Various advantages and disadvantages of laser welding machineThe main advantages of laser welding compared to other welding techniques are:
1, fast, deep, small deformation.
2. It can be welded at room temperature or under special conditions. The welding equipment is simple. For example, if the laser passes through an electromagnetic field, the beam will not be deflected; the laser can be welded in vacuum, air, and certain gas environments, 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 sex material, the effect is good.
4. After the laser is focused, the power density is high. When the high-power device is welded, the aspect ratio can reach 5:1 and the maximum can reach 10:1.
5, can be micro-welding. After the laser beam is focused, it can obtain a very small spot and can be accurately positioned. It can be applied to the mass welding of micro- and small-sized workpieces in large-scale automated production.
6. Can weld inaccessible parts, perform non-contact long-distance welding, with great flexibility. Especially in recent years, the optical fiber transmission technology has been adopted in the YAG laser processing technology, and the laser welding technology has been widely promoted and applied.
7. The laser beam is easy to realize beam splitting according to time and space. It can perform multi-beam simultaneous processing and multi-position processing, providing conditions for more precise welding.
However, laser welding also has some limitations:
1. It is required that the assembly precision of the weldment is high and the position of the beam on the workpiece must not be significantly shifted. This is due to the fact that the spot size of the spot after the laser is focused is small and the weld seam is narrow. It is filled with metal materials. If the workpiece assembly accuracy or the beam positioning accuracy does not meet the requirements, it is very easy to cause welding defects.
2. The cost of lasers and related systems is high, and one-time investment is relatively large.
Laser welding heat conduction
Laser welding irradiates high-strength laser beams onto the metal surface, and the metal melts to form a weld through the interaction of the laser and the metal. In the laser-metal interaction process, metal melting is only one of the physical phenomena. Sometimes light energy is not primarily converted to metal melting, but is manifested in other forms, such as vaporization, plasma formation, and the like. However, to achieve good fusion welding, the metal must be melted to become the main form of energy conversion. To this end, it is necessary to understand the various physical phenomena generated in the interaction between laser and metal, and the relationship between these physical phenomena and laser parameters, so that by controlling the laser parameters, most of the laser energy is converted into the energy of metal melting to achieve welding. purpose.
Laser welding process parameters
1, power density
Power density is one of the most critical parameters in laser processing. With 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 drilling, cutting, and engraving. For lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point and easy to form a good fusion weld. Therefore, in the conduction laser welding, the power density is in the range of 104~106W/cm2.
2, laser pulse waveform
Laser pulse waveform is an important issue in laser welding, especially for thin-film welding. When a high-intensity laser beam hits the surface of the material, 60 to 98% of the laser energy on the metal surface is reflected and lost, and the reflectivity varies with the surface temperature. During a laser pulse, the reflectivity of the metal varies greatly.
3, the laser pulse width
Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter that distinguishes material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.
4. Influence of defocus amount on welding quality
Laser welding usually requires a certain amount of work to do, because the power density at the center of the laser spot at the laser spot is too high and it easily evaporates into holes. The power density distribution is relatively uniform on each plane leaving the laser focus.
There are two types of defocus: positive defocus and negative defocus. The focal plane above the workpiece is positively defocused, whereas the focal plane is negatively defocused. According to the theory of geometrical optics, the power density in the corresponding plane is approximately the same when positive and negative are equal to each other. However, the shape of the molten pool actually obtained is different. In the case of negative defocusing, a greater penetration depth can be obtained, which is related to the formation process of the bath. Experiments show that the laser heating 50 ~ 200us material began to melt, the formation of liquid metal and the emergence of vaporization, the formation of market pressure steam, and spray at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor moves the liquid metal to the edge of the bath, forming a depression in the center of the bath. When negative defocus, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting, vaporization, so that the light energy is transmitted deeper into the material. Therefore, in practical applications, negative defocusing is used when the required depth of penetration is greater, and positive defocusing is preferred when welding thin materials.
Laser welding process
1, the film and the film between the welding. Including butt welding, end welding, center penetration melting welding, center piercing fusion welding and other four kinds of process methods.
2, wire and wire welding. Including wire and wire butt welding, cross welding, parallel lap welding, T-type welding and other four kinds of process methods.
3, welding wire and block components. The use of laser welding can successfully achieve the connection of the wire and the block element, and the size of the block element can be arbitrary. Care should be taken in the welding of the geometry of the filamentary elements.
4, different metal welding. Welding different types of metals to solve the range of weldability and weldable parameters. Laser welding between different materials is only possible with certain combinations of materials.
The connection of some components is not suitable for laser welding. However, lasers can be used as a heat source to perform soldering and brazing. It also has the advantages of laser welding. There are many ways to adopt brazing, among which, laser brazing is mainly used for the soldering of printed circuit boards, and is particularly applied to the chip component assembly technology. The use of laser soldering has the following advantages over other methods:
1. Since it is a localized heating, the component is not prone to thermal damage, and the heat affected zone is small, so soldering can be performed near the thermal element.
2. Use non-contact heating to melt the bandwidth and do not need any auxiliary tools. The double-sided printed circuit board can be equipped with two-sided components after processing.
3, repeated operation stability. The flux has little pollution to the welding tool, and the laser irradiation time and output power are easy to control, and the laser brazing welding yield is high.
4, the laser beam is easy to achieve light, semi-lens, mirrors, prisms, scanning mirrors and other optical components for time and space segmentation, can achieve more simultaneous symmetry welding.
5. Laser brazing uses laser with a wavelength of 1.06um as a heat source and can be transmitted by optical fiber. Therefore, it can be processed in a place where it is not easy to weld in a conventional manner, and the flexibility is good.
6, good focus, easy to implement multi-position device automation.
Deep laser welding
1, metallurgical process and process theory
Laser deep fusion welding metallurgical physical processes and electron beam welding is very similar, that is, the energy conversion mechanism is completed through the "hole" structure. Under irradiation with a sufficiently high power density beam, the material evaporates to form small holes.
This steam-filled hole is like a black body and absorbs almost all of the energy of 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, melting 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 four walls of the small holes surround the molten metal, and the liquid metal surrounds the solid material. The liquid flow outside the hole wall and the surface tension of the wall layer are in keeping with the continuously generated steam pressure in the hole and maintain a dynamic balance. The light beam enters the small hole continuously, and the material outside the small hole continuously flows. As the light beam moves, the small hole is always in a steady state of flow. That is, the small holes and the molten metal surrounding the walls of the holes move forward with the advancement of the leading beam, the molten metal fills the gap left by the removal of the holes and then condenses, and the weld is formed.
2. Influencing factors
The factors that influence the laser deep penetration welding include: laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, focal length of the lens, focus position, laser beam position, laser power rising at the welding start and end points. , gradually drop control.
3, the characteristics of laser deep fusion welding and advantages
(1) High aspect ratio. Because the molten metal is formed around the cylindrical high-temperature steam chamber and extends toward the workpiece, the weld seam becomes deep and narrow.
(2) Minimum heat input. Because the source chamber temperature is high, the melting process takes place very quickly, 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 are conducive to melting the molten pool and gas evolution, resulting in the formation of non-porous penetration welding. The high cooling rate after welding makes it easy to miniaturize the weld microstructure.
(4) Strengthen the weld.
(5) Accurate control.
(6) Non-contact, atmospheric welding process.
(1) Since the focused laser beam has a much higher power density than the conventional method, the welding speed is high, the heat affected zone and deformation are small, and it is also possible to weld hard materials such as titanium and quartz.
(2) Because the light beam is easy to transmit and control, it is not necessary to frequently replace the welding torch and nozzle, and the downtime auxiliary time is significantly reduced, so the load coefficient and the production efficiency are high.
(3) Due to the purification and high cooling rate, the weld seam is strong and the overall performance is high.
(4) Since the balance heat input is low and the machining accuracy is high, the reprocessing cost can be reduced. In addition, the cost of laser welding is relatively low, which can reduce production costs.
(5) It is easy to be automated and can effectively control beam intensity and fine positioning.
4, laser deep fusion welding equipment
Continuous deep-wave CO2 lasers are usually used for laser deep penetration welding. Such lasers can maintain a sufficiently high output power, produce a "small hole" effect, penetrate the entire workpiece cross-section, and form strong weld joints.
As far as the laser itself is concerned, it is only a device that can produce a parallel light beam that can be used as a heat source and has good directivity. If it is directed and effectively directed toward the workpiece, its input power is strongly compatible, making it more adaptable to automated processes.
In order to carry out the welding effectively, the laser and other necessary optics, machinery and control components together form a large welding system. This system includes lasers, beam delivery assemblies, loading and unloading of the workpiece, and control devices. The system can be simply hand-carried and fixed by the operator, or it can include 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.
Laser welding of steel materials
1. Laser welding of carbon steel and ordinary alloy steel
In general, carbon steel laser welding is good, and its welding quality depends on the impurity content. Like other welding processes, sulfur and phosphorus are sensitive factors that produce weld cracks.
In order to obtain a satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%. When different carbon content steels are welded to each other, the torch can be slightly biased toward the low carbon material to ensure the quality of the joints.
Low-carbon steels are not suitable for laser welding because of their high sulfur and phosphorus content. Low-carbon killed steels have very good welding results due to low impurity content.
Medium- and high-carbon steels and ordinary alloy steels can be well laser welded but require preheating and post-weld treatment to eliminate stress and prevent crack formation.
2, stainless steel laser welding
In general, stainless steel laser welding makes it easier to obtain good joints than conventional welding. Sensitization does not become an important issue due to the small heat-affected zone of high welding speed. Compared with carbon steel, the low thermal conductivity of stainless steel makes it easier to obtain deep weld narrow welds.
3, laser welding between different metals
The extremely high cooling rate and small heat-affected zone of laser welding have created favorable conditions for the compatibility of materials with different structures after melting of different metals. It has been proved that the following metals can be used for laser deep penetration welding: stainless steel to low carbon steel, 416 stainless steel to 310 stainless steel, 347 stainless steel to HASTALLY nickel alloy, nickel electrode to cold forged steel, and bimetallic with different nickel contents.