Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Abstract

Serious center segregation greatly limits the application of twin-roll casting (TRC) technology for producing 6xxx alloy strips. Herein, Al-0.9Mg-0.6Si-0.2Cu-0.1Fe (wt.%, 6061) strips with different thicknesses were fabricated by TRC, and we found that the center segregation was well relieved with the thickness increased from 3 mm to 4 mm. To reveal the mechanisms of mitigation of center segregation in the 4 mm strip, various techniques including solidification simulation, crystallographic calculation, electron backscatter diffraction (EBSD), and electron probe micro-analyzer (EPMA) were utilized. The results disclosed that the Fe-containing phase in the 3 mm strip was π-AlFeMgSi, while the counterpart in the 4 mm strip was α-AlFeSi. The α-AlFeSi could serve as nucleation substrates for Mg2Si and Q-AlCuMgSi phases, thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region. Three matching planes between the α-AlFeSi and Q/Mg2Si were examined as: (112¯0)α-AlFeSi // (0001)Q, (0001)α-AlFeSi // (110)Mg2Si, and (112¯0)α-AlFeSi // (11¯0)Mg2Si. Meanwhile, the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region, which is the other reason for the alleviation of center segregation. Owing to the elimination of the center segregation, a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip (∼29%) compared with the counterpart of the 3 mm strip (∼20%). This work may provide a strategy to eliminate the center segregation, thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.