Investigation and modelling of post processing of AlSi10Mg manufactured by LPBF

The integration of additive manufacturing, specifically Laser Powder Bed Fusion (LPBF) with Aluminium alloys, represents promising opportunities for industries such as automotive and aerospace, where components with complex designs and optimal strength-to-weight ratios are crucial. However, the industrial reliability of LPBF technology is limited by the complex and not fully understood process-structure-property relationship in the scope of fabrication. This thesis experimentally explores and demonstrates the interconnected nature of this relationship, exploring the potential for creating numerical models to tailor microstructures in post-process treatment using simple thermal sensors. The irregular response of the LPBFmanufactured component to the thermal load suggests the ongoing microstructural transformation within the specimen induced by the elevated temperature, which, in further context, demonstrates an alteration of the mechanical properties. Moreover, the thesis meticulously studied the effects of the position of the specimens on the building platform when manufacturing more specimens simultaneously on the geometrical dimensions and tolerances of the manufactured cubes. The findings have great industrial significance, especially for the demanding fields requiring high accuracy of the manufactured components. Additionally, this work addresses the persistent issue of defects in LPBF-manufactured Aluminium Alloy. A Finite Element Method (FEM) numerical model was proposed, predicting lack-of-fusion defects in AlSi10Mg components ats mesoscale, which was validated through experiments. Optimal LPBF processing conditions for near fully dense components were identified and will serve as a foundation for further modelling of post-processing treatments. The extensive literature review revealed a lack of standardized guidelines for post-process treatment of LPBF-manufactured AlSi10Mg components. Due to the absence of heat treatments specifically tailored for LPBF-manufactured AlSi10Mg, the industrial practice usually applies heat treatments standardized for cast Aluminium alloys, which are, due to the significant microstructural differences between as-built and as-cast AlSi10Mg, no longer effective. As an innovation, this thesis introduces a novel thermo-mechanical post-process treatment. The here-presented pinless Friction Stir Spot Processing (FSSP) aims to provide microstructural homogenization, porosity reduction, and material softening. The identified optimal processing window yielded low distortions and a significantly higher effective depth compared to traditional methods like shot peening, while maintaining distortion levels comparable to those originating from the manufacturing process. To support the study of that complex thermomechanical process, a new numerical model simulating material flow during treatment was proposed. The numerical model, utilizing a Coupled Eulerian-Lagrangian approach, offers an accurate prediction of phenomena such as flash formation and void suppression. The successful validation of the proposed model suggests that the proposed numerical model can be considered an effective tool in the optimization of the FSSP processing of LPBF-manufactured components. The numerical model works well in synergy with the numerical model simulating the LPBF process that was also introduced in this thesis.The experimental results of the FSSP post-processing method showed that localized microstructural transformation within the TMAZ resulted in mechanical properties that diverge significantly from those of the as-built material. This phenomenon of localized anisotropy, which can manifest differential mechanical behaviour, could be particularly advantageous in numerous industrial applications, such as automotive, aerospace, and healthcare. In the context of different materials, the effect of this post-process treatment might be advantageous for the production of medical implants, where it could effectively reduce the risk of stress shielding by creating areas with brittle and ductile mechanical properties. The synergistic integration of LPBF, renowned for its exceptional design flexibility, with the capability of FSSP to tailor mechanical properties in a localized manner, presents a transformative potential in the post processing of LPBF aluminium alloys. Moreover, the efficacy of the presented post-process treatment, its relative simplicity, long tool life and the low environmental impact of the treatment hold the potential to determine the FSSP as the ideal type of post-processing for LPBF-manufactured AlSi10Mg alloy.