There are four main steps of pre-construction preparation, surface pretreatment, spraying, and post-spraying treatment:

1) Preparation:

Before compiling the process, the actual condition and technical requirements of the workpiece to be sprayed should be understood and analyzed

1. Determine the thickness of the coating. Generally speaking, mechanical processing must be carried out after spraying, so the coating thickness should be reserved for machining allowance, and the thermal expansion and contraction during spraying should also be taken into account.

2. Determination of coating materials. The selection is based on the fact that the coating material should meet the material, matching requirements, technical requirements and working conditions of the workpiece to be sprayed, and the binding layer and working layer material should be selected respectively

3. Determine the parameters: pressure, powder particle size, relative movement speed between the spray gun and the workpiece

2) Pretreatment of the surface of the workpiece

   Surface preparation is an important process to ensure the strength of the coating and the substrate

1. Concave cutting treatment, when there is a fatigue layer and a groove with local severe strain on the surface, turning can be carried out under the premise of strength permitting, so as to provide accommodation space for thermal spraying.

2. Surface cleaning, remove oil, rust, paint layer, etc., so that the surface of the workpiece is clean, and the oily paint can be removed with solvent cleaning agent. If the oil stain has penetrated into the base material, it can be removed by flame heating, and the rust layer can be removed by acid leaching, mechanical sanding or sandblasting.

3. Surface roughening, the purpose is to enhance the bonding force between the coating and the substrate and eliminate the stress effect, and the commonly used ones are sandblasting, slotting, threading, and brushing.

     A: Sandblasting is the most commonly used, and the sand can choose quartz sand, alumina sand, chilled iron sand, etc. The sand material should be sharp and hard, and it must be clean and dry, with sharp edges and corners. Its size, the amount of air pressure, the angle of blasting, distance and time should be determined on a case-by-case basis.

     B: Slotting, threading, rolling. The roughening treatment of the surface of shafts and sleeves can be slotted and spiraled, and the surface roughness of grooves and threads should be RA6.3-12.5, and no coolant and moisturizer are added during the processing process, and they can also be knurled on the surface, but sharp corners are avoided.

     C: Workpieces with high hardness can be roughened by EDM brushing, but thin-coated workpieces should be used with caution. The EDM brushing method uses a thin nickel wire or aluminum wire as an electrode, and under the action of the arc, the electrode material is partially fused with the surface of the substrate to produce a rough surface.

The fresh surface presented after the surface roughening should be prevented from pollution, it is strictly forbidden to touch it by hand, keep it in a clean and dry environment, spray as soon as possible after roughening, and the general spraying time should not exceed two hours.

4. Protection of non-spraying parts

Non-spraying near the sprayed surface needs to be protected and can be shielded with heat-resistant glass cloth or asbestos. If necessary, make the corresponding fixture protection according to the part switch, but pay attention to the fixture material to have a certain strength, and the alloy with a low melting point cannot be used to avoid contaminating the coating. For the keyway, oil holes and other parts on the surface of the substrate that are not allowed to be sprayed, graphite blocks or chalk can be used to plug flat or slightly higher than the surface.

When removing after spraying, pay attention to the coating and the edges and corners should be dull.

3) Spraying process and parameters

(1) Powder characteristics:

At present, powder suppliers provide a wide variety of carbide powders, and the characteristics of powders often show large differences due to different milling processes. Powder characteristics include: powder particle size distribution, particle shape, surface roughness, etc.

For ZB-2700 equipment, the suitable powder particle size is: 15μm-40μm.

(2) Oxygen-gas flow rate and ratio

The temperature and characteristics of the spray flow depend on the oxygen-gas flow rate and mixing ratio. When spraying, the flow of oxygen and gas should first be determined according to the specified requirements of the equipment to ensure that the flame flow of the spray gun reaches the designed power level. There are many factors that can lead to the fluctuation of oxygen-gas ratio in the actual production process, and the oxygen-gas ratio is very important to determine the final coating structure.

Theoretically, the complete combustion of propane requires a ratio of oxygen to propane of 5:1 (C3H8 + 5O2 = 4H2O + 3CO2), and this combustion ratio produces a neutral flame (i.e., all oxygen and gas molecules are exhausted during combustion). If the proportion of gas decreases, the undepleted oxygen molecules in the flame stream will create an "oxidizing" atmosphere, resulting in excessive oxidation of the molten powder particles and an increase in the oxide content of the coating. Too much gas in the mixture will produce a low-temperature oxygen-poor flame, which will increase the number of unmelted particles and pores in the resulting coating, while the oxide content will decrease. In fact, neutral flames do not exist, at high temperatures, the combustion process is not completely reversible, and reactants and reaction products coexist in thermal and chemical equilibrium.

The ZB-2700 supersonic flame spraying system achieves a high-performance coating when the oxygen-gas ratio is between 4.2-5.6.

(3) Spraying distance:

ZB-2700 supersonic flame spraying system, when the powder particles are within 100mm from the gun outlet, the maximum temperature has been reached, with the increase of the spray distance, the particle temperature gradually decreases, in the range of 100-230mm, the particle temperature is reduced by about 60 °C, the reduction is not large, the particles can still maintain a high temperature of about 1775 °C; The particle velocity is a gradual acceleration process within about 190mm from the gun outlet, reaching a maximum speed of more than 580m/s at about 190-200mm from the gun outlet, and basically maintaining the particle velocity above 580m/s at the spray distance of 170-230mm. Considering the adverse effect of high temperature flame flow on the heat transfer of the substrate, the spray distance should be increased as much as possible if possible, so the suitable spray distance for the ZB-2700 supersonic flame spraying system should be: 190-230mm.

Compared to other spraying processes, the spray pitch can be adjusted over a relatively large range, thanks to the high velocity of the particles. The large adjustable range of spray distance is very beneficial to actual production, because the appropriate spray distance can be selected according to the shape, size, coating thickness and other requirements of the workpiece, so as to obtain the coating with the best comprehensive performance.

(4) Powder feeding volume:

For any thermal spray process, powder feed is an important parameter that affects the performance of the coating. A certain powder corresponds to an appropriate powder feeding range under a specific spraying process condition.

If the amount of powder fed is too small, the possible adverse effects are:

1) The sprayed powder is overmelted, the powder is burned, the smoke is large, and the coating is easy to be polluted.

2) Each spraying can not completely cover the path it sweeps, resulting in an increase in the porosity of the coating.

3) The spraying time is prolonged, which is easy to cause overheating of the workpiece, cracking of the coating and the increase of production costs.

If the amount of powder fed is too large, the possible adverse effects are:

1) The powder is not fully melted, the bonding strength of the coating is reduced, and the porosity is increased.

2) The coating stress increases, resulting in cracking of the coating.

3) The powder deposition rate decreases and the production cost increases.

    Using the ZB system, when spraying WC-Co coating, when the powder feeding amount varies between 38-60g/min, the porosity of the coating is between 0.55-1.2%, the microhardness is HV1000-1300, the powder deposition rate is 40-50%, and the coating performance is excellent. When spraying CrC-NiCr coatings: Satisfactory coating quality can be obtained when the powder feed varies between 27-45g/min.

4) Post-spraying treatment

Hole sealing, machining and other processes.

The porosity of the coating accounts for about 5% of the volume, and some of the porosity can be from the surface to the inside. When the parts are friction pairs, the parts can be placed in the lubricating oil while they are hot after spraying, and the use of pore oil storage is conducive to lubrication. However, for parts with hydraulic pressure, pores are easy to leak, and after spraying, the hole should be sealed with a sealer.

Requirements for sealing agent: good permeability, chemical resistance, insolubility, no deterioration. The performance is stable at working temperature, which can enhance the coating performance, and the commonly used ones are paraffin, epoxy, phenolic, etc.

When the dimensional accuracy and surface roughness after spraying cannot meet the requirements, it needs to be machined, which can be turned or grinded.

(The article comes from the Internet)