Microstructural evolution of AA7050 alloy wires during tandem hot rolling (THR) and cold-drawing process

A novel processing route, the combination of two-step tandem hot rolling (THR) and cold drawing, was recently developed to produce AA7050 wire products. The evolution of second phases, grain microstructures, and micropores during the whole process was thoroughly studied. The specific effect of second phases on continuous dynamic recrystallization (CDRX) and grain refinement was also systematically investigated, which was of great significance to optimize the processing parameters and further improve the mechanical properties of AA7050 wires. Firstly, both the strip-shaped microscale and tens-of-nanometer MgZn₂ primary phases are observed in the as-received extruded sample. The strip-shaped primary MgZn₂ phases break and gradually spheroidize throughout the THR process, with some of them redissolving into the Al matrix. However, the nanoscale MgZn₂ phases are obviously coarsened and equiaxed only during the second-step THR process. After the first-step THR process, the coarse columnar grains are refined to thin fibrous grains due to the occurrence of CDRX, and a high number of dislocations are stored in the recrystallized structures. During the second-step THR process, only dynamic recovery occurs and no obvious CDRX is observable. This may be due to the coarsening of nanoscale MgZn₂ phases, which reduces the driving force of recrystallization by attracting and combining with other solute atoms. The following two-step cold-drawing process has little influence on the primary MgZn₂ phases, but promotes the formation of refined and equiaxed grains due to the recurrence of CDRX. This may be because the larger coarsened MgZn₂ phases accelerate the recrystallization nucleation and increase the driving force of recrystallization.