Improving the mechanical properties and corrosion resistance of a highly alloyed Al–Zn–Mg–Cu alloy through pre-treatment at 350°C prior to multi-stage solution treatment

Abstract

This study proposed a new solution treatment process for optimizing the microstructure and properties of Al–Zn–Mg–Cu alloy, by introducing a pre-treatment process that can achieve sufficient recovery and trace recrystallization of the alloy, namely, 350°C pre-treatment (PT-350°C), prior to traditional solution treatment. Research has found that during the PT-350°C/12h process, a large amount of nanoscale η(Mg(Zn,Cu)2) phases in the extruded state dissolved, diffused, re-precipitated on the larger η(Mg(Zn,Cu)2) phases, and formed a large particles, with a size of about 0.74 μm, mainly distributed at the grain boundaries of the matrix. In the subsequent multi-stage solution treatment (MST) process, while fully dissolving, they also had a certain inhibitory effect on the growth of matrix grains. More importantly, the alloy underwent sufficient recovery and slight recrystallization during the PT-350°C/12h, significantly reducing dislocation density, suppressing the degree of recrystallization, and refining the grains of the alloy during MST process. The intragranular aging precipitate changed from the η '+ η phase of MST to the GP zone + η′ phase of PT-350°C/12h + MST, with an increase in content and a decrease in size. The distribution of grain boundary precipitates (GBPs) transformed from continuous to discontinuous. The yield strength, ultimate tensile strength, and elongation increased from 740 MPa, 755 MPa, and 2.9% to 778 MPa, 792 MPa, and 5.8%, respectively. The corrosion resistance of the alloy also had significantly improved, in 3.5 wt%NaCl solution, the corrosion current density decreased from 4.02 × 10−6 A cm−2 to 8.33 × 10−7 A cm−2.

Keywords

• Solid solution treatment
• Highly alloyed Al–Zn–Mg–Cu alloy
• Mechanical properties
• Corrosion resistance