Working Mechanism Of Laser Welding (Heat Transfer Welding And Deep Penetration Welding)

According to the different ways of laser output energy, laser welding can be divided into pulse laser welding and continuous laser welding (including high-frequency pulse continuous laser welding);

According to the different power density on the spot after laser focusing, laser welding can be divided into heat transfer welding and deep penetration welding.

1. Heat transfer welding

The power density of the laser spot used in heat transfer welding is less than 106W / cm2, and the laser heats the metal surface to between the melting point and boiling point.

During welding, the absorbed laser energy is converted into heat energy on the surface of the metal material, so that the metal surface temperature rises and melts, and then the heat energy is transmitted to the interior of the metal through heat conduction, so that the melting zone is gradually expanded, and the welding spot or weld is formed after solidification, and its penetration profile is approximately semi spherical.

The mechanism of heat transfer welding is similar to TIG welding and other tungsten arc welding processes, as shown in fig. 1.

heat transfer welding diagram and weld shape

Fig.1 Heat transfer welding diagram and weld shape

The main characteristic of heat transfer welding is that the power density of laser spot is small. 

A large part of the light is reflected by the metal surface, the light absorption is low, the welding penetration is shallow, and the welding speed is slow.

It is mainly used for welding of thin (thickness < 1mm) and small parts.

2. Deep penetration fusion welding

When the power density in the laser class is large enough (≥ 106W / cm2), the metal is heated rapidly under the irradiation of the laser, and its surface temperature rises to the boiling point in a very short time (10-8 ~ 10-6s), so as to melt and vaporize the metal.

When the metal vaporizes, the generated metal vapor escapes from the molten pool at a certain speed.

The escape of metal vapor generates an additional pressure on the molten liquid metal (e.g. P ≈ 11MPa for aluminum; P ≈ 5MPa for steel), which makes the metal surface of the molten pool concave downward and generates a small pit under the laser spot.

When the light beam continues to heat and vaporize at the bottom of the pit, the generated metal vapor on the one hand compresses the liquid metal at the bottom of the pit to further deepen the pit;

On the other hand, the reaction force of the steam flying out of the pit will discharge the molten metal around the molten pool.

If this process continues, a slender hole is formed in the liquid metal.

When the recoil pressure of the metal vapor generated by the beam energy is balanced with the surface tension and gravity of the liquid metal, the small hole will no longer deepen and form a deep and stable small hole for welding.

Therefore, it is called laser deep penetration fusion welding, which is referred to as deep penetration welding for short (see Fig. 2).

If the laser power is large enough and the material is relatively thin, the small hole formed by laser welding passes through the whole plate thickness and part of the laser can be received on the back.

This welding method can also be called thin plate laser keyhole effect welding.

From the perspective of the mechanism, the premise of deep penetration welding and keyhole effect welding is that there are keyholes in the welding process, and there is no essential difference between them.

Fig.2 Principle and weld shape of deep penetration fusion welding

small hole during laser deep penetration fusion welding

Fig.3 Small hole during laser deep penetration fusion welding

Under the condition of energy balance and liquid flow balance, some phenomena caused by the stable existence of small holes can be analyzed.

As long as the beam has a high enough power density, small holes can always be formed.

The small hole is filled with metal vapor and plasma produced by the welded metal under the continuous irradiation of the laser beam (see Fig. 2a and Fig. 3).

The plasma with a certain pressure erupts into the surface space of the workpiece and forms a certain range of plasma cloud on the small hole.

The small hole is surrounded by liquid metal.

Outside the liquid metal are unmelted metal and some solidified metal.

The gravity and surface tension of molten metal tend to bridge the small hole, while the continuous metal vapor tries to maintain the existence of the small hole.

With the movement of the beam, the small hole will move with the beam, but its shape and size are stable.

When the small hole moves with the beam, an inclined ablation front is formed in front of the small hole.

In this region, with the melting and vaporization of the material, there are pressure gradients and temperature gradients around the small hole.

Under the action of this pressure gradient, the molten material flows around the periphery of the small hole from the front to the back.

In addition, the existence of temperature gradient makes the surface tension of the gas-liquid interface decrease with the increase of temperature, so a surface tension gradient is established along the periphery of the small hole.

The surface tension at the leading edge is small, and the surface tension at the trailing edge is large, which further drives the molten material to flow around the periphery of the small hole from the leading edge to the back, and finally solidifies behind the small hole to form a weld.

The formation of small holes is accompanied by obvious sound and light characteristics.

When laser welding is used to weld steel parts, when no small hole is formed, the flame on the surface of the welding part is orange or white.

Once the small hole is generated, the flame turns blue and accompanied by a burst sound.

This sound is generated when the plasma ejects the small hole.

Using the acoustic and optical characteristics of laser welding, the welding quality can be monitored.

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