Types of Solidification in Metal Casting

Solidification is the phase transformation by which molten metal turns into solid inside a mould cavity. This stage governs the microstructure, mechanical properties, soundness, and defect formation in castings. The manner in which heat is extracted and the way the solid–liquid interface advances determine the type of solidification that occurs.

In metal casting, solidification is broadly classified based on thermal gradient, nucleation behavior, and movement of the solidification front.

1) Directional Solidification

In directional solidification, the metal solidifies progressively from one end to another, ideally from the farthest point toward the riser.

Characteristics:

• Controlled heat flow in one direction

• Promotes feeding of liquid metal from riser

• Minimizes shrinkage cavities

Benefits:

• Produces sound castings

• Preferred in riser design and gating practice

Applications:
Turbine blades, high-integrity structural castings.

2) Progressive (Normal) Solidification

Here, solidification starts at the mould walls and progresses inward toward the center from all directions.

Characteristics:

• Heat extracted uniformly from all sides

• Central region solidifies last

Risk:

• Formation of central shrinkage cavity if not fed properly

Common in:
Sand castings of simple geometry.

3) Simultaneous Solidification

In this type, the entire casting solidifies almost at the same time.

Characteristics:

• Occurs in thin sections

• Very small temperature gradients

• Little feeding possible

Risk:

• High chances of shrinkage porosity throughout

Typical in:
Thin-walled castings and small components.

4) Chilled Solidification

Occurs when a metal chill is placed in the mould to increase the rate of heat extraction at specific locations.

Characteristics:

• Rapid solidification near chill

• Fine grain structure

• Increased hardness at chilled region

Used for:
Controlling grain structure and improving surface hardness.

5) Dendritic Solidification

Most metals solidify in a tree-like (dendritic) structure due to constitutional supercooling.

Characteristics:

• Primary and secondary dendrite arms

• Segregation of alloying elements between dendrites

• Influences mechanical properties

Common in:
All alloy castings.

6) Equiaxed Solidification

Solid grains form uniformly in all directions without preferred orientation.

Characteristics:

• Occurs when cooling is uniform

• Fine, uniform grain structure

• Good mechanical properties

7) Columnar Solidification

Grains grow in a particular direction opposite to heat flow.

Characteristics:

• Long column-like grains

• Occurs under steep thermal gradient

• Lower transverse strength

Seen in:
Large castings and ingots.

8) Skin Formation Solidification

A thin solid skin forms immediately after pouring due to contact with cold mould walls.

Characteristics:

• Rapid initial solid layer

• Inner metal remains liquid longer

• Affects feeding and shrinkage behavior

Factors Affecting Type of Solidification

• Thermal gradient between mould and molten metal

• Cooling rate

• Presence of chills

• Alloy composition

• Mould material

• Pouring temperature

Importance in Casting Design

Understanding solidification type helps in:

• Proper riser placement

• Avoiding shrinkage defects

• Controlling grain structure

• Improving casting quality