1. Laser welding technology：
Laser welding is one of the important aspects of the application of laser processing technology. Laser welding is a process that uses the radiant energy of the laser to achieve effective welding. The working principle of laser welding is: To excite the laser active medium (such as a mixture of CO2 and other gases, YAG yttrium aluminum garnet crystals, etc.) in a specific way, so that it oscillates back and forth in the resonator, thereby forming an excited radiation beam. When the beam comes into contact with the workpiece, its energy is absorbed by the workpiece, and welding can be carried out when the temperature reaches the melting point of the material.
2. Important parameters of laser welding technology：
2.1 Power density：
Power density is one of the most critical parameters in laser processing.Using a higher power density, the surface layer can be heated to the boiling point within a microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is very beneficial for material removal processing, such as punching, cutting, and engraving.For lower power densities, it takes a few milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which can easily form a good melt welding.
2.2 Laser pulse waveform：
When a high-intensity laser beam hits the surface of the material, 60 to 98% of the laser energy on the metal surface will be reflected and lost, especially gold, silver, copper, aluminum, titanium and other materials have strong reflection and fast heat transfer. During a laser pulse signal, the reflectance of the metal changes over time. When the surface temperature of the material rises to the melting point, the reflectance will decrease rapidly. When the surface is in a molten state, the reflection is stable at a certain value.
2.3 Laser pulse width：
Pulse width is an important parameter of pulsed laser welding. The pulse width is determined by the melting depth and the heat-affected zone. The longer the pulse width, the larger the heat-affected zone, and the melting depth increases with the power of 1/2 of the pulse width.However, increasing the pulse width will reduce the peak power. Therefore, increasing the pulse width is generally used for heat conduction welding. The weld size formed is wide and shallow, especially suitable for lap welding of thin and thick plates.
However, lower peak power will lead to excess heat input, and each material has an optimal pulse width that can maximize the melting depth.
2.4 Defocus amount：
Laser welding usually requires a certain amount of defocus, because the power density in the center of the spot at the laser focus is too high, which can easily evaporate into holes. On each plane leaving the laser focus, the power density distribution is relatively uniform.
2.5 There are two ways to defocus：
Positive defocus and negative defocus. The focal plane is located above the workpiece for positive defocus, and vice versa for negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus plane and the welding plane is equal, the power density on the corresponding plane is approximately the same, but in fact the shape of the bath obtained is somewhat different. When negative defocus, a greater melting depth can be obtained, which is related to the formation process of the bath.
2.6 Welding speed：
The welding speed has a greater impact on the melting depth. Increasing the speed will make the melting depth shallow, but if the speed is too low, it will cause the material to melt excessively and the workpiece to be welded through. Therefore, there is a suitable welding speed range for specific materials with a certain laser power and a certain thickness, and the maximum melting depth can be obtained at the corresponding speed value.
2.7 Protective gas：
Inert gases are often used in the laser welding process to protect the bath, while helium, argon, nitrogen and other gases are often used for protection in most applications. The second role of the protective gas is to protect the focusing lens from metal vapor pollution and sputtering of liquid droplets. During high-power laser welding, the ejection is very powerful, and it is more necessary to protect the lens at this time. The third effect of the protective gas is that it can effectively disperse the plasma shielding generated by high-power laser welding. Metal vapor absorbs the laser beam and ionizes it into plasma. If there is too much plasma, the laser beam will be consumed by the plasma to some extent.
3.The unique effect of laser welding technology：
Compared with traditional welding technology, it has four unique effects：
3.1 Weld purification effect：
When the laser beam is irradiated on the weld, the absorption rate of impurities such as oxides in the material to the laser is much higher than that of the metal to the laser. Therefore, the impurities such as oxides in the weld are quickly heated and vaporized to escape, so that the content of impurities in the weld is greatly reduced. Therefore, laser welding not only does not pollute the workpiece, but can purify the material.
3.2 Light burst impact effect：
When the laser power density is very high, the metal in the weld evaporates and vaporizes sharply under the irradiation of a powerful laser beam. Under the action of high-pressure metal vapor, the metal melt in the bath produces an explosive splash, and its powerful shock wave propagates in the direction of the depth of the hole, forming an elongated deep hole. During the continuous movement of the laser welding process, the surrounding molten metal continuously fills the holes and condenses into a solid deep-melt weld.
3.3 The small hole effect of deep melt welding：
Under the irradiation of a laser beam with a power density of up to 107W/cm2, the rate of energy input to the weld is much greater than the rate of heat conduction, convection, and radiation loss, causing the metal in the laser irradiation area to vaporize rapidly, and under the action of high-pressure vapor, small holes are formed in the bath. This kind of hole is like a black hole in astronomy, which can absorb all light energy. The laser beam passes through this hole and hits the bottom of the hole directly. The depth of the hole determines the depth of melting.
3.4 The focusing effect of the side walls of the holes in the bath on the laser：
In the process of forming holes in the bath under laser irradiation, the angle of incidence of the laser beam incident on the side wall of the hole is usually large, so that the incident laser beam is reflected on the side wall of the hole and transmitted to the bottom of the hole, resulting in the phenomenon of superposition of the beam energy in the hole, which can effectively increase the intensity of the beam in the hole. This phenomenon is called the focusing effect of the side wall of the hole. The reason why lasers can be used for welding is based on the results of the above effects.
4. Advantages of laser welding technology：
The unique effect of laser welding makes laser welding have the following advantages：
4.1 The laser irradiation time is short and the welding process is extremely rapid, which is not only conducive to improving productivity, but also the welded material is not easy to oxidize, the heat-affected zone is small, and it is suitable for welding transistor components with strong heat sensitivity. Laser welding has neither welding slag nor the need to remove the oxide film of the workpiece. It can even be welded through glass, especially suitable for welding in miniature precision instruments.
4.2 Lasers can not only weld the same kind of metal materials, but also dissimilar metal materials, and even metal and non-metallic materials. For example, for integrated circuits with ceramics as the substrate, due to the high melting point of ceramics and the inappropriate application of pressure, it is difficult to use other welding methods, but laser welding is more convenient. Of course, laser welding cannot weld all dissimilar materials.
Applicable scenarios and industries of laser welding: 1. Heat conduction welding is mainly used for precision machining, such as metal foil, edge machining, medical technology, etc.; 2. Deep melt welding and brazing are mainly used in the automotive industry, of which deep melt welding is used for car bodies, transmissions, housings, etc.; Brazing is mainly used for car body welding; 3. Laser conduction welding can handle non-metals and has a wide range of applications. It can be used in consumer goods, the automotive industry, electronic housings, medical technology, etc.; 4. Composite welding is mainly suitable for special steel structures, such as ship decks.