A survey of sensing and control systems for machine and process monitoring of directed-energy, metal-based additive manufacturing

Additive manufacturing of metal parts is, even in the simplest of cases, a complex undertaking. Parts typically involve hundreds or thousands of individual laser or electron-beam deposits, each of which involve a complex interaction between energy source, feedstock, and substrate. During deposition, many of the independent process variables that contribute to overall build quality-such as travel speed, feedstock flow pattern, energy distrbution, gas pressure, etc-are subject to perturbations from systematic fluctuations (such as changing build geometry or growing global temperature) and random external disturbances (such as spatter on a cover lens). Such process variations affect final part quality, including dimensional tolerance, microstructure, and properties. Researchers have utilized a wide variety of sensor data and analysis for quality monitoring and real-time control of the component geometry, microstructure, and properties. Process attributes that have been targeted for measurement and control include melt pool geometry, temperature, and layer build-height; process parameters that have been utilized for control include processing-head stand-off, substrate angle, travel speed, material feed-rate, and beam power. Here, we survey many of these methods for laser-based, directed-energy deposition, and briefly discuss recently-introduced methods for real-time, closed-loop control of build-plan.