Mould permitting regulation of the level at which it is in contact with the free surface of the metal in a vertical casting operation

The invention relates to a mould for vertical casting of molten metal and including a means for regulating the level at which the inner surface of the mould contacts the free surface of the metal being cast. The mould is formed at least in its lower part of metal, and the means for regulating the level comprises an electrically insulating material intentionally coated onto the inner surface of the mould, an insert of electrically insulating material disposed along at least one generatrix of the mould, over the entire height and thickness of the mould, and two terminals for connection to a source of alternating current provided on the outer surface of the mould at its lower part, with one terminal provided on each side of the insert. The mould finds application in the casting of metallurgical semi-finished products, in particular of aluminum and aluminum alloys.

The present invention concerns a mould permitting regulation of the level 
at which it is in contact with the free surface of the metal in a vertical 
casting operation. 
When producing metallurgical semi-products by casting light or ferrous 
metals such as aluminium and alloys thereof, the man skilled in the art 
seeks to produce ingots, billets, plates, etc., which have the best 
possible degree of physical and chemical homogeneity, in order to avoid 
the appearance of certain defects when those products are subsequently 
worked to produce sheets, wires, etc. 
Now, most of the casting processes which are used in industry at the 
present time give rise, when the metal goes from the liquid state to the 
solid state, to the formation of homogeneity faults of greater or lesser 
magnitude, which are essentially due to cooling conditions differing from 
one point of the cast products to another. Thus, in the casting process 
which involves casting the metal in a mould with a vertical passage, 
wherein the metal is successively cooled indirectly by way of the mould 
and then directly by means of a sheet of water, an external layer referred 
to as the "primary cortical layer" is found to be present in the 
semi-products. The structure and the composition of that layer differ from 
those of the internal part of the semi-product, the layer resulting from 
indirect cooling of the metal in contact with the mould. Moreover, other 
heterogeneity phenomena which are much less pronounced but which are all 
as troublesome may appear, such as "pocks" or small pits which are due in 
particular to dispersion in the metal material of the layer of oxide which 
is formed at the surface of the liquid metal which is in contact with the 
atmosphere. 
It is true to say that the man skilled in the art has not remained inactive 
when faced with those problems and he has put forward a certain number of 
solutions which are more or less satisfactory, aiming to eliminate or at 
least reduce the seriousness of such heterogeneity. 
Thus, in French Pat. No. 1 509 962, the man skilled in the art has 
recommended using electromagnetic casting, an already conventional 
procedure in which, by virtue of the metal being confined by means of 
forces of electromagnetic origin, it is possible to eliminate the mould 
and thus to avoid the occurrence of the cortical layer since there is no 
longer any indirect cooling effect. 
It is thus possible to arrive at an improvement in the homogeneity of the 
semi-manufactured products. 
However, that procedure suffers from the following disadvantages: 
it is necessary to equip the casting plant with an electrical installation 
of which is relatively complicated and expensive because of the need to 
provide currents at non-industrial frequency (500 to 4000 Hz), in order to 
generate a suitable confining field, 
the risk of heterogeneity due to the formation of pitting is increased on 
the one hand due to the absence of a mould and therefore an increase in 
the surface area of liquid metal which is capable of suffering from 
oxidation, and on the other hand due to the phenomenon of stirring of the 
liquid mass caused by the confining field which contributes greatly to 
dislocation of the film of oxide and the dispersion thereof in the metal, 
it is often difficult to generate a suitable confinement effect when 
starting up the electromagnetic casting operation, and 
the safety of the personnel may be put at risk when casting aluminium and 
alloys thereof as, in the even of an electrical failure, the liquid metal 
which is no longer confined spreads to the outside of the mould and can 
come into contact with the fluid for producing the direct cooling effect, 
thereby causing an explosion. 
Other, simpler solutions have also been proposed for reducing the thickness 
of the cortical layer. For example, French Pat. No. 1 398 526 teaches the 
use of a strip of fiberfrax which is glued to the mould so as to reduce 
the height of the metal in contact with the mould and therefore to reduce 
the effects due to indirect cooling. However, that reduction in height 
cannot be fixed once and for all as it depends in particular on the speed 
of casting. Thus, when that parameter varies, it is necessary either to 
change the mould or at least to alter the height of the strip. That gives 
rise to a lack of flexibility in a solution which ultimately provides only 
for partial suppression of the heterogeneity phenomena. 
French Pat. No. 1 496 241 provides for eliminating the disadvantages of 
indirect cooling by using a non-cooled graphite mould, but that system 
then encounters problems in regard to frequent maintenance and changing of 
the mould due to the fragility of that material. 
Another solution involves using moulds with a grooved or corrugated inside 
surface, by means of which the thickness of the cortical layer is reduced 
by more than 30% when casting for example aluminium 1050. However, besides 
the machining of those moulds which substantially increases the cost 
thereof, that system also suffers from the disadvantages involved in 
adapting the mould and in this case the grooves or corrugations to each 
casting speed. 
Another known process is casting under load with a raised reservoir, 
referred to as "HOT TOP" casting, but that procedure also suffers from the 
disadvantage of resulting in periodic solidification of the meniscus, 
which is the cause of small ripples at the surface of the 
semi-manufactured products, as well as involving difficulties when 
starting up the casting procedure. 
Finally, more recently, French Pat. No. 2 417 357 claimed a process wherein 
the axial length of the portion of the mould in contact with the liquid 
metal is varied by using a sleeve which slides on the inside wall of the 
mould. Such a system has the disadvantage, in the event of the metal 
solidifying in an untimely fashion, that it gives rise to an adhesion 
effect between the mould and the sleeve, thus causing tearing damage to 
those components at the time when the sliding movement is produced. 
It is for that reason that the present applicants, being aware of the 
problems raised by those procedures, in order to produce homogenous 
semi-manufactured products in which the thickness of the cortical layer is 
virtually zero, the material is of refined grain and the skin is free from 
pitting, sought and developed a process which has the following advantages 
over the prior-art processes: 
the use of electrical installations which are less complicated than those 
which are required by the conventional electromagnetic casting process, 
easy transfer from the phase of starting up the casting operation to the 
steady-state phase of casting, 
easy adaptation to variations in parameters such as the speed of casting 
since the process does not require any modification in the equipment such 
as a change of mould, 
application to any type of conventional ingot mould, 
the absence of any apparatus which involves moving components, 
the risks of explosion due to leakage of the liquid metal are less serious 
than with conventional electromagnetic casting. 
In order to arrive at that result, the applicants based themselves on the 
following observations: 
on the one hand, the operation of starting off casting is easier in 
proportion to the level of metal in the mould being higher. Indeed, with a 
low level, the glass cloth filter which regularizes the level and the feed 
of metal to the mould approaches the leading edge of solidification of the 
metal, and gives rise to the risk, when dealing with semi-manufactured 
products of small dimensions, of being clogged by untimely solidification 
of the metal and no longer being able to perform its proper function. 
Likewise, the phenomenon of cambering which occurs with semi-manufactured 
products of substantial width also prevents the casting operation from 
being started up at a low level of metal, and 
on the other hand, in the steady-state phase, it is preferable to cast with 
a height of metal in the mould which is as low as possible as that limits 
the height of the contact between the metal and the wall of the mould and 
accordingly reduces the thickness of the cortical zone which, as has been 
pointed out hereinbefore, is essentially due to cooling of the metal by 
way of the mould. 
Therefore, starting from a conventional mould with its contingencies, that 
is to say, while retaining in the mould a height of metal which is 
constant since it is fixed by the position of the float and which is 
sufficient in order not to interfere with proper operation of the filter, 
it was necessary to be able to limit as far as possible the height over 
which the metal is in contact with the surface of the mould, which in 
short amounted to finding a way of regulating the level of the line of 
contact of the free surface of the liquid metal with the wall of the 
mould. 
That way comprises applying to the liquid in the course of solidification, 
a periodic magnetic field of variable intensity and with a direction 
substantially parallel to the axis of the mould, and adapting the 
intensity thereof in dependence on the desired level. 
It has been found in fact that, by placing around the mould at least one 
circular coil constituted by an electrical circuit formed by one or more 
windings, and by feeding it with an alternating current at adequate 
industrial voltage, it was possible to modify the profile of the metal 
meniscus and in particular to vary the level of the above-mentioned line 
of contact of the metal with the mould, in direct relationship to the 
variations in the feed voltage and correlatively the intensity of the 
field generated. 
Thus, by increasing the strength or intensity of the field, it was possible 
to lower the level and consequently reduce the height of the zone of metal 
contact or, on the other hand, by reducing the strength of intensity of 
the field, it was possible to raise the above-mentioned level and 
consequently increase the above-mentioned height. 
The attraction of such a process is therefore that it makes it possible as 
desired to reduce the height of metal-mould contact and consequently the 
thickness of the cortical layer in a simple manner with a coil which is 
supplied with a current at an industrial frequency of 50 or 60 Hz, with 
the knowledge that the only repercussion of any electrical failure will be 
to vary the height of metal in the mould, that is to say any risk of 
liquid metal leakage will be avoided, which is not the case with 
electromagnetic casting. 
Moreover, while the presence of a mould limits the possibility of oxidation 
of the liquid metal at the level of the meniscus, the mould, by virtue of 
its contact with the metal, prevents any displacement of the film of oxide 
towards the side wall and therefore any danger of pitting at the surface 
of the semi-manufactured product. 
Moreover, the field applied to the metal also has the effect of generating 
forces within the liquid which homogenize the cooling effect and tend to 
cause a refining action in respect of the cast grain. 
However, the solution which involves surround the mould with an annular 
coil, while having the great advantage of in no way altering the 
conventional set-up of the casting installations, does in contrast suffer 
from the disadvantage of involving a substantial level of consumption of 
electrical power. Indeed, the mould absorbs a part of the magnetic field 
which represents approximately 15 to 30% of the total energy consumed. In 
addition, because of the presence of the mould, the coil must be spaced 
away from the liquid metal; now, as the strength of the magnetic field 
decreases in proportion to increasing spacing, that results in an 
additional waste of energy. 
It is for that reason that, being concerned with saving on energy and being 
aware of the fact that the cost of electrical installations increases 
rapidly in dependence on the level of output strength required, the 
applicants sought a way of reducing the electrical strength necessary for 
operation of that type of process. 
The applicants' efforts resulted in the creation of a mould characterised 
in that it is formed at least in its lower part by a metal, that it is 
internally coated with a film of electrically insulating material, that it 
has an insert of electrically insulating material along at least one 
generatrix, over its entire height and over its entire thickness, and that 
it is externally provided at the level of said lower part with two 
terminals disposed on respective sides of said insert and connected to an 
alternating current source. 
The applicants found that such a mould played precisely the same part as 
the coil as it made it possible to generate a magnetic field whose effect 
was to modify the profile of the meniscus of the cast metal and to vary 
the level of the line of contact between the metal and the mould. This 
invention provides that it is possible to eliminate the coil with all the 
disadvantages thereof, in particular the disadvantage of its high level of 
consumption of electrical energy. 
As the current necessary for generating the magnetic field is relatively 
strong, it is necessary for the mould to be made of metal and preferably a 
metal having a low level of electrical resistivity such as copper or 
aluminium or their alloys. That does not give rise to any problems since 
moulds are conventionally formed by such metals. 
However it is not necessary for the mould to be entirely of metal and it is 
even preferable to use composite moulds in which the use of such a 
material is limited to the lower part and the upper part comprising the 
collar portion is produced with a material which, if not electrically 
insulating, is at least a poor conductor of electricity such as for 
example a stainless steel. Indeed, it will be seen hereinafter that better 
results are achieved by thus limiting the height over which the current 
flows in the mould. 
Such composite moulds are constructed by using assembly means which are 
well known to the man skilled in the art. 
The mould according to the invention is characterized in particular in that 
it is internally coated with a film of electrically insulating material in 
order to prevent current from flowing from the mould towards the cast 
metal. The film is completely to cover the internal surface of the mould, 
from top to bottom. It is desirable for the film not to be a good thermal 
insulator as it would interfere with mould-metal heat exchanges and would 
have troublesome repercussions on the quality of the products obtained. It 
is for that reason that the applicants were led to study that problem in 
greater detail and after many tests found that certain films only could be 
used. 
Particularly when the mould is of aluminium or one of its alloys, that 
involves a film of aluminium oxide which is produced by anodisation. In 
fact, irrespective of the type of anodisation used, such a film forms a 
continuous coating which is relatively resistant to the flow of current 
and which is a good conductor of heat since thicknesses which are as small 
as 1 .mu.m permit the application of voltages of close to 100 volts. In 
addition, that kind of coating is highly resistant to abrasion and may be 
impregnated with a lubricating agent (hot grease) in order to facilitate 
the casting operation. Furthermore, that oxide lends itself readily to 
colouring, which makes it possible to detect any deterioration in the 
coating in the course of manufacture. 
Another type of coating which gives the same advantages as the oxide is a 
film of enamel, the production of which involves processes with which the 
man skilled in the art is familiar. 
Mention may also be made of films of fluorocarbon resin with which the 
results are also excellent and whose fragility is largely compensated for, 
by its excellent frictional properties which make it possible to avoid any 
use of a lubricating agent. 
Another solution which is an attractive proposition involves separating the 
film from the cast metal by a layer of graphite which is a few millimeters 
in thickness, thus also avoiding the need to use a lubricating agent. 
The invention is also characterized in that the mould, along at least one 
generatrix, over its entire height and over its entire thickness, has an 
insert of electrically insulating material. Indeed, in order for the mould 
to perform a function similar to that of the coil, it must permit the 
current which flows therethrough to pass in a direction perpendicular to 
the axis of the mould. That is achived by splitting the mould over its 
entire height, inserting an electrical insulation material into that slit 
and feeding the mould with alternating current from two terminals which 
are disposed on its outside wall on respective sides of said insert and at 
the level of the lower metal part thereof. The insulation may be any 
material which is known to the man skilled in the art such as for example 
mica, including the materials constituting the film and in particular in 
the case of a mould of aluminium or one of its alloys, the oxide which is 
produced by anodisation. The two terminals for connection to the 
electrical supply system are of any known type.

Referring to FIG. 1, shown therein is a nozzle 1 for a feed of liquid 
material, a level regulating stopper rod 2, and a mould 3 which is cooled 
directly by a fluid 4 which then cools the metal 5 directly at the point 
6. The right-hand mould half is fitted with a coil 7 which is supplied 
with an a.c. voltage 8 in order to generate the magnetic field in the 
direction 9 and to cause a lowering of the level of the line of contact of 
the surface of the metal with the mould from a point 10 in the prior-art 
casting operation to the point 11 in accordance with the process, which 
point is disposed at the level of the intersection 12 of the leading edge 
of solidification of the metal, as at 13, resulting from indirect cooling, 
and the leading edge of solidification as at 14, which results from direct 
cooling. It will be seen therefore that the height over which the metal is 
in contact with the mould has been reduced from a height h.sub.1 to an 
extremely small height h.sub.2 which can be assimilated to the point 11. 
Referring to FIG. 2, shown therein is a mould 15 of aluminium which is 
internally coated with a film 16 of oxide produced by anodisation, having 
on one of its generatrices a slit 17, the two facing faces of which have 
also been anodised, together with two terminals 18 which are connected to 
an alternating current source (not shown). 
Referring to FIG. 3, shown therein is a composite mould formed by an upper 
part 19 of stainless steel and a lower part 20 of aluminium. The assembly 
is internally coated with a film 21 of fluorocarbon resin. Disposed on the 
lower part of the mould and on the outside thereof are two terminals 22 
connected to an alternating current source (not shown). 
The invention can be illustrated by reference to the following examples of 
use thereof which involve a comparison, for three types of mould, in 
respect of the strength necessary to lower the level of the line of 
contact of the metal with the mould respectively by 15, 30 and 40 mm in 
the case of an aluminium mould with a section measuring 1100.times.300 mm 
and in which the level of metal at the centre was fixed at 60 mm from the 
base of the mould. 
The following types of mould were considered: 
type 1: a mould of internally anodised aluminium, in accordance with the 
invention, measuring 104 mm in height, 
type 2: a composite mould according to the invention of anodised aluminium 
in regard to the lower part, measuring 60 mm in height, and insulating 
material in regard to the upper part, measuring 44 mm in height, and 
type 3: a mould of aluminium of the same total height, which is used in 
accordance with the prior art, that is to say without an internal coating 
or insert, with the field being generated by an annular coil. 
The results are set forth in the following Table: 
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Lowering of the level of the 
Strength in amperes 
line of contact in mm 
Type 1 Type 2 Type 3 
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15 4600 3400 9800 
30 6200 5300 14000 
40 7000 6300 16000 
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It will be seen that, irrespective of the lowering of the level, it is the 
composite mould (type 2) which gives the lowest level of strength, that is 
to say, it is that mould which permits the maximum reduction in the cost 
of the apparatus. Having regard to the electrical losses in the current 
supply means, it is also that mould which, taken overall, consumes the 
smallest amount of electrical energy. Although type 1 has a lower level of 
performance, it nonetheless constitutes a very substantial advance in 
comparison with the moulds of the prior art since the level of current 
strength is practically divided by 2. 
The present invention finds application in the casting of metallurgical 
semi-manufactured products, in particular of aluminium and alloys thereof, 
such as for example lithium-bearing alloys, and in which there is a wish 
to produce both a cortical zone of practically zero thickness, a fine 
grain without the initial addition of refining agents such as ATSB, and an 
absence of pitting.