Solid state recycling of aluminium chips: Multi-technique characterization and analysis of oxidation

Solid-state recycling of metallic alloys is a promising alternative method which avoids remelting and reduces significantly the CO2-emission compared to remelting. Contamination being critical for solid-state recycling, the objective of this study is to quantify and localize oxygen contamination at each step of the process. Pre-compacted AA6060 aluminium alloy machining chips were hot-extruded. The oxygen intake and the formation of oxides were quantified and localized through a multi-technique characterization, including X-ray photoelectron spectroscopy and transmission electron microscopy. The overall oxygen content is found to increase at every step of the recycling process. Analyses reveal that the initial thin Al2O3 layer on the chips gets covered by a MgO layer during pre-annealing and hot extrusion. Thus, alloying elements, as Mg, are involved in oxidation. It results in a tenfold increase of the oxygen content in the extrudate accompanied by a complex network of oxides with an average thickness of 290 nm and a low density. Thus, tuning processing parameters is a promising leverage to limit oxidation and improve chip welding, which will probably depend on the alloying elements.