The effect of chemical grain refinement and low superheat pouring on the structure of NRC castings of aluminium alloy Al-7Si-0.4Mg

The effect of the addition of Al-5Ti-1B (wt.%) chemical grain refiners on the nuclei generation for a range of superheats during pouring in new rheocasting (NRQ of aluminium alloy Al-7Si-0.4Mg (wt.%) has been investigated. The contributions to the grain density by the grain refiner additions and impurity particles were quantified and it was found that the addition of grain refiner provides increasing number of nucleation sites as the superheat is decreased from 105 to 35 degrees C. However, at superheats of 15 degrees C, which are more typical of NRC, the grain density is similar in the alloy both with and without grain refiner additions. At this superheat, the equiaxed grain morphology is globular rather than dendritic and it is postulated that the grain size is dependant upon grain coarsening mechanisms rather than the number of heterogeneous nucleation events. In agreement with previous studies on semi-solid processing, it was found that the achievement of a fine globular grain structure led to a more homogeneous casting being produced. The mechanism of the macrosegregation observed in these castings is discussed and explained by the 'sponge effect'. (c) 2006 Elsevier B.V. All rights reserved.