Dimensional accuracy and internal structure can assess the quality of a casting. Both will influence strength and performance and characterise its value in terms of service.
Foundry practice has learnt that structure control during solidification plays a vital role in the achievement of high integrity castings.
In castings the two most important metallurgical features are:-
GRAIN REFINEMENT: for the macrostructure
A casting is a compact network of individual grains. The size, shape and bonding of these grains will determine the mechanical properties and soundness of the casting, so there is a real need to control their formation.
SILICON MODIFICATION: for the microstructure
Aluminium alloys containing up to 14% Silicon will solidify naturally into coarse, brittle plates of Silicon in a medium of Aluminium.
The mechanical strength and ductility of such a casting will be poor and its use limited.
Modification of AISi into a fine, fibrous microstructure brings a marked improvement in properties.
Fast cooling will achieve the modified AISi structure, but the presence of Strontium produces a similar effect that exists for several hours under normal foundry conditions.
Strontium additions are typically introduced in the form of an aluminium based Master Alloy during melt preparation.