Microstructure and stress evolution in LPBF revealed by in situ X-ray and neutron Diffraction

The development of microstructure and residual stress during laser powder bed fusion (LPBF) is governed by complex thermal histories involving rapid cooling and repeated reheating. To investigate these effects in real time, we applied complementary operando techniques to track phase transformations, strain evolution, and crystallographic texture formation during LPBF. In the first study, high-speed in situ X-ray diffraction was used to examine phase evolution in hypo-eutectoid carbon steel. The measurements revealed a transition in the primary solidification mode from δ-ferrite to austenite under rapid cooling, followed by the formation of martensite once the cooling rate exceeded the transformation threshold. Post-mortem analysis confirmed the presence of a lath martensitic microstructure with high hardness. In addition, both diffraction and simulation data indicated that cementite was the dominant carbide phase formed during intrinsic heat treatment (IHT). These results demonstrate how operando diffraction can resolve the timing and nature of solid-to-solid transformations, including tempering and carbide precipitation, under the unique thermal conditions of LPBF. The second study focused on 2205 duplex stainless steel and employed operando neutron diffraction and Bragg-edge imaging to investigate residual stress and texture formation. Neutron diffraction enabled layer-by-layer tracking of strain evolution during LPBF, showing that stress redistribution occurs primarily within the first few layers beneath the heat-affected zone. Texture analysis revealed strong crystallographic alignment, which significantly influenced stress determination and highlighted the importance of accounting for orientation-dependent elastic constants. Bragg-edge imaging further mapped spatial variations in strain and texture throughout the bulk of the build. Together, these studies demonstrate how real-time X-ray and neutron diffraction provide access to complementary length and time scales, offering a comprehensive view of the processes driving microstructure evolution and residual stress development in metal additive manufacturing.