There are two types of defocusing: positive defocusing and negative defocusing. The focal plane is located above the surface of the workpiece is positive defocusing, and vice versa is negative defocusing. According to geometric optics theory, when the positive and negative defocusing plane and welding plane distance is equal, the corresponding plane of power density is approximately the same, but the actual shape of the molten pool obtained is different.
When the negative defocusing, a greater depth of melt can be obtained, which is related to the formation process of the melt pool. Experiments show that the laser heated to 50~200us material begins to melt, forming liquid metal and appearing partially vaporized, forming high pressure vapor and emitting a dazzling white light with a very high velocity jet. At the same time, the high concentration of vapor causes the liquid metal to move to the edge of the molten pool, forming a depression in the center of the pool. When the negative defocusing, the internal power density of the material is still higher than the surface, which tends to form stronger melting and vaporization, so that the light energy is transmitted deeper into the material. Therefore, in practical applications, when the requirement for a larger depth of melt, the use of negative defocus; welding thin materials, it is appropriate to use positive defocus.
Article written by XING Laser