Aug 03, 2020
Lens focal length
A convex lens is usually used to converge the laser during welding, and a lens with a focal length of 63-254mm is generally used. The focus spot size is proportional to the focal length, the shorter the focal length, the smaller the spot. However, the focal depth increases synchronously with the focal length, so a short focal length can increase the power density, but because the focal depth is small, the distance between the lens and the workpiece must be accurately maintained, and the penetration depth is not large. Due to the influence of spatter and laser mode, the shortest focal depth used in actual welding is mostly 126mm. When you need to increase the weld seam, you can choose a lens with a focal length of 254mm. In this case, in order to achieve the deep penetration pinhole effect, a higher laser output power is required.
When the laser power exceeds 2kW, especially for the 10.6μm CO2 laser beam, in order to prevent the focusing lens from being optically damaged, the reflective focusing method is often used, and the polished copper mirror is generally used as the reflecting mirror. Because of its effective cooling, it is often recommended for high-power laser beam focusing.
When welding, in order to maintain sufficient power density, the focus position is very important. The change of the relative position of the focus and the workpiece surface directly affects the width and depth of the weld. In most laser welding applications, the focal point is usually set at about 1/4 of the required penetration depth below the surface of the workpiece.
Laser beam position
When laser welding is performed, the position of the laser beam controls the final quality of the weld, especially the case of butt joints is more sensitive than the case of lap joints. For example, when a hardened steel gear is welded to a low-carbon steel drum, the correct control of the laser beam position will help produce a weld with low carbon content, which has better crack resistance. In some applications, the geometry of the welded workpiece requires the laser beam to be deflected by an angle. When the deflection angle between the beam axis and the joint plane is within 100 degrees, the workpiece's absorption of laser energy will not be affected.
Control of sudden change of welding laser power
In laser deep penetration welding, no matter the depth of the weld, the pinhole phenomenon always exists. When the welding process is terminated and the power switch is turned off, pits will appear at the end of the weld. In addition, when the laser welding layer covers the original weld, excessive absorption of the laser beam will occur, causing the weldment to overheat or produce air holes.
In order to prevent the occurrence of the above phenomenon, the power start and stop points can be programmed to make the power adjustable at the start and end, that is, the start power is raised from zero to the set power value in a short period of time using electronic methods, and adjusted For welding time, a similar method is adopted when welding is terminated.