What is solidification?

Some articles on solidification:

Directional Solidification
... Directional solidification and progressive solidification describe types of solidification within castings ... Directional solidification describes solidification that occurs from farthest end of the casting and works its way towards the sprue ... Progressive solidification, also known as parallel solidification, is solidification that starts at the walls of the casting and progresses perpendicularly from that surface ...