Effect of macrostructure and microstructure on the viscosity of the A356 alloy in the semi-solid state

Abstract

An A356 alloy was processed from liquid state in different conditions to obtain globular as-cast structures instead of the conventional dendritic ones. These structures were produced in a semi-continuous casting system coupled to an electromagnetic power source. This device allows the variation of the intensity of the stirring by controlling the electromagnetic field, as well as of the cooling rate by controlling the casting velocity. Different cast conditions were used: the alloy was poured at 600°C and left to solidify under the action of a 600, 900 and 1200W electromagnetic field. Cast materials produced in these conditions are suitable for thixo-forming processes after a short re-heating treatment to complete spheroidisation of the primary phase. The generated structures were characterised by macrostructure (grain size) and microstructure (globule size and shape factor). The smaller the grain size, the smaller the globule size and shape factor. The Rheocast Quality Index ((RQI=globule size/grain size×shape factor)) was used to evaluate the morphological evolution during the partial melting treatment necessary to complete spheroidisation of the primary phase. Viscosity of thixotropic slurries obtained by re-heating the cast materials at 580°C for times of 0, 30, 90, 210 and 600s, was also determined by hot compress test. A comparison with a commercial ALTHIX®67S is described.