Aug 03, 2020
Laser welding has a laser energy density threshold. Once this value is reached or exceeded, the penetration depth will be greatly increased, otherwise the penetration depth will be very shallow. Only when the laser power density on the workpiece exceeds the threshold (related to the material), the plasma will be generated, and the stable deep penetration welding can be ensured. If the laser power is lower than this threshold, only surface melting of the workpiece occurs, that is, welding is performed in a stable thermal conductivity type. When the laser power density is near the critical condition for the formation of small holes, deep penetration welding and conduction welding alternately become an unstable welding process, resulting in large fluctuations in penetration. During laser deep penetration welding, the laser power simultaneously controls the penetration depth and welding speed. The welding penetration is directly related to the beam power density, and is a function of the incident beam power and the beam focal spot. Generally speaking, for a laser beam with a certain diameter, the penetration depth increases as the beam power increases.
Beam focal spot:
The beam spot size is one of the most important variables in laser welding, because it determines the power density. But for high-power lasers, its measurement is a difficult problem, although there are many indirect measurement techniques.
The beam focal point diffraction limit spot size can be calculated according to the light diffraction theory, but due to the aberration of the focusing lens, the actual spot size is larger than the calculated value. The simplest actual measurement method is the isothermal profile method, which uses thick paper to scorch and penetrate the polypropylene plate to measure the focal spot and perforation diameter. This method requires measurement and practice to master the laser power and the time of beam action.
Material absorption value
The absorption of laser light by a material depends on some important properties of the material, such as absorptivity, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc. The most important of which is absorptivity.
The factors that affect the absorption rate of the laser beam of the material include two aspects: First, the resistivity of the material. After measuring the absorption rate of the polished surface of the material, it is found that the absorption rate of the material is proportional to the square root of the resistivity, and the resistivity varies with temperature. And change; Secondly, the surface state (or finish) of the material has a more important influence on the beam absorption rate, which has a significant effect on the welding effect.
The output wavelength of CO2 laser is usually 10.6μm. The absorption rate of non-metals such as ceramics, glass, rubber, plastics and so on is very high at room temperature, while the absorption of metal materials is very poor at room temperature, until the material is melted or even gas. Its absorption only increases sharply. The method of using surface coating or generating oxide film on the surface is very effective in improving the absorption of the light beam by the material.
The welding speed has a great influence on the penetration depth. Increasing the speed will make the penetration shallower, but too low a speed will cause excessive melting of the material and weld penetration of the workpiece. Therefore, there is a suitable welding speed range for a certain laser power and a certain thickness of a certain material, and the maximum penetration depth can be obtained at the corresponding speed value.