The effect of defocusing on welding quality
Laser welding sheet metal usually requires a certain amount of defocus, because the power density at the center of the spot at the laser focal point is too high, which will easily evaporates into holes. On each plane away from the laser focus, the power density distribution is relatively uniform.
There are two ways to defocus: positive defocus and negative defocus. The focal plane above the workpiece is positive defocus, otherwise it is negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocusing planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but in fact the shape of the molten pool obtained is different. In the case of negative defocusing, a larger penetration depth can be obtained, which is related to the formation process of the molten pool. Laser heating 50 ~ 200us material begins to melt, forming liquid metal and partial vaporization, thus forming high-pressure steam, and spraying at a very high speed and emitting dazzling white light. At the same time, the high-concentration vapor moves the liquid metal to the edge of the molten pool, forming a depression in the center of the molten pool. In the case of negative defocus, the internal power density of the material is higher than the surface, and it is easy to form stronger melting and vaporization, so that the light energy is transmitted to the depth of the material. Therefore, in practical applications, when the penetration depth is large, negative defocusing is used; when welding thin materials, positive defocusing should be used.
The welding speed will affect the heat input in per unit time. If the welding speed is too slow, the heat input is too large, causing the workpiece to burn through. If the welding speed is too fast, the amount of heat input is too small, resulting in impervious welding of the workpiece.