Laser energy density threshold exists in laser welding machine. If it is below this value, the penetration depth is very shallow. Once this value is reached or exceeded, the permeability will be greatly improved. Plasma is produced only when the laser power density on the workpiece exceeds a threshold (material dependent), which marks the progress of stable deep penetration welding. If the laser power is below this threshold, only the surface of the workpiece will melt, that is, the welding will be carried out in a stable heat conduction type.
When the laser power density is close to the critical condition for the formation of small holes, deep penetration welding and conductive welding alternately become unstable welding processes, resulting in large fluctuation of penetration depth. In laser deep penetration welding, laser power controls both 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 beam focus. In general, for a laser beam of a certain diameter, the penetration depth increases with the increase of the beam power.
The focus of the beam. The beam spot size is one of the most important variables in laser welding because it determines the power density. But for high power laser, although there are many indirect measurement techniques, its measurement is a difficult problem.
The diffraction spot size of the beam focus can be calculated according to the theory of optical diffraction, but due to the aberration of the focusing lens, the actual spot size is larger than the calculated value. The simplest measurement method is the isotherm contour method, which measures the focus and perforation diameter after burning thick paper and penetrating the polypropylene plate. In this method, we should master the laser power size and beam action time through measurement practice.
Material absorption value. The absorption of the laser by the material depends on some important properties of the material, such as absorption rate, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc., the most important of which is the absorption rate.
There are two factors that affect the absorption rate of the laser beam: first, the resistance coefficient of the material. By measuring the absorptivity of the polished surface of the material, it is found that the absorptivity of the material is proportional to the square root of the resistance coefficient, and the resistance coefficient changes with temperature. Secondly, the surface state (or finish) of the material has a significant impact on the beam absorption, and has a significant impact on the welding effect.
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