Laser welding can be performed using either continuous or pulsed laser beams. The principles of laser welding are generally classified into heat conduction welding and laser deep penetration welding.
When the laser power density is below approximately 10⁴–10⁵ W/cm², heat conduction welding occurs. In this mode, the weld penetration is shallow and the welding speed is relatively slow.
When the power density exceeds approximately 10⁵–10⁷ W/cm², the metal surface is rapidly heated and depressed to form a "keyhole", resulting in deep penetration welding. This process is characterized by high welding speed and a large depth-to-width ratio.
1. Heat Conduction Laser Welding Principle
In heat conduction welding, laser radiation heats the surface of the workpiece. The heat then diffuses into the material through thermal conduction. By precisely controlling laser parameters such as pulse width, energy, peak power, and repetition frequency, the material is melted to form a controlled molten pool and achieve welding.
2. Laser Deep Penetration Welding (Keyhole Welding)
Laser welding machines used for applications such as gear welding and metallurgical thin-plate welding primarily rely on laser deep penetration welding.
Deep penetration welding typically uses a continuous laser beam to join materials. Its metallurgical and physical process is highly similar to electron beam welding. The energy conversion mechanism is achieved through a keyhole (Keyhole effect) structure.
Under sufficiently high laser power density, the material at the surface vaporizes and forms a keyhole. This vapor-filled cavity behaves like a black body, absorbing nearly all incident laser energy. The equilibrium temperature inside the keyhole can reach approximately 2500°C [1]. Heat is then conducted outward from the high-temperature cavity walls, melting the surrounding metal.
The keyhole is filled with high-temperature metal vapor continuously generated by evaporation under laser irradiation. The molten metal surrounds the keyhole, while solid metal remains outside the molten zone. (In most conventional welding processes and in laser heat conduction welding, energy is first absorbed at the surface and then transferred inward.)
Outside the keyhole, the flow of molten metal, surface tension, and vapor pressure inside the cavity remain in dynamic balance. As the laser beam continuously moves forward, the keyhole remains stable and travels with the beam. The molten metal continuously flows around the keyhole and fills the void left behind it, solidifying as it cools and forming the weld seam.
This entire process occurs extremely rapidly, allowing welding speeds to easily reach several meters per minute.
