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Welding is a manufacturing or sculpting process that joins metals or thermoplastics. During the welding process, the workpiece and the solder melt to form a molten area (molten pool). After the molten pool cools and solidifies, a connection between the materials is formed. This process usually requires the application of pressure. The difference between ordinary welding and brazing and soldering is that soldering forms a connection by melting a solder with a lower melting point (lower than the melting point of the workpiece itself) without heating to melt the workpiece itself.
There are many energy sources for welding, including gas flame, arc, laser, electron beam, friction and ultrasonic wave. In addition to use in factories, welding can be performed in a variety of environments, such as in the field, underwater, and in space. Wherever welding occurs, it can pose dangers to the operator, so appropriate protective measures must be taken when performing welding. Possible injuries to the human body caused by welding include burns, electric shock, visual impairment, inhalation of toxic gases, excessive ultraviolet exposure, etc.
Before the end of the 19th century, the only welding process was forge welding of metals that blacksmiths had used for hundreds of years. The earliest modern welding technology appeared at the end of the 19th century, first arc welding and oxygen gas welding, and later resistance welding. In the early 20th century, there was a great demand for military equipment in World War I and World War II. Correspondingly, cheap and reliable metal joining processes were valued, which in turn promoted the development of welding technology. After the war, several modern welding technologies have emerged, including the most popular manual arc welding, as well as automatic or semi-automatic welding technologies such as gas metal arc welding, submerged arc welding, flux cored arc welding and electroslag welding. . In the second half of the 20th century, welding technology developed rapidly, and laser welding and electron beam welding were developed. Today, welding robots are widely used in industrial production. Researchers are still delving into the nature of welding, continuing to develop new welding methods, and further improving welding quality.
Arc welding
Arc welding uses a welding power source to create and maintain an arc between the electrode and the welding material, melting the metal on the welding point to form a molten pool. They can operate on direct or alternating current, using consumable or non-consumable electrodes. Sometimes a certain inert or semi-inert gas, a shielding gas, is introduced near the molten pool, and sometimes welding repair material is added.
The arc welding process consumes a large amount of electrical energy, and energy can be supplied through a variety of welding power sources. The most common welding power supplies include constant current power supplies and constant voltage power supplies. In the arc welding process, the applied voltage determines the length of the arc, and the input current determines the output heat. Constant current power supply outputs constant current and fluctuating voltage, and is mostly used for manual welding, such as manual arc welding and gas tungsten arc welding. Because manual welding requires the current to remain relatively stable, in actual operation, it is difficult to ensure that the position of the electrode remains unchanged, and the arc length and voltage will also change accordingly. Constant voltage power supplies output constant voltage and fluctuating current, so they are often used in automatic welding processes such as gas metal arc welding, flux cored arc welding and submerged arc welding. In these welding processes, the arc length remains constant because any fluctuations in the distance between the welding head and the workpiece are compensated for by changes in current. For example, if the distance between the welding head and the workpiece is too close, the current will increase rapidly, causing the heat at the welding point to increase suddenly, and the welding head will partially melt until the distance returns to the original level.
The type of electricity used has a huge impact on welding. Welding processes that consume a lot of power, such as manual arc welding and gas metal arc welding, usually use direct current, and the electrodes can be connected to the positive or negative electrodes. During welding, the part connected to the positive electrode will have greater heat concentration, so changing the polarity of the electrode will affect the welding performance. If the workpiece is connected to the positive electrode, the workpiece will be hotter, and the welding depth and welding speed will also be greatly increased. On the contrary, if the workpiece is connected to the negative electrode, a shallower weld will be produced. Welding processes that consume less power, such as gas tungsten arc welding, can use direct current (using any joint method) or alternating current.
However, the electrodes used in these welding processes only generate arcs and do not provide solder. Therefore, when using direct current, when the positive electrode is connected, the welding depth is shallow, while when the negative electrode is connected, a deeper weld can be produced. Alternating current causes the polarity of the electrode to change rapidly, resulting in a weld with medium penetration. One of the disadvantages of using alternating current is that the arc must be re-ignited every time the voltage changes through the voltage zero point. To solve this problem, some special welding power sources produce a square wave type of alternating current instead of the usual sine wave type. , minimizing the negative impact when the voltage changes through zero.
Manual arc welding
Shielded metal arc welding (SMAW) is the most common welding process. An arc is formed by applying high voltage between the welding material and a consumable electrode, whose core is usually made of steel and covered with a layer of flux. During the welding process, the flux burns to produce carbon dioxide, which protects the weld area from oxidation and pollution. The electrode core directly acts as a filler material, without the need to add additional solder.
This process has a wide range of adaptability and the required equipment is relatively cheap, making it very suitable for on-site and outdoor operations. Operators need only a small amount of training to become proficient. Welding times are slower because the consumable rod electrodes must be replaced frequently. After welding, the slag formed by flux also needs to be removed. In addition, this technique is usually only used for welding ferrous metals. Special electrodes are required for welding cast iron, nickel, aluminum, copper and other metals. Inexperienced operators also often have difficulty mastering welding in special locations.
Gas metal arc welding (GMAW), also known as metal-inert gas welding or MIG welding, is a semi-automatic or automatic welding process. It uses a welding rod to continuously feed wire as the electrode, and uses an inert or semi-inert mixed gas to protect the welding joint. Similar to manual arc welding, the operator can master it with a little training. Because the supply of welding wire is continuous, gas metal arc welding can achieve higher welding speeds than manual arc welding. In addition, because its arc is smaller than that of manual arc welding, gas metal arc welding is more suitable for special position welding (such as overhead welding).
Compared with manual arc welding, the equipment required for gas metal arc welding is much more complex and expensive, and the installation process is also more cumbersome. Therefore, the portability and versatility of gas metal arc welding are not good, and because shielding gas must be used, it is not particularly suitable for outdoor operations. However, gas metal arc welding has a faster welding speed and is very suitable for factory-scale large-scale welding. This process works on a variety of metals, including ferrous and non-ferrous metals.
Another similar technology is flux-cored arc welding (FCAW), which uses similar equipment to gas metal arc welding, but uses steel coated with powder material Welding rod with quality electrode core. This type of wire is more expensive than a standard solid electrode and produces smoke and slag during welding, but it allows for higher welding speeds and greater weld depth.
Gas tungsten arc welding (GTAW), also known as tungsten-inert gas (TIG welding) welding (sometimes mistakenly called helium arc welding), is a manual welding process. It uses a non-consumable tungsten electrode, an inert or semi-inert shielding gas, and additional solder. This process has a stable arc and high welding quality, and is especially suitable for welding sheet metal. However, this process has higher requirements for operators and the welding speed is relatively low.
Gas tungsten arc welding is suitable for almost all weldable metals, and is most commonly used for welding stainless steel and light metals. It is often used to weld products that require higher welding quality, such as bicycles, aircraft and offshore work tools. Similar to this is plasma arc welding (PAW), which uses a tungsten electrode and plasma gas to generate an arc.
The arc of plasma arc welding is more concentrated than that of gas tungsten arc welding, which makes the lateral control of plasma arc welding particularly important. Therefore, this technology has higher requirements for mechanical systems. Due to its relatively stable current, this method has greater welding depth and faster welding speed than gas tungsten arc welding. It can weld almost all metals that gas tungsten arc welding can weld, the only thing it can't weld is magnesium. Automatic welding of stainless steel is an important application of plasma arc welding. A variant of this process is plasma cutting, suitable for cutting steel.
Submerged arc welding (SAW) is a highly efficient welding process. The arc of submerged arc welding is generated inside the flux. Since the flux blocks the influence of the atmosphere, the welding quality is greatly improved. The slag from submerged arc welding often falls off on its own, and there is no need to clean the slag. Submerged arc welding can achieve automatic welding by using an automatic wire feeding device, which can achieve extremely high welding speeds. Because the arc is hidden under the flux and produces almost no smoke, the working environment of submerged arc welding is much better than other arc welding processes. This process is commonly used in industrial production, especially when manufacturing large products and pressure vessels. Other arc welding processes include atomic hydrogen welding (AHW), carbon arc welding (CAW), electroslag welding (ESW), electrogas welding (EGW), and stud welding. (Stud welding) etc.
Using oxy-fuel welding to weld metal parts
The most common gas welding process is oxy-fuel welding, also known as oxyacetylene flame welding. It is one of the oldest and most versatile welding processes, but it has become less common in industrial production in recent years. It is still widely used in making and repairing pipes, and is also suitable for making certain types of metal art. Combustible gas welding can be used not only for welding iron or steel, but also for brazing, soldering, heating metal (for bending and shaping), gas flame cutting, etc.
The equipment required for combustible gas welding is relatively simple and relatively cheap. It generally uses a mixture of oxygen and acetylene to generate a flame with a temperature of about 3100 degrees Celsius. Because the flame is more dispersed than the arc, the weld of combustible gas welding cools more slowly, which may result in greater residual stress and welding distortion, but this feature simplifies the welding of high-alloy steels. A derivative application is called gas flame cutting, which uses gas flames to cut metal [5]. Other gas welding processes include air acetylene welding, oxygen hydrogen welding, and gas pressure welding. The main difference between them lies in the use of different fuel gases. Hydroxygen welding is sometimes used for precision welding of small items, such as jewelry. Gas welding can also be used to weld plastics. Heated air is generally used to weld plastics, and its working temperature is much lower than that of welding metals.
Resistance welding
The principle of resistance welding is: when two or more metal surfaces come into contact, contact resistance will occur on the contact surface. If a large current (1,000-100,000 amperes) is passed through these metals, according to Joule's law, the part with a large contact resistance will heat up, melting the metal near the contact point to form a molten pool. Generally speaking, resistance welding is an efficient and pollution-free welding process, but its application is limited due to equipment cost.
Spot welding machine
Spot welding, also known as resistance spot welding, is a popular resistance welding process used to connect metal plates that are laminated together. The thickness of the metal plate can be up to 3 mm. While the two electrodes fix the metal plate, they also deliver strong current to the metal plate. The advantages of this method include: high energy utilization efficiency, small deformation of the workpiece, fast welding speed, easy automatic welding, and no need for solder. Since the weld strength of resistance spot welding is significantly lower, this process is only suitable for the manufacture of certain products. It is widely used in the automobile manufacturing industry. There are thousands of welding points performed by industrial robots on an ordinary car.
A special spot welding process (Shot welding) can be used for spot welding of stainless steel.
A welding process similar to spot welding is called seam welding, which uses electrodes to apply pressure and current to join metal plates. The electrode used in seam welding is roller-shaped rather than point-shaped. The electrode can roll to transport the metal plate, which allows seam welding to create longer welds. In the past, this process was used to make cans, but it is rarely used now. Other resistance welding processes include flash welding, projection welding, butt welding, etc.
Energy beam welding
Energy beam welding processes include laser welding (Laser beam welding, LBW) and electron beam welding (Electron beam welding, EBW). They are both relatively new processes that are popular in high-tech manufacturing. The principles of these two processes are similar, with the most significant difference being their energy sources. Laser welding uses a highly concentrated laser beam, while electron beam welding uses a beam of electrons emitted in a vacuum chamber. Since both energy beams have high energy density, the penetration depth of energy beam welding is large and the welding spots are small. Both welding processes work very quickly, are easily automated, and are extremely productive. The main disadvantages are that the equipment is extremely expensive (although prices have been falling) and the welds are prone to thermal cracking. A new development in this field is laser-hybrid welding, which combines the advantages of laser welding and arc welding and therefore enables higher quality welds to be obtained.
Solid State Welding
Similar to the earliest welding process, forge welding, some modern welding processes do not require melting of material to form a connection. The most popular of these is ultrasonic welding, which joins sheets and wires made of metal and thermoplastics by applying high-frequency sound waves and pressure. The equipment and principles of ultrasonic welding are similar to resistance welding, except that the input is not current but high-frequency vibration. This welding process welds metal without heating it to melt, and relies on horizontal vibration and pressure to form the weld. When welding plastics, vertical vibration should be applied at the melting temperature. Ultrasonic welding is often used to manufacture electrical interfaces made of copper or aluminum, and is also commonly used to weld composite materials.
Another common solid-state welding process is explosion welding. Its principle is to connect materials under the high temperature and high pressure generated by the explosion. The impact of the explosion causes the material to become plastic in a short period of time, thereby forming a solder joint, and only a small amount of heat is generated in the process. This process is commonly used for welding joining dissimilar materials, such as joining aluminum components on ship hulls or composite panels. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, friction welding (including friction stir welding), and high-frequency welding (High frequency welding), hot pressure welding (Hot pressure welding), induction welding (Induction welding), hot rolled welding (Roll welding).
Joint types
Common welded joint types: (1) I-shaped butt joint; (2) V-shaped butt joint; (3) Lap joint; (4) T-shaped connector.
Welded connections between workpieces can have a variety of joint forms. The five basic joint types are: butt joint, lap joint, corner joint, end joint, and T-joint. There are also some joint forms derived from this, such as double V-shaped butt preparation joints, which are characterized by cutting both materials to be connected into a V-shaped sharp corner shape.
Single U-shaped and double U-shaped butt prepared joints are also very common. Their joints are processed into a curved U shape. Unlike the straight shape of V-shaped joints, lap joints can be used to connect more than two pieces of materials, depending on the Depending on the welding process and material thickness, one lap joint can weld multiple workpieces.
Often, certain welding processes are unable or almost completely incapable of processing certain types of joints. For example, lap joints are often used in resistance spot welding, laser welding, and electron beam welding. However, some welding processes, such as manual arc welding, can use almost any joint type. It is worth mentioning that some welding processes allow multiple weldings: after the weld seam of one weld has cooled, weld it again based on it. This allows thicker workpieces to be welded with a V-shaped butt joint.
In the cross-section of a welded joint, the darkest part is the welding zone or melting zone, the lighter part is the heat-affected zone, and the lightest part is the base metal
After welding, the material near the weld shows several distinct areas. The weld seam is called the fusion zone, more specifically the area filled by the melted flux. The material properties of the fusion zone depend primarily on the flux used and the compatibility of the flux with the base metal. Surrounding the melting zone is the heat-affected zone (HAZ), where the material undergoes changes in microstructure and properties during the welding process. These changes depend on the properties of the base metal in the heated state. The metal properties in the heat-affected zone are often inferior to those of the base metal and melting zone, and residual stress is distributed in this area [28].
[edit] Welding quality
The main indicator of welding quality is the strength of the solder joint and its surrounding materials. There are many factors that affect strength, including welding process, energy injection form, base metal, filler materials, flux, joint design, and the interaction between the above factors. Destructive or non-destructive testing is usually used to check welding quality. The main objects of testing are defects in solder joints, the degree of residual stress and deformation, and the nature of the heat-affected zone. Welding inspection has a complete set of specifications and standards to guide operators to adopt appropriate welding processes and judge welding quality.
[edit] Heat-affected zone
The blue part in the picture shows the metal oxidation caused during the welding process at about 600°C. It is very accurate to judge the temperature during welding by color, but the color area does not represent the size of the heat affected zone. The true heat-affected zone is actually a very narrow area around the weld.
The influence of the welding process on the metal properties near the weld can be calibrated. Different welding materials and welding processes will form heat-affected zones of different sizes and characteristics. The thermal diffusion coefficient of the base metal has a great influence on the properties of the heat-affected zone: a larger thermal diffusion coefficient allows the material to cool faster, forming a relatively smaller heat-affected zone. On the contrary, if the thermal diffusivity of the material is small and heat dissipation is difficult, the heat affected zone will be relatively large. The heat input of the welding process also has a significant impact on the heat affected zone. For example, in oxyacetylene welding, because the heat is not input intensively, a larger heat affected zone will be formed. Processes such as laser welding can concentrate limited heat output, resulting in a smaller heat-affected zone. The heat-affected zone caused by arc welding is located between the two extreme situations, and the operator's level often determines the size of the arc welding heat-affected zone [29][30].
To calculate the heat input of arc welding, the following formula can be used:
Q = \left(\frac{V \times I \times 60}{S \times 1000} \right) \times \mathit{Efficiency}
Where Q is the heat input (kJ/mm), V is the voltage (V), I is the current (A), and S is the welding speed (mm /min). The value of Efficiency depends on the welding process used: 0.75 for manual arc welding, 0.9 for gas metal arc welding and submerged arc welding, and 0.8 for gas tungsten arc welding [31].
[edit] Twisting and Fracture
Because metals are heated to their melting temperature during welding, they shrink as they cool. Shrinkage creates residual stresses and causes longitudinal and circumferential distortion. Distortion may result in loss of product shape. In order to eliminate distortion, a certain offset is sometimes introduced during welding to offset the distortion caused by cooling [32]. Other methods of limiting distortion include clamping the workpiece, but this can result in increased residual stresses in the heat-affected zone. Residual stress can reduce the mechanical properties of the base metal and form catastrophic cold cracks. This problem occurred in many Liberty ships built during World War II[33][34]. Cold cracks are only seen in steel materials. They are related to the formation of martensite when the steel is cooled. Fractures mostly occur in the heat-affected zone of the base metal. In order to reduce distortion and residual stress, the heat input of welding should be controlled and welding on a single material should be completed in one pass rather than in multiple passes.
Other types of cracks, such as hot cracks and hardening cracks, can occur in the fusion zone of welds in all metals. In order to reduce the occurrence of cracks, external force constraints should not be applied when metal welding, and appropriate flux should be used [35].
[edit] Weldability
The quality of welding also depends on the base metal and filler materials used. Not all metals can be welded, and different base materials require specific fluxes.
[edit] Steel
The weldability of different steel materials is inversely proportional to their own hardening properties. Hardening properties refer to the ability of steel to produce martensite during cooling after welding. The hardening properties of steel depend on its chemical composition. If a piece of steel contains a higher proportion of carbon and other alloying elements, its hardening properties will be higher and its weldability will therefore be lower. To compare the weldability of different alloy steels, a method called equivalent carbon content can be used, which reflects the weldability of different alloy steels relative to ordinary carbon steel. For example, chromium and vanadium have a higher impact on weldability than copper and nickel, while the impact factors of the above alloying elements are smaller than carbon. The higher the equivalent carbon content of an alloy steel, the lower its weldability. If ordinary carbon steel and low alloy steel are used to achieve high weldability, the strength of the product will be relatively low - there is a subtle trade-off between weldability and product strength. The high-strength low-alloy steels developed in the 1970s have overcome the contradiction between strength and weldability. These alloy steels have high strength and good weldability, making them ideal materials for welding applications [36 ].
Because stainless steel contains a higher proportion of chromium, its weldability is analyzed differently than other steels. Austenite in stainless steel has better weldability, but austenite is very sensitive to distortion due to its high thermal expansion coefficient. Some austenitic stainless steel alloys are prone to fracture, thus reducing their corrosion resistance. If the formation of ferrite is not carefully controlled during welding, thermal fracture may result. To solve this problem, an additional electrode tip can be used to deposit a weld metal containing a small amount of ferrite. Ferritic stainless steel and martensitic stainless steel also have poor weldability and must be preheated during welding and used with special welding electrodes [37].
[edit] Aluminum
The weldability of aluminum alloys varies greatly depending on the alloying elements they contain. Aluminum alloys are highly sensitive to thermal fracture, so high welding speed and low heat input methods are usually used during welding. Preheating can reduce the temperature gradient in the welding area, thereby reducing thermal cracking. But preheating can also reduce the mechanical properties of the base material and cannot be applied when the base material is fixed. Adopting appropriate joint forms and more compatible filler alloys can reduce the occurrence of thermal fractures. The surface of aluminum alloys should be cleaned before welding to remove oxides, oil and loose impurities. Surface cleaning is very important because when aluminum alloys are welded, too much hydrogen will cause foaming, and too much oxygen will form dross [38].
[edit] Welding in extreme environments
Underwater welding
In addition to working in controlled environments such as factories and repair shops, some welding processes It can also be performed in a variety of environments, such as outdoors, underwater, and in a vacuum (such as space). Manual arc welding is often used in outdoor operations such as building construction and repair work. Welding processes that require shielding gas usually cannot be performed outdoors because the disordered flow of air can cause welding failure. Manual arc welding can also be used for underwater welding, such as welding ship hulls, underwater pipelines, offshore working platforms, etc. The more commonly used processes for underwater welding include flux-cored wire arc welding. Welding in space is also possible: in 1969, Soviet cosmonauts tested manual arc welding, plasma arc welding and electron beam welding in a vacuum environment for the first time. In the decades since, space welding technology has developed greatly. Today, researchers are still trying to transfer different welding technologies to vacuum, such as laser welding, resistance welding and friction welding. These welding technologies played a big role in the construction of the International Space Station. Through vacuum welding technology, the space station sub-modules built on the ground can be assembled in space [39].
[edit] Protection measures
Welders wear protective helmets, gloves and protective clothing to perform arc welding operations
It is very dangerous to perform welding operations without protection. Dangerous and harmful to health. By using new technology and appropriate protective measures, the risk of accidents and deaths while welding can be significantly reduced. Commonly used welding techniques often use an open arc or flame, which can easily cause burns. Welders avoid exposure to high temperatures and flames by wearing additional personal protective equipment, such as rubber gloves, long-sleeved protective jackets, etc. In addition, strong light in the welding area can cause diseases such as electro-optic ophthalmia, because the large amount of ultraviolet rays generated during welding can irritate and damage the cornea and retina. When performing arc welding, you must wear eye protection goggles or a protective helmet. New protective helmets developed in recent years can change the transmittance of the goggles according to the intensity of incident ultraviolet rays. In order to protect people other than the welder who are close to the welding site, the welding work site is often surrounded by a translucent protective curtain. These protective curtains are usually plastic curtains made of polyvinyl chloride, which can protect nearby unrelated personnel from the high-intensity ultraviolet rays generated by the arc, but the protective curtains cannot completely replace goggles and helmets [40].
Welders are also at risk from hazardous gases and flying materials. Welding processes such as flux-cored arc welding and manual arc welding produce fumes containing a variety of oxides that may cause occupational diseases such as metal fume fever. Small particles in welding fumes can also affect workers' health. The smaller the size of the particles, the greater the hazard. In addition, many welding processes produce harmful gases and fumes, such as carbon dioxide, ozone and heavy metal oxides. These gases are very harmful to operators without experience and effective ventilation measures. It is worth noting that the protective gases and raw materials used in many welding processes are flammable and explosive, and appropriate protective measures need to be taken, such as controlling the oxygen content in the air, stacking flammable and explosive materials separately, etc. [41] . Welding fume extraction equipment is often used to extract harmful gases and filter them through high-efficiency baffled air filters.
[edit] Economics and development trends
The economic cost of welding is an important factor affecting its industrial application. There are many factors that affect welding costs, such as equipment, labor, raw material and energy costs. The cost of welding equipment varies greatly for different processes. Manual arc welding and combustible gas welding are relatively low-cost, while laser welding and electron beam welding are relatively expensive. Due to the high cost of some welding processes, they are generally only used to manufacture important components. The equipment cost of automatic welding equipment and welding robots is also very high, so their use is also subject to corresponding restrictions. Labor costs depend on the speed of welding, hourly rate and total working time (including welding and subsequent processing). The cost of raw materials includes the cost of purchasing base metal, weld filling materials, and shielding gas. The energy cost depends on the arc operating time and the energy requirement of the weld.
For manual welding, labor costs often account for a large part of the total cost. Therefore, the reduction of manual welding costs often focuses on reducing the time of welding operations. Effective methods include increasing welding speed, optimizing welding parameters, etc. Slag removal after welding is also a time-consuming and laborious task. Therefore, reducing welding slag can improve safety, environmental protection, reduce costs, and improve welding quality [42]. Mechanization and automation can also effectively reduce labor costs, but on the other hand, they increase equipment costs and require additional equipment installation and debugging time. When products have special needs, the cost of raw materials often rises. The energy cost is usually unimportant since it generally accounts for only a few percent of the total cost [43].
In recent years, in order to reduce the labor cost of welding in high-end products, automatic welding equipment has been widely used in resistance spot welding and arc welding in industrial production (especially in the automotive industry). Welding robots can effectively complete welding, especially spot welding. With the advancement of technology, welding robots have also begun to be used for arc welding. The cutting-edge development areas of welding technology include: welding between special-shaped materials (such as welded connections of iron and aluminum parts), new welding processes, such as friction stir welding, magnetic pulse welding, and thermal conductive seams Welding (conductive heat seam welding) and laser hybrid welding (laser-hybrid welding), etc. Other research focuses on expanding the application range of existing welding processes, such as laser welding in the aerospace and automotive industries. Researchers also hope to further improve welding quality, especially by controlling the microstructure and residual stress of the weld to reduce deformation and fracture of the weld