X-Ray Computed Tomography Inspection in Metal Additive Manufacturing: The Role of Witness Specimens

• Published in: Structural Integrity of Additive Manufactured Materials and Parts pp. 139 - 156
• Main Authors: du Plessis, Anton, Waller, Jess M., le Roux, Stephan G., Yadroitsava, Ina, Yadroitsev, Igor, Els, Johan, Prinsloo, Jacobus
• Format: Book Chapter
• Language: English
• Published: 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 ASTM International 01.09.2020
• Subjects: Laser Powder Bed Fusion; Manufacturing Engineering; Materials & Manufacturing Processes; Metal Additive Manufacturing; Microct; Nondestructive Testing; Porosity; Stop-Start Flaw; Witness Specimen; X-Ray Tomography
• ISBN: 9780803177086; 0803177089
• DOI: 10.1520/STP163120190109

This work highlights the capabilities for high-resolution X-ray computed tomography (CT) inspection of witness specimens, built alongside a complex part, in metal additive manufacturing. Such witness specimens, which can be standardized in their dimensions (fixed diameter of 15 mm, with cylindrical shape built in a vertical orientation), allow X-ray CT inspections with fixed and reproducible workflows. The detection of improper process parameters of the additive manufacturing system is possible, as is demonstrated in this paper. It is also demonstrated how the presence of inclusions/contamination in the powder feedstock can be detected in the witness specimen. A series of Ti6Al4V witness specimens with varying porosity distributions are presented, which were part of a previous study of builds of the same set of parts on different laser powder bed fusion systems. This demonstrates how various process parameter errors are highlighted and proven to be detectable in witness specimens using standardized CT procedures. More importantly, it also allows the potential to detect layered flaws, which can occur horizontally in the build plane. Such layered flaws may originate from reduced laser power, improper powder spreading, or due to complete shutdown and restart of a build. A complex bracket and witness specimen cylinder was built and a layered flaw was artificially induced by shutting down the system and restarting it. The positive detection of the flaw by CT in the witness rod is demonstrated. This witness rod was recently part of a round-robin test, and the layered flaw was successfully identified by all ten participants in the round-robin test. The witness rod and complex part were subsequently sectioned and optical microscopy reported here. This approach is especially useful for inspection of larger parts, which cannot be inspected using X-ray CT at the highest possible resolution due to part size and associated CT scanning time limits.