Note: Descriptions are shown in the official language in which they were submitted.
1089~B6 ~`
This invention relates to a method of making a
composite part of movably interconnected dissimilar
metal elements, such as a piston and connecting rod
insert.
U.S. Patent 3,763,535 discloses the method for
fabricating an article such~as a piston which has a cap-
tured connecti~g rod journaled therein. The piston is
cast around the connecting rod insert end at a pressure
selected to provide, upon cooling, a clearance between
the insert and the piston. The selected casting pressures
are well under 2~00 psi. Die casting pressures in the
order of 3000 to 10,000 psi will produce a casting in
which the connecting rod insert is irretrievably locked
in place.
The present invention in its broadest aspect relates
to a method for making a composite part of movably inter-
connected dissimilar metal elements which comprises
(a) introducing into a casting die a first metal element
having a bearing surface thereon; (b) casting a second
metal element having a lower melting point and a higher
thermal conductivity than said first metal element in
said castlng die to form a composite part in which said
first metal element is captured at least in part in said ~;
second metal element; (c) thermally treating said
composite part to provide a temperature differential
between said first metal element and said second metal
element whereby to free said first metal element and ~
bearing surface from said second metal element and ;
thereby establish a clearance thetebetween; and
,
(d) restoring the elements of said composite part to the
same temperature at which no further significant contraction
or expansion of said elements occurs.
-- 2 --
. .
~...... -.' ' ' ' . ~ , ~
1089186
More particularly it provides a method for die
casting a composite article such as a piston and connect-
ing rod insert, for example, in which normal die casting
pressures of, for example, from 3000 psi. and higher may
be used. Die casting is highly advantageous in making
articles of this nature because the process is faster
and less expensive than other casting processes. Though
the composite casting is ejected from the die with the
parts frozen together, the present invention provides a
method to free them one from the other. We have found
that if a composite aluminum alloy-iron alloy casting is
heat treated by the method of this invention, the initially
frozen parts are freed and a very satisfactory product
results. In more specific terms the procedure can be as
~ollows:
1. Die cast at normal high pressures.
2. Optionally quench in water as the part comes
from the die casting machine if hardness is desired in
the aluminum.
3. Reheat to 850F. to 950F.
.~
4. Hold the heat in step 3 for a period of ten to ~ -~
thirty minutes depending upon the clearance desired.
,~.
5. Quench in water to bring the casting back to
room temperature.
6. Age or harden the aluminum alloy by hea~ing to ;
320F. to 350F. for about four hours.
7. Cool to room temperature.
Apart from the controlled temperature aspects of ~ ~
~ the invention, there is also provided, as a preferred ~;
,~ :.. , -.~.
30 embodiment, a step to obtain a long lasting lubricant ``
film between the working surfaces of the bearing and its
;:
. , .
~: ; , .
~ ., , . - , . , . , - . . , . ~,, . . ~
1089~8~
housing. This step includes the application of a coating
of polytetrafluoroethylene ( hereinafter referred to by
its trademark " Teflon " ) to the bearing surface prior
to casting.
Certain preferred embodiments of the invention
are illustrated by the attached drawings in which:
Figure l is a central, vertical, sectional view
of a piston having a spherical connecting rod insert
cast in place:
Figure 2 is a similar view showing a cylindrical
insert configuration;
Figure 3 is a section on line 3-3 of Figure 2;
Figure 4 is a diagrammatic sectional view of a
part having a cast-in-place conical insert; and
Figure 5 is a diagrammatic sectional view of a
part having a cast-in-place double cone insert.
The present invention constitutes a method for
making any composite part such as those shown in the
drawings wherein a first element such as a connecting rod
is designated 10. The first ~element is received in a
second element such as the body of a piston 12. The
composite part need not be a piston-connecting rod com-
bination, but this combination lends itself to an easily
understandable description of our new method. ~ ;
In manufacturing a composite member, a first element
such as a connecting rod lO is provided with a bearing ll
which ultimately journals the elements of the combination ~-
with respect to each other. The element is inserted into
a die casting die having an appropriate configuration.
If desired, and such is usually the case, the bearing end
11 of the connecting rod is ~irst given a lubricant coating
-- 4 --
.
1089186
by swabbing the exposed end of the insert with, for example,
a colloidal graphite suspension. In some instances a
second lubricant such as a coating of " Teflon " is sprayed
on. Coating may advantageously take place outside the
die casting machine as well as after the insert has been
placed in the die. The exterior of the "Teflon" coating
as it is applied appears to be rough in texture, but
after casting it appears that the coating has been
transplanted to the surrounding casting and that the
first element moves with respect to the coating which,
of course, has a very low coefficient of friction.
It will be assumed that the first element of the
composite part is a ferrous alloy of a composition conven-
tionally used for the desired part. Such an alloy may have
a melting point in the range of 2500F. to 2800F. and a
coefficient of thermal expansion of 8.3 x 10-6 in. per in.
per degree F.
:;`",'i ,
After the first element has been properly coated
and inserted into the die casting machine the die halves
are closed and the shot made. The die casting pressure is
normally from 3000 psi. to 10,000 psi. for an aluminum alloy. ;~-
The aluminum alloy is prefera~ly of a characte~ of the alloy ¦~
known commercially as 380 alloy which has a nominal composition:
.. . .
Si................. ..9% Mg........................ .2% ~-
Fe................. 1.0% Al........................ Balance
Cu................. 3.5%
The melting point of such an alloy is in the range of 1000F.
-~ to 1100F. and its coefficicnt of thermal expansion is
11.7 x 10 6 in. per in. per degree F. -
_ 5 _ ~
:.:. , .
1089~86
By inspection of the drawings it will be seen that
the cast metal surrounds more th2n 180 of the bearing part 11
of the connecting rod so that the first element is captured by
the second. The parts are ejected from the die casting machine
in a locked condition and are incapable of relative movement
because the cast metal has shrunk around the insert.
The composite casting is ejected from the die cast-
ing machine while it is still at a tcmperaturc of about 700F.
to 800F. and i9 pre~erably quenched promptly in water to cool
it to room temperature.
The composite casting is then placed in a
furnace held at a temperature of between 850F. and
950~. where it will be rapidly reheated. The cast-
ing remains in the furnace for a period of ten to
~ twenty minutes depending upon the final clearance de-
; sired for the bearing. Longer reheat time gives
greater clearance for the bearing in the final product~
, ~or example, in a part in which the bearing diameter
was .565 inches and a clearance of 0.0005 inches was
desired a reheat time of 25 minutes was preferred.
.
A~ter reheating, the composite part is again
quenched in water. -At about 600F. the surrounding
aluminum alloy has a greatly reduced tensile strength
and yield point and reheating has carried the aluminum
alloy above this temperature. Quenching now results
in a rapid contraction of the aluminum alloy around
the still hot ferrous alloy element. As the aluminum
cools it appears to take a permanent set at the ex-
.'-
.
' . ' ~ . . . ~ ,
~ .
1089186
panded dimension of the ferrous alloy part which i~cooled l~ter in the quenching cycle because it has been
largely insulated rom the quenching liquid by the
aluminum alloy. When the ferrous ~lloy element finally
reaches thc t~mperature of the quenching medium a ~lear-
ance exists between the elements and the bearing is now
free in its socket.
A preferred final step in the process consists
in hardening the aluminum alloy by soaking at a tempera-
ture of from 320F. to 350~F. for a period of about four
hours. This har~ening or aging of the aluminum alloy is
known to improve its resistance to wear, and its hardness,
and is known as a T-5 heat treatment.
After aging the parts are again cooled and are
now ready for final assembly and the bearing surface 11 is
fully operative to journal one part with respect to the -
other. The graphite and"Teflon"coatings have not been
destroyed by the successive heating and cooling steps but -
are functional to serve as inplace lubricants. The har~
dening treatment has given the aluminum alloy a Brinell
hardness of approximately 100 so that minimum wear occurs.
,.' ~-
As'previously noted, the "Teflon" coating appears
to be stationary with respect to the body 12 and the bearing -
11 moves therein. This transplanting of the "Teflon" coating
from the bearing 11 to the body 12 probably takes place during
the die casting operation itself.
1089186
The configuration of the insert, whether it
be spheric~l, cylindrical or conical is not important
since the method can be employed with any of these
confiourations and others.
It has been found that instead of reheating
the casting as in steps3 and 4 ctated above, a clearance
between the parts can be established by immersing the
casting in liquid nitrogen for a period of 15 to 30
minutes. In this case the aluminum shrinks away from
the ferrous alloy part due to its high heat conductivity
After the shrinkagc step the heat treatment proceeds as
before. This phenomenon would give rise to the proposition
that it is only necessary to establish a temperature differ~
ential between the two alloys to bring about the desired ~ ;
clearance. The time required will depend on the relative
mass of the parts being treated. ;-
;, --' '
!`~
;i~,
i'' ` ~
, ; ~
,' ~
; - 8 - ~
,
:
