Influence of Homogenization Heat Treatment and Cooling Modes on Microstructure of AA7475 Aluminum Alloy Ingot

摘要

Microstructure evolution during the homogenization heat treatment of an Al-Zn-Cu-Mg (AA7475, which is typically used for the manufacture of aircraft design) alloy, was investigated using a combination of light microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ingots after different types (one- or two-steps treatments) of temperatures (from 380 to 510 °C) of homogenization and cooling conditions (cooling with an air or quenching to water) were investigated. The results show that the microstructure of ingot presents a typical microstructure with some isolated Al_7Cu_2Fe particles, which after homogenization almost remains in both the size and morphology. The structure ingot after homogenization below 400 °C contains secondary phases, based on η (MgZn_2), S (Al_2CuMg) and T (Al_2Mg_3Zn_3) are distributed along the grain boundary. In the T (Al_2Mg_3Zn_3) phase copper dissolves up to 30 wt.%. Then the increase in temperature and the complication of heat treatment of homogenization, which led to the complication of the kinetics of the evolution of inter-dendritic phases, were found. The two-steps homogenization has a better effect than a single homogenization, as its completely dissolution of non-equilibrium phases was established.