Laser welding is a highly efficient and precise welding technique that uses a high energy density laser beam as a heat source. This method is a significant aspect of laser material processing technology.
Initially, in the 1970s, laser welding was mainly used for welding thin-walled materials and for low-speed welding operations. This process is a type of heat conduction welding, that is, laser radiation heats the surface of the part and the heat is transferred internally by conduction.
By carefully controlling parameters such as laser pulse width, energy, peak power and repetition frequency, the workpiece is melted and a specific molten pool is formed.
Due to its unique advantages, laser welding has been successfully applied to the precision welding of micro and small parts.
1. Technical principle
Laser welding can be performed using a continuous or pulsed laser beam. The principle of laser welding can be divided into two categories: heat conduction welding and laser deep penetration welding.
When the power density is less than 104 to 105 W/cm 2 , heat conduction welding is considered. At this power density, penetration is shallow and welding speed is slower.
On the other hand, when the power density exceeds 105 to 107 W/cm 2 , the metal surface will become concave and form “holes” due to the heating effect, resulting in deep penetration welding. This type of welding is characterized by its 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, with the heat being diffused internally through heat transfer. The workpiece is then melted to form a specific molten pool by controlling laser parameters such as laser pulse width, energy, peak power, and repetition frequency.
Laser welding machine mainly used for gear welding and metallurgical sheet welding refers to laser deep penetration welding, which typically employs a continuous laser beam to connect materials.
The physical metallurgical process of laser deep penetration welding is similar to that of electron beam welding, where energy conversion occurs through the formation of a “keyhole” structure.
Under high power density laser irradiation, the material evaporates and forms small vapor-filled holes. These holes act like black bodies, absorbing almost all of the energy from the incident beam and reaching an equilibrium temperature of about 2,500 degrees.
Heat is then transmitted from the outer wall of the high-temperature hole cavity to melt the surrounding metal, while the small hole is filled with high-temperature vapor generated by the continuous evaporation of the wall material under light beam irradiation.
In most conventional welding processes and laser conduction welding, energy is deposited on the surface of the part and then transmitted inward. However, in laser deep penetration welding, the four walls of the small hole are surrounded by molten metal, which is surrounded by solid materials.
The liquid flow and wall surface tension outside the hole wall are in dynamic equilibrium with the continuous vapor pressure inside the hole cavity. The light beam continuously enters the small hole while the external material continuously flows out. With the movement of the light beam, the small hole is in a stable flow state and moves forward at the speed of the main beam. The molten metal fills the gap left by the small hole and condenses to form the weld.
All of these processes happen at such a fast pace that the welding speed can easily reach several meters per minute.
2. Principle of heat conduction welding
Heating material surface
The required surface thermal power is greater than the sum of heat reflection and heat divergence.
Heat is conducted both at the surface and deeply.
The surface temperature reaches the melting point (TM, for steel it is 1490°C).
Absorptivity varies with temperature: for a semiconductor laser it varies from 38% at 20°C to 36% at 1500°C. For a CO 2 laser it changes from 5% at 20°C to 12% at 1500°C, at which point a molten pool begins to form.
Weld seam
The feeding motion shapes the weld.
The depth and width of the crack are linked to power and speed.
Typical welding rate: 1-3 meters per minute.
The weld has a smooth surface and is free from impurities.
The depth to width ratio is less than 1.
