Publish Time: 2024-02-15 Origin: Site
Laser Focusing Adjustment
When using new equipment or conducting experiments, the first step is always to adjust the focus. Only by locating the focal plane can other process parameters such as defocus amount, power, and speed be correctly determined, ensuring clarity in the process.
The principle of focusing adjustment is as follows: The energy distribution of the laser beam is not uniform. Typically, it forms a funnel shape around the focusing lens. At the waist of the beam, the energy is most concentrated and strongest. To ensure efficient processing and quality, it's essential to locate the focal plane and adjust the defocus amount accordingly for product fabrication. Without establishing the focal plane, subsequent parameter adjustments cannot proceed effectively. Therefore, identifying the focal plane is fundamental in mastering laser technology.
As shown in Figures 1 and 2, laser beams of different energies exhibit different depth of focus characteristics. Additionally, the distribution of capabilities varies between galvanometer mirrors and single/multi-mode laser sources. Some are more compact, while others are elongated. Consequently, different focusing methods are employed for different laser beams, generally divided into three steps:
Initial Positioning: Use coarse focusing methods to determine the approximate focal distance.
Fine Adjustment: Use precise focusing techniques to accurately locate the focal plane. Typically, adjustments are made based on the observed processing results in the welding area or feedback from laser energy detectors.
Confirming the Focal Plane: Finally, verify the position and stability of the focus point, serving as the basis for subsequent process parameter settings and experimental operations.
By following these steps, effective control of the focal point and assurance of processing quality under various laser beam conditions can be ensured.
Diagonal Line Method:
1.1 Start by using a guide spot to roughly determine the range of the focal plane. Identify the brightest and smallest point of the guide spot to establish the preliminary experimental focal point. 1.2 Platform setup, as shown in Figure 4.
Diagonal Line Method Considerations:
(1) Generally, for steel plates and semiconductor lasers up to 500W, and fiber lasers around 300W are sufficient. Speed can be set between 80-200mm. (2) A steeper angle on the steel plate is preferable, ideally around 45-60 degrees. The midpoint should be positioned at the rough focal point identified by the smallest and brightest spot of the guide beam. (3) Begin drawing diagonal lines. What effect should these lines achieve? Theoretically, these lines will symmetrically distribute around the focal point, with a trajectory that changes from large to small and then back to large, or vice versa. (4) For semiconductor lasers, locate the finest point. On steel plates, the focal point may exhibit a whitish color feature at its position, which can also serve as a focal point reference. (5) Additionally, with fiber lasers, try to control micro-transparency on the backside. Micro-transparency at the focal point indicates the midpoint of the micro-transparency length on the backside. Once this rough positioning of the focal point is completed, proceed with the next step using line laser assistance for positioning.
Flattening the Workpiece Align the line laser with the focal point indicated by the guide beam, confirming the focal point positioning. Then proceed with the final step of focal plane verification.
(1) Validate the focal point by pulsing points. Principle: Sparks splatter at the focal point, characterized by distinct sound features. There is a boundary point between the upper and lower limits of the focal point, where the sound and the splattering sparks are notably different. Record the upper and lower limits of the focal point; the midpoint is the focal point.
(2) Adjust the line laser alignment again to coincide with the focal point. Once the focal point is accurately positioned, the margin of error is within approximately 1mm. Repeat the positioning experiment to enhance accuracy.
Laser Welding Focusing Methods - Different Methods for Different Laser Beams
(Slant Line Method):
This method involves using a slant line to determine the focal point.
Start by identifying the approximate focal plane range through the guiding spot.
Locate the brightest and smallest point of the guiding spot as the initial experimental focal point.
Build the platform as shown in Figure 4.
(Notes on Slant Line Method):
For general use on steel plates, semiconductor lasers up to 500W and fiber lasers around 300W are sufficient.
Speed can be set between 80-200mm.
Aim for a larger angle on the steel plate, ideally around 45-60 degrees.
Place the midpoint at the rough positioning focal point where the guiding spot is brightest and smallest.
Begin drawing lines. The line should ideally symmetrically distribute around the focal point, showing a trajectory that starts small, grows larger, and then becomes smaller again.
The semiconductor should be found at its thinnest point, and the steel plate should still appear white and have significant color characteristics at the focal point, serving as a guide for the focal point.
Additionally, fiber optics should be controlled to the rear for minor penetration, with the midpoint at the focal point in the middle of the minor penetration length. This indicates the rough positioning completion of the focal point, assisting with subsequent laser-assisted positioning for the next step.
(Flattening the Workpiece):
Adjust the line laser until it coincides with the focal point due to the guiding spot, marking the definitive focal point, and subsequently, execute the final step for the focal plane verification.
Verify by applying a pulsating dot method, in which sparks splash at the focal point with distinctive sound characteristics. At the top and bottom limits of the focal point, there is a boundary point between the sound and the splash of sparks. Record the top and bottom limits of the focal point, with the midpoint as the focal point.
Adjust the line laser again for the definitive focal point, where the error is within 1 mm left and right. It is permissible to repeat the experiment for positioning and to improve accuracy.
(Dotting Method):
Suitable for lasers with significant focal depth and varying spot sizes along the Z-axis.
Create a series of dots on the steel plate surface and observe the trend of changes in these dots.
Adjust the Z-axis incrementally by 1 mm each time.
The dots on the steel plate will exhibit a pattern where they start large, become small, and then grow large again. Identify the smallest dot as the focal point.
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