The effect of plasma formation on beam focusing in deep penetration welding with CO₂lasers

Abstract

This paper deals with the absorption and defocusing of a CO2 laser beam by the laser-induced plasma plume in deep penetration welding. To derive the `effective` intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma`s size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO2 lasers.

Laser Welding under Vacuum: A Review

“…The strong inverse bremsstrahlung (I.B.) effect decreases the penetration depth and even stops the CO2 laser welding process due to the optical breakdown [51,52]. The vapor plasma exerts a different state between CO2 laser welding and solid laser welding.…”

Numerical Modelling Of Thermal And Structural Phenomena In Yb:YAG Laser Butt-Welded Steel Elements

“…In the case of laser welding a "keyhole" is created with ionised vapour that absorbs the laser beam power. The beam is afterwards transferred to the walls of the "keyhole" forming the fusion zone [12][13][14]. Moreover, the impact of concentrated heat source on the material causes high temperature gradients as well as different heating and cooling conditions, contributing to the formation of various structures in the weld and heat affected zone, which leads to different mechanical properties [6,10].…”

Characteristics of plasma plume in fiber laser welding of aluminum alloy

“…Usually, laser power is a decisive factor for generating laser-induced plume [5], and laser induced plasma plume point could defocus the laser energy, decrease the weld penetration and even change the welding mode from deep penetration welding to heat conduction welding if the plume is not suppressed appropriately [6][7]. Because of these significant effects, the plasma plume has been therefore considered as one of the most important phenomena for revealing the essence of laser welding.…”