1 Principle of arc spraying
Arc spraying uses two continuously fed metals as consumable electrodes, generates an arc as a heat source at its ends, atomizes the melted wire with compressed air, and sprays it onto the surface of the workpiece at a high speed to form a coating. A thermal spray method. During spraying, the two filamentous metal spray materials are uniformly and continuously fed into the contact nozzles of the arc spray gun through the wire feeding device, and the contact nozzles are respectively connected to the positive and negative poles of the power source to ensure reliable reliability of the two wires before being contacted. insulation. When the ends of the two metal wires are brought into contact with each other due to feeding, a short circuit occurs to generate an arc, and the ends of the wires are instantaneously melted, and the compressed air melts the molten metal into micro-melts and is sprayed onto the surface of the workpiece at a high speed. An arc sprayed layer is formed.
2 characteristics of arc spraying process
Arc spray as a new type of thermal spray method has many significant advantages.
2.1 The coating has high bonding strength, high hardness and low porosity.
When the arc is sprayed, the droplet temperature can reach above 6000 °C, and the micro-melt heating temperature and ejection speed are higher than those in the flame spraying. Therefore, the kinetic energy and thermal energy of the particles are relatively high, thereby obtaining high bonding strength and coating strength. Tests have shown that the combined strength of arc spray is about 2.5 times that of the flame sprayed layer. The hardness of the coating formed by arc spraying is 30%-80% higher than that of the sprayed material. The porosity of the arc spray coating is less than 8%, which is lower than the porosity of the coating obtained by flame spraying.
2.2 High productivity, safe and reliable, the workpiece is not deformed
The productivity of arc spraying is proportional to the spray current, and its high efficiency is manifested by the large amount of metal sprayed per unit time. The test proves that when the spraying current is 300A, the spraying Zn efficiency can reach 30kg/h, and the spraying Al can reach 10 kg/h. The arc spraying technology uses only the power source and compressed air, and does not use flammable gas such as oxygen or acetylene. It has high safety and the workpiece is not deformed.
2.3 High energy efficiency and low spraying cost
The energy-saving effect of arc spraying is very prominent, the energy utilization rate is significantly higher than other spraying methods, and the energy cost is reduced by more than 50%. In addition to its high energy efficiency, the price of electricity is much lower than that of oxygen and acetylene. The cost is usually only 1/5 to 1/10 of the flame spray and 1/5 to 1/20 of the cost of plasma spraying. And its spraying efficiency has nearly doubled.
2.4 equipment cost is low, easy to use and maintain, easy to construct on site
The cost of arc spraying equipment is only 1/4 to 1/6 of that of plasma spraying equipment. Moreover, the arc spraying equipment is small in size, light in weight, convenient in movement, less wearing parts, convenient in maintenance, and easy on-site construction.
3 arc spraying equipment and materials
3.1 Arc spraying equipment
The arc spraying equipment is composed of an arc spray gun, a control box, a power source, a wire feeding device, and a compressed air system.
(1) Arc spray power supply
In recent years, arc spray power sources have adopted flat volt-ampere characteristics. In the past, DC welding machine was used as the arc spraying power source. Because the electric welding machine has the characteristic of steep drop, the arc working voltage is above 40V, which causes the carbon burning loss during the spraying process to reduce the hardness of the coating. The flat volt-ampere characteristic arc spraying power supply can be sprayed at a lower voltage, so that the carbon loss in the coating is greatly reduced (about 50% reduction), the arc length self-regulation can be maintained, and the arc can be effectively controlled. Voltage. The flat characteristic power supply changes rapidly when the wire feed speed changes, maintaining a stable arc spraying process. According to the spraying wire, a certain no-load voltage is selected, and changing the wire feeding speed can automatically eliminate the arc spraying current, thereby controlling the productivity of the arc spraying.
(2) Arc spray gun
Arc spray guns are a key component of arc spray equipment. The function is to intersect the continuously fed wire at a certain angle at the front of the spray gun. Since the wires are connected to the two poles of the direct current power source to generate an arc, the compressed air flow ejected from the nozzle is blown against the molten metal to form a stable Atomized metal particle stream.
3.2 Arc spraying materials
Arc spraying materials mainly include non-ferrous metal wires (metals such as aluminum, zinc, copper, nickel and their alloys) and black metal wires (carbon steel, stainless steel). At present, 2-3mm powder core wire is tried at home and abroad, and the required alloy powder is filled therein to obtain an alloy sprayed layer.
(1) Zinc and zinc alloys
Zinc is a silver-white metal with good corrosion resistance in the atmosphere or water, and also has an electrochemical protection effect on steel substrates. It is not resistant to corrosion in acids, alkalis and salts. When the water contains sulfur dioxide, its corrosion resistance is very poor. The addition of aluminum to zinc improves the corrosion resistance after spraying, so Zn-A1 alloy spray materials are currently used in large quantities. Zinc spray coatings have been widely used in outdoor open-air steel components such as water gates, bridges, towers and containers.
(2) Aluminum and aluminum alloy
When aluminum is used as an anti-corrosion coating, its effect is similar to that of zinc. Compared with zinc, it has a light specific gravity and a low price, and has a good corrosion resistance in an atmosphere containing sulfur dioxide. The addition of rare earth elements to aluminum and aluminum alloys not only improves the bonding strength of the coating but also reduces the porosity. Aluminum can also be used as a heat resistant spray coating. Under the action of high temperature, aluminum can diffuse on the iron matrix and react with iron to form Fe3A1 which is resistant to high temperature oxidation, thereby improving the heat resistance of the steel. Aluminum coatings have been widely used in water storage containers, steel components surrounded by sulfur gas, food storage, combustion chambers, hulls and gates.
(3) Copper and copper alloys
Pure copper is mainly used as a conductive spray coating for electrical switches and electronic components, and as a decorative spray coating for building surfaces such as portraits, crafts, and cement. The brass spray coating is widely used to repair worn and over-machined parts, to repair brass parts with cast blisters and air holes, and as a decorative spray coating. The aluminum bronze coating has high bonding strength, strong resistance to seawater corrosion, and good corrosion fatigue and wear resistance. Mainly used for spraying water pump blades, air lock valves, pistons, bearing surfaces.
(4) Nickel and nickel-chromium alloys
Nickel-chromium alloys are mainly used as coating materials in nickel alloys. These alloys have excellent high-temperature oxidation resistance and can be used at high temperatures of 800 ° C. They are widely used as heat-resistance materials. Nickel-chromium coatings are resistant to corrosion by water vapor, carbon dioxide, carbon monoxide, ammonia, acetic acid, and alkali. Therefore, nickel-chromium alloys are used in large quantities as corrosion-resistant and high-temperature spray coatings. For example, in the United States, 45CT (45% Cr, 4% Ti, and the balance of Ni) wire arc spraying boiler tubes have achieved good results.
(5) Carbon steel and low alloy steel
Carbon steel and low alloy steel are widely used arc spray materials. It has the characteristics of high strength, good wear resistance and low price. During the arc spraying process, carbon and alloy elements are easily burned, which tends to cause porous coating and oxide slag inclusion, which degrades the performance of the coating. Therefore, carbon steel with higher carbon content is used to compensate for the burning of carbon.
(6) Powder core wire
At home and abroad, powder core wire has been used as the arc spraying material. China has applied 3Crl3, 4Crl3, 7Crl3 and other wires as wear-resistant spraying materials, and has good high temperature stability.
4 arc spraying process in the mold manufacturing
4.1 Metal spraying molding is a new technology for rapid mold manufacturing
The application of arc spraying in mold manufacturing is mainly reflected in the rapid manufacturing of molds and also in the repair of molds. In 2003, Xi'an Jiaotong University passed the appraisal of “metal spraying rapid mold manufacturing technology and equipmentâ€, and through technology research, formed a rapid manufacturing integrated system with rapid prototyping technology and metal arc spraying technology as the core, and successfully realized in the enterprise. The result is transformed. The result combines rapid prototyping, metal spraying and brush plating technologies to form a rapid mold manufacturing integration technology. The process includes: spray master preparation, substrate processing, metal arc spraying, filling backing materials, brush plating, etc. Link.
The metal spraying rapid mold is mainly composed of a metal spraying layer, a brush coating layer and a backing material, and exhibits a gradient structure characteristic, wherein the brush plating layer improves the surface hardness of the mold (greater than HV500) and wear resistance (up to 3 to 10 times), and the glass fiber reinforced plastics The layer and epoxy layer improve the overall mechanical properties of the mold, and the high strength cement backing reduces mold manufacturing costs. This technology is suitable for automobile manufacturing and mold manufacturing enterprises. It can realize the rapid manufacturing of large-scale stamping dies and meet the requirements of rapid mold manufacturing for general automotive panels. The manufacturing cost is only 1/10 of that of traditional machining steel molds. The manufacturing cycle is the latter. 1/4~1/5, the economic benefits are very impressive, which is very suitable for prototype production and small batch production in the development of new models.
This technology can also be used for rapid manufacturing of injection molds. In the rapid prototyping of injection molds, the metal arc spray molding method takes the sample mold as the standard. The mold cavity size and geometric precision depend entirely on the sample mold. The cavity surface and its fine pattern are formed at the same time, so the mold making speed is fast. The manufacturing cycle is short, the die life is long, and the cost is low. It is only 1/3~1/5 of the numerical control method for processing steel molds. The mold surface has good surface finish, simple process and low equipment requirements, and is more suitable for injection molding die, die-casting die and board. Rapid manufacturing of the calendering die.
4.2 Metal arc spraying molding method specific process
The process of metal arc spray molding is roughly divided into five steps:
(1) Model preparation. The model can be made of plastic, gypsum, rubber, wood and other materials. First, a three-dimensional model is built, then layered and sliced, and the prototype is made by a rapid prototyping mechanism. The most important part of the model preparation is to clean the surface of the model and apply the release agent.
(2) Metal spraying model. After the release agent is dried, select the best spray parameters, and then start spraying the metal on the mold. When spraying, ensure that the spray gun continuously moves to prevent overheating deformation of the coating. The thickness of the coating is generally controlled at 2~3mm.
(3) Making a mold frame. If the mold is subjected to internal pressure during operation or the mold must be installed on the molding machine, the mold must have a skeleton structure and the finished skeleton should have a filler.
(4) Filling material for the casting mold. The filler material should have high thermal conductivity and low solidification shrinkage as well as high compressive strength and wear resistance. The general choice of filler material is a mixture of epoxy resin and metal powder such as aluminum powder and aluminum particles. .
(5) Demoulding and surface polishing of the mold cavity. After demolding, the release agent remaining on the surface of the metal coating is cleaned, and then the mold is polished and post-production according to different needs.
(6) Assembling the test pattern.
4.3 Limitations of metal arc spray molding
The material used for arc spraying molding should have better mechanical properties and dense coating structure as far as possible under the condition of low melting point and low shrinkage. At present, the wire materials for arc spraying provided by the market include aluminum, zinc, copper, nickel, stainless steel, aluminum bronze, babbitt, composite wire and the like. However, high-melting-point metals, such as steel, nickel, copper, etc., have relatively large coating shrinkage, thermal stress, and porosity during spraying. The coating is easy to crack, warp, and peel off. Therefore, only low-melting zinc and aluminum wires are currently available. Suitable for mold making.
references:
[1]Du Xiaohong. Development and Application of Arc Spray Technology in China [J]. Surface Technology. 2000.29.(5).
[2] Xu Binshi, Ma Shining. Application and development of arc spraying technology [J]. Railway Materials Science Management. 1997.15. (3). 34 ~ 35.
[3] Deng Shijun. Meeting the rise of thermal spraying industry [J]. Material Protection. 2000.33 (1). 78 ~ 82.
[4] Hu Chuanqi. Principle and application of thermal spraying [M]. Beijing. China Science and Technology Press. 1998.
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