High strength Al recycling alloys for additive manufacturing

Abstract

The recycling of aerospace scraps is challenging, as it is composed of different grades of Al alloys, where sorting is laborious and often not economical feasible. One promising strategy for achieving high recycling rates, potentially up to 100 %, is cross-alloying of the two Al grades (2xxx and 7xxx alloys) mainly used in modern airframes. In this way, the formation of the intermetallic T-phase is promoted in a eutectic reaction, which opens up the potential for the creation of fine structures that contribute to high mechanical properties. To explore this approach, thermodynamic calculations were carried out to identify suitable compositions with high recycling potential. Selected compositions where then cast into cylindrical samples, and laser treatments simulating laser powder bed fusion (LPBF) were performed to assess the suitability for additive manufacturing. The microstructure of the laser treated samples showed columnar Al grains oriented along the solidification direction, featuring a very fine dendritic substructure. T-phase forming interconnected networks was observed in the interdendritic regions and at the Al dendrite colony boundaries. It can be anticipated that similar microstructures, which may be detrimental for the mechanical properties, will also be formed during LPBF. To mitigate this, spheroidization heat treatments were carried out, breaking up the interconnected T-phase networks. Furthermore, the laser treatment experiments revealed distinct cracking phenomena. In the almost fully eutectic model alloy, large cracks perpendicular to the scan vector direction were observed, indicating a cold cracking mechanism and a very brittle material behavior. Interestingly, the alloy that allows a 100% recycling rate performed better than alloys with reduced amounts of T-phase, which appeared more prone to hot cracking. Based on these findings, a candidate for a high strength AM alloy was selected. In the next step, pre-alloyed powder will be produced for the validation of the processability by LPBF.