The deep penetration welding of the laser welding machine usually uses a fiber continuous laser beam to complete the connection of materials. The metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the "keyhole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms a small hole. This hole full of steam is like a black body, absorbing almost all the incident beam energy. The equilibrium temperature in the cavity reaches about 2500°C. Heat is transferred from the outer wall of the high-temperature cavity to melt the metal around the cavity. The small hole is filled with high-temperature steam, which is generated by the continuous evaporation of the wall material under the irradiation of the ray beam. The four walls of the small hole are surrounded by molten metal, and the liquid metal is surrounded by solid material (in most conventional welding processes and laser conduction welding, energy is first deposited on the surface of the workpiece and then transferred to the internal transfer). The liquid flow outside the hole wall and the surface tension of the wall layer maintain a dynamic balance with the steam pressure continuously generated in the cavity. The beam continuously enters the small hole, and the material outside the small hole flows continuously. As the beam moves, the small hole is always in a stable flow state. In other words, the keyhole and the molten metal around the hole wall move forward at the forward speed of the guiding beam, and the molten metal fills the gap left by the keyhole and solidifies to form a weld. All of the above processes occur so quickly that the welding speed can easily reach several meters per minute. Main process parameters of laser deep penetration welding (1) Laser power. There is a threshold laser energy density in laser welding. Below this value, the penetration depth is very shallow. Once this value is reached or exceeded, the penetration depth will increase greatly. Only when the laser power density on the workpiece exceeds the threshold (related to the material) will plasma be generated, which marks the progress of stable deep penetration welding. If the laser power is below this threshold, only surface melting of the workpiece occurs, that is, welding is carried out with a stable thermal conductivity type. When the laser power density approaches the critical condition for the formation of a keyhole, deep penetration welding and conduction welding will alternately become unstable welding processes, resulting in large fluctuations in the depth of penetration. In laser deep penetration welding, the laser power controls both the depth of penetration and the welding speed. The welding depth is directly related to the beam power density and is a function of the incident beam power and the beam focus. Generally speaking, for a laser beam with a certain diameter, the penetration depth increases with the increase of the beam power. (2) Beam focus. The beam spot size is one of the most important variables in laser welding because it determines the power density. However, for high-power lasers, its measurement remains a difficult problem despite many indirect measurement techniques. The beam focusing diffraction-limited spot size can be calculated based on the light diffraction theory, but due to the aberration of the focusing lens, the actual spot size is larger than the calculated value. The simplest practical measurement method is the isothermal profile method, which uses thick paper to burn and penetrate a polypropylene plate to measure the focus and perforation diameter. This method should be practiced through measurements to master the laser power and beam action time.