Frequently Asked Questions
The high power densities produced using finely focused, circularly symmetric spots from CO2 and Nd:YAG laser beams have been used for many years in the production of deep penetration, keyhole laser welds. The main materials of interest have been steel, aluminium and titanium alloys. For all of these alloy types, the formation of porosity during laser welding can, however, be a cause for concern.
The occurrence of large-scale (>0.4mm diameter) porosity can be reduced or completely eliminated - particularly when welding aluminium alloys - if two foci are introduced into the beam focal plane. This effect is believed to be due to elongation of the laser welding keyhole, providing a larger surface area for the escape of gas which would otherwise form pores. A single keyhole must still be maintained, so the separation of the two foci can be small (~0.3mm). Reduced porosity has been observed whether the two spots are aligned parallel to, or perpendicular to, the direction of welding.
The fibre optic delivery of Nd:YAG laser beams, and the use of glass optics to manipulate and focus these beams, has meant that compact laser welding heads have been developed which produce two, side-by-side foci from a single laser source.
For CO2 lasers, similar techniques using zinc selenide refractive optics have been developed, as well as techniques using specially manufactured mirrors. An elliptical beam focus, effectively elongating the keyhole, can be formed using a misaligned parabolic focusing mirror.
Twin spot techniques can also be used to enhance the gap bridging capability of laser beams in the production of tailored blanks in the automotive industry. Further modifications to the optics allow different levels of energy in each of the two spots. This technique has proved beneficial in the laser welding of dissimilar thickness material, in the butt joint configuration, where the spot containing the larger amount of energy is biased in position to the thicker side of the joint.
FAQ: Can I focus my laser beam to energy profiles which are something other than simple circular spots?