KEYWORDS: Laser welding, Coherence imaging, Laser processing, Image processing, Temperature metrology, Control systems, Manufacturing, Black bodies, In situ metrology, Backscatter
Directly measuring morphology and temperature changes during laser processing (such as in keyhole welding and selective laser melting) can help us to understand, optimize, and control on-the-fly the manufacturing process. Even with such great potential, the technical requirements for such an in situ metrology are high due to the fast nature of the highly localized dynamics, all the while in the presence of bright backscatter and blackbody radiation, and possible obstructions such as molten ejecta and plumes. We have demonstrated that by exploiting coherent imaging through a single-mode fiber inline with the processing lens, we can image morphology at line rates up to 312 kHz, with sufficient robustness to achieve closed loop control of the manufacturing process. Applied to metal additive manufacturing, inline coherent imaging can directly measure powder layer thickness and uniformity, and formed track roughness including the onset of balling. Inline coherent imaging measures morphology dynamics but that is only part of the story. Temperature is also key to final part quality. Standard thermal imaging exploits blackbody radiation but are plagued by the highly variable emissivity of the region of interest, making quantitative measurement challenging. We were able to exploit the same apparatus used for coherent imaging to collect surface temperature profiles. Since we spectrally resolve a wide signature, we have overcome the emissivity problem to measure absolute temperature on the micron scale during laser processing.
In laser welding and additive manufacturing, melt pool behavior is directly related to final part quality as instability can lead to pore formation and ejected spatter. In transition and keyhole mode welding of metals, the dynamic balance of forces working to open and close the resulting vapor cavity gives rise to melt pool surface oscillations at frequencies on the order of kHz. Existing process monitoring techniques, such as high-speed video, have been extremely useful to image melt pool boundaries, but are limited in their ability to quantifiably track oscillation amplitudes and monitor high-aspect ratio features.
We exploit inline coherent imaging (a through-the-lens technique based on low coherence interferometry) to directly measure melt oscillations through the transition zone (from conduction to keyhole welding). An Yb:fiber laser is used to perform 10ms spot welds on 316 stainless steel with varying laser powers (120-630W). Morphology is measured in situ at a rate of 170kHz. Oscillations are observed starting at the onset of the transition zone (irradiance 0.63MW/cm²) with a frequency of 10.0 ± 0.3kHz. At higher power (keyhole mode, irradiance 1.8MW/cm²), the bottom of the keyhole oscillates with a frequency of 2.5 ± 0.5kHz around a maximum depth of 1mm. This trend agrees with analytic modelling dependent on melt surface tension, density and diameter, as well as complementary experiments that track total laser absorptance using an integrating sphere.
We present a Tm:fiber based broadband ASE source which was used for atmospheric CO2 detection. The average
spectral power of this source was limited to ~6.1 μW/nm which was the main limitation in detection of trace
concentrations of gases. This shortcoming was overcome by using an ultrashort pulsed Raman amplifier system with
maximum of ~127 μW/nm of spectral power density which was able to provide sensitivity better than 300 ppm for CO2.
In addition, improving the average power of the ASE provided an essential tool in lab to characterize optical elements
with sharp spectral features around 2 μm.
We report on a thulium doped silica fiber ASE source for absorption spectroscopy of CO2. The average spectral power
of this source was 2.3-6.1 μW/nm. This low spectral power of this source posed limitation in the sensitivity of the
system which was overcome by using an ultrashort pulsed Raman amplifier system with 50-125 μW/nm average spectral
power. This system produced CO2 sensitivity better than 300 ppm making measurement of CO2 possible at standard
atmospheric concentrations.
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