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Abstract
An in situ process control monitor is presented by way of experimental results and simulations, which utilizes a pulsed laser ultrasonic source as a probe and an optical heterodyne displacement meter as a sensor. The intent is for a process control system that operates in near real time, is nonintrusive, and in situ: A necessary requirement for a serial manufacturing technology such as additive manufacturing (AM). We show that the diagnostic approach has utility in characterizing the local temperature, the area of the heat-affected zone, and the surface roughness (Ra ∼ 0.4 μm). We further demonstrate that it can be used to identify solitary defects (i.e., holes) on the order of 10 to 20 μm in diameter. Moreover, the technique shows promise in measuring properties of materials with features that have a small radius of curvature. We present results for a thin wire of ∼650 μm in diameter. By applying multiple pairs of probe–sensor systems, the diagnostic could also measure the local cooling rate on the scale of 1 μs. Finally, while an obvious application is used in AM technology, then all optical diagnostics could be applied to other manufacturing technologies.
Highlights
Additive manufacturing (AM) is a promising technology that produces working parts out of metals, polymers, and composites
Current AM methods include the use of a laser or electron beam to melt and fuse stock material that is delivered as a powder through a jet nozzle, dispensed as a powder bed with repeated layering, as a thin spool of wire that is just unreeled, or a vat of material with photolytic cross-linking properties that harden upon focused laser irradiation
surface acoustic wave (SAW) can be used to measure a change in temperature within and around a heat-affected zone (HAZ)
Summary
Additive manufacturing (AM) is a promising technology that produces working parts out of metals, polymers, and composites. An increase in the temperature typically decreases velocities of bulk waves and SAWs.[12,13] SAWs may be measured and analyzed to infer frequency (wavelength) content, making it possible to gain information about additional properties, such as roughness, grain size, or defects related to microstructure.[14,15,16,17] SAWs in this work are generated from the thermoelastic response of the material with a laser pulse.[18] SAWs produced by laser pulses can have a higher frequency bandwidth than those generated with piezoelectric transducers. The diagnostic technique that merges the capabilities of two nondestructive evaluation (NDE) disciplines (e.g., optical and ultrasound) does have the speed throughput that could even exceed some conventional or thermal[8] imaging techniques while providing more than one piece of information It can provide information from below the surface and is agnostic to the material and processing approach
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