Effect of Retrogressions on Precipitation Characteristics and Strength Property in an Al-Zn-Mg-Cu Alloy

Abstract

Retrogression treatment is a crucial process to obtain preferential microstructure and mechanical property for aviation Al-Zn-Mg-Cu alloys. In the current work, ramping time, preservation time and temperature were set as variations to investigate property evolution and precipitation characteristics for a commercial Al-Zn-Mg-Cu alloy. The results showed that the strength exhibited a continuously decrease trend with the enhancement of preservation temperature or the extension of preservation time. For a same strength level with different preservation temperature and time matches, GPII zone and η’ phase were detected as the main types of precipitates, which showed apparent coherency and semi-coherency with the matrix, respectively. Phases with relatively large size possessed larger proportions for higher preservation temperatures while the average grain size exhibited an increase trend. As for prolonging ramping time, similar strength degradation was observed under equivalent preservation regimes. For the same strength level with different ramping and preservation time matches, the precipitates were still GPII zone and η’ phase. With the prolonging of ramping time, the proportion of phases with relatively large size exhibited a decrease trend, so as to the average grain size (AGS). The subtle differences of precipitation characteristics and strength property matches were analyzed, which provided a promising way to improve overall performance of Al-Zn-Mg-Cu alloys.