Periodic Reporting for period 4 - ALUFIX (Friction stir processing based local damage mitigation and healing in aluminium alloys) Reporting period: 2021-07-01 to 2022-12-31 Summary of the context and overall objectives of the project Wrought or additively manufactured aluminum alloys are widely used in automotive and aerospace industries and their fracture during service life of critical components could lead to significant safety hazard. Aluminum alloys contain porosities, Si rich networks and iron-rich intermetallic particles that are the source of damage by the fracture of these particles or their decohesion with the Al matrix. This will inevitably lead to the final failure of the material by coalescence of these newly formed cavities. The ALUFIX project aims at local mitigation and healing of damage in Al alloys. Improving the damage resistance of the entire structural component is usually not necessary. In practice, damage is only initiated in regions presenting high stress concentration. A friction stir processing (FSP) treatment can be locally applied to these regions. FSP is a solid-state process (typically 80% of the melting temperature) that locally extrudes and drags material from the front to the back and around the tool pin (Figure 1). The process will lead to microstructure refinement, homogenization and porosity reduction, altogether significantly delaying damage. FSP is also used to integrate finely distributed healing particles in the Al matrix. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In WP1, FSP can be used to improve the ductility of high strength aluminum alloys by a factor 2.5 without loss of strength when performing a post-FSP treatment. Such an improvement cannot be reached by heat treatments alone. The improvement is expectedly associated to the grain size reduction due to FSP and the suppressing of large Mg-Zn precipitates at the grain boundary. This significantly changes the damage mechanism. FSP also improves the ductility of SLM aluminum alloys by a factor 4 and the fatigue life by a factor 100 due respectively to the Si network breakdown and porosity reduction delaying fatigue crack nucleation (Figure 2). This work was even proven to be extendable to titanium alloys.In WP2, FSP can also be used to produce a material with local residual stress by inserting NiTi particles inside an aluminum matrix and trigger shape memory effect (Figure 3 and 4). In the first two years of the project a proof-of-concept material (with Al1050 as matrix material) has been manufactured showing local residual stresses around NiTi particles (Figure 5). Now work is under way to extend the concept to high strength (7xxx series) aluminum alloys (WP4). In WP3, FSP and additive manufacturing were also used to manufacture healable aluminum alloys. Healing was achieved in Mg supersaturated Al alloy. The healing mechanism was evaluated using in-situ heating in high resolution (35 nm) synchrotron nanoholotomography (Figure 6, performed at ESRF Grenoble) and in transmission electron microscopy (collaboration with University of Antwerp). As the process requires 16 passes of FSP, additive manufacturing is now envisioned as a new manufacturing route for these new composites. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The developments in WP1, 2 and 3 using FSP or additive manufacturing are clearly beyond the state of the art. Innovative materials have been developed and the concept of their active solutions towards improving damage or delaying crack propagation have been proven for simple systems. However at this stage it is required to extend these findings to high strength matrix alloys, to fatigue crack propagation and further understand the underlying mechanism toward additional optimization. Figure 4: Sequential steps of the manufacturing of Al/NiTi composites. From Zhao et al. 2019. Figure 2: Porosity reduction after FSP of an SLM sample. From Santos et al. (2019) Figure 5: Detection of internal stresses in Al/NiTi composite. From Zhao et al. 2019. Figure 1: Schematic of Friction stir processing. From Simar et al. (2019) Figure 6: Healing evolution at 400°C for an Al FSP + Mg sample. From Arseenko et al. (2019). Figure 3: Design of Al/NiTi composites presenting local internal stresses. From Zhao et al. 2019.
