Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to the art of castin~ composite
articlesO It is particularly appllcable to castLng improved
lightweight pistons and will be described ~ith plrticular referencQ
thereto. It will be appreciated, however9 that the ln~ention also
finds application in casting other composite articles, partlcularly
those which include wear resistant lnserts.
Aluminum alloys and othe]r lightweight materials are
advantageously used in the manufacture of plstons. One method of
casting aluminum and other alLoys is known as squeeze castingO In
squeeze casting1 a female die cavity is fashioned i~ the shape to be
castO The die is open at the top to allow a molten alloy to be
poured into the dle cavity. The die is closed by a top punch ~hich
is inserted into the upper opening of the die cavity. The punch ex-
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erts a pressure on the molten metal which continuously forces thealloy against the walls of the dle cavity as it solidifies. The top
punrh enters the die cavity further with shrlnkage. An article cast
by the squee~e casting technique has good conformity to the die cav-
ity surface, has a fine microstructure, and rela~i~ely little or no
porosity.
Aluminum and many other light~eight alloys tend to wear
quickly~ This lack of wear resls~ance makes aluminum and other
lightweight alloy pistons undesirable for heavy-duty engines such as
are found in large trucks, large far~ vehicles, and off the highway
equipment. To improve the wear resistance of aluminum and other
lightweight pistons, it has been suggested that a ring of wear re-
sisted material be inserted around the plston. Examples of such
composite plston structures are illustrated in U~S. Patent No.
4,008,051, issued February 15, 1977 to T. M~ Cadle~ U.S. Patent No.
3,533,329, issued October 13, 1970 to E. Galli, U.S. Patent No.
2,956,8469 issued October 18, 1960 to W. E. McCullough, and U.SO
Patent No. 2,550,879, issued May 1, 1951 to C. E~ Stevens, Jr~
These composite pistons are cast in a permanent mold which has
runners, ga~es, or risers for introducing molten metal in~o the mold
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cavity at the proper places and rates. To allow for shrinkage, as
much as 40~ extra alloy is poured lnto the mold, runners, gates, and
risersO When the ~etal solidlfies~ the two or more parts of the
permanent mold are opened and the plston is removed. Varlous
machining steps are needed to cut off the excess metal, ad~ust the
plston dimenslons for shrinkage, and prepare it for preclae flnish
machining. The piston tends to be weakened by porosity and a coarse
microstr~cture, both of which are attributable to shrinkage from the
die walls durlng solidificatlomO
The wear resistant insert ring, commonly an iron alloy, and the
aluminum piston body ha~e diffe~ent physical properties. For e~-
ample, the iron alloy's speciflc gravity is generally 2 to 3 tlmes
that of the aluminum alloy9 the iron alloy~s thermal expanæion
coefficient is generally 1 to 1 1/2 times that of the aluminu~
alloyD the iron alloy's thermal conductivity is generally less than
half that of the aluminum alloy. These different physical pro
perties cause residual stresses in the composite pisto~s.
To resist these stresses, it is essential that a strong bond be
formed between the wear resistant ring and the aluminum alloy. In
it9 normal life cycle, a composite piston is sub~ect to the shocks
and vibrations of innumerable firing cycles as well as numerous
heating and cooling cycles from starting and stopping the engine.
~en a minute crack or bonding failure between the wear resistant
ring and the aluminum alloy can propagate quickly under these
adverse conditlons. The propagation of cracks is accelerated by the
formation of a brittle aluminum-iron intermetallic compound or the
formation of o~ides at the interface. The propagation of cracks can
cause pieces of pistons to break loose resulting in catastrophic en-
gine damage.
Although composite articles ha~e been squeeze cast or molded ~n
the past9 note U.S. Patent No. 3>792~726, issued February 19, 1974
to Sakai et al., U.S. Patent No. 2,157,453, issued May 9, 1939 t~
Jaeger, V.S. Patent No. 1,950,356, issued March 6, 1934 to DeBats~
and Japanese Patent 9557, issued July 4, 1961 to Iwamura et al.
~Chemical ~bstracts 14862(h), 1962)~ squee7e casting of pistons has
not, heretofore, been successful. This may be attributable to the
difficulty in achieving adequate bonding, particularly to the lower
side of the wear resistant ring. This may also be attributable to
the additional internal stresses from flexing of the wear resistant
ring under the forces from the top punch.
One of the principal problems in composite piston casting
techniques is achieving a strong, fracture resistant bond between
the wear resistant ring and the aluminum or other lightweight
alloy.
Yet another problem with casting composite pistons has been
the number of machining steps and other labor processes
required to finish the cast product.
The present invention contemplates a new and improved method
and apparatus for casting composite articles, particularly pistons9
which overcomes the above-referenced problems and others. Yet
it provides a composite piston in which the unlike metal alloys
are strongly bonded, which is crack and fatigue resistant,
and which is finished with fewer machining steps.
t The invention relates to a method of casting a lightweight
f 20 piston with a wear resistant insert.
In accordance with one aspect of the invention there is
provided a method of casting a lightweight piston with a wear
resistant insert comprising: preheating a ring of wear resistant
material; causing the preheated ring to be disposed in a die
~` cavity; pouring a predetermined amount of` a molten lightweight
alloy into the die cavity through a plurality of paths which
are disposed adjacent the outer die surface; closing the mold
cavity with a punch which is recieved in the die cavity; and
applying a force with the punch which tends to compress the
lightweight alloy in the die cavity, wherein the lightweight
alloy is pressed in continuing contact with the ring and the
die cavity as the alloy solidifies.
In this manner the lightweight alloy is pressed into firm
and continuous contact with the ring and the die cavity even
as the alloy shri.nks during solidification.
In accordance with another aspect of the present
invention, there is provided a mold assembly for casting a
lightweight piston which has a wear resi.stant insert.
In particular there is provided a mold assembly for
casting a lightwei.ght piston with a wear resistant insert
comprising: a female di.e having a die cavity with a sub-
stantially cylindrical outer die surface; support means forsupporting the wear resistant insert, the supporting means
being a ledge extending around the outer die surface and being
connected with the cylindrical outer die surface; a movable
pouring disc which is selectively disposed in an upper
portion of the die cavity, the pouring disc having a
plurality of passages arranged circumferentially around its
perimeter and disposed generally above said ledge, whereby
molten metal which is poured on the pouring disc flows
through the passages and into the die cavity adjacent its
2~ outer die surface flows over the wear resistant insert
which is supported on the ledge; and a punch which is mounted
for selectively entering the upper portion of the die cavity
after removal of the pouring disc.
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A principal advantage of the present invention is that it
produces a cornposite piston of a lightweight alloy with a wear
resistant insert in which the bond between the lightweight
alloy and the insert is strong and durable.
Another advantage of the present invention is that it
rninimizes machining steps, labor, and manufacturing time by
casting a composite piston which is in close conformity to the
shape of the final piston product.
The invention may take physical form in certain parts and
arrangements of parts~ a preferred embodiment of which is
illustrated in the accompanying drawings. The drawings are
only for purposes of illustrating the preferred embodiments
of the invention and are not to be construed as limiting it.
Figure 1 is a cross-sectional view of a die cavity in which
a wear resistant insert ring and a pouring disc are disposed
in accordance with the present invention;
Figure 2 is a cross-sectional illustration of the die
cavity of Figure 1 during the pouring of a molten lightweight
alloy into the die cavity;
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Figure 3 is a cross-sectional illustration of the die cavity of
Eigu;es 1 and 2 after pourlng in which ~n upper opening of the die
cavity is closed by a top punch for exerting pressure on the
solidifylng alloy;
Plgure 4 is a cross~sectional illustration of the die cav~ty of
~igures 1-3 after the lightweight alloy has solldified;
Figure 5 is a top plan view of the wear res:Lstant insert rlng
illustrated in Figures 1-4;
Figure 6 is a cross~sectional view through se~tion line 6-6 of
Figure 5,
Figure 7 is a top plan view of the pouring disc illustrated ln
~igures 1 and 2; and
Flgure 8 is a perspective view in partial section of a com
posite piston manufactured in accordance with the present lnven~
tion~
DESCRIPTION 0~ THE PR~FERRED EMBODIMENTS
A mold assembly is illustrated in Figures 1-4 for castlng com-
posite pistons of a lightwelght alloy wlth a wear reslstant insert
as illustrated in Figure 8. The mold assembly includes a two-piece
femala die 10 which has a die cavity 12. The die cavity is shaped
~n conformity to the piston or other structure to be cast~ The die
cavity 12 includes an outer die surface 14 whlch is configured ~o
conform to the exterior of the pistonO The outer die surface 14 ~s
smooth and polished to minimize machining steps to finish the cast
piston. The outer die surface 14 extends peripherally around the
die cavlty along a circular path. In the center of the die cavity
12, a core 16 is disposed for defining a hollow interior to the
piston. Toward the top of the die cavity is a supporting ~eans 18
for supporting a wear reslstant insert 20. In the preferred ~m-
bodi~ent, the supporting means 18 is a peripheral ledge or land
which extends around the outer die su;face 140 Above the peripheral
ledge or supporting ~eans 18, the outer die surface 14 has an en-
larged upper portion 22. This enables the insert 20 to be placed
into the upper portion ~2 of the die cavity 12 through an upper
horizontal openlng 24. The upper portlon 22 has a cross section
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whlch conformg the periphery of the insert 20.
The insert 20 comprises an annular rlng 30 which is shown in
greater detail in Flgure 5. ~he annular ring 30 ls fashioned of a
wear resistant materlal that has e~panslon and contractlon pro-
pertles which are similar to the expanslon and contrac~ion pro-
perties of the lightweight alloy from which the piston is to be
cast. In the preferred embodiment, the wear resistant material is
an austenitic alloy of iron, nickel, chromiuml and copper, par-
tLcularly advantageous i9 the alloy sold by Thomas Foundries, Inc.
of Birmingham9 Alabama under the trademark NI-RESIST . The annular
ring has a peripheral edge 32 whlcll conforms with the exterior dia-
meter of ~he piston to be castO In the preferred embodiment, the
diameter of the peripheral edge 32 is slightly larger than the
Pinished piston to allow the exterior surface to be machined to
close tolerances. Extending outward from the peripheral edge 32 1
at least one pro~ection. In the preferred embodiment, the a~ leas~
one projection is a plurality of tabs 34 whicll are dimensioned to be
received snugly within the upper portion 22 of the die cav:lty and
rest on supporting means 18. The diameter of the periphera1 edge 32
of the annular ring is dimensioned to be substantially the same or
slightly smaller than the diameter of the outer die surface 14. As
illustrated ln Figure 6~ each of the tabs 34 is sufficien~ly thin
that it is frangible under the pressures norntally associated with
squeeze casting. Alternatively) the at least one projection may be
a continuous frangible flange whlch extends peripherally along the
perlpheral edge 32. As yet another alternative9 the tabs 34 may be
substantially the same thickness as the annular rt~g 30.
To cast a composite piston~ the wear resistant annular ring 30
is cleaned, preheated, and coated with a thln coating 40 of ductile
metal. The thlckness of the coating 40 is exaggerated in Flgures 1
and 8 for eas~ of illustration. The coatlng of ductile metal form~
a stress absorbing buffer betwèen the annular ring 30 and the
lightweight alloy. In the preferred embodiment~ the annular ring
immersed Ln a bath of pure aluminum or aluminuDt alloy which ls
heated to within 200 F. of the temperature at which the lightweight
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~lloy is to be cast. The duration whlch the annular ring 30 isimmersed in the aluminum i8 kep~ to a mlni~um to miniml~e the
formation of brittl~ aluminum-lron lntermetalltc compounds~ After
the annular ring 30 is preheated and coated with the ductile metal,
it is plaeed on supporting mean~ 18 in ~he die cavityO
A pouring disc 50 is placed ln the upper opening of the cavity.
The pouring disc 50 is a means for distributing the melted
lightweight alloy into the die cavity during pouring. Particularly~
it distributes the poured melted alloy such that it flow~ evenly
over the insert 20. This inhibits the formation of oxides at the
interface and aæsists in retaining the insert near the temperature
of the ~olten alloy during the pouring operation.
With cont~nued reference to Flgure 1 and further reference to
Figure 7, the pouring disc 50 ls dimensioned to be received in the
upper portion 22 of the die cavity. The pouring disc has a central
portion 52 which in con~unction with part of the upper portion 22 of
the die cavity forms a temporary reservoir for receiving and holding
the molten llghtweight alloy. The depth of the temporary reservoir
determines the pressure head of the molten alloy hence the flow rate
into the d~e cavity.
Arranged peripherally around the pouring disc 50 are a
plurality of passages for channeling the molten alloy into the mold
cavity around its periphery. In the preferred embodiment, th2
plurality of passages is formed by a plurality of circular apertures
54 arranged around the perimeter of the pouring disc.
Alternatively9 the passages may include a plurallty of radial slots
around the periphery, a screen or mesh around the periphery, an
annular gap between the disc and the die cavity, or the like.
Connected with the pouring disc 50 are a plurality of means 56
for supporting and centerlng its central portion 52 in the upper
portion of the die cavityO Projecting upward from the central
portion 52 of the pouring disc is a chimney 58 for allowing air
within the die cavity to escape as molten lightweight alloy is
poured in without aerating or agitating the mol~en alloyO The
chimney 58 i9 a tubular pro~ec~ion of sufficient length to extend
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above tlle molten alloy in the ~emporary reservolr.
Wi~h particular refere~ce to Figure 2, before the preheate~
lnsert 20 cools, a predetermined amount of molten llghtwe:Lght alloy,
denoted by reference nu~eral 607 iS poured from a furnace or cruc-
ible 62 GntO the pouring disc 50. From the temporary reservolr, the
~olten alloy passes through the apertures 54 around the perimeter of
the pouring dlsc and impinges upon the insert 20. The molten alloy
flows over and around the insert lnto the lower portion3 of the dle
cavity. ~en the predetermlned amount of molten alloy has been
introduced into the dle cavity, the pouring disc 50 is removed. The
flow of the molten alloy partially dissolves the thin aluminum coat-
:Lng 40 making it even thinner.
With particular reference to Figure 3, the upper opening of the
die ca~ity is closed by a top punch 70 preferably wlth an annular
pro~ection 72 for forming a combustion bowl in the top of the
pl~ton. The top punch 70 is dimensioned to conform with the cross
section of the upper por~lon 22 of the die cavity within very close
tolerances to prohibit the ~olten alloy from passing between the
punch and the outer die surfaceO Top punch 70 is caused by a
hydraulic cylinder ~not shown) to exert several tons of pressure on
the lightweight alloy in the die cavity. As the molten alloy
solidifies, it contracts. The top punch continues to press the
alloy firmly and continuously agalnst the surfaces of the die cav~
ity, tha punch, and the insert as the lightweight alloy solidifi~sO
The contraction of the lightweight alloy during solidif~cation
allows the top punch 70 to advance into the die cav~ty by a cor-
responding amount. It has been found that the pressure from the top
punch tends to deflect the annular ring which causes interual
stresses after solidificatlonO This deElection is alleviated with
the annular ring construction of Figures 5 and 6 because the tabs 34
space the peripheral edge 32 of the ring a sufflcient distance from
the outer die surface 14 that the lightweight alloy flows between
the ring and the die surface.
With particular reference to Flgure 4, the deflection is
fur~her reduced by the fracture of the frangible ~abs 34
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Under the pressure exerted by the top punch 70, the tabs 34 breakallowin~ the ann~llar ring 30 to shift downward in the die cavlty
wlth the contraction during solidification.
After the lightweight alloy Eolidifies, the top punch i8
withdrawn, the die cavlty opened, and the composite piston removed.
The composlte piston is finished by a machining operation. Because
the exterior surface conforms very cloæely to the outer surface of
the punch and the die, very little machining is required. The top
surface of the piston conforms to the surface of the punch with suf-
ficlent accuracy that, as a rule, no further machining ~8 required
to finish the top surfaceO The exterior surface of the piston i8
~achined to remove the severed tabs 34 and the excess material from
the increased diameter of the upper portion 22 of the die cavity.
The machining may further be used to give tlle piston a circular
cross section with very precise dimensional tolerances. With re-
ference to ~igure 8, the machining operation further includes cut-
ting a plurality of grooves into the piston to receive piston rings~
A top piston ring groove 82 is machined in the annular ring 30.
Additional piston ring grooves 84 and 86 are also machined in the
piston 80. A wrist pin 88 connects the piston 80 with a rod 900
The invention has been described with reference to the pre-
ferred embodiment. Obviously, modifications and alterations will
occur to others upon readin8 and understanding this specification.
It is intended to include all such modifications and alterations
insofar as they come within the scope of the appended claims or the
equivalents thereof.
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