Experimental and numerical investigation of the fatigue crack growth behaviour of residual stress-modified additive manufactured aluminium samples

Additive manufacturing offers new possibilities for the manufacturing of near-net-shape structures. Although additive manufacturing processes are constantly enhanced and more and more materials are introduced, the knowledge about the effect of microstructure and residual stresses on the fatigue behaviour is relatively limited. In this project, residual stress-modified specimens manufactured by wire-based laser metal deposition (LMD) of an Al-Mg alloy will be investigated in regards to the resulting fatigue crack growth behaviour. Wire-based LMD facilitates comparatively high deposition rates. However, considerable higher energy inputs are required for this purpose, causing high temperature gradients that result in residual stresses as well as in coarse-grained structures. Residual stress-modifying techniques, such as laser shock peening (LSP), enable innovative possibilities for the local adjustment of residual stresses and modifications of the microstructure in additive manufactured structures. In this context, the goal of LSP is to improve the mechanical properties in additive manufactured specimens. The focus of this project is on the investigation of the influence of both the initial and modified microstructure as well as residual stresses of additive manufactured specimens on the resulting fatigue crack growth. To answer this question, a coupled experimental-numerical holistic approach is taken. For this purpose, AA5087 structures are manufactured via wire-based LMD, post-processed and subsequently characterised with regard to microstructure and residual stresses. The LMD process is adapted in such a way that two distinct microstructures, intended for the experimental investigation of their influence on the fatigue properties, are generated. To clarify the influence of the residual stresses, LSP treated specimens are analysed. Besides the investigation of the residual stress modification by LSP, which is expected to lead to an improvement of the fatigue crack growth properties, the influence of microstructural changes due to LSP in additive manufactured specimens will be also addressed. Numerical process simulations of the LMD and LSP process will be conducted and combined in a multistep simulation strategy to predict the fatigue crack growth behaviour and to clarify the importance of the microstructure and residual stresses on it. The gained knowledge by the coupled experimental-numerical approach will contribute to the quantification of the influence of the microstructure and the residual stresses in additive manufactured structures on the fatigue behaviour.