In-Process Monitoring and Adaptive Control in Micro Welding with a Single-Mode Fiber Laser

A fiber laser has excellent beam quality, enough to be applied for micro welding of electronics or automobile parts, and thus is regarded as a promising heat source for adaptive control since the laser peak power can be changed within a sub-micro-second period. This research was undertaken with the objective of obtaining a fundamental knowledge of in-process monitoring and adaptive control for the stable bead widths of full penetrations under the conditions that surround heat transfer or rapid change in welding speed when drastically changed in the micro welding of thin metal sheets with a continuous wave (CW) single-mode fiber laser beam. Concerning the surrounding conditions of heat transfer in bead-on-plate welding of a 0.1mm-thick type 304 stainless steel sheet, the bead width was so influenced with or without aluminum heat sink that the bead width increased from 320 μm to 790 μm at 10 mm/s welding speed and 75W laser power. It was found that the heat radiation was sensitive to such a rapid growth of the molten pool diameter. Moreover, the desired bead width was produced regardless of the existence of the heat sink by controlling the laser power adaptively on the basis of heat radiation intensity. As for welding speed in lap seam micro-welding of 0.1mm-thick pure titanium, it was revealed that the geometry of the full penetration weld was drastically changed from narrow to wide, or to partial penetration during rapid deceleration in welding speed from 50mm/s to 10mm/s and the corresponding laser power of 75W to 47W. Prediction of the weld bead width on the bottom surface was difficult when utilizing monitoring of the heat radiation signal. Therefore, the designed stable full-penetration welds were made by controlling the laser power adaptively according to the heat radiation signal in order to adjust the weld bead width on the laser-irradiated surface to the target weld geometry affected by thermal storage. Consequently, it was confirmed that adaptive control was effective with drastic changes of surrounding conditions of heat transfer or welding speed during lap seam micro-welding process with a CW single-mode fiber laser beam