3D spatial reconstruction of thermal characteristics in directed energy deposition through optical thermal imaging

A method to analyze and visualize thermal metrics extracted from coaxial thermal images collected during a 3D directed energy deposition is developed as a non-destructive means to assess thermally driven material characteristics and part quality. Standard practice for part qualification in additive manufacturing is through costly post-process non-destructive methods such as 3D digital computer tomography scans, or destructive cross sectional microstructure analysis. The extraction of thermal metrics throughout the build is useful for correlation to build characteristics such as defects or microstructure for process monitoring and control of additive manufacturing processes. The thermal metrics attained from the coaxial images in this work include the thermal gradient at the solidus-to-liquidus region, the maximum temperature in the melt pool, the melt pool area, and the length-to-width ratio of the melt pool, several of which have been correlated to microstructure by other researchers. To demonstrate the proposed methodology, two Ti-6Al-4V L-shaped parts, representative of typical geometric primitives fabricated with this process, were deposited with a 1-bead wide deposition on one leg of the build and 3-bead wide deposition on the second leg of the build. Image filtering techniques were applied in order to distinguish the melt pool solidus-to-liquidus region. Synchronizing thermal images to build location enables generation of a 3D spatial representation of the calculated thermal metrics. Differences in thermal metric values between separate legs of the L-shaped parts express changes in thermal history that can be expected to result in variation in microstructure.