Investigation by operando synchrotron X-ray diffraction of the phase transformation in 316L-IN625 multi-material direct energy deposition

Stainless steel 316L is widely used and frequently studied in additive manufacturing. Although designed to be fully austenitic, its microstructure, grain morphology, and phase composition vary with different processes and parameters, sometimes displaying a significant fraction of ferrite. Simulations and ex-situ observations suggest that cooling rates, solidification velocities, and local chemical composition gradients play key roles, but no experimental observations could validate that hypothesis and determine the role of each factor. This study uses operando synchrotron X-ray diffraction and ex-situ observations to examine the solidification process of 316L deposited via wire-based laser direct energy deposition, both pure and on Inconel 625, a bi-material combination also common in literature and industry. Real-time diffraction data revealed the formation and the evolution during the cooling of the BCC and FCC phases, along with the temperature profiles. EBSD and feritscope measurements confirmed that ferrite is retained in the microstructure. The results validate that in the case of laser DED, the primary BCC solidification of 316L is determined by a competition between chemical composition and solidification velocity, while the cooling rates influences the phase retention. This study sets a precedent for operando investigations of commercial additive manufacturing processes in medium energy synchrotron X-ray radiation, and contributes to the understanding of the solidification modes of 316L deposited by laser DED.