Influence of isothermal and non-isothermal aging treatments on microstructure and properties of Al-Zn-Mg alloy helical profile

Aging treatment was an important way to improve the mechanical properties of Al-Zn-Mg alloys. In this work, the Al-Zn-Mg (7 N01) alloy helical profile was extruded and treated by different kinds of isothermal and non-isothermal aging processes. It was found that the alloy had the peak strength at 39 h during the isothermal aging treatment. Its precipitation sequence was supersaturated solid solution (SSS) → GP zones → metastable η′ phase, where the stable η phase had not been formed. The peak strength of the alloy during the non-isothermal aging reached at 4-5 h, but the peak strength was lower than those with isothermal aging treatment. The maximum temperature Tmax of non-isothermal aging played a key role on the mechanical properties: the higher the maximum temperature was, the longer time was required, and the lower the strength and the hardness were. The grain boundary structure of the alloy with Tmax = 185 °C was similar to that with isothermal aging, and the grain boundary precipitate (GBP) was rarer than that with Tmax = 200 °C and Tmax = 215 °C, where there was no obvious precipitate free zone (PFZ) in the alloy, which was conducive to improving the corrosion resistance of the alloy.