Unique Laser Welding Technology

Laser welding technology

Laser welding is a key application of laser processing technology. It utilizes the radiation energy of lasers to achieve effective welding. Its operating principle is to excite a laser-active medium (such as a mixture of CO2 and other gases, or YAG yttrium aluminum garnet crystals) in a specific manner, causing it to oscillate back and forth within a resonant cavity. This stimulates the formation of a stimulated radiation beam, which, when it contacts the workpiece, absorbs its energy. When the temperature reaches the material's melting point, welding can occur.

Important parameters of laser welding technology

1. Power Density:

Power density is one of the most critical parameters in laser processing. At higher power densities, the surface layer can be heated to boiling point within microseconds, resulting in significant vaporization. Therefore, high power density is highly advantageous for material removal processes such as drilling, cutting, and engraving. At lower power densities, the surface layer temperature takes several milliseconds to reach boiling point. Before the surface layer vaporizes, the underlying layer reaches its melting point, facilitating a good fusion weld.

2. Laser Pulse Waveform:

When a high-intensity laser beam strikes a material's surface, 60-98% of the laser energy will be reflected and lost. This is particularly true for materials such as gold, silver, copper, aluminum, and titanium, which have strong reflectivity and rapid heat transfer. During a laser pulse, the metal's reflectivity varies over time. When the surface temperature rises to its melting point, the reflectivity drops rapidly. Once the surface is molten, the reflectivity stabilizes at a certain value.

3. Laser Pulse Width:

Pulse width is a critical parameter in pulsed laser welding. It is determined by the depth of penetration and the heat-affected zone (HAZ). Longer pulse widths increase the HAZ, and penetration increases as the pulse width is raised to the power of 1/2. However, increasing pulse width reduces peak power. Therefore, increasing pulse width is generally used for heat conduction welding, resulting in wider, shallower welds, particularly suitable for lap welding of thin and thick plates.

However, lower peak power results in excess heat input. Each material has an optimal pulse width for maximum penetration.

4. Defocus:

Laser welding typically requires a certain amount of defocus, as excessive power density at the center of the laser spot at the focal point can easily cause evaporation and pore formation. The power density distribution is relatively uniform across all planes away from the laser focal point.

5. There are two types of defocus:

positive and negative. Positive defocus occurs when the focal plane is above the workpiece, while negative defocus occurs when the focal plane is above the workpiece. According to geometric optics theory, when the positive and negative defocus planes are equidistant from the weld plane, the power density on the corresponding planes is approximately the same. However, in practice, the resulting molten pool shape differs. Negative defocus achieves greater penetration, which is related to the molten pool formation process.

6. Welding Speed:

Welding speed significantly affects penetration. Increasing the speed reduces penetration, while too low a speed can lead to over-melting and weld penetration. Therefore, for a given laser power and thickness, there is an appropriate welding speed range within which maximum penetration is achieved.

7. Shielding Gas:

Inert gases are often used to shield the weld pool during laser welding. For most applications, helium, argon, nitrogen, and other shielding gases are commonly used. A secondary function of the shielding gas is to protect the focusing lens from metal vapor contamination and splashing of liquid droplets. This is especially true during high-power laser welding, where the ejecta are extremely powerful. A third function of the shielding gas is to effectively dissipate the plasma generated by high-power laser welding. Metal vapor absorbs the laser beam and ionizes into plasma. If there is an excessive amount of plasma, the laser beam will be consumed to some extent by the plasma.

Unique effects of laser welding technology

Compared to traditional welding techniques, laser welding offers four unique effects:

1. Weld Cleaning Effect

When the laser beam strikes the weld, impurities such as oxides in the material absorb the laser much more efficiently than the metal itself. Therefore, these impurities are rapidly heated and vaporized, significantly reducing the impurity content in the weld. Therefore, laser welding not only does not contaminate the workpiece but actually purifies the material.

2. Light Explosion Shock Effect

When the laser power density is high, the powerful laser beam causes the metal in the weld to rapidly vaporize. Under the influence of the high-pressure metal vapor, the molten metal in the weld pool explodes and splashes. The powerful shock wave propagates deep into the cavity, forming a long, narrow hole. As the laser continuously moves through the weld, the surrounding molten metal continuously fills the cavity, solidifying to form a strong, deep-penetrating weld.

3. Keyhole Effect in Deep Penetration Welding

Under laser irradiation with a power density as high as 107 W/cm², the rate at which energy is input into the weld is far greater than the rate at which heat is dissipated through conduction, convection, and radiation. This causes the metal in the laser-irradiated area to rapidly vaporize. Under the action of the high-pressure vapor, a small cavity forms in the molten pool. This cavity, like a black hole in astronomy, absorbs all light energy. The laser beam passes through this cavity and directly strikes the bottom of the hole. The depth of the cavity determines the depth of the melt.

4. Focusing Effect of the Sidewalls of the Hole in the Molten Pool on the Laser

During the process of forming a cavity in the molten pool under laser irradiation, the laser beam typically strikes the cavity sidewalls at a large angle. This causes the incident laser beam to reflect off the sidewalls and propagate toward the bottom of the hole. This results in a phenomenon in which the beam energy in the cavity is superimposed, effectively increasing the beam intensity within the cavity. This phenomenon is known as the cavity sidewall focusing effect. The application of lasers in welding is due to the aforementioned effects.

Advantages of laser welding technology

The unique properties of laser welding offer the following advantages:

1. The laser exposure time is short, resulting in an extremely rapid welding process. This not only improves productivity but also reduces oxidation of the materials being welded, resulting in a small heat-affected zone, making it suitable for welding highly heat-sensitive transistor components. Laser welding produces no slag and requires no removal of oxide films from the workpiece. Laser welding can even be performed through glass, making it particularly suitable for welding micro-precision instruments.

2. Lasers can weld not only homogeneous metals but also dissimilar metals, and even metal and non-metal materials. For example, integrated circuits with ceramic substrates are difficult to weld using other welding methods due to the high melting point of ceramic and the resistance to pressure. However, 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 bodies, transmissions, housings, 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 products, automotive industry, electronic housings, medical technology, etc.;

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