Effects of quenching rate and over-aging on microstructures, mechanical properties and corrosion resistance of an Al–Zn–Mg (7046A) alloy

Abstract

To provide insight to the relationships between quenching rate and aging treatment on the microstructure and properties of Al–Zn–Mg alloys, 7046 A alloy, a low Cu-content commercial Al–Zn–Mg alloy, was selected in the present study. As-received hot-extruded 7046 A alloys were quenched in air and room temperature water and then aged at 120 °C for 0–120 h. The mechanical behavior, corrosion resistance and microstructures of the aged alloy were compared and analyzed. The experiment result shows that quenching rate exhibits a greater influence on the precipitates and precipitation free zones (PFZs) of aged alloys than over-aging treatment and the reasons have been discussed. The η equilibrium phases preferentially take the Al3Zr and Al6Mn dispersoids as the heterogeneous nucleation core and grow quickly inside the grains during the slow cooling process. Over-aging treatment would broaden the grain boundary PFZ and the space of grain boundary precipitates, thus improving the corrosion resistance of water quenched alloys, while it is harmful to air quenched (AQ) alloys. Pitting resistance can be summarized as: AQ24 < AQ120 < AQ60, WQ24 < WQ60 < WQ120. The yield strength, ultimate tensile strength, and elongation the T6 treated WQ alloys (∼120 °C/24 h) are ∼532 MPa, ∼568 MPa, ∼12.4%, respectively; and the corresponding values of T6 treated AQ alloys (∼120 °C/60 h) are ∼475 MPa, ∼521 MPa, ∼11.4%. Long-term over-aging (120 °C/120 h) has little effect on the hardness, while the tensile properties decrease by ∼5–6% compared to T6 treatment.