Introduction to Welding

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Introduction to Welding: Metal welding refers to a method of joining two separate metal objects (of the same or different metals) together by using appropriate means to create atomic (molecular) bonds between them. In various product manufacturing industries, welding and cutting (thermal cutting) is a very important processing technique. According to statistics from industrially developed countries, steel that needs to be welded before use accounts for around 45% of the total steel production each year. Welding can not only be used to join various types of steel, but also to join non-ferrous metals such as aluminum and copper, as well as special metal materials like titanium and zirconium. As a result, it is widely applied in industries such as machinery manufacturing, shipbuilding, ocean development, automobile production, petrochemicals, aerospace technology, nuclear energy, electricity, electronics, and construction. With the demands of modern industrial production and the rapid development of science and technology, welding technology continues to advance. Just in terms of new welding methods, there are already dozens of them to date. When selecting a welding method in production, it is necessary not only to understand the characteristics and applicable ranges of various welding methods but also to take into account the requirements of the product. A preliminary choice must then be made based on factors such as the structure, materials, and manufacturing techniques of the product to be welded. First, the classification of welding methods is discussed, followed by a summary of the characteristics and application ranges of various welding methods. Secondly, it provides a brief introduction to the basic knowledge on how to select the most suitable welding method from a technical and economic perspective, based on the requirements and characteristics of the product, for reference by engineering and technical personnel involved in welding production. Finally, the development of welding technology in various aspects is discussed, in the hope of providing assistance to experienced welding technicians who are interested in new welding technologies when developing or adopting them. I. Classification of welding methods There are a great variety of welding methods, and new ones continue to emerge; therefore, it is a very important issue to classify these welding methods in a scientific manner. Correct classification not only helps readers understand and learn the characteristics and essence of various welding methods, but also provides a solid foundation for scientists to develop new welding techniques. Currently, there are numerous classification systems for welding methods in domestic and international publications, each with its own differences. This manual first provides a brief description and commentary on existing classification systems, and then proposes a new classification method, discussing its principles and advantages. and used it as the basis for writing the chapters of this volume. 1. Family method: This classification approach essentially divides welding methods into several major categories based on certain characteristics of the welding process, and then further subdivides them into smaller categories based on other characteristics, and so on, thereby forming families. This classification system is the most widely used in various works today. In this classification system, welding methods are first divided into three major categories: fusion welding, solid-state welding, and brazing. Secondly, each major category of methods, such as fusion welding, solid-state welding, and brazing. Secondly, each major category of methods, such as fusion welding, is further subdivided into types based on the type of energy used, including arc welding, gas welding, thermite welding, electroslag welding, etc. Then there are methods such as arc welding, which are further divided into welding processes with a consumable electrode and various shielding methods. When classified by the characteristics of welding processes, the number of classification levels can be adjusted as needed, offering flexibility; moreover, the hierarchy among them is quite clear. This is an advantage. However, such classification systems often lack clear and consistent principles for classification; for example, in Table 1-1, the principles used for categorizing into major groups are different from those used for the subsequent levels of classification. There is also no fixed, consistent classification principle among these three categories of characteristics. For example, fusion welding is based on whether melting and crystallization occur during the welding process ； Brazing is primarily classified based on the filler metal. Therefore, for a certain welding method, there may be different classifications depending on the features emphasized, such as spot welding, flash welding, and gas pressure welding. Furthermore, due to the overly rigid boundaries between the primary and secondary categories at each level, cross-category classification is not possible, which results in some welding methods failing to be classified, such as diffusion brazing and thermal spraying. II. Introduction to Welding Methods 1. Arc Welding Arc welding is the most widely used welding method at present. It includes: manual arc welding, submerged arc welding, tungsten inert gas arc welding, plasma arc welding, gas metal arc welding, etc. The vast majority of arc welding uses the arc that burns between the electrode and the workpiece as a heat source. When forming a joint, filler metal may be used or may not be used. The electrode used is one that melts during the welding process, and this type of welding is known as shielded metal arc welding; examples include manual arc welding, submerged arc welding, gas shielded arc welding, and tubular wire arc welding ； The electrodes used are carbon or tungsten rods that do not melt during the welding process; this type of welding is known as non-consumable electrode arc welding, including processes such as tungsten inert gas welding and plasma arc welding. (1) Shielded metal arc welding: Shielded metal arc welding is the welding method that was developed earliest among all arc welding techniques, and it remains the most widely used one to this day. It uses a welding rod coated with flux as the electrode and filler metal, with the arc burning between the end of the welding rod and the surface of the workpiece to be welded. Under the action of arc heat, the coating can produce gases to protect the arc; it can also generate slag that covers the surface of the molten pool, preventing interaction between the molten metal and the surrounding gases. Another important role of slag is to undergo physical and chemical reactions with the molten metal or to add alloying elements, thereby improving the mechanical properties of the weld. Manual arc welding equipment is simple, lightweight, and easy to operate. It can be used for welding short seams in maintenance and assembly, especially for welding in areas that are difficult to access. Manual arc welding, using the appropriate electrodes, can be applied to most industrial carbon steels, stainless steels, cast irons, copper, aluminum, nickel, and their alloys. (2) Submerged arc welding: Submerged arc welding uses a wire fed continuously as both the electrode and the filler metal. During welding, a layer of granular flux is applied over the welding area; the arc burns beneath this flux layer, melting the end of the wire and the local base material to form a weld. Under the action of arc heat, the upper part of the flux melts to form slag, which then undergoes a metallurgical reaction with the liquid metal. The slag floats on the surface of the molten metal pool; it protects the weld metal by preventing contamination from air, and it undergoes physical and chemical reactions with the molten metal, thereby improving the quality and properties of the weld metal ； On the other hand, it can also allow the weld metal to cool down slowly. Submerged arc welding can use a higher welding current. Compared to manual arc welding, its greatest advantage is better weld quality and higher welding speed. Therefore, it is particularly suitable for welding straight and circumferential seams of large workpieces. Moreover, mechanized welding is used in most cases. Submerged arc welding is widely used for welding carbon steel, low-alloy structural steel, and stainless steel. Since slag can reduce the cooling rate of the joint, submerged arc welding can also be used for certain high-strength structural steels, high-carbon steels, etc. (3) Tungsten inert gas arc welding: This is a type of non-fusing electrode gas shielded arc welding in which the arc between the tungsten electrode and the workpiece is used to melt the metal and form a weld. During welding, the tungsten electrode does not melt; it merely serves as an electrode. At the same time, argon or helium is supplied through the nozzle of the welding torch for protection. Additional metals can also be added as needed. It is commonly known internationally as TIG welding. Tungsten inert gas arc welding is an excellent method for joining thin metal sheets and for performing root welding, as it allows for good control of heat input. This method can be used for joining almost all metals, especially suitable for welding metals such as aluminum and magnesium that form refractory oxides, as well as reactive metals like titanium and zirconium. This welding method produces welds of high quality, but its welding speed is slower compared to other arc welding methods. (4) Plasma arc welding: Plasma arc welding is also a type of non-consumable electrode arc welding. It achieves welding by using a compressed arc between the electrode and the workpiece (referred to as a forward transfer arc). The electrodes used are usually tungsten electrodes. The plasma gas used to generate the plasma arc can be argon, nitrogen, helium, or a mixture of two of them. It is also protected by an inert gas through a nozzle. During welding, filler metal can be used or it can be omitted. During plasma arc welding, the arc is straight and has a high energy density, which gives it strong penetration capability. The hole effect that occurs during plasma arc welding enables butt welding without groove preparation for most metals within a certain thickness range, while ensuring full penetration and uniform weld quality. Therefore, plasma arc welding has high productivity and good weld quality. However, plasma arc welding equipment (including nozzles) is relatively complex, and it requires high precision in controlling the welding process parameters. Plasma arc welding can be used for virtually all metals that can be welded by tungsten inert gas arc welding. In contrast, welding extremely thin metals of 1 mm or less can be carried out more easily using plasma arc welding. (5) Gaseous metal arc welding: This welding method uses an arc that is generated between a continuously fed wire and the workpiece as the heat source, with the arc being protected by gas emitted from the torch nozzle. The shielding gases commonly used in gas metal arc welding include argon, helium, CO2, or mixtures of these gases. When argon or helium is used as the shielding gas, it is called metal inert gas shielded arc welding (internationally abbreviated as MIG welding) ； When an inert gas mixed with oxidizing gases (O2, CO2) is used as the shielding gas, or when CO2 gas or a mixture of CO2 and O2 is used as the shielding gas, this process is collectively referred to as metal active gas shielded arc welding (internationally abbreviated as MAG welding). The main advantage of gas metal arc welding is the ability to weld in various positions easily, along with advantages such as a fast welding speed and high deposition rate. GMAW can be applied to most major metals, including carbon steel and alloy steel. GMAW is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium, and nickel alloys. Arc spot welding can also be performed using this welding method. (6) Tubular wire arc welding: Tubular wire arc welding also uses the arc that is generated between the continuously fed wire and the workpiece as a heat source for welding; it can be considered a type of metal inert gas welding. The welding wire used is a tubular type, with flux containing various components inside the tube. During welding, an external shielding gas is used, mainly CO. The flux decomposes or melts when heated, serving functions such as forming slag to protect the molten pool, alloying, and stabilizing the arc. In addition to the advantages of shielded metal arc welding mentioned above, tubular wire arc welding possesses further metallurgical advantages due to the effect of the flux inside the tube. Tubular wire arc welding can be used for welding various joints of most ferrous metals. Tubular wire arc welding has been widely used in some industrially advanced countries. 2. Resistance welding: This is a type of welding method that uses resistive heat as an energy source, including electroslag welding which relies on the resistive heat of slag, and resistance welding which utilizes the resistive heat of solids. Since electroslag welding has more unique characteristics, it is covered later. Here, several types of resistance welding that use solid resistive heat as an energy source are introduced, mainly including spot welding, seam welding, butt welding, and overlay welding. Resistance welding is a welding method in which the workpieces are subjected to a certain electrode pressure, and the resistance heat generated when current passes through the workpieces is used to melt the contact surface between the two workpieces, thereby achieving connection. A larger current is usually used. To prevent arcing at the contact surface and to forge the weld metal, pressure must always be applied during the welding process. When performing this type of resistance welding, the surface condition of the workpieces to be welded is of paramount importance for achieving stable welding quality. Therefore, the contact surfaces between the electrode and the workpiece, as well as between the workpieces themselves, must be cleaned prior to welding. The differences between spot welding, seam welding, and projection welding lie in the fact that the welding current (single-phase) is high (several thousand to several tens of thousands of amperes), the electrical conduction time is short (several cycles to a few seconds), the equipment is expensive and complex, yet the productivity is high; therefore, these methods are suitable for mass production. It is mainly used for welding thin sheet components with a thickness of less than 3 mm. Various types of steel, non-ferrous metals such as aluminum and magnesium and their alloys, as well as stainless steel, can all be welded. 3. High-energy beam welding: This category of welding methods includes electron beam welding and laser welding. (1) Electron beam welding: Electron beam welding is a welding method that utilizes the thermal energy generated by bombarding the surface of a workpiece with a concentrated high-speed electron beam. During electron beam welding, an electron beam is generated and accelerated by an electron gun. Common types of 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 both carried out in a vacuum chamber. The welding preparation time (mainly the vacuum pumping time) is long, and the size of the workpiece is limited by the size of the vacuum chamber. Compared with arc welding, the main advantages of electron beam welding are a large weld depth, a small weld width, and high purity of the weld metal. It can be used for the precise welding of very thin materials as well as for welding thick components (up to 300 mm thick). All metals and alloys that can be welded by fusion welding using other methods can also be welded by electron beam welding. It is mainly used for welding applications that require high-quality products. It can also handle the welding of dissimilar metals, easily oxidizable metals, and refractory metals. But it is not suitable for mass-produced products. (2) Laser welding: Laser welding is a welding process that uses a laser beam, generated by focusing a high-power coherent monochromatic photon stream, as the heat source. This type of welding method typically includes continuous-power laser welding and pulsed-power laser welding. The advantage of laser welding is that it does not require a vacuum environment, while its disadvantage is that its penetration power is weaker than that of electron beam welding. Laser welding allows for precise control of energy, thereby enabling the welding of precision micro-devices. It can be applied to many metals, especially helping with the welding of some difficult-to-weld metals and dissimilar metals. 4. Brazing: The energy source for brazing can be the heat from a chemical reaction, or it can be indirect heat energy. It uses a metal with a melting point lower than that of the materials to be welded as a filler metal; upon heating, the filler metal melts, and through capillary action it is drawn into the gaps at the contact surfaces of the joints. This allows the filler metal to wet the surfaces of the metals being welded, enabling interdiffusion between the liquid and solid phases to form a welded joint. Therefore, brazing is a welding method that involves both a solid phase and a liquid phase. The brazing heating temperature is low; the base material does not melt, and no pressure needs to be applied. However, certain measures must be taken before welding to remove oil, dust, oxide films, and other contaminants from the surface of the workpieces to be welded. This is an important guarantee for achieving good wettability of the workpiece and ensuring joint quality. When the liquidus temperature of the filler metal is higher than 450°C and lower than the melting point of the base metal, it is referred to as hard brazing ； Below 450°C, it is called soft soldering. Depending on the heat source or heating method, brazing can be classified into flame brazing, induction brazing, furnace brazing, dip brazing, resistance brazing, etc. During brazing, since the heating temperature is relatively low, the impact on the properties of the workpiece material is minimal, and the stress deformation of the welded joint is also small. However, the strength of brazed joints is generally low, and their heat resistance is poor. Brazing can be used to weld metal materials such as carbon steel, stainless steel, superalloys, aluminum, and copper, as well as to join different types of metals or metals with non-metals. It is suitable for welding joints that are not subject to high loads or operate at room temperature, and is particularly appropriate for precise, miniature, and complex multi-brazed components. 5. Other welding methods: These welding methods are specialized to varying degrees, and their scope of application is relatively narrow. It mainly includes electroslag welding and high-frequency welding, which use resistive heat as an energy source ； Oxyacetylene welding, gas pressure welding, and explosive welding that use chemical energy as the welding source ； Friction welding, cold pressure welding, ultrasonic welding, and diffusion welding, which use mechanical energy as the welding energy source. (1) Electroslag welding: As mentioned earlier, electroslag welding is a welding method that uses the resistive heat of slag as its energy source. The welding process is carried out in the vertical welding position, within the assembly gap formed by the end faces of the two workpieces and the water-cooled copper sliders on both sides. During welding, electrical current is used to generate resistive heat in the slag, thereby melting the ends of the workpiece. Based on the shape of the electrode used during welding, electroslag welding is divided into wire-electrode electroslag welding, plate-electrode electroslag welding, and nozzle-electrode electroslag welding. The advantages of electroslag welding are: it can weld workpieces with large thicknesses (from 30 mm to over 1000 mm) and offers high productivity. It is mainly used for welding butt joints and T-joints on cross-sections. Electroslag welding can be used for welding various steel structures, as well as for assembling castings. Due to the slow heating and cooling processes, electroslag welded joints have a wide heat-affected zone, a coarse microstructure, and low toughness; therefore, normalizing treatment is generally required after welding. (2) High-frequency welding: Resonant frequency welding uses solid resistive heat as its energy source. During welding, the resistive heat generated within the workpiece by high-frequency current is used to heat the surface layer of the welding area to a molten state or a plastic state close to it; subsequently, a upsetting force is applied (or not) to achieve the bonding of the metals. Therefore, it is a solid-state resistance welding method. High-frequency welding can be divided into contact high-frequency welding and induction high-frequency welding, based on the way in which high-frequency current generates heat in the workpiece. When in contact with high-frequency welding, the high-frequency current is transmitted into the workpiece through mechanical contact with it. During induction high-frequency welding, the high-frequency current generates an induced current within the workpiece through the coupling effect of the external induction coil around it. High-frequency welding is a highly specialized welding method that requires specialized equipment depending on the product. It features high productivity, with a welding speed of up to 30 m/min. It is mainly used for welding the longitudinal or spiral seams when manufacturing pipes. (3) Gas welding: Gas welding is a welding method that uses a gas flame as the heat source. The most commonly used is the oxygen-acetylene flame fueled by acetylene gas. Due to its simple design, it is easy to operate; however, gas welding has a slower heating rate and lower productivity, results in a larger heat-affected zone, and is prone to causing significant deformation. Oxyacetylene welding can be used to weld many ferrous metals, non-ferrous metals, and alloys. It is generally suitable for maintenance and welding of individual thin sheets. (4) Gas pressure welding: Similar to gas welding, gas pressure welding also uses a gas flame as the heat source. During welding, the ends of the two pieces to be joined are heated to a certain temperature, and then sufficient pressure is applied to achieve a strong joint. It is a type of solid-state welding. No filler metal is used in gas pressure welding, which is commonly applied to rail welding and rebar welding. (5) Explosive welding: Explosive welding is another solid-state welding method that uses the heat generated by chemical reactions as its energy source. But it uses the **energy generated by an explosion to achieve metal bonding. Under the effect of the shock wave, two pieces of metal can be accelerated and brought into collision within less than a second, resulting in their fusion. Among various welding methods, explosion welding has the widest range of combinations of dissimilar metals that it can weld. Explosive welding can be used to weld two metallurgically incompatible metals together to form various transition joints. Explosive welding is commonly used for cladding flat plates with a relatively large surface area, and it is an efficient method for manufacturing composite sheets. (6) Friction welding: Friction welding is a solid-state welding process that uses mechanical energy as its power source. It uses the heat generated by mechanical friction between two surfaces to achieve the joining of metals. In friction welding, heat is concentrated at the joint surface, resulting in a narrow heat-affected zone. Pressure must be applied between the two surfaces; in most cases, this pressure is increased at the end of heating, so that the hot metal is forged together, with the joint surface generally not melting. Friction welding offers high productivity, and in principle, almost all metals that can be hot-forged can be friction welded. Friction welding can also be used for welding dissimilar metals. It is suitable for workpieces with a circular cross-section and a maximum diameter of 100 mm. (7) Ultrasonic welding: Ultrasonic welding is also a solid-state welding method that uses mechanical energy as its power source. During ultrasonic welding, the workpieces to be welded are subjected to a low static pressure; the high-frequency vibrations generated by the transducers cause intense friction at the joint surface, which heats it to the welding temperature and thus enables bonding. Ultrasonic welding can be used to weld most metal materials, enabling the welding of metals, dissimilar metals, as well as metals and non-metals. It can be used for the repeated production of metal wire, foil, or thin sheet metal joints of 2–3 mm or less. (8) Diffusion welding: Diffusion welding is generally a solid-state welding method that uses indirect thermal energy as its power source. It is usually carried out under vacuum or a protective atmosphere. During welding, the surfaces of the two workpieces to be joined are brought into contact under high temperature and significant pressure, and held at that state for a certain period of time so that the atomic distances become sufficient; thereafter, the atoms diffuse into one another and bond together. Before welding, it is necessary not only to remove impurities such as oxides from the surface of the workpiece, but also to ensure that the surface roughness is below a certain value in order to guarantee welding quality. Diffusion welding has almost no adverse effect on the properties of the materials to be welded. It can weld many identical and different metals, as well as some non-metallic materials such as ceramics. Diffusion welding can be used to weld complex structures as well as workpieces with large differences in thickness. III. Selection of Welding Methods 1. Product Characteristics (1) Product Structure Types Welded products can be roughly divided into four major categories based on their structural characteristics. 1) Structural types such as bridges, construction projects, petrochemical containers, etc. 2) Institutional parts, such as automotive components. 3) Semi-finished products such as I-beams, pipes, etc. 4) Microelectronic devices. Due to differences in factors such as the length and shape of the welds as well as the welding position, these products with different structures require different welding methods. Submerged arc welding is suitable for regular long welds and circumferential welds in structural products. Glove welding is used for root welding and welding short seams. Joints in mechanical products are generally short; depending on the required accuracy, gas shielded welding (for general thicknesses), electroslag welding, gas-electric welding (suitable for vertical welding of heavy components), resistance welding (for thin sheets), friction welding (for circular cross-sections), or electron beam welding (for applications requiring high precision) are used. The weld joints of semi-finished products are often regular, making it appropriate to use welding methods suitable for mechanization, such as submerged arc welding, gas shielded arc welding, and high-frequency welding. The joints of microelectronic devices require sealing, electrical conductivity, and low heat exposure; therefore, electron beam welding, ultrasonic welding, diffusion welding, brazing, and capacitor discharge welding are suitable methods. As mentioned above, for products with different structures, there are usually several welding methods available; therefore, other characteristics of the product also need to be taken into consideration. (2) Workpiece thickness The thickness of the workpiece can, to a certain extent, determine the suitable welding method. Each welding method has a certain range of suitable material thicknesses due to the different heat sources used. Welding within the recommended thickness range makes it easier to control weld quality and maintain reasonable productivity. (3) Joint type and welding position: Depending on the usage requirements of the product, as well as the thickness and shape of the base material, the designed product can employ joint types such as butt joints, lap joints, and corner joints. Among them, the butt joint type is suitable for most welding methods. Brazing is generally only suitable for joining lap joints with a large contact area and a small material thickness. The positions of various joints in a product are often determined by the product’s structural requirements and stress conditions. These joints may need to be welded at different welding positions, including flat welding, vertical welding, horizontal welding, overhead welding, and all-position welding. Flat welding is the easiest and most common welding position; therefore, during welding, the product joints should be placed in this position as much as possible. This allows for the use of welding methods that can ensure good weld quality while also enabling high productivity, such as submerged arc welding and gas metal arc welding. For vertical weld joints, gas metal arc welding (for thin plates) and gas tungsten arc welding (for medium-thickness plates) are suitable; electroslag welding can be used when the plate thickness exceeds about 30 mm. (4) Properties of the base material 1) Physical properties of the base material: Physical properties such as the thermal conductivity, electrical conductivity, and melting point of the base material directly affect its weldability and welding quality. When welding metals with high thermal conductivity such as copper, aluminum, and their alloys, a welding method with high heat input and strong penetration capability should be chosen, so that the metal to be welded reaches a molten state in the shortest possible time while minimizing deformation of the workpiece. For metals with higher resistivity, resistance welding is more suitable. For heat-sensitive materials, care should be taken to choose welding methods with low heat input, such as laser welding and ultrasonic welding. For high-melting-point refractory metals such as molybdenum and tantalum, electron beam welding is an excellent welding method. For dissimilar metals with significant differences in physical properties, welding methods that are less likely to result in the formation of brittle interphase phases should be used, such as various solid-state welding methods and laser welding. 2) Mechanical properties of the base material: The mechanical properties of the material to be welded, such as strength, plasticity, and hardness, affect the smooth progress of the welding process. Metals with a narrow plasticity temperature range, such as aluminum and magnesium, cannot be welded using resistance spot welding; whereas low-carbon steel, with a wider plasticity temperature range, is easier to weld by this method. Similarly, metals with poor ductility are not suitable for cold welding, which involves significant plastic deformation. Similarly, in explosion welding, the materials to be welded are required to have sufficient strength and ductility, as well as the ability to withstand rapid deformation that occurs during the welding process. On the other hand, various welding methods have different degrees of influence on the microstructure and mechanical properties of the weld metal and the heat-affected zone; as a result, they also affect the performance of the products to varying degrees. The selected welding method should also facilitate achieving joints with mechanical properties similar to those of the base material by controlling heat input in order to regulate the penetration depth, fusion ratio, and heat-affected zone (which is important for controlling plastic deformation during solid-state welding). For example, in processes such as electroslag welding and submerged arc welding, the high heat input results in a decrease in the impact toughness of the welded joint. Furthermore, the heat-affected zone of the weld joint produced by electron beam welding is narrow; compared to conventional arc welding, such joints exhibit better mechanical properties as well as a smaller heat-affected zone. Therefore, electron beam welding is an excellent welding method for certain metals such as stainless steel or heat-treated parts. 3) Metallurgical properties of the base metal: Since the chemical composition of the base metal directly affects its metallurgical properties, it also influences the weldability of the material. Therefore, this is also an important factor that must be considered when choosing a welding method. The most commonly used ordinary carbon steels and low-alloy steels in industrial production can be welded using conventional arc welding methods. The higher the alloy content of steel, especially the carbon content, the worse its weldability tends to be, and the fewer welding methods that can be used. For more reactive non-ferrous metal materials such as aluminum, magnesium, and their alloys, CO2 arc welding and submerged arc welding are not suitable; instead, gas shielded welding methods should be used, such as tungsten inert gas arc welding and metal inert gas arc welding. For stainless steel, methods such as manual arc welding, tungsten inert gas welding, or gas tungsten arc welding can typically be used. In particular, TIG welding offers excellent protection, allows for easy control of the weld composition, and can meet the requirements regarding the corrosion resistance of the welds. For metals such as titanium and zirconium, which have a high gas solubility and tend to become brittle after welding, high-vacuum electron beam welding is the best choice. Furthermore, for metal materials containing a high amount of alloying elements, using different welding methods results in different fusion ratios of the weld, which in turn affects the chemical composition of the weld and thus its properties. Metals with high hardenability should be welded using a slow cooling rate, as this can reduce the tendency for cracking in the heat-affected zone. Resistance welding is not suitable for quenched steel; otherwise, the very fast cooling rate after welding may cause cracks in the weld joints. When welding certain precipitation-hardening stainless steels, electron beam welding can yield joints with good mechanical properties. For dissimilar metals with poor metallurgical compatibility that are difficult to weld using fusion welding, it is advisable to consider some welding methods that do not involve a liquid state, such as brazing, diffusion welding, or explosive welding as introduced in this volume. 2. Production conditions (1) Technical level When selecting a welding method for manufacturing a specific product, the design and technical capabilities of the manufacturer must be taken into account. Among these, the skill level of the welder is particularly important. It is usually necessary to train welders. This includes: manual operations, welding machine use, welding techniques, welding inspection, and welding management. For certain high-demand products such as pressure vessels, welders must undergo specialized training and assessment before repeating welding tasks. Manual arc welding requires certain skill in operating the welding equipment; especially when welding in positions such as vertical, overhead, or horizontal welding, the welder needs even higher levels of skill. Compared to manual shielded metal arc welding, manual tungsten inert gas welding requires welders to undergo longer training and possess more skilled and agile operational abilities. Submerged arc welding and gas metal arc welding are mostly mechanical or semi-automatic welding processes, and their operational requirements are relatively lower compared to manual arc welding. In electron beam welding and laser welding, due to the complexity of the equipment and auxiliary devices, a higher level of basic knowledge and operational skills is required. (2) Equipment: Each welding method requires specific welding equipment. These include: welding power supplies, mechanical systems for enabling mechanized welding, control systems, and other auxiliary equipment. The power of the power supply, the complexity of the equipment, costs, and so on all directly affect the economic efficiency of welding production; therefore, welding equipment is also an important factor that must be taken into consideration when choosing a welding method. Welding current sources fall into two main categories: AC current sources and DC sources. The structure of conventional AC arc welders is relatively simple and their cost is low. The equipment required for manual arc welding is the simplest; aside from a power supply, only a welding cable and a welding torch for holding the electrode are needed. It should be given priority. MIG welding requires mechanical equipment such as an automatic wire feed system and an automatic travel cart. In addition, there is also a gas supply system for delivering the shielding gas, a water supply system for cooling water, and welding torches, etc. Vacuum electron beam welding requires a high-voltage power supply, a vacuum chamber, and a specialized electron gun. Laser welding requires a laser with sufficient power as well as a focusing system. Therefore, both of these welding methods require specialized tooling and auxiliary equipment; such equipment is complex and has high power requirements, which results in higher costs as well. Due to the high voltage of electron beam welders and their X-ray radiation, certain safety measures as well as shielding devices to prevent X-ray radiation are also required. (3) Consumables for welding The consumables used during welding include: welding wire, electrodes or filler metal, flux, soldering flux, solder, shielding gas, etc. All types of shielded metal arc welding require certain consumable materials. Such as coated electrodes used in manual arc welding ； Submerged arc welding and gas shielded metal arc welding both require welding wire ； Electroslag welding requires a welding wire, nozzle, or plate electrode. In addition to electrodes (wires, etc.), both submerged arc welding and electroslag welding require fluxes with specific chemical compositions. In tungsten inert gas welding and plasma arc welding, tungsten electrodes, thoriated tungsten electrodes, or ceriated tungsten electrodes with very high melting points are used as non-melting electrodes. In addition, highly pure inert gases at higher prices are also required. In resistance welding, copper alloys with high electrical conductivity and hardness are typically used as electrodes, so as to provide high electrical conductivity during welding while also enabling them to withstand pressure and wear at high temperatures. IV. New Developments in Welding Technology: With the advancement of industry and science and technology, welding techniques continue to improve. The various sections of this manual only introduce those that are more mature. However, as time passes by at a rapid pace, it is impossible to update the manual contents in a timely manner to reflect the latest advancements in welding technology. To address this shortcoming, this section introduces the development trends in welding technology. 1. Increasing welding productivity is a key driving force behind the development of welding technology. There are two ways to improve productivity. The first is to increase the welding deposition rate. Techniques such as iron powder electrodes, center-of-gravity electrodes, and lying electrodes in manual welding, as well as multi-wire welding and hot-wire welding in submerged arc welding, all fall into this category and yield significant results. For example, in triple-wire submerged arc welding, the process parameters are 2200A×33V, 1400A×40V, and 1100A×45V respectively. A groove with a small cross-section is used, and a shield or backing is employed on the back side; steel plates of 50–60 mm thickness can be fully welded in one pass, with a welding speed of over 0.4 m/min. Its deposition efficiency is more than 100 times higher than that of manual welding. The second approach is to reduce the groove cross-section and the amount of deposited metal; the most significant achievement in the past decade has been narrow-gap welding. Narrow-gap welding is based on gas shielded welding, using single-wire, double-wire, or triple-wire methods for welding. The butt joint style can be used regardless of the joint thickness. For example, the thickness of the steel plate ranges from 50 to 300 mm, and the gap can be set at around 13 mm; as a result, the amount of weld metal required is reduced by several times or even dozens of times, thereby **increasing productivity**. The main technical challenge in narrow-gap welding is how to ensure penetration on both sides and to keep the arc center automatically aligned with the center line of the groove. To address these two problems, countries around the world have developed various solutions, resulting in a wide range of narrow-gap welding methods. In electron beam welding, plasma welding, and laser welding, butt joints can be used without the need for beveling; hence, it is an ideal method for welding narrow gaps, which is one of the key reasons why it has received widespread attention. 2. Improving the level of mechanization and automation in preparation workshops is a key development direction in the world’s advanced industries today. To enhance the efficiency and quality of welded structure production, relying solely on welding techniques has certain limitations. Therefore, countries around the world attach great importance to the technical transformation of workshops. The main processes in the preparation workshop include: material transportation ； Degreasing, sandblasting, and applying protective paint to the material surface ； Steel plate marking, cutting, and beveling ； Component assembly and spot welding. All of the above four processes have been fully mechanized and automated in modern factories. Its advantages lie not only in improving productivity but, more importantly, in enhancing product quality. For example, with the use of computer numerical control technology (CNC) for tasks such as marking steel plates (including marking the centering points and lines during assembly), cutting, and beveling, the dimensional accuracy of the components improves significantly, while the surface roughness of the bevels is greatly reduced. During assembly, the entire structure can be approached using methods similar to those used for assembling mechanical parts; as a result, the geometric dimensions of the grooves are quite accurate. After automatic welding, the entire structure is neat, precise, and aesthetically pleasing, completely eliminating the outdated manual operations that were common in previous riveting and welding workshops. 3. The automation and intelligence of the welding process are important approaches for improving the stability of welding quality and addressing poor working conditions. Given the strict requirements for welding quality and often unfavorable working conditions, automation and intelligence receive special attention. The emergence of robots was quickly met with enthusiastic response from the welding industry. Currently, over 50% of robots worldwide are used in welding technology. It was initially used in the spot welding assembly lines of the automotive industry, and in recent years it has been extended to the field of arc welding. Although a robot is a highly automated device, from the perspective of automatic control, it remains an open-loop control system driven by programs. Therefore, it is impossible to make timely adjustments based on the specific conditions during welding. For this reason, intelligent welding has become a focus of attention in the welding industry today. The first focus of development in intelligent welding lies in the vision system. The visual systems that have been developed enable robots to automatically adjust the trajectory of the welding torch based on the specific conditions during welding; some of them can even adjust the welding process as appropriate according to the size of the groove. However, overall, intelligence is still in its early stages, and development in this area will be a long-term task. 4. The development of emerging industries continues to drive advancements in welding technology. Welding technology has a history of over a hundred years since its invention, and it is now capable of meeting the manufacturing needs for all important products in modern industry, such as those used in the aviation, aerospace, and nuclear energy sectors. However, the development of emerging industries continues to drive welding technology forward in order to meet their needs. For example, the development of the microelectronics industry has promoted the advancement of micro-connection processes and equipment. Similarly, the development of ceramic materials and composite materials has promoted the advancement of brazing, vacuum diffusion welding, spraying, and bonding processes, giving them greater vitality and lifting them to a new level. 5. The research and development of heat sources are the fundamental driving force behind the advancement of welding processes. Welding processes make use of almost all available heat sources in the world, including flames, arcs, resistance heat, ultrasound, friction, plasma, electron beams, laser beams, microwaves, and so on. Throughout history, the emergence of each new heat source has been accompanied by the development of new welding techniques. However, to this day, research and development on welding heat sources have not ceased. The new developments can be summarized in two aspects. On the one hand, it involves improving existing heat sources to make them more efficient, convenient, and cost-effective. In this regard, significant progress has been made in electron beam, especially laser beam welding. On the other hand, it is to develop better and more efficient heat sources. For example, in recent studies, many approaches have employed the combination of two heat sources to achieve a higher energy density, such as adding a laser to a plasma beam or adding a laser to an arc. 6. Energy-saving technologies are a matter of widespread concern, and they represent one of the important areas in the welding industry as well. As is well known, welding processes consume a large amount of energy; for example, manual welders require around 20 kVA per unit, submerged arc welders need about 60 kVA per unit, while resistance welders can require up to several thousand kVA per unit. Many new technologies have been developed to achieve this energy-saving goal. In resistance spot welding, thanks to advances in electronic technology, AC spot welding has been replaced by secondary rectified spot welding, which allows the welding machine’s capacity to be reduced to 200 kVA while still achieving the same welding results. The emergence of inverter welders over the past 10 years is another successful example. Of course, inverter welding machines not only help save electrical energy and improve the power factor, but more importantly, they can significantly reduce the size and weight of the welding machine. In summary, as illustrated by the above introduction, welding technology continues to evolve. We hope that through this brief overview, readers will be able to understand how to discern the significance of the various new processes available today, and how to choose the right ones for development.