Influence of over-aging and sub-zero temperature rolling on strength-ductility balance in AA6061 alloy

Abstract The present investigation aims to achieve a balance between strength and ductility in AA6061 alloy by tailoring the microstructure through sequential combinations of multi-pass rolling near sub-zero temperature and age hardening. At the initial stages of the investigation, it was observed that controlled over-aging can induce a substantial amount of ductility (elongation at failure, TE ∼17%) in the alloy with moderate strength (yield strength ∼143 MPa and ultimate tensile strength ∼272 MPa). Therefore, in order to attain a considerable balance between strength, ductility and strain hardening, the alloy has been subjected to different sequences of sub-zero temperature rolling and over-aging. The enhancement of ductility and work-hardening induced by the thermomechanical processing can be attributed to the combined effect of grain refinement, precipitation, and dynamic recovery. The sub-zero temperature rolling produces fine and elongated grains; whereas, the over-aging process helps in coarsening of the precipitate phase Mg 2 Si as well as recovery. The thermomechanical treatments also result in proliferation of dislocations in the microstructure. Their interactions with the fine grains result in higher strength (maximum yield strength ∼318 MPa and maximum ultimate tensile strength ∼322 MPa), whereas the coarse grains aid their flow to maintain sufficient ductility.