Plasma surfacing

       The plasma arc welding process is a branch of surface strengthening technology. It uses a combined or transfer plasma arc as a heat source, and uses alloy powder or welding wire as a filler metal. During surfacing, the surface of the workpiece and the surfacing material are melted at the same time, and the two materials are mixed with each other to form a molten pool. The molten pool is condensed and crystallized. Form a surfacing layer.



Plasma surfacing technology appeared in the 1960s. In the early stage of development, it was mainly used to repair damaged parts, such as restoring the shape and size of parts. The recovery of shape and size is then gradually combined with surface strengthening and surface modification. After the 1980s, the application field of plasma surfacing technology was further expanded, from surface repair to manufacturing. Since the 1990s, influenced by the development of advanced manufacturing technology, the research on plasma surfacing equipment has become more and more in-depth. Foreign equipment can automatically detect and automatically simulate the main process parameters of the surfacing process, and at the same time establish a database for the test data for microcomputer processing, thereby greatly improving the degree of surfacing automation. In recent years, Near Net Shape technology, which combines plasma surfacing technology with intelligent control technology (CAD/CAM) and precision grinding technology, has attracted widespread attention in the manufacturing industry, and its application range has also been expanding.



Plasma powder surfacing uses the plasma arc as the heat source, and the high temperature generated by the plasma arc is used to rapidly heat the alloy powder and the surface of the substrate to melt, mix, diffuse and solidify together. After the plasma beam leaves, it is self-excited and cooled to form a layer of high-performance alloy. layer, so as to realize the surfacing process of strengthening and hardening of the surface of the part.



Fundamental

Plasma powder surfacing uses the plasma arc as the heat source, and the high temperature generated by the plasma arc is used to rapidly heat the alloy powder and the surface of the substrate to melt, mix, diffuse and solidify together. After the plasma beam leaves, it is self-excited and cooled to form a layer of high-performance alloy. layer, so as to realize the surfacing process of strengthening and hardening of the surface of the parts. Because the plasma arc has high arc temperature, high heat transfer rate, good stability, and strong controllability of penetration depth, by adjusting the relevant surfacing parameters, the surfacing can be adjusted. The thickness, width and hardness of the welding layer can be freely adjusted within a certain range. After plasma powder surfacing, a fusion interface is formed between the base material and the surfacing material, and the bonding strength is high; the surfacing layer has a dense structure, good corrosion resistance and wear resistance; the dilution of the base material and the surfacing material is reduced, and the material properties change little; Using powder as a surfacing material can improve the selectivity of alloy design, especially the refractory material can be smoothly surfacing, and the wear resistance, high temperature resistance and corrosion resistance of the workpiece can be improved. Plasma powder surfacing has high productivity, beautiful forming, and the surfacing process is easy to mechanize and automate.



production and characteristics

The plasma arc is an arc formed by ionizing gas between the cathode and the cold nozzle or between the cathode and the workpiece using a special welding torch (also known as a compression arc). When the arc passes through the small hole of the water-cooled nozzle, it is cooled by the cold air flow and the wall of the water-cooled nozzle hole, and the arc will produce the following three compression effects:



(1) Thermal compression effect



The copper nozzle has good electrical and thermal conductivity, and the temperature of the nozzle hole wall is very low due to water cooling. Therefore, the closer the gas entering the air chamber of the spray gun with a certain pressure is to the hole wall, the lower its ionization degree, and the cold gas layer close to the hole wall is basically not ionized, but a neutral gas, so that the arc and the hole wall are formed. A circle of neutral airflow layer that is both insulating and heat-insulating, the arc current is most concentrated in the center of the gas with high degree of ionization, and the contraction of the arc current cross-section increases the current density, and the whole process is an effective reaction of thermal compression.



(2) Mechanical compression effect



The cold gas layer around the plasma arc column is attached to the wall of the nozzle orifice. Therefore, the nozzle aperture basically determines the diameter of the annular cold gas flow layer, and also determines the thickness of the plasma arc. Obviously, the nozzle aperture and its geometric size not only compress the arc to different degrees through the thermal compression effect, but also play a mechanical compressing role on the arc.



(3) Self-magnetic compression



The arc current has a certain flow direction, and the arc column is equivalent to a beam of parallel conductors with the same current direction. Each current-carrying conductor generates a magnetic field around it, and under the action of electromagnetic force, the arc column is subjected to a compressive force directed toward the center of the arc column, so that the arc column is compressed.



These three compression effects reduce the cross-sectional area of the arc column, increase the density of charged particles, and increase the electric field strength. This compressed arc is called a plasma arc.



l Plasma arc has the following characteristics:



(1) High plasma arc temperature and concentrated heat



Because the plasma arc has a compressive effect, its central temperature can reach 10,000-50,000K, and its energy density can be as high as 10-10W/cm. Due to the high plasma arc temperature, the material being processed is generally not affected by the melting point.



(2) Good thermal stability of plasma arc



Since the gas in the plasma arc is fully ionized, its arc is more stable. Plasma arc current and arc voltage are not sensitive to changes in arc length within a certain range, that is, stable combustion can still be maintained when the arc column is long, and there is no disadvantage of free arc fluttering.



(3) The plasma arc is controllable



The plasma arc can adjust the thermal effect in a wide range. In addition to changing the input power, the thermal energy and temperature of the plasma arc can also be adjusted by changing the type of gas, flow rate and changing the structure size of the nozzle; it can be adjusted by selecting different working gases. Plasma arc working atmosphere; the stiffness of the arc can be adjusted by changing the arc current, gas flow and nozzle compression ratio.



Technical Features

Compared with other surface surfacing methods such as oxyacetylene flame surfacing, electrode arc surfacing, etc., plasma surfacing has many advantages.



(1) High productivity. The plasma arc welding process is carried out under automatic control, which has high productivity and high deposition rate. At present, the current of high-power plasma arc welding in foreign countries is as high as 500A, and the deposition rate can reach 12kg/h.



(2) The dilution rate of the base metal to the alloy is low. Since there are many adjustable specification parameters in the process, the heat input can be actively controlled. The alloy powder is preheated in the arc column and transitions to the molten pool in a spray-like manner, which has a buffering effect on the arc blowing force, and the molten pool is heated evenly, so the penetration depth of the base metal can be controlled and the dilution rate of the base metal to the alloy can be reduced.



(3) The surfacing layer is well formed. Plasma arc surfacing is generally protected by argon gas, and the surfacing layer is formed flat and smooth. By changing the process specification parameters, the forming size can be precisely controlled and the size range is wide.



(4) The quality and process stability of the spray welding layer are good. Due to the good stability of the plasma arc, the interference of external factors has little effect on the electrical parameters and arc stability, so that the process is easy to stabilize. Due to the high plasma arc temperature, concentrated heat, fast spray welding speed, and one-time melting, the heat affected zone of the workpiece is small, the alloy structure of the spray welding layer is fine, the hardness and chemical composition are uniform, and the quality of the spray welding layer is good.



(5) The alloy powder is easy to prepare and has a wide range of materials. The temperature of the plasma arc area is as high as (1.0-5.0) × l0K, so various metals can be surfacing. The alloy powder for surfacing welding is directly atomized into spherical powder after smelting. It is easy to prepare. It can be smelted into alloy powder with different compositions according to the required formula to obtain alloy fusion welding layers with different properties, so as to adapt to the surface properties of parts under different conditions. requirements. There are many types of spray welding alloy materials, such as cobalt-based, nickel-based, iron-based, copper-based and so on. Generally, alloys with high hardness, wear resistance, high temperature resistance and corrosion resistance are difficult to be made into wire rods and surfacing by other process methods, but it is simple and easy to make these alloys into powder by plasma spray welding.



(6) Good controllability. The arc parameters such as the atmosphere and temperature of the plasma arc can be adjusted by changing the power, the type of gas, the flow rate and the structure size of the nozzle, so as to realize the most automatic production.



(7) The temperature of the workpiece to be surfacing is low, and it is generally not easy to deform. In order to further reduce the temperature of the workpiece during surfacing, the workpiece can also be cooled with gas.



shortcoming

(1) Plasma surfacing is not suitable for thin substrates and will cause workpiece deformation



(2) When the hardness of plasma surfacing is higher than that of HRC46, the cracking tendency is large



(3) Although the dilution amount of plasma surfacing welding is small, it will still be affected by the matrix during surfacing welding, resulting in a decrease in hardness after surfacing welding. When high hardness is required, it is necessary to surfacing in stages



Classification

According to the different filling methods of filling materials, plasma surfacing can be divided into:



(1) Cold wire plasma surfacing



Cold wire plasma surfacing uses welding wire as filler material, which is directly sent to the welding area for surfacing without preheating.



(2) Hot wire plasma surfacing



Hot wire plasma surfacing welding is to use the resistance of the welding wire itself to preheat, and then send it into the plasma area for surfacing welding. Single wire or double wire feeding can be used.



(3) Prefabricated plasma surfacing



Prefabricated plasma surfacing is to prefabricate the surfacing alloy into a certain shape and place it on the surface to be surfacing, and then melt it with plasma to form a surfacing layer.



(4) Powder plasma surfacing



Powder plasma surfacing is a surfacing method in which alloy powder is fed into the plasma arc area and melted to obtain a surfacing layer. Most alloys can be made into powder, so powder plasma surfacing has a wide range of materials, and is not limited by material ductility like wire.



Common materials

powder plasma arc welding



Alloy powders commonly used in powder plasma arc surfacing mainly include self-fluxing alloy powders and composite powders.



1. Self-fluxing alloy powder



Self-fluxing alloy powders include nickel-based, cobalt-based, iron-based, copper-based, and the like. Among them, iron-based alloy powder has been widely used due to its wide source of raw materials, low price and good performance. Nickel-based and cobalt-based alloy powders have good comprehensive properties, but nickel and cobalt are rare metals with high cost, and are generally only used for surfacing with special surface performance requirements.



(1) Nickel-based self-fluxing alloy powder: Nickel-based self-fluxing alloys are collectively referred to as nickel-based alloys. It can be divided into two categories: nickel borosilicate series and nickel chromium borosilicate series. Nickel-boron-silicon series is a self-fluxing alloy formed by adding appropriate amounts of boron and silicon elements to nickel; nickel-chromium-boron-silicon series is a nickel-boron-silicon alloy by adding chromium and carbon to form nickel with a wide range of uses and varieties. Chromium borosilicate self-fluxing alloy.



(2) Cobalt-based self-fluxing alloy powder: This series of self-fluxing alloys are based on metal cobalt and are developed on the basis of Stellite alloys. It is formed by adding B and Si elements to cobalt-chromium-tungsten alloy. Cobalt-based self-fluxing alloys have excellent high temperature properties, good thermal strength, corrosion resistance and thermal fatigue resistance, and are more suitable for oxidation-resistant, corrosion-resistant and wear-resistant surfaces that work at high temperatures of 600℃~700℃ coating. Such as the surfacing welding of the sealing surface of high pressure valves.



(3) Iron-based self-fluxing alloy powder: The iron-based self-fluxing alloy powder is mainly composed of iron and consists of several main elements such as iron, chromium, boron, and silicon. This type of alloy is developed on the basis of chromium stainless steel and nickel-chromium stainless steel. It can be divided into two types: austenitic stainless steel type self-fluxing alloy (adding boron and silicon elements to austenitic stainless steel, and adjusting the content of carbon and alloying elements) and high-chromium cast iron type self-fluxing alloy.



(4) Copper-based self-fluxing alloy powder: copper-based alloy has a low friction coefficient and good resistance to seawater and atmospheric corrosion. Copper-based alloys have good scratch resistance, good plasticity and easy processing. At present, there are two main types of copper-based self-fluxing alloy powders developed and produced in my country, one is tin phosphor bronze powder, and the other is cupronickel powder with nickel added.



2. Composite alloy powder



Composite powder is composed of two or more solid phases with different properties. The components that make up the composite powder can be metal and metal, metal (alloy) and ceramic, ceramic and ceramic, metal (alloy) and plastic, metal (alloy) and graphite, etc. The range is very wide, including almost all solid-state engineering materials.



According to the structure of composite powder, it can be generally divided into different types of powders such as coated type, non-coated type and sintered type. The core particles of the coated composite powder are completely covered by the coating material; the core material of the non-coated powder is not completely coated by the coating material, which depends on the proportion of the components. , whether the coated composite powder or the non-coated composite powder, the combination between the components is generally a mechanical combination.



application

Plasma surfacing is mainly used in the following two aspects: repairing various mechanical parts damaged due to wear, corrosion, etc.; manufacturing bimetal parts.



1. Repair mechanical parts



Repair all kinds of mechanical parts damaged due to wear, corrosion, etc., such as: wire tie rollers, shafts, various molds, etc. They are often scrapped due to local wear, shapes and sizes that do not meet the requirements, so that they cannot be used and are scrapped. Using the surfacing process to repair the old parts can not only save costs but also greatly improve the service life.



2. Manufacture of bimetal parts



In the manufacture of bimetal parts, there are often different requirements for the surface of the workpiece and the substrate in production. The surfacing method is used to properly solve the contradiction between the different performance requirements of the workpiece surface and the substrate, which not only saves a lot of precious metals and alloys, but also improves parts. usage performance. For example, high-temperature molybdenum-based alloys must be used on the top of seamless steel pipe plugs to meet the requirements. Bimetal composite plugs can be manufactured by surfacing welding instead of expensive molybdenum plugs. Superalloys with high molybdenum content can be prepared and surfacing by powder plasma arc surfacing process.



l Development trend of plasma arc surfacing



The future development trend of plasma arc surfacing technology is to improve production efficiency and reduce labor costs. Through this technology, a high-performance surfacing layer can be obtained to maximize the service life of mechanical parts.



This trend determines that the future development of this technology will focus on the following aspects:



(1) The improvement and perfection of plasma arc surfacing equipment. Compared with other surfacing technologies, the current plasma arc surfacing equipment is more complicated and costly. Strengthening the research on surfacing equipment, especially the development of surfacing gun body and supporting system, in order to improve the reliability of production under continuous automatic operation conditions, further improve the quality of surfacing layer, surfacing efficiency and reduce costs is the A future development trend of technology.



(2) Development of new high-performance surfacing composite powder. The development of modern industry has put forward higher and higher requirements for the surface properties of mechanical parts. According to different working conditions, plasma arc surfacing powders with higher performance should be continuously developed.



(3) Research on surfacing process suitable for new materials. Different surfacing materials have different performance characteristics, and the corresponding process research should be strengthened to determine reasonable plasma surfacing process parameters, so as to improve the surfacing quality, obtain the expected performance, make it a mature technology, and apply it to engineering practice as soon as possible. .



(4) Development of basic theoretical research. The in-depth study of the basic theory will provide a theoretical basis for the wider application of plasma surfacing technology. The main research hotspots in recent years are: thermal behavior analysis of surfacing materials in plasma arc column; microstructure, performance characteristics and changing laws of surfacing layers formed by new surfacing materials; interrelationship between process and performance.



At present, foreign plasma surfacing equipment, materials and related products are expensive and have high technical added value. Our country should strengthen the research on this technology, continuously develop and improve the technology, accelerate the popularization and popularization of plasma surfacing technology in engineering practice, and make it play an important role in the modernization of industrial production in our country.



my country's surfacing technology has been widely used in industries such as mold repair and pressure vessels, and has achieved remarkable results, making outstanding contributions to the development of my country's manufacturing industry. However, my country is not a strong country in the development of surfacing technology, the surfacing materials are not serialized enough, the surfacing methods are not detailed and economical, and the surfacing equipment is not advanced enough. In order to make surfacing technology better contribute to the "circular economy"