SOLIDIFICATION AND FABRICATION
1. Introduction
This practical has two purposes - the direct observation of solidification phenomena
and an introduction to some of the casting techniques in industrial use.
When a liquid metal or alloy is poured into a mould and allowed to solidify
undisturbed, the grain structure of the resulting ingot usually consists of zones
containing chill, columnar or equiaxed grains. The relative proportion of these zones
varies with the cooling rate and the composition of the alloy. The first experiment is
a study of how these zones develop, using a transparent model-system. The
phenomenon observed is precipitation from a supersaturated solution, rather than a
conventional solidification process, but many of the features of the grain structure are
similar in the two situations.
The microstructure of cast metal is profoundly affected by the morphology of the
liquid/solid interface. In many cases, this is dendritic. The shape and size of
individual dendrites, and the extent of the mushy zone composed of dendrites and
surrounding liquid, is dependent on the atomic structure of the interface, solute
redistribution and heat flow. The second experiment involves the study of various
interfacial structures, again using transparent model-systems. Two materials will be
studied. In one of these the entropy of fusion is relatively low, as in most metallic
systems, giving rise to an interface which is rough on an atomic scale and hence to
dendrites that are rounded. In the other, the entropy of fusion is higher, as in many
non-metallic systems, leading to atomically smooth interfaces, a tendency for certain
crystallographic planes to be preferentially exposed and hence the formation of
dendrites with facets.
In the last part of the practical, there will be an opportunity to look at castings
produced using a variety of techniques. Details of the microstructure, surface finish
and defects such as porosity and macrosegregation may be correlated with the
solidification conditions during casting. The techniques to be described include sand
casting, gravity die casting, pressure die casting, squeeze casting, investment casting
and centrifugal casting. One objective is to identify the constraints imposed by each
technique in terms of shape complexity, size, wall thickness, soundness, surface
finish and cost.
2. Observation of Grain Structure Development
(a) SAFETY GLASSES MUST BE WORN WHEN USING LIQUID NITROGEN.
(b) DO NOT TOUCH ANY OBJECT THAT HAS BEEN COOLED IN LIQUID
NITROGEN
Three brass moulds with transparent sides are cooled by pouring liquid nitrogen into
the dish in which the mould stands, but not into the mould itself. When frost has
formed on the mould, clear the outside of the perspex window facing you by spraying
with alcohol from a wash bottle, but do not get alcohol into the mould. Illuminate the
mould from behind in order to see the structures being formed.
Part IB
AP2/2
Make up a saturated solution of ammonium chloride in a beaker by stirring in the
crystals at 60˚C on a hot-plate. Continue until dissolution stops. Pour some of this
solution into a cooled mould. Observe that many fine crystals are formed during
initial contact with the mould wall (“big bang” nucleation) and that these become
redistributed throughout the liquid. This often occurs during the casting of metals.
Those in the bulk of the liquid may quickly remelt, depending on the pouring
superheat. This is desirable since they may grow and block the feed of liquid metal
needed to compensate for the freezing contraction. In the present experiment, such
remelting is difficult because of the lower thermal conductivity of non-metallic
systems. Depending on whether the solution was fully saturated, you may observe
that many of the “big bang” crystallites survive and grow to form equiaxed grains. In
any event, those which remained adjacent to the mould walls after pouring stay
unmelted and form the chill zone.
The remelting of the “big bang” crystallites is promoted by a high pouring superheat.
This can be simulated by heating the saturated solution from 60˚C to 90˚C before
pouring into the mould. You should then see the liquid clear quickly after pouring, as
most of the precipitates are taken back into solution. The development of the
columnar zone should then be clearly visible. In some cases, this will extend across
the complete section of the casting. In practice, it might be arrested by the
development of an equiaxed zone as a result of solid fragments surviving ahead of
the advancing columnar grains. A common source of solid fragments to form the
equiaxed zone is the free surface, where crystallisation is stimulated by heat loss.
These surface crystals sediment down into the interior. You can promote this process
by blowing gently on the free surface. Another mechanism by which equiaxed
crystals form in castings is grain multiplication, for example by the detachment of
dendrite arms at the advancing front as a result of mechanical and/or thermal
disturbances. This cannot be readily promoted in the present experiment, since the
growing crystals do not have the branched dendritic morphology which favours this.
You may be able to promote grain multiplication by (gently!) tapping the windows.
3. Observation of Dendritic Structures
3.1 Dendritic Growth
The interface remains planar during crystal growth from a pure melt with a positive
temperature gradient in the direction of growth. Although a positive thermal gradient
is usually present, the melt is never entirely pure. An impurity which partitions into
the liquid leads to an accumulation of solute in the liquid adjacent to the growing
crystal, thereby depressing its freezing point. There is then a larger driving force for
solidification of liquid ahead of the interface than at the interface, even though the
former is hotter. Such liquid is said to be “constitutionally” undercooled, to highlight
the fact that it is its composition, rather than its temperature, which is responsible for
its having a strong tendency to freeze. In this unstable situation, protrusions on the
growth front grow rapidly into the supercooled liquid, giving familiar dendritic (from
Greek for “tree”) structure.
Part IB
AP2/3
Constitutional undercooling can usually only be avoided at very slow growth rates.
The details of the growth morphology tend to vary with the strength of the
constitutional undercooling. If the effect is weak, then cellular structures are formed,
composed of arrays of parallel prisms. As the strength of the undercooling increases,
these cells start to develop side branches and also to exhibit a stronger tendency to
grow along well-defined crystallographic directions (the so-called “easy growth”
directions). For cubic metals, these are the <100> directions. The precise reasons
why this occurs are still not entirely clear, but the effect is thought to be due to
anisotropy of the atomic addition kinetics at the interface. (Non-metals, most of
which have atomically flat interfaces, tend to exhibit this growth anisotropy over the
complete range of growth conditions.) The reorientation to the nearest easy growth
direction is often taken as marking the transition from a cell to a dendrite. Further
changes occur as constitutional undercooling increases, with side arms forming and a
highly branched morphology developing. A change in growth rate tends to have two
separate effects. It may alter the degree of constitutional undercooling, and hence the
dendrite morphology, it also affects the scale of the structure, with faster cooling
giving rise to finer dendrite spacings.
3.2 Experimental Procedure
view with
microscope
camphene or
salol liquid
glass
slides
sealed with glue
around edge
heater
cooler
Experimental arrangement for study of dendritic growth
The set-up is shown in the figure. Specimens will have been left for some time
beforehand on each apparatus, with heaters and coolers switched on, to reach thermal
equilibrium. The liquid/solid interface should be approximately planar and be located
somewhere around the centre of the glass slide in the viewing field of the microscope.
Growth can be stimulated by perturbing the thermal field. The easiest method of
doing this is to slide the specimen towards the cooler. (The specimen is simply
resting on both the heater and the cooler.) This should cause the growth front to
advance. The growth rate can be controlled by changing the distance the specimen is
moved. A degree of fine control can be exercised by blowing gently on the specimen.
Part IB
AP2/4
Two types of specimen are provided. One is camphene (melts at 51˚C) and the other
is salol (melts at 42˚C). In both cases, impurity content is such that constitutional
undercooling is readily stimulated. Camphene forms dendrites in a similar manner to
metals. This is because it has a similarly low entropy of fusion since the molecules
can move from the liquid to the crystal in a number of alternative orientations. This
is analogous to a (monomolecular) metal, the atoms of which do not need to rotate as
they enter the crystal structure.
A number of the features outlined above can be studied with this specimen. The
breakdown of a planar front to cells, followed by reorientation to the easy growth
directions and the development a branched dendritic structure can be observed. It is
also possible to study the competitive growth between neighbouring grains which is
responsible for the development of the columnar zone. It will be seen that the
dendrites of a grain in which one of the easy growth directions is approximately
parallel to the heat flow direction will grow faster than those of a less favourably
oriented neighbour, which will gradually be excluded from further growth.
The other specimen, salol, provides an analogue for the growth of faceted dendrites.
It has a relatively high entropy of fusion, typical of materials with strong directional
bonds and with molecular structures in which reorientation, as well as translation, are
necessary as transfer takes place from liquid to solid. (Faceted dendrites can also
arise with metallic phases, provided the entropy of fusion is high for some reason,
e.g. Al dendrites in a tin-rich Sn-Al alloy. The entropy of fusion is high because the
Al is so dilute in the melt, an unusual situation for a primary metallic phase.) The
structures observed with the salol are often a little less obviously dendritic than the
camphene, since the facets tend to dominate the appearance. Nevertheless, a plane
front tends to break down to a dendritic structure in a similar manner as the
camphene. The transition is more sluggish and reorientation is not observed, since
growth only occurs in an easy direction. Both of these effects are consequences of
the relatively high undercoolings needed for any interfacial advance.
References
1. W.Kurz and D.J.Fisher, "Fundamentals of Solidification", Trans Tech., (1986)
[Ng100]
2. www.msm.cam.ac.uk/phase-trans/dendrites.html
3. www.msm.cam.ac.uk/phase-trans/phase.field.models/movies2.html
4. www.msm.cam.ac.uk/phase-trans/phase.field.models/movies.html
The last two references are computer generated or real movies of solidification, showing
all of the features studied in this practical. You should feel free to download references
2-4 on to your own computers for future reference.
HDB/IB/00