U.S. patent application number 09/808498 was filed with the patent office on 2001-07-26 for apparatus for simplified production of heat treatable aluminum alloy castings with artificial self-aging.
Invention is credited to Garza-Ondarza, Oscar, Mojica-Briseno, Juan Francisco, Valtierra-Gallardo, Salvador.
Application Number | 20010009170 09/808498 |
Document ID | / |
Family ID | 23820440 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009170 |
Kind Code |
A1 |
Garza-Ondarza, Oscar ; et
al. |
July 26, 2001 |
Apparatus for simplified production of heat treatable aluminum
alloy castings with artificial self-aging
Abstract
Apparatus for simplified heat-treatment in making aluminum
alloys castings of the type improved by aging, especially for
automotive engine heads, etc. The newly solidified castings have an
end product (workpiece) portion and a riser portion (ultimately cut
off as waste). The apparatus transfers the casting oriented to
selectively quench the workpiece portion from solution temperatures
down to about 120.degree. C. (e.g. by water misting the workpiece,
while maintaining the essentially unsprayed riser portion at
relatively significantly higher temperatures). After the quench,
the residual heat retained by the riser portion reheats the
workpiece portion (by internal heat conduction) and maintains it
for an effective time within the temperature range for artificial
aging (obviating any need for the prior art's aging furnace).
Preferably, the quench occurs immediately after the casting mold
extraction (without the standard natural cooling, reheating, and
solution heat-treatment, all prior to quenching), thus also
eliminating any solution heat-treatment furnace.
Inventors: |
Garza-Ondarza, Oscar; (San
Pedro Garza Garcia, MX) ; Valtierra-Gallardo, Salvador;
(Saltillo, MX) ; Mojica-Briseno, Juan Francisco;
(Monterrey, MX) |
Correspondence
Address: |
FROMMER LAWRENCE & HAUG
745 FIFTH AVENUE
NEW YORK
NY
10151
US
|
Family ID: |
23820440 |
Appl. No.: |
09/808498 |
Filed: |
March 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09808498 |
Mar 14, 2001 |
|
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09458347 |
Dec 10, 1999 |
|
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6224693 |
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Current U.S.
Class: |
148/698 ;
266/114; 266/249 |
Current CPC
Class: |
C22F 1/043 20130101;
C22F 1/04 20130101; B22D 11/0405 20130101 |
Class at
Publication: |
148/698 ;
266/249; 266/114 |
International
Class: |
C21D 001/00; C21D
001/62; C21D 009/00; C22F 001/04 |
Claims
What is claimed is:
1. An apparatus for quenching and artificially aging a hot aluminum
alloy casting having a riser portion and a workpiece portion,
comprising: a quench unit; an aging unit; transfer means for moving
at least one casting successively through said units and for
positioning such casting in said quench unit in an oriented
position; cooling means for rapidly and selectively quenching the
workpiece portion of the casting so oriented in said quench unit
largely to the exclusion of the riser portion, which latter will
thus initially remain at a relatively higher temperature.
2. An apparatus according to claim 1, wherein said units are part
of a continuous processing line; said quench unit is a spray
containment housing; said cooling means is at least one liquid
quench sprayer directed to impinge directly on essentially only the
workpiece portion of the casting presented to it by the transfer
means in the oriented position; and said aging unit is an insulated
tunnel housing.
Description
RELATED APPLICATIONS
[0001] This is a division of application Ser. No. 09/458,347, filed
Dec. 10, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
making aluminum alloys castings, wherein the heat-treatment
processes of the prior art are simplified, by actually eliminating
some traditional steps and equipment. The invention is applicable
for example in the production of cylinder heads, motor blocks and
the like, for automotive engines. The invention provides many
advantages over the prior art heat-treatments, with an increased
productivity of the casting plants, and lower capital and operation
costs as well. The invention is particularly useful for producing
aluminum alloys of the 3xx.x series of the classification of the
Aluminum Association (AA), especially for T6 and T7 properties.
[0003] This invention is broadly applicable to the production of
any aluminum alloy casting which in the past has derived meaningful
benefit from quenching and artificial aging in an aging furnace.
The invention eliminates the need for an aging furnace, while
retaining the benefits thereof. This improvement has been styled
herein as artificial self-aging (to distinguish from natural aging
at ambient temperatures and from prior art artificial aging, which
requires an aging furnace).
[0004] For a good background discussion and definitions of "heat
treatable aluminum alloy castings", "artificial aging" (see also
"precipitation hardening"), "quenching", "solution heat treatment",
"casting series 3xx.x", and "T6 & T7 tempers" see the ASM
Handbook series; particularly Volume 4 (1991), entitled "Heat
Treating" (especially pages 841-879; see p.841 for "heat
treatable", p. 851 et seq. for "quenching" and p. 859 for "age
hardening") and Volume 2 (1990), entitled "Properties and
Selection: Nonferrous Alloys and Special Purpose Materials",
(especially pages 15-41; see p.39 for "heat treatable", p. 40 for
"artificial aging") both being tenth editions, and also Volume 15
(1988), entitled "Casting" ninth edition (especially pages 757-761,
see pp. 760-1 for "quenching" and "aging"); all published by ASM
International; the contents of which (including also the patents
cited below) are incorporated herein by reference.
[0005] The present invention uses a selectively directed spray
quench in a manner which eliminates expensive equipment and reduces
significantly the overall production time. The castings are
preferably so quenched promptly after demolding in accordance with
applicants' own recent patent to obtain the properties of a
conventional "solution" heat treatment (such as the properties
required by a T6 temper) but without the usual "solution" heat
treatment in a furnace.
[0006] This invention is broadly applicable to the production of
any aluminum alloy casting of the type having significant
precipitation hardening with meaningful benefit from "solution"
heat treating and aging.
BACKGROUND OF THE INVENTION
[0007] In the production of cast parts made of aluminum alloys it
has always been thought in the past to be necessary for many such
castings (especially with a T6 or T7 temper) to undergo an
elaborate heat-treating process in order to impart to the cast
parts the necessary mechanical properties (like hardness and
tensile strength required for the demanding working uses of said
parts).
[0008] It is known that the degree of hardness and other mechanical
properties of the cast parts depend on the thermal history of the
cast parts after having been cast in the mold. The Aluminum
Association (AA) has classified the most used aluminum alloys and
the several standard heat treatments used in the industry. Examples
of such standard heat-treatments those denominated T6 and T7, which
designate a standard set of mechanical properties developed by
certain castings of primarily silicon-copper-aluminum alloys.
[0009] The automotive industry throughout the world demands very
strict quality standards. Casting plants making aluminum motor
parts must therefore be able to produce cast parts which
consistently comply with the minimum levels of mechanical
properties specified for each part. Since quality is a must, the
casting plants follow those procedures and processes which are well
tested and have proven reliable for many years. The production
process currently followed in the industry comprises filling a mold
with liquid aluminum alloy, cooling the cast part in the mold in
order to obtain a solidified casting, extracting the casting from
the mold, and allowing the cast part to cool-down naturally to
ambient temperatures, and then subjecting batches of such cooled
castings to the aforementioned "solution" heat-treating process.
One way to reduce the heat load in the solution heat treatment
furnace, has been to remove the sand cores and riser portions of
the castings after natural cooling and before the "solution" heat
treatment. The heat treating of the prior art comprises heating the
preferably trimmed castings in a furnace to temperatures above
about 470.degree. C. (typically in the range between 480.degree. C.
and 495.degree. C.) for a certain period of time, usually in the
range between at least 2 to 7 hours. This treatment is performed in
order to bring back into solid solution the copper and/or other
alloying elements that give the castings their hardness. It is
known that, while the casting metal is in the molten state, the
alloying elements are in solution in the aluminum substrate. During
the cooling process, particularly if the cooling is carried out at
a slow rate, there is a tendency for the different elements to
become segregated. Therefore, traditionally the casting is
re-heated in a "solution" heat treatment furnace for several hours,
and only then is quenched, i.e. rapidly cooled down by a fluid
quench from a temperature for example about 480.degree. C. to
around 85.degree. C., so that the solid solution is preserved
(before segregation can occur). Such post solution-treatment quench
cooling may commonly be continued in a manner sufficient to bring
the castings down to any of a number of different temperatures and
at different rates according to the final properties of the alloy
to be emphasized.
[0010] This quenching step produces a supersaturated solid solution
that causes the alloy to harden naturally as time passes. Finally,
in order to accelerate and improve this age hardening, the quenched
castings are maintained at temperatures of about 200.degree. C. in
an "aging" furnace for about 2 or more hours. The time spent in the
"aging" furnace at elevated artificial aging temperatures brings
the alloy to at least a partial coherency in its structure giving
the required hardness and strength properties.
[0011] U.S. Pat. No. 5,788,784 to Koppenhoefer et al. discloses a
process for heat-treating light metal castings that requires "a
solution heat treatment furnace 2, an adjoining quenching device 3,
as well as an aging furnace 4", all particularly for cylinder heads
of piston engines. In the U.S. Pat. No. 5,788,784 process, after
solidifying and removing the casting from the mold, said castings
unconventionally are not naturally cooled, yet are still solution
heat treated (claiming the advantage of using the residual heat of
the casting present at the approximate 530.degree. C. temperature
of such treatment). Thereafter, the castings are quenched with an
air/water mixture down to 130.degree. C. to 160.degree. C., and
then aged in a furnace at approximately 170.degree. C. to
210.degree. C. (thus taking advantage of some relatively minor
residual heat carryover into the aging furnace), and are then
finally cooled to room temperature after, for example, four hours
of furnace aging. The castings are individually quenched with a
mist-type fine mixture of air and water, which is "nozzle sprayed
on all sides" of the casting.
[0012] Koppenhoefer asserts a number of advantages by reason of
quenching the castings with an air-water spray, for example that a
uniform and low-distortion cooling is achieved, that the adhering
core sand is not wetted at the elevated quenching temperatures and
can be collected clean and reused after regeneration, and that the
residual heat of the casting remaining at 130.degree. to
160.degree. C. can be used to aid in the subsequent furnace aging
step (by not cooling down the casting too much and leaving some
heat in said casting). Quenching the casting by directing the
sprayed water on all sides of the casting suggests that most of the
residual heat is lost, with that amount retained being mainly in
the inner portion of the casting. This also suggests that a large
temperature gradient would have to be maintained between the
interior and the surface of the casting in order for the amount of
retained residual heat carried over into the aging step to be
meaningful. Such large differentials in temperature across the
casting (particularly the end product portion thereof) is one of
the problems to be avoided while quenching a piece in order to
avoid stresses and achieve the T6 or T7 properties and also to
avoid spheroidization of the alloying elements. Furthermore,
Koppenhoefer does not teach or suggest applicants' invention of
selectively quenching only the end product portion of the casting
in order to use eventually the unquenched retained residual heat
from the sprue and from any other temporarily retained waste
portion of the casting (including sand cores) in order to enable
aging of said casting without need for an aging furnace. In
contrast, Koppenhoefer teaches decoring the resin bonded sand cores
from the castings by being "pyrolytically destroyed" during
solution heat treatment and further removed during quenching, all
prior to aging.
[0013] U.S. Pat. No. 5,112,412 to Plata et al. teaches a process
for cooling large cast billets of aluminum after a temperature
homogenization (re-heat) annealing step. Annealing is a softening
process for aluminum (just the opposite of the strengthening and
hardening heat treatment of the present invention), and this Plata
patent is silent on how the cooling is to be done to accomplish a
particular result (mainly mentioning only that it be "in accordance
with the alloy composition" and describing how the "automated and
controlled manner of spraying can be adjusted to different shaped
billets, as they may differ from the usual round shape"). This
patent first describes cooling the annealed billet with a spray on
all sides. This decreases the temperature at the surface of said
billet, while the center portion (inaccessible to the spray)
necessarily cools more slowly and thus initially remains at a
relatively higher temperature. The billet leaves the spray and is
allowed to equalize its internal and external temperatures in an
insulated chamber. In another embodiment, Plata et al describes a
process modification in the case of a so-called (but otherwise
unidentified) "hard" alloy to continue spraying until the billet
has achieved an equalized temperature. An example of this
temperature is given as "310 C.-350 C. in AlMgSi alloys" (a range
above most age hardening but typical of softening annealing). The
teaching includes the possibility of varying the intensity of the
continuous spray, but only for the purpose of achieving a "better
balanced heat flow" and a temperature zone "preferably distributed
homogeneously during cooling so that no or only minimal
deformations, stresses or cracks form". For example, the patent
states that circular billets are sprayed evenly, but a rectangular
billet may be sprayed with different intensity along it periphery.
This difference in spray intensity is to achieve uniformity of
cooling during the quench (just the opposite of subjecting the
casting to a significant differential or complete absence of quench
cooling of a specific waste portion of the casting in order to
maintain such portion at a significantly higher temperature during
the quench of the work portion (and much less to identify such a
waste portion which is accessible to the spray, but is not to be so
spray cooled). Thus, even though one of the embodiments discloses a
spray process involving a difference in the temperature between
certain portions of the billet which later reach an equalized
temperature, there is no disclosure of differential quenching of
selected portions of the casting to promote rather than minimize an
initial significant heat differential between selected different
portions of the casting (particularly with the division being
between equally exposed waste and workpiece portions). Furthermore,
Plata et al. teaches a process of cooling the surfaces of the
workpiece (billet) on all sides, while the inner portion of the
workpiece remains hot. If this process is applied to the workpiece
portion of the castings for cylinder heads or blocks for engines,
it will cause a different distribution of the alloying elements and
thus it will fail to achieve the objects of the present invention
(which provides a quenching step to produce uniform properties such
as those obtained with a T6 treatment, all with accelerated aging
but without the need for an aging furnace). In applicants' casting,
the unquenched portion is an existing waste portion that is put to
a useful interim purpose but whose ultimate alloy and physical
properties are irrelevant. Engine castings, if made by the Plata
process would be rejected.
[0014] Applicants' recent U.S. Pat. No. 5,922,147 (to Valtierra et
al.), mentioned above, discloses an improved heat-treating method
whereby the castings are quenched immediately after having been
extracted from the mold, thus eliminating "solution" heat treatment
and avoiding the need for a solution heat treatment furnace; while
nevertheless producing castings with similar properties to those
that undergo the traditional solution heating step. The U.S. Pat.
No. 5,922,147 patent process provides a casting plant with greatly
improved productivity and significant savings in capital and
operational costs. This patent, however, does not teach or suggest
a method capable of eliminating also the aging furnace.
OBJECTS AND SUMMARY OF THE INVENTION
[0015] The present invention improves upon the applicants'
aforementioned U.S. Pat. No. 5,922,147 patent by simplifying even
further the overall heat-treatment of the castings, although
broadly it can be applied separately. The invention dispenses with
the aging furnace, in addition to preferably also dispensing with
the solution furnace. Therefore, it provides a method and apparatus
for producing the casting in a considerably shorter time, with less
capital, and lower production costs, while maintaining and even
improving on required mechanical properties of the castings.
[0016] In order to better describe the invention, the applicants
specifically identify the two major parts of the casting as a riser
portion (the riser portion being that portion which is subsequently
removed and discarded) and a workpiece portion (the workpiece
portion being the portion used for the end product). A "riser" is a
reservoir of liquid metal used to largely compensate for shrinkage
of a casting as it cools in its mold. The term "riser" also
commonly has the meaning used in this application, namely the
solidified metal portion of the casting remaining in the reservoir
after the casting is cooled. "Riser portion" is intended to include
at least the riser and additionally in its broader sense can
include other similar waste attachments such as sprue, runners,
gates, etc. formed as part of the original casting. When the
casting is demolded from the typical water-cooled mold, the
workpiece commonly has a temperature of about 400.degree. C. and
the riser one of about 500.degree. C. The invention achieves its
advantages by a selective quench of only the workpiece portion to
surface temperatures preferably in the range from above about
100.degree. C. to about 130.degree. C., at a rate sufficient to
achieve a supersaturated solution of the hardening element
(typically copper) in the aluminum alloy of the workpiece at the
atomic level. To perform this selective quench, spray nozzles are
set to direct the water spray or mist on the workpiece and minimize
any impingement on the riser. This workpiece-directed quench
permits the riser (subjected only to natural or at most a minimized
indirect cooling) to maintain a significantly higher temperature
typically above about at least 300.degree. C. to 350.degree. C.
during the workpiece quenching step. Thereafter, when the quenching
is finished, the residual heat in the sprue portion is used as a
heat reservoir to slightly re-heat the workpiece and maintain it
(by conductive phenomena) in the artificial aging temperature range
of between 140.degree. C. and 250.degree. C., and preferably about
180.degree. C. to about 220.degree. C., for an adequate time
period, to thus achieve the desired properties for the workpiece.
The invention dispenses with the need to supply furnace heat for
re-heating and maintaining the whole casting in the artificial
aging temperature range and simplifies the casting plant by thus
rendering the aging furnace unnecessary. The final quench
temperature should not be so low that the residual heat from the
riser is too little to maintain the workpiece in the required aging
temperature range for the necessary length of time. Also, if the
workpiece surface temperature is maintained high enough above the
boiling point of the spray liquid (typically water) throughout the
quench, then liquid overflow onto the riser can be more easily
minimized or avoided altogether and the latent heat of can be
utilized and concentrated on the workpiece. A copious flow of a
fine water mist is especially effective, since the mist particles
evaporate immediately and there is no liquid wetting of the hot
workpiece surface that can flow over onto the riser.
[0017] It has been found that the existing riser mass as dictated
by ordinary foundry practice is sufficient to achieve this result
(i.e. provides an adequate heat reservoir for the artificial age
hardening without need for an aging furnace); however, it would be
within the scope of this invention to increase the mass as needed
for the desired inventive result.
[0018] Even though the temperature of the workpiece and the riser
greatly depends on the mass and the surface area of both portions,
the quenching temperature can be regulated to achieve the
advantages of the invention at different temperature paths. Also
the aging position of the casting plant can be insulated to prolong
the artificial aging step at elevated temperatures for a more
extended time period as may be needed (or even make use of a heat
exchanger to take advantage of other residual or excess heat
sources that may be available elsewhere in the casting plant
system), all as an aid to avoid the need for the added expense of
an aging furnace.
[0019] It is therefore an object of the present invention to
provide method and apparatus for producing aluminum alloy castings
having similar mechanical properties as those produced by the prior
art methods while avoiding the necessity of an aging furnace, and
preferably also of a solution heat furnace.
[0020] It is a further object of the invention to increase the
productivity of a casting plant and to reduce its capital costs and
its operating costs significantly.
[0021] Other objects of the invention will be evident to those
skilled in the art or will be pointed out hereafter.
[0022] The invention is herein described as applied to the
production of cylinder heads for automotive motors using generally
silicon-based aluminum alloys of the AA 3xx.x series, having T6 and
T7 properties (such as particularly A319), but it will be evident
to those skilled in the art, that the invention can in its broader
aspects be also applied to other metal alloys and to the
heat-treating of other castings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In this specification and in the accompanying drawings, some
preferred embodiments of the invention are shown and described and
various alternatives and modifications thereof have been suggested;
but it is to be understood that these are not intended to be
exhaustive and modifications can be made without departing from the
scope of the invention.
[0024] FIG. 1 is a graph showing the different casting temperature
paths followed over time during the heat treatment according to the
conventional prior art, according to applicants' most recent prior
art process (shown on U.S. Pat. No. 5,922,147 to Valtierra et al.);
and according to the present invention with respect to the
workpiece portion and also to the riser portion.
[0025] FIG. 2 is a schematic side elevational view of a preferred
embodiment according to the present invention illustrating a series
of stations making up the quench portion of a casting production
line, showing some castings (each comprised of both the workpiece
portion and the riser portion) and the spray nozzles used for
quenching only the workpiece portion oriented uppermost.
[0026] FIG. 3 is a schematic frontal view of the embodiment shown
in FIG. 2, showing the nozzles directing the spray or mist
selectively onto the workpiece portion of the casting.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0027] FIG. 1 is a graph showing the different temperature paths
vs. time of various castings; with the prior art processes in
dotted lines and the present invention in continuous lines.
Nowadays, the most common practice of the prior art (shown in the
graph by the thinner dotted line) includes, after demolding, the
steps of subjecting the casting to: natural cooling, reheating and
maintaining in a solution heat furnace, quenching, and reheating
and maintaining in an aging furnace. Another illustrated prior art
temperature path (shown by the bold dotted line) is the heat
treatment disclosed in applicants' own very recent U.S. Pat. No.
5,922,147 (wherein the solution treatment is omitted entirely, with
the quenching of the casting occurring without natural cooling
after demolding, and preferably immediately).
[0028] Also shown in the graph in FIG. 1, are the casting
temperature paths of the workpiece portion and of the riser portion
according to the present invention (shown in respective continuous
lines). As it can be seen, there is a selective quenching of the
workpiece portion of the casting. At the same time, the riser
remains essentially unquenched, with any cooling typically
occurring only naturally and at a much lower rate; so that when the
quenched workpiece has a temperature of about 120.degree. C., the
riser still has a temperature about 350.degree. C. At about that
point, before the quench reaches ambient temperature, the quenching
step is stopped and the casting (riser and workpiece together) is
allowed to homogenize its temperature with the workpiece being
mainly in the range of from about 160.degree. C. to about
220.degree. C. (initially towards the high end of the range,
preferably). This essentially duplicates the conventional
temperature profile of a casting maintained in an aging furnace
after the quenching step, while surprisingly eliminating the need
for any aging furnace. This is possible, because there is
sufficient mass in the riser to function as an adequate reservoir
of heat available for a sufficient duration to achieve complete
aging.
[0029] FIG. 2 schematically shows a preferred embodiment of the
apparatus used for the quenching step to perform the invention.
Immediately after demolding, the casting 20, broadly comprising
riser 24 and the workpiece 22, is placed on a conveyor 16 by means
of a feeding robot 12. The conveyor 16 has structural supports 18
(such as rollers)located through the quenching unit 10. Also
provided in the quenching unit 10 are an air header 26 and a water
header 28, both being connected to spray nozzles 30. Spray nozzle
30 projects a water spray or an air driven mist 32 that is directed
to impinge mainly on the workpiece portion 20 of the casting 22
positioned on the conveyor 16. Since rapid cooling rates are quite
important to achieve the desired properties in the workpieces 20,
and the nozzles 30 in this embodiment are in a fixed position
within the quenching unit 10, the conveyor 16 is operated
discontinuously in order to transport castings 20 from one
quenching station 33 to the next in a step-wise mode (over a
distance 34). After the castings 20 have traveled along the length
of the conveyor 16 within the quenching unit 10, the residence time
needed for quenching the workpieces 22 to the desired temperatures
is completed. Finally, a withdrawing robot 14 transports the
quenched castings to a place to be aged artificially at still
elevated temperatures over an extended time utilizing the reservoir
of heat remaining in the riser 24. In order to improve the
quenching operation of the quenching unit 10, a fan 13 can be
supplied to extract the vapor produced by the evaporation of the
sprayed water while quenching the workpieces 22 of castings 20.
[0030] FIG. 3 shows an end view of the quenching unit 10. The same
elements bear the same reference numerals of FIG. 2. Additionally
shown is an air supply 25 for air header 26, preferably at high
pressure in order to achieve a better water spray or mist. Liquid
supply 27 feeds header 28, which can handle water or any other
suitable liquid cooling medium.
[0031] Even though the process described on FIGS. 2 and 3 teaches a
quenching unit for processing castings 20 in a step-wise mode and
with the riser 24 oriented as the base of the casting 20, it will
be evident to those skilled in the art that the quenching unit 10
of the invention could be operated continuously preferably with
moving headers and spray nozzles or in a batch processing system as
well. Another change that can be performed to the embodiments of
the invention without departing from its scope comprises locating
the spray nozzles below the castings thus directing the spray 32
upwardly. In this case, the sprue portion 24 of the castings 20
would be positioned above the workpiece 22.
[0032] As previously indicated, the invention in its broader
aspects can be applicable to other aluminum alloys and heat
treating processes wherein the aging furnace step is normally used,
including those prior art systems still using a conventional
solution heat treatment with a subsequent quenching step.
* * * * *