U.S. patent application number 13/109033 was filed with the patent office on 2012-04-26 for sand casting a diesel piston with an as-cast, reentrant combustion bowl.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Joseph C. Kopchick, Mark A. Osborne, Richard D. Ricchi.
Application Number | 20120097354 13/109033 |
Document ID | / |
Family ID | 45971969 |
Filed Date | 2012-04-26 |
United States Patent
Application |
20120097354 |
Kind Code |
A1 |
Kopchick; Joseph C. ; et
al. |
April 26, 2012 |
SAND CASTING A DIESEL PISTON WITH AN AS-CAST, REENTRANT COMBUSTION
BOWL
Abstract
A diesel engine piston and a method of making the piston. The
method involves casting the piston with complex geometries,
including undercut features. The casting uses an aggregate
disposable mold that can be removed from the as-cast part. In one
form, the complex geometry includes an undercut combustion bowl
formed in the piston dome, while in another, it may include an
internal cooling passage. The undercut bowl and internal passages
may be produced using the aggregate disposable mold.
Inventors: |
Kopchick; Joseph C.;
(Warren, MI) ; Osborne; Mark A.; (Grand Blanc,
MI) ; Ricchi; Richard D.; (Lapeer, MI) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC
Detroit
MI
|
Family ID: |
45971969 |
Appl. No.: |
13/109033 |
Filed: |
May 17, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61405739 |
Oct 22, 2010 |
|
|
|
Current U.S.
Class: |
164/15 |
Current CPC
Class: |
B22D 15/02 20130101;
B22C 9/00 20130101; F02F 2003/0007 20130101; B22D 30/00 20130101;
B22D 29/002 20130101 |
Class at
Publication: |
164/15 |
International
Class: |
B22D 25/02 20060101
B22D025/02; B22D 30/00 20060101 B22D030/00; B22C 9/00 20060101
B22C009/00 |
Claims
1. A method of making a diesel piston comprising: providing a
pattern for the piston, the pattern including a dome and a
reentrant bowl; forming a piston mold around the pattern, the mold
comprising an aggregate material and a binder; removing the pattern
from the piston mold; introducing molten metal into the piston
mold; contacting the piston mold with a solvent for the binder and
removing the binder and the aggregate; cooling the molten metal;
and solidifying the molten metal to form the piston with the dome
and the reentrant bowl without need for post-casting
processing.
2. The method of claim 1 wherein the binder is water soluble, and
wherein the solvent is water.
3. The method of claim 1 wherein cooling the molten metal and
solidifying the molten metal are controlled separately.
4. The method of claim 3 wherein cooling the molten metal and
solidifying the molten metal are controlled separately by applying
the solvent to one area of the piston before other areas or by
applying different amounts of solvent to different areas of the
piston.
5. The method of claim 1 wherein the aggregate material is silica
sand, zircon sand, chromite sand, ceramic micro spheres, or
combinations thereof.
6. The method of claim 1 wherein cooling the molten metal comprises
contacting a shell of solidified metal around the molten metal with
the solvent.
7. The method of claim 1 further comprising: providing a mold for
an internal cooling passage, the mold for the internal cooling
passage comprising a second aggregate material and a second binder;
placing the mold for the internal cooling passage in the piston
mold before introducing the molten metal into the piston mold; and
contacting the mold for the internal cooling passage with a solvent
for the second binder and removing the second binder and the second
aggregate.
8. The method of claim 6 wherein the second aggregate material and
the second binder are the same as the aggregate material and the
binder for the piston mold.
9. The method of claim 1 wherein contacting the piston mold with a
solvent for the binder comprises spraying the piston mold with the
solvent.
10. A method of making a diesel piston comprising: providing a
pattern for the piston, the pattern including a dome and a
reentrant bowl; forming a piston mold around the pattern, the mold
comprising an aggregate material and a water-soluble binder;
removing the pattern from the piston mold; introducing molten metal
into the piston mold; contacting the piston mold with water and
removing the binder and the aggregate; cooling the molten metal;
and solidifying the molten metal to form the piston with the dome
and the reentrant bowl without need for post-casting
processing.
11. The method of claim 10 wherein cooling the molten metal and
solidifying the molten metal are controlled separately.
12. The method of claim 11 wherein cooling the molten metal and
solidifying the molten metal are controlled separately by applying
water to one area of the piston before other areas or by applying
different amounts of water to different areas of the piston.
13. The method of claim 10 wherein the aggregate material is silica
sand, zircon sand, chromite sand, ceramic micro spheres, or
combinations thereof.
14. The method of claim 10 wherein cooling the molten metal
comprises contacting a shell of solidified metal around the molten
metal with water.
15. The method of claim 10 further comprising: providing a mold for
an internal cooling passage, the mold for the internal cooling
passage comprising a second aggregate material and a second binder;
placing the mold for the internal cooling passage in the piston
mold before introducing the molten metal into the piston mold; and
contacting the mold for the internal cooling passage with a solvent
for the second binder and removing the second binder and the second
aggregate.
16. The method of claim 15 wherein the second aggregate material
and the second binder are the same as the aggregate material and
the binder for the piston mold.
17. The method of claim 10 wherein contacting the piston mold with
water comprises spraying the piston mold with water.
Description
STATEMENT OF RELATED CASES
[0001] This application claims the benefit of Provisional
Application Ser. No. 61/405,739, filed Oct. 22, 2010, entitled Sand
Casting A Diesel Piston With An As-Cast, Undercut Combustion Bowl,
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to devices and
methods for casting engine components, and more particularly to an
advanced diesel piston with complex features and a method of
casting the same.
[0003] Pistons used in internal combustion engines are typically
made up of a head (also called a dome), skirt, one or more ring
grooves, and land between the grooves. More stringent emissions and
efficiency requirements dictate that pistons will need to operate
at closer to stoichiometric conditions, higher cylinder pressures,
and in tighter packaging regimes in the future, which in turn will
necessitate higher component loading and operating temperature
conditions. This is particularly true for pistons used in diesel
engines, which are increasingly being used to power passenger
vehicles, in addition to being the predominant engine form for
larger, commercial vehicles Likewise, more sophisticated dome
designs are being developed to achieve a more thorough, efficient
combustion process, which raises temperatures in the combustion
chamber even more. These advanced dome designs, with their
three-dimensional (3D) profiles, require more complex mold shapes,
increasing the difficulty of casting the pistons, or necessitating
the use of excessive machining to achieve the desired dome
geometry.
[0004] Higher operating temperatures require the use of materials
capable of operating under those conditions. Cooling schemes have
been employed as a way to enhance the life of pistons and related
high-temperature components. One way of achieving cooling is
routing engine lubricant, which is already present, through
passages formed in the piston. There is a significant benefit to
cooling the dome because it is exposed directly to the combustion
process. However, adding cooling channels further increases the
difficulty of casting the piston.
[0005] Traditional nonexpendable forms of casting, such as
permanent mold casting, are not well-suited to forming complex
shapes. In particular, the permanent nature of the mold, coupled
with the reentrant regions present in some shapes, such as the 3D
dome shapes discussed above, limit the ability to retract the part
once the casting has solidified. As a result, extensive machining
of the casting is needed to produce the complex shapes. This
additional machining contributes to the complexity and cost of
permanent mold casting approaches. Part quality may also suffer
with permanent mold casting. The presence of risers may impact the
piston's microstructure and the amount of grain refinement that is
achievable. For example, the relatively large thermal mass of the
large risers tends to slow down solidification of the cast
component, whereas faster cooling and the accompanying improved
mechanical properties as evidenced by the presence of smaller
secondary dendrite arm spacing (SDAS) are often desirable. There
can be as much as a 20% difference in mechanical properties
comparing the in-gate location to the riser sides.
[0006] One embodiment of the top view of a piston as cast is shown
in FIG. 1. FIG. 2 shows a perspective view of the piston after
machining, clearly evidencing the extensive machining required to
produce the dome.
[0007] To overcome the shape limitations of permanent mold casting,
other approaches may be employed, including various forms of
expendable-mold castings, such as investment (or lost-wax) casting,
sand casting, or the like. A variant of the sand casting approach,
known as salt core casting, employs techniques generally similar to
sand casting, except that a water-soluble salt is employed for
internal mold geometries, rather than a green or dry sand. Once the
part is made, the salt mold can be washed away with water without
subjecting the part to any additional thermal loads. Salt core
casting has storage and handling concerns that lessen its
appeal.
[0008] Another approach that has been used to create an as-cast
dome is semi-permanent mold casting. This approach still requires
that the combustion bowl be machined. As with the permanent mold
castings discussed above, a significant amount of risering off the
dome is also required. Furthermore, higher output (i.e.,
high-performance) pistons may also require bowl rim remelting.
These additional processes add significantly to the cost of the
piston. This is especially true for remelting work and bowl
machining costs, and even more so for pistons configured to operate
under high temperature and pressure conditions, such as high output
diesel pistons. Such pistons may operate at about 200 bar cylinder
pressure and about 400 C, and involve the use of steel forgings at
an even more significant cost impact.
SUMMARY OF THE INVENTION
[0009] One aspect of the invention is a method of making a diesel
piston. In one embodiment, the method includes providing a pattern
for the piston, the pattern including a dome and a reentrant bowl;
forming a piston mold around the pattern, the mold comprising an
aggregate material and a binder; removing the pattern from the
piston mold; introducing molten metal into the piston mold;
contacting the piston mold with a solvent for the binder and
removing the binder and the aggregate; cooling the molten metal;
and solidifying the molten metal to form the piston with the dome
and the reentrant bowl without need for post-casting
processing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The following detailed description of specific embodiments
can be best understood when read in conjunction with the following
drawings, where like structure is indicated with like reference
numerals and in which:
[0011] FIG. 1 is top view of a diesel piston of the present
invention in the current as-cast condition where the dome is
completely cast over, showing excessive risers; and
[0012] FIG. 2 is a perspective view of the diesel piston of FIG. 1
after final machining.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] An ablation casting approach can be used to produce a piston
with a dome incorporating an as-cast, reentrant bowl, and an
optional internal cooling passage. Ablation casting uses inorganic
(i.e., water soluble) cores, and water is sprayed on the mold which
slowly washes away (hence the term "ablation"), rapidly cooling the
casting. The rapid cooling results in improved material properties.
Ablation casting allows complex parts to be produced with a fine
solidification microstructure. The application of water allows
component solidification and cooling to be controlled separately
from one another (e.g., by applying water to specific areas of the
casting before others or by applying different amounts of water to
different areas). By providing the high solidification rates and
refined microstructure that are often needed to achieve the
through-section higher mechanical properties (such as tensile and
fatigue properties at room temperature and elevated temperatures),
ablation casting allows complex parts to be formed, such as those
combining both thin and thick sections, as well as those with
complex internal cores. The through-section properties are superior
compared to those made using bowl rim re-melting, which only
provides the desirable fine microstructure to a depth only slightly
below the surface (e.g., a few mm).
[0014] The ablation casting process is described generally in U.S.
Pat. No. 7,121,318, which is incorporated by reference herein. A
pattern is formed from a material, and a mold is formed around at
least a portion of the pattern. The mold is made of aggregate
material and a binder. The pattern is removed from the mold, and
molten metal is then introduced into the mold. The mold is
contacted with a solvent, and the molten metal is cooled so that it
at least partially solidifies to form a casting. The cooling step
includes contacting a shell of solidified metal around the molten
metal with the solvent.
[0015] Ablation casting has not been used to cast diesel pistons in
general, and more particularly diesel pistons with a reentrant
bowl.
[0016] U.S. Pat. Nos. 7,164,963, 7,618,823, and 7,225,049 describe
analysis methods for lost foam casting (a type of ablation
casting), each of which is incorporated by reference herein.
[0017] The use of ablation casting offers the possibility of
casting the piston with a near-net-shape dome without the
significant post-casting processing and/or machining required by
other processes. Thus, the dome and combustion bowl can be cast
simultaneously. In a particular form, the reentrant bowl and
internal passages would be produced by means of an aggregate
disposable mold that could be produced by conventional core
technology with retractable tooling in the molding die. In the
present context, the aggregate form of the molding media includes,
but is not limited to, silica sand, zircon sand, chromite sand,
ceramic micro spheres, or the like.
[0018] The benefits associated with the present invention include,
but are not limited to, one or more of the following: reduced
machining costs, refined as-cast microstructure for improved
mechanical properties, taking advantage of a sand (or related)
molding process to tailor a reentrant region in the dome, reducing
casting weight, and eliminating the need for an internal salt core.
The traditional salt core could be replaced with an aggregate core
of the same material as the piston mold.
[0019] In one form, sand casting can be used to produce the piston
of the present invention. This process would substantially reduce
the massive risers that are typically used with permanent mold
casting, resulting in improved material yield. Furthermore, the
high cooling rate inherent in ablation casting makes it easier to
tailor the process to achieve a refined microstructure with
improved material properties. These improved material properties
should provide a stronger piston as a whole, as well as the needed
piston bowl rim strength (without having to re-melt this area) to
pass rigorous head gasket validation tests consistently.
[0020] In one particular form, the mold can be made from sand that
is capable of ablation casting such that complex dome shapes,
including those with undercut and internal cooling passage
features, may be easily and inexpensively manufactured. As such, a
sand mold may be used as part of the ablation process.
[0021] Improved material yield can be realized by eliminating the
large risers that are often used as part of a permanent mold
casting operation. In particular, by using an ablation casting
approach, the inherently high cooling rate can allow the piston
being formed to have a homogeneous microstructure and related
structural properties.
[0022] In addition to the finer microstructure and enhanced piston
properties, use of the ablation process permits much finer details
to be cast into the part, including intricate cooling channels. The
process reduces or eliminates the need for post-cast machining in
the area around the dome, particularly as it relates to the
reentrant region. Because the ablation casting is production-ready,
scaling up to manufacture large quantities of pistons or related
components is comparatively simple. An aggregate disposable mold
could be employed to allow the combustion bowl of the piston dome
and a lubricating and cooling oil gallery to be formed as part of
the casting.
[0023] Employing ablation casting for diesel pistons helps to
achieve a significant microstructural refinement by reducing or
eliminating the need for expensive secondary post-casting
processing such as machining or remelting. In situations where a
refined microstructure is desired, such as the bowl edge or other
complex 3D regions of the piston, tungsten inert gas (TIG) or laser
remelting can be done locally (in the dome reentrant region).
Subsequent machining, such as to yield proper shape of the bowl
edge, may be similarly reduced or eliminated.
[0024] This invention takes advantage of the ablation casting
process to eliminate the need for large risers at the piston dome.
The ability to cool the dome more quickly and uniformly should
enhance mechanical properties. In particular, the disposable
aggregate mold should allow the combustion bowl to be formed
as-cast. Furthermore, the aggregate mold material may also be used
to form the oil gallery behind the top ring groove, eliminating the
need for a salt core. This additionally allows for rapid
prototyping of pistons, which can improve general development
testing.
[0025] It is noted that terms like "preferably," "commonly," and
"typically" are not utilized herein to limit the scope of the
claimed invention or to imply that certain features are critical,
essential, or even important to the structure or function of the
claimed invention. Rather, these terms are merely intended to
highlight alternative or additional features that may or may not be
utilized in a particular embodiment of the present invention.
[0026] For the purposes of describing and defining the present
invention it is noted that the term "device" is utilized herein to
represent a combination of components and individual components,
regardless of whether the components are combined with other
components. For example, a "device" according to the present
invention may comprise an electrochemical conversion assembly or
fuel cell, a vehicle incorporating an electrochemical conversion
assembly according to the present invention, etc.
[0027] For the purposes of describing and defining the present
invention it is noted that the term "substantially" is utilized
herein to represent the inherent degree of uncertainty that may be
attributed to any quantitative comparison, value, measurement, or
other representation. The term "substantially" is also utilized
herein to represent the degree by which a quantitative
representation may vary from a stated reference without resulting
in a change in the basic function of the subject matter at
issue.
[0028] While certain representative embodiments and details have
been shown for purposes of illustrating the invention, it will be
apparent to those skilled in the art that various changes may be
made without departing from the scope of the invention, which is
defined in the appended claims.
* * * * *