Fatigue crack propagation in AA5083 structures additively manufactured via multi-layer friction surfacing

Multi-layer friction surfacing (MLFS) is a layer deposition technique that allows building structures from metals in solid state. As approach for additive manufacturing, the re-heating during subsequent deposition processes is significantly lower compared to fusion-based techniques. Available research work presents promising properties of MLFS structures from aluminum alloys, reporting no significant directional dependency in terms of tensile strength. The present study focuses on the fatigue crack propagation behavior and the role of layer-to-substrate (LTS) as well as layer-to-layer (LTL) interfaces. Compact tension specimens were extracted in different orientations from the MLFS stacks built from AA5083. The crack propagation parallel and perpendicular to the LTL interfaces as well as from the substrate material across LTS interface into the MLFS deposited material was investigated. The results show that LTL interfaces play no significant role for the crack propagation, i.e. specimens with LTL interfaces perpendicular and parallel to the crack presented no significant differences in terms of their fatigue crack propagation behavior. The specimens where the crack propagated from the substrate material across the LTS interface into the MLFS deposited material showed higher fatigue life than the specimens with crack propagation in the MLFS deposited material only. Crack retardation can be observed as long as the crack propagates within the substrate material, which is associated with compressive residual stresses introduced in the substrate during the layer deposition process.