Insight into 3D strain partitioning in Al alloy designed for Additive Manufacturing using synchrotron X-ray nano-tomography

The microstructures of aluminum alloys designed for LB-PBF can be strongly heterogeneous. A bimodal grain structure is often observed with fine-grained regions (FG) consisting of submicron grains near the melt pool boundaries, and coarser columnar grains (CG) in the melt pool interiors. This specific grain arrangement leads to a 3D architecture at the mesoscale. The mechanical response of such microstructures was found to be influenced by the microstructure heterogeneity using digital image correlation based on SEM images collected during in situ tensile tests [Buttard2024]. The response of such heterogeneous microstructures is affected by the relative mechanical behaviour of the FG and CG regions. However, the complexity of the microstructure topology requires a 3D approach to further improve our understanding of the mechanical response of such Al alloys. Here we determined the 3D strain maps, at the scale of few melt pools, in a high strength aluminum alloy designed for LB-PBF. To this end, 3D images were collected using synchrotron X-ray nano-tomography (voxel size of 25nm3) at the ESRF (ID16B Beamline) after different strain increments. The high resolution of the 3D images allowed capturing the fine intermetallic network decorating the microstructure. This enabled to perform high resolution digital volume correlation (DVC) since these intermetallics act as a natural speckle and allow subsets as small as 2µm3 to be used. The evolution of the 3D strain fields as a function of the macroscopic stress is considered and the heterogenous distribution of strain is explored. Links to the microstructure heterogeneities are discussed as well as the dependences on the applied stress level. [Buttard2024] Buttard M. et al., Influence of microstructure heterogeneity on the tensile response of an Aluminium alloy designed for laser powder bed fusion. Acta Materialia 269, 119786 (2024).