The Uniqueness of Laser Welding Technology

01 Laser welding technology

Laser welding is one of the important aspects of laser processing technology application. Laser welding is a process that uses the radiation energy of laser to achieve effective welding. Its working principle is: to stimulate the laser active medium (such as a mixture of CO2 and other gases, YAG yttrium aluminum garnet crystal, etc.) in a specific way, so that it oscillates back and forth in the resonant cavity, thereby forming a stimulated radiation beam. When the beam contacts the workpiece, its energy is absorbed by the workpiece, and welding can be performed when the temperature reaches the melting point of the material.

02 Important parameters of laser welding technology

1.Power density

Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to 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 drilling, cutting, and engraving. For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which is easy to form a good molten weld.

2.Laser pulse waveform

When a high-intensity laser beam is shot to the surface of a material, 60-98% of the laser energy will be reflected and lost on the metal surface, especially for materials such as gold, silver, copper, aluminum, and titanium, which have strong reflection and fast heat transfer. During a laser pulse signal, the reflectivity of the metal changes with time. When the surface temperature of the material rises to the melting point, the reflectivity will drop rapidly. When the surface is in a molten state, the reflection is stable at a certain value.

3. Laser pulse width

Pulse width is an important parameter of pulse laser welding. Pulse width is determined by the penetration depth and heat affected zone. The longer the pulse width, the larger the heat affected zone, and the penetration depth increases with the 1/2 power of the pulse width. However, the increase in pulse width will reduce the peak power, so increasing the pulse width is generally used for heat conduction welding, and 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 penetration depth.

4. Defocus

Laser welding usually requires a certain amount of defocus, because the power density at the center of the laser spot is too high, which is easy to evaporate into holes. The power density distribution is relatively uniform on each plane away from the laser focus.

5. There are two defocusing modes

Positive defocusing and negative defocusing. When the focal plane is above the workpiece, it is positive defocusing, and vice versa, it is negative defocusing. According to the theory of geometric optics, when the distances between the positive and negative defocusing planes and the welding plane are equal, the power density on the corresponding planes is approximately the same, but in fact, the shape of the molten pool obtained is somewhat different. When the defocus is negative, a greater melting depth can be obtained, which is related to the formation process of the molten pool.

6. Welding speed

Welding speed has a great influence on the depth of penetration. Increasing the speed will make the depth of penetration shallower, but too low a speed will lead to excessive melting of the material and welding through the workpiece. Therefore, there is a suitable welding speed range for a certain laser power and a certain thickness of a specific material, and the maximum depth of penetration can be obtained at the corresponding speed value.

7. Shielding gas

Inert gas is often used to protect the molten pool during laser welding. For most applications, helium, argon, nitrogen and other gases are often used for protection. The second function of the shielding gas is to protect the focusing lens from metal vapor contamination and liquid droplet sputtering. In high-power laser welding, the ejecta are very powerful, and it is even more necessary to protect the lens. The third function of the shielding gas is to effectively dispel the plasma shielding generated by high-power laser welding. Metal vapor absorbs the laser beam and ionizes into plasma. If there is too much plasma, the laser beam will be consumed by the plasma to some extent.

03 Unique effects of laser welding technology

Compared with traditional welding technology, it has four unique effects

1. Weld Purification Effect

When the laser beam is irradiated on the weld, the oxides and other impurities in the material absorb the laser much more than the metal does. Therefore, the oxides and other impurities in the weld are quickly heated and vaporized, which greatly reduces the impurity content in the weld. Therefore, laser welding will not pollute the workpiece, but can purify the material.

2. Light explosion shock effect

When the laser power density is very high, the metal in the weld evaporates and gasifies rapidly under the irradiation of the powerful laser beam. Under the action of high-pressure metal vapor, the molten metal in the molten pool produces explosive splashes, and its powerful shock wave propagates in the depth direction of the hole, forming a long and thin deep hole. In the process of laser continuous movement welding, the surrounding molten metal continuously fills the hole and condenses into a solid deep fusion weld.

3. Pinhole effect of deep fusion welding

Under the irradiation of a laser beam with a power density of up to 107W/cm2, the rate of energy input into 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. Under the action of high-pressure steam, small holes are formed in the molten pool. This hole is like a black hole in astronomy, which can absorb all light energy. The laser beam passes through this hole and directly hits the bottom of the hole. The depth of the hole determines the depth of melting.

4. Focusing effect of the side wall of the hole in the molten pool on the laser

In the process of forming holes in the molten pool under laser irradiation, the incident angle 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 beam energy superposition in the hole, which can effectively increase the beam intensity in the hole. This phenomenon is called the focusing effect of the side wall of the hole. The reason why laser can be used for welding is based on the above effect.

04 Advantages of laser welding technology

The unique effect of laser welding gives laser welding the following advantages

1. The laser irradiation time is short and the welding process is extremely fast, which is not only conducive to improving productivity, but also the welded material is not easy to oxidize, and the heat-affected zone is small, which is suitable for welding transistor components with strong heat sensitivity. Laser welding has no welding slag and does not need to remove the oxide film of the workpiece. It can even weld through glass, which is especially suitable for welding in micro-precision instruments.

2. Laser can not only weld the same metal materials, but also weld dissimilar metal materials, and even weld metal and non-metal materials. For example, integrated circuits with ceramic as the substrate are difficult to weld using other welding methods because of the high melting point of ceramics and the difficulty in applying pressure, 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 processing, such as visible edge processing of metal sheets, medical technology, etc.;

2. Deep penetration welding and brazing are mainly used in the automotive industry, among which deep penetration welding is used for car body, transmission, housing, etc.; brazing is mainly used for car body welding;

3. Laser conduction welding can process non-metals and has a wide range of applications. It can be used in consumer goods, automotive industry, electronic housing, medical technology, etc.;

4. Composite welding is mainly suitable for special steel structures, such as ship decks.