title: Investigation of Residual Stress and Microstructure Effects on the Fatigue Behaviour of an Aluminium-Silicon Eutectic Alloy Produced by Laser Powder Bed Fusion
Creators: name: Roveda, Ilaria
Publisher: name: UNSPECIFIED
Year: 2023

description: The advent of additive manufacturing (AM) techniques has paved the way for new possibilities in the production of topologically optimized, near-net shape components. In recent decades, research triggered numerous developments in the field of metal additive manufacturing. Among the AM techniques, Laser Powder Bed Fusion (PBF-LB/M) is a powder-based technique, in which a laser beam selectively melts the material. PBF-LB/M is widely used for the production of near-eutectic Al-Si alloy components, highly demanded material in the aerospace, automotive and biomedical fields due to attractive properties such as excellent corrosion resistance, processability and thermal conductivity at a low price. Whilst the PBF-LB/M technology exhibits high potential, at the same time, it presents major challenges that still prevent its wide and safe industrial application. In particular, fatigue properties are essential for load-bearing applications and are less studied in the literature than static properties. The PBF-LB/M process is characterised by localized thermal cycles, with extremely rapid heating and cooling, as well as a simultaneous melting of the top powder layer and re-melting of the previously solidified layers. The complex cycles can lead to a unique multi-scale microstructure with a high content of manufacturing defects and high-level residual stress (RS). These aspects influence in determining the fatigue resistance of a component. Therefore, a prediction of the material behaviour is not possible without experimental data on the microstructure, defect distribution, and RS fields. For this reason, the aim of the study is to give a comprehensive characterisation of the microstructure and residual stress state as a function of the heat treatments. Gathering this information, in addition to the analysis of the defect content, the fatigue behaviour of PBF-LB/M AlSi10Mg can be more accurately interpreted. Several techniques have been applied for this purpose: optical and scanning electron microscopy, x-ray micro-computed tomography (μCT), diffraction methods for residual stress analysis, as well as static and cyclic testing. As the nanometric PBF-LB/M AlSi10Mg as-built microstructure significantly differs from the as-cast material, its evolution during post-processing heat treatments will also differ greatly. In fact, some studies observed that already at temperatures below 300°C, microstructural and RS modifications are triggered. Therefore, this study focuses on evaluating the effects of two low temperature heat treatments (265°C for 1 hour, HT1, and 300°C for 2 hours, HT2), which have been still scarcely studied in the literature. It is shown herein that the microstructure and residual stress state are subject to significant changes at these temperatures. A precipitation of the supersaturated silicon from the aluminium matrix was observed at 265°C. At 300°C, the fragmentation of the eutectic silicon network, present in the as-built condition, occurred. Various techniques were combined to give a full picture of the changes in the residual stress state: energy-dispersive laboratory x-ray diffraction (EDXRD) provided information on the near-surface volume, while synchrotron x-ray diffraction (SXRD) and neutron diffraction (ND) allowed the investigation of the bulk. These measurements showed that the microstructure changes are accompanied by a partial relaxation of the residual stresses in the bulk, superior in the case of the 265°C heat treatment (-55%) than at 300°C (-35%). The lower RS reduction in the case of HT2 is ascribed to the spheroidization of the silicon phase. These microstructure and RS modifications can have a beneficial effect on the fatigue properties. An extensive experimental campaign was deployed to cover High Cycle Fatigue (HCF) testing at stress ratio 0.1, and fatigue crack growth tests at stress ratios -1, 0.1 and 0.8. This allowed for the characterisation of the fatigue life through the description of S-N curves, as well as the investigation of the fatigue crack propagation threshold in the physically short (cyclic R-curve) and long (da ⁄ dN − ΔK curve) crack regime. Regarding fatigue crack propagation properties, it is observed that the microstructure is the prevailing factor influencing the fatigue resistance. The near-threshold regime is controlled by closure phenomena: the progressive increase in ductility after HT1 and HT2 leads to the development of plasticity-induced crack closure effects, which hinders fatigue crack propagation. Differently, considering the total fatigue life by means of S-N curves, the beneficial effect of the heat treatments was overshadowed by the effect of manufacturing defects. Prior to the fatigue testing, the defect distribution was analysed by means of μCT. Subsequently, the peak over threshold method was successfully applied to provide a prediction of the killer defects. Since defects play a governing role in the fatigue behaviour, defect-tolerant fatigue design has proven to be crucial. The effect of defects on crack initiation and early propagation is considered by deploying fracture mechanics approaches, which cover the specific characteristics of so-called short fatigue cracks. These approaches have been adopted to re-elaborate the experimental data, taking defects into account. This allowed the threshold stress level to be related to the defect size. The Kitagawa-Takahashi diagram, obtained by various methods such as the El-Haddad model, the Murakami’s √area approach and the cyclic R-curve analysis, proved to be a good approach to establish the maximum permissible load, below which no fatigue crack growth occurs.