Influence of Processing Severity During Equal-Channel Angular Pressing on the Microstructure of an Al-Zn-Mg-Cu Alloy

A commercial Al-Zn-Mg-Cu alloy, Al 7075, was overaged at 553 K (280 A degrees C) for 5 hours and processed by equal-channel angular pressing (ECAP) using route B-C. Different temperatures and number of passes, which determine the processing severity, were considered. The processing severity has been estimated by the maximum stress (sigma (Proc)) recorded during each ECAP pass. The higher the number of passes or the lower the processing temperature, i.e., the higher is the processing severity, the finer the (sub)grain size is obtained. A minimum ultrafine (sub)grain size of approximately 150 nm after three passes at 353 K (80 A degrees C) or eight passes at 403 K (130 A degrees C) was obtained. The microhardness exhibited an instant increase from 76 HV for the overaged initial state to 115 HV after only the first pass. The coarsened precipitates in the overaged alloy lead to larger structural refinement than in pure aluminum.