Transient development of residual stresses in laser beam melting - A neutron diffraction study

Additive manufacturing (AM) is increasingly used for the production of functional parts. In order to ensure product reliability in challenging load cases and environments, a valid knowledge of the residual stress state is crucial. Since typical, complex AM geometries necessitate simulative efforts for this prediction, suitable validation data are essential. This study presents results from neutron diffraction measurements on different stages of a build-up of a simple cuboid structure by laser beam melting. Strains are generally measured in three perpendicular directions, coinciding with the symmetry axes of the structure, so that the stresses in these directions can be calculated using Hooke's Law. The strain-free reference is obtained from measurements on small matchstick geometries cut from an analogously manufactured cuboid at the respective measurement spots. By providing quasi-transient data of the evolution of residual stresses in both the base plate and the part, simulation models can be investigated towards their structural validity. Lastly, at two positions, strain measurements were obtained from six directions in order to determine an estimation of the full strain tensor. Results indicate that the assumption of negligible shear strains may not be justifiable.