Shrinkage of alloys

The concept of shrinkage: the phenomenon of reduction in volume or size of an alloy as it cools from the liquid state to room temperature. It is an inherent physical property of metals.

Three stages of shrinkage:

• Liquid Shrinkage:

Shrinkage between the pouring temperature and the solidification onset temperature.

Volume shrinkage is manifested by a decrease in the liquid level in the cavity.

The higher the pouring temperature, the greater the degree of superheat, the more serious the liquid shrinkage.

• Solidification shrinkage:

Shrinkage from the solidification start to the solidification termination temperature.

The larger the solidification temperature range, the more serious solidification shrinkage.

• solid state shrinkage:

From the solidification termination temperature to the room temperature between the contraction.

It is mainly manifested in the reduction of external dimensions.

Factors affecting shrinkage

• Chemical composition: steel shrinkage is the largest, gray cast iron shrinkage is the smallest (precipitation of graphite volume expansion to offset some of the shrinkage).
• Pouring temperature: the higher the pouring temperature, the greater the superheat, the greater the liquid shrinkage.
• Casting structure and mold conditions:

The resistance to shrinkage comes from two sources:

One is the uneven wall thickness of the casting.

The second is the casting mold and core mechanical resistance to shrinkage.

Shrinkage on the casting quality effects

1) Shrinkage and loosening

Liquid alloy in the condensation process, if its liquid shrinkage and solidification shrinkage of the volume shrinkage can not be supplemented, the casting in the last solidified parts of the formation of some holes. The large and concentrated ones are called shrinkage holes, and the small and scattered ones are called shrinkage.

(2) shrinkage and shrinkage hazards and prevention

Hazard: shrinkage and shrinkage can reduce the effective area of the casting, and in the piece of stress concentration, reducing mechanical properties; shrinkage can also make the casting leakage.

Preventive measures: to prevent shrinkage and loosening of commonly used process measures is to control the solidification of the casting order, so that the casting to achieve “sequential solidification”.

Sequential solidification

Sequential solidification is to use a variety of measures to make the casting from the riser to away from the riser between the temperature gradient gradually decreasing, so that away from the riser first solidified, and then to the direction of the riser sequential solidification.

Risers and cold iron joint complementary shrinkage

Place the cold iron in the gradually thick part to speed up the cooling rate.