Utility of optical heterodyne displacement sensing and laser ultrasonics as in situ process control diagnostic for additive manufacturing

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.