1. Power density

Power density is one of the most important parameters in laser machining. With a higher power density, the surface can be heated to the boiling point in microsecond time, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processes such as drilling, cutting, and engraving. For lower power densities, it takes a few milliseconds for the surface temperature to reach the boiling point. Before the surface vaporization, the bottom layer reaches the melting point, easily forming a good fusion welding. Therefore, in the conductive laser welding, the power density is in the range of 104~106W/cm2.

Laser welding process

2. Laser pulse waveform

Laser pulse waveform is an important problem in laser welding, especially for wafer welding. When the high intensity laser beam is fired at the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. The reflectivity of metals varies greatly during the course of a laser pulse.

3. Laser pulse width

Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter different from material removal and material melting, but also a key parameter to determine the cost and volume of processing equipment.

4. Influence of defocus on welding quality

Laser welding usually requires a certain amount of defocus because of the high power density in the center of the spot at the focal point of the laser, which tends to evaporate into holes. In each plane away from the laser focus, the power density distribution is relatively uniform.

There are two ways of defocusing: positive defocusing and negative defocusing. If the focal plane is located above the workpiece, it is positive defocus; otherwise, it is negative defocus. According to geometrical optics theory, when the positive and negative separations are equal, the power density on the corresponding plane is approximately the same, but in fact, the shape of the molten pool is different.

In the case of negative defocusing, a larger melting depth can be obtained, which is related to the formation process of molten pool. The experimental results show that the laser heating 50~200us material began to melt, forming liquid phase metal and interphase vaporization, forming the market pressure vapor, and jet at a high speed, emitting dazzling white light. At the same time, the high concentration vapor makes the liquid metal move to the edge of the molten pool and forms a depression in the center of the molten pool.

When the negative defocus, the material internal power density is higher than the surface, easy to form a stronger melting, vaporization, so that the light energy to the material more deep transfer. Therefore, in practical application, when the required melting depth is large, negative defocusing is used. When welding thin materials, it is appropriate to use positive defocusing.