A method is presented that combines pulsed laser ultrasonics with laser heterodyne interferometry for possible application as an in-situ process control for additive manufacturing. The method implements two lasers that are focused near the build area (i.e. heat affected zone : 1) a pulsed laser that excites a surface acoustic wave (the probe signal) at or near the build area and 2) a CW laser heterodyne interferometer operating as a sensor that measures the time resolved features of the propagated waves. We have conducted investigations on the utility of this type of in situ probe-sensor system for real time measurement of the local temperature, surface defects, surface roughness, and grain boundary (grain size) determination. The all-optical method allows for remote in-situ process control that can be tailored for different build situations and materials. The current setup utilizes a UV (355 nm) or visible (532 nm) pulse laser and a narrow band 488 nm CW laser. By measuring the surface displacements with sub nm accuracy and by conducting analyses on the arrival time of the signal and frequency, the interferometric technique can characterize materials akin to non-destructive evaluation (NDE). Prior, we have presented evidence on the utility of the technique to measure local temperature, we now present evidence for surface defects/roughness and grain boundary identification. In addition, we are now exploring the utility of laser ultrasound to monitor changes in residual stress.
Anthony J. Manzo and Henry Helvajian, "Possible application of laser heterodyne and ultrasonics as in situ process control for additive manufacturing (Conference Presentation)," Proc. SPIE 10523, Laser 3D Manufacturing V, 1052314 (Presented at SPIE LASE: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2296437.5751465996001.
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