Taking DJ type high-speed flame spraying system as an example, the influence of process parameters on the coating performance is introduced.
(1) Powder properties. At present, the powder suppliers provide a wide range of carbide powder, and the powder characteristics often show great differences due to the different powder making process. The powder characteristics include: particle size distribution, particle shape, surface roughness, etc. For DJ2700 equipment, the appropriate powder size is: 15 μ m-40 μ m. Search wococarbide or sales-manager202 (@)wococarbide.com for more tungsten carbide powder information.
(2) Oxygen gas flow and ratio. The flame temperature and characteristics of high-speed flame spraying depend on the oxygen gas flow and mixing ratio. When spraying high-speed flame, the flow of oxygen and gas shall be determined according to the requirements of equipment first to ensure that the flame flow of the spray gun reaches the designed power level. There are many factors in the actual production process which can cause the fluctuation of oxygen gas ratio, and oxygen gas ratio is very important to determine the final coating structure. Theoretically, the ratio of oxygen to propane is 5:1 (c3h8+5o2=4h2o+3co2) in theory. The combustion ratio produces a neutral flame (that is, the oxygen and gas molecules are exhausted during combustion). If the ratio of gas decreases, the oxygen molecules in the flame flow will produce an "oxidation" atmosphere, which will lead to the excessive oxidation of molten powder particles and the increase of oxide content in the coating. The flame with low temperature and oxygen deficiency will be produced when the gas is too much in the mixture. The molten particles and holes in the coating increase, while the oxide content decreases. In fact, the neutral flame does not exist. At high temperature, the combustion process is not completely reversible. The reactants and the reaction products coexist in the way of thermal and chemical equilibrium. The results show that the high performance coating can be obtained when the oxygen gas ratio is between 4.2-5.6 for DJ high-speed flame spraying system.
(3) Distance of spraying. The results show that: the DJ high-speed flame spraying system, when the powder particles reach the maximum temperature within 100mm from the exit of the spray gun, the particle temperature decreases gradually with the increase of spray distance. In the range of 100-230mm, the particle temperature decreases by about 60 ℃, and the reduction range is not large, and the particles can still maintain a high temperature of about 1775 ℃; The particle velocity is a gradual acceleration process within 190mm from the exit of the gun. The maximum velocity is over 480m/s at 190-200mm from the exit of the gun. On the 170-230mm spray distance, the particle velocity is basically maintained at more than 480m / s. 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. Therefore, for DJ type high-speed flame spraying system, the suitable spray distance should be 190-230mm. Compared with other spraying processes, the adjustable range of spray distance of high-speed flame spraying is relatively large, which is due to the high speed of particles. The larger adjustable range of spray distance is very beneficial to the actual production, because the suitable spray distance can be selected according to the shape, size and coating thickness of the workpiece, so as to obtain the best coating with comprehensive performance.
(4) Powder delivery. For any thermal spraying process, powder feeding is an important parameter that affects the coating performance. Under a specific spraying process, a certain powder has a suitable range of powder delivery.
If the powder delivery is too small, the possible adverse effects are:
1) It is too melted by spraying powder, and the powder is burnt, and the smoke is large, which is easy to pollute the coating.
2) Each spray can not completely cover the path of the coating, which results in the increase of the porosity of the coating.
3) The coating cracking and production cost are increased easily after the spraying time is prolonged.
If the powder delivery is too large, the possible adverse effects are:
1) The powder melting is not enough, the bonding strength of the coating decreases and the porosity increases.
2) The coating stress increases, which leads to the cracking of the coating.
3) The deposition rate of powder decreased and the production cost increased.
The results show that when the powder delivery rate changes from 38-60g/min, the porosity of the coating is between 1.12-2%, the microhardness is hv1000-1300, the deposition rate of powder is 40-50%, and the coating performance is excellent. When spraying CRC NiCr coating, the satisfactory coating quality can be obtained when the powder feeding quantity changes between 27-45g/min.
Search wococarbide or sales-manager202 (@)wococarbide.com for more metal powder information.
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