Influence of microstructural refinement on cold deformation behavior of nanocrystalline Al-Zn-Mg alloy processed by solid-state mechanical milling and hot pressing

This research work was aimed at studying the workability behavior of Al-Zn-Mg alloys during a cold upsetting process. Nanocrystalline Al 7017 alloys were synthesized by mechanical milling (MM) at different milling times and consolidated by hot-pressing. The nanocrystalline Al 7017 alloy samples were incrementally deformed with a load step of 0.5 ton in a universal testing machine. Artificial image capturing and comparison techniques were introduced in this study and deformed images were captured using a high-resolution camera. For each incre-mental load (without removing the sample from the platens), different parameters such as the height of the deformed samples, and the corresponding contact diameters (top and bottom) were measured and analyzed using Matlab software (image processing). Cold upsetting was performed until the first crack formed. The influence of milling time on cold workability and strain hardening behavior was investigated under triaxial stress state conditions. The results revealed that the Al 7017-10 h nanocrystallite alloy exhibited improved deformation properties in terms of various tri-axial stress, strain hardening, formability stress index, and stress ratio pa-rameters owing to the fine grain size, lower amount of dislocation pile-up, and fewer migrated atoms near the grain boundaries compared to other samples (after10 h). The microstructure and phase evolution were studied using an X-ray diffractometer and various electron microscopes. Several strengthening mechanisms are also discussed among which the dislocation strengthening mechanism plays a major role in the deformation behavior. Finite element analysis (FEA) was performed to examine the equivalent stress distribution during cold-upset forging.