Flame spraying process
Flame spraying is one of the earliest and most widely used thermal spraying techniques. It utilizes a high-temperature flame generated by gas combustion to melt the spray material. Compressed air then atomizes the molten material into fine particles, which are then sprayed at high speed onto a pretreated substrate, forming a coating with specific properties. This process is simple to operate and has low equipment costs. It is suitable for spraying a variety of materials, including metals, ceramics, and plastics. It effectively improves the substrate’s surface’s wear resistance, corrosion resistance, and high-temperature resistance, playing a vital role in industries such as machinery manufacturing, petrochemicals, and electric power.
The core equipment of the flame spraying process includes a spray gun, fuel supply system, oxygen supply system, feeder, and compressed air system. The spray gun is a key component in flame spraying and can be categorized as either acetylene-oxygen or propane-oxygen, depending on the type of fuel and the form of the sprayed material. The fuel and oxygen mix and burn within the spray gun, generating a high-temperature flame (typically between 2000-3000°C). The spray material (powder or wire) is fed into the flame through the feeder, where it is instantly heated to a molten or semi-molten state. Compressed air then atomizes the molten material into particles approximately 10-100 microns in diameter, which are then sprayed onto the substrate surface at a speed of 50-150 m/s.
The flame spraying process primarily consists of three stages: substrate pretreatment, spraying, and post-treatment. Substrate pretreatment is essential for ensuring coating quality. Its purpose is to remove oil, rust, and scale from the substrate surface, increase surface roughness, and improve the adhesion between the coating and the substrate. Pretreatment typically includes degreasing, pickling, and sandblasting. Degreasing uses organic solvents or alkaline solutions to remove surface grease; pickling uses dilute hydrochloric acid or sulfuric acid to remove scale and rust; and sandblasting uses aluminum oxide or quartz sand to roughen the surface to a roughness of Ra 3.2-6.3 microns. Large or complex workpieces also require preheating to reduce thermal stress between the coating and the substrate.
The spraying stage is the core of the flame spraying process, and the process parameters need to be strictly controlled to ensure the quality of the coating. The main parameters include flame temperature, spraying distance, spraying speed and feed rate. The flame temperature is determined by the ratio of fuel to oxygen. Different spraying materials require different flame temperatures. For example, when spraying metal materials, the flame temperature can be appropriately lowered, while when spraying high-melting-point materials such as ceramics, a higher flame temperature is required. The spraying distance is generally controlled at 100-200 mm. If the distance is too close, the substrate will overheat and deform. If the distance is too far, the particles will cool too quickly, affecting the adhesion of the coating. The spraying speed and feed rate need to match each other to ensure uniform coating thickness. Usually, the thickness of a single spray pass is 50-100 microns, and the required total thickness (generally 0.1-5 mm) can be obtained through multi-pass spraying.
Post-treatment is the final step in the flame spraying process, and its purpose is to further enhance the performance of the coating. Common post-treatment methods include sealing, heat treatment, and machining. Sealing uses resin, metal, or ceramic slurry to fill the pores of the coating, improving the density and corrosion resistance of the coating, and is suitable for coatings used in corrosive environments; heat treatment eliminates residual stress within the coating through low-temperature annealing (200-400°C), reducing the risk of cracking; and machining (such as grinding and polishing) is used to improve the surface finish and dimensional accuracy of the coating to meet the requirements for the use of precision parts. The choice of post-treatment process must be determined based on the application scenario and performance requirements of the coating to fully utilize the flame spray coating.
