Process-influenced fatigue behavior of AISI 316L manufactured by powder- and wire-based Laser Direct Energy Deposition

Because of the enormous potential of Laser Direct Energy Deposition (L-DED) regarding the production and maintenance of components with complex geometries, this type of Additive Manufacturing processes is of great industrial and scientific interest. As two principals of L-DED, i.e., wire-based (L-DED-W) and powder-based (L-DED-P) processes, are commonly used, it is indispensable to thoroughly analyze the influence of the raw material as well as process conditions on the resulting material properties. Therefore, in the present work specimens made of AISI 316L and manufactured via L-DED-P and L-DED-W were investigated. To characterize the cyclic properties of the produced material volume, instrumented cyclic indentation tests (CITs) as well as uniaxial fatigue tests were performed. The cyclic deformation behavior obtained in fatigue tests indicate a significantly higher fatigue strength of L-DED-W material, correlating with a higher δ-ferrite fraction and smaller grain size. This is caused by the different process conditions, whereby the increased δ-ferrite fraction of L-DED-W results from the difference in chemical composition. However, the S-Nf curves show a higher fatigue limit at 2 × 106 cycles for L-DED-P, which is caused by the significantly larger process-induced nonmetallic inclusions observed in L-DED-W specimens. In summary, the present work shows significant differences between the material produced with L-DED-P and L-DED-W, and demonstrates a strong influence of process-induced defects on the fatigue behavior of additively manufactured materials.