Essence of Laser Welding

Laser welding can be divided into pulsed laser spot welding and continuous laser welding (including high frequency pulsed continuous welding) according to the different ways of laser output energy. According to the power density of the focused spot, it can be divided into melt welding and keyhole welding. Melting welding is a process where the power density of laser spot is not high, and the surface of metal material will not exceed its boiling point when heated. After the absorbed laser energy is converted into heat energy, the workpiece is melted by heat conduction, and its penetration profile is approximately hemispherical. This process of heat transfer melting welding is similar to that of non-molten arc welding.

When the power density of laser spot is high enough, the metal is heated rapidly under laser irradiation. The surface temperature of the metal rises to the boiling point in a very short time, and the metal is gasified. Metal vapor leaves the surface of molten pool at a certain speed. An additional pressure reacts on the molten metal, causing it to sink downward, creating a small pit under the action of a laser spot. As the heating process proceeds, the laser can be directly injected into the bottom of the pit to form a slender hole. When the back pressing force of metal vapor is balanced with the tension and gravity of the liquid metal surface, the orifice will not continue to penetrate. When the spot power density is high, the holes will penetrate the whole thickness of the plate, forming a penetrating weld (or solder joint). In continuous laser welding, the keyhole moves along the welding direction with the beam relative to the workpiece. The metal melts in front of the keyhole, flows around the keyhole to the rear, and then re-solidifies to form a weld.

When plasma cloud is used in laser welding under the condition of high power density, it can be found that in the area of laser-metal interaction, metal evaporation is very intense, and red metal vapor escapes from small holes continuously, while there is a blue plasma cloud above the molten pool of metal surface, which is accompanied by small holes. The formation of plasma cloud is related to laser power density, properties of welded metal and protective gas. And it will have adverse effects on the welding process, which will reduce the laser energy on the metal surface, reduce the welding penetration, widen the weld surface, and make the welding process unstable.

A common method of overcoming the effects of plasma clouds during welding is to blow an inert gas through the nozzle to the surface of the bath. The mechanical blow of the gas can be used to drive the plasma cloud away from the bath. It is also possible to use a lower temperature gas to lower the temperature of the high temperature gas above the molten pool and to suppress the high temperature conditions in which the plasma cloud is generated. High-frequency pulsed laser welding can also be used, so that the heating time of each laser pulse is less than the time of formation of the more ion cloud (about 0.5 ms), and the plasma cloud has not yet been generated and the welding heating has ended. In addition, it is also possible to use high-speed welding or laser welding with a wavelength of a segment, which also has a certain effect on alleviating the interference of the plasma cloud on the welding process. There is now also a device for removing a plasma cloud, comprising a control host and an induction coil electrically connected to the control host, the induction coil being located directly above the plasma cloud, the host control induction coil generating an alternating magnetic field for rapidly removing the plasma cloud .

Long focal length focusing mirrors should not be used for laser welding and cutting. That is to say, the beam diameter d on the focal plane is related to the focal length f of the lens and the divergence angle θ of the beam (d=fθ). In the laser welding and cutting, in order to obtain a sufficiently high power density, the beam divergence angle is generally not allowed. Large, do not use a long focal length focusing mirror. Laser welding is not suitable for welding in universities. Because laser welding transfers laser energy to the inside of the workpiece through thermal conduction, the laser energy utilization rate is low, the welding speed is slow, the penetration depth is shallow, and it is difficult to weld thick plate workpieces. Laser deep-fusion welding is characterized by the generation of small holes. The laser entering the small hole is almost completely absorbed by the multiple reflection of the plasma and the hole wall. The energy utilization rate is high, the weld penetration is deep, and the welding speed is fast. An efficient welding method for thick plate welding and high speed sheet welding with small hole effect.

For safety reasons, appropriate warning signs should be placed at the entrance to the laser welding site. The beam path of the laser is required to be higher than the height of the person and transmitted in a closed duct. When the laser is turned on or the capacitor of the pulser is charged, an audible and visual warning is given. When the laser is working, avoid eye contact and skin exposure to direct or diffuse reflection of laser radiation. When conditions permit, the work site should be closed, leaving only the observation hole. The observation hole is equipped with an attenuation device, and the operator is outside the enclosure or remotely operated. . The workplace should be equipped with ventilation and fire safety devices.

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