**4.1 Stage I**

*Casting Processes and Modelling of Metallic Materials*

crystals. These are:

*3.8.1 Crystal multiplication*

i. Turbulence during pouring

cooler surface region,

promoted by the following:

of the melt

morphology of the casting.

**4. Mould feeding issues**

liquid is higher than that of the solid.

a.Heterogeneous nucleation in situ

b.Crystal multiplication and

**3.8 Further factors influencing the cast structure**

favourable sides in the liquid and grow into equiaxed grains.

*3.8.2 The transportation of these fragmented or detached crystal may occur by*

Two further factors, other than the thermal and constitutional undercooling, also influence the cast structure particularly concerning the formation of equiaxed

Crystal multiplication is a consequence of the fragmentation of dendritic arms in the columnar zone due to local factors like thermal fluctuations and change in growth rates, etc. Also the nuclei formed on the inside of the mould wall may get washed off when further metal is being poured into the mould. Some of these detached nuclei may vanish being unstable, while some may get transported to

ii.Thermal convective currents between the hot central portion and the relatively

To sum up the formation of the equiaxed zone consisting of equiaxed grains, is

c.Transportation of crystallites by gravity or by mass movement into the interior

Obviously, the above form certain factors which influence the crystallographic

For a casting to be produced, it is essential that appropriate technique to be adopted for the liquid metal to be fed into the mould cavity. It is an issue inviting special considerations since the viscosity of the melt increases with drop of temperature making its flow sluggish and time taking which may result in considerable solidification prior to the completion of the feeding process. Also, the metal/alloy shrink on solidification producing solidification shrinkages in the casting which are discontinuities in the casting. For a healthy casting production ample facilities must be made be made available for compensating for these shrinkage. On these considerations, we can take the case of a pure metal which solidifies at a constant

iii.Gravitational separation due to the difference in densities between the solid and the liquid. In general the fragmented dendrules in the melt tend to go to the bottom of the melt due to density differences between the solid dendrules and the surrounding liquid melt. The only exception is 'Be'. Here the density of the

**34**

In the early stages, growing crystal bodies are suspended in the liquid. Free movement of the liquid across the crystals is possible. Thus, any contraction can be easily compensated by a feeder head with the general lowering of the free liquid surface.
