Continuous semi-liquid casting process and a furnace for performing the process

A metal alloy, in particular a light alloy, is continuously cast in the semi-liquid state by bringing it up to conditions such as to cause segregation of a solid phase within the interior of the liquid alloy and making the alloy pass through a static mixer adapted to mix the solid phase uniformly upon formation, with the liquid alloy, so as to obtain at the output from the mixer a suspension which, once solidified, provides a material with valuable microstructural characteristics; the alloy is supplied continuously to the static mixer by introducing it in a discrete manner into a sealed furnace through an externally heated barometric column and, simultaneously, by pressurizing the interior of the furnace by the introduction into it of a flow of gas at a pressure value such as to cause the said alloy to flow through the static mixer in stationary laminar conditions.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for casting a metal alloy 
continuously in a semi-liquid state, in particular for casting a light 
alloy usable for casting components of the fuel supply system of a heat 
engine. The invention further relates to a furnace for performing this 
process. 
A static mixer is known from Italian Pat. No. 1,119,287 filed Jun. 20, 1979 
and entitled: "Process for the preparation of a mixture comprising a solid 
phase and a liquid phase of a metal alloy and device for performing this 
process", the static mixer being of the type formed by a cylindrical 
casting channel within which are disposed in succession a series of 
helically wound blades or paddles, by means of which it is possible to 
cast a metal alloy by obtaining partial solidification, during casting, 
within the passage of the static mixer, with simultaneous mixing of the 
solid phase, upon formation, with the remaining liquid phase in such a way 
as to form at the output from the static mixer a solid/liquid mixture in 
which the solid phase separating out from the liquid alloy is uniformly 
dispersed in suspension within the liquid alloy itself. The mixture thus 
obtained is stable for a sufficiently long period of time to permit 
collection in a ladle and subsequent casting in moulds to obtain castings 
having particular and valuable microstructural characteristics. 
To be able to obtain these characteristics, however, the solid/liquid 
mixture must be obtained in stationary fluid dynamics conditions and it is 
necessary to be able to control with precision and speed the physical and 
dynamic parameters of the casting (temperature gradient of cooling of the 
alloy, speed of transit through the static mixer, etc); this necessity 
involves, on the one hand, having to effect casting by the use of 
pressurised furnaces so that the casting cannot be performed continuously, 
but only as a batch process; and on the other hand it involves the 
necessity of rejecting not inconsiderable quantities of alloy and, above 
all, of having to dismantle and clean the static mixer in the interval 
between one casting and the next; bearing in mind that the furnaces 
cannot, for practical reasons, have a very high capacity (for example 
greater than 1000 Kg) this latter disadvantage involves high maintenance 
costs and, finally, a high cost per unit of cast alloy and a low overall 
productivity of the system. 
SUMMARY OF THE INVENTION 
The object of the invention is that of providing a semi-liquid casting 
process which makes use of the known static mixer described above but 
which can however be performed continuously. It is a further object of the 
invention to provide a furnace which can be coupled with a static mixer to 
perform this semi-liquid casting process continuously. 
The said objects are achieved by the invention in that it relates to a 
continuous semi-liquid casting process in which a metal alloy in the 
liquid state is brought into conditions such as to produce separation of a 
solid phase within the body of the liquid and in which the alloy is 
moreover made to pass through a static mixer adapted uniformly to mix the 
solid phase, upon formation, with the liquid alloy in such a way as to 
obtain at the output from the mixer a temporarily stable suspension; 
characterised by the fact that it comprises the following stages: 
introducing the molten alloy into a furnace through a barometric column 
surmounting the said furnace and dipping into the interior thereof, the 
interior of the said furnace being maintained closed with a fluid-tight 
seal and connected hydraulically to the said static mixer; and 
supplying the alloy continuously to the said static mixer by pressurising 
the interior of the furnace to a pressure value such as to cause the said 
alloy to flow through the said static mixer in stationary laminar 
conditions. 
The present invention further relates to a furnace for the continuous 
semi-liquid casting of a metal alloy by making it pass through a static 
mixer connected in a fluid-tight manner to a casting aperture of the said 
furnace, characterised by the fact that the said furnace is sealed in a 
fluid-tight manner and has a monolithic refractory lining adapted to 
contain a fluid bath of the said alloy, the said monolithic refractory 
lining being formed alongside the said casting aperture, means for 
introducing a pressurised gas into the interior of the furnace, above the 
said monolithic refractory lining, and a barometric column of 
predetermined height surmounting the said furnace and dipping into the 
interior of the liquid bath.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIGS. 1 and 2 the reference 1 indicates a furnace for 
casting a metal alloy 2 in a semi-liquid state, in particular for casting 
a light alloy, by passing this through a static mixer 3 of known type, 
illustrated only schematically as a cylindrical tube; the furnace 1, which 
is internally clad with a known refractory lining, not illustrated for 
simplicity, comprises a body 6 in the bottom of which are formed, in 
adjacent positions, a refractory monolithic lining material 4 and a 
casting aperture 5 connected, with a fluid-tight seal, to a static mixer 
3, which is fixed to the body 6 in question immediately beneath the 
casting aperture 5; above the refractory monolithic lining material 4 and 
the casting aperture 5, which defines a lower portion of the furnace 1, 
the body 6 delimits a chamber 61 defining the upper portion of the furnace 
1 and housing, in a known manner not illustrated for simplicity, suitable 
heating means, for example electrical resistances. The chamber 61 is 
closed in a fluid-tight manner by a cover 7 traversed by a tube 8 and, 
according to the invention, by a barometric column 10 of predetermined 
height surmounting the furnace 1 and dipping into the interior of the 
monolithic refractory lining 4; this latter is adapted to contain, in use, 
a fluid bath 12 of the metal alloy 2 of a height such as to ensure the 
immersion within it of a lower end 13 of the barometric column 10; the 
furnace 1 is of the rocking type and is therefore adapted to be inclined 
in use, during the casting stage, by a predetermined angle such as to 
cause displacement of the fluid bath 12 of alloy 2, by gravity, partially 
out from the monolithic refractory lining 4 to cover and fill the casting 
aperture 5, in such a way as to permit the fluid alloy 2 to flow out from 
the furnace 1 itself, by gassing through the casting aperture 5 and, from 
there, through the static mixer 3 connected fixedly to it; the barometric 
column 10 is disposed in a position such as to remain always immersed, 
even in this inclined configuration of the furnace 1, in the fluid bath 12 
and, therefore, once inclined, the furnace 1, with the refractory 
monolithic lining filled with fluid alloy 2 is sealed in a fluid-tight 
manner from the external environment, communicating with the outside only 
through the tube 8; this is normally connected in a known manner not 
illustrated for simplicity with a source of pressurised, preferably inert 
gases, such as argon or nitrogen, in such a way that the interior of the 
furnace 1 can be pressurised to any predetermined pressure value by 
introducing a flow of the said pressurised gas into its interior through 
the tube 8. 
In accordance with the invention the barometric column 10 comprises a first 
tube 20 made of graphite and disposed within the furnace 1, fixed so as to 
pass through the cover 7 and traversing the chamber 61 to terminate within 
the interior of the monolithic refractory lining 4 close to the bottom 
wall of this, and a second tube 22 surmounting the cover 7 outside the 
furnace 1 and provided with a funnel shaped upper end 24; this second tube 
22 is provided externally with heating means defined, in the specific 
example, by an electrical resistance 26 wound helically around it, and is 
connected at its end, in a fluid-tight manner, to the first tube 20 by 
means of a flanged joint 28 disposed in correspondence with the cover 7. 
In this way the barometric column 10 is able to contain the alloy 2 in the 
fluid state within its interior. 
According to the process of the invention, the furnace 1 is charged with a 
predetermined quantity of fluid alloy 2 equal to the capacity of the 
furnace (for example 500 Kg) in a conventional manner and, then, it is 
closed and assumes the conditions illustrated in FIG. 1; the furnace 1 is 
then inclined bringing it into the position of FIG. 2, in such a way as to 
cover the casting aperture 5 with the fluid bath 12 and at least partially 
to fill the static mixer 3; in this condition casting has not yet started 
in that the furnace 1 is dimensioned and disposed in such a way that the 
head of fluid alloy 2 which is created above the mixer 3 is negligible and 
insufficient to overcome, on its own, the pressure drop which the alloy 2 
experiences in the partial solidification stage in passage through the 
mixer 3. This head of fluid alloy 2 has in fact only the purpose of 
putting the barometric column 10, which is immersed in a fluid-tight 
manner in the bath 12, into hydraulic, sealed, communication with the 
static mixer 3, which is covered and at least partially filled by the 
fluid bath 12. Subsequently the interior of the furnace 1 is pressurised 
by the introduction into an upper portion thereof not occupied by the 
fluid bath 12, in the specific example the chamber 61, by a flow of inert 
gas 30, indicated by the arrows (FIG. 2); this flow of gas brings the 
interior of the furnace 1 up to a pressure P greater than the value of the 
pressure drop associated with the passage of fluid alloy 2 through the 
mixer 3 and causes casting to commence: the alloy 2 flows from the 
aperture 5 and passes through the mixer 3; according to the invention, 
during this phase, the alloy 2 is carried (for example by suitably 
adjusting its rate of flow by adjustment of the pressure P, regulating its 
temperature as it leaves the furnace 1 by regulating the said heating 
means in the chamber 61, and regulating its exit temperature from the 
mixer 3, along which the alloy 2 experiences a cooling) in conditions such 
as to produce the segregation within the body of the liquid alloy of a 
solid phase (not illustrated for simplicity); moreover, the passage of the 
fluid alloy 2 through the static mixer 3 causes, as described in Italian 
Pat. No. 1,119,287 cited above, a uniform mixing of the solid phase upon 
formation with the liquid alloy 2 in such a way as to obtain at the output 
from the mixer 3 a temporarily stable suspension, or rather a suspension 
which is stable for a sufficient time for its use for the production of 
castings of the desired shape and dimensions. As described in the Patent 
cited above, the suspension at the output from the mixer 3 is collected 
for use, for example by suitable ladles not illustrated, only when the 
fluid-dynamic conditions of the alloy 2 along the mixer 3 are stationary, 
that is as soon as the initial casting transients have terminated. 
According to the invention, for the purpose of indefinitely extending the 
casting once the stationary conditions have been reached, the rate of flow 
of alloy 2 which leaves the furnace 1 through the mixer 3 is balanced by 
an equal flow of new liquid alloy 2, which is introduced to the interior 
of the pressurised furnace 1, without reducing the pressurisation thereof, 
or rather by maintaining the interior of the furnace 1 sealed, through the 
barometric column 10; for example, the liquid alloy 2 is introduced in a 
discrete manner by pouring a predetermined quantity of it at intervals 
from a ladle 4 into the funnel 24 in such a way as to form and maintain 
within the barometric column 10 a head 41 of molten metal alloy of height 
such as to overcome the pressure P within the interior of the furnace 1. 
This fluid head 41 forms partially, thanks to the presence of the 
barometric column 10 and its immersion in the fluid bath 12 upon 
pressurisation of the furnace 1 by the flow of inert gas 61; the 
pressurisation P, which acts on the surface of the fluid bath 12, in fact 
urges into the barometric column 10, according to the well known laws of 
fluid statics, a part of the fluid alloy 2 forming the bath 12 until it 
forms a fluid head of height such as to balance the pressure P; the 
introduction of new alloy 2 through the funnel 24 causes an increase in 
the height of the fluid column present in the barometric column 10, with 
the formation of the head 41 which has a height such as to permit a part 
of the alloy contained in the column 10 to descend into the interior of 
the furnace 1 in such a way as to maintain substantially constant the 
quantity of alloy 2 which forms the fluid bath 12. Since this latter 
presents a very much greater surface than the section of the column 10, 
furthermore, this introduction of new fluid alloy 2 takes place without 
altering the stationary conditions of efflux of the alloy 2 through the 
casting aperture 5 in that upon writing the Bernoulli equation for the 
alloy 2 at the base of the column 10 it is easy to understand that the 
energy velocity component at this point is zero. For the purpose of 
compensating the dissipation of heat which can take place in the column 10 
and thus permit the alloy 2 remaining in it to stay fluid even for 
relatively long periods of time the part of the column 10 which is outside 
the furnace 1, that is the tube 22, is heated from the outside by the 
resistance 26 during the whole of the casting operation; the part of the 
column 20 within the furnace, that is the tube 20, does not need suitable 
heating since it is heated by radiation from the heating means within the 
furnace 1. 
Finally, according to the process of the invention, the value of the 
pressure P is chosen in such a way as to make the alloy 2 flow through the 
static mixer 3 in rigorously laminar conditions so that the mixer 3 can 
operate correctly. 
From what has been described the advantages connected with the invention 
are evident; thanks to a furnace of very simple structure, which is easy 
to operate and control, and of economic construction, the casting 
operation which, according to the state of the art was only possible in a 
discontinuous manner, can be made continuous entirely without loss of the 
particularly beneficial microstructural characteristics of the material 
subjected to treatment through the mixer 3. The possibility of performing 
continuous casting, which can continue for tens of hours, moreover, makes 
it possible to reduce to the minimum or even to eliminate entirely the 
necessity for maintenance of the mixer 3; bearing in mind that this is in 
any case subject to an inevitable wear so that it must be replaced after a 
certain number of castings, it is possible to design static mixers 3 
having dimensions such as to offer a durability equal to that of an 
individual continuous caster; the process of the invention therefore makes 
it possible to utilise "disposable" mixers thus eliminating any necessity 
for maintenance of the casting installation save for the ordinary 
maintenance of the refractory lining of the furnace 1.