Wire flame spraying
Wire flame spraying is one of the earliest and most mature processes in thermal spraying. It uses metal or alloy wire as the spray material. A flame melts the wire, and compressed air atomizes the molten metal into fine particles, which are then sprayed onto the substrate at high speed to form a coating. This method features simple equipment, easy operation, and low cost. It is widely used in areas such as corrosion protection, wear resistance, and repair, and is particularly suitable for surface treatment of large steel structures, pipelines, bridges, and other workpieces. The core equipment for wire flame spraying includes a spray gun, an oxygen-fuel gas supply system, a wire feed mechanism, and a compressed air system. The spray gun is a key component, and its structural design directly affects the quality and efficiency of the coating. Compared to powder flame spraying, wire flame spraying offers higher material utilization (over 80%), regular wire shape, stable feed, and improved coating uniformity, offering significant advantages in mass production.
The wire flame spraying process can be divided into four stages: wire melting, particle atomization, particle flight, and coating formation. Control of each stage is crucial to coating performance. During the wire melting stage, the spray gun’s flame (typically an oxygen-acetylene flame at approximately 3000°C) heats the wire end to a molten state. The wire is continuously fed through a feed mechanism to ensure a continuous and stable melting process. The melting temperature must be precisely controlled. Too low a temperature will result in incomplete melting of the wire, resulting in a loose coating; too high a temperature may cause metal oxidation and coarse grains, reducing coating performance. During the particle atomization stage, compressed air (pressure 0.4-0.6 MPa) is ejected from the spray gun nozzle at high speed, breaking the molten metal stream into fine particles with a diameter of 10-100 μm. The atomization effect directly affects the particle size distribution and flight velocity. During the particle flight phase, the atomized particles, propelled by the flame and airflow, are accelerated to 50-150 m/s and fly toward the substrate surface. During flight, the particles further cool and oxidize, so the flight distance must be controlled (typically 100-200 mm) to minimize energy loss and oxidation. During the coating formation phase, the particles impact the substrate surface, undergoing plastic deformation and bonding with the substrate, accumulating layer by layer to form a coating. The substrate surface cleanliness and roughness significantly influence bonding strength, typically requiring a surface roughness Ra of 3.2-6.3 μm.
The wire flame spraying method can be used with a wide variety of materials, primarily pure metal wire, alloy wire, and composite wire, suitable for different application scenarios. Pure metal wires such as zinc, aluminum, copper, and nickel are commonly used for anti-corrosion and conductive coatings. For example, coatings sprayed on zinc wire form sacrificial anodes on the surface of steel structures, providing cathodic protection and offering salt spray resistance of over 2,000 hours. Aluminum wire coatings offer excellent weather resistance and can extend the service life of steel structures by 10-15 years in atmospheric environments. Alloy wires such as aluminum-zinc alloys, nickel-chromium alloys, and iron-chromium-aluminum alloys combine the advantages of multiple metals. Aluminum-zinc alloy coatings (e.g., 85% Al-15% Zn) offer superior corrosion resistance to pure aluminum or zinc coatings and are widely used in marine environments. Coatings sprayed on nickel-chromium alloy wires offer excellent high-temperature and oxidation resistance and can be used in environments above 600°C. Composite wires such as nickel-clad aluminum and aluminum-clad ceramics provide special properties through the synergistic effect of different materials. Nickel-clad aluminum wire will produce an exothermic reaction when sprayed, which improves the bonding strength between the coating and the substrate and is often used for primer coating.
Optimizing process parameters for wire flame spraying is crucial for ensuring coating quality. Key parameters include flame power, wire feed speed, compressed air pressure, and spray distance. Flame power is determined by the flow rates of oxygen and acetylene. Typically, the volume ratio of oxygen to acetylene is 1.2-1.5:1. Excessive power can easily lead to overburning and oxidation of the wire, while insufficient power can result in incomplete melting. The wire feed speed must match the flame power. For a 2mm diameter aluminum wire, the typical feed speed is 5-10m/min. Faster speeds result in incomplete melting, while slower speeds lead to inefficient coating deposition. Compressed air pressure influences atomization and particle velocity. Excessive pressure causes particles to cool too quickly, resulting in incomplete deformation. Excessive pressure results in poor atomization, coarse particles, and a rough coating. The spray distance must be adjusted based on the flame length and particle flight characteristics. For an oxygen-acetylene flame, the optimal distance is 150-200mm. Too close can overheat the substrate, while too far can lead to insufficient particle kinetic energy and reduced bonding strength. By optimizing parameters, a pipeline anti-corrosion project increased the bonding strength of the zinc-aluminum coating from 30MPa to 45MPa and reduced the porosity from 8% to below 3%.
Wire flame spraying has extensive industrial application, particularly in the corrosion protection and repair of large components. For corrosion protection of bridge steel structures, arc spraying or flame spraying zinc-aluminum wire creates an 80-120μm thick coating. Combined with a sealant coating, this can extend the corrosion life of bridges to over 20 years. A cross-sea bridge using this technology has shown no significant corrosion for 10 years, reducing maintenance costs by 60%. In oil pipeline repair, nickel-based alloy wire can be applied to worn or corroded pipe surfaces using a specialized rotary spray gun, restoring the pipe’s dimensions and improving wear resistance. In one oil pipeline repair project, the coated pipe’s service life was extended to 15 years, far exceeding the 5-8 years of a new pipe replacement. In agricultural machinery repair, the worn surfaces of tractor transmission gears are flame sprayed with low-carbon steel wire, which is then mechanically restored to shape. This repair costs only one-third of the cost of a new gear and provides 80% of the service life of a new part. With the introduction of automation technology, wire flame spraying has achieved robot operation and online quality monitoring, further improving the consistency and production efficiency of the coating, making it maintain lasting vitality in industrial applications.
