The solidification of metals continues to be a phenomenon of great interest to physicists, metallurgists, casting engineers and software developers. It directly affects the production cycle time, internal quality of castings and material utilization (yield). We will briefly review the solidification phenomenon in castings and focus on three major influencing factors affecting: freezing range, cooling rate and thermal gradient. Finally, we will list the different types of solidification shrinkage related defects and see why it is important to achieve controlled progressive directional solidification.

When molten metal enters a mould cavity, its heat is absorbed by and transferred through the mould wall. In the case of pure metals and eutectics, the solidification proceeds layerby- layer (like onion shells) starting from the mould wall and proceeding inwards. The moving isothermal interface between the liquid and solid region is called the solidification front. As the front solidifies, it contracts in volume, and draws molten metal from the adjacent (inner) liquid layer. When the solidification front reaches the innermost region or the hot spot, there is no more liquid metal left and a void called shrinkage cavity, is formed . This is avoided by attaching a feeder designed to solidify later than the hot spot. The shrinkage cavity shifts to the feeder, which is cut off after casting solidification and recycled. Understanding the solidification phenomenon will help us in predicting the type and location of shrinkage defects, and in overcoming them successfully by appropriate design of feeders.

The temperature history of a location inside the casting with respect to the neighbouring locations governs the formation of shrinkage cavity as well as the macrostructure. This is difficult to determine even for a simple shape, since all modes of heat transfer are involved during casting solidification: by convection within the molten metal; by conduction in the solidified portion of the casting; by convection and radiation at the metal-mould interface; and by conduction in the mould material. Also, the release of latent heat has to be addressed; it increases the casting temperature at that instant and location, and has the effect of delaying the solidification.

The most important factor affecting the rate of heat transfer from the casting to the mould is the interface heat transfer coefficient. It depends on the thickness of the oxide layer and the air gap at the interface. Both are not constant, but gradually grow during casting solidification. The air gap depends on the amount of gas generated (and retained) after metal-mould reaction, the roughness of the mould surface and the expansion of the mould and cores. The air gap is more at external surfaces at the top of the mould, and it grows till the end of solidification.

Let us study three important factors that govern the solidification characteristics of castings: freezing range (F), thermal gradients (G) and cooling rate (R). As we will see, these factors are primarily influenced by the casting metal, process and geometry, respectively.

Feeders are designed to compensate the solidification shrinkage of a casting, so that it is free of shrinkage porosity. Feeder design parameters include the number, location, shape and dimensions of feeders. We will first review the concept of feed path and feeding distance, which influence the location and number of feeders. Different options for feeder position, type and shape are described, followed by the design criteria for determining the dimensions of feeder and its neck, and finally the design of feedaids.

The direction of solidification inside a casting starts from end regions that solidify first, to intermediate regions, and ends at the last freezing regions. The feed metal flows in the reverse direction: from regions at a higher temperature (containing liquid metal) to adjacent solidifying regions. The entire path, starting from a local hot spot to an end region is referred to as the feed path. It follows that any intermediate point on a feed path has only one adjacent point with a higher temperature. The exception is the hot spot, which is a local temperature maxima. The hot spot effectively feeds all regions along the feed paths starting from it. Ideally, the hot spot must be inside a feeder, so that the casting is defect-free. The distance from a feeder to the farthest point along the feed path is referred to as the feeding distance.

Several researchers such as Pellini and Bishop have experimentally established the relationship between feeding distance and section thickness for simple shaped steel castings in sand moulds. The feeding distance is represented by two terms: feeder effect and end effect. For steel plate castings in sand moulds, the total feeding distance is given by 4.5 t (from the edge of feeder), where t is the section thickness. Of this, the feeder effect is 2 t and end effect is 2.5 t. Other researchers have expressed feeding distance in terms of modulus instead of thickness. The feeding distance is not very well established for other metals, particularly long freezing range alloys, and does not appear to directly relate to section thickness (as in the case of steel plate castings).

In complex shaped castings, it is difficult to estimate the feeding distance by the above relationships. One way to overcome this is by dividing the casting into a number of simple shaped regions and calculating the modulus of each (the ratio of volume to cooling surface area). If two adjacent regions have different modulus, then the one with the higher modulus may be assumed to feed the adjacent region.

The shape of the feeder neck depends on the feeder shape, feeder position and the connected portion of the casting. The most widely used neck shapes are cylindrical (for top cylindrical feeders) and rectangular (mainly for side feeders). The neck may be tapered down towards the casting. A single or double V-notch may be included in the neck to facilitate fettling. This does not affect the neck modulus (or its solidification time) because of low heat transfer from the sharp reentrant corner.

Another major feeder design parameter is the use of insulating or exothermic sleeves and covers. They essentially increase the effective modulus of the feeder, so that a smaller feeder can be used and the yield is increased. The shape of the feedaid depends on the feeder shape. Often the reverse is true, since feedaids are available in standard shape/size.