Mechanical anisotropy in additive manufactured materials

Abstract

Metal additive manufacturing (AM) is an innovative fabrication technique that enables processing of components with complex geometries and near-net shape dimensions providing design freedom and efficiency at unprecedented levels. The potential of AM has attracted significant interest from aerospace, automotive and medical device industries. While AM-processed materials can have superior mechanical properties compared to their conventionally processed counterparts, the layer-by-layer deposition of material often results in unique micro and mesostructural features that can lead to mechanical anisotropy. The focus of this thesis is to further our understanding of the impact of i) AM-processing techniques (laser-powder bed fusion (LPBF) and directed energy deposition (DED)) and ii) alloy systems (Al-Si alloys and a maraging steel) on structure formation and corresponding mechanical performance. A comprehensive investigation of LPBF processed Al-Si alloys indicates that the micro and mesostructural features significantly impact their tensile stress-strain and crack resistance curve (R-curve) behavior. As failure initiates along the melt pool boundaries, tensile samples that are tested along the build direction and fracture toughness samples that are tested with a crack propagating perpendicular to the build direction show reduced mechanical performance compared to their orthogonal counterparts. The differences between the directions, however, can be altered by changing processing parameters such as layer thickness, hatch spacing, and scan strategy. To understand the influence of AM technique on the anisotropy characteristics of the material, dual-wire arc DED was used to fabricate a compositionally graded Al-Si alloy. Results indicate that at low Si concentrations the tensile stress-strain behavior is impacted by the orientation of the samples with those tested along the build direction resulting in lower failure strain whereas the R-curves tested in different orientations were essentially identical. With increasing Si-concentration, however, the degree of anisotropy in both tensile and R-curve characteristics increase indicating an impact of chemical composition on mechanical performance. Compared to the Al-Si materials, results from the LPBF maraging steel show near-isotropic properties in both tensile stress-strain response and R-curve behavior due to the strong bonding characteristics between the individual layers. Although post-processing thermal treatments improve the mechanical performance of the material thereby enabling levels of damage tolerance that cannot be achieved with conventional processing methods, they additionally introduce anisotropy in R-curve behavior due to the formation of austenite along the melt pool boundaries