Today we will review the influence of various parameters on welding quality in the laser welding process, and also include some actual verification results.

Laser welding is an efficient and precise welding method that uses a high-energy-density laser beam as a heat source. Laser welding is one of the important aspects of the application of laser material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process is a heat conduction type, that is, the laser radiation heats the surface of the workpiece, and the surface heat diffuses to the inside through heat conduction. By controlling the width, energy, peak power and repetition frequency of the laser pulse, the workpiece is melted to form a specific molten pool. Due to its unique advantages, it has been successfully used in the precision welding of micro and small parts.

Technical principle

Laser welding can be achieved by continuous or pulsed laser beams. The principle of laser welding can be divided into heat conduction welding and laser deep penetration welding. When the power density is less than 10(4)~10(5) W/cm(2), it is heat conduction welding, in which the melting depth is shallow and the welding speed is slow; when the power density is greater than 10(5)~10(7) W/cm(2), the metal surface is heated and concave into a "hole", forming a deep penetration weld, which has the characteristics of fast welding speed and large depth-to-width ratio.

Among them, the principle of heat conduction laser welding is: laser radiation heats the surface to be processed, and the surface heat diffuses to the inside through heat conduction. By controlling the laser parameters such as the width, energy, peak power and repetition frequency of the laser pulse, the workpiece is melted to form a specific molten pool.

The laser welding machine used for gear welding and metallurgical thin plate welding mainly involves laser deep penetration welding.

Laser deep penetration welding generally uses a continuous laser beam to complete the connection of materials. Its metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the "key-hole" structure. Under the irradiation of laser with high enough power density, the material evaporates and forms a small hole. This small hole filled with steam is like a black body, absorbing almost all the energy of the incident beam. The equilibrium temperature in the hole cavity reaches about 2500 0C. The heat is transferred from the outer wall of the high-temperature hole cavity, melting the metal surrounding the hole cavity. The small hole is filled with high-temperature steam generated by the continuous evaporation of the wall material under the irradiation of the beam. The four walls of the small hole surround the molten metal, and the liquid metal is surrounded by solid materials (while in most conventional welding processes and laser conduction welding, the energy is first deposited on the surface of the workpiece and then transferred to the inside). The liquid flow outside the hole wall and the surface tension of the wall layer are in dynamic balance with the steam pressure continuously generated in the hole cavity. The beam continuously enters the small hole, and the material outside the small hole is continuously flowing. As the beam moves, the small hole is always in a stable state of flow. That is to say, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam. The molten metal fills the gap left after the small hole is removed and condenses, and the weld is formed. All of this happens so quickly that welding speeds can easily reach several meters per minute.

Laser classification

Lasers are currently mainly divided into infrared lasers and green lasers, with IPG and TRUMPF as typical representatives, with IPG having a slightly higher market share.

The main differences are:

1. Different laser wavelengths

The wavelength of red light is 650-660nm, and the wavelength of green light is 532nm. Due to their different wavelengths, their colors and performances are different. At the same power, the green light is brighter than the red light, and the light column is more obvious than the red light.

2. Different light-emitting principles

The red laser pen mainly relies on the red laser diode for light emission. Its structure is the simplest, and only one battery is needed as energy. The green laser pen uses a wavelength of 808nm infrared laser to excite nonlinear crystals as a light source. It can generate 1064nm infrared light, and then generate 532nm green light through frequency doubling. It belongs to solid laser.

3. Different scope of use

The green laser pen has a wider range of use. At night, even low-power green light can be seen due to Rayleigh scattering of atmospheric molecules, but the red light effect is much worse. Both green laser pens and red laser pens can be used for electronic teaching and presentations, but green laser pens can be used to point out stars and constellations, while red laser pens cannot.

4. Different prices

The general price of laser products is more than 100 yuan. Compared with green laser pens, red laser pens are not as good as green laser pens in terms of performance and use. The market retail price is much lower, which is very suitable for mid- and low-end consumers.

The difference between red and green laser

In the development of semiconductor solid-state lasers, red lasers and infrared lasers were the first to mature, so the use of these two wavelengths of lasers is the most convenient and mature.

Green lasers are obvious and not easily affected by meteorological conditions, but they are easy to be discovered. Because green lasers can be seen in the air, while red lasers are more hidden.

The difference between red and green lasers

1. The wavelength of red light is 650nm, 635nm, and the wavelength of green laser is 532nm. The wavelengths of lasers are different, and the light colors displayed are different.

2. At the same power, the light of green light is brighter than that of red light.

3. Some people believe that at the same power, the burning property of red light is slightly greater than that of green light.

From the perspective of welding effect, green lasers will have less spatter, a smoother surface, and a smaller heat-affected area.

Main influencing factors

Laser welding: It is an efficient and precise welding method that uses a high-energy-density laser beam as a heat source. Laser welding is one of the important aspects of the application of laser material processing technology. As one of the two basic modes of laser welding (the other is heat conduction welding), laser deep penetration welding is increasingly widely used.

Deep penetration welding, or deep penetration welding, is common in welding thicker materials with high laser power. In deep penetration welding, the laser is focused together to form a very high power density on the workpiece. In fact, the part where the laser beam is focused vaporizes the metal, resulting in a blind hole (the deep penetration hole) in the metal pool. The metal vapor pressure blocks the surrounding molten metal, so that the blind hole remains open during the welding process. The laser power is mainly absorbed by the melt at the vapor-melt boundary and at the deep penetration hole wall. The focused laser beam and the deep penetration hole are continuously moved along the welding trajectory. The welding material melts in front of the deep penetration hole and re-solidifies behind it to form a weld.

The factors that affect the effect of laser deep penetration welding are:

1. Laser power density

The premise of deep penetration welding is to focus the laser spot so that it has a sufficiently high power density, so the laser power density has a decisive influence on the weld formation. The laser power controls the penetration depth and welding speed at the same time. For a laser beam of a certain diameter, when the laser power is increased, the penetration depth deepens and the welding speed increases.

There is generally a critical value for the laser power that reaches a certain welding penetration depth. When this critical value is reached, the molten pool boils violently, and when it exceeds the critical value, the penetration depth will decrease sharply. In addition, due to the force of metal vapor, small holes will be formed in the molten pool, and small holes are the key to deep penetration welding.

The focus power density is not only proportional to the laser power, but also related to the laser beam and focusing optical path parameters.

2. Welding speed

During deep penetration welding, the welding speed is inversely proportional to the penetration depth. If the welding speed is increased while keeping the laser power unchanged, the heat input will decrease and the penetration depth will also decrease. Therefore, appropriately reducing the welding speed can increase the penetration depth, but too low a speed will cause excessive melting of the material and weld penetration of the workpiece. Therefore, for a specific laser power and a specific thickness and type of material, there is a suitable welding speed range to obtain the maximum penetration depth.

3. Focal point position

In deep penetration welding, the focus position is crucial to maintain sufficient power density. The change in the relative position between the focus and the workpiece surface directly affects the width and depth of the weld. Only when the focus is located at a suitable position on the workpiece surface can the weld form a parallel section and obtain the maximum penetration depth.

4. Shielding gas

The shielding gas has two functions: 1) remove the air in the local welding area and protect the working surface from oxidation; 2) suppress the generation of plasma cloud during high-power laser welding.

5. Workpiece joint gap

The workpiece splicing gap and assembly gap are directly related to the penetration depth and weld width of the welded workpiece. In deep-melting welding, if the joint gap exceeds the spot size, welding cannot be performed; if the joint gap is too small, sometimes the process will produce undesirable effects such as overlapping of the joint plates and difficulty in fusion; if the joint gap is too large, it is very easy to weld through; slow welding can make up for some weld defects caused by excessive gaps, while high-speed welding narrows the weld and has stricter assembly requirements.

7. Fiber diameter

The smaller the fiber diameter, the greater the energy density provided, and the smaller the output power of the equipment required to produce the same penetration depth. At present, the latest fiber diameter of infrared fiber lasers can reach 14um.

8. Swing diameter

The swing diameter can be understood as the width of the theoretical weld and can be adjusted according to application needs.

9. Swing frequency (overlap rate)

Swing frequency refers to the number of times the laser beam swings per unit time and per unit distance. The higher the swing frequency, the closer the two adjacent swing diameters are, and the higher the overlap rate.

10. Path

Path refers to the trajectory that the laser beam follows during the welding process. Common paths can be found in the table below.

Welding effect analysis method

The analysis method here mainly refers to the verification of welding effects, which usually includes two categories: destructive testing and non-destructive testing. Destructive testing is the most commonly used method in product design and development, which can help everyone quickly iterate and optimize.