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 Mg2Si 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. Keywords: Al-Mg-Si alloy, Over-aging, Thermomechanical treatment, Strain hardening exponent, Work-hardening rate, Strength-ductility balance