Cold spray equipment system and process parameters
The cold spray equipment system is the core vehicle for cold spray technology. It consists of four main components: the air source system, powder delivery system, spray gun system, and control system. These components work together to ensure high-quality coating deposition. The air source system provides high-pressure gas for the cold spray process, typically nitrogen, helium, or a mixture of the two. After being compressed by a compressor, the gas enters a gas heater where it is heated to a set temperature (generally 200-600°C) to increase the gas’s kinetic energy and the plasticity of the powder particles. The powder delivery system ensures a uniform and stable delivery of powder particles to the spray gun. Common powder delivery methods include gravity feeding, pressure feeding, and screw feeding. Pressure feeding is particularly suitable for industrial production due to its high powder delivery stability. The spray gun system is a key component of cold spray equipment and consists of a nozzle, powder hose, and gun body. The nozzle design directly affects the airflow velocity and powder acceleration. The commonly used Laval nozzle can accelerate the airflow to supersonic speeds (Mach 1-3). The control system uses a PLC or industrial computer to precisely control parameters such as gas pressure, temperature, powder feed rate, and spray gun movement speed, ensuring stability and repeatability during the spraying process. An automated cold spray system produced by a cold spray equipment manufacturer boasts parameter control accuracies of up to ±0.01 MPa for gas pressure, ±5°C for gas temperature, and ±1 g/min for powder feed rate, ensuring the production of high-quality coatings.
Gas parameters are crucial in the cold spray process, directly impacting the acceleration of powder particles and the resulting coating quality. Gas pressure is a key factor in determining gas velocity and is typically adjusted within the range of 0.5-5 MPa. Higher pressure and greater gas velocity increase the kinetic energy imparted to the powder particles, facilitating effective bonding with the substrate. However, excessively high pressure increases equipment energy consumption and may cause powder particles to break. Gas temperature significantly affects the plasticity of powder particles. Properly increasing the gas temperature (e.g., from room temperature to 400°C) can reduce the yield strength of the powder material, enhance its plastic deformation capacity, and help improve the density and bonding strength of the coating. For example, when spraying titanium alloy powder, increasing the gas temperature from 200°C to 500°C can increase the coating density from 90% to over 98%. The choice of gas type is also crucial. Helium, with its low molecular weight, allows for higher gas velocities, but is more expensive. Nitrogen, with its lower cost, is suitable for applications where velocity requirements are less critical. In practical applications, the appropriate gas type and parameters are often selected according to the characteristics of the powder material. For example, nitrogen can be used when spraying materials with good plasticity such as aluminum and copper; helium or a helium-nitrogen mixture is required when spraying high-strength materials such as titanium alloys and nickel-based high-temperature alloys.
Powder parameters, including powder material, particle size distribution, morphology, and purity, have a decisive influence on the performance of cold spray coatings. The choice of powder material should be determined based on the coating’s performance requirements. For example, tungsten carbide-cobalt composite powders can be used for wear-resistant coatings, while zinc, aluminum, and their alloys can be used for corrosion-resistant coatings. Powder particle size is typically controlled within the 1-50μm range. Powders with smaller particle sizes (<5μm) are easily carried away by the airflow, resulting in low deposition efficiency. Powders with larger particle sizes (>50μm) lack sufficient kinetic energy and are difficult to form effective bonds. Research has shown that for copper powder, a particle size of 10-30μm achieves optimal coating density and bonding strength. Spherical or quasi-spherical powder morphology is preferred. Powders with this morphology offer excellent flowability, are less likely to clog during powder feeding, and achieve uniform acceleration in the airflow. Irregularly shaped powders can lead to unstable powder feeding, affecting coating uniformity. Powder purity cannot be ignored either. The presence of impurities will reduce the bonding strength and corrosion resistance of the coating. Therefore, the purity of the powder is generally required to be above 99%. For high-end fields such as aerospace, the powder purity must reach above 99.9%.
Spraying process parameters are crucial for ensuring a stable cold spray process and coating quality. These parameters primarily include spray distance, spray angle, powder feed rate, and spray gun traverse speed. The spray distance refers to the distance between the spray gun nozzle outlet and the substrate surface, typically ranging from 10-50 mm. A distance too close results in significant powder particle rebound, resulting in a rough coating surface. A distance too far results in excessive kinetic energy loss, reducing bonding strength. The optimal spray distance varies for different materials. For example, for spraying aluminum powder, the optimal distance is 20-30 mm, while for spraying titanium alloy powder, the optimal distance is 15-25 mm. The spray angle should generally be maintained between 80° and 90° (with respect to the substrate surface normal). A smaller angle results in particles impacting the substrate at an angle, reducing coating density and bonding strength. The powder feed rate must be aligned with the gas flow rate and spray gun traverse speed. A too fast feed rate results in insufficient acceleration of the powder particles, resulting in reduced coating density. A too slow feed rate results in low deposition efficiency and increased production costs. The speed of the spray gun determines the thickness and uniformity of the coating. Too fast a speed can result in insufficient and uneven coating thickness, while too slow a speed can cause localized buildup of coating and stress concentration. In actual production, these process parameters must be optimized through extensive testing to develop the optimal process solution for specific materials and workpieces.
The development trend of cold spray equipment systems is towards efficiency, intelligence, and integration. In terms of efficiency, by optimizing nozzle design and increasing gas pressure, supersonic high-pressure cold spray equipment has been developed, which has increased the coating deposition efficiency from 1-5kg/h of traditional cold spray to 10-20kg/h, meeting the needs of large-scale industrial production. In terms of intelligence, machine vision, sensors, and adaptive control technologies are introduced into the equipment system to achieve real-time monitoring and dynamic adjustment of the spraying process. For example, the substrate temperature is monitored in real time by an infrared thermometer. When the temperature exceeds the set value, the gas temperature is automatically lowered or the spray gun movement speed is increased; the coating thickness is measured in real time by a laser thickness gauge, and the powder feed rate and spray gun movement speed are automatically adjusted to ensure uniform coating thickness. In terms of integration, cold spray equipment is integrated with robotic systems, pre-treatment equipment, and post-treatment equipment to form a complete automated production line. An automotive parts manufacturer’s automated cold spray production line automates the entire process, from workpiece loading, surface pretreatment, cold spraying, coating inspection, to unloading. This increases production efficiency by more than five times compared to manual operations, significantly improving coating quality consistency. With the continued advancement of these technologies, cold spray equipment systems will continue to perform better and become easier to operate, laying a solid foundation for the widespread application of cold spray technology.
