Influence of thermomechanical treatments on the production of rheocast slurries by partial melting

Abstract

This work deals with the influence of thermomechanical treatments on the production, by partial melting, of rheocast structures of Al–3.35wt.%Cu alloy. Two types of treatments are performed: recrystallization and partial melting (RAP), in which the alloy is submitted to cold work at room temperature prior to heating to the semi-solid state; and overaging treatment (OAT), where the alloy is submitted to homogenisation and controlled precipitation of second phase followed by deformation at room temperature and heated to temperatures above Tsolidus. The influence of initial grain size, degree of cold work and overaging conditions in the resulting microstructure is investigated. Results are presented in terms of Cu distribution, solid fraction, globule diameter and shape factor. For both processes there are substantial reductions in the globular primary phase size compared to initial grain sizes, this reduction being more significant for rheocast produced via the OAT than the RAP route. The degree of previous deformation only influences the final globule diameter of the rheocast structure obtained by the OAT process, while initial grain size influences globule diameter in both processes. Changes in shape factor with process parameters show that different mechanisms are operative during rheocasting, such as recrystallization coarsening and coalescence; the predominance of one of them depending on the initial microstructure of the material. In general, OAT produces more rounded and smaller globules than RAP, mainly from higher deformed and coarser raw material, due to the predominance of recrystallization.