Casting properties

Foundry is a process of producing metal castings.
Casting is an operation of shaping metal by pouring it in the liquid state into a mold followed by solidification.
Casting is also a metal detail, produced as a result of pouring a metal into a mold.
In some cases casting is the only method of shaping a metal or alloy: when the alloy is not malleable and therefore it’s plastic deformation is not possible or when a large detail of complex shape is to be produced.
Metal castings quality is determined by the casting properties of the metal:

Fluidity

Fluidity – an ability of metals and alloys to flow through the gating system filling the cavity of the casting mold and conforming its shape.
Factors determining fluidity:

• Solidification interval (the difference between liquidus and solidus temperatures of the alloy).

The wider solidification interval of an alloy, the lower fluidity of the alloy, as a certain part of the solidifying alloy has an increased viscosity caused by the presence of solid primary crystals, formed in course of cooling the alloy below the liquidus temperature.
The fluidity of pure metals is better than that of their alloys.
Eutectic alloys, characterized by nil solidification interval, possess good fluidity.

• Viscosity of the alloy (the lower viscosity of an alloy, the better its fluidity). As viscosity decreases with temperature, metals and alloys cast at increased temperature fill better the mold cavity.

• Surface tension of the alloy. Low surface tension causes better filling of the mold cavity.

Shrinkage

Shrinkage is a contraction of alloy volume caused by:

• Contraction of the melt when it cools down to the liquidus temperature.

• Contraction of the alloy owing its solidification (cooling from liquidus temperature to solidus temperature).

All metals except bismuth have higher density in solid state, than in liquid.

• Contraction of the solid alloy cooling from the solidus temperature to the ambient temperature.

Shrinkage is determined by the temperature of the cast alloy, its chemical composition and by the conditions of its solidification (cooling rate, mold shape).

Shrinkage cavity

When a large isolated region of liquid phase remains within solid, surrounding it, shrinkage cavity will form in this region. The common mold structure includes a riser – a “head”, in which the melt solidifies last and “feeds” the main casting with liquid alloy, compensating the casting shrinkage.

Shrinkage porosity

This shrinkage defect is a characteristic for the central regions of castings (ingots) of the alloys with a wide temperature range of solidification. In these castings “feeding” melt is not able to infiltrate through the interlacing dendrites. The local micro-spaces between the dendrites arm remain isolated from the melt in riser forming micro-cavities or shrinkage porosity.

Surface quality

Surface quality is determined by:

• the surface quality of the mold walls,

• the mold coating material properties,

• gassing of the mold or coating materials,

• chemical reaction between the mold material and the alloy,

• casting pressure.

Resistance to cracks formation

Casting cracks are result of internal stresses caused by a non-uniform cooling of the casting.
Alloys having low ductility are the most susceptible to cracks formation.
Hot cracks form during solidification of the alloy within the solidification interval.
Cold cracks form in completely solidified casting during its consequent cooling.