Process analysis in laser welding technology

1. Process parameters of laser welding:

　　 1. Power density. Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point within a microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processing, such as punching, cutting, and engraving. For lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, which makes it easy to form a good fusion weld. Therefore, in conductive laser welding, the power density is in the range of 104~106W/cm2.

　　2, laser pulse waveform. The laser pulse waveform is an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam hits the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. During a laser pulse, the reflectivity of the metal changes greatly.

　　 3. Laser pulse width. Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter different from material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.

　　 4. The influence of defocusing amount on welding quality. Laser welding usually requires a certain amount of defocus, because the power density in the center of the spot at the laser focus is too high and it is easy to evaporate into a hole. On each plane away from the laser focus, the power density distribution is relatively uniform. There are two defocusing methods: positive defocus and negative defocus. If the focal plane is above the workpiece, it is a positive defocus, otherwise it is a negative defocus. According to geometrical optics theory, when the positive and negative disjunctions are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. When the defocus is negative, a greater penetration depth can be obtained, which is related to the formation process of the molten pool. Experiments have shown that the laser heating 50~200us material begins to melt, forming liquid metal and vaporizing, forming city pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor causes the liquid metal to move to the edge of the molten pool, forming a depression in the center of the molten pool. When the defocus is negative, the internal power density of the material is higher than that of the surface, and it is easy to form stronger melting and vaporization, so that the light energy can be transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocus is used; when welding thin materials, positive defocus should be used.

　　 2. Laser welding process method:

　　 1. Welding between slices. Including four process methods: butt welding, end welding, center penetration fusion welding, center perforation fusion welding.

　　2. Wire and wire welding. Including wire-to-wire butt welding, cross welding, parallel lap welding, T-shaped welding and other 4 process methods.

　　3. Welding of metal wire and block components. Laser welding can successfully realize the connection between the metal wire and the block element, and the size of the block element can be arbitrary. Attention should be paid to the geometric dimensions of the wire-like components during welding.

　　 4. Welding of different metals. Welding different types of metals must solve the range of weldability and weldability parameters. Laser welding between different materials is only possible with certain material combinations. Laser brazing Some components are not suitable for laser welding, but a laser can be used as a heat source to perform soldering and brazing, which also has the advantages of laser welding. There are many ways to use brazing. Among them, laser soldering is mainly used for the welding of printed circuit boards, especially for chip component assembly technology.

　　 3. Compared with other methods, laser soldering has the following advantages:

　　1. Because of the local heating, the components are not easy to be thermally damaged, and the heat-affected zone is small, so soldering can be performed near the heat-sensitive components.

　　2. It uses non-contact heating to melt the bandwidth without any auxiliary tools. It can be processed after the double-sided components are equipped on the double-sided printed circuit board.

　　3. Good stability for repeated operation. Flux has little pollution to welding tools, and the laser irradiation time and output power are easy to control, and the laser brazing yield is high.

　　4. The laser beam is easy to split light, and can be divided into time and space with optical elements such as half-lens, mirror, prism, scanning mirror, etc., which can realize multi-point simultaneous symmetric welding.

　　5. Laser brazing mostly uses a laser with a wavelength of 1.06um as a heat source, which can be transmitted by optical fiber, so it can be processed in parts that are not easy to weld by conventional methods, and has good flexibility.

　　6. Good focus, easy to realize the automation of multi-station devices.

　　4. Laser deep penetration welding:

　　 1. Metallurgical process and process theory. The metallurgical physical process of laser deep penetration welding is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the "small hole" structure. Under the irradiation of a sufficiently high power density beam, the material evaporates to form small holes. This steam-filled small hole is like a black body, which absorbs almost all the energy of the incident light, and the equilibrium temperature in the cavity reaches about 25,000 degrees. Heat is transferred from the outer wall of this high-temperature cavity, melting the metal surrounding the cavity. The small hole is filled with high-temperature steam produced by continuous evaporation of the wall material under the irradiation of the light beam. The four walls of the small hole are surrounded by molten metal, and the liquid metal is surrounded by solid materials. The liquid flow outside the pore wall and the surface tension of the wall layer are maintained in a dynamic balance with the continuously generated steam pressure in the cavity. The light beam continuously enters the small hole, and the material outside the small hole is continuously flowing. As the beam moves, the small hole is always in a stable state of flow. That is to say, the small hole and the molten metal surrounding the hole wall move forward with the advance speed of the leading light beam, and the molten metal fills the gap left by the small hole after it is removed and condenses, and the weld is formed.

2. Influencing factors. Factors affecting laser deep penetration welding include: laser power, laser beam diameter, material absorptivity, welding speed, shielding gas, lens focal length, focal position, laser beam position, and laser power at the start and end points of welding. , Gradually drop control.

　　3, the characteristics of laser deep penetration welding:

　　 Features: (1) High aspect ratio. Because the molten metal is formed around the cylindrical high-temperature steam cavity and extends to the workpiece, the weld becomes deep and narrow. (2) Minimum heat input. Because the temperature of the source cavity is very high, the melting process takes place extremely fast, the heat input to the workpiece is extremely low, and the thermal deformation and heat-affected zone are small. (3) High density. Because the small holes filled with high-temperature steam are conducive to the stirring of the welding pool and the escape of gas, resulting in the formation of non-porous penetration welding. The high cooling rate after welding is easy to make the welded seam microstructure. (4) Strengthen the weld. (5) Precise control. (6) Non-contact, atmospheric welding process.

4. Advantages of laser deep penetration welding: (1) Because the focused laser beam has a much higher power density than conventional methods, the welding speed is faster, the heat-affected zone and deformation are small, and it can also be used for difficult welding of titanium, quartz, etc. Material. (2) Because the beam is easy to transmit and control, and there is no need to frequently replace the welding torch and nozzle, it significantly reduces the auxiliary time of shutdown, so the load factor and production efficiency are high. (3) Due to the purification effect and high cooling rate, the weld is strong and the overall performance is high. (4) Due to the low balance heat input and high processing accuracy, reprocessing costs can be reduced. In addition, the moving cost of laser welding is relatively low, which can reduce production costs. (5) It is easy to realize automation, and can effectively control the beam intensity and fine positioning.

　　5. Laser deep penetration welding equipment: Laser deep penetration welding usually uses continuous wave CO2 lasers. This type of laser can maintain a high enough output power, produce a "small hole" effect, penetrate the entire section of the workpiece, and form a strong welded joint. As far as the laser itself is concerned, it is just a device that can generate a parallel beam that can be used as a heat source and has good directivity. If it is directed and shot to the workpiece after effective processing, its input power will have strong compatibility, making it better adapted to the automated process. In order to effectively implement welding, the laser and other necessary optical, mechanical and control components together form a large welding system. This system includes lasers, beam delivery components, workpiece loading and unloading and moving devices, as well as control devices. This system can be simply manual handling and fixing of the workpiece by the operator, or it can include the automatic loading, unloading, fixing, welding and inspection of the workpiece. The overall requirement for the design and implementation of this system is to obtain satisfactory welding quality and high production efficiency.

　　 5. Laser welding of steel materials:

　　 1. Laser welding of carbon steel and ordinary alloy steel. Generally speaking, the laser welding effect of carbon steel is good, and the welding quality depends on the impurity content. Just like other welding processes, sulfur and phosphorus are sensitive factors for welding cracks. In order to obtain satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%. When steels with different carbon content are welded to each other, the welding torch can be slightly biased to the side of the low-carbon material to ensure the quality of the joint. Low-carbon boiling steel is not suitable for laser welding due to its high content of sulfur and phosphorus. Low-carbon killed steel has a very good welding effect due to its low impurity content. Both medium and high carbon steel and ordinary alloy steel can be well laser welded, but preheating and post-welding treatment are required to eliminate stress and avoid crack formation.

　　2. Laser welding of stainless steel. In general, stainless steel laser welding is easier to obtain high-quality joints than conventional welding. Since the heat-affected zone of high welding speed is small, sensitization does not become an important issue. Compared with carbon steel, the low thermal conductivity of stainless steel makes it easier to obtain deep penetration narrow welds.

　　 3. Laser welding between different metals. The extremely high cooling rate and small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after melting of many different metals. It has been proved that the following metals can be successfully welded by laser deep penetration: stainless steel ~ low carbon steel, 416 stainless steel ~ 310 stainless steel, 347 stainless steel ~ HASTALLY nickel alloy, nickel electrode ~ cold forged steel, bimetallic strips with different nickel content.