Q: After reading your September column, we now understand the importance of the correct value of power density of the laser beam as focused on the material surface. We searched for devices to measure the power density directly and safely but were not able to find one. In searching for other devices that measure the laser beam characteristics, we found the cost of those devices to be quite high; high enough to have to budget it for the next fiscal year. What can we do in the meantime? Is there a way to either directly measure power density or the characteristics of laser beams at a reasonable cost to get the approximate values of power density? A: Let me reemphasize the importance of the power density of the focused laser beam. Practical experience indicates that in more than 90% of the cases, weld defects are directly related to an incorrect power density value and the laser beam quality. The remaining weld defects originate from the weld geometry, shielding gas (composition and delivery), metallurgical composition of the metals, etc. As far as I know, there are no commercially available devices that directly and safely measure the power density of the focused laser beam. What we can measure are the laser beam characteristics that allow computation of the power density. There is one more point to consider. In general practice, laser users talk about the laser beam’s power density at focus. In everyday welding, especially in conduction welding, it is more important to know the power density of the laser beam on the material’s surface rather than at the focal point. Let’s look at the requirements for measuring the quality of the focused laser beam, per Paragraph 8 of AWS C7.4/C7.4M:2008, Process Specification and Operator Qualification for Laser Beam Welding. “Par 8. System calibration. All equipment utilized in qualification and production welding shall be on a maintenance and calibration schedule, which assures the equipment performs satisfactorily on each welding sequence. The maintenance and calibration schedule may include measuring the laser power, laser spot diameter, and the spatial profile of the laser beam at the focal plane. Verification of calibration shall be made at least once a year or after any major component of the laser generator, beam delivery hardware and optics, or motion system has been repaired or replaced.” The upcoming version of C7.4 expands upon the above requirement and provides additional details: “Par. 9.1 Suggested equipment qualification schedules “9.1.1 Full qualification…when the equipment is installed, relocated, or after significant maintenance or repair activity. LASER WELDING Q&A 24 WELDING JOURNAL / NOVEMBER 2016 BY SIMON L. ENGEL For info, go to aws.org/adindex
Welding Journal | November 2016
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