How Does The Laser Defocus Affect The Beam Quality And Welding Effect?

1. Preface

Laser technology is the four major technological discoveries in the 20th century that are as famous as computers, semiconductors and atomic energy technologies.

After decades of development, it has been widely used in the fields of optical communications, medical treatment, testing, and material processing.

Laser has developed rapidly in the field of material processing in recent years, such as laser marking, cutting, drilling, welding, etc., especially in welding, due to its unique advantages, it is widely replacing traditional argon arc welding, resistance welding and other methods.

The advantages of laser welding are mainly reflected in the small thermal influence range, easy to obtain welds with large aspect ratios, high welding strength, and the joint strength can reach or exceed the strength of the base material. The laser beam can be flexibly transmitted through high-energy optical fibers in space, which is easy to achieve automated manufacturing of welding.

Lasers used in laser welding generally include CO2 lasers, disc lasers, Nd: YAG lasers, fiber lasers and semiconductor lasers.

Among them, the fiber laser is a new type of laser developed in recent years. The photoelectric conversion efficiency is extremely high, reaching 30%, with a small size and good energy stability.

This laser has almost no consumables and has a long life. It has been widely used in stainless steel welding and aluminum alloy welding.

Quasi-continuous pulse lasers are fiber lasers, which can output high peak power and pulse widths up to ms level, which are very suitable for metal welding and other material processing.

It is a new type of laser welding light source developed by the American IPG company in the last two years. It is currently used in the field of electronic precision welding, but there are few reports on its detailed welding process research.

In this paper, the defocus, which is the key factor affecting the welding process, is used as the starting point to study the difference in laser beam quality under different defocus conditions, and then to study its influence 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 parameters of the laser are shown in Table 1.

Table 1 Laser technical parameters

Average power /W150
Peak power /W500
Pulse width /ms0.2-20
Frequency/ Hz0-2500
Cooling methodAir cooling
Beam quality BPP/mm*mrad1-2

The laser processing head moves relative to the workpiece under the drive of the X/Y/Z mobile platform to complete the welding of the track.

The laser processing head and the laser output signal are linked by the motion control board, that is, after moving to a certain position, the laser emits light for welding.

(2) Welding materials

This article uses 304 stainless steel as the test material, the welding method is lap welding, the upper material thickness is 0.2mm, the lower material thickness is 0.5nim, and the length×width of the material is 100x50mm.

Before welding, the material is cleaned with acetone and alcohol to remove surface impurities such as oil stains.

The self-made fixture is used to compress the upper and lower layers of materials to reduce the gap between the two layers of materials and ensure the consistency and reliability of the welding test results.

(3) Confirm the laser focus position

The main factors that affect the effect of laser welding are laser peak power, pulse width and defocus (the distance between the laser focus and the surface of the workpiece), and the defocus is a very critical factor.

Define the focus above the surface of the workpiece as positive focus, and below the surface of the workpiece as negative focus.

The more effective method to determine the position of the laser focus is: the triangle stainless steel laser calibration method.

Specifically, a small laser energy (50W) is used to make a spot on the stainless steel, and the spot with the strongest spark is near the laser focus. Then place a stainless steel triangle block near the laser focus, use a laser beam to draw a line on the stainless steel triangle block, and draw a line about 2mm apart from 0.5mm. Use a microscope to measure the narrowest line width, which is the laser focus.

3. The effect of defocus on beam quality

Use beam analyzer, laser probe and laser attenuator to test the quality of laser beam.

First put the laser probe at the laser focus for testing, and then lift the laser processing head upwards by 1mm each time, that is, set the defocus to 0mm, 1mm, 2mm, 3mm, 4mm.

The beam distribution obtained by the test is shown in Figure 1.

Changes in beam quality with defocus

Figure 1 Changes in beam quality with defocus

When the defocus is 0mm, the laser energy is mainly concentrated in the center of the spot. With the increase of the defocus, the distribution of the laser energy on the spot is gradually uniform. When the defocus is 3mm, the distribution of the laser energy on the spot is the most uniform . When the defocus is increased to 4mm, the distribution of laser energy on the spot is uneven.

4. The effect of defocusing on the welding effect

(1) The amount of defocus affects the solder joints

Place the workpiece at the laser focus position, set the laser peak power and pulse width, and make a spot on the stainless steel sample.

The power and pulse width are gradually increased until obvious traces can be seen on the back of the underlying material. At this time, the laser peak power is 500W and the pulse width is 3ms.

Keep the peak power, pulse width and other parameters unchanged, adjust the defocus amount, adjust 1mm each time, the appearance of the solder joint is shown in Figure 2.

The appearance of solder joints changes with the amount of defocus

Figure 2 The appearance of solder joints changes with the amount of defocus

It is found that when the defocus is 0~lmm, the solder joint is the smallest and the welding spot has spatter. This may be because when the defocus is 0~1mm, the laser energy is mainly concentrated in the center of the spot, resulting in the laser power at the center of the solder joint. The density is too high, causing welding spatter.

As the amount of defocus continues to increase, the solder joints have no spatter and the solder joints become more uniform. It should be because the laser beam distribution is more uniform as the amount of defocus increases.

But when the defocus is more than 4mm, the roundness of the solder joint becomes inconsistent, and the size of the solder joint is reduced to a certain extent. This may be caused by the uneven distribution of laser energy on the spot when the defocus is increased to 4mm.

It was also observed that when the defocus amount increased from 0mm to 3mm step by step, the size of the solder joint gradually increased, and the diameter of the solder joint increased from 0.4mm to 0.5mm.

This is because as the amount of defocus increases, the laser spot on the surface of the material becomes larger, resulting in larger solder joints.

When the defocus is increased to 4mm, the size of the solder joints decreases instead.

It may be because the distribution of the laser beam has changed, and the energy at the edge of the spot where the laser is in contact with the material is low, which leads to the phenomenon that although the spot on the surface becomes larger, the size of the solder joint decreases.

Figure 3 shows the relationship between the diameter of the solder joint and the amount of defocus.

Relationship between solder joint diameter and defocus

Figure 3 Relationship between solder joint diameter and defocus

(2) The effect of defocusing amount on solder joint penetration

Use a slicer to cut along the edge of the laser solder joint.

After rough grinding, fine grinding and polishing.

Observe while polishing until the center of the solder joint is polished.

After corrosion (nitric acid and alcohol solution) treatment, test the change of solder joint penetration under different defocusing conditions.

It was found that when the defocus amount is 0~lmm, the solder joint has the deepest penetration and penetrates the underlying material.

When the defocus is 2~3mm, the weld penetration becomes smaller and penetrates 1/2 of the thickness of the underlying material.

When the defocus is 4mm, the weld penetration depth is significantly smaller, and the penetration depth is 1/3 of the thickness of the underlying material, as shown in Figure 4.

Change of solder joint penetration with defocus

Figure 4 Change of solder joint penetration with defocus

(3) The effect of defocusing amount on welding strength

Use a tensile machine to test the strength of a single solder joint, fix the lower material and pull the upper material upward.

In order to ensure the accuracy of the tensile test data, 3 samples were tested for each set of parameters and the average value was taken.

The defocus amount is 0mm, 1mm, 2mm, 3mm, 4mm corresponding to the solder joints; the strength is 7N, 8N, 11N, 15N, 6N, respectively.

Generally, with the increase of the defocus, the tensile strength of the solder joints gradually increases.

This is because as the amount of defocus increases, the size of the solder joints increases, especially the width of the contact between the upper layer material and the lower layer material gradually increases, which increases the tensile strength.

But when the defocus is increased to 4mm, the tensile strength decreases.

It should be because as the amount of defocus increases, the beam quality becomes worse and the spot becomes larger, and the laser power density decreases, which leads to a decrease in the penetration depth of the solder joint, and thus the strength of the solder joint.

According to the experimental data, when the defocus is 3mm, the tensile strength of a single solder joint reaches the maximum 15N.

5. Conclusion

This article tested the laser beam distribution under different defocus conditions and concluded that:

With the increase of the defocus, the distribution of laser energy on the spot gradually becomes uniform, but when the defocus exceeds 4mm, the energy distribution becomes uneven.

Through the stainless steel lap welding process test, under the premise of other factors unchanged, adjust the defocus amount, test the appearance, size, penetration and tensile strength of the solder joint, and comprehensive appearance and strength requirements. The conclusion is:

  • As the amount of defocus increases, the quality becomes better, and the tensile strength of the solder joints gradually increases;
  • When the defocus is 3mm, the solder joints are consistent and the tensile strength is the largest;
  • However, when the defocus amount is increased, the strength and quality of the solder joints will decrease again.

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