By Richard Green, Product Manager, Concoa
Edited by Richard Mandel
, senior editor

Now that hybrid laser welding - the combination of laser welding with traditional gas metal arc welding (GMAW) - has become a commercially available product, many manufacturing markets will likely see an increase in throughput and productivity. The hybrid process embodies the benefits of laser welding - increased travel speeds, limited heat affected zone, narrow weld joint, and excellent bead appearance. GMAW, as a secondary energy source to the weld pool area, improves overall process energy efficiency, lowers the cost of fit ups with the enhanced ability to bridge gaps, slows cooling rates, and improves the energy coupling efficiency of aluminum. It is necessary to examine the effect of gas selection on laser beam interaction, shielding effectiveness, bead characteristic, and the equipment required to deliver the correct gas mixture and flow.

The GMAW arc creates plasma and vaporizes the base material to create concavity in the front edge of the weld pool as the filler wire fills the trailing edge. This dimple in the molten weld pool reduces the overall depth that the laser beam must penetrate to improve penetration.

Vapor particles evacuating the key hole or weld area attenuate (scatter and absorb) the laser beam, reducing the amount of beam energy coupled to the base material 1 and, in turn, reducing the travel speed and depth of the weldment. 2 The larger the particles, the greater the attenuation effect, according to Greses.

Table 1 suggests that pure helium is the best choice to control the vapor particle size for CO2 or YAG laser welding. Granted, helium does have higher ionization and lower plasma formation potential than argon, but helium's lower molecular weight requires a greater flow rate to evacuate effectively the metal vapor out of the laser beam path. This increases the average cost per foot of a weld because helium costs more per unit than argon.

To optimize the shielding gas for plasma suppression, vapor particle evacuation and unit cost, a mixture of up to 40 percent to 50 percent argon should be considered. The heavier argon mixture requires less flow to evacuate the vapor particles, and provides an inert atmosphere for a longer duration as the weld pool solidifies. This facilitates greater travel speeds, and reduces the amount of trapped gases to lessen scrap rate from porosity.

Table 2, part of an Air Liquide test in nozzle design, 3 illustrates shielding efficiency and travel speed as a function of N2 (ppm) level in the weldment. Assuming that all parameters are equal, the table infers that a mixture of helium and argon enable higher productivity while maintaining weld integrity.

Properly selecting minor additions of CO2 and/or O2 to the mixture, or using them as a secondary shielding gas for the GMAW process can enhance bead characteristics further. Helium-argon mixtures tend to produce higher arc voltages, which subsequently yield wider bead profiles and greater arc instability. For this reason, using a mixture with 3 percent to 10 percent CO2 stabilizes the transfer and constricts the arc. In some cases, using a mixture containing 1 percent to 5 percent O2 may achieve superior arc stability and better tie-in (wetting) at the weld edge. Oxygen tends to provide a wider but shallower penetration profile, as compared to CO2 mixtures, based on its lower ionization and higher thermal conductivity properties.

Once the mixture has been specified for desired quality and productivity standards, the gas must be delivered as economically as possible to its point of use. Onsite mixing of a gas mixture requires a blending system capable of accurate adjustments. A system that helps maintain quality may include placing an analyzer on the blender outlet to monitor and to provide a warning if the mixture ratio is out of tolerance. Software and alarm systems are available that send such information to a local desktop computer, even forward the information to a remote location by means of fax or email. Attention to shielding gas variables such as type, flow and angle of impact will enhance weld quality and reduce beam absorption and scattering effects. With a properly designed delivery system, the user will realize higher travel speeds and greater productivity.

Footnotes:

  1. A. Matsunawa and T Ohnawa, "Beam-Plume Interaction in Laser Materials Processing," Trans. JWRI 21, 1 (1991)
  2. J Greses, P.A. Hilton, C.Y. Barlow, and W. M. Steen, "Plume attenuation under high power Nd:yttritium-aluminum-garnet laser welding," J. Laser Appl. Vol. 16, 1 (2004)
  3. F. Briand, K. Chouf, E. Verna, G. Caillibotte, and C. Caristan, "Front and Back Gas Shieldings for CO2 Laser Welding," Proceedings of ALAC 2004, Volume 2, p. 117