1 . Preface
Laser technology is one of the four greatest technological advances of the 20th century, alongside computers, semiconductors and atomic energy technologies. Over the years, it has been widely adopted in the areas of optical communications, medical treatments, testing and materials processing.
In recent years, the development of laser technology in the area of materials processing has been particularly impressive, with applications such as laser marking, cutting, drilling and welding. Among these, laser welding has become especially popular as it has several unique advantages over traditional welding methods such as argon arc welding and resistance welding.
The benefits of laser welding include a small range of thermal influence, the ability to produce welds with large aspect ratios, high welding strength and joint strength that can meet or exceed the strength of the base material. Furthermore, laser beams can be easily transmitted through high-energy optical fibers, making it possible to automate the welding process.
Laser welding typically employs CO2 lasers, disk lasers, Nd:YAG lasers, fiber lasers, and semiconductor lasers. Among them, fiber lasers are a relatively new development in laser technology, with high photoelectric conversion efficiency of 30% and compact size. They have low maintenance requirements, long service life, and are commonly used for welding stainless steel and aluminum alloys.
Quasi-continuous pulse fiber lasers are a new type of laser source developed by the American company IPG in recent years. They offer high peak power and pulse widths of up to milliseconds, making them suitable for metal welding and processing of other materials. Although they are widely used for precision electronic soldering, there is limited research on the detailed soldering process.
In this study, focus, which is a critical factor in the welding process, is used as a starting point to investigate the difference in laser beam quality under different focus conditions and its impact on the welding effect.
2. Welding equipment and test preparation
(1) Welding equipment
This article uses a 150W quasi-continuous pulse fiber laser as the welding light source. The technical specifications of the laser can be found in Table 1.
Table 1 Laser technical parameters
| Average power /W | 150 |
|---|---|
| Peak power /W | 500 |
| Pulse width /ms | 0.2-20 |
| Frequency/ Hz | 0-2500 |
| Cooling method | Air cooling |
| Beam quality BPP/mm*mrad | 1-2 |
The laser processing head is moved relative to the workpiece through the operation of the X/Y/Z moving platform to perform track welding. The laser processing head and the laser output signal are connected through a motion control board, which means that after being positioned in a specific location, the laser emits light for welding.
(2) Welding materials
In this article, 304 stainless steel is used as the test material, with an overlap welding method applied. The thickness of the upper material is 0.2mm, the thickness of the lower material is 0.5mm, and the dimensions of the material are 100mm x 50mm.
Before welding, the surface of the material is cleaned with acetone and alcohol to remove impurities such as oil stains. A self-made fixture is used to compress the top and bottom layers of material, reducing any gaps between the two layers and ensuring the accuracy and reliability of welding test results.
(3) Confirm the laser focus position
The main factors that have an impact on the result of laser welding are laser peak power, pulse width and defocus (the distance between the laser focus and the surface of the part), with defocus being a particularly crucial factor.
Defocus is defined as positive when the focus is above the surface of the workpiece and negative when it is below the surface.
The most reliable method for determining the position of the laser focus is the stainless steel triangle laser calibration method. This method involves using a low-energy (50W) laser to make a point on the stainless steel, with the strongest spark indicating the location of the laser's focus. A triangular block of stainless steel is then placed near the laser focus, and a laser beam is used to draw a line on the block, spaced approximately 0.5 mm apart. The narrowest linewidth is measured using a microscope and this measurement represents the focus of the laser.
3. The effect of defocus on beam quality
The quality of the laser beam is tested using a beam analyzer, a laser probe and a laser attenuator. The laser probe is first placed at the laser focus for testing, and then the laser processing head is lifted 1mm at a time, with defocus set at 0mm, 1mm, 2mm, 3mm and 4mm .
The test results, showing the beam distribution, are presented in Figure 1.
Figure 1 Changes in beam quality with defocus
When the defocus is set to 0 mm, the laser energy is mainly concentrated in the center of the spot. As the defocus increases, the distribution of laser energy across the spot becomes increasingly uniform. With a defocus of 3 mm, the distribution of laser energy at the spot is more balanced. However, when the defocus is increased to 4 mm, the laser energy distribution becomes uneven.
4.T The effect of defocus on the welding effect
(1) The amount of blur affects solder joints
The workpiece is positioned at the laser focus and the laser peak power and pulse width are defined. A spot is then made on the stainless steel sample by gradually increasing the power and pulse width until clear traces are visible on the back of the underlying material. In this case, the peak laser power was 500 W and the pulse width was 3 ms.
With the peak power, pulse width, and other parameters unchanged, the amount of defocus was adjusted by 1 mm at a time and the appearance of the solder joint was recorded. These results can be seen in Figure 2.
Figure 2 The appearance of solder joints changes with the amount of blur
The results showed that when the blur was adjusted between 0mm and 1mm, the weld joint was smaller and showed welding spatter. This is probably because, in this defocus range, the laser energy was mainly concentrated in the center of the spot, resulting in a high laser power density in the center of the weld joint, causing spatter.
As the defocus continued to increase, the solder joints became more uniform and spatter-free, likely due to the more even distribution of the laser beam. However, when the defocus was greater than 4 mm, the circularity of the solder joint became inconsistent and the size of the solder joint was reduced to a certain extent, possibly due to the uneven distribution of laser energy at the site.
The results also showed that as the defocus increased from 0 mm to 3 mm, the solder joint size gradually increased, with the solder joint diameter growing from 0.4 mm to 0.5 mm. This is because as the defocus increased, the laser spot on the surface of the material increased, leading to larger solder joints.
However, when the defocus was increased to 4 mm, the size of the solder joints decreased. This may be due to altered distribution of the laser beam, with low energy at the edge of the spot where the laser was in contact with the material, resulting in a larger spot on the surface but a smaller weld joint.
The relationship between the solder joint diameter and the amount of defocus is shown in Figure 3.
Figure 3 Relationship between solder joint diameter and defocus
(2) The effect of defocus amount on solder joint penetration
A cutter was used to cut along the edge of the laser weld joint. After undergoing grinding, fine grinding and polishing, the center of the weld joint was observed during polishing. Finally, after undergoing anti-corrosion treatment with nitric acid and alcohol solution, the change in penetration of the weld joint was tested under different blurring conditions.
The results showed that when the defocus was set between 0 mm and 1 mm, the weld joint had the deepest penetration and reached the underlying material. When the defocus was set between 2 mm and 3 mm, the weld penetration became shallower and only penetrated 1/2 the thickness of the underlying material. However, when the defocus was set to 4 mm, the penetration depth of the weld was significantly reduced and only penetrated 1/3 of the thickness of the underlying material, as shown in Figure 4.
Figure 4 Change in solder joint penetration with defocus
(3) The effect of defocus amount on welding strength
A tensile machine was used to test the strength of a single weld joint by clamping the bottom material and pulling the top material up. To ensure accurate tensile test data, 3 samples were tested for each set of parameters and the average value was obtained.
The defocus amount was set to 0mm, 1mm, 2mm, 3mm and 4mm, corresponding to solder joints with strengths of 7N, 8N, 11N, 15N and 6N, respectively.
As a general trend, the tensile strength of solder joints increased as defocus increased. This was because as the defocus increased, the size of the solder joints also increased, particularly the width of the contact between the top and bottom material, leading to an increase in tensile strength. However, when the defocus was increased to 4 mm, the tensile strength decreased, probably due to worsening beam quality and larger spot size, which led to a decrease in laser power density and therefore laser power density. penetration depth and weld strength. articulation.
Based on the experimental data, the tensile strength of a single solder joint reached its maximum value of 15N when the defocus was set at 3mm.
5 . Conclusion
This paper investigated the laser beam distribution under different defocus conditions and found that as the defocus increased, the laser energy distribution at the site became more uniform, but when the defocus exceeded 4mm, the energy distribution it became irregular.
By testing the stainless steel lap welding process, the study concluded that under constant other factors, adjusting the amount of defocus affected the appearance, size, penetration, and tensile strength of the weld joint, as well as the general appearance and strength requirements.
The conclusions were:
- As the defocus increased, the quality of solder joints improved and the tensile strength of solder joints gradually increased.
- When the blur was set to 3 mm, the solder joints were consistent and had the highest tensile strength.
- However, when the blur was increased further, the strength and quality of the solder joints decreased again.
