RESUMO
We describe the development of a real-time nonintrusive monitor to detect degradation of a gas shield condition during laser welding by use of on-axis spectrally resolved detection of light emitted from the workpiece. Failure of gas shielding to the point at which there is a risk of contamination from the air is revealed by the marked increase in the intensity of a spectral feature around 426 nm. To avoid unwanted sensitivity to the overall intensity of the radiation, the intensity at 426 nm is normalized by that at 835 nm, where the spectrum is insensitive to gas shielding. We collected the radiation by using the same optics as are used to deliver the processing beam, and thus the detection process is entirely nonintrusive. We demonstrate successful operation for welding stainless steel and titanium under both helium and argon gas shielding.
RESUMO
Recent improvements in design have made it possible to build Nd:YAG lasers with both high pulse energy and high beam quality. These lasers are particularly suited for percussion drilling of holes of as much as 1-mm diameter thick (a few millimeters) metal parts. An example application is the production of cooling holes in aeroengine components for which 1-ms duration, 30-J energy laser pulses produce holes of sufficient quality much more efficiently than with a laser trepanning process. Fiber optic delivery of the laser beam would be advantageous, particularly when one is processing complex three-dimensional structures. However, lasers for percussion drilling are available only with conventional bulk-optic beam delivery because of laser-induced damage problems with the small-diameter (approximately 200-400-mum) fibers that would be required for preserving necessary beam quality. We report measurements of beam degradation in step-index optical fibers with an input beam quality corresponding to an M(2) of 22. We then show that the laser-induced damage threshold of 400-mum core-diameter optical fibers can be increased significantly by a CO(2) laser treatment step following the mechanical polishing routine. This increase in laser-induced damage threshold is sufficient to propagate 25-J, 1-ms laser pulses with a 400-mum core-diameter optical fiber and an output M(2) of 31.
RESUMO
Laser beam characteristics are altered during propagation through large-core optical fibers. The distribution of modes excited by the input laser beam is modified by means of mode coupling on transmission through the fiber, leading to spatial dispersion of the profile and, ultimately and unavoidably, to degradation in the quality of the delivered beam unless the beam is spatially filtered with consequent power loss. Furthermore, a mismatch between the intensity profile of a typical focused high-power laser beam and the profile of the step-index fiber gives rise to additional beam-quality degradation. Modern materials processing applications demand ever higher delivered beam qualities (as measured by a parameter such as M(2)) to achieve greater machining precision and efficiency, a demand that is currently in conflict with the desire to utilize the convenience and flexibility of large-core fiber-optic beam delivery. We present a detailed experimental investigation of the principal beam-quality degradation effects associated with fiber-optic beam delivery and use numerical modeling to aid an initial discussion of the causes of such degradation.
