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What Is the Effect of Defocus on Laser Welding?

1. Effect of defocus on laser welding

Defocus refers to the distance between the workpiece surface and the focus.

Effect of defocus on laser welding

If the focus is above the workpiece, it is called positive defocus;

The focus is located below the workpiece, which is called negative defocus.

Effect of defocus on weld shape

2. Selection of defocus amount

The power density of the spot center at the laser focus is too high, which is easy to evaporate into holes.

The power density distribution is relatively uniform on each plane away from the laser focus.

When the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding plane is approximately the same in theory, but the weld penetration and shape are different in practice.

During negative defocus, the internal power density of the material is higher than that of the surface, which is easy to form stronger melting and vaporization, so that the light energy can be transmitted deeper into the material, so as to obtain greater penetration.

3. Influence of defocus on welding

The defocus amount should be negative 1-2mm.

Effect of defocus on weld penetration, weld width and cross section

Effect of defocus on weld shape

Material: 6mm thick; stainless steel;
Power: 5kW;
Welding speed: 96cm/min

The defocusing amount decreases, the penetration becomes larger and enters the deep penetration welding stage;

When the defocus amount is large, the penetration is very small and is in the stage of heat conduction welding.

defocus on weld penetration, weld width and cross section

Effect of defocus on weld penetration, weld width and cross section

4. Summary

The defocus amount is the distance between the focus and the workpiece surface.

If the focus is above the workpiece, it is positive defocus, otherwise it is negative defocus; Negative defocusing is beneficial to increase weld penetration.

Influence on the welding quality.

Laser welding usually requires a certain amount of defocus, because the power density of the spot center at the laser focus is too high, which is easy to evaporate into holes.

The power density distribution is relatively uniform on each plane away from the laser focus.

According to the geometrical optics theory, when the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but in fact, the shape of the molten pool is different.

When negative defocusing, greater penetration can be obtained, which is related to the formation process of the molten pool.

The experiment shows that negative defocus is adopted when the penetration depth is required to be large; When welding thin materials, positive defocus should be used.

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