Critical points of laser welding
Critical points of laser welding
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1. Power density
Power density is one of the most critical parameters in laser processing. Higher power density, which can heat the surface layer to a boiling point within microseconds, resulting in a large amount of vaporization. The high power density thus benefits material removal processes such as drilling, cutting, and engraving. For lower power densities, it takes milliseconds for the surface temperature to reach the boiling point. Before the surface layer is gasified, the bottom layer reaches the melting point, and it is easy to form a good fusion weld. Therefore, in conductive laser welding, the power density is in the range of 10^4~10^6W/CM^2.
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2. Laser pulse waveform
The laser pulse waveform is an important issue in laser welding, especially thin plate welding. When the high-intensity laser beam hits the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the change in the surface temperature. During a laser pulse, the reflectivity of the metal varies greatly.
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3. Laser pulse width
Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter different from material removal and material melting but also a key parameter that determines the cost and volume of processing equipment.
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4. Influence of defocus amount on the welding quality
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Laser welding usually requires a certain distance, because the power density in the center of the spot at the laser focus is too high, and it is easy to evaporate into holes.
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On each plane away from the laser focus, the power density distribution is relatively uniform.
There are two defocus methods: positive defocus and negative defocus. Positive defocus if the focal plane is above the workpiece, negative defocus otherwise.
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According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding surface is equal, the power density on the corresponding planes is roughly the same. Still, the shape of the weld pool obtained is different. When the defocus is negative, a larger penetration depth can be obtained, which is related to the formation process of the molten pool.
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Experiments have shown that when the laser is heated for 50~200us, the material begins to melt, forming liquid phase metal and vaporizing, creating industrial pressure steam, which is ejected at a very high speed and emits dazzling white light. At the same time, the high-concentration gas moves the liquid phase metal to the edge of the molten pool, forming a depression in the center of the molten pool.
When the defocus is negative, the power density inside the material is higher than that on the surface, and it is easy to form stronger melting and vaporization so that the light energy can be transmitted to the deeper part of the material.
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Therefore, in practical applications, when a large penetration depth is required, negative defocus should be used; when welding thin materials, positive defocus should be used.?#laser?#lasermachine?#laserwelding?#lasertechnology