Analysis: The difference processes is mainly the difference in the absorption rate of laser energy.
(1) Fiber lasers have a shorter wavelength and produce less plasma in the welding process, with a higher and more concentrated energy density.
The utilization of laser energy is higher, and the recoil pressure of the metal vapor during the welding process will be higher, making it difficult to find a balance between the threshold of penetration and non-penetration.
(2) In the CO2 laser welding process, heat transfer loss of power, that is, the inclination of the front wall of the small hole, is also large, generating a large number of plasma to balance and regulate the distribution and absorption of laser energy in the welding process.
Therefore, there is a wide process window between penetration and non-penetration.
- Fiber laser: wavelength 1.06 μm, spot diameter 0.6 mm
- CO2 laser: wavelength 1.06μm, spot diameter 0.86mm
There is a significant difference between fiber and CO2 laser welding in weld seam formation.
Some data suggest that this difference is related to the coupling properties between the laser and the material at different wavelengths.
The melting efficiency of the laser-material connection can be measured in laser welding.
A comparison of the melting efficiency of fiber laser and CO2 laser welding is presented below:
The melting efficiency of a weld seam can be determined using its cross-sectional area.
The results of the calculation are shown in the figure below.
The melting efficiency of both laser welds first increases and then decreases as the welding speed increases.
Fiber laser welding has a maximum melting efficiency of roughly 10 m/min welding speed.
CO2 laser welding, on the other hand, has a maximum melting efficiency at a welding speed of roughly 4 m/min.
The energy coupling behavior in laser welding is related to the fluctuation of melting efficiency with welding speed.
The total absorption rate AK of the deep perforation to the incident laser can be stated as follows using the principle of energy conservation:
AK= (PF+ PEY+ Po+ PL)/P
where PEV is the power required to evaporate part of the metal during welding, Po is the power consumed by overheating the molten pool metal, and PL is the power lost by heat conduction.
According to the study, the mass MeV evaporated by laser welding is very small, so that PEV can be neglected.
The variation law of the melt pool superheat power Po with the welding speed is similar to that of the melting efficiency, but the percentage of superheat power in the laser output power is very small.
A portion of the heat transfer power Pl through the melting front is used for plate melting, and another portion is lost to the base metal due to heat transfer.
The power lost through heat conduction at the melting front can be expressed as:
where 2r0 is the weld width and S is the cross-sectional area of the weld.
The experimentally measured weld cross-sectional area and melting width can be substituted into the above equation to obtain the variation law of PL with welding speed, as shown below.
The heat transfer loss power reduces as the welding speed increases, with a greater decrease at low welding speeds and a lesser decrease at high welding rates.
Deep perforation of the total absorption rate of the two lasers AK with the welding speed variation law is shown below. It can be seen that the total absorption rate of the two laser welding with the welding speed is similar to the law of change, which are first slowly decreasing and then rapidly decreasing.
The critical speed of the total absorption rate from slowly decreasing to rapidly decreasing, on the other hand, is not the same. Fiber laser welding for 10m/min, CO2 laser welding for 4m/min.
The difference in total absorption rate between the two types of laser welding is related to whether the entire beam completely enters the deep penetration hole.
Because the laser beam entirely enters the deep penetration hole when the welding speed is low, the overall absorption rate is less impacted by the welding speed.
When the welding speed is higher, the front part of the spot can no longer vaporize the front point of the small hole, so this part of the beam can no longer enter the small hole, resulting in a rapid decrease in the total absorption rate of the incident laser by the small hole as the welding speed increases.
Conclusion
The total absorption rate and the heat transfer loss power are the main factors that determine the melting efficiency.
In terms of melting efficiency, fiber lasers are more suitable for medium and high speed welding. In contrast, CO2 lasers are more suitable for low-speed welding when the same welding process.
