In Situ Tracking of Phase Changes Within an Additively Manufactured Steel via Laser Ultrasound

Author(s):

Frank Kellogg, Michael Rupinen, Alexander Butler, Andelle Kudzal, Joshua Taggart-Scarff, and Daniel Field

Abstract:

Metals-based laser powder bed fusion (LPBF) additive manufacturing (AM) involves the rapid melting, solidification, and reheating of metallic powder as parts are built layer-by-layer. This process can lead to the development of residual stress within the part, which can be large enough to cause the sample to tear itself apart when being removed from the build plate, creating the need for stress-reducing heat treatments after each build. In this study, microstructural changes to a low-alloy, high-strength steel produced via LPBF were monitored in situ during simulated low-temperature stress reliefs using a Laser Ultrasound Measurement (LUMet) add-on to a Gleeble 3800 system. Ultrasound techniques are influenced by several factors including stress state, microstructure, and temperature; thus, LUMet testing was complemented with X-ray diffraction, dilatometry, and microscopy to further elucidate the mechanisms leading to changes in signals. Based on the combined results, it appears that the LUMet can track the decomposition of retained austinite in the LPBF AM samples during stress-relief simulations on the Gleeble.