Laser powder bed fusion AlSi10Mg damage and fatigue resistance improvement by post-processing
(2021)
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- Authors
- Supervisors
- Simar, Aude
- Abstract
- The laser powder bed fusion (LPBF) additively manufactured AlSi10Mg alloy is a promising material for lightweight structural applications in the aerospace industry. It presents a very fine microstructure which grants high mechanical strength. However, there is still a gap in the understanding of the correlation between the microstructure and the mechanical behaviour. Besides, the critical fatigue issue still poses a significant hurdle in relation to the nature of this production method. Despite the relative flexibility of the LPBF additive manufacturing process, there still exists a limitation in the possibility of tailoring parameters to obtain a satisfactory combination of porosity and surface roughness for adequate fatigue resistance. From this quandary arises the interest for post-treatments that could help overcome this limitation. First, a comprehensive microstructural characterisation was carried out. It started by looking at the as built state. A specific processing parameter, the build platform temperature, stands out for its significant influence in the microstructure, as it affects the cooling rate and thermal gradient during manufacturing. Setting the build platform temperature to 200°C yields a negligible residual stress level. However, the strength is lower compared to that obtained using a build platform temperature of 35°C, with a similar fracture strain. A detailed 3D microstructural analysis involving focused ion beam/scanning electron microscopy tomography was performed to describe the connectivity and size of the Si-rich eutectic network and link it to the strength and fracture strain. The coarser microstructure of the 200°C build platform material is more prone to damage. The α-Al cells as well as the Si-rich precipitates present a larger size in the 200°C material, the latter thus having a lower strengthening effect. The Si-rich eutectic network is also less interconnected and has a larger thickness in the 200°C material. An analytical model is developed to exploit these microstructural features and predict the strength of the two materials. To complete the study on the as built condition for 35°C build platform temperature LPBF AlSi10Mg, damage nucleation sites and fracture path were analysed and compared for two loading directions. For the first time, the orientation of the melt pool border is shown to present negligible effect on a ductility exceeding 10%, in contrast to previous works. Through observation and statistical analysis of damage, it is found for both loading directions that the melt pool border does not exhibit damage localisation. When the melt pool border is perpendicular to tensile load, the crack propagates frequently along the coarse melt pool due to easier damage growth in this relatively softer region. Nevertheless, the ductility is barely compromised owing to delayed damage coalescence across larger α-Al cells. Post-treatments, aiming at eliminating inherent defects related to LPBF such as residual stresses, porosity or inhomogeneity, result in significant changes in the microstructure and impact both the hardening and the damage mechanisms of the post-treated material. The fracture of LPBF AlSi10Mg was studied under as built and three post-treatment conditions, namely two stress relieve heat treatments and friction stir processing (FSP). It is found that the interconnected Si network fosters damage at low strain due to the brittleness of the Si phase. The onset of damage transfers load to the enclosed Al phase which then fractures quickly under high stress, thus leading to low material ductility. In contrast, when the Si network is globularised into Si particles, the ductility is highly increased even in the case where the porosity and inhomogeneity of the microstructure remain after the post-treatment. The ductility enhancement results from the delay in void nucleation on the Si particles as well as from the tolerance for void growth in the Al matrix. The fatigue crack initiation of as built laser powder bed fusion AlSi10Mg alloys is highly affected by the presence of large porosities inherent to the process. Hot isostatic pressing (HIP) is a potential source of reduction of this detrimental porosity. Classic high temperature HIP treatments at about 500°C lead to significant strength loss that is partially recovered after additional solution and ageing heat treatments. In this work, a 350°C HIP treatment performed under a pressure of 300 MPa for 2 hours allowed to supress these large porosities and lead to higher fracture strain than after the classic HIP treatment combined with the multiple step heat treatment. However, this condition is not sufficient to lead to any improvement of the fatigue life compared to the as built condition. Friction stir processing (FSP) is a promising post-treatment solution to tackle the porosity and fatigue issues. FSP of laser powder bed fused AlSi10Mg leads to the globularisation of the Si-rich eutectic network, microstructural homogenisation and porosity reduction, proving to be a viable solution to improve the fatigue behaviour. Indeed, thanks to this post-process that reduces overall porosity and eliminates critical defects while keeping a satisfactory fine microstructure and static mechanical behaviour, the fatigue resistance of the material was significantly enhanced. In contrast, the other studied post-process, stress relieve heat treatment, did not have a significant impact on porosity, which remained the main factor behind fatigue crack nucleation, the cyclic life of the material being thus little affected with respect to the as built state. Furthermore, both post-treatments decreased the fatigue crack growth rate by an order of magnitude, with plasticity induced crack closure and crack branching probably playing a notable role. This thesis establishes a comprehensive microstructure characterisation of laser powder bed fusion AlSi10Mg in as built and post-processed conditions. Coupled with the mechanical behaviour evaluation, it allows to determine the influence of post-treatments and to postulate friction stir processing as a useful tool to remediate issues specific to LPBF AlSi10Mg and extrapolable to other additive manufactured materials.
- Affiliations
Citations
Santos Macias, J. G. (2021). Laser powder bed fusion AlSi10Mg damage and fatigue resistance improvement by post-processing. https://hdl.handle.net/2078.5/114510