Power density. Power density is one of the most important parameters in laser processing. With high power density, the surface layer can be heated to the boiling point within microsecond time range, resulting in a large number of vaporization. Therefore, high power density is advantageous for material removal processes such as piercing, cutting and carving. For lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, which often forms a good fusion weld. Therefore, in the conduction laser welding, the power density is in the range of 10 ^ 4 ~ 10 ^ 6W / cm ^ 2
The influence of defocusing on welding quality. Laser welding usually needs a certain degree of defocusing, because the power density in the center of laser spot is too high, and it is easy to evaporate into holes. In the plane away from the laser focus, the power density distribution is relatively uniform. There are two defocusing methods: positive defocusing and negative defocusing. The focal plane jogs directly above the workpiece, and vice versa. According to the theory of geometrical optics, when the distance between the positive and negative focal plane and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. In the case of negative defocusing, greater penetration force can be obtained, which is related to the formation of molten pool. The experimental results show that the materials heated by laser for 50-200us begin to melt, form liquid phase metal and evaporate partially, form high-pressure steam, and then spray at a very high speed, emitting dazzling white light. At the same time, the high concentration vapor moves the liquid metal to the edge of the molten pool and forms a depression in the center of the molten pool. When negative defocusing occurs, the internal power density of the material is higher than that of 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. Therefore, in practical application, when the penetration depth is required to be large, negative defocus is used. When welding thin materials, positive coke shall be used.