Effect of annealing on oxidation during solid-state recycling of aluminium chips

Solid-state recycling of AA6060 chips by hot extrusion is investigated. This study focuses on understanding the formation of oxides at the former chip boundaries, that could limit sound welding of the chips. The impact of the time and temperature of the annealing treatment prior to extrusion is analysed ex-situ by coupling TGA and XPS . The results show that for temperatures above 400 °C, oxidation kinetics follows a parabolic law . On top of the 10 nm Al 2 O 3 native oxide layer , MgO islands first grow up to a thickness of 60 nm until reaching a covering rate of about 90 %, and then thicken. Limiting time and temperature of the annealing treatment is favourable to reduce oxide formation. During extrusion of the pre-compacted annealed chips, a new oxygen increase is measured. It results in larger oxides, with a thickness between 165 and 300 nm, depending on the annealing conditions. This enhanced oxidation combined with an increase of chip surface results in a covering rate of prior chip boundaries of 25 %. Extrusion was described into two sub-steps: first, densification of the chips in the billet occurs, during which oxides increase their covering rate if it was’nt yet saturated, or thicken; second, an elongation of the billet where oxides mostly grow on newly formed surfaces. A model using results from TGA , XPS , total oxygen concentration and TEM imaging is developed to quantitatively describe the sub-steps, to propose a complete description of the oxide network and to guide future selection of processing parameters. • Aluminium chips were annealed at 450 or 550°C and hot-extruded. • Decreasing the temperature limits oxidation at each step of solid-state recycling. • The oxide covering rate is quantified by coupling XPS , TGA and TEM . • It is of 90 % after annealing and of 25 % after extrusion.