Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
Method and Apparatus for Manufacturing Strong Thin-Walled Castings
RELATED APPLICATION
Benefit is claimed of the prior filing date of provisional application no.
60/531,278, filed
December 18, 2003 in accordance with 37 CFR ~ 1.78(4) and 35 USC ~ 119(e).
FIELD OF THE INVENTION
The invention broadly relates to the art of manufacturing thin-walled castings
and is
particularly useful for the manufacture of cast aluminum automotive engine
blocks with very
thin interliner walls.
BACKGROUND OF THE INVENTION
The description will be mainly in terms of its applicability to aluminum
engine blocks where
there is a constant push to increase the power to weight ratio, which results
in the desire to
utilize a minimum amount of light weight materials and small dimensions, yet
with maintained
strength and integrity, and thus effectiveness and reliability.
In recent years for the manufacture of engine blocks, particularly for
automotive
applications, several processes are available; among which we can list (1) the
sand package
either low-pressure or gravity filled, wherein a sand mold comprising sand
cores defining
cavities of predetermined shapes, is filled with liquid aluminum alloy, which
after
solidification form the motor block, or (2) the semi-permanent low pressure
molds or (3)
gravity filled metallic molds with sand cores to form the interior features of
the block.
The design of the engine blocks has been changing over the time with a
tendency to
increase the power of engines. The dimensions of the motor blocks tend to be
fixed by the
dimensions of the car body. The blocks need to accommodate cylinders of larger
volume,
meaning larger diameter, within the same block volume. These designs pose a
challenging
problem to block manufacturers, because the cylinder liners (usually made of
iron) are sought
to be so close together that the aluminum wall formed as a web in the gap
between said
cylinder liners is becoming ever thinner (less than about 3 mm).
With the gap between adjacent pairs of cylinder liners being so thin, the
liquid
aluminum volume filling such interliner cavity is relatively small and rapidly
loses heat upon
contacting said iron liners and consequently solidifies prematurely thus
plugging the cavity and
preventing liquid aluminum from filling the rest of said cavity.
It has also been found that increasing the pressure in the aluminum alloy
holding
furnace does not solve the above-described problem, because the space through
which the
liquid aluminum must flow is too small.
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This uncontrolled solidification appears even when the iron liners are
preheated
to a temperature close to that of the liquid aluminum alloy.
See U.S. Patent 5,421,397 issued June 6, 1995 to Robert K. Hembree et al,
which discusses this same problem and teaches an elaborate system for not just
preheating the
liners, but actually using a casting plug 14 for forming the cylinder bore
mold which has a
computer temperature-controlled fluid pumped through it to delay the premature
solidification
at the thin interliner walls. But even this proposal, which is not likely to
be cost effective, fails
effectively to address the internal tension stresses that build up in the
particularly vulnerable
thin walls due to the simultaneous cooling of different materials in cohesive
contact with one
another (iron and aluminum).
In other words, even if the aluminum is forced to fill the small interliner
cavities
another problem nevertheless arises, which is that the thin aluminum wall
between the cylinder
liners fails or develops cracks due to the thermal stresses generated by the
rapid heat transfer
from the relatively small amount of aluminum alloy between the liners and the
relatively larger
and colder mass of said liners.
Thus, where the walls have been thinned by design considerations to be just a
few millimeters, this cooling occurs while in contact with the liners which
have different
coefficients of expansion and contraction. This sets up stresses in the thin
aluminum walls
between the adjacent iron liners of the cylinder block, which thin walls crack
on cooling, or
later upon machining, or when the completed engine goes into service. This can
cause oil
leaks, compression loss, and other significant impairment of the engine.
Even if the premature cooling does not plug the interliner cavity, the control
of the
cooling rate is nevertheless adversely and unpredictably affected and can
result in undesirable
differences in the crystalline structure of the cooled casting.
There is also often the design need to provide for additional or more
effective engine
cooling passages in the interliner areas, because there is now less material
to dissipate the heat
from the cylinders; yet just maintaining even past cooling flow rates is
becoming more difficult
as the interliner gap becomes smaller. Current designs now seek aluminum wall
interliner
thicknesses of about 2 to 3 mm. Thus, the creation of a cooling passage
reduces the aluminum
wall thickness in such passages to about only 1 mm.
Older techniques, such as drilling out passages in the interliner web do not
work in such
small dimensions (because the small diameter bits of such relatively long
length would be
costly, easy to break, and difficult to control without wandering). Also
merely one round hole
for forming the passages would be only about 1 mm in diameter and provide
insufficient flow.
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To get sufficient flow, there needs to be several passages and/or the vertical
height needs to be
several times the horizontal width.
Some current proposals for providing such cooling passages involve special
core-
making techniques, either using sand or other breakable materials like glass.
These processes
are not ideal, being typically of high cost and could be hazardous.
Another current practice involves casting a solid section between the cylinder
liners and
then opening or machining a very thin gap in the interlines web, the top of
which gap is later
closed and sealed by welding (thus forming a cooling passage). This procedure
requires
expensive cutting/machining equipment and tools.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention overcomes the above drawbacks of the current art by
introducing
a pre-formed web insert, with the required shape to fit in the narrowest space
between the
adjacent pairs of cylinder liners, at least along where the width is 3 mm or
less. This insert is
preferably made of aluminum of the same alloy or optionally of a different
alloy or even of
some other suitable material (such as bronze or copper) and is positioned in
the desired place at
the same time that the sand core defining the water jacket of the block is
produced.
This insert can have an hourglass shape similar to the pre-formed core
discussed in U.S.
Patent No. 6,298,899 and also similarly has the advantage of avoiding the
"angle of
inclination" discussed therein. Though similar in shape due to its
positioning, the '899 core
serves a very different purpose (involving a different concept).
The applicants' pre-formed insert is significantly different with many more
advantages.
The '899 core is an impermanent sand core, while in contrast Applicants' pre-
formed insert is a
permanent solid structure that effectively remains in place as part of the
casting after the water
jacket core is removed.
The Applicants' pre-formed insert can be a solid, or can additionally
incorporate a thin-
walled pipe, or preferably can alternatively have an integrally formed passage
in it. The pipe
or passage in such an insert is thus able to provide an effective stable
cooling fluid passage
through the ultra-narrow interlines gap.
It is therefore an object of the invention to provide a method for
manufacturing engine
blocks of aluminum alloys, which method eliminates the problems of unfilled
spaces, stresses,
and/or uncontrolled cooling rates in the interlines webs between closely
aligned pairs of
cylinder block liners (or in any other casting, automotive or not, having the
necessity of casting
a strong thin-wall portion that otherwise would have flow, cooling, and/or
differential stress
problems), while preferably providing for an adequate cooling fluid
passageway.
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It is another object of the invention to provide an engine block for
automotive
applications made of aluminum alloys wherein the web between adjacent cylinder
liners is
formed of a relatively stress-free pre-formed insert of a shape to fill the
gap and a content
suitable to function as part of the casting and to bond well chemically
(including by some
surface melting, possibly aided by an agent such zinc) and/or mechanically
with the remainder
of the casting and with the liners, preferably, being of the same aluminum
alloy as the
remainder of the casting.
It is another object of the invention to provide a method for manufacturing an
engine
block of aluminum alloys wherein a cooling passage is formed in the narrow
interlines gap of
said block by providing a pre-formed insert with a cooling passage already
formed therein,
which is of a suitable material and shape so as to fit in said interlines gap,
and which passage is
placed so that it remains in flow communication with separate areas of the
cooling-fluid jacket
of said engine block.
In one embodiment of the present invention, a method of casting a cast product
is
provided comprising pre-formed solid elements having at least one dimension
thinner than
about 3mm and separately casting thereafter the rest of the product with
liquid metal with the
pre-formed elements already in place thereby to form the final cast product.
In another embodiment of the present invention, a method of manufacturing an
engine
block of aluminum alloy having cylinder liners comprising providing a mold for
casting said
engine block; inserting pre-formed solid elements each between a pair of said
cylinder liners
and filling said mold with molten aluminum alloy to form said motor block.
In a further embodiment of the present invention, an engine block made of
aluminum
alloy is provided, comprising relatively stress-free pre-formed solid elements
between said
cylinder liners and the rest of said engine block being cast from said
aluminum alloy forming
an integral block after solidification of said casting.
In further embodiments of the invention, the solid elements can incorporate a
cooling
passage pipe, or the pre-formed solid insert can have a cooling passage
already formed therein
(optionally with an in-place removable core).
Brief description of the drawings
Figure 1 is an isometric view of the core of a mold for forming the water
jacket (i.e.
cooling passages) of an aluminum engine block incorporating three pre-formed
solid
aluminum inserts (according to one preferred embodiment of the present
invention), with each
such insert having a bridging cooling passage core and each being incorporated
by means of
the bridging core with the jacket core at a respective interlines gap
position,
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Figure 2 shows a top stylized view of an engine cylinder block with a
cylindrical iron
liner closely fitted into each of the cylinder bores and with a solid aluminum
insert (having no
passages therein) positioned in the interlines gap of each pair of liners in
the finished block
(thus showing the relationship of engine parts in which the invention can
effectively be
practiced).
Figure 3 shows a stylized vertical section taken along section line A-A of
Figure 2.
Figure 4 shows a top stylized view similar to figure 2, but according to a
different
preferred embodiment of the present invention wherein the pre-formed inserts
each incorporate
a bridging cooling fluid flow passage (shown in dotted lines).
Figure 5 shows a stylized vertical section taken along section line A-A of
Figure 4..
Figure 6 is an isometric view a solid pre-formed insert showing two bridging
cores,
passing through and extending out from either end of the upper portion of the
pre-formed
insert, for forming interlines-gap-bridging cooling-fluid flow passages.
Figure 7 is an isometric view showing two iron liners mounted on the core of a
mold
with a solid pre-formed insert (according to the preferred embodiment of
figures 2 and 3) fitted
in the interlines gap.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The solution to the problems of filling a casting mold for an engine-block 8
having thin
interlines walls 9 with a liquid aluminum alloy comprises incorporating a
solid insert 10 made
of a suitable material, for example the same aluminum alloy which will form
the rest of the
cast block. Other suitable materials for the pre-formed insert include, for
example, bronze,
copper and alloys and equivalents thereof. The pre-formed insert (designated
generally by
reference number 10) may be forged or extruded.
The pre-formed insert 10 is placed between the cylinder liners 12 before
introducing
the liquid aluminum alloy into said mold. The insert 10 will be contained and
locked in place
by the liquid aluminum which solidifies forming the rest of the block 8. In a
preferred
embodiment, the perform insert (see inserts 10a or 10b) has grooves 14 to
engage the liquid
aluminum, thereby providing a better bond between the insert and the aluminum
when the
liquid aluminum cools and sets. Bonding may also occur from surface melting of
the insert 10
during the casting pour. This may be aided by the addition of a bonding agent
such as low
melting zinc.
In a preferred embodiment, because of the same or similar coefficients of
expansion
and cooling between the pre-formed 10 insert and the liquid aluminum alloy
used in pouring
into the block casting mold, one of the advantages of the present invention is
that the pre-
formed inserts 10 do not result in residual tension.
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In addition, even when the inserts 10 are not made of the same alloy, little
or no
thermal stresses are induced in the pre-formed inserts 10 due to shrinking of
aluminum or due
to expansion of the iron cylinder liners 12 (since the inserts 10 are already
cooled to solid
stress &ee form). Thus, cracking is minimized or eliminated. Further, the
invention
advantageously resists tensions or cracking caused by post-machining,
procedures such as
boring.
In a further embodiment of the invention, a cooling-liquid passage 15 is
preferably
formed in or as part of a pre-formed insert 10; for example, by placing an
insert l Ob or l Oc
with a conduit for said cooling-fluid having the required shape to fit in the
gap between the
cylinder liners. The passage 1 S can take the form of an embedded thin walled
pipe 17 (made
of steel or the like), see Figure 1, or may be integrally form with the insert
by use of a bridging
core 16, see Figure 6. To facilitate the joinder of the insert cooling passage
in the insert lOb to
the remainder of the outer water jacket cooling passages (formed by the water
jacket core 6),
the removable bridging core 16, when used to form the passage 15 in the pre-
formed insert lOb
1 S will remain in place until after the casting of the engine block is
completed (and thereafter be
removed with the removal of the jacket core 6). Known examples of such
bridging cores
include salt, carbon, or glass (see U.S. Patent No. 6,205,959).
The present invention can be practiced, for example, with current processes
and
equipment, which can comprise a holding furnace for liquid aluminum alloy, a
source of
pressurized gas, normally nitrogen, which is injected into said holding
furnace for pushing
upwardly said liquid aluminum alloy through a suitable connecting conduit into
a gate of a
mold placed on top of said holding furnace. The liquid alloy is forced to
enter all the mold
cavities and after the mold is filled up, the flow of liquid is stopped by a
suitable device, for
example a slide valve or gate and said mold is then disconnected from the
holding furnace and
the process is repeated by a subsequent mold to be filled up. A mass of a heat-
absorbing
material also known as a chill (or a heat sink device) can be currently placed
under a
proprietary process of the assignee of this application in contact with the
liquid aluminum alloy
in order to direct the solidification in the desired direction in order to
produce good-quality
castings. Such controlled cooling can be disrupted by the small interliner
gaps that are now
encountered in modern aluminum automotive engine blocks; but are overcome by
use of the
present invention.
Less than 3 mm is a preferred condition for using this invention, but the
method applies
as well to thicker interliner dimensions when there is a need to provide
interliner cross sections
with reduced residual stresses due to the solidification process or for any of
the other reasons
discussed above.
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***
Those skilled in the art will recognize, or be able to ascertain without undue
experimentation any of the numerous equivalents to the embodiments of the
invention
described herein. All such equivalents are considered to be within the scope
of the instant
invention and are encompassed by the claims that follow.
Unless otherwise explained, all technical and scientific terms used herein
have the same
meaning as commonly understood by one of ordinary skill in the art to which
the invention
belongs. Although methods and materials similar or equivalent to those
described herein can
be used in the practice or testing of the present invention, suitable methods
and materials are
described herein. All publications, patent applications, patents, and other
references mentioned
herein are incorporated by reference in their entirety. In case of conflict,
the present
specification, including explanations of terms, will control. In addition, the
materials,
methods, and examples are illustrative only and not intended to be limiting.
Although preferred embodiments of the present invention and modifications
thereof
have been described in detail herein, it is to be understood that this
invention is not limited to
those precise embodiments and modifications, and that other modifications and
variations may
be affected by one skilled in the art without departing from the spirit and
scope of the invention
as defined by the appended claims.
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