The principle composition and technological characteristics of high-speed flame spraying

       High-speed flame spraying is commonly known as supersonic flame spraying in China, and its English abbreviation is HVOF. High-speed flame coating is a new type of thermal coating technology developed on the basis of explosive coating. It was first successfully developed in the early 1980s by the American Browning Company with the introduction of a commercial spraying equipment called the KOTE jet. After the advent of high-speed flame spraying technology, with its ultra-high flame speed and relatively low temperature, it has shown obvious advantages in spraying metal carbides and metal alloys. The technology has developed rapidly with active promotion by thermal spray companies around the world. At present, high-speed flame coating technology has replaced plasma coating and other coating technologies in coating metal carbides and metal alloys, and has become an important technical method of thermal coating.



1. The principle of high-speed flame spraying.

High-speed flame spraying is a kind of continuous combustion of combustion gas and combustion gas in the combustion chamber. The combustion flame generates high pressure in the combustion chamber. The high-speed flame flow is generated through the expansion nozzle connected to the outlet of the combustion chamber, and the spray material is sent into the high-speed jet for heating and spraying. A method of forming a coating on the surface of a pretreated substrate. Acetylene, propane, propylene and hydrogen can be used as gases, but also as liquid fuels such as diesel or kerosene.

The kerosene and oxygen enter the combustion chamber through the small holes, and are mixed and burned stably and uniformly in the combustion chamber. The monitor is used to monitor the pressure in the combustion chamber to ensure stable combustion. The powder spray rate is proportional to the combustion chamber pressure. The design of the combustor outlet accelerates the rapid expansion of the high-speed airflow, creating a supersonic region and a low-pressure region. The powder is radially sprayed at multiple points in the low pressure zone, the powder is uniformly mixed and sprayed in the air stream at an accelerated speed. The flame speed of high-speed flame spraying reaches 1500m/s-2000m/s, generally 5-8 obvious Mach cones can be observed, and the particle speed reaches 300-650m/s..

2. Equipment composition.

High-speed flame coating equipment generally consists of spray gun, powder feeder, control system, spray gun cooling system and air supply system. At present, most of the high-speed flame coating equipment used in China are imported, and the most used models are SulzerMetco's DJ-2700 and Praxair's JP-5000. JP-5000 was successfully developed by the original Hobart Law Company, which was merged into Praxair Corporation. These two types of equipment are also widely used abroad, representing the development level of high-speed flame coating technology in the world today.

(1) Praxair JP-5000. The equipment uses kerosene as fuel and features high combustion chamber pressure (>10 bar), high power and high flame exit velocity (2100 m/s). The powder is injected radially from the low pressure area of the burner nozzle, so that the powder is uniformly and fully heated. Compared with other processes, the powder particle size suitable for spraying is coarser, which is beneficial to reduce costs. So far, the coating performance of WC-Co coated by JP-5000 is slightly better than other HVOF methods, but the consumption of oxygen and kerosene is astonishing.

(2) SulzerMetcoDJ-2700. This equipment is produced by SulzerMetco and uses propane or propylene as fuel gas. Propane is used more domestically as fuel gas. The equipment is divided into manual control type and automatic control type. The manual equipment is composed of spray gun, powder feeder and flow controller, which has good operability and can be used for on-site spray production. Compared with JP-5000, DJ-2700 has the characteristics of simple and practical configuration, convenient operation and low oxygen consumption.



3. Coating and process characteristics.

Due to its unique characteristics, high-speed flame spraying technology has ultra-high flame speed and relatively low temperature.

(1) High flame and jet particle velocity. The flame speed exceeds 1800m/s, and the particle speed is 300-650m/s. ..

(2) Heat the powder evenly. The sprayed powder particles are sprayed into the combustion chamber in the axial or radial direction, so that the powder stays in the flame for a long time, fully melts, and produces a concentrated spray beam.

(3) The contact time between the powder particles and the surrounding atmosphere is short, the flight speed of the powder particles is fast, the contact time between the powder particles and the surrounding atmosphere is short, the reaction with the atmosphere is small, and the combustion loss of active elements in the material is small. This is particularly advantageous for carbide materials, where decomposition decarburization can be avoided.

(4) The spray powder is fine and the coating is smooth. The particle size of the powder used for high-speed flame spraying is generally 10-45 μm, which is a fine-grained powder. At the same time, when the sprayed particles form the coating, the speed is fast, the melting is sufficient, and the deformation is sufficient, so that the surface roughness of the coating is small.

(5) The coating is dense and has high bonding strength. The coating porosity of high-speed flame coating is generally below 2%, and the bonding strength is above 70MPa.



4. Main process parameters.

Taking the DJ high-speed flame spraying system as an example, the effects of process parameters on the coating properties are introduced.

(1) Powder characteristics. Currently, powder suppliers offer a variety of carbide powders, but the properties of the powders tend to vary widely due to differences in grinding techniques. Powder properties include: powder particle size distribution, particle shape, surface roughness, etc. For the DJ2700 device, the appropriate powder particle size is 15mm-40μm.

(2) Oxygen flow and ratio. The flame temperature and characteristics of high velocity flame spraying depend on the oxygen flow rate and mixing ratio. When high-speed flame spraying, the flow of oxygen and gas should be determined according to the specified requirements of the equipment to ensure that the flame flow of the spray gun reaches the design power level. In the actual production process, there are many factors that cause fluctuations in the oxygen ratio, which 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 (ie, the oxygen and gas molecules are completely depleted during combustion). The gas ratio decreases, the unconsumed oxygen molecules in the flame flow create an oxidizing atmosphere, and the molten powder particles are over-oxidized, increasing the content of oxides in the coating. Too much gas in the mixture will produce a low-temperature oxygen-depleted flame, increase unmelted particles and pores in the coating, and reduce oxide content. In fact, neutral flame does not exist. At high temperatures, the combustion process is not fully reversible, and the reactants and reaction products coexist in thermal and chemical equilibrium. The experimental results show that the DJ high-speed flame spraying system can obtain high-performance coatings under the conditions of oxygen and gas ratios ranging from 4.2 to 5.6.

(3) Spraying distance. The research shows that the DJ high-speed flame spraying system reaches the maximum temperature when the powder particles are within 100mm of the spray gun outlet, and the particle temperature gradually decreases with the increase of the spraying distance. In the range of 100-230mm, the temperature of the particles is reduced by about 60°C, and the decrease is not large, and the particles can still maintain a high temperature of about 1775°C; however, the particle velocity is a gradual acceleration process about 190mm away from the spray gun outlet, and 190 mm away from the spray gun outlet. -200mm to reach the maximum speed of more than 480m/s, and the spraying distance between 170-230mm 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 spraying distance should be increased as much as possible. Therefore, the suitable spraying distance for the DJ high-speed flame spraying system should be 190-230 mm. Compared with other spraying processes, the adjustable range of spraying distance of high-speed flame spraying is relatively large, which is due to the high-speed movement of particles. The large adjustable range of spraying distance is very beneficial for actual production, because the appropriate spraying distance can be selected according to the shape, size and coating thickness of the workpiece, so as to obtain a coating with good comprehensive performance.

(4) Powder feeding amount. As with any thermal spray process, powder feed rate is an important parameter affecting coating performance. Under specific spraying process conditions, a certain powder should have a suitable powder feeding range.



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

1) The spraying powder is excessively melted, the powder is burned out, and thick smoke is generated, which is easy to contaminate the coating.

2) Each spray cannot completely cover the sweep path, resulting in increased coating porosity.

3) The prolonged spraying time will easily lead to overheating and cracking of the coating and increase the production cost.



If too much powder is delivered, the possible adverse effects are:

1) Insufficient powder melting will reduce the bond strength of the coating and increase the porosity.

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

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

The research shows that when the DJ system is used to spray the tungsten carbide-cobalt coating, the coating porosity is 1.12-2%, the microhardness is HV1000-1300, the powder deposition rate is 40-50%, and the coating performance is excellent. When spraying CrC-NiCr coating, when the powder feeding rate varies between 27-45g/min, satisfactory coating quality can be obtained.