Mechanical response and structural development during the hot extrusion of a rapidly solidified Al-20Si-7.5Ni-3Cu-1Mg alloy powder

Abstract

The consolidation of an air-atomized Al-20Si-7.5Ni-3Cu-1 Mg alloy powder was performed utilizing hot extrusion, to determine its extrudability and understand its structural development in relation to process parameters. One of the main features exhibited by the material in this process was a high degree of softening over a peak extrusion pressure, which has been explained by the simultaneous onset of dynamic recovery and recrystallization during deformation. The peak extrusion pressure was shown to be strongly dependent upon the temperature applied, and this dependence has been described with temperature compensated strain rate. It was also observed that the process parameters had a fairly narrow range applicable to the extrusion of the powdered alloy and a significant influence on the deformation behaviour of the powder particles. The combination of heating and deformation, primarily used to convert the loose powder particles into an engineering material, resulted in the decomposition of the meta-stable aluminium matrix and transformations of constituent phases, initially formed in the rapidly solidified powder. Additionally, it was found that the extrusion temperature had an effect on the lattice size and perfection of the as-extruded matrix in the material. Three intermetallic dispersoids containing nickel were detected in the consolidated material, independent of extrusion temperature, and their formation was promoted by hot deformation. The silicon crystal phase in the extruded material was reshaped, and its size was insensitive to the extrusion temperature, which is thought to be caused by a high volume fraction of the coexistent dispersoids. The dispersions of the silicon crystals and intermetallic compounds with various sizes in the matrix substantially modified the deformation mode of the alloy. Evidence of dynamic recrystallization was found, which co-operated with dynamic recovery during deformation, giving rise to a duplex microstructure in the extruded material.