Aluminum casting alloys compositions are much similarly wrought aluminum alloys compositions. Their hardening and desired properties are achieved also through the addition of alloying elements and through heat treatment. However work hardening plays no significant role in the development of casting properties. So the use and purposes of some alloying elements differ in casting and wrought alloys.
Wrought products are typically produced in simple round and rectangular cross sections. Aluminum casting processes minimize the depth and maximize the uniformity of the solidification front. Thus solidification in engineered castings with various complex shapes present different demands on alloy solidification behavior.
Cracking during and after solidification and internal shrinkage dictate alloys for shape casting that minimize these tendencies. The term castability is not precisely defined. It is used to estimate the suitability of a composition for solidification in a specific process to produce defect-free, sound castings.
For gravity casting, the components of castability are generally considered to be fluidity as the measure of mold-filling capability, resistance to hot cracking during and after solidification, and feeding characteristics that promote the flow of metal during solidification to avoid or minimize the formation of shrinkage voids. For pressure die castings, the criteria of castability are resistance to hot cracking, fluidity, die soldering and surface finish.
Fluidity is a complex function that can be quantified and mathematically defined. Fluidity is most strongly affected by temperature above the liquidus or degree of superheat. More fluid compositions at conventional pouring temperatures are those of eutectic or near-eutectic composition.
Improved feeding characteristics are usually associated with narrow solidus-liquidus ranges and in greater percent liquid at the eutectic temperature. The tendency for solidification and postsolidification cracking is dominated by element effects on elevated-temperature strength and on solidification rate. The most commonly used castability ratings were based on practical experience: from A to F or from 1 to 10.
The casting alloys used in the greatest volumes contain silicon in excess of that of most wrought alloys. Solidification results in shaped casting are improved by fluidity, elevated-temperature resistance to cracking, and feeding characteristics that sufficient amounts of silicon impart.
