In-situ Al–Al2O3 composite from recycled Al-7075 machining chips: Processing, microstructure and hardness evolution

Production of powders by grinding has recently been proposed as an energy efficient strategy for recycling short machining chips. Considering that low energy ball milling of Al-7075 machining chips produces crystalline aluminum oxide in addition to the metallic phase, this research investigates whether alumina can act as reinforcement and clarifies how ball milling-induced porosity and microstructural parameters affect hardness. For this, machining milling chips of Al-7075 were ground through low energy ball milling, with ratios of grinding bodies to chips volume of 5.0:1, 7.2:1 and 8.4:1. The obtained powders were die-pressed and sintered in Ar atmosphere. They were characterized by ICP-OES, hot extraction with carrier gas, SEM-EDS, DSC and XRD. TEM and Vickers hardness tests were also used to characterize the sintered samples. The formation of an Al-Al 2 O 3 composite with homogeneously distributed alumina particles throughout the metallic matrix was corroborated, and hardness was demonstrated to be function of the crystalline alumina content, as well as of porosity, which depends on the mean particle size of the powder. It is also evidenced that the material goes through different changes at nanoscale and microscale during the processing.