Analysis of penetration depth fluctuations in single-mode fiber laser welds

By: Jung-Ho Cho, Dave F Farson, Matt J Reiter

The Ohio State University, Laboratory for Multiscale Processing & Characterization,

Single-mode fiber lasers produce high power beams with nearly perfect beam quality, meaning that they can be focused to a very small diameter spot with relatively long focal length optics. It is becoming increasingly clear that these laser beam optical qualities are not always ideal for welding. Welds made with these lasers are uniquely sensitive to a defect known as penetration spiking. Such abrupt fluctuations in weld penetration depth have long been a problem in electron beam welding but have not been observed in laser welding before the advent of high power single-mode lasers. In this work, the effect of laser power, travel speed and focus length and spiking severity was studied and techniques for reducing spiking were demonstrated. As an initial step, the frequency response of the weld penetration depth to sinusoidal power modulations was quantified. It was found that the laser weld keyhole responded as a second-order dynamic system for modulation frequencies in the range from 100Hz to 1000Hz. Thus, at upper end of this range, the sinusoidal response of the laser weld penetration to the sinusoidal power modulation was practically undetectable above the background noise of natural “random” spiking fluctations. However, the frequency response tests also showed that  power modulation in the frequency range from 900Hz to 3kHz had the good effects,  significantly decreasing the magnitude of the spiking penetration fluctuations. At some frequencies, the sinusoidal power modulation was able to completely eliminate spiking, but the effect was very sensitive to parameters and hence not very reliable for actual applications.  A second technique for spiking suppression in electron beam welds is “beam stirring”, where the focus spot is scanned in small circles at high frequency as it is scanned along the weld joint at the welding travel speed. This beam stirring technique was investigated for spiking suppression in the single-mode fiber laser welding process using a galvanometer scanner to produce simultaneous circular oscillation and linear travel of the focus spot. This spiking suppression technique was found to be much less sensitive to parameter settings and nearly eliminated spiking over broader ranges of circular oscillation frequency and diameter.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here