Note: Descriptions are shown in the official language in which they were submitted.
WO91/19578 2 0 8 3 ~ ~ ~ PCT/CA91/00216
A~aratus and Process for ~irect Chill Casting of Metal
Inaots
Technical Field
This invention relates to an apparatus and process
5 for improving internal macro and microstructures and
homogeneity of metal ingots and, more particularly, to
reducing macrosegregation in the central region of
aluminum ingots produced by direct chill casting.
Backaround Art
The primary technique used today for producing
aluminum ingots is direct chill (D.C.) casting. Direct
chill casting is effected in an axially vertical mould
which is initially closed at its lower end by a downwardly
movable platen. Molten aluminum is introduced to the
15 upper end of the mould, which is ~hilled by continuous
supply of coolant fluid to its external surface, and as
the molten metal solidifies in the region adjacent to the
periphery of the mould, the platen is moved downwardly.
With effectively continuous downward movement of the
20 platen and correspondingly continuous supply of molten
metal to the mould, there is produced an ingot of the
desired length. For economy of scale, the thickness of
these ingots keeps increasing such that only a few years
ago an 18" ingot was considered large, while today ingots
25 with a thickness of 26" to 30" are becoming commonplace.
Macrosegregation is one parameter used to measure the
properties of a finished ingot so as to determine its
future usefulness. Changes in macrosegregation across
commercial size castings, particularly aluminum alloy
30 ingots produced by direct chill casting, make it difficult
to maintain a particular concentration of alloying
elements within specification throughout the entire cross-
section or thickness of a casting. The degree of
macrosegregation in a casting is determined to a large
35 extent by the casting thickness, casting speed, the
alloying elements and their concentration, and by the
procedure used in casting the ingots. However, the most
WO91/19578 ~ 4 ~ PCT/CA91/00216
influential parameter affecting macrosegregation is the
thickness or diameter of the ingot being cast, and as the
thickness or diameter increases beyond 18", macrosegre-
gation becomes an extremely serious proble~. With these
5 large dimension ingots, the macrosegregation is a direct
result of unavoidable variations in the solidification
brought about by varying heat extraction rates from
location to location within the ingot cross-section and by
convective forces in the liquid and mushy zones of the
lO ingot.
In the direct chill casting process for aluminum
alloy ingots, it is customary to have a liquid zone or
sump followed by a mushy zone which in turn is followed by
a solid zone, all of which are arranged in a vertical
lS orientation. The mould is water cooled and therefore the
outer surface tends to solidify prior to the central
portion adjacent to the longitudinal centre line. Thus,
there is created in the upper end of the forming ingot a
liguid zone or sump of molten metal ~urrounded by
20 solidi~ied metal. Dendrites wh~ch are lean in eutectic
elements tend to grow about the periphery of the liquid
zone or sump. The presence of strong convection currents
within the liquid zone are believed to cause the tips of
the dendrites to detach and be carried by the convection
25 currents to the centre of the liquid zone. The use of
grain refiners is also known to result in the formation of
unattached crystals. As the dendrites move toward the
centre, they grow isothermally within the thermal boundary
layer and are finally frozen adjacent to the longitudinal
30 centre line. Since the dendrites are lean in alloying
elements, they cause the final product to exhibit a low
concentration of alloying elements in its central portion.
~his low concentration causes a large variation in
macrosegregation of the ingot itself which is undesirable
35 for the reasons stated above.
~ here have been various attempts to reduce macro-
segrega*ion adjacent to the longitudinal centre line of
WO91/19578 PCT/CA91/~216
3 20838~4
commercial size ingots. U.S. Patent No. 4,709,747, Yu et
al, describes the use of a mechanical damper positioned
within the liquid zone which is intended to control the
magnitude of the flow currents within the liquid zone. It
5 was hoped that this would reduce macrosegregation in the
solidified casting. U.S. Patent 3,672,431, Bryson,
describes the use of a baffle structure placed beneath the
metal delivery tube to a mould, this baffle being adapted
to direct a major flow of molten aluminum laterally and a
lO minor portion of the molten flow downwardly. Another
attempt at solving the problem can be found in U.S. Patent
3,506,059, Burkhart et al, in which a core member or
displacer is positioned within the mould cavity vertically
downwards to a location within the liquid zone after
15 casting has co enced. Again in this case the displacer
is intended to direct the incoming hot liquid metal to the
periphery of the mould cavity.
All of the above techniques have been found to have
only a marginal effect on the problem of macrosegregation.
~0 It i5 the object of the present invention to provide a new
apparatus and process which substantially reduces
macrosegregation.
Disclosure of the Invention
This invention relates to an apparatus and process
25 for reducing macrosegregation and generally improving the
uniformity of an ingot formed by direct chill casting of
metals, such as al~minum alloys. It has been found
according to the present invention that very significant
improvements can be achieved if a flow directing means is
30 provided which directs a major portion of a molten metal
feed centrally downwardly and into substantially the
bottom of a liquid sump within the forming ingot, then
preferably outwardly and upwardly along an interface
between the liquid sump and surrounding solidified metal.
35 This system works by ensuring that the hottest feed metal
is fed to the central core of the ingot, and generally
contrary to existing practices which result in the hottest
~ 3 ~ 3 ~ PCT/CA91/~0216
metal being fed to the surface region of the ingot and
dispersed laterally. The system of the present invention
opposes the naturally occurring buoyancy driven flows
which would normally occur in the direct chill casting of
5 large ingots, and inhibits the transport of alloy depleted
dendrites to the central core of the ingot. Additionally,
hot fluid is supplied to those regions of the ingot where
there is the greatest thickness of mushy zone, and thus is
believed to steepen the local temperature gradient and
10 decrease the mushy zone thickness.
Thus, the apparatus of the present invention in its
broadest aspect includes:
(a) a mould having a wall defining an axially
vertical casting zone with an open lower end:
~b) means for feeding a flow of molten metal
downwardly into an upper region of the casting zone; and
(c) means for closing the lower end of the mould,
thi~ means being positioned and adapted to support the
lower extremity of an ingot being cast in the casting
20 zone, and being movable downwardly away from the mould for
effecting continuous downward advance of the ingot during
casting such that the portion of the ingot emerging from
the mould includes a central liquid sump or zone
surrounded by solidified metal. The inventive feature
25 comprises flow directing means for directing a major
portion of the molten metal feed centrally downwardly and
into substantially the bottom of the liquid sump.
This can be achieved in a number of different ways,
such as a baffle or vertically movable dip tube for
30 directing the molten metal feed to substantially the
bottom of the liquid sump, or means may be provided for
increasing the velocity of the molten metal feed
sufficiently to force a major portion of the molten metal
feed centrally downwardly through the liquid sump to
35 substantially the bottom thereof. The molten metal feed
is also preferably directed from the bottom of the sump
outwardly and upwardly along an interface between the
WO91/19578 PCT/CA91/00216
~83~4~
liquid sump and surrounding solidified metal.
When the baffle is used, it is positioned within the
upper region of the casting zone and adapted to project
downwardly into the liquid sump, this baffle means having
5 substantially closed side walls and at least one bottom
opening for directing a major portion of the molten metal
feed centrally downwardly through the bottom opening and
into substantially the bottom of the liquid sump and then
outwardly and upwardly along an interface between the
10 liquid sump and the surrounding solidified metal.
The interface between the liquid sump and solidified
metal generally extends in a downward and inward direction
within the forming ingot and the baffle is preferably
shaped to generally conform to the shape of the liquid
15 sump. Accordingly, the baffle is preferably downwardly
and inwardly tapered. It may be in the form of a fixed
baffle, or an array of ba~les, the position of which may
be independently ad~usted during casting to impose a
preferred interface pro~ile, dynamically. The baf~le may
20 be a solid metal, e.g. alloy steel, structure, but is
preferably designed as a disposable unit with walls formed
of glass cloth. This glass cloth may typically be woven
glass fibre fabric screen which is impermeable to liquid
metal passage in the present application in the absence of
25 a pressure differential across the screen. The same type
of fabric is permeable to liquid metal passage in the
presence of such a pressure drop in the known application
in a flowing stream of metal in e.g. a transfer trough
used for screening such metal to remove coarse inclusions.
30 According to one preferred embodiment, the baffle can be
accordion folded and then deployed downwardly stagewise as
the liquid sump forms in the start up o~ a direct chill
casting procedure. In the casting of a large commercial
ingot, the liquid sump may be in the order of two to three
35 feet deep and the baffle-preferably extends down into a
bottom region of the sump.
The top of the baffle typically has a width
W091/19578 ~ PCT/CA91/00216
approximately 75% of the width of the ingot, with the
walls tapering inwardly generally parallel to the side of
the liquid sump. It may be used with any of the usual
~hapes of casting moulds, including rectangular, square,
5 oblong, round, etc.
The baffle device of the invention may be pushed
deeper and deeper into the ingot head during casting and
in some instances it may be pushed to a position below the
original solidus position. By pushing the baffle into the
lO solidifying metal during casting, the liguidus and solidus
isotherms dynamically adjust themselves to fit an imposed
profile. Holes or slots may be provided in the side walls
of the baffle and the bottom openings and side wall
openings are preferably configured to ensure that flow
15 through them is fast enough to reduce greatly the
diffusive penetration of the sub-baffle cooling into the
hotter liquid maintained above the baffle.
Consequently, the baffle enables metal to be supplied
¢lose to the ~olldification front at temperatures much
20 hotter, e.g. more than 20'K hotter, than would normally
occur. This has the effect of increasing or steepening
the local temperature gradient ahead of and within the
mushy (solid and liquid) region thereby decreasing the
local solidification time. It is well known that the
25 scale of the microstructure (secondary dendrite arm
spacing and size of second phase particles) depends on
local solidification time and the baffle arrangement of
the present invention has profound effects on the
structure.
The baffle functions well whether it is made of heat
conductive or insulating material. In either form, it
serves to thermally isolate the liquid sump from the
external cooling and it is found that there is typically a
50-70-C temperature difference within the liquid sump on
35 each side of the baffle.- Thus, the molten metal is
exposed to the full effect of the external cooling only
when it has passed down through the baffle to the bottom
WO9t/19578 PCT/CA91/00216
7 2~83~
of the liquid sump.
In place of the above baffle, it is also possible to
utilize a vertically movable trough and down spout or
manifold, preferably having a changing cross-section, for
5 directing a major portion of the molten metal feed
centrally downwardly and into substantially the bottom of
the liquid sump. With the vertically movable down spout,
the formation of the ingot is commenced with the bottom
end of the down spout in an upper position in the upper
lO portion of a DC mould. After a steady state casting
condition has been reached with a liquid sump fully
fGrmed, the lower end of the down spout is lowered down
into and immersed in the liquid sump whereby the molten
metal feed is directed to substantially the bottom of the
15 liquid sump. The flow rate of the metal and the
positioning of the down spout can dynamically adjust the
liquidus and solidus isotherms to fit an imposed profile.
As with the baffle described above, the vertically movable
down spout enables the feed metal to be supplied close to
20 the solid~fiGation front at temperatures much hotter than
would normally occur.
Another way of directing the molten metal feed to
substantially the bottom of the liquid sump is to increase
the velocity of the molten metal feed sufficiently to
25 force a major portion of the molten metal feed centrally
downwardly through the liquid sump to substantially the
bottom thereof. One convenient way of achieving this is
to provide an electromagnetic device surrounding a portion
of the down spout. With this arrangement, the outlet of
30 the down spout is positioned in an upper region of the DC
casting mould. The electromagnetic device is not turned
on until the casting has reached a steady state with a
liquid sump fully formed. At that point, the device is
turned on and is adjusted such that the liquid metal feed
35 travels to substantially the bottom of the liquid sump.
Metal hotter than the average sump temperature will then
impinge on the interface between the liquid sump and the
W091/19578 ~3~ ~4 P~T/CA91/00216
surrounding solidified metal. The velocity of the liquid
metal can be adjusted to maintain the liquidus and solidus
isotherms within an imposed profile. While an
electromagnetic velocity increasing means is particularly
S desirable, the velocity can be increased by other means
such as hydraulic or pneumatic systems.
The invention also relates to a novel product, namely
an aluminum-magnesium alloy ingot of improved homogeneity.
Thus, a direct chill cast aluminum-magnesium alloy ingot
10 is obtained having a maximum variation in magnesium
content across a section of the cast ingot between +5% and
-5% of the mean magnesium content of the alloy.
Brief Description of the Drawinas
Certain preferred embodiments of this invention are
15 illustrated by the attached drawings in which:
Figure l is a simplified sectional elevational view
of a direct chill casting apparatus embodying the present
invent~on in a particular form;
Figùre 2 i5 a top plan view of the baffle of the
20 invention shown in Figure 1;
Figure 3 is a simplified sectional elevational view
of a direct chill casting apparatus embodying a tilting
baffle unit according to the invention;
Figure 4 is a perspective view of one half of the
25 baff}e shown in Figure 3;
Figure 5 is a simplified sectional elevational view
of a direct chill casting apparatus embodying a folding
baffle according to the invention with the baffle fully
folded;
Figure 6 is a simplified sectional view of the
arrangement of Figure 5 with the baffle partly unfolded:
Figure 7 is a simplified sectional elevational view
showing the baffle fully unfolded;
Figure 8 is a simplified sectional elevational view
35 of a casting apparatus with a vertically movable down
spout;
Figure 9 is the same view as Figure 8 with the down
WO9l/19578 PCT/CA9l/00216
2~838~
spout lowered;
Figure 10 is a simplified section view of a casting
apparatus with an electromagnetic velocity accelerator;
and
Figure 11 is a graph showing variations in magnesium
content across the thic~ness of an ingot.
Best Mode For Carryina Out the Invention
Referring to Figure 1, a direct chill ingot casting
mould lO is shown for forming an elongated rectangular
10 ingot 17. For all practical purposes, this moulding
device can be any type of continuous or semi-continuous
casting mould for producing elongated ingots having
circular, oblong, square or rectangular cross-section.
The mould includes a water cooling chamber 11, a mould
15 face 12 and a cooling water discharge outlet 18. Molten
aluminum feed 13 is fed from a launder 14 fed into the
mould 10 via dip tube 15 and controlled by stopper rod 23.
At the start of the casting operation, the lower end
of the casting zone between the mould faces 12 is closed
20 by a platen 19 supported by a hydraullc ram. As the
molten aluminum in the casting zone solidifies adjacent
the moulding face 12, the platform 19 is drawn slowly
vertically downward by operation of the hydraulic ram and
the solidifying base of the ingot being cast, resting on
25 the platform, then begins to emerge from the lower end of
the casting zone. Water spray from the outlets 18 is
sprayed onto the emerging solidified ingot surface
immediately below the moulding faces 12. This spray of
water, striking the ingot surface, acts to enhance the
30 cooling and consequent solidification of the ingot 17 as
it moves downwardly away from the mould.
As will be seen from Figure 1, in the region of the
forming ingot within the casting zone and immediately
below the casting zone, there is a sump or pool 16 of
35 molten metal surrounded ~y solidified metal 20. The
interface 21 between the solidified metal 20 and the
molten sump 16 tapers downwardly and inwardly because of
WO 91/19578 ~33 PCT/CA91/0021~
the cooling action of the mould faces 12 and water spray
18. Below the region of the liquid sump 16 is the fully
formed ingot 17.
A feature of the present invention is the baffle
5 arrangement 25 which is designed to direct the molten
metal flowing out of dip tube 15 downwardly in an axial
direction and through bottom outlet 27 such that it
travels to the lower end of the liquid sump 16 and then
travels upwardly outside the baffle and adjacent the
10 interface 21. ThiS baffle has inwardly sloping sidewalls
26 which are generally parallel to the interface 21 and
end walls 28. It is preferably fabricated as a disposable
unit having a steel frame with sidewalls 26 and end walls
28 formed of glass cloth~ At the end of each cast, the
15 baffle is pulled out of the liguid sump, e.g. by a winch,
and the glass cloth portions are removed and replaced by
fresh fabric.
Figure~ 3 and 4 show àn alternate form of baffle
device in which the baf~le 30 comprises two half sections
20 30a and 30b. Each of these sections 30a and 30b is
pivotally mounted by arms 34 pivotally suspended from
brackets 35. Each baffle half section includes a pair of
sidewalls 31 and an end wall 32. The bottom is open and
the walls are preferably tapered.
At the start of the casting operation, the baffle
sections 30a and 30b are swung up into the position shown
by the dotted lines in Figure 3 and as a depth of molten
sump begins to form, the baffles can gradually be tilted
downwardly such that they eventually assume the position
30 shown by the solid lines in Figure 3. In this position,
the baffle is fully functional for the remainder of the
casting of the ingot. This baffle can also be made
disposable by building the sections with steel frames and
glass cloth walls.
Another form of baffle arrangement is shown in
Figures 5 to 7. In this arrangement, the baffle 40 has
flexible glass cloth fabric walls 41 connected by support
W091/19578 PCT/CA9l/00216
112~)838~
rods 42 such that the entire baffle can be accordion
folded within a container 43 as shown in Figure 5.
Sections of the baffle can then be deployed as shown in
Figures 6 and 7 as the liguid sump builds up to its
S maximum depth as shown in Figure 7. The deployment of the
baffle in this manner can be fully automated by the use of
solenoids to lower the baffle sections and controlling the
solenoids by a computer.
Another flow directing embodiment of the invention is
10 shown in Figures 8 and 9. In this embodiment, a down
spout or manifold 15 of substantial length is used and
this distribution arrangement and launder 14 are
vertically movable as indicated.
Figure 8 shows the beginning of a casting with a
15 liquid sump 16 beginning to form. As the molten metal in
the casting zone solidifies adjacent the moulding face 12,
the platform 19 is moved slowly vertically downward with
the solidifying base of the ingot being cast resting on
the platform. Eventually a steady ~tate is reached as
20 shown in Figure 9 with a relatively deep sump or pool 16
of molten metal surrounded by solidified metal 20. The
interface 21 between the solidified metal 20 and the
molten sump 16 tapers downwardly and inwardly because of
the cooling action of the mould ~aces 12 and water spray.
25 Below the region of the liquid sump 16 is the fully formed
ingot 17.
When the casting operation has reached the steady
state as shown in Figure 9, the down spout 15 and launder
14 are lowered as shown such that the lower end of down
30 spout 15 is deeply i ersed within the liquid sump 16. In
this location, the hot feed liquid is directed down
through the down spout 15 and discharges in a lower region
of the liquid sump 16.
An example of a casting device of the invention with
35 a flow accelerator is shown in Figure 10. This utilizes
the same basic casting system as shown in Figures 8 and 9
with a down spout 15 and launder 14. However, in the
WO91/19S78 ~ 12 PCT/CA9l/00216
embodiment of Figure lO, the down spout 15 is surrounded
by an electromagnetic device 50 adapted to increase the
velocity of the flow of molten metal downwardly through
down spout lS. The casting operation with the
5 electromagnetic device is commenced in the same manner as
in Figure 8 with the outlet of the down spout 15 located
in an upper region within the direct chill mould 10. With
the electromagnetic device turned off, the casting
operation is commenced with the platform l9 being lowered.
10 When the casting operation has reached a steady state with
a sump fully formed as shown in Figure 10, the device is
turned on and thé metal emerging from the down spout is
given a sufficient velocity to force the molten metal feed
downwardly through the liquid sump 16 to substantially the
15 bottom thereof as shown in Figure 10.
A preferred embodiment of the invention is
illustrated by the following non-limiting example.
ExamDle
An actual plant trial was conducted using a direct
20 chill caster and baffle as shown in Figures 1 and 2. The
mould was rectangular and was dimensioned to cast an ingot
635 mm by 1350 mm. The metal which was cast was an
aluminum alloy AA3004 containing a nominal 1.15% Mg and
typically used for beverage can bodies.
Casting procedures were carried out both with and
without the baffle and the results were compared as shown
in Figure 11. It will be seen that without the baffle,
the magnesium variation was sufficient to exceed the
Aluminum Association limits for 3004 alloy. ~he measured
30 variation was about +4% to -11% about the nominal 1.15%
Mg.
By comparison, the ingot cast with the baffle shown
in Figures 1 and 2 exhibited a magnesium variation of
about +1% to -5%, falling well within the Aluminum
35 Association limits and close to the nominal 1.15% Mg
across the entire cross-section of the ingot.
Whil~ this invention has been described in
W091/19578 PCT/CA91/00216
13 20838~4
conjunction with certain specific embodiments, it is
evident that many alternatives, modifications and
variations will be apparent to those skilled in the art.
For instance, Figures 1 to 10 are all based upon a
5 rectangular mould with a corresponding rectangular shaped
baffle. The invention functions equally well with moulds
of ~quare, circular etc. cross-sections. For instance,
when the mould is circular, the baffle has a corresponding
truncated conical shape. Without being bound by any
10 single theory to explain the complex macrosegregational
effects occurring in various DC cast ingots, the inventors
believe that the unique combination of fluid flow and heat
flux brought about by the system of the invention controls
the major macrosegregational patterns which appear in DC
15 cast ingots. Thus, it will be seen that the invention
embraces all such modifications, alternatives and
variations as fall within the spirit and broad scope of
the appended claims.
