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The main applications of laser welding in industry Laser welding is one of the important aspects of the application of laser material processing technology. In the 1970s, it was primarily used for welding thin-walled materials and at low speeds. The welding process relied on heat conduction: the laser radiation heated the surface of the workpiece, and the heat from that surface spread inward through conduction. By controlling parameters such as the width, energy, peak power, and repetition frequency of the laser pulses, the workpiece could be melted to form a specific molten pool. Due to its unique advantages, it has been successfully applied to the precision welding of micro and small parts. The emergence of high-power CO2 and high-power YAG lasers has opened up new fields in laser welding. Deep penetration welding, which is based on the pore effect as its theoretical foundation, has found increasingly widespread use in industries such as machinery, automotive, and steel manufacturing. Compared to other welding techniques, the main advantages of laser welding are: 1. Fast speed, deep penetration, and minimal deformation. 2. It can be welded at room temperature or under special conditions, and the welding equipment is simple to set up. For example, since the laser relies on an electromagnetic field, the beam does not deviate ； Laser welding can be performed in a vacuum, air, and certain gas environments, and it is also possible to weld through glass or materials that are transparent to light beams. 3. It can weld refractory materials such as titanium and quartz, and performs well when welding dissimilar materials. 4. After laser focusing, the power density is high; when welding high-power devices, the depth-to-width ratio can reach 5:1, and in some cases even 10:1. 5. Micro welding is possible. After being focused, a laser beam can produce a very small spot that can be positioned with precision, making it suitable for the welding of tiny and small components in large-scale automated production. 6. It can weld areas that are difficult to access, enabling non-contact remote welding and offering great flexibility. Especially in recent years, the use of fiber-optic transmission technology in YAG laser processing techniques has led to a wider adoption and application of laser welding technology. 7. Laser beams allow for the spatial and temporal separation of light beams, enabling simultaneous processing with multiple beams as well as multi-station processing, thus providing the conditions for more precise welding. However, laser welding also has certain limitations: 1. It requires high precision in the assembly of the components to be welded, and the position of the beam on the workpiece must not deviate significantly. This is because after laser focusing, the spot size becomes extremely small, resulting in a narrow weld seam, and filler metal needs to be added. If the assembly accuracy of the workpiece or the beam positioning accuracy does not meet the requirements, it is easy to result in welding defects. 2. Lasers and their related systems are costly, requiring a large one-time investment. Laser welding heat conduction. Laser welding involves directing a high-intensity laser beam at the metal surface; through the interaction between the laser and the metal, the metal is melted to form a weld. During the interaction between a laser and metal, metal melting is just one of the physical phenomena that occur. Sometimes, light energy is not primarily converted into metal melting, but manifests in other forms such as vaporization or plasma formation. However, to achieve good fusion welding, it is necessary to make the melting of metal the primary form of energy conversion. To this end, it is necessary to understand the various physical phenomena that occur during the interaction between a laser and metal, as well as the relationship between these phenomena and the laser parameters. By controlling these laser parameters, it is possible to convert the majority of the laser energy into energy used for melting the metal, thereby achieving welding. Process parameters for laser welding. 1. Power density. Power density is one of the most critical parameters in laser processing. By using a high power density, the surface layer can be heated to its boiling point within microseconds, resulting in extensive vaporization. Therefore, high power density is advantageous for material removal processes such as drilling, cutting, and engraving. At lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point; before vaporization occurs at the surface, the bottom layer reaches its melting point, facilitating good fusion welding. Therefore, in conductive laser welding, the power density ranges from 104 to 106 W/CM2. 2. Laser pulse waveform. The laser pulse waveform is an important issue in laser welding, especially for welding thin sheets. When a high-intensity laser beam hits the surface of a material, 60–98% of the laser energy is reflected and lost from the metal surface, with the reflectivity varying depending on the surface temperature. During the duration of a laser pulse, the metal’s reflectivity changes significantly. 3. Laser pulse width. Pulse width is one of the important parameters in pulsed laser welding; it is not only a parameter that distinguishes this process from material removal and material melting, but it is also a key factor determining the cost and size of the processing equipment. 4. The influence of defocus amount on welding quality. Laser welding usually requires a certain distance, as the power density at the center of the light spot at the laser focus is too high, making it easy for the material to evaporate and form holes. On the planes away from the laser focus, the power density distribution is relatively uniform. There are two types of defocus: positive defocus and negative defocus. A focal plane located above the workpiece results in positive defocus, while the opposite case results in negative defocus. According to geometric optics theory, when the positive and negative separations are equal, the power density on the corresponding plane is approximately the same; in reality, however, the shape of the molten pool obtained is different. With negative defocus, a greater penetration depth can be achieved, which is related to the formation process of the molten pool. Experiments show that when the material is heated by a laser for 50–200 us, it begins to melt, forming a liquid metal phase; vaporization occurs, producing steam at atmospheric pressure which is ejected at extremely high speeds, emitting a bright white light. At the same time, the high-concentration gas causes the liquid metal to move toward the edge of the molten pool, creating a depression at the center of the molten pool. When there is negative defocus, the power density inside the material is higher than that at the surface, facilitating stronger melting and vaporization and enabling light energy to penetrate deeper into the material. Therefore, in practical applications, when a larger penetration depth is required, negative defocus is used ； When welding thin materials, positive defocus is advisable. Laser welding process methods 1. Welding between sheets. It includes four process methods: butt welding, end welding, center-penetrating fusion welding, and center-penetrating fusion welding. 2. Welding of wires together. It includes four welding methods: wire-to-wire welding, cross welding, parallel lap welding, and T-joint welding. 3. Welding of wire and solid component parts. Laser welding can successfully achieve the connection of wires and solid components, with the size of the solid components being arbitrary. Attention should be paid to the geometric dimensions of the wire components during welding. 4. Welding of different metals. When welding different types of metals, it is necessary to address weldability and the range of suitable welding parameters. Laser welding between different materials is only possible for certain specific combinations of materials. Laser brazing: For the connection of some components, laser welding is not suitable, but the laser can be used as a heat source for soft and hard brazing, offering the same advantages as laser welding. There are various methods of brazing, among which laser soft brazing is mainly used for welding printed circuit boards, and it is particularly useful in chip component assembly techniques. Laser soft soldering has the following advantages over other methods: 1. Since heating is localized, the components are less prone to thermal damage, and the heat-affected zone is small; therefore, soft soldering can be performed near heat-sensitive components. 2. It uses contactless heating to melt the solder ribbon; no additional tools are required, and it can be used to process components on both sides of a double-sided printed circuit board after they have been installed there. 3. Good stability in repeated operations. Flux causes little contamination of the welding tools, and the laser irradiation time and output power are easy to control, resulting in a high yield for laser brazing. 4. The laser beam can be easily split; optical elements such as half-reflectors, mirrors, prisms, and scanning mirrors can be used for temporal and spatial separation, enabling simultaneous symmetric welding at multiple points. 5. Laser brazing typically uses a laser with a wavelength of 1.06 um as the heat source, and it can be transmitted through optical fibers; therefore, it enables processing in areas that are difficult to weld using conventional methods, offering great flexibility. 6. It has good focus, making it easy to automate multi-station devices. Laser deep penetration welding 1. Metallurgical process and process theory. The metallurgical physical process of laser deep penetration welding is very similar to that of electron beam welding, namely the energy conversion mechanism is achieved through a \"hole\" structure. Under irradiation with a beam of sufficiently high power density, the material evaporates to form pores. This steam-filled pore acts like a black body, absorbing almost all of the energy from the incident light, with the equilibrium temperature inside the pore reaching around 25,000 degrees. Heat is transferred from the outer wall of this high-temperature cavity, causing the metal surrounding it to melt. The small hole is filled with high-temperature steam generated by the continuous evaporation of the wall material under the illumination of a light beam; the four walls of the hole enclose the molten metal, with solid material surrounding the liquid metal. The fluid flow outside the pore wall, together with the surface tension of the wall layer, counteracts the steam pressure generated continuously within the pore cavity, maintaining a dynamic equilibrium. The beam continuously enters the aperture, while the material outside the aperture is in continuous flow; as the beam moves, the aperture remains in a stable state of flow. In other words, the hole and the molten metal surrounding its walls move forward as the leading light beam advances; the molten metal fills the space left behind by the moving hole and then solidifies, thereby forming a weld. 2. Influencing factors. Factors that affect laser deep penetration welding include: laser power, laser beam diameter, material absorptivity, welding speed, shielding gas, lens focal length, focus position, laser beam position, and the control of gradual increases and decreases in laser power at the start and end of welding. 3. Characteristics and advantages of laser deep penetration welding. Features: (1) High aspect ratio. Since the molten metal forms around the cylindrical high-temperature steam chamber and extends toward the workpiece, the weld becomes deep and narrow. (2) Minimum heat input. Because the temperature of the source chamber is very high, the melting process occurs extremely rapidly; the heat input to the workpiece is very low, resulting in minimal thermal deformation and heat-affected zone. (3) High density. This is because the pores filled with high-temperature steam facilitate mixing of the weld pool and gas escape, resulting in a pore-free, fully penetrated weld. The high cooling rate after welding also tends to refine the weld microstructure. (4) Strengthen welds. (5) Precise control. (6) Contactless, atmospheric welding process. Advantages: (1) Since the focused laser beam has a much higher power density compared to conventional methods, welding speeds are fast, the heat-affected zone and deformation are minimal, and it is also possible to weld difficult-to-weld materials such as titanium and quartz. (2) Since light beams are easy to transmit and control, and there is no need to frequently replace welding torches or nozzles, the downtime for maintenance is significantly reduced, resulting in high load factors and production efficiency. (3) Due to purification and high cooling rates, the weld has high strength and excellent comprehensive properties. (4) Due to the low balanced heat input and high processing precision, reprocessing costs can be reduced. Additionally, the operating costs of laser welding are relatively low, which helps to reduce production costs. (5) It is easy to automate, and effective control over beam intensity and precise positioning is possible. 4. Laser deep penetration welding equipment. For laser deep penetration welding, a continuous-wave CO2 laser is typically used; such lasers are capable of maintaining a high enough output power to create a \"pocket\" effect, allowing penetration through the entire cross-section of the workpiece and resulting in a strong and durable weld joint. As for the laser itself, it is merely a device that can generate a parallel beam of light with good directionality, which can be used as a heat source. If it is directed and effectively processed before being directed at the workpiece, its input power exhibits strong compatibility, enabling it to adapt better to automated processes. To enable effective welding, the laser, along with other necessary optical, mechanical, and control components, together form a comprehensive welding system. This system includes a laser, beam transmission components, devices for loading, unloading, and moving the workpiece, as well as control devices. This system can either involve the operator simply manually handling and fixing the workpiece, or it can include automatic functions for loading, unloading, fixing, welding, and inspecting the workpiece. The overall requirements for the design and implementation of this system are to achieve satisfactory welding quality and high production efficiency. Laser welding of steel materials 1. Laser welding of carbon steel and ordinary alloy steel. Generally speaking, carbon steel performs well in laser welding, and its welding quality depends on the impurity content. Like other welding processes, sulfur and phosphorus are sensitive factors that cause welding cracks. To achieve satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%. When welding steels with different carbon contents together, the welding torch can be slightly tilted toward the low-carbon material to ensure joint quality. Low-carbon boiling steel is not suitable for laser welding due to its high sulfur and phosphorus content. Low-carbon mild steel achieves good welding results due to its low impurity content. Medium and high carbon steels, as well as ordinary alloy steels, can be welded well by laser welding, but preheating and post-weld treatment are required to relieve stress and prevent crack formation. 2. Laser welding of stainless steel. Under normal circumstances, stainless steel laser welding yields higher-quality joints more easily than conventional welding. Due to the high welding speed, the heat-affected zone is very small, so sensitization does not pose a significant problem. Compared to carbon steel, the lower thermal conductivity of stainless steel makes it easier to achieve deep, narrow welds. 3. Laser welding between different metals. The extremely high cooling rate and very small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after welding and melting. It has been proven that the following metals can be successfully laser deep penetration welded: stainless steel to low-carbon steel, 416 stainless steel to 310 stainless steel, 347 stainless steel to HASTALLY nickel alloys, nickel electrodes to cold-forged steel, and bimetallic strips with different nickel contents.