatings and large-scale additive manufacturing at relatively low temperatures. In a cold spray process, particles are accelerated to supersonic speeds by a propellant gas and impact a substrate, thus producing a strong bonding with the substrate and subsequently forming a deposit via layer-by-layer buildup. The scalability and low cost of this method make it promising for many applications in industry, such as metal component surface repair/enhancement/restoration and functional coatings for electrical, thermal, biomedical, energy storage, and nuclear plant applications. However, cold sprayed deposits usually require post process treatments to further modify their microstructures and mechanical properties in order to obtain the desired performances. A number of studies have been carried out on this topic. Here, recent progress in different post process treatments on cold sprayed deposits is reviewed, including heat treatment, friction-stir processing, shot peening, and laser re-melting. The effects of these post treatments on the microstructure, residual stress and mechanical properties of cold sprayed deposits are discussed.

Cold spray technology using micron-sized particles to produce coatings is increasingly used for reparative tasks in various industries. In a cold spray setup, the gun is usually connected to a robotic arm to deposit coatings on components with complex geometries. For these components, the standoff distance used in the cold spray process has to be large enough for easy maneuverability of the gun around a small radial feature. However, a small standoff distance is commonly found in most studies, which is thought to prevent a velocity drop of the particles over a larger distance. Here, a study was carried out by measuring the Inconel 625 particle velocity at different spray distances, ranging from 3 to 40 cm. The highest average velocity of 781 m/s was found at a spray distance of 8 cm. Furthermore, a study with varying powder feed rates was also conducted. An increase in the powder feed rate was found to have a minimal effect on the particle velocity. Inconel 625 coatings deposited at the optimum standoff distance (8 cm) were found to have low porosity and high hardness. The results in this study demonstrate that a larger standoff distance can be applied without a significant drop in velocity for cold spray applications requiring high maneuverability.

Cold spray is an emerging additive manufacturing process that allows particles to be coated onto the surface of a base material without melting. It is suitable to repair components made from temperature-sensitive materials, such as grey cast iron, which cannot be easily restored using conventional methods like welding or thermal spray. In this study, the nickel-based alloy Inconel 625 was successfully coated onto a grey cast iron (GJL250) using a cold spray process, and extensive experiments were carried out to study the effects of diffusion between the coating and the substrate after heat treatment at 400, 600, 850 and 1050 °C for 3 and 6 hours durations. The coatings in all conditions were dense (0.25% to 3%) and had defect-free interfaces. Under heat treatment, the diffusion layer increased in thickness with increasing temperature and duration due to atomic diffusion. The Inconel 625 coating is also shown to be effective against oxide growth as compared to grey cast iron. The hardness of the coatings is also stable at high temperatures. The heat-treated coatings at 600 °C achieved a peak hardness of around 500 HV, which is 30% and 60% higher than the as-sprayed coating and grey cast iron substrate, respectively, because of the possible formation of recrystallized nanostructured grains and strengthening precipitates. These findings demonstrate the potential application of using cold spray on nickel-based alloy coatings for restoration and surface enhancement of grey cast iron components, such as engine blocks and pump housings.