Effectively mitigated macro-segregation and improved tensile properties of twin-roll cast Al-Mg-Si (6022) alloy strips via Zr addition

It is a great challenge to avoid the central macro-segregation during the solidification process of twin-roll casting (TRC), which seriously limits the mechanical properties of 6xxx series Al alloys. In this study, we find that the trace addition of 0.08 wt% Zr to Al-1.0Mg-1.2Si (6022) alloy can effectively promote the columnar-to-equiaxed transition and refine the solidification microstructure (particularly & alpha;-Al dendrites and & pi;-AlFeMgSi phase). More importantly, the macro-segregation of the as-cast TRC strip is well mitigated with Zr addition. The refinement of & alpha;-Al dendrites and & pi;-AlFeMgSi phase may be attributed to the formation of (Al, Si)3Zr phases when introducing Zr element, which can act as heterogeneous nucleation sites for both of them. The secondary dendrite arm spacing (SDAS) in the central region of strips is sharply decreased from -33 to -16 & mu;m, which facilitates the incorporation of more solute elements into the Al matrix, thus increasing the partition coefficients of Mg (kMg) and Si (kSi) elements. As a result, there are fewer solute atoms in the residual liquid phase during the final stage of solidification, leading to the alleviation of central macrosegregation. The tensile properties of the as-cast, solid solution treated (T4) and paint baking treated (T6) alloys are synchronously optimized with Zr addition. In particular, the fracture elongation of the T4 alloy is increased from -30.9% to -33.4%, indicating excellent formability. The yield strength of the T6 alloys is increased from -227 MPa to -239 MPa, satisfying the more demanding service requirements. This work can provide a strategy for the effective control of macro-segregation and produce alloy strips with high performance by the TRC process.