Views: 2 Author: Site Editor Publish Time: 2021-05-19 Origin: Site
The deep penetration welding of laser welding machine usually uses fiber continuous laser beam to complete the connection of materials. The metallurgical physical process is very similar to electron beam welding, that is to say, the energy conversion mechanism is completed through the "keyhole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms small holes. This steam-filled hole is like a black body, absorbing almost all of the incident beam energy.
The equilibrium temperature in the cavity reaches approximately 2500°C. Heat is transferred from the outer wall of the high temperature cavity to melt the metal around the cavity. The small holes are filled with high-temperature steam, which is produced by continuous evaporation of the wall material under the irradiation of the radiation beam. The four walls of the small hole are surrounded by molten metal, and the liquid metal is surrounded by solid materials (in most conventional welding processes and laser conduction welding, energy is first deposited on the surface of the workpiece and then transmitted to its surface).
The liquid flow outside the pore wall and the surface tension of the wall layer maintains a dynamic balance with the steam pressure continuously generated in the cavity. The light beam continuously enters the small hole, and the material outside the small hole flows continuously.
With the movement of the beam, the small hole is always in a stable flow state. In other words, the small hole and the molten metal surrounding the hole wall move forward at the forward speed of the guided beam, and the molten metal fills the gap left by the small hole and condenses to form a weld. All the above processes happen so fast that the welding speed can easily reach a few meters per minute.
(1) Laser power. There is a laser energy density threshold in laser welding. Below this value, the penetration depth is very shallow. Once this value is reached or exceeded, the penetration depth will be greatly increased. 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 lower than the threshold, only surface melting of the workpiece occurs, that is, welding is performed with a stable thermal conductivity type.
When the laser power density is close to the critical condition for forming small holes, deep penetration welding and conduction welding will alternately become unstable welding processes, resulting in large fluctuations in penetration. In laser deep penetration welding, the laser power simultaneously controls the penetration depth and welding speed. Weld penetration is directly related to beam power density and is a function of incident beam power and beam focus. Generally speaking, for a laser beam with a certain diameter, the penetration depth increases as the beam power increases.
(2) Beam focus. The beam spot size is one of the most important variables in laser welding, because it determines the power density. But for high-power lasers, although there are many indirect measurement techniques, its measurement is still a difficult problem.
The beam focusing diffraction limit spot size can be calculated according to 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 actual measurement method is the isothermal profile method, which uses thick paper to scorch and penetrate a polypropylene plate to measure the focal point and perforation diameter. This method should be practiced through measurement to master the laser power and beam action time.