Laser welding is to project a focused laser beam with a high power density (106 ~ 1012W / cm2) on the metal material to be welded, through the interaction of the light beam and the material to be welded, the light energy is absorbed by the material and finally converted into heat energy, thereby making the metal material Special welding method for melting.
Laser welding is a high-energy density beam welding with high welding speed and low line energy. Therefore, it has the characteristics of small solder joints or narrow weld seams, small heat affected zone, small welding deformation and smooth weld bead. In addition, the directivity of the focused laser beam is very stable, independent of the influence of electricity, magnetic field and airflow, and the focal spot position of the beam can be accurately positioned in advance, so the laser welding is particularly suitable for the assembly welding requirements of precision structural parts and heat sensitive devices.
The high cost of laser welding equipment and low energy replacement rate are its shortcomings, but the high productivity of laser welding and the ease of production automation make it possible to make the production cost of each product relatively low in large-scale production. . Where laser welding is equivalent to or slightly higher than conventional production costs, if laser welding products can achieve better technical performance, such as longer service life, good product appearance, less post-weld surface treatment time, etc. Laser welding is still suitable. For those heat sensitive components that are not laser-welded and precision structural parts that require minimal welding distortion, the cost of welding will no longer be a decisive factor in considering the trade-offs.
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 distance between the positive and negative defocus planes and the welding plane 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.
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