Cooling-Water Circulation Core for Casting Hollow Ingots
By Takahiko Ichiki *, Katsumi Miyamoto * and Kazumi Katou *Today it is required to make multilateral efforts to streamline the production costs
for current aluminum extrusion processes. We designed and developed an innovative core, which is superior in quality and tactful with attaching smoother inner surfaces to hollow
ingots for extruded mandrel pipes, in the process of hot-top casting.
This core serves to bring about mass improvement in the surface smoothness inside a hollow
ingot and the inner- surface microstructure. It turns out that lesser aluminum chips result fromboring a hollow, preparatory to extrusion work, thereby creating a raise in the yield or the boring
rate, and a drop in the production cost. Concurrently, the merits of hot-top casting remain
equipped. Accordingly, its superiority to float casting (conventional technique) will be unaffected,
namely, the inward quality of a hollow ingot to be produced.
This paper points out features and drawbacks of the hollow-ingot casting techniques, whichhave been put into practice or under study. It's another content covers structural standouts of thedeveloped core and results of what was cast, using the same core. It is also discussed how toform smooth inner-surface texture with certainty and efficiency: the making of a core, setup of acore, and casting conditions.

ABSTRACT1.

INTRODUCTION

Use of a hollow billet is a rational approach to provision
against variation in thickness over an extruded mandrel
pipe. Hollow billets are made out of hollow ingots, whichshould have been cast, followed by having their hollows
bored.
What ought to be improved in streamlining the production cost, is the hollow boring rate or the production yield.
In brief, it will create the most efficiency to raise the boringrate or lessen the thickness to be bored. In particular, acertain measure of thickness must be provided, because
the inner-surface smoothness or the surface microstructure of a hollow ingot cannot be made ultimately perfect.
Our task was to attempt an authentic design change inthe conventional core, which had been used to form a hollow through an ingot, in order to solve the above point at
issue. In general, a core (inner mold) to form a hollow
inside an ingot is of a metal cylinder, into which coolingwater is fed, with the result that the cooling water is injected directly onto the inside of the formed hollow. The same
applies to hot-top casting. Looking back on the past,graphite cores with no cooling water and water-cooled
hot-top cores were tried out, in this context. However, theReprinted with permission from the Proceedings of theSeventh International Aluminum Extrusion Technology
Seminar, published by the Aluminum Extruders Council and
The Aluminum Association.
*

Production Engineering Group, Oyama Tubes and Bars
Plant, Light Metals Company

former turned out to be unstable in internal cooling, andthe latter ended up with rough inner-surface smoothness,due to overcooling. Taking those bitter experiences into
consideration, we contrived a method of circulating cooling water in a core, till being drained out, allowing no cooling water to be in direct contact with the inner surface ofthe hollow ingot being formed.
This method has turned out to be practical. To be brief,
appreciable improvement has taken shape in the innersurface smoothness or the surface microstructure in a hollow ingot, leading up to a lesser measure of thickness tobe bored.

2.

HOLLOW INGOT CASTING TECHNIQUE

Hollow-ingot casting methods are outlined in Figures 1, 2,
3 and 4. This topic covers each mechanics and points ofquestion. Float casting as sketched in Figure 1 is incapable of driving out oxides assignable to spout or float,
and is not free from a threat to fluent feed melting 1).
Figure. 2 picturizes "Hot-Top Casting" for hollow casting, in which melting is fed as leveling out, and is free fromthe same faults as in float casting, corresponding to soldingot casting. But the water-cooled aluminum core carries
through in the midmost of the mold. Consequently, thesump in the hollow gets rather deep or long, where liquation results from remelting of a solidified shell --the mostserious drawback, as shown in Figure 5. The remelting ofa solidified shell is a key factor in roughening the innersurface smoothness 2).

95

Water inlet

Melt
Float

Trough

Core

Melt

Mold
Cooling-water
Ingot

Figure 1

Water

Sump

Melt

Sump

Liquation from remelting

Water inlet

Refractory-header

Water

Ingot
Cooling-water

Figure 2

Figure 5

Float casting (conventional model)

Mold

Hot-top casting with aluminum alloy core

Water inlet

Refractory-header

advantage: being free from variation in the level of molten
metal. Yet on the contrary, a lead-off point of solidification
reaching the not-lubricated refractory header will tear thesolidified shell --a threat to safety (causing a breakout of
melting): this fashion has not been commercialized.
Figure 4 picturizes hot-top casting with a graphite core,in which the solidification proceeds invariably from outer toinner reaches and is blocked out by the graphite surface,
thus forming a hollow. It is characterized in that the intrinsic lubricity of graphite is utilized not to necessitate anyother lubrication; the inner-surface smoothness is fine andsmooth 2). Nevertheless, graphite has one factor in roughening cast surfaces, due to its being eroded in melting andanother factor in varying a boundary of solidification due
to its being sizable in thermal conductivity: graphite cores
have not been commercialized yet.

Melt

3.
Sump
Cooling-water
Water
Ingot

Figure 3

Water

Aluminum alloy
core

Mold

Hot-top casting with aluminum alloy core and refractory header

Refractory-header

Melt

Graphitecore

Sump
Cooling-water
Ingot

Figure 4

Mold

Hot-top casting with graphite core

Figure 3 picturizes hot-top casting with a core in a hottop scheme, where the water-cooled aluminum-alloy coreis shrouded with a refractory header. The water-cooled
segment, being fairly short, is least likely to induce liquation. Moreover, the upward refractory header acts to

96

COOLING-WATER CIRCURATION CORE

Guidelines to design a faultless core that were extracted
from the referenced four cores:-to apply hot-top casting-to set and fix a lead-off point of solidification at a lowest possible level-to furnish adequate cooling to a hollow being formed
In line with the above, a scheme to circulate coolingwater in a core was contrived in combination with hot-top
casting.
Figure 6 draws a brief scheme of the forgoing core,
which can be characterized as follows:
1) The upper part is a refractory header and the lower isof an aluminum alloy; Two-fold structure to moderate
the measure of remelting.
2) The aluminum-alloy part is positioned lower than theouter mold. Besides, the core is cooled down, onlywith the water circulating in itself; therefore, the innersurface of a hollow ingot later as designed. The intent
here is to slacken the measure of remelting, and toretard the chilling effect from the core for evasion oflead-off solidification on the refractory header.
3) Hot-top casting is least likely to involve oxides due toturbulent melting or result in giant compounds.
4) The in-hollow area is structured to make the outermold fed continuously with a lubricant.

Furukawa Review, No. 20

2001Refractory-header

Water outlet
Water outlet

Melt

Sump

Core joint

Cooling-water
Circulate water

Figure 6

Mold1) 2 mm from inner surface

Brief scheme of water circulation core5) The inflow and outflow ducts of cooling water are
coaxially laid out to make the water flow as impartial
as possible.
6) Because no cooling water is injected directly onto thein-hollow area, if a breakout of melting should occur
there, no threat of vaporizing burst will emerge: superiority in safety over others.
The objective of our task was to enlarge the inner diameter of a hollow ingot using this core, to reduce the measure of thickness to be bored and to raise the boring rate.
The diametrical enlargement was designed by thinning thecoarse cell-boundary inside the hollow, in order to reachthe following target:
Target inner diameter: 190 mm (inner diameter asbored: 200 mm)
50% improvement in boring rate
Present level: 175 mm (inner diameter as bored: 200mm)4.

2) 4 mm from inner surface

CASTING WITH A COOLING-WATERCIRCURATION CORE AND THE INGOT
MICROSTRUCTURE

3) 12 mm from inner surface

Based on the casting results from the past trial cores, thecore border level was set to be 60 mm lower than theupper end of the outer mold. The taper angle of the corewas θ degrees (intentionally veiled) and the casting speed
was 45 mm/min. The other conditions (water feeding rate,
temperature of molten metal, etc.) remained the same as
ever. As a result, no breakout of melting at the outset of
casting took place. Nor did any hanging due to shrinkage4) 15 mm from inner surface

Figure 8

Figure 7

Inner surface smoothness, water-circulation core

Inner-surface microstructure1), 2) water-circulation core; 3), 4) hot-top castingwith Al-alloy corein solidification. This setup turned out to bear practical
casting. Notably, preferable, fine ripples inside the hollow
ingot were caught by sight --a great deal of improvement
as contrasted with others (see Figure 7). The ingot measured 190 mm in diameter.

97

Depth of sump from core-joint
Inner diameter
Coarse cell-boundary thickness
195

Inner diameter (mm)Thickness or depth (mm)

40

30

20

10

0

20

40

193
191
189
187
185

60

Core joint to outer mold (mm)1070
Inner diameter (mm)Thickness or depth (mm)

20

10

20

40

193

189
187
185

60

Inner diameter (mm)Thickness or depth (mm)

40

60

195

20

10

20

40

191
189
187
185

60

4.3

193

Core joint to outer mold (mm)
5052

20

40

60

Core joint to outer mold (mm)
5052

30

195

Inner diameter (mm)Thickness or depth (mm)20Core joint to outer mold (mm)
3003

30

20

10

0

193
191
189
187
185

20

40

60

Core joint to outer mold (mm)
6063

20

40

60

Core joint to outer mold (mm)
6063

30

195

Inner diameter (mm)Thickness or depth (mm)Figure 10 Inner-surface smoothness of a 3003 ingot, withwater-circulation core, core joint to outer mold: 20mm, core taper angle: θ, and casting rate: 45
mm/min.

191

Core joint to outer mold (mm)
3003

20

10

0

193
191
189
187
185

20

40

60

Core joint to outer mold (mm)6061

Figure 9

60

195

30

0

40

Core joint to outer mold (mm)
1070

40

0

20

20

40

60

Core joint to outer mold (mm)6061

Coarse cell-boundary thickness, sump depth or innerdiameter vs. core-joint level

As for microstructure, the coarse-cell boundary was
found uniform to the depth of 3 mm, circumferentially. The
other parts were also found fairly sound with no inclusionof giant compounds. These findings ensured a lesser
measure of boring thickness (see Figure 8).
4.1 Upgraded Boring Rate
We succeeded in reducing the measure of boring thickness, as planned: from 175 mm to 190 mm in diameter for3003 (7.5 mm in thickness). This result represents 53% of
improvement in the boring rate.
4.2

Practicability in Combination with 1070, 3003,
5052, 6061 or 6063
Aluminum alloys 1070, 5052, 6063 and 6061 were cast,using the same method and hardware, where all combinations of casting turned out to be practicable under the conventional conditions, with no initial faults emerging.

98

Core-joint level, taper angle, casting rate vs.
inner-surface smoothness
4.3.1 Core-Joint Level
Distances from the top of the outer mold to the joint of thecore, were on trial adjusted to 20, 40 and 60 mm for 1070,
3003, 5052, 5056, 6061 or 6063, respectively. Parameters
were determined to judge the levels of smoothness:(depth of an inner-surface cell boundary), inner diameterof a hollow, depth of a sump. The taper angle was given
as θ; the casting rate was adjusted to 45 mm/min for all
the casting trials. Figure 9 plots determined values for theparameters. Each of the alloys shows a decline of remelting as the core joint lowers, thinning the cell-boundary
thickness, improving the inner-surface smoothness. By
contrast, the inner diameters remained almost constant.
Figure 10 presents the smoothness inside a hollow ingotof aluminum alloy 3003, with the core joint being 20 mm
lower. As compared with Figure 7, given a core joint: 60
mm lower, taper angle: θ, a casting rate: 45 mm/min., theinner surface shows larger ripples, and traces of facial turbulence due to slight remelting.
4.3.2 Core taper angle
If the core taper angle is more increased, a more air gap
arises, interposing between the core and ingot: a thicker
thermal insulation forms to make remelting easier tooccur. The intent of this particular is to judge the influenceof core taper angles on the inner-surface smoothness.
Cores attached with a core taper angle of θ or 2θ were
fabricated to cast hollow ingots of 1070, 3003, 5052, 6061
and 6063. Those ingots were measured in the depth of an
inner-surface coarse cell boundary, the length of a sump,and inner diameter. Notably, the core-joint level was
adjusted to be 60 mm lower than the top of the outermold; the casting rate was fixed at 45 mm/min.
Figure 11 plots the magnitudes of influence of coretaper angles with reference to the respective checkpoints.
Each alloy shows a coarse cell-boundary thinner andmore even at θ than at 2θ, and the inner surface smoothness being fine and fair. The depth of a sump is less at θ
than at 2θ. There are scarcely any differences between
both inner diameters. Figure 12 presents the inner-surface
smoothness of a 3003 hollow ingot cast at 2 θ , which

Furukawa Review, No. 20

2001Depth of sump from core-joint

Depth of sump from core-joint

Inner diameter

Inner diameter

Coarse cell-boundary thickness

Coarse cell-boundary thickness
195

185

2θ

θ

Core taper angle
1070

Core taper angle
1070

10

θ

193
191
189
187
185

2θ

θ

Core taper angle
3003

0

θ

191
189
187

θ

Core taper angle
5052

5

θ

Core taper angle
6063

189
187

θ

2θ

5

Core taper angle
6061

2θ

45

191
189
187

θ

2θ

Core taper angle
6061

Figure 11 Inner-surface coarse cell-boundary thickness, sump
depth or inner diameter at core taper angles of θ and2θ

Figure 12 Inner-surface smoothness of a 3003 ingot, withwater-circulation core, core joint to outer mold: 60
mm, core taper angle: 2θ and casting rate: 45
mm/min.

underwent remelting, and no doubt appears worse than in
Figure 7. On analysis, the core taper angle should produce significant influence.

50

55

Casting rate (mm/min)
3003

10

188
186
184
182
5052
180

45

50

55

45

50

55

Casting rate (mm/min)
5052
190

15

10

5

188
186
184
182
180

50

45

55

Casting rate (mm/min)
6063

50

55

Casting rate (mm/min)
6063
190

20

193

185

θ

182

190

45

Thickness or depth (mm)

10

184

55

0

195

15

50

20

Core taper angle
6063

Inner diameter (mm)Thickness or depth (mm)

191

2θ

20

0

193

185

0

186

180
45

20

Thickness or depth (mm)

Inner diameter (mm)Thickness or depth (mm)

10

188

Casting rate (mm/min)
5052

195

15

55

190

30

0

2θ

50

Casting rate (mm/min)
1070

10

Core taper angle
5052

20

45

40

193

185

2θ

182

Casting rate (mm/min)
3003
Thickness or depth (mm)

Inner diameter (mm)Thickness or depth (mm)

10

184

55

20

0

2θ

195

20

50

30

Core taper angle
3003

30

186

180
45

40

Thickness or depth (mm)

20

188

Casting rate (mm/min)
1070

195

Inner diameter (mm)Thickness or depth (mm)

30

0

5
0

2θ

Inner diameter (mm)

187

10

Inner diameter (mm)

θ

189

15

Inner diameter (mm)

0

191

20

Inner diameter (mm)

10

193

Inner diameter (mm)

20

190

25

Thickness or depth (mm)

Inner diameter (mm)Thickness or depth (mm)

30

15

10

5

188
186
184
182
180

0
45

50

Casting rate (mm/min)
6061

55

45

50

55

Casting rate (mm/min)
6061

Figure 13 Coarse cell-boundary thickness, sump depth or innerdiameter vs. casting-rate

4.3.3 Casting rate
The most significant factor in affecting ingot microstructure
or proceedings of solidification is a casting rate. Aluminumalloys 1070, 3003, 5052, 6061 and 6063 were cast at
casting rates of 45, 50 and 55 mm/min. Each hollow ingot
was measured in the depth of an inner-surface coarse
cell-boundary, the depth of a sump, and inner diameter.
Figure 13 plots the magnitudes of influence of casting
rates with reference to the respective checkpoints. Eachalloy shows a coarse cell-boundary thinner and more evenat a greater sinking rate than at a less casting rate. But
the inner diameters became smaller. Talking about the
inner-surface smoothness, greater casting rates resulted
in least remelting. On the contrary, 1070, 3003, 5052,
6061 and 6063 showed slight roughness; each sump
more deepened in step with greater casting rates.
Figure 14 presents the inner-surface smoothness of a3003 hollow ingot cast at 55 mm/min.

99

of the core and the inner surface of the hollow ingot:
remelting should have been moderated, and a better
inner-surface smoothness should have come into existence. Yet in fact, the inner surface showed lengthwise
wrinkles, which did not seem to have resulted from remelting. It is inferred that the inner-surface roughness is
assignable to a solidified shell distorted by the compressive stress from the shrinkage in solidification more grownat the greater casting rate.
Figure 14 Inner-surface smoothness of a 3003 ingot, withwater-circulation core, core joint to outer mold: 60
mm, core taper angle: θ and casting rate: 55
mm/min.

5.

DISCUSSION

Factors in affecting the inner-surface smoothness of a hollow ingot were studied out, based on the findings from thetrial casting with a water circulation core, as follows:
1) Correlation between core-joint level and outer mold
The joint of the upward refractory header of a coreand its lower aluminum-alloy part should be at a lowest possible level, or as distant as possible from the
top outer mold. This setup moderates remelting,which makes the inner-surface smoothness of a hollow ingot, fine and smooth. It is because the coreapproaching the bottom of a sump refers to going farther away from the high-temperature region of melting: it gets less likely to remelt a solidified shell.
2) Casting conditions
The most significant factor in affecting ingot
microstructure or proceedings of solidification is acasting rate. In reality, the finding points out that agreater casting rate results in rougher inner-surface
smoothness or ingot microstructure; concurrently, theinner diameter gets smaller.
Shrinkage in solidification takes place in casting a hollow ingot as in a solid one: compressive strength acts
toward the center. Along this line, a rise in the casting rate
led to some rise in the cooling rate, where the shrinkage in
inner diameter was even greater than expected. The innerdiameter shrinkage just as described should have served
to largely lessen the air gap between the aluminum alloy

100

6.

CONCLUSION

Hot-top casting combined with a water circulation core
was designed and made, bringing out the items as given
below:
1) Being successful in carrying out mass-production ofhollow ingots with certainty and efficiency. The innersurface smoothness has far improved.
2) Forming a uniform coarse cell boundary with a depthof 3 mm in the inner surface of a hollow ingot, andmaking the outer surface preferable and free fromgiant compounds. It is proven that the measure ofboring thickness can be 3 mm to each side in diameter, much less than 12.5 mm of usually required thickness.
3) Being capable of boring a hollow of an ingot 53%
faster than conventional methods.
4) Preferable casting conditions:-longer distance between the joint of an aluminumalloy part of a water-circulation core and its outermolding-inner surface texture vs. core taper angle and casting rate (each influence on surface roughness)

REFERENCES
1) Mamoru Matsuo: "Aluminum Continuous Casting", Journal ofJapan Institute of Light Metals (1989), Vol. 39, pp.24-38. (inJapanese)
2) Yoh Ishii, Atsumi Takasugi, Masami Abe, and Kazumi Kato:Japanese patent, "Fabrication Method of Hollow Billets", 1630402

Manuscript received on July 4, 2000.

Furukawa Review, No. 20

2001