Solidification and Casting
Nucleation, growth, segregation, feeding and porosity in cast components.
Solidification begins with nucleation
A solid phase forms when the thermodynamic driving force overcomes the energetic cost of creating an interface. In industry, mould walls, inoculants and inclusions often provide heterogeneous nucleation sites.
Growth creates morphology
Temperature gradients and interface stability produce planar, cellular or dendritic growth. Dendrites reject or absorb solute differently, leading to microsegregation.
The casting must be fed
Most metals contract on solidification and cooling. Risers, directional solidification and pressure control are used to prevent shrinkage cavities. Dissolved gas and turbulent filling can create porosity and oxide films.
Structure follows thermal history
Cooling rate controls grain size, secondary dendrite-arm spacing and phase distribution. Local section thickness therefore creates local property variation.
Takeaway
A casting is a thermal-history map. Defects and microstructure must be interpreted in relation to heat flow and liquid feeding.