Laser welding is one of the important aspects of the application of laser material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process is heat-conducting type, that is, laser radiation heats the surface of the workpiece, and the surface heat is diffused to the inside through heat conduction, by controlling the width, energy, peak power and repetition frequency of the laser pulse. The parameters are such that the workpiece melts to form a specific molten pool. Due to its unique advantages, it has been successfully applied to precision welding of small and small parts. >> Into the colorful world of laser welding Â
The emergence of high-power CO 2 and high-power YAG lasers has opened up a new field of laser welding. The deep-fusion welding based on the small hole effect has been obtained, and it has been widely used in the industrial fields of machinery, automobile and steel.
Compared with other welding techniques, the main advantages of laser welding are: 1. Fast speed, large depth and small deformation. 2. It can be welded at room temperature or under special conditions, and the welding equipment is simple. For example, the laser passes through an electromagnetic field, and the beam does not shift; the laser can be soldered in vacuum, air, and a certain gas environment, and can be welded through glass or a material that is transparent to the beam. 3, can weld refractory materials such as titanium, quartz, etc., and can weld the opposite material, the effect is good. 4. After laser focusing, the power density is high. When welding high-power devices, the aspect ratio can reach 5:1, and the maximum can reach 10:1. 5. Micro-welding can be performed. The laser beam is focused to obtain a small spot, and can be accurately positioned, and can be applied to the mass welding of small and small workpieces that are automatically produced in large quantities. 6. It can weld non-accessible parts and perform non-contact long-distance welding, which has great flexibility. Especially in recent years, the optical fiber transmission technology has been adopted in the YAG laser processing technology, which has made the laser welding technology more widely popularized and applied. 7. The laser beam can easily realize beam splitting according to time and space, and can perform multi-beam simultaneous processing and multi-station processing, which provides conditions for more precise welding.
However, laser welding also has certain limitations: 1. It requires high precision assembly of the weldment, and requires that the position of the beam on the workpiece cannot be significantly offset. This is because the spot size of the spot is small after the laser is focused, and the weld is narrow, which is filled with a metal material. If the workpiece assembly accuracy or beam positioning accuracy does not meet the requirements, it is easy to cause soldering defects. 2. The cost of the laser and its related systems is relatively high, and the one-time investment is large.
Laser welding heat conduction.
Laser welding is to radiate a high-intensity laser beam to a metal surface, and the metal is melted to form a weld by the interaction of the laser with the metal. Metal melting is only one of the physical phenomena during the interaction of laser and metal. Sometimes light energy is not mainly converted into metal melting, but is expressed in other forms such as vaporization, plasma formation, and the like. However, to achieve good fusion welding, the metal must be melted into the primary form of energy conversion. To this end, it is necessary to understand the various physical phenomena generated by the interaction between laser and metal and the relationship between these physical phenomena and the laser parameters, so that by controlling the laser parameters, most of the laser energy is converted into the energy of metal melting to achieve the welding. purpose.
Process parameters for 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 in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is advantageous for material removal processing such as punching, cutting, and engraving. For lower power density, the surface temperature reaches the boiling point and it takes several milliseconds. Before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion weld. Therefore, in conducted laser welding, the power density is in the range of 104 to 106 W/cm2.
2. Laser pulse waveform.
Laser pulse waveforms are an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam is incident on the surface of the material, the metal surface will be reflected by 60 to 98% of the laser energy, and the reflectance changes with the surface temperature. The reflectance of the metal changes greatly during the action of one laser pulse.
3. Laser pulse width.
Pulse width is one of the important parameters of pulsed laser welding. It is an important parameter that is different from material removal and material melting. It is also a key parameter that determines the cost and volume of processing equipment.
4. The influence of the amount of defocus on the welding quality.
Laser welding usually requires a certain amount of disengagement because the power density at the center of the spot at the laser focus is too high and it is easy to evaporate into holes. The power density distribution is relatively uniform across the planes that exit the laser focus.
There are two ways to defocus: positive defocusing and negative defocusing. The focal plane is located above the workpiece for positive defocusing, and vice versa for negative defocus. According to the theory of geometric optics, when the positive and negative separations are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. In the case of negative defocusing, a greater penetration can be obtained, which is related to the formation of the molten pool. Experiments show that the laser heating 50~200us material begins to melt, forming liquid phase metal and appearing vaporization, forming commercial pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor moves the liquid phase metal to the edge of the molten pool, forming a depression in the center of the molten pool. When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing should be used.
Laser welding process
1. Welding between sheets and sheets. It includes four methods: butt welding, end welding, center penetration fusion welding, and central piercing fusion welding.
2. Welding of wire and wire. Including wire and wire butt welding, cross welding, parallel lap welding, T-welding and other four methods.
3. Welding of wire and block components. The connection between the wire and the block element can be successfully achieved by laser welding, and the size of the block element can be arbitrary. The geometry of the filamentary elements should be noted in the welding.
4. Welding of different metals. Welding different types of metals requires addressing the range of weldability and weldability parameters. Laser welding between different materials is only possible with certain combinations of materials.
Laser brazing
Some components are not suitable for laser welding, but lasers can be used as a heat source for soldering and brazing, which also has the advantage of laser welding. There are many ways to use brazing, among which laser soldering is mainly used for soldering of printed circuit boards, especially for chip component assembly technology. Laser soldering has the following advantages over other methods:
1. Since it is localized heating, the component is not susceptible to thermal damage and the heat affected zone is small, so soldering can be performed near the heat sensitive component.
2, non-contact heating, melting bandwidth, without any auxiliary tools, can be processed on both sides of the double-sided printed circuit board.
3. Repeated operation is stable. The flux has little contamination to the welding tool, 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 realize the splitting, and the optical components such as the semi-lens, the mirror, the prism, the scanning mirror can be used for time and space division, and the multi-point simultaneous symmetric welding can be realized.
5. Laser brazing uses a laser with a wavelength of 1.06um as a heat source, which can be transmitted by optical fiber. Therefore, it can be processed in a place that is difficult to solder in a conventional manner, and has good flexibility.
6, good focus, easy to achieve automation of multi-station devices.
Laser deep penetration welding
1. Metallurgical process and process theory.
Laser deep penetration welding metallurgical physical processes are very similar to electron beam welding, that is, the energy conversion mechanism is completed by a "small hole" structure. Upon irradiation with a sufficiently high power density beam, the material evaporates to form small holes. This steam-filled hole is like a black body, almost all of which absorbs the energy of the incident light. The equilibrium temperature in the cavity is about 25,000 degrees. Heat is transferred from the outer wall of the high temperature cavity to melt the metal surrounding the cavity. The small holes are filled with high-temperature steam generated by continuous evaporation of the wall material under the irradiation of the light beam, and the small holes surround the molten metal, and the liquid metal surrounds the solid material. The liquid flow outside the pore wall and the surface tension of the wall layer are maintained and maintain a dynamic balance with the vapor pressure continuously generated in the pore cavity. The beam continuously enters the small hole, and the material outside the small hole flows continuously. As the beam moves, the small hole is always in a flowing stable state. That is, the small holes and the molten metal surrounding the walls of the holes move forward with the advancement speed of the leading beam, and the molten metal fills the gap left after the small holes are removed and condenses, and the weld is formed.
2. Influencing factors.
Factors affecting laser deep penetration welding include: laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, lens focal length, focus position, laser beam position, laser power rise at the start and end points of the weld Gradually control.
3. Features and advantages of laser deep penetration welding.
Features: (1) high aspect ratio. Because the molten metal forms around the cylindrical high temperature steam cavity and extends toward the workpiece, the weld becomes deep and narrow. (2) Minimum heat input. Because the source cavity temperature is very high, the melting process occurs extremely fast, the input workpiece heat 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 facilitate the welding pool agitation and gas escape, resulting in the formation of non-porous penetration welding. The high cooling rate after welding also makes the weld microstructure fine. (4) Strong welds. (5) Precise control. (6) Non-contact, atmospheric welding process.
Advantages: (1) Since the focused laser beam has a much higher power density than the conventional method, the welding speed is fast, the heat-affected zone and the deformation are small, and it is also possible to weld difficult-to-weld materials such as titanium and quartz. (2) Because the beam is easy to transmit and control, it does not need to change the torch and nozzle frequently, and the downtime auxiliary time is significantly reduced, so the load factor and production efficiency are high. (3) Due to the purification effect and high cooling rate, the weld seam is strong and the overall performance is high. (4) Due to the low balance heat input and high processing accuracy, the rework cost can be reduced. In addition, the cost of laser welding is relatively low, which can reduce production costs. (5) It is easy to automate and effectively control the beam intensity and fine positioning.
4. Laser deep penetration welding equipment.
Laser deep-fusion welding usually uses continuous-wave CO 2 lasers, which maintain a high enough output power to create a “small hole†effect that penetrates the entire workpiece section to form a tough welded joint.
As far as the laser itself is concerned, it is simply a device that produces a parallel beam of light that can be used as a heat source. If it is directed and effectively processed and then directed toward the workpiece, its input power is highly compatible, making it better suited to the automation process.
In order to effectively perform the welding, the laser and other necessary optical, mechanical and control components together form a large welding system. This system includes a laser, a beam delivery assembly, a loading and unloading and moving device for the workpiece, and a control device. This system can be simply manually handled and fixed by the operator, or it can be automatically loaded, unloaded, fixed, welded, and inspected. The general requirement for the design and implementation of this system is to achieve satisfactory weld quality and high production efficiency.
Laser welding of steel materials
1. Laser welding of carbon steel and common alloy steel.
In general, carbon steel laser welding has a good effect, and its welding quality depends on the impurity content. Like other welding processes, sulfur and phosphorus are sensitive factors in the production of weld cracks.
In order to obtain a satisfactory weld quality, preheating is required when the carbon content exceeds 0.25%. When steels with different carbon contents are welded to each other, the torch can be slightly biased to the side of 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 killed steel has good welding results due to low impurity content.
Medium and high carbon steels and common alloy steels can perform good laser welding, but require preheating and post-weld treatment to eliminate stress and avoid crack formation.
2. Laser welding of stainless steel.
In general, stainless steel laser welding is easier to obtain a quality joint than conventional welding. Sensitization does not become an important issue due to the high heat transfer zone of the welding speed. Compared to carbon steel, the low thermal conductivity of stainless steel makes it easier to obtain deep-welded welds.
3. Laser welding between different metals.
The extremely high cooling rate of laser welding and the small heat-affected zone create favorable conditions for the compatibility of materials with different structures after many different metal welds are melted. It has been proved that the following metals can be smoothly laser deep-welded: stainless steel ~ low carbon steel, 416 stainless steel ~ 310 stainless steel, 347 stainless steel ~ HASTALLY nickel alloy, nickel electrode ~ cold forged steel, bimetallic strip with different nickel content.
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