Decomposition kinetics of machining oil and oxidation of A356.2 aluminium alloy in aluminium recycling process

Abstract

The recycling of scrap aluminium turnings or chips requires preprocessing steps such as thermal degreasing, drying, and removal of other metal impurities on the surface of the scrap. The present study was performed using a thermogravimetric technique to understand the decomposition kinetics of water-machining oil mixtures. The weight loss curves of a water-machining oil mixture showed two distinct inflections over the temperature range studied (room temperature to 500°C), with a residue of oil still remaining at 500°C. Oxidation kinetics were studied and chemical analysis conducted of the oxide surface formed in both dry air and moist air for A356.2 aluminium alloy (Al-7 wt-%Si-O.4 wt-%Mg) using a thermogravimetric technique and Auger electron spectroscopy/X-ray diffraction respectively. The results showed different oxidation rate behaviour between oxidation in dry air and oxidation in moist air at temperatures above 525°C. That is, oxidation in moist air showed a transition of parabolic to linear behaviour which was not observed for oxidation in dry air. Auger electron spectroscopy surface analysis and X-ray diffraction revealed that only aluminium oxide wasformed at 300°C while a duplex film of MgO/Al₂O₃ formed on the surface of the matrix at relatively high temperatures (550°C).