(1) The wavelength of fiber laser is shorter, the plasma produced in the welding process is less, and the energy density is larger and more concentrated.
The utilization rate of laser energy is higher, the recoil pressure of metal vapor will be greater in the welding process, and it is difficult to find a balance point at the critical point of penetration and non-penetration;
(2) In the process of CO2 laser welding, the heat conduction loss power is large, that is, the inclination angle of the front wall of the small hole is also large, and a large amount of plasma is generated in the welding process to balance and adjust the distribution and absorption of laser energy.
Therefore, the process window between penetration and non-penetration is wide.
References are as follows:
- Fiber laser: wavelength 1.06 μm, Spot diameter 0.6mm
- CO2 laser: wavelength 1.06 μm, Spot diameter 0.86mm
There are great differences in weld formation between fiber laser and CO2 laser welding.
Some data show that this difference is related to the coupling characteristics between different wavelengths of laser and materials.
In laser welding, the coupling between laser and material can be characterized by melting efficiency.
The following is a comparative analysis of the melting efficiency of fiber laser and CO2 laser welding.
The melting efficiency can be calculated by using the cross-sectional area of the weld.
The calculated results are shown in the figure below.
The melting efficiency of both laser welding increases first and then decreases with the increase of welding speed.
The melting efficiency of fiber laser welding reaches the maximum when the welding speed is about 10m/min, while the melting efficiency of CO2 laser welding reaches the maximum when the welding speed is about 4m/min.
The variation of melting efficiency with welding speed is related to the energy coupling behavior in laser welding.
According to the principle of energy conservation, the total absorptivity AK of the deep penetration hole to the incident laser can be expressed as:
AK=（PF+ PEY+Po+ PL）/P
Where PEV is the power required for partial metal evaporation during welding, Po is the power consumed by overheating of molten pool metal, and PL is the power lost by heat conduction.
According to the research, the mass MeV of laser welding evaporation is very small, so PEV can be ignored.
The variation law of molten pool superheat power Po with welding speed is similar to that of melting efficiency, but the proportion of superheat power in laser output power is small.
Part of the heat conduction power pl passing through the melting front is used for plate melting, and the other part is lost to the base metal due to heat conduction.
The power lost by heat conduction through the melting front can be expressed as:
Where 2r0 is the weld width and S is the cross-sectional area of the weld.
The variation law of PL with welding speed can be obtained by substituting the cross-sectional product and fusion width of the weld measured by the experiment into the above formula, as shown in the figure below.
It can be seen that the power of heat conduction loss will decrease with the increase of welding speed, and the decrease range is larger at low welding speed and smaller at high welding speed.
The variation law of the total absorptivity AK of the deep penetration hole to the two lasers with the welding speed is shown in the figure below. It can be seen that the variation law of the total absorptivity with the welding speed in the two laser welding is similar, which decreases slowly first and then rapidly.
However, the critical speed of the total absorption rate from slow reduction to rapid reduction is different. It is 10m / min for fiber laser welding and 4m/min for CO2 laser welding.
The difference of total absorptivity between the two kinds of laser welding is related to whether the whole beam completely enters the deep penetration hole.
When the welding speed is low, the laser beam completely enters the deep penetration hole, so the total absorption is less affected by the welding speed;
When the welding speed is high, the front part of the spot can no longer vaporize the front point of the small orifice, so this part of the beam can no longer enter the small hole, resulting in the rapid decrease of the total absorption rate of the small hole to the incident laser with the increase of the welding speed.
The total absorptivity and heat conduction loss power are the main factors determining the melting efficiency.
Judging from the melting efficiency, when the welding process is basically the same, fiber laser is more suitable for medium and high-speed welding, while CO2 laser is more suitable for low-speed welding.