Introduction to common welding processes

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1. Introduction to welding process

welding process : usually refers to the welding of metals. It is a forming method that uses heating or pressure, or both at the same time, to create an inter-atomic bonding force between two separate objects and connect them into one body.

Classification: According to the difference in heating degree and process characteristics during the welding process, welding methods can be divided into three major categories.

Fusion welding: non-fusion welding is a common metal joining process that heats metal above its melting point to melt it and form a connection on the contact surface. During the welding process, metal is heated to a sufficient temperature to melt it using a suitable heat source (such as arc, flame, laser, etc.). In the molten state of the metal, the metal parts to be connected are brought into contact with each other through appropriate technical means and a connection is formed. Subsequently, it cools and solidifies to form a welded joint. Fusion welding is mainly divided into arc welding, gas welding, laser welding, plasma welding, electron beam welding and other methods.

Pressure welding: Pressure welding is a metal welding process that heats the contact surfaces of metal workpieces to the welding temperature by applying a certain amount of pressure, and then maintaining pressure on them for a certain period of time to achieve welding. Pressure welding usually requires auxiliary heating sources, such as resistance heating, friction heating, arc heating, etc., to heat the joint surface to the welding temperature. During the welding process, pressure not only helps ensure the fit of the joint surfaces, but also helps promote the diffusion of metal atoms, resulting in a strong welded joint.

Brazing: brazing welding is a metal joining process that heats and melts a metal or alloy called brazing material (or flux), allowing it to flow into the joints of the connected parts, and then forms a Strong connection. Unlike welding, brazing welding operates at temperatures below the melting point of the parts being joined, so it does not melt the parts being joined.

2. Characteristics of welding process production:

(1) Diversified application fields:

Welding technology is widely used in various industrial fields, including automobile manufacturing, aerospace, shipbuilding, construction engineering, electronic equipment manufacturing, petrochemical industry, etc. Different industries have different requirements for welding and need to adapt to different application environments and process requirements.

(2) High degree of customization:

The welding needs of each product may be different and require customized design and processing based on the requirements of the specific product. This includes determining welding process parameters, selecting appropriate welding materials, determining the best welding method, etc.

(3) High technical requirements:

Welding production requires operators to have a certain technical level and experience. Especially in complex welding processes and high-demand products, operators need to have high skills and professional knowledge, and be able to master various welding methods and equipment operations.

(4) Key to quality control:

Welding quality directly affects the safety and reliability of products, so quality control is an important link in welding production. It is necessary to strictly control everything from material selection, equipment debugging to operating procedures and quality inspection to ensure that the welding quality meets standards and requirements.

(5) Strong safety awareness:

Welding production involves dangerous factors such as high temperature and high pressure, and operators need to have good safety awareness. Strictly abide by relevant safety regulations and operating specifications, and take necessary protective measures to ensure the safety of personnel and equipment.

(6) Balance between production efficiency and cost:

In welding production, there is a need to balance production efficiency and cost control. Improving production efficiency can reduce costs, but not at the expense of quality. It is necessary to find a balance between quality and efficiency and optimize production processes and management methods.

(7) Increased environmental protection requirements:

With the increasing awareness of environmental protection, welding production is also facing higher requirements on environmental impact. Waste gas, waste water, etc. generated during the welding process need to be effectively treated to reduce environmental pollution. Enterprises need to actively take environmental protection measures to reduce resource consumption and emissions and achieve sustainable development.

3. Introduction to various welding processes

(1) Welding Process Hand arc welding

Manual arc welding is the earliest developed and still the most widely used welding method among various arc welding methods. It uses an externally coated welding rod as the electrode and filler metal, and the arc burns between the end of the welding rod and the surface of the workpiece to be welded. The coating can generate gas under the action of arc heat on the one hand to protect the arc, 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 function of slag is to react physically and chemically with the molten metal or add alloying elements to improve the metal properties of the weld. Manual arc welding equipment is simple, lightweight and flexible to operate. It can be used for welding short seams in maintenance and assembly, especially in hard-to-reach areas. Hand arc welding with corresponding welding rods can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys.

(2) Welding Process Submerged arc welding

Submerged arc welding is a melting electrode welding method that uses granular flux as a protective medium and the arc is hidden under the flux layer. The welding process of submerged arc welding consists of three links: 1. Apply sufficient granular flux evenly at the seam of the welding piece to be welded; 2. The contact tip and the welding piece are respectively connected to two levels of welding power to generate a welding arc; 3. Automatic Feed the wire and move the arc to weld.

Characteristics of submerged arc welding

1. Unique arc performance

(1) The quality of the weld is high, the slag is well insulated from the air, the main component of the arc zone is CO2, the nitrogen and oxygen content in the weld metal are greatly reduced, the welding parameters are automatically adjusted, the arc travel is mechanized, and the molten pool lasts 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. The slag isolating arc light is conducive to welding operations; mechanized walking has low labor intensity.

2. The arc column electric field intensity is higher than that of gas metal arc welding. It has the following characteristics:

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

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

3. High production efficiency. Due to the shortened conductive length of the welding wire, the current and current density are significantly increased, which greatly improves the penetration capacity of the arc and the deposition rate of the welding wire; and due to the heat insulation effect of the flux and slag, the overall thermal efficiency is greatly increased. , greatly increasing the welding speed.

Metallurgical reaction: The flux participates in the metallurgical reaction, Si and Mn are reduced, C is partially burned, limiting the removal of impurities S and P to H, and preventing the generation of hydrogen pores.

Droplet transfer: slag wall transfer

Power supply: DC power supply is used for small current conditions, constant speed wire feeding, and self-arc adjustment; large currents generally use AC power supply, variable speed wire feeding (SAW welding wire is generally thicker), and arc voltage feedback arc adjustment. Welding materials: welding wire and flux. The selection of welding wire and flux must ensure a high-quality welding joint while reducing costs as much as possible. Pay attention to the applicable current type and polarity.

Scope of application: Due to its large penetration depth, high productivity and high degree of mechanical operation, submerged arc welding is suitable for welding long welds of medium and thick plate structures. It is widely used in shipbuilding, boilers and pressure vessels, bridges, overweight machinery, nuclear power plant structures, marine structures, weapons and other manufacturing sectors. It is one of the most commonly used welding methods in today’s welding production. In addition to being used to connect components in metal structures, submerged arc welding can also be used to weld wear-resistant or corrosion-resistant alloy layers on the surface of base metals. 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 certain 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) Restrictions on the welding position. Due to flux retention, if special measures are not taken, submerged arc welding is mainly used for horizontal welding seam welding, while It cannot be used for horizontal, vertical or overhead welding; (2) Limitations of welding materials. It cannot weld aluminum, titanium and other highly oxidizing metals and their alloys, and is mainly used for welding ferrous metals; (3) It is only suitable for long welding seam welding. cutting, and cannot weld welds with limited space; (4) The arc cannot be directly observed; (5) It is not suitable for thin plates and low current welding.

(3) Welding Process Gas tungsten arc welding

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 functions as an electrode. At the same time, argon or helium gas is fed into the nozzle of the welding torch for protection. Additional metal can be added as needed. It is commonly known as TIG welding internationally. Gas tungsten arc welding is an excellent method for joining thin sheet metal and backing welds due to its ability to control heat input very well. This method can be used to join almost all metals, and is especially suitable for welding metals such as aluminum and magnesium that can form refractory oxides, as well as active metals such as titanium and zirconium. This welding method produces high-quality welds, but its welding speed is slow compared to other arc welding methods.

(4)Welding Process Gas metal arc welding (GMAG)

(GMAG) 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 wire metal and the base metal are mixed into the molten pool. After the arc heat source is removed, the molten pool crystallizes to form a weld. And connect the separated base materials through metallurgical methods.

Features of CO2 welding:

(1) Under the high temperature of the welding arc, CO2 will decompose into CO, O2 and O, which has a strong compressive effect on the arc. As a result, the arc shape of this welding method has a small arc column diameter, a small arc heel area and is often difficult to It has the characteristics of covering all the droplets at the end of the welding wire, so the transition resistance (spot force) encountered by the droplets is large, which makes the droplets coarsen, the axiality of the transition path becomes worse, and the spatter rate is high;

(2) The welding area is well protected. The density of CO2 is the largest among commonly used protective gases. In addition, after CO2 gas is thermally decomposed, its volume increases, 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) The process and technology also include good visibility of the welding zone, easy to observe and operate; small welding heat-affected zone and welding deformation; small molten pool volume, fast crystallization speed, good welding performance in all positions; sensitivity to rust and contamination Low advantages.

Metallurgical properties:

(1) Oxidation of alloy elements During CO2 welding, CO2 will decompose into CO, O2 and O under the high temperature of the arc. Under welding conditions, CO is insoluble in the metal and does not participate in the reaction, while CO2 and O have strong The oxidizing property oxidizes Fe and other alloying elements.

(2) Deoxidation and alloying of weld metal? A certain amount of deoxidizer is usually added to the welding wire for deoxidation. In addition, the remaining deoxidizer remains in the weld as an alloying element to compensate for the oxidation and burning loss and ensure the welding quality. The chemical composition requirements of the seam.

Droplet transfer:

(1) Short-circuit transition (short arc, filament, small current) is suitable for all-position welding of thin plates;

(2), fine particle transition, thick wire, long arc, high current welding;

(3) Submerged arc droplet transfer (rarely used).

Power supply: flat characteristic power supply (single knob adjustment), DC reverse connection, constant speed wire feeding Welding materials: CO2 gas and welding wire

Scope of application: Currently, 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 is not suitable for welding non-ferrous metals and stainless steel. Although there is data showing that CO2 gas shielded welding can be used for welding stainless steel, it is not the first choice for welding stainless steel.

(5)Welding Process Plasma arc welding

Measures such as water-cooling nozzles can reduce the cross-sectional area of the arc column area of the arc, and significantly increase the arc temperature, energy density, and plasma flow rate. This type of arc that uses external constraints to compress the arc column is called a plasma arc.

Plasma arc is a special form of arc. It is an arc with high energy density and is still a gas conductive phenomenon. Plasma arc welding is a method that uses the heat of the plasma arc to heat & melt the workpiece and base metal to achieve welding.

Classification: perforation plasma arc welding and micro-beam plasma arc welding.

Perforated plasma arc: The welding current is between 100 and 300A, and the joints do not need to be beveled and do not leave any gaps. 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. When the arc moves, the small hole moves with it, and a weld is formed behind it, thus achieving single-sided welding. Double-sided forming at one time. The upper limit of plate thickness that can be welded by this method is: 7mm for carbon steel and 10mm for stainless steel.

Micro-beam plasma arc: welding current is 0.1~30A, welding thickness is 0.025~2.5mm. In addition, there is also fusion plasma arc welding suitable for welding copper and copper alloys, and melting electrode plasma arc welding which can be used for deep penetration welding of thick plates or high-speed welding of thin plates as well as surfacing, which can solve the problems of plasma arc welding of aluminum alloys ( Variable polarity) plasma arc welding and other process methods. The main process parameters of plasma arc welding include welding current, welding speed, shielding gas flow, ion gas flow, welding gun nozzle structure and aperture, etc.

Plasma arc cutting: a cutting method that uses the high-temperature and high-speed arc flow of the plasma arc to partially melt and evaporate the metal in the cut, and blows the molten material away from the matrix with the help of high-speed air or water flow to form the cut.

Features:

(1) Plasma arc has high energy density, high arc column temperature, and strong penetration ability. Steel with a thickness of 10 to 12 mm does not need to be grooved, and can be welded through on both sides at one time. It has fast welding speed, high productivity, and small stress deformation.

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

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

(4) The arc is stable at 0.1A and still has relatively flat static characteristics. Equipped with a constant current source, it can weld thin plates (0.1mm) very well.

(5) The tungsten electrode is retracted to prevent tungsten from being caught in the weld seam

(6) Use small hole welding technology to achieve single-sided welding and double-sided forming.

(7) The equipment is relatively complex and consumes large amounts of gas, so it is only suitable for indoor welding. The accessibility of the welding gun is worse than TIG.

The arc diameter is small and the welding gun axis and the weld center line need to be more accurately aligned.

Metallurgical reaction: single, only evaporation

Power supply: steep drop power supply, DC positive connection; when welding aluminum and magnesium, use AC, steep drop power supply, 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 and cutting-edge industrial technologies, such as titanium alloy missile casings, Some thin-walled containers on airplanes, etc.

(6)Welding Process Tubular wire arc welding

Tubular wire arc welding also uses the arc burning between the continuously fed wire and the workpiece as the heat source for welding. It can be considered a type of gas metal arc welding. The welding wire used is a tubular welding wire, and the tube is filled with flux of various components. When welding, an external protective gas is added, mainly CO2. The flux decomposes or melts when heated, and plays the role of forming slag to protect the molten pool, infiltrating the alloy and stabilizing the arc. In addition to the above-mentioned advantages of gas metal arc welding, tubular wire arc welding also has more advantages in metallurgy due to the effect of flux in the tube. 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 industrially advanced countries. “Tubular welding wire” is now known as “flux cored wire”

2. Fusion welding

(1) Gas welding

Gas welding: A fusion welding method that uses the heat generated when combustible gas burns in oxygen to melt the base metal welding joint to achieve connection. Gas welding is a welding method that uses gas flame as the heat source. The most commonly used flame is the oxygen-acetylene flame using acetylene gas as fuel. Because the equipment is simple and easy to operate, gas welding heating speed and productivity are low, the heat affected zone is large, and it is easy to cause large deformation. Gas welding can be used for welding 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°C when burned in oxygen. There are three types of oxyacetylene flames:

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

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

③ Oxidation flame: The volume mixing ratio of oxygen to acetylene is greater than 1.2, and there is excess oxygen. It is suitable for brazing of brass and bronze.

Gas welding has low flame temperature, slow heating speed, wide heating area, wide welding heat affected zone, large welding deformation, and during the welding process, the molten metal is poorly protected and the welding quality is not easy to guarantee, so its application has been rare. However, gas welding has the characteristics of no need for power supply, simple equipment, low cost, easy movement, and strong versatility. Therefore, it has practical value in situations without power supply and when working in the field. At present, it is mainly used for the welding of thin steel plates (thickness 0.5~3mm), copper and copper alloys and the repair welding of cast iron.

(2) Air pressure welding

Gas pressure welding is the same as gas welding. Gas pressure welding also uses gas flame as the heat source. During welding, the ends of the two butt workpieces are heated to a certain temperature, and then sufficient pressure is applied to obtain a strong 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.

(3) Electroslag welding

Electroslag welding is a welding method that uses the resistance heat of molten slag as energy. The welding process is carried out in the vertical welding position, within the assembly gap formed by the end surfaces of the two workpieces and the water-cooled copper sliders on both sides. During welding, the resistance heat generated by electric current passing through the slag is used to melt the end of the workpiece. According to the shape of the electrode used during welding, electroslag welding is divided into wire electrode electroslag welding, plate electrode electroslag welding and molten nozzle 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 workpiece that can be welded is large (from 30mm to more than 1000mm), and the productivity is high. Mainly used for welding of butt joints and T-joints in cross-sections. Electroslag welding can be used for welding various steel structures and can also be used for assembly welding of castings. Because electroslag welding joints are heated and cooled slowly, the heat-affected zone is wide, and the microstructure is coarse and tough, so normalizing treatment is generally required after welding.

Limitations of electroslag welding:

Due to the large welding pool, slow heating and cooling, it is easy to overheat and form coarse structures in the weld and heat-affected zone. Therefore, electroslag welding is usually normalized after welding to eliminate coarse grains in the joint.
Electroslag welding is always carried out in vertical welding mode, and flat welding is not allowed. 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 electrode electroslag welding is the most commonly used electroslag welding method. It uses welding wire as the 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 to 60 mm. When the thickness of the weldment is greater than 60mm, the welding wire must swing laterally; three wire swings can weld weldments up to 450mm thick. Wire electrode electroslag welding is mainly used for welding weldments with a thickness of 40 to 450mm and weldments with long welds. It can also be used for girth welds of large weldments.

Application: Mainly used in heavy machinery manufacturing industry to manufacture forged-welded structural parts and cast-welded structural parts, such as machine bases of heavy machine tools, high-pressure boilers, etc. The thickness of weldments is generally 40 to 450mm, and the materials are carbon steel, low Alloy steel, stainless steel, etc.

(4) Electron beam welding

Electron beam welding is a method of welding using 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 include: 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, the main characteristics of electron beam welding are large weld penetration depth, small weld width, and high purity of weld metal. It can be used not only for precision welding of very thin materials, but also for 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 products requiring high quality. It can also solve the welding of dissimilar metals, easily oxidized metals and refractory metals. But it is not suitable for high-volume products.

Sub-beam welding machine: The core is the electron gun, which is a 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), ejected through the anode hole, and then focused The coil converges into an electron beam with a diameter of 0.8~3.2mm and shoots towards the weldment, and converts kinetic energy into heat energy on the surface of the weldment, causing the joint of the weldment to melt rapidly and form a weld after cooling and crystallization.

According to the different degrees of vacuum in the welding studio (where the welding parts are placed), electron beam welding is classified:

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

Low vacuum electron beam welding. The working room and 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.

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

Advantages of vacuum electron beam welding:

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

2.When welded 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 has a large adjustable range, flexible control, and strong adaptability. It can weld thin parts of 0.05mm and thick plates of 200 to 700mm.

Application: Especially suitable for welding some refractory metals, reactive 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 accuracy is high, and it is easy to excite X-rays, the welding auxiliary time is long, and the productivity is low. These weaknesses limit the wide application of electron beam welding.

(5) Laser welding

Laser welding is a welding process 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 pulse 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 power is not as strong as electron beam welding. Laser welding can achieve precise energy control, thus enabling 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 of exactly the same wavelength, frequency, and direction.

Characteristics of laser: It has the characteristics of good monochromaticity, good directionality and high energy density. After the laser is transmitted or focused by a 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 for welding, Heat source for cutting, drilling and surface treatment. The substances that generate lasers include solids, semiconductors, liquids, gases, etc. Among them, the main ones used for industrial processing such as welding and cutting are yttrium aluminum garnet (YAG) solid laser and CO2 gas laser.

The main advantages of laser welding are:

Laser can be bent and transmitted through optical methods such as optical fibers and prisms. It is suitable for welding micro-parts and parts that are difficult to reach by other welding methods. It can also be welded through transparent materials.

It has high energy density and can achieve high-speed welding. The heat-affected zone and welding deformation are very small. It is especially suitable for welding of heat-sensitive materials.

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

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

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

3. Pressure welding

(1) Resistance welding

This is a type of welding method that uses resistance heat as energy, including electroslag welding that uses slag resistance heat as energy and resistance welding that uses solid resistance heat as energy. Since electroslag welding has unique characteristics, it will be introduced later. Here we mainly introduce several types of resistance welding using solid resistance heat as energy source, including spot welding, seam welding, projection welding and butt welding. Resistance welding is generally a welding method that puts 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 to achieve connection. Usually larger currents are used. To prevent arcing at the contact surfaces 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 contact surfaces between the electrode and the workpiece and between the workpiece and the workpiece must be cleaned before welding.

Advantages: 1) When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple. 2) The heating time is short and the heat is concentrated, so the heat affected zone is small, the deformation and stress are also small, and there is usually no need to arrange correction and heat treatment processes after welding. 3) There is no need for filler metals such as welding wires and electrodes, as well as welding materials such as oxygen, acetylene, and argon, so the welding cost is low. 4) It is simple to operate, easy to implement mechanization and automation, and improves working conditions. 5) It has high productivity and no noise or harmful gases. In mass production, it can be integrated into the assembly line together with other manufacturing processes. However, flash butt welding needs to be isolated due to spark splash.

Disadvantages: 1) There is currently a lack of reliable non-destructive testing methods. Welding quality can only be checked by destructive testing of process samples and workpieces, and guaranteed by various monitoring technologies. 2) Point and seam welded lap joints not only increase the weight of the component, but also cause the angle formed around the nugget between the two plates, resulting in low tensile strength and fatigue strength of the joint. 3) The equipment has high power and a high degree of mechanization and automation, which makes the equipment cost high and maintenance difficult, and the commonly used high-power single-phase AC welding machine is not conducive to the normal operation of the power grid.

Scope of application: It is widely used in automobiles, aircraft, instruments, home appliances, steel bars for construction, and other industries. It is suitable for a wide range of materials, but the resistance welding of easily oxidized metals is slightly poor. Mainly used for welding thin plate components with thickness less than 3mm. All types of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel, etc. can be welded.

(2) Friction welding

Friction welding is solid phase welding that uses mechanical energy as energy source. 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. Pressure must be applied between the two surfaces. In most cases, the pressure is increased when heating is terminated, so that the hot metals are forged and bonded. Generally, the bonding surface does not melt. Friction welding has high productivity. In principle, almost all metals that can be hot forged can be friction welded. Friction welding can also be used to weld dissimilar metals. It is suitable for workpieces with a circular cross-section and a maximum diameter of 100mm.

It is a solid phase pressure welding method that uses the heat generated by the friction between the contact end faces of the weldment to bring the end faces to a thermoplastic state, and then quickly applies upsetting force to achieve welding.

Friction welding has the following advantages:

1. The welding quality is stable, the dimensional accuracy of the welded parts is high, and the joint scrap rate is lower than that of resistance butt welding and flash butt welding.

2. High welding productivity, 5 to 6 times higher than flash butt welding.

3. Suitable for welding dissimilar metals, such as the connection between carbon steel, low alloy steel and stainless steel, high-speed steel, and the connection between copper-stainless steel, copper-aluminum, aluminum-steel, steel-zirconium, etc.

4. Low processing cost, energy saving, no special cleaning required for welded parts.

5. It is easy to realize mechanization and automation, and the operation is simple. There are no sparks, arc light and harmful gases in the welding work site.

Disadvantages: It relies on the rotation of the workpiece, and it is difficult to weld non-circular cross-sections. Disc-shaped workpieces and thin-walled pipe fittings are difficult to weld because they are difficult to clamp. Limited by the power of the spindle motor of the welding machine, the maximum cross-section that can be welded by friction welding is currently 20,000mm2. Friction welding machines have large one-time investment costs and are suitable for mass production.

Application: Dissimilar metal and dissimilar steel products, such as copper-aluminum transition joints in the power industry, high-speed steel-structural steel tools for metal cutting, etc.; structural steel products, such as power station boiler serpentine tubes, valves, tractor bearings, etc.

(3) Diffusion welding

Diffusion welding is generally a solid phase welding method that uses indirect heat as energy source. This is usually done under vacuum or protective atmosphere. During welding, the surfaces of the two workpieces to be welded are brought into contact under high temperature and greater pressure and kept warm for a certain period of time to achieve the distance between atoms and combine through simple mutual diffusion of atoms. Before welding, it is not only necessary to clean the oxide and other impurities on the surface of the workpiece, but also the surface roughness must be below a certain value to ensure welding quality.

Diffusion welding is under the protection of a vacuum or protective atmosphere, and under certain temperature (lower than the melting point of the base metal) and pressure conditions, so that the flat and smooth surfaces to be welded that are in contact with each other are in close contact after microscopic plastic rheology, and the atoms diffuse each other. After a long period of time, the original interface disappears and a complete metallurgical bonding welding method is achieved.

Diffusion welding has the following advantages:

It can achieve welding between various types of same materials and dissimilar materials without almost damaging the properties of the materials to be welded, and can be used to manufacture double-layer or multi-layer composite materials.
Able to weld workpieces with complex structures and large differences in thickness.

The joint composition and structure are uniform, reducing the tendency of stress corrosion.

The welding deformation is small and the joint precision is high. It can be used as the final assembly and connection method of components.

It can be carried out simultaneously with other processing techniques (such as vacuum heat treatment, etc.), and the welding of multiple joints can be completed at the same time, thereby improving productivity.

Disadvantages: Diffusion welding has high requirements for surface processing and cleaning of welded parts, long welding time, low productivity, high cost, and large equipment investment.

Application: Welding between dissimilar metals with large melting point differences or metallurgical incompatibility, welding of metals and ceramics, and welding of titanium, nickel, and aluminum alloy structural parts. It is not only used in cutting-edge technology fields such as atomic energy, aerospace and electronic industries, but also has been extended to general machinery manufacturing industry sectors.

4. Brazing

The energy source for brazing can be chemical reaction heat or indirect heat energy. It uses a metal with a lower melting point than the melting point of the material to be welded as the solder. After heating, the solder is melted, and the capillary action draws the solder into the gap between the joint contact surfaces, moistening the surface of the metal to be welded, so that the liquid phase and the solid The phases diffuse into each other to form a brazed joint. Therefore, brazing is a solid phase and liquid phase welding method.

(1) Characteristics and applications of brazing

Brazing uses an alloy with a lower melting point than the base metal as the filler metal. When heated, the filler metal melts, fills and remains in the joint gap by wetting and capillary action, while the base metal is in a solid state, relying on the liquid filler metal and solid parent material. The mutual diffusion between materials forms a brazed joint. Brazing has little impact on the physical and chemical properties of the base metal, has small welding stress and deformation, can weld dissimilar metals with large differences in performance, can complete multiple welds at the same time, has a beautiful and neat joint appearance, simple equipment, and low production investment. However, the strength of the brazed joint is low and the heat resistance is poor.

Applications: Carbide cutting tools, drilling bits, bicycle frames, heat exchangers, conduits and various containers, etc.; In the manufacturing of microwave waveguides, electron tubes and electronic vacuum devices, brazing is even the only possible connection method.

(2) Solder and flux

Filler metal is the filler metal used to form a brazed joint, and the quality of the brazed joint depends largely on the filler metal. The brazing material should have a suitable melting point, good wettability and gap-filling ability, and can diffuse with the base metal. It should also have certain mechanical properties and physical and chemical properties to meet the performance requirements of the joint. According to the different melting points of solder, soldering is divided into two categories: soft soldering and hard soldering.

Soft soldering. Soldering where the melting point of the solder is lower than 450°C is called soft soldering. The commonly used solder is tin-lead solder. It has good wettability and conductivity and is widely used in electronic products, electrical appliances and auto parts. The joint strength of soft soldering is generally 60~140MPa.

Brazing with a solder melting point higher than 450°C is called brazing. Commonly used solders are brass solder and silver-based solder. Joints using silver-based solder have high strength, conductivity and corrosion resistance. The melting point of the solder is low and the processability is good. However, the price of the solder is higher. It is mostly used for welding parts with higher requirements. Generally, there are many welding parts. Use brass solder. Brazing is mostly used for brazing steel and copper alloy workpieces that are subject to greater stress, as well as tools. The joint strength of brazing is 200~490MPa.

Note: The contact surface of the base material should be very clean, so flux should be used. The function of the flux is to remove oxides and oily impurities on the surface of the base metal and the solder metal, protect the contact surface between the solder metal and the base metal from oxidation, and increase the wettability and capillary fluidity of the solder metal. The melting point of the flux should be lower than that of the solder, and the flux residue should be less corrosive to the base metal and joints. The commonly used flux for soft soldering is rosin or zinc chloride solution, and the commonly used flux for brazing is a mixture of borax, boric acid and alkaline fluoride.

According to different heat sources or heating methods, brazing can be divided into: flame brazing, induction brazing, furnace brazing, dip brazing, resistance brazing, etc. Since the heating temperature is relatively low during brazing, it has less impact on the performance of the workpiece material and the stress deformation of the weldment is also small. However, the strength of brazed joints is generally low and their heat resistance is poor.

Brazing heating method: Almost all heating sources can be used as brazing heat sources, and brazing is classified accordingly.

Flame brazing: Heating with gas flame, used for brazing carbon steel, stainless steel, carbide, cast iron, copper and copper alloys, aluminum and aluminum alloys.

Induction brazing: Resistance heating welding parts that use an alternating magnetic field to generate induced current in parts. It is used for welding parts with symmetrical shapes, especially for brazing pipe shafts.

Dip brazing: The weldment is partially or completely immersed in the molten salt mixture or solder melt, and the brazing process is realized by the heat of these liquid media. It is characterized by rapid heating, uniform temperature, and small deformation of the weldment.

Furnace brazing: A resistance furnace is used to heat the weldment. The resistance furnace can protect the weldment by vacuuming or using reducing gas or inert gas.

In addition, there are soldering iron soldering, resistance soldering, diffusion soldering, infrared soldering, reaction soldering, electron beam soldering, laser soldering, etc.

Brazing can be used to weld carbon steel, stainless steel, high-temperature alloys, aluminum, copper and other metal materials, and can also connect dissimilar metals, metals and non-metals. It is suitable for welding joints with low load capacity or working at normal temperature. It is especially suitable for precision, micro and complex multi-brazing welding parts.

5. Introduction to other welding processes

(1) High frequency welding

High-frequency welding uses solid resistance heat as energy source. During welding, the resistance heat generated by high-frequency current in the workpiece is used to heat the surface of the welding area of the workpiece to a molten or close to plastic state, and then upsetting force is applied (or not applied) to achieve the bonding of metals. 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 based on the way high-frequency current generates heat in the workpiece. When contacting high-frequency welding, high-frequency current is introduced into the workpiece through mechanical contact with the workpiece. During induction high-frequency welding, the high-frequency current generates an induced current in the workpiece through the coupling effect of the induction coil outside the workpiece. High-frequency welding is a highly specialized welding method, and special equipment must be equipped according to the product. High productivity, welding speed can reach 30m/min. Mainly used for welding longitudinal seams or spiral seams when manufacturing pipes.

(2) Explosion welding

Explosion welding is also another solid-state welding method that uses chemical reaction heat as energy. But it uses the energy generated by the explosion of explosives to achieve metal connections. Under the action of the explosion wave, the two pieces of metal can be accelerated and collided to form a metallic bond in less than a second. Among the various welding methods, explosion welding can weld the widest range of combinations of dissimilar metals. Explosive welding can be used to weld two metallurgically incompatible metals into various transition joints. Explosion welding is mostly used for cladding flat plates with considerable surface areas and is an efficient method for manufacturing composite panels.

(3) Ultrasonic welding

Ultrasonic welding is also a solid-phase welding method that uses mechanical energy as energy source. During ultrasonic welding, the welded workpiece is under low static pressure, and the high-frequency vibration emitted by the sonotrode can cause strong cracking friction on the joint surface and heat it to the welding temperature to form a bond. Ultrasonic welding can be used for welding between most metal materials, and can achieve welding between metals, dissimilar metals, and metals and non-metals. It is suitable for the repeated production of metal wires, foils or thin plate metal joints below 2~3mm.