Correlative evolution of microstructure, particle dissolution, hardness and strength of ultrafine grained AA6063 alloy during annealing

Simultaneous improvements of the strength and ductility of fine-grained AA6063 alloy with increasing number of ECAP passes

“…In a previous investigation on the annealing of the AA6063 alloy [14], it was found that the tensile elongation of AA6063 increased if a large fraction of the alloying elements were kept in the α-Al solid solution. This was attributed to a stronger work-hardening effect contributed by a higher concentration of the alloying elements in the α-Al solid solution, which postponed localized deformation and necking during tensile testing.…”

“…To produce a fine initial grain structure, the billets were subjected to two ECAP passes and then a thermal treatment at 500 C for 10 s. With this thermal treatment, most of the alloying elements were brought into the α-Al solid solution and no significant grain growth could occur [14]. The solution-treated billets were fed into the ECAP machine and deformed through six passes at room temperature.…”

“…Apparently, a lower thermal treatment temperature (but above the recrystallization temperature of the AA6063 alloy -320 0 C) would result in a finer recrystallized grain structure. However, such a treatment would require a longer time for recrystallization to complete, e.g., 10 min at 420 C, which would cause the alloying elements in the α-Al matrix to lose partially and coarse second-phase particles to precipitate [14]. Consequently, the alloy would become softer as a result of reduced solution hardening and its precipitation hardenability to be exhibited during aging would be reduced as well.…”

“…This was attributed to a stronger work-hardening effect contributed by a higher concentration of the alloying elements in the α-Al solid solution, which postponed localized deformation and necking during tensile testing. In addition, the elongation to failure increased, when a fine initial grain structure was created, because of a greater capacity of grain boundaries to accommodate dislocations formed during tensile deformation and to postpone fracture [14]. A fine grain structure is usually obtained when thermal exposure temperature is low, e.g., at 350 or 420 C.…”

“…The key to realizing the improvements lay in creating a fine initial grain structure with a high degree of supersaturation. It was based on the earlier finding [14] that the tensile elongation of the AA6063 alloy could be enhanced, if a fine grained structure was produced with the alloying elements (mainly Mg and Si) in the α-Al solid solution and the gained elongation would decrease with increasing extent of the precipitation of second-phase particles. In the light of this finding, in the present research, a fine initial grain structure with large amounts of the alloy elements in the α-Al solid solution was produced by applying ECAP and a thermal treatment to allow recrystallization to take place and prevent second-phase particles from precipitating.…”