Laser welding process

Process parameters of laser welding

1. Power density

Power density is one of the most critical parameters in laser processing. By using a higher power density, the surface can be heated to boiling point within microseconds, producing a large amount of vaporization. Therefore, high power density is advantageous for material removal processing, such as drilling, cutting, and carving. For lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, making it easy to form a good melt weld. Therefore, in conductive laser welding, the power density ranges from 104 to 106 W/cm2.

2. Laser pulse waveform

The waveform of laser pulses is an important issue in laser welding, especially for thin film welding. When a high-intensity laser beam is irradiated onto the surface of a material, 60-98% of the laser energy will be reflected off the metal surface and lost, and the reflectivity will vary with the surface temperature. During a laser pulse, there is a significant change in the reflectivity of the metal.

3. Laser pulse width

Pulse width is one of the important parameters in pulsed laser welding. It is not only an important parameter that distinguishes it from material removal and melting, but also a key parameter that determines the cost and volume of processing equipment.

4. The influence of defocus on welding quality

Laser welding usually requires a certain amount of distance because the power density at the center of the laser spot is too high, which can easily evaporate into holes. On the planes away from the laser focus, the power density distribution is relatively uniform.

There are two ways of defocusing: positive defocusing and negative defocusing.

If the focal plane is located above the workpiece, it is positive defocusing, otherwise it is negative defocusing.

According to the theory of geometric optics, when defocused, the power density on the corresponding plane is approximately the same, but in reality, the shape of the molten pool obtained is different. When negative defocusing occurs, a greater melt depth can be obtained, which is related to the formation process of the melt pool.

Experiments have shown that materials heated by laser at 50~200us begin to melt, forming liquid phase metals and undergoing fractional vaporization, resulting in the formation of atmospheric pressure vapor, which is ejected at extremely high speeds and emits dazzling white light.

At the same time, the high concentration of vapor causes the liquid metal to move to the edge of the melt, forming a depression in the center of the melt. When negative defocusing occurs, the internal power density of the material is higher than the surface, making it easier to form stronger melting and vaporization, allowing light energy to be transmitted deeper into the material.

So in practical applications, when a large melting depth is required, negative defocusing is used; When welding thin materials, it is advisable to use positive defocusing.