What are the main process parameters that affect the spraying quality?

       When the spraying equipment and working gas have been determined, the main process parameters that affect the spraying quality are: the temperature of the substrate, the spraying distance, the flow of the main gas and the powder feeding gas, the powder feeding amount and the electric power, etc.



    1. Gas parameters (flow rate)



    The flow rate of the main gas is one of the important process parameters, which directly affects the enthalpy and velocity of the plasma flame, which in turn affects the spraying efficiency and coating porosity. When the spraying power is constant, too large or too small main air flow will lead to the decrease of spraying efficiency and the increase of coating porosity. If the air flow is too large, the ion concentration will decrease. The excess gas will cool the plasma flame, which is not conducive to the heating of the powder. The powder is not fully melted, which will reduce the spraying efficiency, the coating structure will be loose, and the porosity will increase. Otherwise, the main air flow is too small. , will make the flame weak and weak, and the relative content of secondary gas in the working gas will increase, resulting in an increase in the enthalpy and temperature of the jet, and the spray powder will be overmelted.



    The flow change of secondary gas is mainly reflected in the change of spraying voltage.



    The pressure and flow of the powder feeding gas also have a great influence on the coating quality. For external powder feeding spray guns, the influence of powder feeding gas on the coating quality is particularly serious. As shown in the figure, if the powder feeding pressure and flow rate are too small, it will be difficult for the powder to reach the center of the flame, and if it is too large, the powder will pass through the center of the jet, resulting in a serious "boundary effect", resulting in loose coating and reduced bonding strength. For the internal powder feeding spray gun, if the powder feeding gas pressure and flow rate are too large, the powder cannot be sent into the flame core. If the powder feeding gas pressure and flow rate are large enough to send the powder into the flame core, it is necessary to check the air tightness of the powder feeding system and whether it is leaking.



    Therefore, the pressure and flow of the powder feeding gas should be selected according to the size of the powder feeding amount, the specific gravity of the powder, the fluidity of the powder, the performance of the powder feeding system, and the power and rigidity of the jet.



    (a) The powder feeding pressure and flow rate are too large



    (b) The powder feeding pressure and flow are just right



    (c) The pressure and flow of powder feeding air are too small



    Influence of the flow rate of powder feeding air on the powder feeding effect



    2. Electrical parameters



   (1) Power



    The input power must first be able to melt the powder well.



    In general, it is better to use a higher power value. The power commonly used in general plasma spraying is 20~35 kW, while the power commonly used in HEPJet high-efficiency supersonic plasma spraying is 45~65 kW.



   (2) Voltage and current



    The plasma arc voltage is determined by the gun structure and the working gas. The arc voltage can be adjusted by adjusting the distance between the cathode and the nozzle and changing the composition of the working gas. When the structure of the spray gun has been selected and the main gas flow rate is a certain value, the adjustment of voltage and current can be adjusted by changing the power regulator and H2 flow rate. It should be noted that when the voltage or current is changed, the flow of the main gas will also change accordingly. Therefore, in order to ensure stable spraying parameters, when adjusting the voltage and current, it is necessary to adjust and maintain the main gas flow in time.



After the power is determined, the higher voltage and lower current should be selected as much as possible, which is beneficial to improve the thermal efficiency of the spray gun.



    3. Spraying distance



    The spraying distance refers to the straight-line distance from the nozzle end face to the substrate surface. It takes a period of time for the powder to heat and accelerate in the plasma flame, so there should be a suitable spraying distance. If the spraying distance is too close, the coating quality will be affected due to the short heating time of the powder and the insufficient impact deformation, and the parts will also be affected. Due to the influence of the plasma flame, the temperature rises rapidly and severe oxidation occurs, causing the coating to fall off. If the spraying distance is too long, the powder that has been heated to a molten state will cool down when it comes into contact with the part, and the flying speed will begin to decrease, which will also affect the quality of the coating, and the spraying efficiency will be significantly reduced. The spraying of plasma spraying is usually 70-150mm.



    4. Powder feeding rate



    The powder feeding rate refers to the powder feeding amount per unit time, which directly affects the spraying efficiency and coating quality. The powder feeding amount should match the heat source parameters. For powders of the same grade and particle size, different input powers should be applied under different powder feeding rates. When the powder feeding amount is constant, if the heat source power parameter is too small, the powder will not be melted well, and there will be too much raw powder in the coating. Deterioration of coating quality. On the other hand, if the power parameter of the heat source is too large, although the melting and impact deformation of the powder are good, the powder is seriously oxidized and ablated by heat, and there is more smoke and dust in the coating, and the molten particles splash seriously, which will also reduce the deposition efficiency and reduce the coating quality. decline. Therefore, for a certain grade of powder with a certain particle size, the size of the powder feeding amount and the parameters of the heat source should be adapted.



    5. Spray angle



    The spraying angle refers to the angle between the axis of the spraying jet and the tangent to the surface of the substrate. The spraying angle is generally 60°~80°, and when the spraying angle is not less than 45°, the structure and deposition efficiency of the coating will not be greatly affected. It is generally believed that spraying is not allowed if the spray angle is less than 30°.



    When the spraying angle is too small, fine powder particles stick to the sprayed surface, hindering the particles that continue to be sprayed, and as a result, a kind of "mask" is formed behind it, so that a porous coating with many irregular cavities is formed. . Such cavities not only weaken the coating, but also collect fine species containing high oxides from the jet, changing the coating's chemical composition.



    When the spraying angle is less than 45°, the "shadowing effect" of spraying will appear, which affects the interlayer bonding of the coating and greatly reduces the bonding strength of the coating and the substrate.



    6. Preheating and temperature control of sprayed workpiece



    In winter or the parts with more complex structure and inner hole parts should be preheated before spraying, and the preheating temperature is generally between 80 and 150 °C. The purpose is to remove the moisture on the surface of the substrate, improve the activation state of the surface of the substrate, reduce the cooling rate of the sprayed particles to the surface of the substrate, and reduce the thermal stress generated when the sprayed particles are cooled.



    The temperature rise of the sprayed workpiece should be controlled during the spraying process. It is not only necessary to control the temperature of the entire workpiece, the maximum temperature should not exceed 200 ℃, but also to prevent local overheating of the spraying part. Compared with the overall overheating, local overheating has a greater impact on the coating, especially in the preparation of ceramic coatings, the coating is very easy to crack. Auxiliary air cooling is usually used to control the temperature of the workpiece.



    7. The moving speed of the gun



    The speed of the gun movement is generally based on the diameter of the beam spot. Because of different spraying process methods, the beam spot diameter is different, usually 30%~50% of the gland spot, not less than 30%. After the moving speed of the spray gun is determined, it must match the rotational speed of the workpiece, so that the coating thickness of each spraying can meet the requirements. Under a certain amount of powder feeding, the speed of the spray gun or the relative speed between the spray gun and the workpiece is slow or fast, which means how much the spray gun sweeps over the workpiece area or the thickness of each sprayed layer in a unit time. Therefore, adjusting the moving speed of the spray gun actually It is to control the thickness of each sprayed layer. The thickness of each spray should not be too thick. In general, for thin coatings below 0.15 mm, the thickness of each spray should not exceed 0.02 mm. In addition, the moving speed of the spray gun also affects the temperature rise of the workpiece. In order to prevent the local temperature rise of the substrate from being too high and causing thermal deformation or excessive thermal stress, the method of slightly increasing the linear speed of the workpiece can be adopted to speed up the moving speed of the spray gun.



    8. Spray atmosphere control



    During the spraying process, the flying particles will react with the gas or the atmosphere, resulting in oxide inclusions in the coating, which are related to the atmosphere of the heat source and the influence of the atmospheric environment. Some low-pressure atmosphere spraying can improve the oxidation degree of particles, such as low-pressure plasma spraying, which can be used to prepare coatings of easily oxidizable metals and their alloys; use inert gas to protect the particle beam.



    In the entire spraying environment, dust is unavoidable. It is necessary to do a good job of ventilation and dust removal in the entire environment to minimize the impact of inclusions in the coating.