Effects of Spraying Distance on Bonding Strength, Porosity and Deposition Efficiency

Spraying distance refers to the distance between the spray gun and the surface of the workpiece. It can directly influence the flight speed and particle temperature.

The wire is melted in the arc area, and then atomized by the atomizing air, and finally strike the workpiece surface to form a coating. At the nozzle of the spray gun, the flow rate of compressed air is the largest and the droplet velocity is the lowest. As the spraying distance increases, the sprayed particles are gradually accelerated, while the velocity of the atomized gas stream is gradually reduced. For example, when spraying a wire, spraying particles at around 50 mm has the fastest speed; the flight velocity of the sprayed particles begins to decrease due to air resistance and the weakening of accelerated airflow. According to the principle of fluid mechanics, the velocity of the particle depends on its size under certain atomizing gas pressure and flow rate. As the sizes of the particles are different, they are accelerated to different degrees. Figure 2 shows how spraying distance effects atomizing air velocity and particle flight speed when arc spraying carbon steel with different sizes. We can find that the spray particles with the 50 ~ 200 mm spray distance has a higher flight speed.

To obtain high-quality coating, it not only needs to spray particles with a high flight speed, but also requires the particles to have a high enough temperature.

1. Bonding Strength

With enough temperature the particles in melting or softening state can be fully deformed when hitting the workpiece surface, ensuring each spray particles mosaic well. Therefore the coating can have sufficient cohesive strength and bonding strength. Figure 3 shows the effect of the spraying distance on the bonding strength of the 3Crl3 coating.     

2. Porosity

During the spraying process, the spray particles in a highly overheated state are easily oxidized, leading to a large specific surface area. The smaller the particle size, the greater the specific surface area per unit volume, and the greater the chance of oxidation. In the normal spray distance, spray particles only need 1-2 ms to reach the workpiece surface. Although the particles flight time is very short, the oxidation of particles is often very serious due to the large specific surface area of particles and sufficient oxygen supply. For steel materials, the oxidation process will bring many adverse effects on the coating, such as the burning of carbon, the increase of the oxide content and the increase of the porosity. The change of the carbon content directly affects the mechanical properties of the coating. Figure 4 shows the effect of the spray distance on the porosity and oxide content of the SS420 coating.

3. Deposition Efficiency

For arc sprayed zinc or aluminum anti-corrosion coating, the commonly used spray distance should be 150-250 mm. In this range, the spray particles can get a high enough flight speed and particle temperature, resulting in good coating quality and mechanical properties. With the increase of the spraying distance, some slow-moving and severely oxidized particles lose enough kinetic energy and plasticity. Therefore, they cannot be deposited on the workpiece, thus affecting the coating deposition efficiency.