Classification of thermal spraying As a new useful engineering skill, there is no standard classification method, which is generally classified according to the type of heat source, the shape of spraying data and the function of coating. For example, according to the function of the coating, it can be divided into corrosion resistance, wear resistance, heat insulation and other coatings. According to the heating and bonding methods, it can be divided into spraying and spray melting: the former is that the body does not melt, and the coating and the substrate form a mechanical combination; The latter is the re-heating and remelting of the coating, and the coating and the substrate are mutually soluble and dispersed to form metallurgical bonding.   A classification method that is frequently used in the past is based on the type of heat source in the heating and spraying data, which can be divided into: ① Flame, including flame spraying, blasting spraying and supersonic spraying; ② Arc type, including arc spraying and plasma spraying; ③ Electrothermal method, including electric explosion spraying, induction heating spraying and capacitive discharge spraying;   ④ Laser: laser spraying.     ... Flame spraying 1. Flame spraying Flame spraying includes wire flame spraying and powder flame spraying. Wire flame spraying method: It is an ultra-early spraying method. It is to send the metal wire into the spray gun at a certain speed to melt the end in the high temperature flame, and then use compressed air to atomize and blow it away, and accumulate it on the surface of the pretreated workpiece. The spraying source is the nozzle, and the metal wire passes through the center of the nozzle, passes through the annular flame formed by the nozzle and the hood, and the metal wire is continuously heated to its melting point. Then, the compressed air passing through the hood atomizes it into ejected particles, which are accelerated to the substrate by relying on the air flow, and then the molten particles are cooled to the plastic or semi-molten state, which also causes a certain degree of oxidation. When the particles collide with the substrate, they flatten and bind to the surface of the substrate, and the subsequent particles collide with the substrate also flatten and bind to the particles previously bonded to the substrate, and then accumulate into a coating. The wire is transported by the air turbine or electric motor in the spray gun, whose speed can be adjusted to control the wire feeding speed. The spray gun with air turbine is difficult to fine-tune the wire feeding speed, and its speed is difficult to be stable due to the influence of compressed air, but the spray gun is light and suitable for manual operation; The spraying equipment using electric motor to transmit wire can be stable and stable, and the spraying automation is high, although the wire feeding speed is slightly adjusted, but the spray gun is clumsy and only suitable for mechanical spraying. In the wire flame spray gun, the gas flame is mainly used for wire melting, and the diameter of metal wire suitable for spraying is generally 1.8~4.8mm. However, sometimes bars with larger diameter and even some strips can also be sprayed, but special spray guns must be equipped at this time. Powder flame spraying method: the difference between it and wire flame spraying is that the spraying data is not wire but powder. In flame spraying, the combination of acetylene and oxygen is generally used to provide heat, and methyl acetylene, propylene (MPS), propane, hydrogen or natural gas can also be used. Flame spraying can spray metal, ceramics, plastics and other materials. It is very sensitive to use. The spraying equipment is simple, simple, mobile, and cheaper than other spraying equipment. It is a widely used method in spraying technology. However, flame spraying also has significant deficiencies. For example, the velocity of the particles sprayed is small, the flame temperature is low, the adhesive strength of the coating and the inductive strength of the coating itself are relatively low, and the porosity obtained by other methods is also lower. In addition, the flame center is an oxidizing atmosphere, so attention should be paid to the application of high melting point data and oxidizable data. In order to improve the lack of flame spraying and improve the bonding strength and coating density, the equipment that can speed up the compressed air or air flow can be selected to improve the particle speed; You can also choose to change the compressed air flow from air to inert gas to reduce the degree of oxidation, but this also improves the cost. 2. Blasting spraying and blasting spraying: oxygen and acetylene gas are used to incinerate and incinerate to form gas expansion and burst, releasing heat energy and shock wave, which melt the spraying powder, and the shock wave makes the molten powder spray to the surface of the workpiece at a speed of 700~800m/s to form a coating. Figure 5 is the schematic diagram of blasting spray gun. Figure 5 Schematic diagram of blasting spraying The basic characteristics of the composition of blasting coating are generally believed to be still the result of high-speed molten particles hitting the substrate. The super-large characteristics of blasting spraying are high particle flying speed and large kinetic energy, so the blasting spraying coating has the following characteristics: ① high bonding strength between the coating and the substrate, ② dense coating, low porosity, ③ low surface roughness of the coating after processing, ④ low surface temperature of the workpiece. Blasting spraying can spray metal, cermet and ceramic materials. However, due to the high price, loud noise, oxidizing atmosphere and other reasons, the equipment is not widely used at home and abroad. The super-successful blasting spray coating applied on the surface was patented by Linde Branch of United Carbide in 1955, and its equipment and process parameters remain confidential. In China around 1985, the blasting and spraying equipment was successfully developed by the Institute of Aeronautical Information of the Ministry of Aerospace Industry of China. In terms of the function of Co/WC coating, the spraying function is close to the level of United Carbide of the United States. In blasting spraying, when the content of acetylene is 45%, the oxygen-acetylene mixture can have a spontaneous combustion temperature of 3140 ℃, but it may exceed 4200 ℃ under blasting conditions, so most of the powder can be melted. The length of powder transported in the high-speed gun is much longer than that of the plasma gun, which is also the reason for its high particle velocity. 3. Supersonic spraying In order to resist the blasting and spraying of American Carbide Company, in the early 1960s, American J. Browning created the supersonic flame spraying technology, called "Jet-Kote", and obtained the US patent in 1983. In recent years, foreign supersonic flame spraying technology has developed rapidly, and many new equipment have emerged, replacing traditional plasma spraying in many areas. In China, Wuhan Institute of Data Maintenance, Beijing Iron and Steel Research Institute, Beijing Titan New Technology Information Co., Ltd. and other companies are also carrying out this research and producing supersonic spraying equipment with their own characteristics. The fuel aviation kerosene and combustion-supporting agent (O2) are introduced into the incinerator in a certain proportion for mixing and explosive incineration. The high temperature gas generated by incineration passes through the expansion tube at high speed to obtain supersonic speed. The powder gas (Ar or N2) is injected together, and is quantitatively injected into the high-temperature gas along the tungsten carbide central sleeve in the incineration head, and sprayed together on the workpiece to form a coating. The flame flow velocity at the nozzle outlet of the spraying machine is generally 4 times of the sound speed, that is, about 1520m/s, and the superelevation can be as high as 2400m/s (details are related to the type of incineration gas, mixing proportion, flow rate, powder quality and powder flow rate). The speed of the powder hitting the surface of the workpiece is estimated to be 550-760m/s, which is appropriate for blasting spraying. The reason why Jet-Kote method can achieve such high speed is that a nozzle, called Laval tube expansion tube, is reasonably planned and manufactured according to the principle of fluid mechanics. As long as the pipe planning is reasonable, the fluid can reach the sound speed at a certain section of the pipe (such as AB) after satisfactory compaction when the speed is low. After passing this section, the supersonic speed will be achieved. The supersonic spraying method has the following characteristics: ① The temperature of powder particles is low and oxygen is light (this is mainly due to the short duration of powder particles in high temperature and exposure time in the air, so the content of oxide in the coating is low and the chemical composition and phase composition have strong stability), but it is only suitable for spraying metal powder, Co-Wc powder and low-melting point TiO2 ceramic powder;   ② The particle movement speed is high.   ③ Small particle size (10~53>; μ m) The dispersion scale is narrow, otherwise it cannot be melted.   ④ The coating has high bonding strength, high density and no delamination. ⑤ The surface roughness of the coating is low. ⑥ The spraying interval can be changed in a large scale without affecting the spraying quality. ⑦ A thicker coating can be obtained than blasting spraying, and the residual stress can also be improved. ⑧ High spraying power and convenient operation. ⑨ The noise is large (more than 120dB), and sound insulation and protective equipment are required. Arc spraying 1. Arc spraying: Arc spraying: an arc is generated between two metal materials in the form of welding wire. The heat of the arc causes the metal welding wire to melt gradually, and the molten part is sprayed to the surface of the substrate by compressed air flow to form a coating. Arc spraying can be divided into DC arc spraying and AC arc spraying according to the arc power supply. DC: stable operation, dense coating arrangement, high power. Communication: loud noise. The temperature of the arc is related to the arc gas medium, electrode data type and current (such as Fe material, current 280 A, arc temperature 6100 K). However, in general, the arc spraying powder particles contain more heat than the flame spraying powder particles, and the particle flying speed is also faster. Therefore, when the molten particles hit the substrate, the possibility of forming part of the micro-metallurgical bonding is much greater. Therefore, the bonding strength between coating and substrate is 1.5~2.0 times higher than that of flame spraying, and the spraying power is also higher. Arc spraying can also make alloy coating or "pseudo alloy" coating conveniently. Different alloy compositions can be obtained by using two wires with different compositions and using different feed speeds. Similar to flame spraying equipment, arc spraying has the advantages of low cost, less one-time investment and convenient use. However, there is a significant lack of arc spraying, and the spraying materials need to be conductive welding wires, so only metal can be used instead of ceramic, which limits the application scale of arc spraying. In recent years, in order to further improve the function of arc spraying coating, foreign countries have made great improvements in equipment and technology, and issued many patents. For example, add methane and the like to the compressed air as atomizing gas to reduce the oxygen content of the coating. Japan also changed the traditional round wire into square to improve the spray rate and improve the bonding strength of the coating.   2. Plasma spraying: Plasma spraying: including atmospheric plasma spraying, maintenance atmosphere plasma spraying, vacuum plasma spraying and water stabilized plasma spraying. Isoparticle spraying technology is a new multi-purpose fine spraying method vigorously developed after flame spraying, which has: ① Ultra-high temperature characteristics, easy to spray high melting point data.   ② The spray particles have high speed, dense coating and high bonding strength. ③ Because inert gas is used as the working gas, the spraying data is not easy to oxidize. Composition of plasma (taking N2 as an example) After gas ionization, there are not only atoms, but also positive ions and free electrons in space, which is called plasma. Plasma can be divided into three categories: ① High temperature and high pressure plasma, with 100% ionization degree and temperature of hundreds of millions of degrees, is used for nuclear fusion research;   ② Low temperature and low pressure plasma, the ionization degree is less than 1%, and the temperature is only 50-250 degrees; ③ High temperature and low pressure plasma, about 1% of the gas is ionized, with a temperature of tens of thousands of degrees. The kinetic energy of ions, free electrons and uninionized atoms is close to the thermal equilibrium. This kind of plasma is used in thermal spraying. Spraying principle The iso-particle spraying is carried out by using plasma arc, and the ion arc is a compact arc. Compared with the free arc, its arc column is thin, its current density is high, and its gas ionization degree is high. Therefore, it has the characteristics of high temperature, concentrated energy, and good arc stability. According to different methods of power connection, there are three ways of plasma arc: ① Non-handling arc: refers to the plasma arc between cathode and nozzle. In this case, the positive electrode is connected to the nozzle, the workpiece is not charged, and an arc occurs between the cathode and the inner wall of the nozzle. The working gas is heated by the arc between the cathode and the nozzle, forming full or partial ionization, and then the nozzle sprays out to form a plasma flame (or plasma jet). This type of plasma arc is selected for equal particle spraying. ② Handling arc: the plasma arc that the arc is removed from the spray gun and transported to the processed part. In this case, the nozzle is not connected to the power supply, and the workpiece is connected to the positive pole. The arc flies between the cathode and the anode (workpiece) of the spray gun. The working gas is sent around the arc and then sprayed out from the nozzle. Plasma cutting, plasma arc welding and plasma arc smelting use this kind of plasma arc. ③ Combined arc: the non-carrying arc ignites the carrying arc and heats the metal powder, and the carrying arc heats the workpiece to form a molten pool on its surface. In this case, the nozzle and the workpiece are connected to the positive pole. This plasma arc is selected for plasma spray welding. In the process of equal particle spraying, a DC arc is first generated between the cathode and anode (nozzle), which heats and ionizes the imported working gas into high-temperature plasma, and sprays it out from the nozzle to form a plasma flame. The temperature of the plasma flame is very high, its central temperature can reach 30000 ° k, and the temperature of the nozzle outlet can reach; 15000～20000°k。 The flame flow velocity can reach 1000~2000m/s at the nozzle outlet, but it decays rapidly. The powder is sent into the flame by the powder feeding gas to be melted, and the flame flow accelerates to obtain a speed higher than 150 m/s, which is sprayed onto the base material to form a film. Isoparticle spraying equipment: plasma spraying equipment mainly includes: ① Spray gun: actually, it is a non-transporting arc plasma generator, which is a super-critical component. It concentrates the electricity, gas, powder, water, etc. of the whole system. ② Power supply: used to supply DC power to the spray gun. Generally, it is full-wave silicon rectifier equipment. ③ Powder feeder: equipment used to store spray powder and deliver powder to spray gun according to process requirements.   ④ Heat exchanger: It is mainly used to make the spray gun obtain useful cooling and achieve the purpose of prolonging nozzle life.   ⑤ Gas supply system: including the supply system of working gas and powder gas. ⑥ Control box: used to regulate and control water, electricity, gas and powder.   Isoparticle spraying process In the process of equal particle spraying, there are many process parameters that affect the coating quality, mainly including: ① Plasma gas: The selection criteria of gas are mainly based on availability and economy. N2 gas is cheap, and the ion flame has high enthalpy and fast heat transfer, which is conducive to the heating and melting of powder. However, the powder or matrix that is prone to nitriding reaction cannot be selected. Ar gas has low ionization potential, stable and easy ignition of plasma arc, short arc flame, and is suitable for spraying small or thin parts. In addition, Ar gas has good maintenance effect, but Ar gas has low enthalpy and is expensive. The size of the gas flow directly affects the enthalpy and flow rate of the plasma flame flow, and then affects the spray power, coating porosity and adhesion. If the flow rate is too high, the gas collar will take away useful heat from the plasma jet and increase the speed of the spray particles, reducing the "residence" time of the spray particles in the plasma flame, resulting in the particles not reaching the semi-melting or plastic conditions necessary for deformation, resulting in poor adhesion strength, density and hardness of the coating, and a significant decrease in the stacking rate; On the contrary, it will make the arc voltage value inappropriate and greatly reduce the speed of particles emitted. Under extreme conditions, the spray data will be overheated, forming excessive melting or vaporization of the spray data, causing the molten powder particles to gather in the nozzle or powder nozzle, and then accumulate in the coating in a large spherical shape, forming a large hole.    ② Arc power: if the arc power is too high and the arc temperature rises, more gas will change into plasma. Under the condition of high power and low operating gas flow, almost all operating gas will change into active isoparticle flow, and the flame temperature of isoparticle is also high, which may vaporize some spraying materials and cause the coating to form