Method and equipment for bringing metal alloy ingots, billets and the like to the semisolid or semiliquid state in readiness for thixotropic forming

Equipment for bringing ingots of thixotropic metal alloy to the semisolid or semiliquid state including a heat chamber for holding the solid ingots introduced at ambient temperature, and a source generating air currents within the chamber to heat the ingots principally by convection, and a unit for controlling the temperature of the ingots. The ingots are supported and conveyed through a circular path internally of the heat chamber by a set of radial platforms revolving between an infeed zone and an outfeed zone.

BACKGROUND OF THE INVENTION 
The present invention relates to a method by which solid metal alloy 
castings such as ingots, billets and the like destined for thixotropic 
forming are brought to the semisolid or semiliquid state and to equipment 
for its implementation. In accordance with the invention the castings are 
typically alloys of aluminum, magnesium and copper. 
A recent addition to the range of processes adopted, for the shaping of 
metal alloys, typically pressure diecasting, forming, and others, is the 
method known as thixotropic forming. Such a method employs ingots prepared 
from a metal alloy that is brought to the semisolid or semiliquid state 
before being shaped and the alloy exhibits a particular structure of a 
homogeneous arrangement of solid crystals, globules or granules immersed 
in a liquid phase. Accordingly, this type of process requires a partial or 
total fusion of those phases of the alloy with a lower melting point, 
while the globular phases determining the thixotropic nature of the alloy 
must be maintained in the solid state. 
In practice, the resulting structure is composed of solid globules 
distributed homogeneously within a liquid phase, with no dendrites, i.e. 
devoid of crystals growing arborescently around nuclei. In this type of 
process it is essential that the proportions between solid phase and 
liquid phase are able to be reproduced at will in any ingot cast as a 
thixotropic starting material, compatibly with the type of alloy and the 
forming process adopted, so as to ensure that the behavior of the alloy in 
forming and the specifications of the end product can be maintained 
constant. 
Referring to the accompanying drawings, the state of a metal alloy suitable 
for thixotropic forming is indicated schematically by the graph of FIG. 1, 
the part of the curve to the left of point A represents material entirely 
in the liquid state, whereas the part to the right of point C represents 
material entirely in the solid state. The parts of the curve between 
points A and C indicate the semisolid or semiliquid material and, more 
exactly, the part between B and C represents a material composed of solid 
crystals or granules or globules immersed in a liquid phase, which is the 
eutectic. 
Progressing from point C to point B, the percentage of eutectic in the 
liquid state as opposed to solid crystals increases from 0 to 100. From 
point B to point A, on the other hand, it is the percentage of crystals in 
solid solution passing to the liquid state the increases from 0 to 100. In 
the case of thixotropic alloys, the areas of interest are generally B-C, 
where one has solid crystals together with eutectic in the liquid state, 
and apart of B-A depending on the liquid fraction effectively required. 
An ingot of such a material will behave as a solid when conveyed or 
handled, but behaves in the manner of a liquid when subject to any type of 
forcible shaping operation. 
In summary, an ingot in the thixotropic state is devoid of dendrites 
tending to jeopardize its homogeneous composition and mechanical strength. 
Again referring to FIG. 1, and in particular to the part of the curve 
between B and C, it will be noted that the mere application of heat is not 
enough to induce the required semisolid or semiliquid state of the 
material. But, in practice, the material must be maintained at the 
requisite temperature for a given length of time. 
Conventionally, an ingot of any given description in the solid state and at 
ambient temperature is brought to the semisolid or semiliquid state using 
induction furnaces, in which the heat is produced by generating a magnetic 
field whose flux lines directly envelop the ingot. The correct heating 
action, in terms of obtaining the requisite temperature and maintaining 
the ingots at the same temperature for the correct duration of time, 
usually is determined by trial and error, whereupon the conditions which 
are seen to produce the desired end result must be repeated exactly. 
The typical induction furnace consists essentially of a cylindrical 
crucible accommodating a single ingot surrounded by induction coils 
disposed in such a manner as to generate a magnetic field with flux lines 
impinging on and enveloping the ingot. 
Clearly, any variation in value and frequency of the magnetic field will 
cause a corresponding variation in the temperature applied to heat the 
ingot and cause a different distribution of heat between the skin and the 
core of the ingot. By regulating and monitoring the value of the magnetic 
field in the appropriate manner, the type of heating action applied to the 
ingot can be controlled selectively, targeting areas further and further 
in toward the ingot core. 
The time taken by such furnaces to bring each ingot to the desired 
temperature will naturally depend on the dimensions of the ingot. 
For a better illustration of the problem addressed by the present 
invention, reference may be made to a specific example: to bring an ingot 
some 150 mm in diameter and 380 mm in height to the semisolid or 
semiliquid state in the correct manner using an induction furnace of 
conventional type, a time of approximately 18 minutes is required. This 
may be acceptable in an experimental situation, but is not acceptable for 
industrial scale manufacture. 
Considering a production rate of one ingot per minute as acceptable, a 
battery of 18 conventional furnaces would be required to achieve such a 
rate. First of all, there are serious problems of economy associated with 
the operation of so many furnaces, given their high overall power 
consumption. What is more, there is the drawback of the considerable size 
exhibited by the equipment, given that an induction furnace able to heat 
the size of ingot in question will have an external diameter of some 600 
mm, to which the dimensions of the electrical panels must also be added. 
The size of the furnace is augmented further by being associated, 
necessarily, with an automatic or semi-automatic device for changing the 
ingot. The overall dimensions of the installation could be reduced 
somewhat by utilizing a single change device serving all the furnaces, 
though this would lead to notable structural complexities. 
The prior art considers one particular multiple type of induction furnace, 
albeit designed for use with smaller ingots, especially in the transverse 
dimensions, which comprises a platform rotatable about a vertical axis and 
supporting a plurality of ingots spaced apart around the axis of rotation 
at equidistant intervals. Located above the platform is a support capable 
of movement in the vertical direction and carrying a plurality of open 
bottomed induction furnaces, the number of the induction furnaces being 
identical to the number of ingots carried by the platform. The support is 
designed to alternate between a lowered position in which the induction 
furnaces each encompass a relative ingot, the open bottom ends engaging in 
a close fit with the platform, and a raised position in which the platform 
is able to index through one angular step, corresponding to the distance 
between any two adjacent ingots. The furnaces are put into operation in 
such a way that they rotate around the axis of rotation and can be divided 
substantially into three zones of difference temperature, including one in 
which the temperature and the structure of the ingots is made uniform. 
Not even this special multiple furnace can meet the requirements stated 
previously. However, such a furnace able to heat ingots of the dimensions 
indicated above would be unacceptable because of excessive dimensions and 
similarly excessive operating costs. Accordingly, the object of the 
present invention is to provide a method and equipment by means of which 
ingots can be heated to the semisolid or semiliquid state both swiftly and 
at reasonable cost. 
SUMMARY OF THE INVENTION 
The aforementioned object is realized in a method for bringing ingots or 
billets of thixotropic metal alloy to the semisolid or semiliquid state, 
in readiness for forming, which comprises the steps of introducing the 
ingots into a heat chamber in their solid state and heating the air within 
the chamber, generating convectional air currents internally of the 
enclosure in such a manner that the ingots are heated principally by 
convention, then controlling the temperature of the ingots, and finally 
removing the ingots from the chamber after being raised to a given 
temperature which is maintained for a predetermined time sufficient to 
induce the semisolid or semiliquid state. 
The softening ingots are set in motion within the heat chamber through the 
agency of conveying and positioning means by which they are supported and 
advanced from an infeed zone to an outfeed zone. 
In a preferred embodiment, the air internally of the chamber can be heated 
by means of a fluid fuel burner, which will also serve to generate the 
convectional air currents, the heat chamber in this instance affording 
vents through which the fumes emitted by the burner are exhausted. 
Alternatively, the necessary heat can be produced by electrical resistance 
heaters associated at least with the side walls of the chamber and 
operating in conjunction with a forced ventilation system. In this case 
the heat output from the resistance heater can be proportioned so that the 
requisite temperature is initially obtained and thereafter maintained 
along the path followed by the ingots between the infeed and outfeed 
zones. 
The invention also relates to equipment capable of implementing the method 
for bringing metal alloy ingots to the semisolid or semiliquid state as 
outlined above. Such equipment comprises a heat chamber, and installed and 
operating internally of the chamber, conveying and positioning means: such 
as will transfer a plurality of ingots from an infeed zone of the chamber, 
at which the ingots are introduced in the solid state, to an outfeed zone 
at which the ingots are removed ultimately in the semisolid or semiliquid 
state. The equipment also includes heating means operating in conjunction 
with forced ventilation means internally of the heat chamber in such a way 
as to generate convectional air currents by which the ingots are enveloped 
and heated. 
The equipment further comprises mean to sense the temperature of the ingots 
which are connected to a monitoring and control unit of which the 
functions are to control the operation of the conveying and positioning 
means, the forced ventilation means and the heating mens, and also to 
memorize the temperature of the ingots and the rate at which the ingots 
are advanced through the chamber by the conveying and positioning means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIGS. 2, 3 and 4, the present invention relates to 
equipment capable of bringing ingots or billets or similar castings of a 
metal alloy, denoted 2, to the semiliquid or semisolid or plastic state. 
The alloy in questions can be of aluminum or magnesium or copper and 
formulated in such a way as to respond to heat as indicated, by way purely 
of example, in the graph of FIG. 1. 
As a first step of the method to which the present invention relates, 
ingots 2 in the solid state are introduced into a heat chamber 4 and 
exposed within the relative enclosure to convectional currents, or 
streams, of hot air. The ingots 2 are thus heated primarily by convection. 
The temperature of the solid ingots 2 at the moment of introduction into 
the heat chamber 4 will of course be substantially the same as the ambient 
temperature outside the chamber 4. Thereafter, the temperature of the 
alloy is monitored continuously within the chamber 4 and ingots 2 will be 
removed after being heated to a predetermined temperature and held at this 
same temperature for a predetermined duration sufficient to induce the 
semisolid or semiliquid state. Inside the heat chamber 4, the ingots 2 are 
set in motion through the agency of conveying and positioning means 3, and 
transferred from an infeed zone 5 of the chamber 4 to an outfeed zone 6 of 
the chamber 4. 
The chamber 4 can be heated by means of a fluid fuel burner 11, which also 
serves to generate the convection hot air currents. The fumes produced by 
the burner 11 are exhausted through vents 12 positioned above and 
substantially in alignment with the ingots 2. 
Alternatively, the heat can be generated by a plurality of electrical 
resistance heater elements 13 arrayed at least along the side walls 19 of 
the chamber 4. The electrical resistance elements 13 can be made to 
operate selectively in such a way as to create zones of different 
temperature within the chamber 4, and more precisely, in such a way that 
the temperature gradually increases along the path followed by the ingots 
2 in their progress from the infeed zone 5 to the outfeed zone 6. 
The equipment capable of implementing the method according to the present 
invention, denoted 1 in its entirety, comprises conveying and positioning 
means 3 installed within the operating internally of the heat chamber 4 of 
which the side walls 19, the bottom wall 22 and the top wall 15 are lined 
with a refractory material. The ingots 2 are advanced by the conveying 
means 3 from an infeed zone 5 to an outfeed zone 6, both of which situated 
internally of the chamber 4. Ingots 2 supplied to the infeed zone 5 at 
ambient temperature are taken up by the conveying means 3, and removed 
subsequently from the equipment 1 at the outfeed zone 6 having been 
conditioned to the desired semisolid or semiliquid state. 
The equipment comprises means 7 by which to heat the ambient air, operating 
within the chamber 4, and forced ventilation means 8 serving to generate 
convectional currents or streams of hot air which are, played over the 
ingots 2. Also located within the chamber 4 are temperature sensing means 
9 by which the temperature of the ingots 2 is monitored continuously. 
The output of the temperature sensing means 9 is connected to the input of 
a monitoring and control unit 10 that controls the operation of the 
equipment 1 overall. In effect, this same unit 10 controls the heating 
means 7, the forced ventilation means 8 and the conveying and positioning 
means 3. The control unit 10 is programmed in such a way that the desired 
temperature and timing conditions are maintained internally of the chamber 
4. Timing in this context signifies the duration of the period for which 
the ingots 2 remain inside the chamber 4. 
Considering the two embodiments of FIGS. 2, 3 and 4 in greater detail, the 
chamber 4 has the geometry of a cylinder with a vertically disposed axis, 
and is formed by side walls 19 and a bottom wall 22 combining to create a 
crucible substantially in the form of a bucket. There also is an upper 
wall 15 acting as a lid. The conveying and positioning means 3 includes a 
rotor 33 disposed coaxially with the chamber 4 and comprising a hollow 
shaft 14 that is inserted through and supported by the lid 15 in such a 
way as to allow rotation about its own axis. 
The bottom end of the hollow shaft 14 is associated with a circumferential 
flange 16 serving to support ingots 2. 
The structure of the flange 16 can be either continuous or, preferably, 
discontinuous as indicated in FIG. 4, which illustrates a flange 16 
embodied as a plurality of individual platforms 17 carried by respective 
radial arms 20 extending from the hollow shaft 14. Each platform 17 
affords an arcuate element 23 serving to restrain the relative ingot 2. 
The hollow shaft 14 is accommodated by the lid 15 in an airtight fit and 
carries a plurality of freely revolving radial wheels 24, each with a 
peripheral groove designed to engage in rolling contact with a circular 
projection 25 extending from the lid 15. The hollow shaft 14 is set in 
rotation about its own axis by a geared motor 26 that is mounted to the 
lid 15, in a manner not shown in the drawings, and meshes with a gear 27 
keyed to the hollow shaft 14. The operation of the geared motor 26 is 
controlled by the monitoring and control unit 10. 
The side walls 19 of the chamber 4 afford at least one access door 32 
situated next to the infeed and outfeed zones 5 and 6. The embodiment of 
FIG. 4 shows only one such access door 32, so that the positions of the 
infeed and outfeed zones 5 and 6 coincide. 
The equipment operates in conjunction with means (not illustrated) by which 
to change the ingots 2, located externally of the heat chamber 4. 
In the embodiment of FIG. 2, the heating means 7 is shown as a fluid fuel 
burner 11 supported by a superstructure 28 mounted to the lid 15. The 
flame of the burner 11 is directed down the bore of the hollow shaft 14 in 
such a way that the fumes emerge from the bottom end and then flow 
upwardly and around the ingots 2 supported by the platforms 17. 
The lid 15 has a plurality of vents 12 located above and substantially in 
vertical alignment with the platforms 17, and connecting externally of the 
chamber 4 with an annular chamber 29 into which the fumes are channelled. 
The side walls 19 may also support electrical resistance heater elements 
13, as illustrated in FIG. 2, designed to operate in conjunction with the 
burner 11. 
In the embodiment of FIG. 3, the heating means 7 are shown as electrical 
resistance heater elements 13 carried at least by the side walls 19 of the 
heat chamber 4. In this instance, the superstructure 28 supports a motor 
30 which drives a fan 31 located near the bottom end of the hollow shaft 
14, thus forming the forced ventilation means 8. 
While the lid 15 has no vents 12 in the embodiment of FIG. 3, the hollow 
shaft 14 has radial holes 18 located above the level of the fan 31 and 
providing air inlet ports for the forced ventilation means 8. 
By proportioning the output of the resistances 13 in a suitable manner and 
adopting an appropriate arrangement of the radial holes 18, the interior 
of the heat chamber 4 can be divided into different temperature zones, and 
more exactly, zones in which the temperature increases gradually along the 
path followed by the ingots 2. 
Utilizing equipment 1 embodied in the manner thus described, ingots 2 are 
introduced singly into the chamber 4 via the access door 32, exposed to 
the convection hot air currents circulated forcibly within the enclosure, 
heated up to a predetermined temperature and maintained at this same 
temperature for a given duration, then removed singly from the chamber 4 
likewise via the access door 32. The monitoring and control unit 10 serves 
to vary the maximum temperature at which the ingots 2 are destined to 
soften, and more importantly, the duration for which the ingots remain in 
the chamber 4. With regard in particular to the length of time the ingots 
2 are kept inside the heat chamber 4, it is sufficient to adjust the speed 
of rotation of the hollow shaft 14. 
The advantages afforded by the present invention are discernible in the 
constructional simplicity and compact dimensions of a practical and 
reliable piece of equipment 1. In particular, the use of the rotor 33 
operating inside the heat chamber 4 is instrumental both in reducing 
dimensions and in allowing several ingots 2 to be heated at once. 
A further advantage of the invention is reflected in the operational 
versatility of the equipment 1. With convention heat as the principal 
means of raising temperature, it is a comparatively simple matter to heat 
even ingots 2 of non-cylindrical geometry, for example of square or 
rectangular or polygonal section. In addition, the resistances 13 can be 
controlled in such a way as to create zones maintained at different 
temperatures, so that even non-cylindrical ingots 2 can be heated 
correctly. 
Yet another advantage of the equipment 1 is that of economy in operation, 
gained through the adaption of heating means 7 of a type more conventional 
and easier to manage than induction furnaces. 
Also advantageous is the use of a single access door 32, as in FIG. 4, 
since with fewer openings in the chamber 4 the risk is minimized that 
these will upset the conditions of thermal equilibrium established 
internally by the convectional hot air currents.