Welding plays an irreplaceable role in people’s production and life, and the following is a brief introduction to the commonly used welding methods in industry.
1.Introduction to welding
Welding: Usually refers to the welding of metal. It is a forming method by heating or pressurizing, or using both at the same time, so that two separated objects generate interatomic bonding force and join into one.
Classification: According to the heating degree and process characteristics of the welding process, the welding method can be divided into three categories.
- Fusion welding: The workpiece is partially heated to a molten state at the welding point to form a molten pool (usually with filler metal), which is cooled and crystallized to form a weld, which is combined into an inseparable whole by the welder. Common fusion welding methods are gas welding, arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding and so on.
- pressure welding: In the welding process, regardless of heating or not, pressure welding method is required. Common pressure welding is resistance welding, friction welding, cold pressure welding, diffusion welding, explosive welding and so on.
- Brazing: The filler metal (filler metal) with a melting point lower than the welded metal is melted, and the joint gap is filled and diffused with the welded metal to realize the connection. The welded parts do not melt during brazing, and there is generally no plastic deformation.
Characteristics of welding production:
- Save metal materials, light structural weight.
- To create heavy and complex machine parts, simplify casting, forging and cutting processes, and obtain the best technical and economic results.
- The welded joint has good mechanical properties and sealing property.
- The bimetal structure can be manufactured to make full use of the properties of the material.
Application: Welding technology is widely used in machine manufacturing, shipbuilding industry, construction engineering, power equipment production, aviation and aerospace industry.
Shortcomings: Welding technology also has some shortcomings, such as the welding structure is not removable, which brings inconvenience to maintenance; There will be welding stress and deformation in the welding structure. The microstructure and properties of welded joints are often uneven, and welding defects will occur.
2.Various welding techniques are introduced
（1）Electrode arc welding
The hand electrode arc welding equipment is simple, light and flexible. It can be used for the welding of short joints in maintenance and assembly, especially for the welding of parts that are difficult to reach. Welding rod with the corresponding welding rod can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys. Arc welding is one of the most basic skills that welders master, if the skills are not in place, there will be a variety of defects in the welding seam.
Welding arc refers to the strong and persistent discharge phenomenon between two electrodes or between electrodes and weldparts, which is supplied by welding power supply and has a certain voltage, in the gas medium. When one end of the electrode is in contact with the weldment, it causes a short circuit and produces high temperature, so that the metal in contact quickly melts and produces metal vapor. When the electrode is quickly lifted 2-4mm, under the action of an electric field, the cathode surface begins to produce electron emission. These electrons collide with atoms in gas molecules and metal vapors in the process of high speed movement towards the sun, resulting in ionization of the medium and metal.
Free electrons and negative ions produced by ionization head towards the anode, while positive ions head towards the cathode. During their movement and when they reach the poles, they constantly collide and recombine, turning kinetic energy into heat energy and generating large amounts of light and heat. Its macroscopic manifestation is a strong and persistent discharge phenomenon, that is, an arc.
Welding arc
The welding arc consists of three parts: cathode region, anode region and arc column region.
- Cathode region: At the end of the cathode, it is the part that emits electrons. The emission of electrons requires a certain amount of energy, so the heat generated in the cathode region is not much, and the emission heat accounts for about 36% of the total arc heat.
- Anode region: At the end of the anode, is the part that receives electrons. Because the anode is bombarded by electrons and absorbs electrons, it obtains a lot of energy, so the temperature and heat emitted by the anode region are higher than that of the cathode, accounting for about 43% of the total heat of the arc.
- Arc column region: The gas space region between the anode region and the cathode region, the length of which is equivalent to the entire arc length. It is composed of electrons, positive and negative ions, and the heat generated accounts for about 21% of the total heat of the arc. Most of the heat in the arc column area is lost to the surrounding air through convection and radiation.
The temperature of each part of the arc varies depending on the electrode material. For example, when welding carbon steel welds with carbon steel welding rods, the temperature of the cathode region is about 2400k, the temperature of the anode region is about 2600k, and the temperature of the arc center is as high as 5000-8000k.
Polarity and application of welding arc: due to DC welding, welding arc positive and negative electrode heat is different, so the use of DC power supply has a positive connection and reverse connection. The so-called positive connection means that the electrode is connected to the negative electrode of the power supply, and the weldment is connected to the positive electrode of the power supply. At this time, the weldment obtains more heat, has high temperature, deep melting pool, is easy to weld through, and is suitable for welding thick parts. The so-called reverse connection means that the electrode is connected to the positive electrode of the power supply, and the weldment is connected to the negative electrode of the power supply. At this time, the weldment has less heat, low temperature, shallow melting pool, and is not easy to weld through. It is suitable for welding thin parts. If AC welding equipment is used when welding, due to instantaneous alternating changes in arc polarity, the polar heating is the same, and the polar temperature is basically the same, and there is no problem of positive connection and reverse connection.
（2）Tungsten gas arc welding
TIG Welding (Tungsten Inert Gas Welding), also known as non-molten inert gas arc welding. Whether it is manual welding or automatic welding 0.5 ~ 4.0mm thick stainless steel, TIG welding is the most commonly used welding method. The method of TIG welding with filler wire is often used for the bottom welding of pressure vessels, because the better air tightness of TIG welding can reduce the porosity of the weld during the welding of pressure vessels. The heat source of TIG welding is DC arc, the working voltage is 10 ~ 95 volts, but the current can reach 600 A. The correct connection method of the welding machine is to connect the workpiece to the positive electrode of the power supply, and the tungsten electrode in the welding torch is used as the negative electrode. The inert gas is usually argon.
To share with you a few argon arc welding precautions:
- Tungsten needle to often sharpen sharp, blunt current is not concentrated flowering is finished.
- The distance between the tungsten needle and the weld is close to sticking together, and the arc bloom is far away, and the bloom is burned black, and the tungsten needle is fast bald, and the radiation is strong to itself. Closer is better.
- The control of the switch is an art, especially thin plate welding, which can only be pointed one by one, which is not an automatic welding machine that automatically moves and automatically gives the wire, and continuously burns it through.
- To give the silk, this is a feel, advanced welding wire, is to use a shearing machine to cut 304 plates, do not buy bundles, of course, in the wholesale point, you can find good.
- Try to work in ventilated conditions, equipped with leather gloves, clothing, automatic dimming mask.
- The ceramic head of the welding gun should be shielded from the arc, specifically, the tail of the welding gun should be oriented towards his face as far as possible.
- If you can intuit the temperature of the pool, the size of the pool, the action of the switch, you’re an advanced technician.
- Try to use yellow or white marked tungsten needle, so the requirements of the craft are high.
（3）Melt-electrode gas Shielded arc welding
Mig welding can be welded by short circuit transition, jet transition and pulse jet transition, and can obtain stable welding process performance and good welding joints, suitable for flat welding, vertical welding, horizontal welding, upward welding and all-position welding, especially suitable for carbon steel, alloy steel and stainless steel and other ferrous metal welding. Especially when combined with the pulse power supply, the output current of the welding power supply changes with a certain frequency and amplitude to control the rhythmical transition of the melt drop to the melt pool. The jet transition can be obtained under the condition that the average current is less than the critical current value, and the ideal state of one (or more) droplet transition with one pulse can be stably realized – droplet transition without spatter. And has a wide range of current regulation, suitable for plate thickness δ≥1.0mm workpiece all-position welding, especially for those thermal sensitive materials, can effectively control the heat input, improve the joint performance. Because pulsed arc has a strong stirring effect in molten pool, it can change the metallurgical performance of molten pool, and is conducive to eliminating welding defects such as porosity and non-fusion. The pulse condition reduces the interlayer grinding time and the weld shape is beautiful.
When an inert gas (Ar, He) and an oxidizing gas (O 2, CO 2) mixture is used as a protective gas, or a CO2 gas or a CO2 + O 2 mixture is used as a protective gas, or a CO2 gas or a CO2 + O 2 mixture is used as a protective gas, Collectively known as melt-electrode active gas shielded arc welding (referred to internationally as MAG welding).
Argon plus oxygen (Ar+O2)
The common mixing ratio of the gas mixture formed by adding oxygen to argon is: Ar > 95% ~ 99%,O2 < 1% ~ 5%. Can be used for carbon steel, stainless steel and other high alloy steel and high strength steel welding. It can overcome the problems such as high liquid metal viscosity, high surface tension, easy porosity, poor wettability of weld metal, and unstable arc caused by biting edge and floating cathode spots when welding stainless steel with pure argon. When welding low carbon steel and low alloy steel with Ar/O: 80%/20% gas mixture, the performance of the welded joint is better than when welding with Ar/CO: 80%/20% gas mixture.
Argon plus carbon dioxide gas and oxygen (Ar+CO2+O2)
The welding of low carbon steel and low alloy steel with Ar+CO2+O2 as the protective gas is better than that with the above two mixed gases as the protective gas welding, the weld formation, joint quality, metal drop transition and arc stability are better.
The main advantages of MIG arc welding are that it can be easily welded in various positions, and it also has the advantages of fast welding speed and high deposition rate.
Melt-active gas arc welding can be applied to most major metals, including carbon steel and alloy steel. Mig welding is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium and nickel alloys. This welding method can also be used for arc spot welding.
Some welders think that the two guarantee welding is the simplest, because it is the easiest to get started, the easiest to learn, generally a novice who has not been exposed to welding, if there is a master to teach him for two or three hours, basically simple position welding can be operated. Two guarantee welding learning has several key points: the hand should be stable, the current and voltage will be adjusted, the welding speed can be controlled, the gesture can also master a lot of videos, and then master the welding sequence, basically can cope with more than half of the work requirements.
（4）Plasma arc welding
Plasma arc welding is also a non-melting electrode arc welding. It is the use of the electrode and the workpiece between the compression arc (called the transfer arc) to achieve welding. The electrodes used are usually tungsten. The plasma gas that produces the plasma arc can be argon, nitrogen, helium, or a mixture of both. It is also protected by inert gas through the nozzle. Welding can be performed with or without filler metal.
In plasma arc welding, because of its straight arc and high energy density, the arc penetration ability is strong. The keyhole effect produced by plasma arc welding can be used for most metals within a certain thickness range without bevelling, and can ensure that the penetration and weld are uniform.
Therefore, plasma arc welding has high productivity and good weld quality. However, the plasma arc welding equipment (including nozzle) is more complex, and the control requirements of welding process parameters are higher.
Most of the metals that can be welded by tungsten gas arc welding can be welded by plasma arc welding.
In contrast, the welding of extremely thin metals below 1 mm can be easier with plasma arc welding.
（5）Cored (tubular) wire arc welding
Tubular wire arc welding also uses the burning arc between the continuous wire and the workpiece as a heat source for welding, which can be considered as a type of MIG welding. The welding wire used is a tubular welding wire with a flux of various components in the tube.
Welding, plus protective gas, mainly CO 2. The flux is decomposed or melted by heat, and plays the role of slag-forming, protecting the solution pool, alloying and stabilizing arc.
In addition to the above advantages of gas shielded arc welding, tubular wire arc welding has more advantages in metallurgy due to the action of tube flux. Tubular wire arc welding can be applied to the welding of various joints of most ferrous metals. Tubular wire arc welding has been widely used in some industrial advanced countries.
“Tubular wire” is now called “flux-cored wire”.
（6）Resistance welding
This is a kind of welding method with resistance heat as energy, including electroslag welding with slag resistance heat as energy and resistance welding with solid resistance heat as energy. Because electroslag welding has more unique characteristics, it is introduced later. Here mainly introduces several solid resistance heat as the energy of resistance welding, mainly spot welding, seam welding, projection welding and butt welding.
Resistance welding is generally a welding method that makes the workpiece under a certain electrode pressure and uses the resistance heat generated when the current passes through the workpiece to melt the contact surface between the two workpieces and realize the connection. A larger current is usually used.
In order to prevent arcing on the contact surface and to forge the weld metal, pressure is always applied during welding. When performing this type of resistance welding, the surface quality of the workpiece to be welded is of Paramount importance for obtaining stable welding quality. Therefore, the electrode and the workpiece and the contact surface between the workpiece and the workpiece must be cleaned before welding.
The contradiction between spot welding, seam welding and projection welding is that the welding current (single phase) is large (thousands to tens of thousands of amps), the power time is short (a few weeks to a few seconds), the equipment is expensive, complex, and the productivity is high, so it is suitable for mass production. Mainly used for welding thin plate components with thickness less than 3mm. All kinds of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel can be welded.
（7）Electron beam welding
Electron beam welding is a method of welding the thermal energy generated when the concentrated high-speed electron beam bombards the workpiece surface.
During electron beam welding, the electron beam is generated by the electron gun and accelerated. Common electron beam welding: high vacuum electron beam welding, low vacuum electron beam welding and non-vacuum electron beam welding. The first two methods are carried out in a vacuum chamber. The welding preparation time (mainly the vacuum time) is long, and the workpiece size is limited by the size of the vacuum chamber.
Compared with arc welding, electron beam welding is characterized by large weld penetration, small weld width and high weld metal purity. It can be used for precision welding of very thin materials, as well as for welding of very thick (up to 300mm) components.
All metals and alloys that can be fused by other welding methods can be welded by electron beam welding. It is mainly used for welding of products requiring high quality. It can also solve the welding of dissimilar metals, easily oxidized metals and refractory metals. But not suitable for large quantities of products.
（8）Laser welding
Laser welding is an efficient and precise welding method using high energy density laser beam as heat source. Laser welding is one of the important applications of laser material processing technology. In the 1970s, it was mainly used for welding thin-wall materials and low-speed welding, the welding process is heat conduction type, that is, laser radiation heating the surface of the workpiece, the surface heat is diffused internally through thermal conduction, and the workpiece is melted by controlling the width, energy, peak power and repetition frequency of the laser pulse and other parameters to form a specific melt pool. Laser welding is a welding process that uses a laser beam focused by a high-power coherent monochromatic photon stream as a heat source. This welding method usually has continuous power laser welding and pulsed power laser welding.
The advantage of laser welding is that it does not need to be carried out in a vacuum, and the disadvantage is that the penetration is not as strong as electron beam welding.
Laser welding can achieve precise energy control, so it can realize the welding of precision micro devices. It can be applied to many metals, especially to solve the welding of some difficult metals and dissimilar metals.
（9）Brazeing
Brazing is a method of using the metal material with lower melting point than the base metal as a filler metal, heating the solder and the filler metal to a temperature higher than the melting point of the filler metal and lower than the melting temperature of the base metal, wetting the base metal with liquid filler metal, filling the joint gap and diffusing with the base metal to realize the connection of the solder. Brazing deformation is small, the joint is smooth and beautiful, suitable for welding precision, complex and composed of different materials, such as honeycomb structure board, turbine blades, carbide cutting tools and printed circuit boards. Depending on the welding temperature, brazing can be divided into two categories. Welding heating temperature below 450 ° C is called brazing, higher than 450 ° C is called brazing.
The energy source of brazing can be chemical reaction heat or indirect heat energy. It is the use of metal with a melting point lower than the melting point of the material to be welded as a solder, after heating the solder to melt, * capillary action will solder and into the gap of the joint contact surface, wetting the welded metal surface, so that the liquid phase and solid phase mutual diffusion between the formation of brazing head. Therefore, brazing is a solid phase and liquid phase welding method.
The brazing heating temperature is low, the base material does not melt, and no pressure is required. However, before welding, certain measures must be taken to remove the oil, dust and oxide film on the surface of the welder. This is an important guarantee for making the workpiece wettability good and ensuring the quality of the joint.
When the liquidus humidity of the filler metal is higher than 450℃ and lower than the melting point of the base metal, it is called brazing. Below 450 ° C, it is called brazing. According to different heat sources or heating methods brazing can be divided into: flame brazing, induction brazing, furnace brazing, dip brazing, resistance brazing and so on.
Because the heating temperature is relatively low during brazing, it has less influence on the performance of the workpiece material, and the stress deformation of the weldment is also small. However, the strength of the brazed joint is generally low, and the heat resistance is poor.
Brazing can be used to weld carbon steel, stainless steel, superalloy, aluminum, copper and other metal materials, and can also connect dissimilar metals, metals and non-metals. Suitable for welding joints with small load or working at room temperature, especially suitable for precision, micro and complex multi-brazed welds.
（10）Electroslag welding
Electroslag welding is a welding method using the resistance heat of slag as the energy source. The welding process is carried out in the vertical welding position, in the assembly gap formed by the end faces of the two workpieces and the water-cooled copper sliders on both sides. During welding, the end of the workpiece is melted by the resistance heat generated by the current passing through the slag. According to the electrode shape used in welding, electroslag welding is divided into wire electroslag welding, plate electroslag welding and nozzle electroslag welding.
The advantages of electroslag welding are: the thickness of the weldable workpiece is large (from 30mm to more than 1000mm), and the productivity is high. Mainly used in the section butt joint and T-joint welding.
Electroslag welding can be used for the welding of various steel structures, and can also be used for the welding of castings. Due to the slow heating and cooling of the electroslag welding head, the heat affected zone is wide, the microstructure is coarse, and the toughness, so it is generally necessary to normalize after welding.
（11）High-frequency welding
High frequency welding uses solid resistance heat as energy source. During welding, the resistance heat generated in the workpiece by the high-frequency current is used to heat the surface surface of the workpiece welding zone to a molten or close plastic state, and then the metal bonding is achieved by applying (or not applying) the forging force. Therefore, it is a solid phase resistance welding method.
High-frequency welding can be divided into contact high-frequency welding and induction high-frequency welding according to the way that high-frequency current generates heat in the workpiece. In contact high-frequency welding, high-frequency current is passed into the workpiece through mechanical contact with the workpiece. During induction welding, the high frequency current generates induction current in the workpiece through the coupling action of the induction ring outside the workpiece.
High-frequency welding is a highly specialized welding method, and special equipment should be equipped according to the product. High productivity, welding speed up to 30m/min. It is mainly used for welding longitudinal or spiral seams when making pipes.
（12）Gas welding
Oxygen fuel gas welding, referred to as: OFW, is the use of flame to heat the metal and the welding wire at the joint of the metal workpiece, so that it is melted, to achieve the purpose of welding. The commonly used combustible gases are mainly acetylene, liquefied petroleum gas and hydrogen, and the commonly used combustion gas is oxygen. Gas welding is a welding method using gas flame as heat source. The most widely used oxyacetylene flame is acetylene gas as fuel. Because the equipment is simple and convenient to use, but the heating speed and productivity of gas welding are low, the heat affected zone is large, and it is easy to cause large deformation.
Gas welding can be used for the welding of many ferrous metals, non-ferrous metals and alloys. Generally suitable for maintenance and single sheet welding.
（13）Gas pressure welding
Like gas welding, gas pressure welding also uses a gas flame as a heat source. When welding, the ends of the two jointed workpieces are heated to a certain temperature, and then enough pressure is applied to obtain a strong joint. It’s a solid phase welding. Pneumatic welding without filler metal, often used in rail welding and steel welding.
（14）Explosive welding
Explosive welding is also another solid phase welding method using chemical reaction heat as energy source. But it uses the energy generated by explosive explosions to achieve metal connections. Under the action of the explosion wave, two pieces of metal can be accelerated in less than a second to form a metal bond.
Among various welding methods, explosive welding can weld the widest range of dissimilar metal combinations. Two metallurgically incompatible metals can be welded into various transition joints by explosive welding. Explosive welding is mainly used for flat plate coating with large surface area and is an efficient method for manufacturing composite plates.
（15）Friction welding
Friction welding is a solid phase welding with mechanical energy. It uses the heat generated by mechanical friction between two surfaces to realize the connection of metals. The heat of friction welding is concentrated at the joint surface, so the heat affected zone is narrow. The pressure between the two surfaces must be applied, and in most cases the pressure is increased at the end of the heating, so that the hot metal is combined by the top forging, and the joint surface is generally not melted.
Friction welding productivity is high, in principle, almost all metals that can be hot forged can be friction welding.
Friction welding can also be used for welding dissimilar metals. It is suitable for workpieces with a maximum diameter of 1.5m and a circular cross-section.
Under the action of pressure, it is under the action of constant or increasing pressure and torque that the relative motion between the welding contact end faces is used to generate friction heat and plastic deformation heat in the friction surface and its nearby area, so that the temperature in the nearby area rises to a temperature range close to but generally lower than the melting point, the deformation resistance of the material is reduced, the plasticity is increased, and the oxide film at the interface is broken. Under the action of the overforging pressure, the solid state welding method is realized through the molecular diffusion and recrystallization at the interface with the plastic deformation and flow of the material.
Friction welding usually consists of the following four steps: (1) conversion of mechanical energy into thermal energy; (2) plastic deformation of the material; (3) forging pressure under thermoplasticity; (4) Intermolecular diffusion recrystallization.
（16）Ultrasonic welding
Ultrasonic welding is also a solid phase welding method using mechanical energy. When ultrasonic welding is carried out, the welding workpiece under low static pressure, the high-frequency vibration emitted by the acoustic pole can make the joint surface produce strong cracking friction and heat to the welding temperature to form a joint.
Ultrasonic welding can be used for welding between most metal materials, and can realize the welding between metal, dissimilar metal and metal and non-metal. Suitable for repeated production of wire, foil or sheet metal joints below 3mm.
Ultrasonic welding is the use of high-frequency vibration waves to transfer to the surface of two objects to be welded, under pressure, so that the surface of the two objects friction with each other to form a fusion between molecular layers. The main components of an ultrasonic welding system include the ultrasonic generator/transducer/amplitude converter/welding head triad/mold and frame.
（17）Diffusion welding
Diffusion welding is generally a solid phase welding method using indirect heat energy as energy source. This is usually done in a vacuum or protected atmosphere. When welding, the surface of the two welded parts is contacted at high temperature and greater pressure and kept warm for a certain time to achieve the distance between the atoms, and the atoms are combined through simple mutual diffusion. Before welding, it is not only necessary to clean the impurities such as oxides on the surface of the workpiece, but also the surface roughness should be lower than a certain value to ensure the welding quality.
Diffusion welding has almost no harmful effect on the properties of the materials to be welded. It can weld many homologous and dissimilar metals as well as some non-metallic materials, such as ceramics. Diffusion welding can be used to weld complex structures and workpieces with large thickness differences.
（18）Submerged arc welding
Submerged arc welding is a welding method using arc as a heat source. Due to the deep penetration of submerged arc welding, the welding quality is good: because of the protection of molten slag, the melted metal is not in contact with the air, and the degree of mechanization operation is high, so it is suitable for welding long welds of medium and thick plate structures.
