The Fundamental Principles of Laser Welding Explained

Laser welding is the process of directing a high-intensity laser beam onto the surface of metal. Through the interaction between the laser and the metal, the metal absorbs the laser and converts it into heat, causing the metal to melt and then cool and crystallize, forming a weld seam. During laser welding, the laser is directed onto the surface of the material to be welded, where it interacts with the material.

Some of the laser is reflected, while some is absorbed by the material. In the case of metal, the laser is absorbed and transformed into heat energy within a thickness range of 0.01 to 0.1 μm on the metal surface, resulting in an increase in the surface temperature of the metal, which then propagates into the interior of the metal.

The principle of laser welding is as follows: photons bombard the metal surface, causing the metal to evaporate and form vapor, which prevents the remaining energy from being reflected off the metal. If the vapor pressure on the surface of the metal being welded is high, a greater depth of melting can be achieved.

The reflection, transmission, and absorption of the laser on the material’s surface are essentially the result of the interaction between the electromagnetic field of the light wave and the material. When the laser light wave is directed onto the material, the charged particles in the material vibrate in step with the electric vector of the light wave, converting the radiant energy of the photons into the kinetic energy of the electrons.

When a material absorbs the laser, the excess energy is first manifested as the excess energy of certain particles, such as the kinetic energy of free electrons, the excitation energy of bound electrons, and sometimes an excess of phonons. These primary excitation energies are then converted into heat energy through a certain process. The principle of laser welding is illustrated in Figure 2-1.

Principle of Laser Welding
Figure: 2-1 Principle of Laser Welding

a) Pure laser welding
b) Laser wire filling welding
c) Laser-arc hybrid welding

Three types of laser welding seam cross-sections are shown in Figure 2-2.

Figure 2-2 Cross-section of three types of laser welding seams

a) Pure laser welding seam
b) Laser wire filling welding seam
c) Laser-arc hybrid welding

Laser is a novel type of light source, it is an electromagnetic wave like other light sources, but it possesses unique characteristics such as high directionality, high brightness (photon intensity), high monochromaticity, and high coherence. During laser processing, the conversion of light absorbed by the material into thermal energy is completed in an extremely short time (approximately 10-9 seconds).

Within this time frame, thermal energy is confined to the laser-irradiated area of the material, and then, through heat conduction, heat is transferred from the high-temperature zone to the low-temperature zone.

The absorption of laser by metals is mainly related to factors such as the laser wavelength, material properties, temperature, surface condition, and laser power density. Generally, the absorption rate of metals for lasers increases with temperature and with increasing electrical resistivity.

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