UC Davis

A total of five different types of specimens were additively manufactured by directed energy deposition (DED) process. The specimens have a functionally graded material (FGM) structure, which has been deposited with variation of chemical composition of ferritic and austenitic steel powders in each interlayer on a steel substrate. Residual stress distributions were experimentally measured through the thickness of the specimens by the contour method, neutron diffraction, and deep/incremental center hole drilling. Neutron diffraction provided three orthogonal stress components in each FGM part and the results were compared to the two-dimensional stress map obtained by the contour method and confirmed its criticalities from the highly spatial resolved depth profile by the hole drilling method. Significant variations from tension to compression (up to 950 MPa) in the sine-wave stress profile were alleviated to about 430 MPa when the FGM were deposited with orthogonal or island DED scanning strategies with interlayers. Gradual changes (16.3–12.1 × 10−6/°C) of the thermal expansion coefficient were measured among the inserted DED FGM parts and grain structure with defects along the interface was three dimensionally examined by neutron tomography.