Mastering Laser Welding Process Parameters

(1) Power density

Power density is a crucial parameter in laser processing. A higher power density allows the surface layer to reach boiling point within microseconds, resulting in significant vaporization. This makes high power density advantageous for material removal processes like punching, cutting, and engraving.

In contrast, lower power densities take several milliseconds for the surface temperature to reach boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, making it easier to form a strong weld.

Thus, for conduction laser welding, the power density is typically in the range of 10^4~10^6W/CM^2.

(2) Laser pulse waveform

Laser pulse waveforms play a crucial role in laser welding, particularly in sheet welding. When a high-intensity laser beam is directed at the surface of a material, the metal surface reflects 60-98% of the laser energy, with the reflectivity varying with the surface temperature.

During a laser pulse, the metal reflectivity changes significantly.

(3) Laser pulse width

The pulse width is a critical parameter in pulse laser welding. It plays a key role in determining the difference between material removal and melting and also influences the cost and size of processing equipment.

(4) The effect of the defocus amount on the weld quality

Laser welding typically requires a certain level of defocusing because the power density at the center of the laser focus is too high and can cause holes to evaporate. The power density distribution is more uniform across the planes outside the laser focus. There are two methods of defocusing: positive and negative.

In positive defocusing, the focal plane is above the workpiece, while in negative defocusing, it is below the workpiece. According to geometric optics, when the distance between the positive and negative defocus planes and the welding plane are equal, the power density on each plane is approximately the same, but the resulting molten pool shape is different.

Negative defocusing results in greater penetration, which is related to the formation of the molten pool. Experiments have shown that 50-200us after laser heating, the material begins to melt and form liquid metal, partially vaporizing and creating high-pressure steam that sprays at a high speed, emitting bright white light. The high-concentration vapor pushes the liquid metal to the edge of the pool and creates a depression in the center.

In negative defocusing, the internal power density of the material is higher than the surface, leading to stronger melting and vaporization, which helps transmit light energy to deeper parts of the material. As a result, negative defocusing is used when a deep penetration depth is desired, while positive defocusing is preferred when welding thin materials.

(5) Welding speed

The welding speed affects the heat input per unit time. If the welding speed is too slow, the heat input will be excessive and cause the workpiece to burn through. On the other hand, if the welding speed is too fast, the heat input will be insufficient, making it difficult to properly weld the workpiece.