Week 2 - Casting Fundamentals 1

Mech4304 God Help Me

Types of nucleation

Homogenous - rare, needs 20% undercooling with no external help

Heterogeneous - Dominant in real life, external surface helps solidifcation

Free energy equation

Seen below, essentially, must get over maximum radius r* to nucleate, otherwise breaks back down. Surface energy grows with r but the volume energy lowers it.

Wetting

Essentially just nucleus sticking to the wall. Doesn’t change r* but G* does.

Faceted vs Non-Faceted Growth

Depending on the value, faceted or non-faceted growth occurs.

• Faceted → Flat interface, much more crystaline (occurs in metals)

• Non-faceted → Continuous curvy interface, present in silicons.

Constitutional Under cooling

In compositions, where it cools lower then it theoretically should. Occurs as solute is rejected into liquid, raising its composition and lowering the melting point, forming latices and interesting shapes.

Growth rate During Solidification effects on crystal structure

Planar → Forms when temperature gradient (G) is steeper then liquid temperature gradient

Cellular → When G is slightly less then temperature gradient, forms cells

Dendritic → When G is much less → Get tree branches

Mushy zone

Liquid and solid coexist, between liquid temp (TL) and Solid temp (TS), alloys have a wide range causing more of these range. Problems is, sometimes dendrites link, with not enough liquid in between, causing hot tearing, shrinkage porosity etc.

Undercooling

Things to know:

• DeltaT = Tm - Tactual

• Increasing deltaT → Smaller r*, smaller G*, faster nucleation.

• Increasing deltaT → Lower atomic mobility → slower nucleation

Therefore, balance gives TTT curve attached, where fastest is mix of both.