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IN SITU PROCESS MONITORING A Perspective on the Role of In Situ Process Monitoring in the Certification of Additive Manufactured Space Hardware BY ERIN LANIGAN In situ process monitoring refers to any technology that monitors an additive manufacturing (AM) process. The range of technologies is as broad as the range of nondestructive evaluation (NDE) methods and can even extend to machine health monitoring more traditionally associated with process control (McCann et al. 2021). For example, voltage, current, and pressure sensors can be used to detect if something abnormal occurs in the regular operations of the AM machine, including the machinery, laser or arc, ventilation, wire feed, or powder recoating processes. If these sensors detect an off- nominal condition, that region of the AM build can be investigated by checking the data streams from other process monitoring technologies, or after the build using NDE. For metal AM, systems that monitor the fusion process, such as real-time melt pool monitor- ing and optical or thermal tomography, are common. These systems include on-axis or off-axis photodiodes and visible or near- infrared cameras, which can be used for high- speed or time-delay image capture (Grasso and Colosimo 2017). There are a few potential applications of this data. Thermal modeling of the AM process can be useful for under- standing the thermal history and predicting microstructure, properties, and residual stress distributions. Melt pool monitoring can con- tribute real-time information about the melt pool emission and geometry. Abnormal melt pool signatures can also be a good indicator of a change in the process that may result in a material anomaly. This can be used to guide focused NDE. There have also been attempts to correlate monitoring data with computed tomography or metallography data to under- stand if monitoring indications can reliably predict material defects (Snow et al. 2021). 24 M A T E R I A L S E V A L U A T I O N • A P R I L 2 0 2 2 PHOTO CREDIT: NASA