A common classification method is classified according to the type of heat source of heating spraying materials, which is divided into: ① flame, including flame spraying, explosive spraying and supersonic spraying; (2) Arc, including arc spraying and plasma spraying; (3) electrothermal method, including electric explosion spraying, induction heating spraying and capacitor discharge spraying; ④ Laser category: laser spraying. 1. flame spraying: flame spraying includes metal wire flame spraying and powder flame spraying.
& amplt； 1。 gt； Metal wire flame spraying method: it is the earliest spraying method invented. It sends the metal wire into the spray gun at a certain speed, so that the end is melted in the high-temperature flame, and then it is atomized and blown away by compressed air and deposited on the pretreated workpiece surface.
Figure 1 Schematic diagram of silk flame spraying device.
Fig. 2 schematic diagram of metal wire flame spraying.
Figure 1 shows the wire flame spraying device. Fig. 2 is a sectional view of a metal wire flame spray gun, which shows the basic principle of metal wire flame spraying. The injection source is a nozzle, and the metal wire passes through the center of the nozzle, and the tip of the metal wire is continuously heated to its melting point through the annular flame formed around the nozzle and the gas hood. Then it is atomized by compressed air passing through the gas hood into sprayed particles, which are accelerated by airflow and sprayed on the substrate, so that the molten particles are cooled to a plastic or semi-molten state and oxidized to a certain extent. When the particles collide with the substrate, they flatten and adhere to the surface of the substrate, and then the particles that collide with the substrate flatten and adhere to the particles that have previously adhered to the substrate, thereby accumulating into a coating.
Wire feeding depends on the rotation of the air turbine or motor in the spray gun, and its rotation speed can be adjusted to control the wire feeding speed. Using air turbine spray gun, it is difficult to fine-tune the wire feeding speed, and its speed is difficult to be constant due to the influence of compressed air, but the spray gun is light in weight and suitable for manual operation. The spraying equipment using motor to transport wire is easy to adjust, can be kept constant and has a high degree of spraying automation, but the spray gun is large and only suitable for mechanical spraying. In the wire flame spray gun, the gas flame is mainly used to melt the wire, and the diameter of the wire suitable for spraying is generally 1.8 ~ 4.8 mm, but sometimes it is also possible to spray bars with larger diameter or even some strips, but a special spray gun must be equipped at this time.
& amplt； 2 & ampgt； Powder flame spraying method: the difference between it and wire flame spraying is that the spraying material is powder instead of wire. Figs. 3 and 4 are schematic diagrams of powder flame spraying device and principle, respectively.
Fig. 3 Typical device for powder flame spraying
Fig. 4 schematic diagram of powder flame spraying
In flame spraying, acetylene and oxygen are usually combined to provide heat, and methylacetylene, propylene (MPS), propane, hydrogen or natural gas can also be used. Flame spraying can spray metals, ceramics, plastics and other materials, and its application is very flexible. Spraying equipment is light, simple and mobile, and its price is lower than other spraying equipment. It is a widely used method in spraying technology at present. However, flame spraying also has obvious disadvantages. For example, the jet particle velocity is low, the flame temperature is low, the bonding strength of the coating and the comprehensive strength of the coating itself are low, and the porosity obtained by other methods is higher than that obtained by other methods. In addition, the flame center is an oxidizing atmosphere, so attention should be paid to high melting point materials and oxidizable materials when using. In order to improve the deficiency of flame spraying and improve the bonding strength and coating density, compressed air or airflow can be used to accelerate the particle speed; It is also possible to reduce the degree of oxidation by changing the compressed air flow from air to inert gas, but this also increases the cost.
2. Explosive spraying
Explosive spraying: ignite and burn with oxygen and acetylene gas to make the gas expand and explode, releasing heat energy and shock wave. The thermal energy melts the sprayed powder, and the shock wave causes the melted powder to be sprayed on the surface of the workpiece at a speed of 700 ~ 800 m/s to form a coating. Fig. 5 is a schematic diagram of an explosion spray gun.
Fig. 5 schematic diagram of explosive spraying
Generally speaking, the basic feature of explosive coating formation is still the result of collision between high-speed molten particles and matrix. The biggest feature of explosive spraying is the high flying speed and kinetic energy of particles, so the explosive spraying coating has the following characteristics: ① the bonding strength between the coating and the substrate is high; ② dense coating with low porosity; ③ The surface roughness of the coating after processing is low; ④ The surface temperature of the workpiece is low. Explosive spraying can spray metal, cermet and ceramic materials, but it is not widely used at home and abroad because of its high price, high noise and oxidizing atmosphere. At present, the most successful explosive spraying in the world is the patent obtained by Linde Branch of United Carbide Company of the United States in 1955, and its equipment and process parameters are still confidential. In China, explosive spraying equipment was successfully developed by China Aerospace Industry Institute of Aeronautical Materials around 1985. As far as the performance of Co/WC coating is concerned, the spraying performance is close to that of United Carbide Company.
In explosive spraying, when the content of acetylene is 45%, the free combustion temperature of oxygen-acetylene mixture can be 3 140℃, but it may exceed 4200℃ under explosive conditions, so most of the powder can be melted. The transmission length of powder in high-speed gun is much longer than that in plasma gun, which is also the reason for its high particle velocity.
3. Supersonic spraying
In order to counter the explosive spraying of American carbide company, in the early 1960s, American J.Browning invented the supersonic flame spraying technology, which was called Jet-Kote, and obtained the American patent at 1983. In recent years, foreign supersonic flame spraying technology has developed rapidly, and many new types of equipment have appeared, which are replacing traditional plasma spraying in many fields. In China, Wuhan Institute of Material Protection, Beijing Iron and Steel Research Institute and Beijing Taide New Technology Materials Co., Ltd. are also conducting research in this field and producing their own supersonic spraying devices.
Fig. 6 supersonic flame spray gun
Fuel aviation kerosene and combustion improver (O2) are introduced into the combustion chamber in a certain proportion to mix and explode, and the high-temperature gas produced by combustion passes through the expansion tube at high speed to obtain supersonic speed. At the same time, powder feeding gas (Ar or N2) is introduced, and high-temperature gas is quantitatively fed along the center sleeve of tungsten carbide in the combustion head, which is sprayed on the workpiece to form a coating.
Generally, the speed of flame flow generated at the nozzle outlet of spray coater is four times of the speed of sound, that is, about 1520m/s, and the highest speed can reach 2400m/s (specifically related to the type of combustion gas, mixing ratio, flow rate, powder quality and powder flow rate, etc.). ). The impact velocity of powder on the workpiece surface is estimated to be 550-760m/s, which is equivalent to explosive spraying. The key to the high speed of Jet-Kote method lies in the rational design and manufacture of a nozzle according to the principle of fluid mechanics, which is called the expansion tube of Laval tube.
Fig. 7 Laval tube
According to fluid mechanics, for one-dimensional compressible fluid, there are: ds/s = (m&; sup2- 1)dv/v
Where: s refers to the cross-sectional area of the pipeline; M=v/v sound (Mach number); V- fluid velocity
From the formula, we can see that when V&; gt； V, that is, m &；; gt； At 1, the signs of dv and ds are the same, that is, the fluid velocity increases with the increase of the cross-sectional area of the pipeline (ds is positive). When v &；; lt； V, that is, m &；; lt； At 1, the sign of dv is opposite to that of ds, that is, with the decrease of pipeline cross-sectional area (ds is negative), the fluid velocity also increases. Therefore, as long as the pipeline design is reasonable, the fluid can reach the speed of sound at a certain section of the pipeline (such as AB) at low speed, and after this section, it will get the speed of sound. 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 residence time of powder particles at high temperature and short exposure time in air, so the oxide content in the coating is low and the chemical composition and phase composition are stable), but it is only suitable for spraying metal powder, Co-Wc powder and low melting point TiO2 ceramic powder;
② Powder particles move at high speed.
③ The powder particle size is small (10 ~ 53 >; Micron), the distribution range is narrow, otherwise it cannot be melted.
④ The coating has high bonding strength, compactness and no delamination.
⑤ The coating surface roughness is low.
⑥ The spraying distance can be changed in a wide range without affecting the spraying quality.
⑦ A thicker coating can be obtained than explosive spraying, and the residual stress is also improved.
⑧ High spraying efficiency and convenient operation.
Pet-name ruby noise (120dB above), need to have sound insulation and protection device. 1, arc spraying:
Arc spraying: an arc is generated between two kinds of filamentary metal materials, and the metal wire gradually melts due to the heat generated by the arc, and the melted part is sprayed on 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 arc power supply. Direct current: stable operation, compact coating structure and high efficiency. Communication: loud noise. The temperature generated by arc is related to arc gas medium, electrode material type and current (for example, Fe material, current is 280 A, and arc temperature is 6 100K). But generally speaking, arc spraying powder particles contain more heat and fly faster than flame spraying powder particles. Therefore, when molten particles hit the substrate, it is much more likely to form local micro-metallurgical bonding. Therefore, the bonding strength between coating and substrate is 1.5 ~ 2.0 times higher than that of flame spraying, and the spraying efficiency is also higher. Arc spraying can also make alloy coatings or "pseudo-alloy" coatings conveniently. By using two wires with different compositions and using different feed rates, different alloy compositions can be obtained. Arc spraying is similar to flame spraying equipment, and it also has the advantages of low cost, less one-time investment and convenient use. However, due to the obvious shortcomings of arc spraying, the spraying material must be conductive welding wire, so only metal can be used, not ceramics, which limits the application scope of arc spraying. In recent years, in order to further improve the performance of arc spraying coating, the equipment and technology abroad have been greatly improved, and many patents have been published. For example, methane is added to compressed air as an atomizing gas to reduce the oxygen content of the coating. Japan also changed the traditional round wire into square wire to improve the spraying speed and the bonding strength of the coating.
2, plasma spraying:
Plasma spraying: including atmospheric plasma spraying, protective atmospheric plasma spraying, vacuum plasma spraying and water-stable plasma spraying. Isoparametric spraying technology is a new multi-purpose precision spraying method developed after flame spraying, which has the following characteristics: ① ultra-high temperature characteristics, which is convenient for spraying high melting point materials. ② The spraying speed of particles is fast, the coating is compact and the bonding strength is high. (3) Because inert gas is used as working gas, the spraying material is not easy to be oxidized.
& amplt； 1。 gt； The formation of plasma (taking N2 as an example)
Fig. 8 is a schematic diagram of the plasma generation process.
At 0 K, the two atomic ranges of N2 molecule are dumbbell-shaped and only move in X, Y, Z, Y and Z directions.
When it is greater than 10 k, it starts to rotate;
When it is greater than 10000 k, the atoms vibrate and the molecules collide with each other, and the molecules will dissociate and become monatomic;
N2+Ud——& amp； gt； Where Ud is the dissociation energy.
If the temperature rises again, atoms will ionize: n+ui-&; gt； N++e where Ui is ionization energy.
After gas ionization, there are not only atoms in space, but also positive ions and free electrons. This state is called plasma.
Plasma can be divided into three categories: ① high temperature and high pressure plasma with ionization degree 100% and temperature of several hundred million degrees, which is used for the research of nuclear fusion; ② Low temperature and low pressure plasma with ionization degree less than 65438 0% and temperature only 50 ~ 250 degrees; ③ High temperature and low pressure plasma, about 1% gas is ionized, and the temperature is tens of thousands of degrees. The kinetic energy of ions, free electrons and non-ionized atoms is close to thermal equilibrium. This kind of plasma is used in thermal spraying.
& amplt； 2 & ampgt； Spraying principle:
Fig. 9 Principle of Isoparticle Spraying
Plasma arc is used for plasma spraying, and ion arc is compression arc. Compared with the free arc term, the arc column is thin, the current density is high, and the gas ionization degree is high, so it has the characteristics of high temperature, concentrated energy and good arc stability.
According to the different power connection modes, there are three forms of plasma arc:
① Non-transfer arc: refers to the plasma arc generated between cathode and nozzle. In this case, the anode is connected to the nozzle, and the workpiece is not charged, so that an arc is generated between the cathode and the inner wall of the nozzle. The working gas is heated by the arc between the cathode and the nozzle, resulting in total or partial ionization, and then ejected from the nozzle to form a plasma flame (or plasma jet). This plasma arc is used for plasma spraying.
② Transfer arc: the arc leaves the spray gun and transfers to the plasma arc on the machined part. In this case, the nozzle is not connected to the power supply, and the workpiece is connected to the anode. The arc flies between the cathode and anode (workpiece) of the spray gun, and the working gas is fed around the arc and then ejected from the nozzle. This kind of plasma arc is used for plasma cutting, plasma arc welding and plasma arc melting.
③ Combined arc: non-transfer arc ignites transfer arc to heat metal powder, and transfer arc heats workpiece to produce molten pool on its surface. In this case, the nozzle and the workpiece are connected to the positive electrode. This plasma arc is used for plasma spray welding.
During plasma spraying, a DC arc is first generated between the cathode and the anode (nozzle), and the introduced working gas is heated and ionized into high-temperature plasma, which is ejected from the nozzle to form a plasma flame. The temperature of plasma flame is very high, the center temperature can reach 30000 k, and the nozzle outlet temperature can reach; 15000 ~ 20000 K. The flame speed can reach 1000 ~ 2000 m/s at the nozzle outlet, but it decays quickly. Powder is melted by powder feeding gas, accelerated by flame flow to a speed higher than 150m/s, and sprayed on the base material to form a thin film.
Figure 10 Temperature Distribution of Plasma Flame
& amplt； 3 & ampgt； Isoparticle spraying equipment: plasma spraying equipment mainly includes:
① Spray gun: It is actually a non-transfer arc plasma generator, which is the most critical component, and it concentrates the electricity, gas, powder and water of the whole system.
② Power supply: used to provide direct current to the spray gun. Usually a full-wave silicon rectifier.
③ Powder feeder: a device for storing spraying powder and conveying the powder to the spray gun according to the process requirements.
④ Heat exchanger: it is mainly used to effectively cool the spray gun and prolong the life of the nozzle.
⑤ Gas supply system: including the supply system of working gas and powder feeding gas.
⑥ Control box: used to adjust and control water, electricity, gas and powder.
& amplt； 4 & ampgt； Isoparticle spraying process;
In the process of plasma spraying, there are many process parameters that affect the coating quality, mainly including:
① Plasma gas: The selection principle of gas is mainly based on availability and economy. N2 gas is cheap, the enthalpy of ion flame is high, and the heat transfer is fast, which is beneficial to the heating and melting of powder, but it is not suitable for powder or matrix which is prone to nitriding reaction. Ar gas has low ionization potential, stable plasma arc and short arc flame, and is suitable for spraying small or thin parts. In addition, Ar gas has a good protective effect, but Ar gas has a low enthalpy and is expensive. The gas flow rate directly affects the enthalpy and velocity of plasma flame flow, thus affecting the spraying efficiency, coating porosity and bonding force. If the flow velocity is too high, the gas will take away useful heat from the plasma jet, which will increase the speed of spraying particles and reduce the "residence" time of spraying particles in the plasma flame, resulting in the particles not reaching the semi-molten or plastic state necessary for deformation. This leads to poor bonding strength, density and hardness of the coating, and the deposition rate will also be significantly reduced. On the contrary, it will make the arc voltage value inappropriate and greatly reduce the speed of spraying particles. In extreme cases, it will lead to overheating of the spraying material, excessive melting or vaporization of the spraying material, and the molten powder particles will gather in the nozzle or powder nozzle, and then be deposited in the coating in a larger ball shape, forming a larger cavity.
② Arc power: If the arc power is too high and the arc temperature rises, more gas will be converted into plasma. Under the condition of high power and low working gas flow, almost all working gas will be converted into active particle flow, and the flame temperature of particles is also very high, which may cause some spraying materials to vaporize and change the coating composition, and the vapor of spraying materials will condense between the substrate and the coating or between the coatings, resulting in poor adhesion. In addition, the nozzle and electrode can be ablated. However, if the arc power is too low, some low-temperature ionic gases and plasma flames will be obtained, resulting in insufficient heating of particles and low bonding strength, hardness and deposition efficiency of the coating.
③ Powder supply: the powder supply speed must be adapted to the input power. If it is too large, raw powder (unmelted) will appear, which will reduce the spraying efficiency; If it is too low, the powder is seriously oxidized and the matrix is overheated. The feeding position will also affect the coating structure and spraying efficiency. Generally speaking, powder must be sent to the center of the flame to get the best heating and the highest speed.
④ Spraying distance and spraying angle: The distance from the spray gun to the workpiece affects the speed and temperature when spraying particles collide with the substrate, and the characteristics of coating and spraying materials are very sensitive to the spraying distance. When the spraying distance is too large, the temperature and speed of powder particles decrease, and the binding force, porosity and spraying efficiency decrease obviously. If it is too small, the temperature of the substrate will rise too high, and the substrate and the coating will be oxidized, which will affect the bonding of the coating. If the engine temperature rise allows, the injection distance should be smaller.
Spraying angle: refers to the angle between the flame axis and the surface of the workpiece to be sprayed. When the angle is less than 45 degrees, due to the influence of "shadowing effect", the coating structure will deteriorate to form holes, resulting in loose coating.
⑤ Relative moving speed of the spray gun and the workpiece: The moving speed of the spray gun should ensure that the coating is flat and there is no trace of back spray. In other words, the width of each stroke should completely overlap. On the premise of meeting the above requirements, the spraying operation generally adopts a higher moving speed of the spray gun, which can prevent local hot spots and surface oxidation.
⑥ substrate temperature control: the ideal spraying workpiece is to preheat the workpiece to the temperature to be reached in the spraying process before spraying, and then take air-jet cooling measures to keep the original temperature of the workpiece in the spraying process. In recent years, several new plasma spraying technologies have been developed on the basis of plasma spraying, such as:
3. Vacuum plasma spraying (also known as low-pressure plasma spraying)
Vacuum plasma spraying is a spraying technology carried out in a sealed room with controlled atmosphere of 4 ~ 40kpa. Because the working gas after plasma is ejected while expanding its volume in a low-pressure atmosphere, the ejection speed is supersonic, which is very suitable for materials that are highly sensitive to oxidation.
4. Water-stable plasma spraying
The working medium of plasma spraying mentioned above is gas, but the working medium of this method is water instead of gas. It is a high-power or high-speed plasma spraying method. Its working principle is that high-pressure water flows into the spray gun and forms eddy current on the inner wall of the barrel. At this time, a DC arc is generated between the cathode at the back of the gun body and the rotating anode at the front of the gun body, so that a part of the inner wall surface of the gun barrel is evaporated and decomposed into a plasma state, and a continuous plasma arc is generated. Due to the bunching effect of rotating vortex water, its energy density is improved and combustion is stable. Therefore, high melting point materials, especially oxide ceramics, can be sprayed with high spraying efficiency. 1. electric explosion spraying: apply a large instantaneous current at both ends of the metal wire to make the metal wire melt and explode. This method is specially used to spray the inner surface of cylinder.
2. Induction heating spraying: heating the metal wire with high-frequency eddy current, and then atomizing and accelerating with high-pressure gas.
3. Capacitive discharge heating: the metal wire is heated by capacitive discharge, and then atomized and accelerated by high-pressure gas. The laser beam with high-density energy is directed in the direction close to the surface of the substrate of the part, and the substrate is heated by the auxiliary laser heater. At this time, fine powder is blown into the laser beam at an inclined angle. Figure 1 1 laser spraying
Melt and bond to the surface of the substrate to form a thin surface coating and form a good combination with the substrate (the spraying environment can be atmospheric atmosphere, inert gas atmosphere or vacuum).