Effect of a novel semi-solid middle-layer structure formed during twin-roll casting on the mechanical properties of AA6022 alloy

Abstract

During a twin-roll casting (TRC) process with low-rolling force and near-liquidus melt treatment, a novel semi-solid middle structure is produced. The formation of this structure and its influence on the alloy mechanical properties were investigated using polarized light microscopy, electron backscattered diffraction, scanning electron microscopy, and transmission electron microscopy. The middle structure was generated by rolling a semi-solid melt with high thixotropic strength. The size of its sedimentary grains strongly affected the thixotropic strength by influencing the grain boundary density, which promoted different grain movements in the rolling process. The large grains slid along each other, whereas the small grains were deformed, inhibiting their macrosegregation. Compared with the conventional TRC structure, the semi-solid middle layer containing small sedimentary grains increased the slab elongation from 17.5% to 25.4% without reducing the tensile strength; however, the slab with large sedimentary grains exhibited low plasticity. The effect of the middle structure on the alloy mechanical properties is elucidated in terms of dislocation density, grain size, texture type, and resistance to crack propagation. The obtained results can help develop new approaches to designing TRC slab structures with desired characteristics.