The deep penetration welding of laser welding machine usually uses fiber optic continuous laser beams to complete the connection of materials. The metallurgical physical process is very similar to electron beam welding, which means that the energy conversion mechanism is completed through a "keyhole" structure.
When irradiated with a sufficiently high power density laser, the material evaporates and forms small holes. This hole filled with steam is like a blackbody, absorbing almost all the energy of the incident beam. The equilibrium temperature in the cavity reaches approximately 2500 ° C. Heat is transferred from the outer wall of the high-temperature chamber to melt the metal around the chamber. The small hole is filled with high-temperature steam, which is generated by continuous evaporation of the wall material under the irradiation of a ray beam. The four walls of the small hole are surrounded by molten metal, while 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 the surface. 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 hole flows continuously. As the beam of light moves, the small hole remains 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 guiding beam, and the molten metal fills the gap left by the small hole and condenses, forming a weld seam. All of the above processes occur very quickly, so that the welding speed can easily reach a few meters per minute.
Main process parameters of laser deep penetration welding
1.Beam focus.
The size of the beam spot is one of the most important variables in laser welding, as it determines the power density. However, for high-power lasers, despite many indirect measurement techniques, their measurement remains a challenge.
The maximum spot size for beam focusing and diffraction can be calculated based on the theory of light diffraction, but due to the aberration of the focusing lens, the actual spot size is greater than the calculated value. The simplest practical measurement method is the isothermal contour method, which uses thick paper to burn and penetrate the polypropylene plate to measure the focal point and perforation diameter. This method should be practiced through measurement to grasp the laser power and beam action time.
2.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 greatly increase. Plasma is only generated when the laser power density on the workpiece exceeds the threshold, marking the progress of stable deep penetration welding. If the laser power is below this threshold, only surface melting of the workpiece occurs, i.e. welding is carried out with a stable thermal conductivity type. When the laser power density approaches the critical condition for forming small holes, deep penetration welding and conduction welding will alternately become unstable welding processes, leading to significant fluctuations in penetration depth. In laser deep penetration welding, the laser power controls both penetration depth and welding speed. The welding penetration depth is directly related to the beam power density and is a function of the incident beam power and beam focus. Generally speaking, for laser beams with a certain diameter, the penetration depth increases with the increase of beam power.
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