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Laser welding commissioning - focusing

Views: 0     Author: 胡     Publish Time: 2022-12-31      Origin: Site

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Summary of laser focusing

Every time the laser contacts a new device or conducts a new experiment, the first step must be focusing. Only when you find the focal plane, can you correctly determine other process parameters, such as defocusing amount, power, speed, etc. In the field debugging, the first thing is to fix the focus. If the focus is not right, it will directly lead to subsequent defects. The direction of improvement is impossible to talk about, or even to the wrong direction. Therefore, finding the focal plane is the first priority of process debugging, proofing and experiment.


The principle of focusing is as follows:

First of all, the energy of the laser beam is not evenly distributed. Since the left and right of the focusing mirror are in an hourglass shape, the energy is most concentrated and strongest at the waist, that is, the plane where the focus is located. As shown in Figures 1 and 2, the focal depth characteristics of laser beams with different energies are different, and the galvanometer, single-mode and multimode lasers are also different, which is mainly reflected in the spatial distribution of capabilities, some are compact, some are slender (Rayleigh lengths are different), but the energy is symmetrically distributed around the focal point.

激光焊接调试——定焦点

激光焊接调试——定焦点

Therefore, the basis for finding the focus is to perceive the change of energy. Generally, small power, about 200-500W, is used to find the position coordinates of two welding phenomena (acousto-optic) symmetrically distributed in the laser direction in a certain way, and the median value is calculated as the focal plane position. As shown in the figure, the material surface is extremely sensitive to the laser unit energy density, and once it is lower than a certain threshold, there will be no reaction. This is simple. As long as we find two positions in the laser direction, as shown in Figure 1 and Figure 2, there should be the same sound, spark and splash at the same position in the two directions from the focus, so as to determine that the focus is in the middle of these two positions.

激光焊接调试——定焦点

激光焊接调试——定焦点

Waveform setting:

1. Welding mode Select QCW mode.

2. The maximum power is set to about 0.3, and the power should not be set too high.

3. The waveform pulse width is set as 0.1 ms for the first segment, 100% power, 2.5 ms for the second segment, and 100% power. The pulse width cannot be set too long.

4. Set the frequency to 1~1.5, and do not set the frequency too high.

5. Start focusing


There are different focusing methods for different laser beams, which are generally divided into three steps.


The first method: diagonal marking:

1. First, determine the approximate range of the focal plane through the guide spot, and find the brightest and smallest point of the guide spot to determine the initial experimental focus;

激光焊接调试——定焦点

2. Platform construction, as shown in Figure 4

激光焊接调试——定焦点

2. Precautions for diagonal line

(1) Generally, steel plate is used, semiconductor is less than 500W, and optical fiber is about 300W; 80-200mm can be set for speed

(2) The larger the inclined angle of the steel plate is, the better. It should be about 45-60 degrees as far as possible. The midpoint should be set at the coarse positioning focus position with the smallest and brightest guide spot;

(3) Then start to line up. What effect should the line up achieve? Theoretically, the line will be symmetrically distributed around the focus, and the track will change from large to small and then become large, or from small to large and then become small;

(4) The semiconductor finds the most detailed point, and the steel plate will turn white at the focal point with obvious color characteristics, which can also be used as the basis for positioning the focal point;

(5) Secondly, the optical fiber shall control the back micro penetration as much as possible, and the micro penetration at the focus position indicates that the focus is at the midpoint of the back micro penetration length. At this time, the coarse positioning of the focus is completed, and the line laser assisted positioning is used for the next step.

激光焊接调试——定焦点

激光焊接调试——定焦点

3. The next step is to level the workpiece, adjust the line laser to coincide with the focus guide spot, that is, to locate the focus, and then perform the focal plane verification of the last step: that is, the dotting method, which is also the second method

(1) The principle is to splash sparks at the focal point with obvious sound characteristics. There is a dividing point at the upper and lower limits of the focus. The upper and lower limits of the focus shall be recorded if the sound is obviously different from the splash and sparks. The midpoint is the focus,

(2) Then adjust the line laser coincidence, and the focus is positioned with the error within 0.5mm. Be sure to repeat the experimental positioning to improve the accuracy and avoid the system error caused by a single time.

激光焊接调试——定焦点

There are also many kinds of dotting methods, and each manufacturer has different usage habits: it is suitable for optical fiber lasers with large focal depth and large change of spot size in the Z-axis direction. By hitting a row of dots, you can see the change trend of the points on the steel plate surface. Each time you adjust the Z-axis change by 0.5mm, the imprint on the steel plate changes from large to small, and then from small to large. The smallest point is the focus.

Some people judge the focus by marking on the photo paper, generally using small power, and looking at the size of the ablation point of the photo paper. The ablation point becomes larger from large to small, and the smallest is the focus. This error is relatively large, generally about 0.5-1 mm;

In addition, dot the steel plate at an interval of 0.5mm by moving the Z-axis height. Through microscope and magnification, the smallest point is the focus. This usually requires a larger interval of about 10mm. Otherwise, the heat of the steel plate will also affect the absorption rate of the steel plate to the laser, which will lead to errors. This accuracy is also between 0.5 and 1mm;

激光焊接调试——定焦点

激光焊接调试——定焦点

激光焊接调试——定焦点

In addition, as mentioned above, each person selects signals according to their own habits and sensitivity to these characteristics. Each inching is 0.2-0.5mm, and observe the changes of target signals. Generally, sparks come from nothing, or sounds come from nothing, and splashes come from nothing. This boundary is defined as the upper boundary, and then the lower boundary is found. Sparks come from nothing, splashes from nothing, and sounds from nothing, Another intermediate value is the focal plane.

It should be noted that there are many punches here. Generally, there is a special style of punches. Generally, it is used directly with a graphic blade and alcohol to remove oil stains. Attention shall be paid to that the dotting positions shall not coincide, otherwise the surface roughness and temperature will affect the absorptivity, which will affect the abnormal characteristic signal and lead to false judgment; Also, pay attention to the greater spacing between the dots to prevent the blade from being deformed and warped, which may cause misjudgment; The average value is calculated for many times and the error is smoothed. In this way, the accuracy can be controlled within 0.3mm.

Goggles must be worn during the dotting process to prevent eye injuries caused by strong light and splash.

The third type of spiral line is mainly used on the galvanometer. It is mainly the galvanometer pulse dotting. The characteristics are not obvious and the speed is too fast. Therefore, the spiral line is used. The principle is the same as above. The median value of two critical points is taken.

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