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The most complete welding technology knowledge summary

Views: 0     Author: hu     Publish Time: 2022-05-26      Origin: dapeng

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1. Introduction of welding technology

Arc: A strong and lasting gas discharge phenomenon, there is a certain voltage between the positive and negative electrodes, and the gas medium between the two electrodes should be in an ionized state. When igniting the welding arc, the two electrodes (one pole for the workpiece and the other pole for the filler wire or electrode) are usually connected to the power supply, briefly contacted and quickly separated, and a short circuit occurs when the two poles contact each other to form an arc. This method is called contact arc ignition. After the arc is formed, the burning of the arc can be maintained as long as the power source maintains a certain potential difference between the two poles.


Arc characteristics: low voltage, high current, high temperature, high energy density, good mobility, etc. Generally, the voltage of 20-30V can maintain the stable combustion of the arc, and the current in the arc can range from tens of amperes to thousands of amperes. To meet the welding requirements of different workpieces, the temperature of the arc can reach more than 5000K, which can melt various metals.


Arc composition: cathode area, anode area, arc column area three parts


Arc welding power source: The power source used for welding arc is called arc welding power source, which can usually be divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source and inverter arc welding power source.


DC positive connection: When a DC welding machine is used, when the workpiece is connected to the anode and the electrode is connected to the cathode, it is called DC positive connection. At this time, the workpiece is heated more, which is suitable for welding thick and large workpieces;


DC reverse connection: when the workpiece is connected to the cathode and the electrode is connected to the anode, it is called DC reverse connection. At this time, the workpiece is less heated and suitable for welding thin and small workpieces. When welding with an AC welding machine, because the polarities of the two poles change alternately, there is no problem of positive connection or reverse connection.


Hand arc welding technology

Manual arc welding is the earliest and most widely used welding method among various arc welding methods. It uses an electrode coated with paint on the outside as electrode and filler metal, and the arc burns between the end of the electrode and the surface of the workpiece to be welded. On the one hand, the coating can generate gas to protect the arc under the action of the arc heat, and on the other hand, it can generate slag to cover the surface of the molten pool to prevent the interaction between the molten metal and the surrounding gas.

The more important role of slag is to produce physical and chemical reaction with molten metal or add alloying elements to improve weld metal performance. The hand arc welding equipment is simple, lightweight and flexible in operation. It can be applied to the welding of short seams in maintenance and assembly, especially for the welding of hard-to-reach places. Hand arc welding with corresponding electrodes can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys.


Submerged arc welding technology

Submerged arc welding is a molten electrode welding method in which granular flux is used as the protective medium and the arc is hidden under the flux layer. The welding process of submerged arc welding consists of three steps: 1. Evenly deposit enough granular flux at the weld to be welded; 2. The contact tip and the weldment are respectively connected to two stages of welding power to generate a welding arc; 3. Automatically Feed the wire and move the arc to weld.

The main characteristics of submerged arc welding are as follows:

1. Unique arc performance

(1) The welding seam quality is high, the slag has good air protection effect, the main component of the arc area is CO2, the nitrogen content and oxygen content in the weld metal are greatly reduced, the welding parameters are automatically adjusted, the arc walking is mechanized, and the molten pool exists for a long time. , the metallurgical reaction is sufficient, and the wind resistance is strong, so the weld composition is stable and the mechanical properties are good;

(2) Good working conditions Slag isolation arc is beneficial to welding operation; mechanized walking, low labor intensity.

2. The electric field strength of the arc column is higher than that of MIG/MAG welding, which has the following characteristics:

(1) The equipment has good adjustment performance. Due to the high electric field intensity, the sensitivity of the automatic adjustment system is high, which improves the stability of the welding process;

(2) The lower limit of welding current is higher.

3. High production efficiency As the conductive length of the welding wire is shortened, the current and current density are significantly increased, which greatly improves the penetration ability of the arc and the deposition rate of the welding wire; The welding speed is greatly improved.

Scope of application:

Due to the large penetration depth, high productivity and high degree of mechanical operation, submerged arc welding is suitable for welding long welds of medium and heavy plate structures. It has a wide range of applications in shipbuilding, boilers and pressure vessels, bridges, overweight machinery, nuclear power plant structures, marine structures, weapons and other manufacturing sectors, and is one of the most commonly used welding methods in welding production today. In addition to being used for the connection of components in metal structures, submerged arc welding can also overlay a wear-resistant or corrosion-resistant alloy layer on the surface of the base metal. With the development of welding metallurgy technology and welding material production technology, the materials that can be welded by submerged arc welding have developed from carbon structural steel to low-alloy structural steel, stainless steel, heat-resistant steel, etc., as well as some non-ferrous metals, such as nickel-based alloys, Titanium alloy, copper alloy, etc.

Due to its own characteristics, its application also has certain limitations, mainly as follows:

(1) Due to the limitation of welding position, due to the reason of flux retention, if special measures are not taken, submerged arc welding is mainly used for welding seam welding in horizontal position, but not for horizontal, vertical and overhead welding;

(2) Due to the limitation of welding materials, strong oxidizing metals such as aluminum and titanium and their alloys cannot be welded, and are mainly used for welding ferrous metals;

(3) It is only suitable for welding long welds, and cannot weld welds with limited space;

(4) The arc cannot be directly observed;

(5) Not suitable for thin plate and small current welding.


Gas tungsten arc welding technology

This is a non-melting electrode gas shielded arc welding, which uses the arc between the tungsten electrode and the workpiece to melt the metal to form a weld. The tungsten electrode does not melt during the welding process and only acts as an electrode. At the same time, argon or helium is fed into the nozzle of the torch for protection. Additional metals may be added as required. Internationally known as TIG welding. Gas tungsten arc welding is an excellent method for joining sheet metal and for backing due to its well-controlled heat input. This method can be used for almost all metal connections, especially for metals such as aluminum, magnesium, which can form refractory oxides, and active metals such as titanium and zirconium. This welding method produces high-quality welds, but the welding speed is slow compared to other arc welding.


MIG Arc Welding Technology

Gas metal arc welding is a fusion welding method that uses an arc as a heat source. The arc is established between the continuously fed welding wire and the molten pool. The molten welding wire metal and the base metal are mixed. The molten pool crystallizes after the arc heat source is removed. Welds are formed and metallurgically joined separate base metals.

Features of CO2 welding:

(1) Under the high temperature of the welding arc, CO2 will be decomposed into CO, O2 and O, which has a strong compression effect on the arc, resulting in the arc shape of the welding method having a small arc column diameter and a small arc heel area and is often difficult. The characteristics of covering all the droplets at the end of the welding wire, so the transition resistance (spot force) of the droplets is large, which makes the droplets coarsen, the axiality of the transition path becomes poor, and the spatter rate is large;

(2) The welding area is well protected, the density of CO2 is the largest among the commonly used protective gases, and the volume of CO2 gas increases after thermal decomposition, so the protection is better;

(3) The energy is relatively concentrated and the penetration ability is large;

(4) Low production cost and energy saving.

(5) In terms of technology and technology, it also has good visibility of the welding zone, which is easy to observe and operate; the welding heat-affected zone and welding deformation are small; the volume of the molten pool is small, the crystallization speed is fast, and the welding performance at all positions is good; the sensitivity to rust stains low advantage.

Metallurgical characteristics: (1) During the oxidation of alloying elements in CO2 welding, under the high temperature of the arc, CO2 will be decomposed into CO, O2 and O. Under welding conditions, CO is insoluble in metal and does not participate in the reaction, while CO2 and O All have strong oxidizing properties, which oxidize Fe and other alloying elements. (2) Deoxidation and alloying of weld metal? Usually, a certain amount of deoxidizer is added to the welding wire for deoxidation. In addition, the remaining deoxidizer is left in the weld as an alloying element to make up for the loss of oxidation and burning and ensure welding. chemical composition requirements of the seam.

Droplet transfer: (1), short-circuit transfer (short arc, thin wire, small current) is suitable for all-position welding of thin plates; (2), fine particle transfer, thick wire, long arc, high current welding; (3), latent welding Arc drop transition (rarely used).

Scope of application: At present, CO2 gas shielded welding is widely used in locomotive manufacturing, shipbuilding, automobile manufacturing, coal mining machinery manufacturing and other fields. It is suitable for welding low carbon steel, low alloy steel and low alloy high strength steel, but not suitable for welding non-ferrous metals and stainless steel. Although there is information that CO2 gas shielded welding can be used for stainless steel welding, it is not the first choice for welding stainless steel.


Plasma arc welding technology

Measures such as water-cooling nozzles can reduce the cross-sectional area of the arc column area of the arc, and the temperature, energy density, and plasma flow rate of the arc can be significantly increased.

Plasma arc is a special form of electric arc, which is an arc with high energy density and is still a gas conduction phenomenon. Plasma arc welding is a method of using the heat of the plasma arc to heat and melt the workpiece and base metal to achieve welding.

Classification: perforated plasma arc welding and microbeam plasma arc welding.

Perforated plasma arc: the welding current is 100-300A, the joint does not need to be grooved, and no gap should be left. During welding, the plasma arc can completely penetrate the weldment and form a small through hole, the molten metal is squeezed around the small hole, the arc moves, the small hole moves along with it, and a weld is formed at the rear, thereby realizing single-sided welding. Double-sided forming at one time. The upper limit of plate thickness that can be welded by this method is: carbon steel 7mm, stainless steel 10mm.

Microbeam plasma arc: the welding current is 0.1~30A, and the welding thickness is 0.025~2.5mm. In addition, there are fusion plasma arc welding suitable for copper and copper alloy welding, which can be used for deep penetration welding of thick plates or high-speed welding of thin plates and melting electrode plasma arc welding of surfacing welding, which can solve the AC ( Change polarity) plasma arc welding and other process methods. The main process parameters of plasma arc welding are welding current, welding speed, shielding gas flow, ion gas flow, torch nozzle structure and aperture, etc.

Plasma arc cutting: a cutting method in which the high-temperature and high-speed arc flow of the plasma arc is used to partially melt the metal of the incision so that it evaporates, and the molten material is blown away from the substrate with the help of high-speed airflow or water flow to form an incision.

Features:

(1) The plasma arc has high energy density, high arc column temperature, and strong penetrating ability. Steels with a thickness of 10 to 12 mm can be grooved without grooves, and can be welded through both sides at one time. The welding speed is fast, the productivity is high, and the stress deformation is small.

(2) The cross-section of the weld is in the shape of a wine cup, and there is no finger-shaped penetration problem.

(3) The arc straightness is good, and the fluctuation of the molten pool is small due to the fluctuation of the arc length.

(4) The arc is stable at 0.1A, and it still has relatively flat static characteristics. Equipped with a constant current source, it can be used for thin plate welding (0.1mm).

(5) The tungsten electrode is retracted to prevent tungsten inclusion in the weld

(6) Adopt small hole welding technology to realize single-sided welding and double-sided forming.

(7) The equipment is more complicated and the gas consumption is large, so it is only suitable for indoor welding. The accessibility of the welding torch is worse than that of TIG.

(8) The diameter of the arc is small, and the axis of the welding torch needs to be more accurately aligned with the centerline of the weld.

Metallurgical reaction: single, only evaporation

Power supply: steep drop power supply, DC positive connection; AC, steep drop power supply is used when welding aluminum and magnesium, and arc ignition and arc stabilization measures are required. Welding materials: shielding gas, tungsten electrode

Scope of application: Widely used in industrial production, especially the welding of copper and copper alloys, titanium and titanium alloys, alloy steel, stainless steel, molybdenum and other metals used in aerospace and other military industries and cutting-edge industrial technologies, such as titanium alloy missile casings, Some thin-walled containers on airplanes, etc.


Tubular Wire Arc Welding Technology

Tubular wire arc welding also uses the arc burning between the continuously fed welding wire and the workpiece as the heat source for welding, which can be considered as a type of MIG welding. The welding wire used is a tubular wire with various compositions of flux inside the tube. When welding, a protective gas is added, mainly CO2. The flux is decomposed or melted by heat, which plays the role of slag formation to protect the molten pool, alloy infiltration and arc stabilization. In addition to the above-mentioned advantages of gas shielded arc welding, tubular wire arc welding has more advantages in metallurgy due to the effect of flux in the tube. Tubular wire arc welding can be applied to most ferrous metals and various joints. Tubular wire arc welding has been widely used in some industrially advanced countries. "Tubular wire" is now called "flux cored wire"


Gas welding technology

A fusion welding method that uses the heat generated by the combustion of a combustible gas in oxygen to melt the weld of the base metal to realize the connection. Gas welding is a welding method that uses a gas flame as a heat source. The most widely used oxy-acetylene flame is acetylene gas as fuel. Due to the simple and convenient operation of the equipment, 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 welding of many ferrous metals, non-ferrous metals and alloys.

Combustible gas: acetylene, liquefied petroleum gas, etc. Taking acetylene as an example, its flame temperature can reach 3200℃ when it burns in oxygen. There are three types of oxyacetylene flames:

①Neutral flame: The volume mixing ratio of oxygen and acetylene is 1 to 1.2, and the acetylene is fully burned, which is suitable for welding carbon steel and non-ferrous alloys.

②Carbon flame: The volume mixing ratio of oxygen and acetylene is less than 1, and the acetylene is excessive. It is suitable for welding high carbon steel, cast iron and high speed steel.

③Oxidizing flame: The volume mixing ratio of oxygen and acetylene is greater than 1.2, and the oxygen is excessive, which is suitable for brazing of brass and bronze.

The gas welding flame temperature is low, the heating speed is slow, the heating area is wide, the welding heat-affected zone is wide, the welding deformation is large, and the molten metal is poorly protected during the welding process, and the welding quality is not easy to ensure, so its application has been seldom. However, gas welding has the characteristics of no power supply, simple equipment, low cost, convenient movement, and strong versatility, so it has practical value in places without power supply and field work. At present, it is mainly used for welding of thin steel plate (thickness 0.5-3mm), copper and copper alloy and repair welding of cast iron.


Air pressure welding technology

Gas pressure welding, like gas welding, also uses gas flame as the heat source. During welding, the ends of the two butted workpieces are heated to a certain temperature, and then sufficient pressure is applied to obtain a firm joint. It is a solid phase welding. No filler metal is added during gas pressure welding, and it is often used for rail welding and steel bar welding.


Electroslag Welding Technology

Electroslag welding is a welding method that uses 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 electric current passing through the slag. According to the shape of the electrodes used in welding, electroslag welding is divided into wire electroslag welding, plate electroslag welding and melting tip electroslag welding.

Characteristics of electroslag welding: In the welding process of electroslag welding, except for an arc process in the initial stage, the rest are stable electroslag processes, which are essentially different from submerged arc 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 for welding of butt joints and T-joints at the cross section. Electroslag welding can be used for the welding of various steel structures, as well as for the assembly welding of castings. Due to the slow heating and cooling of the electroslag welded joint, the heat affected zone is wide, the microstructure is coarse and tough, so normalizing treatment is generally required after welding.

Limitations of electroslag welding:

(1) Due to the large welding pool and slow heating and cooling, it is easy to overheat in the weld and heat-affected zone to form a coarse structure, so electroslag welding is usually normalized after welding to eliminate the coarse grains in the joint.

(2) Electroslag welding is always carried out by vertical welding, not flat welding. Electroslag welding is not suitable for workpieces with a thickness of less than 30mm, and the welding seam should not be too long.

Classification and application of electroslag welding

Classification of electroslag welding: wire electroslag welding, plate electroslag welding, nozzle electroslag welding and tube electroslag welding, etc.

Wire electroslag welding is the most commonly used electroslag welding method. It uses a welding wire as an electrode. Depending on the thickness of the weldment, one or more welding wires can be used. The thickness of the weldment that can be welded by single wire welding is 40-60mm. When the thickness of the weldment is greater than 60mm, the welding wire should swing laterally; the three-wire swing can weld weldments with a thickness of 450mm. Wire electroslag welding is mainly used to weld weldments with a thickness of 40-450mm and weldments with long welds, and can also be used for girth welds of large weldments.

Application: Mainly used in the heavy machinery manufacturing industry to manufacture forged-welded structural parts and cast-welded structural parts, such as the base of heavy machine tools, high-pressure boilers, etc. Alloy steel, stainless steel, etc.


Electron beam welding technology

Electron beam welding is a method of welding by the heat energy generated when a concentrated high-speed electron beam bombards the surface of the workpiece. During electron beam welding, an electron beam is generated and accelerated by an electron gun. Commonly used electron beam welding are: high vacuum electron beam welding, low vacuum electron beam welding and non-vacuum electron beam welding.

The first two methods are performed in a vacuum chamber. The welding preparation time (mainly vacuuming time) is long, and the size of the workpiece is limited by the size of the vacuum chamber. Compared with arc welding, electron beam welding is mainly 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 welding of very thick (up to 300mm thick) components.

All metals and alloys that can be fusion welded by other welding methods can be welded by electron beam. 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 high-volume products.

Electron beam welding machine: The core is the electron gun, which is the device that completes the generation of electrons, the formation and convergence of electron beams, and is mainly composed of filament, cathode, anode, focusing coil, etc. The filament is energized to heat up and heat the cathode. When the cathode reaches about 2400K, electrons are emitted. Under the action of the high-voltage electric field between the cathode and the anode, the electrons are accelerated (about 1/2 the speed of light), and are emitted through the anode hole, and then focused. The coil converges into an electron beam with a diameter of 0.8-3.2mm and shoots it towards the weldment, and converts the kinetic energy into heat energy on the surface of the weldment, so that the joint of the weldment is rapidly melted, and the weld is formed after cooling and crystallization.

According to the different vacuum degrees of the welding studio (where the weldment is placed), the classification of electron beam welding:

(1) High vacuum electron beam welding. The studio and the electron gun are in the same room, and the vacuum degree is 10-2~10-1Pa, which is suitable for precision welding of refractory, active, high-purity metals and small parts.

(2) Low vacuum electron beam welding. The working room and the electron gun are divided into two vacuum chambers. The vacuum degree of the working room is 10-1~15Pa, which is suitable for larger structural parts and refractory metals that are not sensitive to oxygen and nitrogen.

(3) Non-vacuum electron beam welding. An additional inert gas shield or nozzle is required, and the distance between the weldment and the electron beam outlet should be controlled at about 10mm to reduce the scattering caused by the collision between the electron beam and gas molecules. Non-vacuum electron beam welding is suitable for the welding of carbon steel, low alloy steel, stainless steel, refractory metal, copper, aluminum alloy, etc., and the size of the weldment is not limited.

Advantages of vacuum electron beam welding:

(1) The energy density of the electron beam is high, up to 5×108W/cm2, which is about 5000 to 10000 times that of the ordinary arc. The heat is concentrated, the thermal efficiency is high, the heat affected zone is small, the welding seam is narrow and deep, and the welding deformation is extremely small.

(2) When welding in a vacuum environment, the metal does not interact with the gas phase, and the joint strength is high.

(3) The electron beam focus radius can be adjusted in a large range, with flexible control and strong adaptability. It can weld thin parts of 0.05mm and thick plates of 200-700mm.

Application: It is especially suitable for welding some refractory metals, active or high-purity metals and metals with strong heat sensitivity. However, the equipment is complex, the cost is high, the size of the weldment is limited by the vacuum chamber, the assembly precision is high, and it is easy to excite X-rays, the welding assistance time is long, and the productivity is low. These weaknesses limit the wide application of electron beam welding.


Laser Welding Technology

Laser welding is a welding that uses a laser beam focused by high-power coherent monochromatic photon flow as a heat source. This welding method usually includes 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, but the disadvantage is that the penetration is not as strong as electron beam welding.

Laser welding can carry out precise energy control, so it can realize the welding of precision micro-devices. It can be applied to many metals, especially the welding of some difficult-to-weld metals and dissimilar metals.

Generation of laser: After the material is excited, it produces a beam with the exact same wavelength, frequency and direction.

Features of laser: It has the characteristics of good monochromaticity, good directionality and high energy density. After the laser is focused by transmission or mirror, it can obtain an energy beam with a diameter of less than 0.01mm and a power density of up to 1013W/cm2, which can be used as welding, Heat source for cutting, drilling and surface preparation. The substances that generate lasers include solids, semiconductors, liquids, gases, etc. Among them, yttrium aluminum garnet (YAG) solid lasers and CO2 gas lasers are mainly used for industrial processing such as welding and cutting.

The main advantages of laser welding are:

(1) The laser can be bent and transmitted by optical methods such as optical fibers and prisms. It is suitable for the welding of micro parts and other parts that are difficult to reach by welding methods, and can also be welded through transparent materials.

(2) High energy density, high-speed welding can be achieved, heat-affected zone and welding deformation are very small, especially suitable for welding of heat-sensitive materials.

(3) The laser is not affected by electromagnetic fields, does not generate X-rays, and does not require vacuum protection, and can be used for welding of large structures.

(4) Insulated conductors can be welded directly without stripping off the insulating layer in advance; dissimilar materials with greatly different physical properties can also be welded.

The main disadvantages of laser welding technology are: expensive equipment, low energy conversion rate (5% to 20%), and high requirements for welding interface processing, assembly, and positioning. Currently, it is mainly used for micro devices in the electronics industry and instrumentation industry. welding, as well as welding of silicon steel sheets, galvanized steel sheets, etc.


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