Atmospheric plasma spraying principle and equipment composition

Atmospheric plasma spraying principle and equipment composition

Plasma Spraying

Plasma spraying is a thermal spraying method that uses a non-transfer plasma arc as the heat source and the spray material is powder. Plasma spraying technology has developed rapidly in the past decade. At present, atmospheric plasma spraying, controlled atmosphere plasma spraying, solution plasma spraying and other spraying technologies have been developed. Plasma spraying has become an important process method in thermal spraying technology.

Principles of atmospheric plasma spraying

Atmospheric plasma spraying is referred to as plasma spraying. Plasma spraying is achieved through a plasma spray gun. The nozzle (anode) and electrode (cathode) of the spray gun are connected to the positive and negative poles of the power supply respectively. The working gas is passed between the nozzle and the electrode, and the arc is ignited with the help of high-frequency sparks. The arc heats the gas and ionizes it, producing a plasma arc. The thermal expansion of the gas causes a high-speed plasma jet to be ejected from the nozzle. The powder feeding gas feeds the powder from inside the nozzle (internal powder feeding) or outside (external powder feeding) into the plasma jet, where it is heated to a molten or semi-molten state, accelerated by the plasma jet, and sprayed into the pretreated material at a certain speed. A coating is formed on the surface of the substrate. Commonly used plasma gases include argon, hydrogen, helium, nitrogen or their mixtures.

Atmospheric plasma spraying principle and equipment composition

Atmospheric plasma spraying schematic diagram

Atmospheric plasma spraying equipment composition

The plasma spraying equipment currently used in our country is divided into two categories: imported and domestic. The imported ones mainly come from the products of two major foreign companies. One is SulzerMetco, such as Metco7M and SulzerMetco. 9M, etc.; the second is Praxair Company, whose representative products include 3620, 4500 and 5500 etc. Domestic equipment is mainly imitations, mostly imitations of Metco7M and SulzerMetco9M. The following mainly refers to the SulzerMetco9M equipment to introduce its composition.

SulzerMetco9M plasma spray equipment consists of six parts: spray gun, rectifier power supply, control system, heat exchange system, powder feeder, and water and electricity transfer box, as shown in Figure 2-27. Auxiliary equipment includes: compressed gas supply system, working gas (argon, hydrogen, nitrogen) supply system, etc.

(1) Spray gun The plasma spray gun is a plasma arc generator that integrates water, electricity, gas and powder. It is the key to the entire system. The design and processing quality of the spray gun directly affect whether the spray system can work normally and the quality of the spray coating is excellent. inferior. The spray gun is mainly composed of nozzle, electrode, electrode rod, insulator, water channel head, etc.

1) Nozzle.

The nozzle is a key component of the spray gun and a wearing part. The material is pure copper. There are four types of nozzles commonly used in 9M spray guns, including two types of nitrogen: G and GE; two types of argon: GH and GP. G and GH are low-speed nozzles with larger opening angles and apertures, thicker flame beams, and are suitable for spraying high melting point materials, such as ZrO2, Al2O3, Cr2O3, W, etc. GE and GP are high-speed nozzles with small opening angles and apertures, and thin flame beams, which are suitable for spraying materials with lower melting points.

Atmospheric plasma spraying principle and equipment composition

Schematic diagram of plasma spraying equipment

2) Electrode.

The electrode is also a key component and wearing part of the spray gun. The head material is tungsten-thorium or tungsten-cerium alloy, and the base material is pure copper. During use, the temperature of the electrode head reaches over 20,000K, and the local heat load is quite high, making it easy to burn. In order to improve the life of the electrode, the cooling effect of the cooling water on the electrode must be enhanced.

3) Insulator. On the one hand, it plays an insulating role between the nozzle and the electrode, and on the other hand, it also serves as a gas and water channel. The material is made of engineering plastics with good heat resistance, such as polysulfone and polyformaldehyde, and the gas distribution ring material at the end of the insulator is polyimide.

4) Water channel head. It is not only the conductor of the spray gun waterway, but also the installation positioning sleeve for the nozzle, electrode and insulator. Material is brass or bronze. The inner hole is processed with high precision, and its processing quality directly affects the concentricity and cooling effect of the nozzle and electrode.

(2) Rectified power supply The rectified power supply used for spraying is a device that provides electrical energy to the spray gun. Its external and dynamic characteristics and power supply parameters should meet the requirements for the spray gun to generate plasma arc. At present, most power supplies use thyristor rectification, and the power supply is mainly composed of transformers, reactors, rectifiers, contactors, etc.

Performance requirements and technical parameters of plasma spray power supply

Atmospheric plasma spraying principle and equipment composition

(3) Control system The control system is the control center of the entire equipment and generally consists of three parts: electrical control system, gas circuit control system and working status monitoring system (or safety alarm system). The control system and the water and electricity transfer box work together to control, adjust and monitor (fault display) the action procedures and process parameters of the entire spraying process.

The electrical control system includes: operation action program control circuit, power supply current trigger control circuit, signal display circuit, etc. The operation action program control circuit is a logical combination (programming) of each input condition signal, and outputs a control signal to control the actuator (electrical component) so that it operates in the sequence required by the process, and has the ability to automatically operate in abnormal working conditions. Shutdown protection function.

At present, there are three forms of domestic equipment program control circuits:

1) Fixed program relay control circuit. Program control is achieved through the series and parallel connection of relay contacts. At present, some domestic equipment still uses this method.

2) Programmable controller PLC program control circuit. A programmable controller is used to form a program control circuit. At present, some domestic and imported equipment adopt this method.

3) Computer control system. A microcomputer is used to automatically control and store the operating procedures and process parameters of the entire spraying system, and the working status and process parameters are displayed through the monitor. Some imported equipment adopts this method.

(4) Powder feeder The powder feeder is a device that quantitatively supplies powder to the spray gun during the spraying process. General requirements: the particle size range of the powder that can be fed is 5 ~ 200 μm; the powder feeding amount is continuously adjustable: 5 ~ 150g/min; powder feeding Accuracy: ±1%. Currently, fluidized bed powder feeders and volumetric powder feeders are commonly used. The parameter variables when the powder feeder is working are the carrier gas flow rate and the working pressure of the powder cylinder (referred to as back pressure), as well as the model of the powder feeding pick-up shaft. Generally, the two variables of the pickup shaft and the carrier gas are fixed, and the powder feeding is changed by adjusting the back pressure. Quantity size. The powder feeding amount per unit time of a certain powder needs to be calibrated to determine. Calibration work is not done once and for all. Differences in powder particle size, gas pressure fluctuations, wear of the pickup shaft, etc. will all cause changes in the powder feeding amount, so the decision must be made based on the actual situation. Whether to recalibrate. Generally, the cause of powder feeder failure is "air tightness" problem. Aging of the carrier gas pipeline and damaged O-rings of the joints may cause "air tightness" problems.

Atmospheric plasma spraying principle and equipment composition

Working principle diagram of fluidized bed powder feeder

(5) Heat exchange system There are two types of heat exchange systems, one is water pump refrigeration, and the other is Freon refrigeration or refrigeration unit. 1) Water pump refrigeration heat exchanger. In the structure of this type of heat exchanger, the heat exchange process is divided into internal and external circulations. The internal circulating fluid is distilled water, and its circulation path is: water storage tank → water pump → spray gun → heat exchanger → water storage tank. The external circulating fluid is reservoir water, and its circulation path is: reservoir water → water pump → heat exchanger → reservoir water. The basic principle is: use distilled water to cool the spray gun, and then use water from the reservoir to cool the distilled water. This form of heat exchange system is mainly used in northern my country, because the water temperature in the northern summer reservoir can be below 25°C after being cooled by the cooling water tower, while the south does not have this condition.