The effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of AA6063 aluminum alloy was investigated. For this purpose, samples of AA6063 aluminum alloy were deformed up to 10 passes using ECAP and the evolution of microstructure, texture and dislocation density was investigated. It was found that the dislocation density increased and cells mostly surrounded by low angle boundaries (LABs) formed after 2 passes ECAP. Increasing the dislocation density continued with further processing and reached a maximum at the 4th ECAP pass. With further deformation to 6 passes, the dislocation density reduced, the fraction of high angle boundaries (HABs) increased and the averages of cell size and grain size reduced, significantly, which indicated the occurrence of grain refinement. However, a slight increase in dislocation density, average cell size (ACS) and average grain size (AGS), indicating coarsening, occurred when 10 passes of ECAP are imposed. Hardness, yield (YS) and ultimate tensile strength (UTS) increased continuously although the most significant enhancement occurred after 2 passes of ECAP. Investigating the correlation between the microstructure and mechanical properties indicated that Hall-Petch relationship was valid for ultrafine grained AA6063 before the coarsening started provided that the property of interest was correlated to average cell size but not the average grain size.

In this research, grain refinement through severe plastic deformation (SPD) in combination with a thermal treatment to create a fine initial grain structure with a high degree of supersaturation was taken as a viable approach to achieving simultaneous increases in the hardness, strength and ductility of the aluminum alloy AA6063 during further SPD. A recrystallized structure with grain sizes around 20 µm was obtained after two passes of equal channel angular pressing (ECAP), followed by a thermal treatment at 500 °C for 10 s and water quenching. The alloy with the supersaturated α-Al matrix was subjected to further ECAP processing up to six passes to create a cellular structure on a nano scale. Hardness and tensile tests revealed the changes of hardness, strength and ductility along with increasing number of ECAP passes. It was found that after two ECAP passes, the ductility of the alloy decreased from the value after the prior two-pass ECAP and thermal treatment. However, by further ECAP processing up to six passes, the ductility increased along with the increases in hardness and strength. The remarkable improvement in ductility was attributed to a nanosized cellular structure with a large area of high-angle grain boundaries developed from the fine initial grain structure formed during the two-pass ECAP and thermal treatment applied earlier.