Morphological analysis of SSM Al-4.5wt.% Cu measured by the rheocast quality index
Abstract There is a direct relationship between grain size obtained from macrostructure characterisation and the globule size and shape factor, obtained from microstructure characterisation: the smaller the grain size, the smaller the globule size and shape factor. Due to this a rheocast quality index ( RQI = globule size /( grain size × shape factor) ) was used to evaluate the Al–4.5 wt.% Cu alloy produced by strain induced melting activation (SIMA). The structure that presents the smallest grain size, the smallest shape factor, and the most homogeneous and globular size of the primary phase, has the best RQI and consequently the best behaviour in the semi-solid forming. Four different as-cast structures were cold rolled to obtain 20 and 40% deformation and were partially melted at 635 °C at holding times of 5 min. Both, macro- and microstructure were characterised.
Semisolid metal processing
AbstractSemisolid metal processing is a relatively new method for forming alloys in the semisolid condition to near net shaped products. It relies on the behaviour of semisolid slurries in which the solid exists in the form of spheroidal particles: if left undisturbed such slurries are stiff and may be handled, but flow like liquids on being sheared. Slugs of semisolid alloy may therefore be injected into a die (thixocasting) or shaped between closed dies (thixoforging) to produce components near-to-shape with good surface finish, free from porosity, and possessing fine uniform microstructures, which may be heat treated to give superior mechanical properties. The rheology of non-dendritic alloy slurries and the recent ideas which have been proposed to account for their pseudoplastic and thixotropic behaviour are considered. This is followed by a description of the technologies available for producing non-dendritic structures and the processes for subsequently shaping the semisolid alloy. Finally, some ind...
Solute redistribution in a duplex dendritic structure
A duplex dendritic structure consists of co-existing regions of coarse and fine dendrites. Such a structure in aluminium-4,5 wt % copper alloy was produced by rapid quenching from the partially solid-partially liquid state. The concentration of solute was found to be low and constant within the coarse structure, which contains a smaller amount of non-equilibrium secondary phase than does the fine dendritic structure. The solute distribution profiles within both the coarse and fine dendritic structures were found to be fairly accurately described by the Scheil equation for non-equilibrium solidification. Complete dissolution of the secondary phase occurred within the coarse structure after a shorter solutionizing time than it did within the fine dendritic structure. A spherical diffusion model accurately described solution kinetics within the coarse structure and a cylindrical diffusion model within the fine structure.