Ceramic mould material

A ceramic material for making ceramic moulds and core for metal casting is described comprising basically granular or bubble refractory material, e.g. alumina or mullite, bound together by hardened ceramic slurry. Moulds for lost wax casting are built-up by dipping a wax pattern in ceramic slurry and then applying granules of bubble alumina in an all over coating. A plurality of such coats may be applied by allowing the slurry to harden between applications. The moulds are more insulating than those using tubular alumina grits, for example, and produce castings with smoother surface finishes.

The invention relates to improvements to ceramic moulds. In particular it 
concerns the materials used to make the moulds and methods of producing 
the moulds. 
In the manufacture of moulds for investment casting of metals, the mould 
shell is built up around a wax pattern by dipping it into a slurry of 
ceramic material and stuccoing or raining coarse refractory grit onto the 
wet slurry. The wet slurry coat may be dried or hardened and the above 
procedure repeated several times to build up a coating of sufficient 
thickness, for mould strength and integrity, before the green mould is 
fired. 
Several refractory materials, such as fused silica, fused alumina, tabular 
alumina and fused or sintered alumina silicates are used as stucco 
materials. They are produced by bulk fusion or sintering and are then 
crushed and sieved to separate-out grits of required sizes. Purified and 
graded natural sands, for example zirconium silicate and quartz sands are 
sometimes also used. Characteristically these materials consist of 
particles which are angular in shape with a tendency to have sharp edges 
and corners and a degree of uneven packing occurs in the stuccoed layers. 
These stucco grits preground more finely to provide a flour of suitable 
particle size distribution are usually used for slurry fillers. 
In multi-layered moulds the first or prime coat slurry, because it forms 
the internal surface of the mould in contact with the cast metal, usually 
has a higher viscosity than subsequent coats and the stucco refractory 
grit is of finer particle size so as to produce as smooth a cast surface 
as possible. Subsequent coats are produced using coarser grit sizes and 
lower viscosity slurries. 
Moulds need to be dimensionally stable, inert, and to have good thermal 
shock characteristics depending on the type of alloy being cast, the 
geometry of the cast article and the nature of the metallurgical 
structure. In equiaxed casting, where molten alloy is poured into 
preheated moulds and allowed to solidify relatively quickly, mould surface 
temperatures may reach around 1300.degree. C. maximum for short periods of 
time. In directionally solidified and single crystal alloy casting the 
mould is heated above the alloy melting point so that the casting may be 
progressively solidified over a relatively longer period of time. Thus, a 
mould must be dimensionally stable and able to withstand temperatures of 
up to around 1650.degree. C. Without adequate refractoriness a mould or 
mould system can distort during the pouring and solidification stages 
leading to poor control of casting dimensions. 
Good casting surface finish is also required and for this a smooth surface 
of the prime coat is essential. If the initial slurry viscosity is 
unsuitable, or the wax pattern is overdrained, the grits or sands in the 
prime coat stucco can penetrate the wet slurry coat too deeply causing an 
air pocket to form at or near the metal/mould interface leading to 
penetration of the cast metal into the mould surface, producing a rough 
casting surface. Even when a rough finish to the casting is desired the 
process by which it is produced must be controllable to achieve 
consistency. 
Mould thickness consistency is also important for strength and predictable 
thermal behaviour. Mould shell strength must be sufficiently high to avoid 
mould failure on one hand and on the other hand it must be low enough, and 
the shell sufficiently crushable, to avoid stressing tearing or cracking 
of the solidifying casting and to facilitate easy shell removal. 
In equiaxed casting a mould must also exhibit good thermal characteristics 
to ensure it is at and maintains the correct temperature when molten metal 
is poured. A temperature which is too low, particularly for castings with 
thin sections can cause premature chilling of the metal and local 
variations in mould temperature resulting in variable solidification rates 
which can produce undesirable metallurgical structures in the finished 
casting. To avoid this, for example, when casting thin section equiaxed 
turbine blades, moulds are usually wrapped in additional external 
insulation to maintain a correct mould temperature and avoid cooling 
before metal is poured if separate ovens are used to heat the moulds 
causing a delay. 
Hollow cavities in cast articles are produced using preformed ceramic cores 
located within the mould cavity. Using for example the lost wax pattern 
process these cores are formed separately, fired and incorporated within 
the expendable pattern prior to building-up the external mould shell. 
These cores can be produced in a similar manner to external shell moulds 
but on the internal surfaces of a core die which can be split to remove a 
hardened "green" core. Other core forming methods used mainly involve 
casting and injection moulding. However, in common with the described 
shell building process these methods also use a hardenable liquid of 
flowable binder with a refractory grit or powder of suitable particle 
size. 
Such internal cores also need high temperature stability, inertness and 
crushability. Simple core shapes can be removed by mechanical means but 
complex shapes may need to be leached from the casting. The latter 
requirement restricts the choice of usable materials principally to silica 
or alumina based ceramic compositions or the like. 
The present invention has for its object to provide ceramic moulds which 
will overcome the problems and difficulties discussed above. In particular 
the invention is intended to produce moulds the shells of which are of 
very even thickness, and of consistently reproducible thickness; to 
produce moulds having good thermal insulating properties a high degree of 
dimensional stability, are easily removed after casting and where 
necessary possess good "crushability" but which are free, or largely free, 
of surface voids which could be penetrated by molten alloy and are thus 
able to produce good surface finishes. 
In its most general form the invention provides a ceramic shell mould or 
core material comprising refractory material in bubble form. 
According to one aspect of the invention a ceramic mould or core material 
for use in casting metals contains hollow grains or bubbles of refractory 
material bound together by a hardened ceramic slurry. 
The hollow grains or bubbles of refractory material have a closed cell 
structure and comprises alumina, preferably, or mullite. The ceramic 
slurry consists of a liquid binder and powdered refractory material. 
In a preferred form of the invention a ceramic shell mould for casting 
molten metal has a plurality of layers of bubble material bonded by 
hardened ceramic slurry. The viscosity of the wet ceramic slurry used to 
produce the first of said layers is relatively higher than the viscosity 
of the slurry used in subsequent layers. 
A method of producing a ceramic shell mould of the kind already described 
involves coating a wax pattern of an article to be cast with said ceramic 
slurry and while it is still wet applying to said coating a layer of the 
hollow sphere or bubble refractory materials, and subsequently hardening 
the ceramic slurry to bind together the bubbles or spheres of refractory 
material. To produce shell moulds having a plurality of layers of said 
bubble or hollow sphere material the described process step is repeated an 
appropriate number of times. Preferably, the viscosity of the ceramic 
slurry used for the first layer is relatively higher than that used for 
the subsequent layers.

EXAMPLE 1 
Ceramic Shell Mould 
A ceramic shell mould for a solid cast article, for example a turbine 
blade, without internal cavities or cores was built-up on a wax pattern 
assembly of the article by dipping it repeatedly into a ceramic slurry and 
applying stucco coatings of hollow grains of bubble alumina. The diagram 
of FIG. 3 shows a section through part of such a mould and indicates the 
composition of the constituent layers of the mould. The primary ceramic 
slurry composition, set out in more detail hereinafter, was more viscous 
than the slurry used for the multiple secondary coats and the particle 
size of the primary coating stucco was finer than the secondary coatings 
thereby providing a smoother finish to the internal surface of the mould. 
The wax turbine blade pattern assembly was dipped into a vat containing the 
primary coat slurry and allowed to drain sufficiently to leave an even 
coating on the pattern. The primary coat stucco material of bubble alumina 
grains or hollow particles was then sprinkled over the still wet slurry 
coat, ensuring that the entire surface was covered. It was then left in 
air for one to two hours to dry. 
After drying, seven additional secondary coats were applied by dipping the 
primary coated pattern into the secondary coating ceramic slurry, allowing 
it to drain and then applying the secondary coat stucco of larger size 
grains of bubble alumina. At each stage the coating slurry was left to 
harden by a three step process which consisted of air drying for one half 
hour, followed by ten minutes in an atmosphere of ammonia and then a 
further period of one half hour in air before the next dip. Finally, after 
the required number of layers had been applied, the shell was sealed by 
dipping in the secondary slurry mix and, without a further application of 
stucco material, allowing the shell to dry in air for roughly twelve 
hours. 
When the ceramic shell mould was thoroughly dried the wax was removed in a 
steam autoclave. The dewaxed "green" ceramic mould was then fired in a gas 
oven at a temperature of 850.degree. for one hour. The finished shell 
ready for casting weighed only two-thirds the weight of a more 
conventional mould produced using similar slurry composition and tabular 
alumina grits. Insulation tests also showed that the moulds produced using 
bubble alumina were relatively much more insulating as well as 
substantially lighter. Shells produced this way were also found to have 
good resistance to cracking. Tests carried out by filling the shells with 
isopropanol coloured with methylene blue dye revealed no cracks, and 
proved to be dimensionally stable, judged by measurement of the dimensions 
of cast components, while at the same time the moulds were easy to remove 
after casting. 
A batch of shell moulds made in accordance with the above detailed method 
were tested in a directional solidification process. The mould was heated 
inside a vacuum furnace to a temperature of 1470.degree. C. An alloy 
charge was then melted and the molten metal poured into the mould and 
progressively solidified over a period of ninety minutes, according to 
known directional solidification techniques. The mould proved easy to 
remove and the cast component showed good dimensional control. Also, the 
surface finish of the component was smooth with no metal penetration 
defects or rough casting surfaces. 
However, the enhanced insulating properties possessed by moulds made in 
this way are not necessarily ideal for directional solidification and 
single crystal casting where a longer thermal time constant could make it 
more difficult to control progress of the crystal solidification front 
during the withdrawal/cooling stage. On the other hand these properties 
are found positively beneficial in equiaxed casting where it is desirable 
to retain heat in some parts of a mould to prevent premature 
solidification of, for example, extremities and thinner sections of the 
article. 
Primary Coat Slurry 
The ingredients of the primary coat slurry were as follows: 
Binder - Aqueous colloidal silica solvent containing 30% w/w silica. 
Filler - 200 mesh zirconium silicate flour at a nominal loading of 4.8 
kg/liter of binder. plus 
Wetting agent at 10 ml/liter of binder, and 
Antifoam agent at 5 ml/liter of binder. 
The viscosity of the slurry was adjusted to 30 seconds to empty the first 
70 ml using a BS 3900 B5 flow cup. 
Primary Coat Stucco 
Bubble alumina having a particle size range 0.25 mm-0.50 mm diameter. 
Secondary Coat Slurry 
The ingredients of the secondary coat slurry were as follows: 
Binder - Hydrolyzed ethyl silicate with isopropanol solvent containing 25% 
w/w silica. 
Filler - 200 mesh zirconium silicate flour at a nominal loading of 3.6 
liter of binder. 
The viscosity of the slurry was adjusted to 40 seconds to completely empty 
a BS 3900 B4 flow cup. 
Secondary Coat Stucco 
Bubble alumina having a particle size range 0.50 mm-1.00 mm diameter. 
EXAMPLE II 
Dimension Test Specimens. 
Test specimens of bubble alumina shell were prepared by the method 
described above in Example I. Rectangular wax coated strips of metal, 
measuring 110 mm .times. 23 mm .times. 2 mm where coated using the same 
slurry mixes as previously noted. After shell build up was completed and 
the specimens dried the edges of each specimen were ground away and to 
release two flat ceramic test pieces or strips. Similarly sized test 
pieces were also built up using tabular alumina grit, instead of bubble 
alumina, for back-to-back testing. 
Thermal expansion tests were carried out in air. The test pieces were 
heated at a rate of 10.degree. C./minute from room temperature 20.degree. 
C. to 1500.degree. C., then held for 15 minutes dwell time at 
substantially constant maximum temperature 1500.degree. C., and afterwards 
allowed to cool at a rate of 10.degree./minute. The measurement results 
for each of the two types of test pieces are illustrated graphically in 
FIGS. 1 and 2 of the accompanying drawings. 
A prolonged dwell approximately 15 minutes at the maximum temperature is 
preferred as a means of revealing the dimensional stability of the shell 
material at high temperature. As will be seen from comparison of the 
results, the bubble alumina shell material exhibits excellent stability 
throughout the whole temperature range, but the tabular alumina shell 
starts to sinter at 1450.degree. C. and shrinks during the dwell at 
1500.degree.. Whereas a mould made using tabular alumina material would 
shrink substantially on cooling, a similar mould made using bubble alumina 
would shrink very little on cooling thereby subjecting a casting to much 
lower stresses. 
EXAMPLE III 
Ceramic Core Material. 
A ceramic material of similar type to that described in Example I for use 
as core material comprises the following ingredients: 
Binder - Low viscosity polyester resin having a viscosity of 250 
centistokes at 20.degree. C. containing a peroxide catalyst and cobalt 
naphenate accelerator. This mixture has a cure time of approximately 10 
minutes. 
Filler - A powder blend containing 200 mesh fused alumina flour, and bubble 
alumina having nominal particle size range 0-0.25 mm mixed in the ratio of 
powder to bubble alumina of 30:70 by weight. 
The liquid binder and blended filler were mixed in the ratio of filler to 
binder of 4.5:1 by weight. The resulting slurry was then introduced into 
the cavity of a core die by gravity feeding gently assisted by vibration, 
and allowed to cold cure to full hardness. The hardened "green" core, 
after being stripped from the die was then fired in a furnace in air using 
the following heating cycle: 
20.degree. C.-180.degree. C. at a rate of 10.degree. C./minute 
180.degree. C.-450.degree. C. at a rate of 2.degree. C./minute 
450.degree. C.-1550.degree. C. at a rate of 10.degree. C./minute 
The temperature of the furnace was then held at 1550.degree. C. for four 
hours before being allowed to cool. 
Cores made in this way will be found to be dimensionally stable and to 
possess an excellent smooth surface finish with high refractoriness. In 
addition the cores may be easily removed post-casting by chemical leaching 
in accordance with the techniques described in British Patent Nos. 
GB2,126,569B and GB2,126,931B. 
The basis of the leaching technique described in these patents is the 
provision in the substance of the core of a quantity of hydrogen which it 
was found greatly enhanced the leachability of ceramic cores by anhydrous 
caustic salts. In the context of the present invention the hydrogen donor 
may be provided by the gases trapped within the alumina bubbles during 
their formation. This atmosphere may be controlled or adjusted to vary the 
leachablility of the final core.