Reducing the sensitivity of high-silicon Al-Mg-Si(Cu) alloys to intergranular corrosion

The mechanical properties and susceptibility to intergranular corrosion (IGC) of Al-Mg-Si(Cu) aircraft alloys containing 1.4-1.5%Si were studied. A different Mg/Si ratio and a different phase content of Mg2Si and Si, as a consequence, were realized for them by varying the chemical composition. It is shown, that the strength of the alloys increases and the resistance to MGC decreases as the chemical composition of the alloy moves away from the quasi-binary cross-section and the amount of residual silicon Si+ above the limit required for Mg2Si formation is enhanced. Marked No. 2 the strongest alloy and the least corrosion-resistant, at the same time, which has UTS ≥350 MPa and MGC penetration depth of more than 100 μm was determined. It contains 0.7 % Si+ and, among the other experimental alloys, this one is the closest to the upper solubility limit of 1.85 % Mg2Si in aluminum. The negative effect of Cu and Fe on MGC, as well as the temporary delay, if any between quenching and strengthening heat treatment (artificial aging), was demonstrated. A series of isothermal curves, which characterize the strengthening of alloy No. 2 during artificial aging, was obtained, and it is shown, there is no isothermal processing within the temperature range of 145-200 °С, which would increase its resistance to MGC. Analyzing the generally accepted sequence of phase transformations, which takes place during the decomposition of a supersaturated solid solution after quenching, two-stage treatment modes of 145 °С, 4 h + 220 °С, 0.5 h and 145 °С, 2 h + 165 °С, 4 h were found and tested. They provided decreasing the maximum depth of MGC penetration by approximately 1.5-2.5 without significant worsening of the mechanical properties for alloy No. 2. Keywords: aircraft alloys, strength, intergranular corrosion, residual silicon, artificial aging, multi-stage processing.