EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

A method to avoid hot cracking phenomena for aluminium alloys in Laser Beam Melting (LBM) has been developed, here focused on the 6061 alloy. 6061 is a precipitation-hardened aluminium alloy, containing magnesium and silicon as its major alloying elements. This alloy is particularly prone to hot cracking, in particular during LBM processing. The proposed solution to remove cracks is to induce grain refinement to avoid the development of large columnar structures. To this end, various quantities of Yttrium Stabilized Zirconia (YSZ) are added to Al6061 base powder using a dry mixing (Turbula®) procedure. Experiments highlight a grain refinement effect depending on the added YSZ quantity. From 1 vol% on, SEM and EBSD images reveal an equiaxed-columnar bimodal grain microstructure. Results show that the addition of 2 vol% allows to fully avoid cracks due to a continuous equiaxed band at each melt pool boundaries. TEM investigations provide new insights into the becoming of added particles. A clear variation of nucleant sites density with melting pool depth explains the two successive microstructures transition (columnar to equiaxed and equiaxed to columnar). The evolution of the microstructures of this new 6061+2vol% YSZ alloy after post-processing heat treatments is investigated and rationalized. Subsequently, room temperature tensile tests are carried out. More precisely, the influence of the building direction, the use of recycled powder (mixing process adhesion robustness) and the effect of different heat treatments highlighted many interesting results. Charpy tests complete the study.