Laser powder bed fusion (LPBF) is an additive manufacturing (AM) process involving local fast solidification and repeated intrinsic heat treatments due to the small heat source building up residual stresses because of large temperature gradients and phase transformations. Residual stresses affect the performance and reliability of the manufactured part due to crack formation, part distortion and their apparent mechanical properties. Thus, the understanding of the formation of residual stresses during the LPBF process is of key importance to ensure reliable manufactured parts.
    Neutron diffraction is a non-destructive technique used to investigate residual stresses. Thanks to the deep penetration of neutrons within metals, compared to synchrotron radiation, residual stresses can be investigated in the bulk of metallic samples scaling up the comparison with industrial applications. For LPBF AM process, residual stress measurements with neutrons are performed at the end of the manufacturing and simulations are often used to study the transient stress evolution without in situ validation. One of the reasons for the lack of such studies is the absence of available LPBF machines designed for neutron studies.
    The present work takes advantage of the new LBPF machine designed for neutron studies to investigate the development of residual stresses in a 2205 duplex stainless steel cuboid sample of size 12×12×13.5 mm3 during LPBF AM. The sample was printed in situ at the Pulse-OverLap-DIffractometer (POLDI) at the Swiss spallation neutron source. The residual stresses were measured along the building and one of the two transverse directions at three different positions along the transverse direction. Three sequences of 4.5 mm were printed and residual stresses were investigated for each of them. In addition, when possible, real-time diffraction measurements were performed while printing.
    Thanks to the quasi-transient as well as the real-time evolution, our study provides insights into the mechanisms governing the formation of residual stresses in textured AM materials. Overall, the present study highlights the potential of the newly developed LPBF machine combined with neutron techniques as a powerful tool for studying AM. Leveraging the forthcoming upgrade of the POLDI beamline, higher spatial and temporal resolutions as well as signal-to-noise ratio are expected to push forward the possibilities of the technique and our understanding of AM.