Microstructure and rheology of an AA2024 aluminium alloy in the semi-solid state, and mechanical properties of a back-extruded part

Abstract

The effect of deformation mode (rolling and extrusion) on the efficiency of microstructural conditioning and microstructural evolution of a semi-solid AA2024 alloy was studied by optical microscopy and X-ray diffraction. Results show that although the extruded sample produced a very small initial particle size, its initial growth rate is faster, a behaviour ascribed to its accentuated texture. However after 600 s in the semi-solid state, rolled and extruded samples exhibit the same degree of texture and very similar growth rates, whose rate-controlling mechanism was found to be particle coalescence. A separate study on rolled material showed that the higher the deformation ratio, the smaller the initial particle size and that a thickness reduction of 30% is sufficient to obtain a fine distribution of Al-α particles. Simple compression tests showed that the rheological behaviour of the AA2024 alloy is adequately described by the power law. Moreover, the alloy exhibited solid-like behaviour before fracture of the solid skeleton and pseudoplastic behaviour after that. Finally, a sound and crack-free demonstration part was produced by back-extrusion. The item displayed both macro- and microsegregation but solution heat treatment was reasonably effective to equalise solute concentration. Tensile properties were equivalent to those of conventionally forged material although local differences throughout the part were observed. Results have important implications on the production of suitable semi-solid microstructures and on the mechanical behaviour of thixoformed parts.