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What Is the Principle of Heat Transfer in Laser Welding?

Laser welding is an efficient and precise welding method using high energy density laser beam as heat source.

Laser welding is one of the important aspects of the application of laser material processing technology.

In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding.

The welding process belongs to the heat conduction type, that is, laser radiation heats the surface of the workpiece, and the surface heat diffuses internally through heat transfer.

By controlling the parameters such as laser pulse width, energy, peak power and repetition frequency, the workpiece melts and forms a specific molten pool.

Because of its unique advantages, it has been successfully applied to the precision welding of micro and small parts.

1. Technical principle

Laser welding can be realized by the continuous or pulsed laser beam.

The principle of laser welding can be divided into heat conduction welding and laser deep penetration welding.

When the power density is less than 104 ~ 105 w/cm2, it is heat conduction welding. At this time, the penetration is shallow and the welding speed is slow;

When the power density is greater than 105 ~ 107 w/cm2, the metal surface will be concave into “holes” under the action of heating to form deep penetration welding, which has the characteristics of fast welding speed and large depth width ratio.

The principle of heat conduction laser welding is:

The surface to be processed is heated by laser radiation, the surface heat is diffused internally through heat transfer, and the workpiece is melted to form a specific molten pool by controlling the laser parameters such as laser pulse width, energy, peak power and repetition frequency.

The laser welding machine used for gear welding and metallurgical sheet welding mainly relates to laser deep penetration welding.

Laser deep penetration welding generally uses continuous laser beam to connect materials.

Its metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the “key hole” structure.

Under the irradiation of sufficiently high power density laser, the material evaporates and forms small holes.

The small hole filled with steam is like a blackbody, which absorbs almost all the incident beam energy.

The equilibrium temperature in the hole cavity reaches about 2500 degrees.

The heat is transmitted from the outer wall of the high-temperature hole cavity to melt the metal surrounding the hole cavity.

The small hole is filled with high-temperature steam generated by the continuous evaporation of wall material under the irradiation of light beam.

The four walls of the small hole are surrounded by molten metal and the liquid metal is surrounded by solid materials (in most conventional welding processes and laser conduction welding, energy is first deposited on the surface of the workpiece and then transmitted to the interior by transmission).

The liquid flow and wall surface tension outside the hole wall are in dynamic equilibrium with the continuous steam pressure in the hole cavity.

The light beam continuously enters the small hole, and the material outside the small hole is flowing continuously. With the movement of the light beam, the small hole is always in a stable state of flow.

That is, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam, the molten metal fills the gap left after the small hole is removed and condenses, and the weld is formed.

All of the above processes happen so fast that the welding speed can easily reach several meters per minute.

2. Principle of heat conduction welding

Heating material surface

Surface heat required, power > reflection + heat divergence

Heat is conducted to both sides and deep

The surface temperature reaches the melting point (TM, steel = 1490 ° C)

Absorptivity changes with temperature: – semiconductor laser: 38% (20 ° C)

To 36% (1500 ° C) – CO2 laser: 5% (20 ° C) to 12% (1500 °C) start to generate molten pool.

Heating material surface

Weld seam

The feed motion forms the weld.

The depth and slit width are related to power and speed.

Typical welding rate: 1-3m / min

The weld is smooth and free of debris.

The ratio of depth to width < 1

Weld seam

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