Effect of plasma plume produced by vacuum laser welding on energy transmission

The correlation between plasma plume and laser energy transmission during vacuum laser welding of aluminum alloy was discussed in this study. The electron temperature and density of the plasma plume were calculated by spectroscopic analysis. The refraction and attenuation of laser energy were studied with a probe laser. The scattering effect of nanoparticles in the plasma plume on laser energy was studied. The influence of plasma plume on laser energy transmission mainly includes two ways: the changes in the area and location of the laser spot caused by laser refraction and the attenuation of laser energy mainly affected by inverse bremsstrahlung absorption and scattering. It was found that all these factors were sensitive to ambient pressure and decreased with the decrease of ambient pressure. For the refraction effect, the average area of laser and center position deflection angles were subject to a decrease as the ambient pressure decreased. When ambient pressure was 10⁵ Pa, the average area and center position deflection angle of the laser was 5.48 mm² and 1.22 mrad. While they would reduce to 3.12 mm² and 0.118 mrad when ambient pressure was declined to 10¹ Pa. When the ambient pressures decreased from 10⁵ Pa to 10¹ Pa, the attenuation ratio of the laser energy would reduce from 13.5% to 0.1% which had a great relationship with the plasma plume. For the inverse bremsstrahlung absorption effect, when the ambient pressure was 10⁵ Pa, an 8.3% attenuation ratio of laser energy loss was produced while it would decrease to 0.027% with a further decrease of ambient pressure to 10³ Pa. For the scattering effect, it was mainly determined by the size of the generated nanoparticles during welding. When ambient pressure dropped from 10⁵ Pa to 10¹ Pa, the average diameter of nanoparticle also declined from 100 nm to 27 nm, reducing the attenuation ratio to decrease from 2.8% to 0.001%. The main attenuated mechanism of plasma plume to laser energy was inverse bremsstrahlung absorption when the ambient pressure was 10⁵ Pa to 10³ Pa while it was caused by other reasons under a lower ambient which a further investigation was needed.