On the suppression of Lüders elongation in high-strength Cu/Zn modified 5xxx series aluminum alloy

Abstract

The deterioration in surface quality due to the formation of Lüders lines is a typical phenomenon in 5xxx series Al–Mg alloys. Traditional efforts show that an increase of the grain size results in the suppression of the Lüders elongation. This paper presents that the suppression of the Lüders elongation can be realized at a smaller grain size by the addition of Zn/Cu elements in the 5xxx series alloys and by pre-aging treatment. The alloys with (Zn + Cu)/Mg ratios below 1.0 result in an increase of the strength by the precipitation of T-Mg 32 (AlZnCu) 49 phase, and quite different from 7xxx series Al–Zn–Mg alloys strengthened by η-MgZn 2 precipitates. The decisive measurement for suppressing Lüders elongation is based on the depletion of Mg atom concentrations in the matrix due to the formation of GP zones after pre-aging treatment. • The 5xxx series Al alloy is modified with (Zn + Cu)/Mg ratios below 1.0. • The Lüders elongation can be suppressed in a smaller grain size at T4P temper. • The Hall-Petch coefficient for Lüders increases with the concentrations of Mg atoms. • The suppression of Lüders elongation is due to the formation of T precipitates.