Microstructure and Residual Stress Evolution During Cyclic Elastoplastic Deformation of Aisi 316l Fabricated Via Laser Powder Bed Fusion

Abstract

In metal additive manufacturing (MAM), microstructural properties such as texture, residual stresses and dislocation density have emerged as key-factors ruling the resulting mechanical performances. In this study, cylindrical AISI 316L specimens, fabricated with laser powder bed fusion (LPBF), were tested under cyclic elastoplastic (EP) deformation using a constant strain amplitude to highlight the evolution of residual stresses (RS), dislocation density and texture with increasing number of EP cycles, N, in the attempt to find correlations between relevant microstructural parameters and macroscopic properties. The structural and microstructural analysis is carried out through whole powder pattern modelling (WPPM) of neutron diffraction (ND) data and Electron Back-Scattering Diffraction (EBSD) analysis. A dual cyclic hardening-softening (H/S) behaviour is observed depending on N, with the hardening regime terminating in less than 10 cycles. The trend of the maximum tensile stress correlates closely with the trend of WPPM-calculated total dislocation density, suggesting a major role of dislocations on the macroscopic mechanical properties under the EP regime. EBSD analysis reveals rearrangement of geometrically necessary dislocations (GND) into cellular structures, and moderate grain refinement, which are deemed to be responsible for the quick fading of RS in the very early stage of EP loading (within 5 cycles). ND-based texture analysis reveals a (220) preferential orientation retained throughout the EP tests, but with orientation density functions (ODFs) changing non-monotonically with N, suggesting a preliminary partial randomization of grains around the deformation axis followed by the recovery of crystallographic anisotropy and more localized ODFs.

Keywords: AISI 316L stainless steel, Metal additive manufacturing, residual stress, microstructure, Low cycle fatigue, disloca-tions density.