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To eliminate or reduce the defects of laser welding, some other heat sources and laser hybrid welding processes are proposed, mainly including laser and arc, laser and plasma arc, laser and induction heat source hybrid welding, and multi-beam welding. In addition, various auxiliary process measures have been proposed, such as laser filler wire welding (which can be subdivided into cold wire welding and hot wire welding), external magnetic field-assisted enhanced laser welding, shielding gas controlled penetration laser welding, laser-assisted friction stir welding, etc.
The Principle And Heat Source Classification of Laser Arc Hybrid Welding
The Principle of Laser Arc Hybrid Welding
The laser arc hybrid welding technology was first proposed by the British scholar WM Steen in the late 1970s. The main idea is to effectively use the arc heat. The “arc” mentioned here mainly refers to the tungsten argon arc (TIG) and the melting argon arc. (MIG/MAG), also known as laser-TIG/MIG; hybrid welding technology, the “laser-MIG hybrid welding” equipment exhibited by Austrian Fronuis at the Essen International Welding Exhibition in 2001, has caused a great interest. In recent years, due to the needs of industrial production, it has gradually become the focus of attention of the international welding community and has received attention.
The laser arc hybrid welding technology is to simultaneously act on the welding area by laser and MIG arc. Through the mutual influence of laser and arc, a good composite effect is produced, and a larger welding penetration is obtained under a smaller laser power condition, and the laser is improved at the same time. The adaptability of the welding to the joint gap enables a high-efficiency and high-quality welding process. Figure 1.1 shows the principle of laser-arc hybrid welding and the typical weld cross-sectional shape.
The laser acts on the metal surface, and a photo plasma cloud is generated above the weld due to the action of the laser. The absorption and scattering of the incident laser by the plasma cloud will reduce the utilization rate of laser energy. After the arc is applied, the low-temperature and low-density arc plasma cause the photo-induced plasma to be diluted, and the laser energy transmission efficiency is improved; at the same time, the arc heats the base material, so that the temperature of the base material is increased, and the absorption rate of the base material to the laser is increased. The penetration increases.
The melts the metal provides free electrons for the arc, reduces the resistance of the arc channel, and increases the energy utilization rate of the arc. Finally, the total energy utilization rate is increased, and the penetration depth is further increased. When the laser beam passes through the MIG, its ability to penetrate metal is significantly enhanced than in the general atmosphere. The laser beam also focuses and guides the arc, which makes the arc more stable during the welding process.
In laser hybrid welding technology, the laser arc hybrid welding technology is more widely used. The main purpose is to effectively use the arc energy to obtain a larger penetration depth and reduce the assembly accuracy of laser welding. For example, laser TIG/MIG hybrid welding composed of laser and TIG/MIG arc can realize large penetration welding under the condition of lower laser power, and the heat input is greatly reduced compared with TIG/MIG arc.
When welding metal parts, the energy density of the laser beam output by the YAG laser is about 106W/cm2. When the laser beam hits the surface of the material, the heated surface immediately reaches the evaporation temperature and due to the action of the flowing metal vapor, pits are generated in the welded metal, can get a larger welding aspect ratio. The energy density of the MIG arc is slightly greater than 104W/cm2, which can obtain a wider weld with a small aspect ratio. From the principle of laser-arc hybrid welding [Figure 3.1(a)], it can be seen that the laser beam and the arc are combined in the same area of the welding place, and the two influence each other, which improves the energy utilization rate. The weld morphology of laser-MIG hybrid welding is shown in Figure 3.1(b), which is better than the welding effect of a single energy source.
When single laser welding, the diameter of the laser beam is small, and the groove assembly gap is required to be small. The welding seam tracking accuracy is required to be high, and the thermal efficiency is very low when the molten pool is not formed. The laser arc hybrid welding can just make up for these shortcomings, which can be reflected in the following aspects.
- Laser welding and MIG welding are combined, the width of the molten pool is increased, the requirement for groove assembly is reduced, and the welding seam is easy to track.
- The MIG welding arc first heats the surface of the weldment to form a molten pool, which can increase the absorption rate of laser radiation; the airflow of MIG welding can also protect the metal vapor excited by the laser beam; the liquid metal produced by the molten wire of MIG welding can fill the weld, avoid undercut.
- The plasma generated by the laser enhances the ignition and maintenance ability of the MIG arc, and the laser-MIG composite arc is more stable.
In short, the interaction between laser and MIG arc is complementary and strengthened, and better welding results can be obtained.
For example, at a welding speed of 2m/min, laser beams with a power of 0.2KW and a TIG arc with a welding current of 90A can be combined to weld a weld with a penetration depth of 1mm, which usually requires a laser beam with a power of 5KW. The same effect. In addition, when the continuous laser beam is 3~5mm away from the arc centerline, it can attract the arc and make it burn stably, which can increase the laser welding speed. The combination of laser and arc is not a simple superposition of the two welding processes. It not only enables the two heat sources to give full play to their respective advantages but also makes up for each other’s shortcomings, realizing the “1+1>2” synergistic effect, making it become one of the most promising high-efficiency welding technologies in industrial production.
Laser-arc Composite Heat Source Classification
The combination of laser and arc can give full play to the advantages of the two heat sources, and make up for each other’s shortcomings, forming a new, high-quality, high-efficiency, and energy-saving heat source. Under the same conditions, laser arc hybrid welding has stronger adaptability than single welding or TIG/MIG welding, and the weld formability is better. Laser arc hybrid welding has entered the stage of industrial application in developed countries such as Germany and Japan.
Lasers used in laser arc composite heat sources generally include CO2 gas lasers, YAG solid-state lasers, semiconductor lasers, and fiber lasers. According to the different types of arcs, laser and arc composite heat sources mainly include TIG composite, laser-MIG/MAG, laser-double composite, laser-plasma composite, and so on.
Laser-TIG Composite Heat Source
The earliest research on laser-arc composite heat sources started from the paraxial recombination of CO2 laser and non-melting electrode TIG. The composite process of laser and TIG arc is relatively simple. The beam and arc can be arranged coaxially or on a side axis. The angle between the beam, the size of the arc current and the input-form, the laser power, the direction of arrangement, the distance of the action, the height of the arc, the flow of shielding gas, etc. are the main factors that affect the effect of hybrid welding.
Figure 1.2 shows a schematic diagram of laser-TIG hybrid welding. The laser-TIG composite heat source can obtain a stable arc under fast welding conditions, and the weld seam is beautifully formed while reducing welding defects such as pores, inclusions, and undercuts. Especially at low current, high welding speed, and long arc, the welding speed of laser-TIG composite heat source can even reach more than twice that of single laser welding, which is difficult for conventional TIG welding. The laser-TIG arc composite heat source is mostly used for high-speed welding of thin plates, and can also be used for welding of butt welds of unequal thickness plates. Filler metal can be used when welding large gap plates.
Studies have shown that when the welding speed is 0.5-5m/min, the welding penetration with a 5KW laser beam and TIG arc is 1.3~2 times that of a 5KW laser beam alone, and there is no undercut or porosity in the weld. defect. After the arc composite laser, its current density has been significantly improved.
Laser-MIG/MAG Composite Heat Source
Laser-MIG/MAG hybrid welding is a widely used composite heat source welding method, which has been applied in the fields of automobile and shipbuilding. Laser MIG/MAG hybrid welding uses the advantages of MIG/MAG welding filler wire, which can improve weld metallurgy and structural properties while increasing welding penetration and adaptability.
Figure 1.3 shows a schematic diagram of laser-MIG/MAG hybrid welding. Because laser-MIG/MAG composite heat source welding has problems such as wire feeding and droplet transfer, its physical process is more complicated than laser-TIG or laser-PAW composite heat source welding, and most of them use side-axis composite welding.
Figure 1.4 shows two different types of laser MIG composite welding torch heads. Some companies specialize in the design and manufacture of laser-MIG/MAG composite welding torch heads. MIG welding wire and a shielding gas are fed into the welding area at a certain angle and obliquely. The welding wire melted by forms an axially transitioned droplet, and then the droplet and the base metal are heated and melted by the laser and arc together to form a welding pool. Due to the existence of the filler wire, it can increase the welding penetration, enhance the adaptability of the process and improve the weld structure and properties.
If the process parameters are set improperly, the welding wire and molten droplets can easily cause interference to the laser and affect the welding quality. If the irradiance of the laser on the surface of the workpiece reaches the critical irradiance of material vaporization, the pinhole effect and photo-induced plasma will be generated to realize the deep penetration welding process. Compared with laser-TIG hybrid welding, laser-MIG/MAG hybrid welding has a good application prospect, which can be welded with larger plate thickness and stronger welding adaptability. Especially because MIG/MAG arc has the advantages of strong directionality and cathode atomization, it is suitable for welding large-thickness plates and aluminum alloys, and other laser-resistant metals.
Laser-MIG hybrid welding uses the advantages of filler wire to improve the metallurgical properties and microstructure of the weld metal and is often used for welding medium and thick plates. Therefore, this method is mainly used in shipbuilding, pipeline transportation, and heavy-duty automobile manufacturing. In Germany, this composite technology has been developed to the practical stage. For example, the Fraunhofer Research Institute has developed a laser-MIG composite heat source welding oil storage tank welding system, which can effectively weld oil tanks with a thickness of 5-8mm.
Compared with laser-TIG or laser-PAW hybrid welding, laser MIG/MAG hybrid welding uses side-axis composite welding due to the presence of welding wire.
The structure can be welded with a larger current density, the cladding efficiency is higher, and a larger penetration depth and width can be obtained. Used for welding of thick and large plates, it is less sensitive to workpiece gaps, wrong edges, and moderate deviations, stronger adaptability, and higher welding efficiency. In addition, laser MIG/MAG hybrid welding can also add beneficial elements to the weld metal by selecting the appropriate welding wire to improve the metallurgical properties and microstructure of the weld, which can reduce the tendency of weld cracks and ensure impact toughness and strength. It is more suitable for welding high-strength structural steel, aluminum alloy, and other materials. It is based on these characteristics that laser-MIG/MAG hybrid welding has become a highly respected laser-arc hybrid heat source welding method at home and abroad.
Laser-double Arc Composite Heat Source
Laser-double arc hybrid heat source welding is a welding process that combines laser and two MIG arcs at the same time. Both MIG welding torches have independent power supply and wire feeding mechanisms and share the welding torch head through their own power supply system. Each MIG welding torch can be adjusted arbitrarily relative to the other welding torch and the position of the laser beam, as shown in Figure 1.5.
Since the three heat sources have to act in one area at the same time, the arrangement of each other is particularly important. To make the position of the hybrid welding head relative to the laser beam repositionable in the vertical direction, it is necessary to carefully consider the size of the MIG welding gun and the laser beam focus when researching and designing the test device.
For gapless joint welding, the welding speed of laser-double arc hybrid welding is about 30% higher than that of general laser-MIG hybrid welding, and about 80% higher than that of submerged arc welding. The heat input per unit length is about 25% less than that of conventional laser-MIG hybrid welding, and about 80% less than that of submerged arc welding, and the welding process is very stable, far exceeding the welding efficiency of conventional laser-MIG hybrid welding.
Since there are MIG heat sources before and after the laser, the limitation of the welding direction of a single laser-MIG composite heat source is avoided, and it is easier to realize the automatic welding of large thickness plates.
Laser-plasma Composite Heat Source
Plasma arc has the advantages of good rigidity, high temperature, strong directionality, good arc ignitability, narrow heating zone, and low sensitivity to the outside world, which is good for composite heat source welding. The application of plasma arc and composite for thin plate butt welding, unequal thickness plate connection, galvanized plate lap welding, aluminum alloy welding, cutting, and surface alloying have achieved good results.
The use of laser-plasma hybrid welding for high-speed welding of galvanized sheets with a thickness of 0.16 mm shows that the arc is very stable during welding, even at 90 m/min, and there will be no defects in welding alone. When laser welding alone is 48m/min, there will be arc instability and welding defects.
Like laser-TIG hybrid welding, laser-plasma (PAW) hybrid welding can be combined side-by-side or coaxially.
Features of Laser-arc Hybrid Welding
Laser-arc hybrid welding combines the advantages of the two independent heat sources of it (for example, the laser heat source has high energy density, excellent directivity, and the characteristics of transparent medium conduction; arc plasma has high heat-electricity conversion Low efficiency, low equipment cost and operating cost, mature technology development and other advantages), avoid the shortcomings of both (such as the loss of laser energy caused by the high reflectivity of metal materials to the laser, the high cost of laser equipment, and the low electricity- Light conversion efficiency, etc.; lower energy density of the arc heat source, poor arc stability during high-speed movement, etc.).
At the same time, the organic combination of the two has derived many new features (high energy density, high energy utilization, high arc stability, low tooling accuracy and surface quality of the workpiece to be welded, etc.), making it a great application Prospective new welding heat source.
It can be used for composite lasers; CO2 lasers, YAG lasers, semiconductor lasers, fiber lasers, etc. Can be used for composite welding arc heat sources: TIG, MIG, MAG, plasma arc, etc.
The above-mentioned laser and arc can be combined in any way without limitation to construct a composite heat source. Its composite technology has the following main features.
Arc Preheating Improves Laser Thermal Efficiency
The optical properties of metal materials are closely related to the test temperature. When the temperature rises, the metal’s absorption rate of laser energy increases nonlinearly. In the hybrid welding process, the arc heating causes the workpiece to heat up and melt, and the laser beam passes through the arc to directly act on the surface of the liquid metal, which greatly reduces the reflectivity of the workpiece to the infrared laser (especially the CO2 laser with a larger wavelength), and improves the workpiece Absorption rate of the laser.
In addition, the temperature and ionization degree of the arc plasma is relatively low, which has a diluting effect on the photo-induced plasma to reduce the electron number density, thereby reducing the absorption and refraction of the laser by the photo-induced plasma, and increasing the incidence on the surface of the workpiece. Laser energy. But this kind of influence is more complicated, when the welding current is large, it may have a negative effect.
In laser and arc hybrid welding, the TIG or MIG arc first melts the base material, and then irradiates the molten metal with a laser to increase the absorption rate of the base material to the laser, which can effectively use the arc energy and reduce the laser power. The interaction of it will increase the welding efficiency, and the welding speed can reach 9m/min. Due to the effect of the arc, a laser with a lower power can be used to achieve a good welding effect. Compared with laser welding, it can reduce production costs, meet the requirements of “high efficiency, energy-saving, and economy”, and can effectively use laser energy.
Improve Arc Heat Flux And Welding Stability
The laser energy density is extremely high, which causes the metal to evaporate during the welding process and forms a large amount of metal plasma, thereby providing a good conductive path for the arc, and has a strong attraction and contraction effect on the arc, which can reduce the arc ignition pressure and reduce the field strength. Enhance the stability of the arc. Due to the arc stabilization effect of the laser, the arc drift or arc breaking phenomenon is not easy to occur during the high-speed welding of the composite heat source, so that the entire welding process is very stable and the spatter is extremely small. Because the laser shrinks the arc, the heat flux density of the arc increases, and the laser compresses the arc root, further increasing the penetration depth.
When TIG or MIG alone, the welding arc is sometimes unstable, especially in the case of a small current, when the welding speed increases to a certain value, it will cause the arc to drift, making the welding process impossible. When using laser-arc hybrid welding technology, the plasma generated by the laser helps stabilize the arc, and the laser acts on the molten pool to form a keyhole, which attracts the arc and also increases the welding stability; and the keyhole will Compress the root of the arc, thereby increasing the utilization of arc energy. The composite arc increases the weld width (especially the MIG arc), reduces the heat source’s requirements for assembly accuracy, the amount of misalignment and the centering sensitivity of the joint gap, and reduces the workload of the workpiece butt processing and assembly. Welding is realized under a larger joint gap, which improves production efficiency.
The base material is melted to form a molten pool under the action of the electric sol, and the laser beam acts on the bottom of the arc to form the molten pool. The liquid metal has a high absorption rate of the laser beam, so the penetration depth of hybrid welding is greater than that of simple laser welding, which is equivalent to that of laser welding. Compared with laser welding at high power, the penetration depth of composite heat source welding can be doubled, especially in the welding of narrow gaps and large thick plates. When using laser arc hybrid welding, the arc can dive into the depth of the weld under the action of the laser. Reduce the amount of filler metal deposition and realize deep penetration welding of large and thick plates.
Improve Welding Efficiency And Reduce Costs
Due to the preheating effect of the arc, the absorption rate of the laser energy by the workpiece is increased, thereby increasing the weld penetration depth. In addition, the arc heat can also act on the interior of the workpiece through the small holes generated by the low light, which further increases the penetration depth. The comparison of the cross-sectional shape of the welding seam of the 6mm thick 1Cr18NigTi stainless steel plate with different welding processes (laser welding, MIG welding, and laser-MIG hybrid welding) is shown in Figure 1.6.
The interaction of laser and arc makes the energy effect of hybrid welding greater than the sum of the energy effects of two separate heat sources, and also makes laser-arc hybrid welding have obvious advantages compared to a single welding process. Under the condition of the same penetration depth, the welding speed can be increased by 1 to 2 times, which greatly improves the welding efficiency, reduces the requirements for laser power, and reduces equipment investment and production costs.
Combination of Laser And Arc
There are two ways to combine laser and electric arc welding. One is along the welding direction, the distance between the laser and the arc is large, and they are arranged in series before and after. The two-act as independent heat sources on the workpiece. The main purpose is to use the arc heat source to preheat or post heat the weld to improve the laser absorption rate. The purpose of improving the weld structure and performance.
Due to the short wavelength of solid-state lasers, it has unique advantages in material processing, especially welding processing (for example, the material has a high absorption rate of the laser, the beam is easy to transmit through the optical fiber, and it is easy to achieve welding flexibility and automation, etc.), so that the solid laser + Arc hybrid heat source welding has received more and more attention.
In production practice, laser-arc composite heat sources mostly use CO2 lasers and YAG lasers (semiconductor fiber lasers have been gradually promoted in recent years). According to the relative position of the laser and the arc, there are paraxial composites and coaxial composites, as shown in the figure. As shown in 1.7.
Paraxial recombination means that the laser beams and the arc act together at the same position of the workpiece at a certain angle, that is, the laser passes through the outside of the arc to reach the surface of the workpiece, and can be sent in front of the arc or behind the arc, as shown in the figure as shown in 1.8(a). Paraxial recombination is easier to achieve. It can use non-melting electrode tungsten argon arc welding (TIG) arc or molten electrode gas shielded welding (MIG) arc, which is currently a widely used method.
Coaxial recombination means that the laser and the arc coaxially act on the same position of the workpiece, that is, the laser passes through the center of the arc or the arc passes through the center of the ring laser beam to reach the surface of the workpiece, as shown in Figure 1.8 (b). Coaxial composite is more difficult and the process is more complicated, so TIG arc or PAW (Plasma Arc Welding, plasma arc welding) arc is often used.
The influence of the front and rear position of the laser
The relative position of the laser and the arc will affect the weld surface formation and microstructure properties. Studies have shown that the upper surface of the welding seam is uniformly formed and full and beautiful before the laser beam is in the arc, especially when the welding speed is high, the effect is more obvious; while the arc is in front of the laser beam, the upper surface of the welding seam will appear grooves.
By analyzing the composition and performance of the weld, it is known that the element content increases from the upper to the lower part of the weld in both cases. The hardness of the upper part of the weld before the arc by the laser is less than that of the lower part, and the hardness of the upper part of the weld after the laser is greater than the hardness of the lower part.
The reason for this is that when the arc is behind, the heat source has a large area of action, and the heat source is removed after the welding The slow cooling of the seam is conducive to the escape of the gas in the molten pool and good formation; after the arc heat source acts on the laser, it is equivalent to a tempering of the weld, and the heat cannot be transmitted to the deeper part of the weld, so the lower part is not tempered, So the hardness of the upper part of the weld is less than the lower part.
Not only does the difference between the front and back of the laser and the arc have an impact on the welding process, but the difference between the distance also has an impact on the welding process. The distance between them affects the droplet transfer of hybrid welding. The droplet transfer is very unstable during high-speed MIG welding. In laser MIG-hybrid welding, due to the thermal radiation effect of the laser-plasma on the droplet and the absorption of the arc The shape of it and the force state of the droplet have been changed, and the droplet transfer has been changed. Different welding currents have different optimal laser and arc distances. Under the optimal spacing, the droplet transfer form is a single stable jet transfer, the current and voltage are constant, and the weld seam is formed well.
In Figure 1.9(a), the arc is located in the middle of the two laser beams. After the YAG laser beam is output from the optical fiber, it is divided into two beams and refocused through a set of lenses. The electrode and the arc are placed under the lens, and the focus point of its radiation point is coinciding. At this time, 8 tungsten electrodes are used, which are uniformly distributed at 45° on a circular ring with a certain diameter.
The tungsten electrodes are powered by independent power sources. During the welding process, the two pairs of electrodes in the corresponding direction are controlled to work according to the direction of the torch movement, forming a heat source in the front and rear directions. Designing the hollow tungsten electrode to make the laser beam pass through the center of the circular is also a common method for coaxial recombination. It coaxial composite solves the problem of the directionality of the paraxial composite and is suitable for welding three-dimensional structural parts. The difficulty is that the negation of the welding gun is more complicated.
Changes in Voltage And Current of Laser-arc Hybrid Welding
The interaction between laser and arc forms a welding method that enhances adaptability. It avoids the shortcomings of a single welding method. It has the advantages of increasing energy, increasing penetration, stabilizing the welding process. It also reducing assembly requirements, and realizing welding of highly reflective materials. Many advantages.
Figure 1.10 shows the waveforms of arc voltage and welding current in pure TIG welding and laser-TIG hybrid welding. In Figure 1.10(a), the welding speed is 135cm/min and the TIG welding current is 100A. It can be seen that the arc voltage and welding current are significantly increased during laser-TIG hybrid welding. In Figure 1.10(b), the welding speed is 270cm/min, and the TIG welding current is 70A. It can be seen that the arc voltage and welding current are unstable in TIG welding alone. It is difficult to weld, while the voltage in TIG hybrid welding and the welding current are very stable, and welding can be carried out smoothly.
Laser wire welding refers to filling the welding seam with welding wire while performing laser welding. There are two purposes for adding welding wire: normal welding can still be carried out when the joint gap is not ideal so that the weld can be formed well. The second is to change the composition and structure of the weld so that the weld can meet certain performance requirements.
When using wire deep penetration welding, care should be taken not to add the welding wire too fast. It won’t damage the small holes in the molten pool. Experiments have shown that when using wire laser welding, under other welding conditions unchanged. The width of the weld is narrower than when no wire is added. This is because the melting of the filler wire consumes part of the light under the same heat input. Energy, the energy used to melt the base material is correspondingly reduced.