The Effect of Increased Fe, Cu and Zn on the Microstructure and Properties of Recycling Friendly 6082 and 6005 Aluminium Alloys

To assess the effects of using post-consumer scrap in common wrought aluminium alloys, simulated recycled 6082 and 6005 alloys were produced with added common impurity elements Fe, Cu and Zn in amounts of 0.1 to 0.2 wt pct. They were investigated and compared to their primary alloy counterparts with respect to microstructure, mechanical properties and intergranular corrosion resistance. Optical microscopy, scanning electron microscopy, transmission electron microscopy and atom probe tomography were used to measure grain structure, primary- and dispersoid particles, hardening precipitates and solute distribution in the microstructure. Through process modelling was conducted to rationalize the effects of the impurities on the microstructure constituents. In both alloy groups Fe was responsible for an increase in the density and volume fraction of primary particles. The increased Cu level in 6082 led to a finer microstructure of hardening precipitates in the peak hardness condition and to a small hardness increase. Low amounts of Cu and Zn incorporate into the hardening precipitate phases. However, most Zn remained in solid solution, including in the precipitation free zones close to grain boundaries. This has been associated with a deterioration of intergranular corrosion resistance due to the galvanic coupling with Cu segregated at the grain boundary planes.