Things You Should Know About Multifocal Laser Welding

1. Generation of multi focus

After the laser beam is focused, the diameter of the laser focus is very small.

There must be a precise clamping device to keep the weld gap within a small range, otherwise the laser may directly pass through the weld or deviate from the weld, affecting the welding quality.

In addition, the temperature gradient in the heat-affected zone is very large, which has an adverse impact on the performance of weldments and is prone to undercut and other welding defects.

Multifocal technology can effectively prevent the occurrence of the above defects.

Multifocal can be generated by dividing a beam into several identical beams, or by coupling several separate beams together, as shown in figure 6-46.

In CO2 laser welding, the method shown in Fig. 1a is generally used, and in Nd∶YAG laser welding, the method shown in Fig. 1b is generally used.

By selecting different spectroscopes and different optical fiber numbers and layouts, we can get different distributions of focus on the weldment, and it is also easy to get multiple focus.

The basic principle of multifocal technology is:

Different combinations of lenses and mirrors are used to decompose a beam emitted by the laser into multiple channels (or a simple method of directly using multiple transmitters), so that multiple focal points will be formed on the weld surface.

2. Advantages of multifocal Technology

1) Increase the laser welding power.

Due to the large output laser power of CO2 laser, there are few applications to increase laser power through beam superposition.

Beam superposition is mainly used in Nd∶YAG laser welding.

The power, laser mode and focusing parameters of the two laser beams can be controlled separately, and finally focused on the same point of the weldment through their own focusing system, or produce very adjacent bifocals for welding.

Because the laser power of Nd∶ YAG laser is generally about 4kw.

The superposition of energy transmitted through multiple optical fibers can increase the energy reaching the weldment.

It overcomes the limitation of low transmission energy of single optical fiber.

Through the superposition of laser power, the welding speed of laser welding can be significantly improved, as shown in fig. 2.

schematic diagram of multifocal generation

a) Beam splitting     b) power superposition

Fig. 1 Schematic diagram of multifocal generation

comparison of welding speed between single focus and double focus laser welding

Fig. 2 Comparison of welding speed between single focus and double focus laser welding (double focus is distributed on both sides of the weld)

2) It can flexibly improve the distribution of energy on the weld.

The energy distribution on the weld can be adjusted by changing the distance between focal points and the distribution of focal points.

3) Improve the stability of welding process.

When the two focal points are parallel to the weld, the weld width ratio is large, and it is not easy to produce the phenomenon that the focus deviates from the weld.

4) Improve weld quality.

Fig. 3 shows when the laser power P is equal to 2 × 2 kW, photos of weld section when the distance between focus points is 0.36mm and 1mm respectively.

It can be seen from Fig. 3 that when the distance between the focal points is large enough, the bifocal technology can significantly increase the weld width.

In this way, even if the gap between welds is relatively large, the welding can be completed better and the assembly quality requirements of weldments can be reduced.

effect of distance between focal points on weld shape

a) Distance between focal points: 0.36mm

b) Distance between focal points 1mm

Fig. 3 Effect of distance between focal points on weld shape (P = 2×2 kW,v=2m/min)

In the process of bifocal welding, the expansion of small hole determines the shape of the final weld section, and the expansion of small hole will cause shrinkage stress when cooling.

By changing the geometry of the small hole, the stress state of the weld can be improved.

When welding with single focus or double focus along the weld, the shape of the weld pool is a narrow triangle.

When the bifocals are bridged on both sides of the weld, the shape of the molten pool is semi elliptical, so the formation speed and crystal structure of grains in the molten pool will be greatly improved.

During bifocal welding, due to the large melting width, the evaporated metal and the gas in the small hole can overflow the molten pool smoothly, which greatly reduces the number of bubbles and pores.

3. Bifocal laser welding technology

Double focus laser welding method is mainly used to solve the adaptability of laser welding to assembly accuracy, improve the stability of welding process and improve weld quality, especially for thin plate welding.

Common welding defects such as melting hole and burn through can also be avoided in the welding process of aluminum alloy materials.

Bifocal can be generated by dividing a beam into two identical beams.

The beam from the laser is reflected by a plane mirror and reflected to the roof beam splitter. The roof beam splitter divides the beam into two beams with equal energy.

The two beams of light are focused and reflected by the paraboloid focusing mirror respectively, so as to form two laser spots near the surface of the weldment.

Or by coupling several separate beams together to produce two beams, as shown in fig. 4.

In general, due to the high power of CO2 laser, CO2 laser welding adopts spectroscopic technology to obtain double focus, so as to obtain the same amount of two lasers.

In Nd∶YAG laser welding, because the laser power transmitted by optical fiber is relatively small (generally about 4kw), beam coupling technology is mostly used to realize bifocal technology.

The characteristic is that the two focal points can be adjusted respectively.

The most commonly used is dual beam laser (i.e. bifocal).

This paper only discusses the bifocal technology produced by the excessive light technology of CO2 laser welding.

By selecting different beam splitting mirrors, the distribution of two adjacent focal points can be flexibly adjusted and arranged.

The two focal points of the double beam laser can be distributed on both sides of the weld in parallel, or on the center line of the weld in front and back, or arranged at any angle with the weld, as shown in Fig. 5.

Parallel bifocal laser welding can be carried out under wide weld, which can effectively increase the weld width and reduce the assembly requirements.

When two focal points are distributed on both sides of the weld, the weld gap that can be bridged is about twice that when the focal point is distributed in the weld center;

When the front and rear arrangement is adopted, the temperature gradient in the heat affected zone will be greatly reduced and the welding defects such as undercut will be avoided.

After using the bifocal technology, the shape of small holes can be changed, the weld collapse can be effectively improved, and the formation of pores in the weld can be reduced to a great extent.

4. Double focus laser welding process

The weld morphology of double beam laser welding is different from that of ordinary single beam laser welding.

The surface of the weldment welded by double beam laser welding is wide and narrows rapidly downward, like the shape of a nail.

Double beam laser welding can reduce the cooling rate of the molten pool and significantly improve the weld quality for steels with high carbon content.

The surface melting of double beam laser welding is more stable and has less fluctuation, which is conducive to the formation of stable weld quality and the reduction of defects such as pores.

formation principle of bifocal

Fig. 4 Formation principle of bifocal

distribution of bifocals

Fig. 5 Distribution of bifocals

Whether it is single focus or double focus laser welding, there are usually two welding modes: one is heat conduction fusion welding;

The other is deep penetration welding.

The main parameters of bifocal laser welding include laser power, welding speed, defocus amount, type and flow of shielding gas, etc.

(1) Laser power

In bifocal laser welding, when the welding speed and defocus amount remain unchanged, the penetration depth and width of the welding pool increase with the increase of laser power.

However, when the laser power increases to a certain extent, the aspect ratio basically does not change.

With the increase of laser power, the welding heat affected zone becomes larger and larger, and the microstructure becomes thicker, while the microstructure of the weld zone changes little.

(2) Welding speed

In deep penetration welding, the welding speed has a great influence on the penetration depth and width of the weld, and also causes the change of microstructure.

When the laser power and defocus amount remain unchanged in the process of bifocal laser welding, with the increase of welding speed, the penetration depth and width of the welding pool decrease, as shown in Fig. 6.

effect of welding speed on weld shape

Fig. 6 Effect of welding speed on weld shape (P = 1kW, Δ f=0)

a)v=0.9m/min

b)v=1.5m/min

c)v=2.1m/min

(3) Defocus amount

When the defocus of bifocal laser welding is negative, the weld penetration is larger.

(4) Distance between focal points

The distance between focal points has a great influence on the welding stability.

In bifocal laser welding, two small holes are formed on the weldment, which are connected on both sides of the gap to obtain a wider weld.

When the two focal points are distributed along the weld, the front focal point plays the role of preheating, and the rear focal point is welded, so that the temperature gradient in the fusion zone is small, which is conducive to obtaining a better weld crystalline structure.

However, the distance between the two focal points should be able to effectively span the fusion zone of two small holes.

When the distance between the two focal points is small, the weld surface is rough and the spatter is serious.

When the distance between the two focal points is large, the weld will be very different.

The metal vapor has a large overflow opening, which is easy to overflow, and the weld surface is smooth and beautiful.

Too much distance is not conducive to obtaining high-quality welds.

The distance between focal points and the number of focal points are very important parameters to determine the shape of the weld section.

During laser deep penetration welding of aluminum, it is found that there are many pores and serious spatter in the weld.

However, the use of bifocal welding technology can reduce the pores in the weld to a great extent.

(5) Distribution of focus along weld

If the laser power of a single focus is 4kw and the laser power of both focuses is 2kW, the laser energy transmitted to the weldment is basically the same.

There are two cases of bifocal welding:

One is that two focuses are distributed on both sides of the weld;

The other is that two focuses are distributed on the centerline of the weld.

In these two cases, there is little difference in the weld surface, and the weld formation is mainly determined by the distance between the two focal points.

Generally, the focus diameter is about 0.3mm.

When the distance between two focal points is 0.36mm, the weld is basically not affected by the layout of bifocals and focal points on the weld.

When the two focuses are distributed on both sides of the weld, the penetration depth is slightly smaller.

This is because the two focuses are distributed on both sides of the weld, the weld width is larger, and the heat is not all distributed in the center of the weld, so the penetration is slightly reduced.

The double focus series arrangement is adopted, and the two focuses one before and one after are more conducive to maintaining the stability of the molten pool.

Therefore, the maximum penetration of double focus laser welding is greater than that of single focus.

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