What Is Plasma Arc Welding?

Plasma arc welding refers to a welding method using a plasma arc high energy density beam as a welding heat source. Plasma arc welding has the characteristics of energy concentration, high productivity, fast welding speed, small stress and deformation, stable electrical isolation, and is suitable for welding thin plates and boxes. It is particularly suitable for various refractory, oxidizable and thermally sensitive metal materials ( Such as tungsten, molybdenum, copper, nickel, titanium, etc.).

(1) Microbeam plasma arc welding can weld foils and sheets.

(2) It has a small hole effect, which can better achieve single-sided welding on both sides

Plasma arc works in two ways. One is "non-transfer arc". The arc burns between the tungsten electrode and the nozzle. It is mainly used for plasma spraying or heating non-conductive materials.

The other is the "transfer arc". The arc consists of

Widely used in industrial production, especially for welding of copper and copper alloys, titanium and titanium alloys, alloy steel, stainless steel, molybdenum and other metals used in military and advanced industrial technologies such as aerospace, such as titanium alloy missile shells, aircraft Some thin-walled containers etc.

Type of plasma arc

According to different power connection methods, there are three types of plasma arcs: non-transfer type, transfer type and joint type.

(1) Non-transfer plasma arc tungsten electrode is connected to the negative end of the power source, and the nozzle is connected to the positive end of the power source. The plasma arc body is generated between the tungsten electrode and the nozzle. Under the pressure of the plasma gas, the arc column is ejected from the nozzle to form a plasma. flame.

(2) The transfer type plasma arc tungsten electrode is connected to the negative end of the current, and the weldment is connected to the positive end of the current. The plasma arc is generated between the tungsten electrode and the weldment. Because the transfer arc can transfer more heat to the weldment, almost all metal welding and cutting use transfer plasma arcs.

(3) The non-transferred arc and the transfer arc coexist when the combined plasma arc works, so it is called combined plasma arc. The non-transferred arc plays the role of stabilizing the arc and supplementary heating. The transfer arc directly heats the weldment, melting it for welding. Mainly used for micro-beam plasma arc welding and powder surfacing.

Transfer plasma arc

In order to establish a transfer type plasma arc, the tungsten electrode should be connected to the negative electrode of the power supply, and the nozzle and the weldment should be connected to the positive electrode at the same time. The schematic diagram of the transfer type arc is shown in Figure 24. First connect the circuit between the tungsten electrode and the nozzle, ignite the arc between the tungsten electrode and the nozzle, then quickly switch on the circuit between the tungsten electrode and the weldment, so that the arc is transferred to the tungsten electrode and the weldment to burn directly. At the same time, the circuit between the tungsten electrode and the nozzle is cut off, and the transfer type plasma arc is officially established.

Under normal working conditions, the nozzle is not charged, and the plasma arc generated at the time of ignition is only used as an intermediate medium for establishing the transfer arc.

Arc welding method

Commonly used basic methods of plasma arc welding include small-hole type plasma arc welding, penetration type plasma arc welding, and micro-beam plasma arc welding.

(1) Small-hole type plasma arc welding uses a large welding current, which is usually 50 ~ 500A, transfer type arc. When welding, the compressed plasma flame flow speed is fast, the arc is slender and powerful, and a small hole is formed for the front end of the molten pool to penetrate the weldment. . As the welding gun moves forward, the small hole moves forward, and the molten metal behind it solidifies into a weld. Because there is a certain limit to the improvement of plasma arc energy density, small-hole plasma arc welding can only be performed in a limited thick plate.

(2) Penetration type plasma arc welding When the plasma gas flow is small and the arc column compression is weak, this type of plasma arc only melts the weldment during the welding process without producing a small hole effect. The welding seam forming principle and tungsten electrode Similar to argon arc welding, it is called penetration plasma arc welding, which is mainly used for single-sided welding of double-sided forming of thin plates with a thickness of less than 2 to 3 mm and multilayer welding of thick plates.

(3) Microbeam plasma arc welding Welding current below 30A is called microbeam plasma arc welding. Using small-bore compression nozzle (0.6mm 1.2mm) and combined arc, when the welding current is less than 1A, the arc can still burn stably, and it can weld filament and foil ^[1]

Like tungsten hydrogen arc welding, according to the operation mode, plasma arc welding equipment can be divided into two types: manual welding and automatic welding. Manual welding equipment consists of welding power source, welding torch, control circuit, gas circuit and water circuit. Automatic welding equipment is composed of welding power source, welding torch, welding trolley (or rotating fixture), control circuit, gas circuit and water circuit.

Welding power

Rectifier power sources or arc welding generators with descending or vertical descending characteristics can be used as plasma arc welding power sources. When pure hydrogen is used as the ion gas, the no-load voltage of the power supply is only 65-80V; when using hydrogen and hydrogen gas, the no-load voltage is 110-120V.

High-current plasma arcs use plasma arcs. Non-transferred arcs are ignited with high frequency and then transferred to transfer arcs.

For small current microbeam plasma arc welding below 30A, a hybrid arc is used, and the arc is drawn back with high frequency or contact short circuit. Since non-transfer arcs cannot be cut off during very welding, two separate power sources are generally used.

Pneumatic system

The gas supply system of the plasma arc welding machine should be able to supply adjustable ion gas, protective gas, and protective gas on the back. In order to ensure the welding quality at the arc starting and extinguishing positions, the ion gas can be supplied in two ways, one of which can be vented through the gas valve to achieve the ion airflow attenuation control.

Control System

The control system of the manual plasma arc welding machine is relatively simple, as long as it can ensure that the ion gas and the protective gas are passed first, and then the arc is started. The control system of the automatic plasma arc welding machine usually consists of a high-frequency generator and a trolley. Filling the welded joints with a backward-moving circuit and a program-controlled circuit. The program-controlled circuit should be able to meet the requirements of advance air supply, high frequency arc starting and arc turning, ion gas increasing, delayed walking, current and airflow attenuation and quenching. Control requirements such as delayed outages.

A newly developed welding moment system for plasma arc welding, using reverse polarity electrodes and using a welding current of 100 ~ 200A, can economically and effectively weld aluminum parts with good welding quality. Welding tests on various aluminum-magnesium alloys have shown that when welding 2-8mm plates, fusion and keyhole welding techniques can be used.

Plasma arc welding with keyhole technology with variable electrode polarity can be used to weld circumferential welds, such as AlMg3 pipes, flanges, and various shaped parts made of GK-AlSi7Mg cold-cast alloy, capable of 8mm wall thickness materials Bevel-free butt-welded connection. The newly developed special gas control system can be used to complete the final welding of circumferential seams without defects. Since pores are generated only on one side of the casting, the atomic hydrogen content of the molten metal of the casting must be determined. If the hydrogen content in the molten metal of the casting is less than 0.3mL / 100g, the porosity generated by the weld is very small. The total length of the weld to be repaired by this method can reach 39m, accounting for 27.2% of the total length of the weld.

Under the conditions of research and development of the most modern power supply and control technology, the use of plasma arc welding technology is a connection process with the best quality, cost-effectiveness and good repeatability. In addition, by adjusting the current, it is ensured that the sensor system that generates a keyhole during welding of the thick plate plasma arc butt joint, the conductive molten pool support is insulated from the welded plate, and the current is measured by the charged frame when the plasma arc penetrates, Its movement.

Compared with TIG welding, this new process has the following characteristics:

(1) The specific process advantages when using plasma arc welding are not only mainly reflected in the range of sheet thickness of micro plasma arc welding, but also involve the use of keyhole technology.

Applications include: surfacing, spray coating and welding. By using a low-pulse welding current with adjustable frequency, plasma arc welding can better control the amount of arc energy, and can reliably monitor the implementation of various set values through modern control systems. Transistor welding power sources, such as the AUTOTIG series, can operate accurately in accordance with the technical specifications.

(2) When welding thin plates and pipes by powder plasma arc welding, it has the advantages of fast welding speed, small heat input and small deformation.

(3) When plasma arc welding, the advantages of keyhole technology are clearly reflected in the welding of materials with a thickness of 10mm. In applied technology, powder plasma arc welding has a solid market position. This new process will also be applied to robots ^[2]

Classification of plasma arc welding:

1. Small hole type plasma arc welding

Small hole welding is also called perforation, keyhole or penetration welding. Utilizing the characteristics of high plasma arc energy density and strong plasma current force, the workpiece is completely melted through and a small hole penetrates the workpiece. The molten metal maintains equilibrium under the interaction of arc suction, liquid metal gravity and surface tension. When the welding torch is advanced, the small hole is locked behind the arc, forming a fully fused weld.

The perforation effect can be formed only with sufficient energy density. Increased plate thickness: The required energy density also increases. Because there is a certain limit to the increase of the plasma arc energy density, the small hole type plasma arc welding can only be performed within a limited plate thickness.

2. Penetration type plasma arc welding

When the ion gas flow is small and the arc compression resistance is weak, this plasma arc only melts the workpiece during the welding process without producing a pinhole effect. The principle of welding seam formation is similar to that of tungsten hydrogen arc welding. This method is also called fusion arc welding or plasma arc welding. Mainly used for thin plate plus single-sided welding, double-sided forming and thick plate multi-layer welding.

3.Microbeam plasma arc welding

Fuse plasma arc welding below 15-30A is usually called micro-beam plasma arc welding. Due to the constraint of the nozzle and the existence of the arc current, the plasma arc with small current can be very stable, and it has become an effective method for welding thin metal foil. In order to ensure the welding quality, a precise welding fixture should be used to ensure the quality of the assembly and prevent welding deformation. Particular attention should be paid to the cleanliness of the workpiece surface. To facilitate observation, an optical magnification observation system can be used.

Ultra-thin wall tube

The ultra-thin wall seam pipe is manufactured by welding process. The strip is rolled into a round pipe and then welded.

Ultra-thin walled tube, micro-beam plasma arc welding

Ultra-thin wall tubes have been widely used in many industrial sectors. They can be used to manufacture metal hoses, corrugated tubes, torsion tubes, heat exchanger tubes for heat exchangers, and resonance tubes for instruments. Used to transport various corrosive media under vibration and alternating loads. The ultra-thin wall seam pipe is manufactured by welding process. The strip is rolled into a round pipe and then welded.

This method has a simple process, high productivity, and low cost (about 50% of seamless pipes), and has received great attention from domestic and foreign manufacturers.

Microbeam plasma arc is a highly concentrated heat source. The arc is compressed, its stability is much better than a free arc (such as an argon arc), and the working arc length can be longer than a free arc. Therefore, it is convenient to observe the welding process. Micro-beam plasma arc welding is commonly used for ultra-thin-walled pipes.

Micro-beam plasma arc welding of ultra-thin walls has the following advantages:

a. The thickness of the strip to be welded is smaller than that of argon arc welding. · The thickness is usually 0.1 0.5mm. It can be welded without crimping, and the welding quality is good.

b. When the pipe is continuously and automatically welded, the change of the plasma arc length has little effect on the welding quality. This is different from the argon arc welding, which has a great influence on the welding quality.

c. When the welding current is very small (less than 3A), the stability of the microbeam plasma arc is good, but the argon arc sometimes swims, and the stability is poor.

d. Due to the concentration of heat in the microbeam plasma arc, the welding speed is higher than that of argon arc welding and the productivity is high.

e. Capable of welding a variety of metals, including stainless steel, non-ferrous metals and refractory metals.

The continuous automatic micro-beam plasma arc welding of ultra-thin-walled pipes is similar to the closed compression arc welding process. Install an insulation sleeve between the welding mold sleeve and the welding torch to reliably insulate the plasma welding torch from metal parts. At the same time, the protective argon gas is enclosed in a small chamber. In addition to the welding current, welding speed, and shielding gas flow, there are also the working gas flow, the composition of the shielding gas, and the ratio of the shielding gas flow to the working gas flow. These parameters all affect the welding quality.

The working gas flow is large, the arc stiffness is good, the arc is easy to lead out of the nozzle, and the transfer arc is easy to establish; the working gas flow is small, the arc stiffness is poor, and the transfer arc is difficult to establish. However, the flow of working gas should not be too large. If it is too large, it will cause cutting and poor weld formation. Hydrogen-argon mixed gas is used as the protective gas to protect the gas. Generally, 5% hydrogen is used, and the rest is argon. Sometimes helium is also added, but helium is expensive and is only used when welding to certain non-ferrous metals. Experience has shown that there is an optimal ratio of the protective gas flow to the working gas flow, which must be determined through experiments.

Experience shows that the most important process parameters that affect the productivity of ultra-thin-walled pipes are the welding current, the flow rate of the working gas, and the orifice diameter of the nozzle.

The welding process of copper and its alloy ultra-thin wall pipes has many similarities with the welding process of stainless steel pipes. However, due to their different physical performance characteristics, such as high coefficient of linear expansion and thermal conductivity, high tendency to form pores in welds, alloy elements zinc (brass), beryllium (beryllium bronze) are easy to burn, etc., the following additional must be taken during welding Measures (other process measures are the same as stainless steel).

a. A closed cell must be established at the welding place, and helium gas is used as a protective gas to avoid oxidation of the molten pool and improve the protection effect.

b. Replace the copper nozzle with a molybdenum nozzle. Because the thermal conductivity of the molybdenum nozzle is quite low (2.7 times smaller than copper), it is hot pink when heated to high temperature, which hinders the evaporation and deposition of zinc and beryllium, and can reduce the burning loss of zinc and beryllium.

c. Ultrathin-walled pipes must be manufactured from soft strip.

Ultra-thin walled tubes of titanium and zirconium can also be welded by microbeam plasma arc using helium as shielding gas in closed cells ^[1]

The stability of the plasma arc directly affects the cutting quality. The instability of the plasma arc can cause defects such as uneven cuts and built-up tumors, and also reduce the life of related components of the control system, and frequent replacement of nozzles and electrodes. For this phenomenon, analyze and propose some methods.

1. Air pressure or flow is too low

When the plasma arc cutting machine is working, if the working pressure is far lower than that required in the instruction, this means that the discharge speed of the plasma arc is weakened, and the input air flow is less than the specified value. At this time, a sufficient amount of high energy, High-speed negative ions, resulting in poor incision quality, impervious cuts, and incision accumulation. The reasons for the insufficient air pressure are: insufficient input air pressure or flow rate, too low pressure adjustment of the cutting machine's air regulating valve, oil in the solenoid valve, and unsmooth air paths.

The solution is: pay attention to the output pressure display of the air compressor before use. If it does not meet the requirements, you can adjust the pressure or repair the air compressor. If the input air pressure has reached the requirements, you should check whether the adjustment of the air filter pressure reducing valve is correct, and the gauge pressure shows whether it can meet the cutting requirements. Otherwise, routine maintenance should be performed on the air filter pressure reducing valve to ensure that the input air is dry and free of oil. If the quality of the input air is poor, oil pollution will occur in the solenoid valve, the valve core will be difficult to open, and the valve port cannot be completely opened. In addition, the torch nozzle's air pressure is too low, and the solenoid valve needs to be replaced; the smaller the cross section of the gas path will also cause the air pressure to be too low, you can replace the air pipe according to the instructions.

2. High air pressure

If the input air pressure far exceeds 0.45 MPa, after the plasma arc is formed, the excessive airflow will blow away the concentrated arc column, dispersing the energy of the arc column, and weakening the cutting strength of the plasma arc. The causes of high air pressure are: improper input air conditioning, excessively high air filter regulator adjustment, or failure of the air filter regulator.

The solution is to check whether the pressure of the air compressor is adjusted properly and whether the pressure of the air compressor and the air filter pressure reducing valve is out of regulation. After turning on, if the air filter pressure reducing valve is turned, the gauge pressure does not change, indicating that the air filter pressure reducing valve has failed and needs to be replaced.

3. Burning of torch nozzle and electrode

Due to improper installation of the nozzle, such as the screw is not tightened, the gears of the equipment are not adjusted properly, the torch that needs to be cooled by water during operation, the cooling water is not supplied as required, and frequent arcing will cause premature nozzle damage.

The solution is to adjust the gears of the equipment correctly according to the technical requirements of cutting the workpiece, check whether the torch nozzle is installed firmly, and the nozzle that needs to pass the cooling water should circulate the cooling water in advance. When cutting, adjust the distance between the torch and the workpiece according to the thickness of the workpiece.

4. Poor contact between ground wire and workpiece

Grounding is an essential preparation before cutting. Without using special grounding tools, insulation on the surface of the workpiece and long-term use of the aging ground wire, etc., will make the ground wire and the workpiece poor contact. Use special grounding tools and check for

Anti-shock

Plasma arc welding and cutting power supplies have a high no-load voltage, and there is a danger of electric shock, especially in manual operation. Therefore, the power supply must be reliably grounded when in use, and the welding gun body or torch gun body must be reliably insulated from the hand touching parts. A lower voltage can be used to ignite a non-transferred arc and then switch on a higher voltage transfer arc circuit. If the starter switch is installed on the handlebar, an insulated rubber sleeve must be placed on the exposed switch to avoid direct contact with the hand. Use automated methods whenever possible.

2. Protection against arc light radiation

The intensity of arc light radiation is large, it is mainly composed of ultraviolet radiation, visible light radiation and infrared radiation. Plasma arc has a stronger light radiation intensity than other arcs, especially ultraviolet light intensity, so it causes serious damage to the skin. Operators must wear good masks and gloves when welding or cutting, and it is best to add ultraviolet absorbing lenses. During automatic operation, protective screens can be set on the operator and the operating area. When plasma arc cutting, water cutting can be used to absorb water radiation.

3.Dustproof

Plasma arc welding and cutting processes are accompanied by a large amount of vaporized metal vapor, ozone, nitride, etc. Especially when cutting, due to the large gas flow, a large amount of dust and dust on the work site is raised, and these smoke and dust have a serious impact on the respiratory tract and lungs of the operator. When cutting, an air exhaust device can also be set under the grid table, or a water cutting method can be adopted.

4.Anti-noise

Plasma arcs produce high-intensity, high-frequency noise, especially when high-power plasma arc cutting is used, which is more noisy, which is very harmful to the auditory system and nervous system of the operator. Its noise energy is concentrated in the range of 2000 ^ 8000Hz. Operators are required to wear earplugs. Where possible, use automated cutting as much as possible to enable the operator to work in a sound-proof operating room. Water cutting can also be used to use water to absorb noise ^[1] .