2. • • Segregation is a result of separation of
impurities and alloying elements in different
casting regions.
• • The liquids that are frozen to form industrial
alloys usually contain , in addition to solute
elements added intentionally for their beneficial
effects.
• • Many impurity elements find their way into
liquid metal by different routes.
• • Impurity elements present in the ores are partly
removed during smelting and refining.
3. • Refractory brick lining of furnaces and the
gases in the furnaces atmospheres may be
other sources.
• In the latter case, elements enter the liquid
metal in the form of dissolved gases.
• The various elements dissolved in liquid
metal can and often do react with each
other to form compounds( Oxides, Silicates,
etc. ).
• In many cases the latter becomes less
dense than the liquid and rise to surface and
join the slag on the top of liquid metal.
4. • On the other hand, it is quite possible for
small impurity particles to exist in the liquid.
• These particles will act as a nucleation
centers for heterogeneous nucleation.
• Grain size of casting can be controlled by
inoculating liquid metals with elements that
combine to form nucleation catalysts.
• Increasing the number of nucleation centers
will produce finer size in the solidified
casting.
5. • When an alloy is frozen, solute elements ,
whether present as alloying elements or as
impurities are more soluble in the liquid
state than in solid state.
• This fact usually leads to a segregation of
the solute elements in the finished casting.
6. • There are two basic ways of looking at the
resulting non uniformity of the solute.
• First, because the liquid becomes
progressively richer in the solute as freezing
progress.
• The solute concentrations in a casting tend
to rise in those regions that solidify last.
• Second, gravitational effects are often a
factor in producing segregation.
7. • The crystals that form freely in the liquid
often have a different density from that of
liquid.
• As a result, they may either rise towards the
surface of the casting, or settle towards the
bottom.
8. There are two types of Segregation.
1- Macro Segregation
2- Micro Segregation
1- Macro Segregation:
Long-range
composition fluctuations.
In general, macro segregation refers to the
change in the average composition of the
metal as one moves from place to place in
an ingot.
9. 2- Micro Segregation:
Localized composition
variations on a scale smaller than the crystal
size.
• A much more frequent form of micro
segregation, commonly known as coring.
• Coring is caused by dendritic freezing in alloys.
• The original dendrite arms, which shoot out into
the liquid metal, freeze as relatively pure metal.
• Rapid solidification can drastically reduce
segregation.
