Enhanced plasticity and corrosion resistance of high strength Al-Zn-Mg-Cu alloy processed by an improved thermomechanical processing

An improved thermomechanical processing double step hot rolling (DHR) was proposed to manufacture fine-grained Al-Zn-Mg-Cu alloys based on pre-deformation, short time intermediate annealing and final hot rolling. The corresponding microstructure evolution, mechanical properties and corrosion resistance were investigated. The DHR processing can produce high-quality sheets with finer grains than the conventional hot rolling (CHR). The grain refinement is mainly proceeded via dislocation rearrangement and low angle grain boundary transition. The grain boundary area (with coarse grains) is small in the CHR alloy and enormous solute atoms could move from intra-granular area to grain boundaries. Therefore, coarse particles precipitated continuously along grain boundaries. However, larger grain boundary area is obtained in DHR alloy by finer grain structures leading to the formation of discontinuous grain boundary precipitates. The results reveal that the present DHR treated alloy possesses improved tensile plasticity and corrosion resistance than the CHR alloy because of refined grains and discontinuous grain boundary precipitates. Thus, the present DHR processing is a promising manufacturing process for obtaining fine grained heat-treatable Al alloy sheets.