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Pig*2.33. Maorosegregation phenomenal
a)Normal and inverse segregation in solute9
b)Foroes and factors promoting inverse segregation*
Foundry Department |
'"Casting defects and |
Dr* V* Bastrakev |
measures of their prevention" |
Clearly the extent of normal segregation is dependent on
extent of coring of individual crystaleand hence on the
rate of cooling*
This kind of segfgation frequently leads to an overall solute
difference of |
the |
order of 0.555 to 155 |
between the surface |
and the center |
of |
the easting* |
|
|
, |
normal segregation is |
seldom observed |
in practice* |
|
|
|
This is entirely due to the fact that additional factors oome into play during the solidification* One such additi
onal factor is the volumetrio contraction of liquid metal
on freezing* As the crystals grow invards, so the liquid
alloy flows outwards to compensate for the contraction in volume on freezing. This solute-rioh liduid becomes even
more enriched as it flows outwards (fig |
*2*3-3*a) « |
The |
|||||
swrage |
content |
of |
solute in the crystals at the outer skin |
||||
of |
the |
casting |
is |
thus raised |
above the |
value in the |
orig i |
nal |
liquid alloy* |
Therefore, |
the final |
macrocomposition |
|||
gradient in the solute is opposite to thet of normal segre gation* This is defined as inverse segregation*
As the volumetric freezing contraction is of an appr eciable magnitude in most industrially used alloys,
Inver ^©rather than normal segregation Is muoh more frequently observed in eastings*
Metal Struetore»
After pouring an alloy into the mould, it begins to solidify forming the proper crystalline structure. The
fundamental concpts of nucleation and crystal growth apply
to a ll metals and alloys irrespective |
of their |
composition. |
|
But different metals |
and alloy phases |
may not |
only belong |
to different ^rystal |
systems,but also |
the morphology of |
|
crystal growth may be affected by thermal conditionsl as well as by the composition of the alloy.
Thus, the number of possible types of crystal or grain structure in cast metals is very large.
Correspondingly, different structural defects can occur
and affect the properties of castings depending onthe vari able factors. Main imperfections of casting structure may be related to two distinct groups which determine the cast
ings property:
In the first greup are various imperfections which arise through physical phenomena occuring during s o lid ific - ation 4 la ttice, submicro, micro and macro defects). The
oi^in of |
these defects is more often stipulated by voiHunetric |
|
contraction or shrinkage during crystallisation. Such v |
||
cavities |
are closely related to crystal |
growth as well as |
to the pattern of crystallisation of the |
casting. |
|
• Block Irregularities due to the presence «5.f crystal
/blocks (mosaics) of slightly different orientation w within one crystal,
•Non-homogeneity of ordering or disordering i‘n the distr ibution of solute atoms or of discrete sub-microsoopio
particles (i.e . crystals of another kind) within the main
ooym atpix cr y s ta l,
-Submicroscopic cavities COP voids due to either to gases or to solidification eryfctal growth phenomena (volumetric contraction)*
Some of these imperfection are illustrated in fig .
2.3». |
|
|
In principle » crystals free |
from most or all |
such defects |
can be grown for theoretical |
study and even |
for some indu |
strial requirments (crystal fibres or whiskers). But crystals formed during solidifcation in industrial processes have