U.S. patent application number 11/505019 was filed with the patent office on 2008-02-21 for solidification microstructure of aggregate molded shaped castings.
This patent application is currently assigned to Alotech Ltd. LLC. Invention is credited to John Campbell, John R. Grassi.
Application Number | 20080041499 11/505019 |
Document ID | / |
Family ID | 39082327 |
Filed Date | 2008-02-21 |
United States Patent
Application |
20080041499 |
Kind Code |
A1 |
Grassi; John R. ; et
al. |
February 21, 2008 |
Solidification microstructure of aggregate molded shaped
castings
Abstract
A shaped metal casting made in an aggregate mold comprises fine
solidification microstructure that is finer than the solidification
microstructure of an analogous metal casting made from conventional
molding processes. The solidification microstructure may be up to
five times finer than the solidification microstructure of a
conventionally prepared casting. In preferred embodiments, as a
result of directional solidification, the fine solidification
microstructure is substantially continuous from a distal end of the
casting to a proximal end of the casting, and exhibits greatly
enhanced soundness. Because of the control of the uniformity of
freezing of the casting, its properties are substantially
uniform.
Inventors: |
Grassi; John R.; (Kennesaw,
GA) ; Campbell; John; (Ledbury, GB) |
Correspondence
Address: |
FAY SHARPE LLP
1100 SUPERIOR AVENUE, SEVENTH FLOOR
CLEVELAND
OH
44114
US
|
Assignee: |
Alotech Ltd. LLC
|
Family ID: |
39082327 |
Appl. No.: |
11/505019 |
Filed: |
August 16, 2006 |
Current U.S.
Class: |
148/400 ;
148/437; 148/902 |
Current CPC
Class: |
B22D 30/00 20130101;
C22C 21/00 20130101; B22D 46/00 20130101; C22F 1/002 20130101; B22D
27/04 20130101; C22F 1/04 20130101; B22D 21/007 20130101 |
Class at
Publication: |
148/400 ;
148/437; 148/902 |
International
Class: |
C22C 21/00 20060101
C22C021/00 |
Claims
1. A shaped metal casting formed in a mold comprising an aggregate
by an ablation casting process, the casting comprising a fine
solidification microstructure that is finer than the microstructure
of a casting having a similar metal of a similar weight or section
thickness that is produced by a conventional aggregate molded
casting process, wherein the fine microstructure comprises one or
more of grains, dendrites, eutectic phases or combinations
thereof.
2. The shaped metal casting according to claim 1, wherein the fine
solidification microstructure is about five times finer than the
microstructure of a casting of a similar metal that is produced by
a conventional aggregate casting process.
3. The shaped metal casting according to claim 1, wherein the
casting comprises about 100% fine solidification
microstructure.
4. The shaped metal casting according to claim 1, wherein the fine
solidification microstructure is continuous from a distal end of
the casting to a proximal end thereof.
5. The shaped metal casting according to claim 1, wherein the fine
solidification microstructure is substantially continuous from a
distal end of the casting to a proximal end thereof.
6. The shaped metal casting according to claim 1, wherein the
casting is substantially free of porosity.
7. The shaped metal casting according to claim 1, further
comprising a dual solidification microstructure region comprising
(i) a coarse solidification microstructure portion, and (ii) a fine
solidification microstructure portion interspersed within the
coarse solidification microstructure portion.
8. The shaped metal casting according to claim 7, wherein the dual
solidification microstructure region is substantially continuous
from a distal end of the casting to a proximal end thereof.
9. A metal casting formed in a mold comprising an aggregate via a
rapid cooling process and exhibiting a cast microstructure, the
microstructure comprising: a first region located adjacent a
surface of the metal casting, the first region comprising a coarse
solidification microstructure; and a second region located internal
to the first region, the second region comprising a fine
solidification microstructure.
10. The metal casting according to claim 9, wherein the second
region is substantially continuous from a distal end of the casting
to a proximal end thereof.
11. The metal casting according to claim 9, wherein the fine
solidification microstructure of the second region is finer than a
fine solidification microstructure of a casting of a similar metal
having a similar weight or section thickness that is formed by a
conventional aggregate casting process.
12. The metal casting according to claim 9, wherein the second
region comprises dendrites having a dendrite arm spacing of about
20 micrometers or less.
13. The metal casting of claim 9, wherein the second region
comprises dendrites having a dendrite arm spacing of about 5 to
about 15 micrometers.
14. The metal casting of claim 9, wherein the first region
comprises dendrites having a dendrite arm spacing of about 20 to
about 200 micrometers.
15. The metal casting of claim 9, wherein the casting is
substantially free of at least one of (i) shrinkage porosity, and
(ii) damaging iron-rich platelets.
16. The metal casting of claim 9, wherein the first region
comprises approximately 100% course solidification microstructure,
and the second region comprises approximately 100% fine
solidification microstructure.
17. The metal casting of claim 16 further comprising a third region
located between the first and second regions, wherein the third
region comprises a dual solidification microstructure comprising
(i) one or more coarse solidification microstructure portions, and
(ii) one or more fine solidification microstructure portions.
18. The metal casting of claim 17, wherein the one or more coarse
solidification microstructure portions of the dual solidification
microstructure region comprises dendrites having a dendrite arm
spacing of about 20 to about 200 micrometers and the one or more
fine solidification microstructure portions of the dual
solidification microstructure region comprises dendrites having a
dendrite arm spacing of about 15 micrometers or less.
19. The metal casting of claim 17 wherein the third region is
substantially continuous throughout a shape of the metal
casting.
20. A metal casting formed from a eutectic-containing alloy, the
casting comprising a dual solidification microstructure region,
wherein the dual solidification microstructure region comprises (i)
one or more fine dendrite portions; and (ii) one or more fine
eutectic portions.
21. The metal casting of claim 20 further comprising a coarse
solidification microstructure region located between the dual
solidification microstructure region and a surface of the metal
casting.
22. The metal casting of claim 21 further comprising a fine
solidification microstructure region located adjacent the dual
solidification microstructure region.
23. The metal casting of claim 20, wherein the one or more fine
dendrite portions has a dendrite arm spacing of about 5 to about 30
micrometers, and the one or more fine eutectic portions has a
dendrite arm spacing of 1 micrometer or less.
24. The metal casting of claim 20, wherein the casting is
substantially free of shrinkage porosity.
25. The metal casting of claim 20, wherein the casting is
substantially free of damaging iron-rich platelets.
26. A shaped metal casting made in a mold comprising an aggregate,
the casting comprising a dual solidification microstructure region,
wherein the dual solidification microstructure region comprises: at
least one coarse solidification microstructure portion having a
dendrite arm spacing in the range of about 50 to about 200
micrometers; and at least one fine solidification microstructure
portion having a dendrite arm spacing of less than about 15
micrometers.
27. The metal casting of claim 26, wherein the at least one fine
solidification microstructure region is substantially continuous
from a distal end of the metal casting to a proximal end
thereof.
28. The metal casting of claim 26, wherein the at least one fine
solidification microstructure region comprises a first fine
solidification microstructure region located adjacent a distal end
of the metal casting and a second fine solidification
microstructure region located adjacent a proximal end thereof.
29. The metal casting of claim 26, further comprising a
substantially continuous fine solidification microstructure region
internal to and distinct from the dual solidification
microstructure region.
30. The metal casting of claim 26, wherein the at least one fine
solidification microstructure portion has a dendrite arm spacing in
the range of about 5 to 15 micrometers.
31. The metal casting of claim 26, wherein the casting is
substantially free of at least one of (i) shrinkage porosity, and
(ii) damaging iron-rich platelets.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to metal castings. More
particularly, the present invention relates to aggregate shaped
metal castings having a fine solidification microstructure.
BACKGROUND
[0002] In the traditional casting process, molten metal is poured
into a mold and solidifies, or freezes, through a loss of heat to
the mold. When enough heat has been lost from the metal so that it
has frozen, the resulting product, i.e., a casting, can support its
own weight. The casting is then removed from the mold.
[0003] Different types of molds of the prior art offer certain
advantages. For example, green sand molds are composed of an
aggregate, sand, that is held together with a binder such as a
mixture of clay and water. These molds may be manufactured rapidly,
e.g., in ten (10) seconds for simple molds in an automated mold
making plant. In addition, the sand can be recycled for further use
relatively easily.
[0004] Other sand molds often use resin based chemical binders that
possess high dimensional accuracy and high hardness. Such
resin-bonded sand molds take somewhat longer to manufacture than
green sand molds because a curing reaction must take place for the
binder to become effective and allow formation of the mold. As in
clay-bonded molds, the sand can often be recycled, although with
some treatment to remove the resin.
[0005] In addition to relatively quick and economical manufacture,
sand molds also have high productivity. A sand mold can be set
aside after the molten metal has been poured to allow it to cool
and solidify, allowing other molds to be poured.
[0006] The sand that is used as an aggregate in sand molding is
most commonly silica. However, other minerals have been used to
avoid the undesirable transition from alpha quartz to beta quartz
at about 570 degrees Celsius (.degree. C.), or 1,058 degrees
Fahrenheit (.degree. F.), that include olivine, chromite and
zircon. These minerals possess certain disadvantages, as olivine is
often variable in its chemistry, leading to problems of uniform
control with chemical binders. Chromite is typically crushed,
creating angular grains that lead to a poor surface finish on the
casting and rapid wear of tooling. Zircon is heavy, increasing the
demands on equipment that is used to form and handle a mold and
causing rapid tool wear.
[0007] In addition, the disadvantages created by the unique aspects
of silica and alternative minerals, sand molds with clay and
chemical binders typically do not allow rapid cooling of the molten
metal due to their relatively low thermal conductivity. Rapid
cooling of the molten metal is often desirable, as it is known in
the art that such cooling improves the mechanical properties of the
casting. In addition, rapid cooling allows the retention of more of
the alloying elements in solution, thereby introducing the
possibility of eliminating subsequent solution treatment, which
saves time and expense. The elimination of solution treatment
prevents the quench that typically follows, removing the problems
of distortion and residual stress in the casting that are caused by
the quench. Related to the mechanical properties, the fineness of
the cast microstructure is related to the rate of cooling and
solidification. Generally, as the rate of cooling and
solidification increases, the solidification microstructure of the
casting becomes finer.
[0008] As an alternative to sand molds, molds made of metal or
semi-permanent molds or molds with chills are sometimes used. These
metal molds are particularly advantageous because their relatively
high thermal conductivity allows the cast molten metal to cool and
solidify quickly, leading to advantageous mechanical properties in
the casting. For example, a particular casting process known as
pressure die casting utilizes metal molds and is known to have a
rapid solidification rate. Such a rapid rate of solidification is
indicated by the presence of fine dendrite arm spacing (DAS) in the
casting. As known in the art, the faster the solidification rate,
the smaller the DAS. However, pressure die casting often allows the
formation of defects in a cast part because extreme surface
turbulence occurs in the molten metal during the filling of the
mold. The presence of fine dendrite arm spacing may also be
achieved by cooling the casting by a local chill or fin. Such
techniques include the localized application of solid chill
materials, such as metal lump chills or moldable chilling
aggregates, and the like, that are integrated into the mold
adjacent to the portion of the casting that is to be chilled. These
methods, however, only provide a localized effect in the region
where the chill is applied. This localized effect contrasts with
the benefits of the invention discussed in this application, in
which the benefits of fine microstructure can apply, if the
invention is implemented correctly, extensively throughout the
casting. This is an important aspect of the current application,
because the ultimate benefit is that the casting displays
properties that are not only generally higher, but are also
essentially uniform throughout the product, and thus of great
benefit to the designer of the product. The product now essentially
enjoys the uniformity normally associated with forgings.
[0009] One variety of known permanent mold process in which the
residual liquid phase in the structure may be subject to rapid
cooling includes some types of semi-solid casting. In this process
the metallic slurry is formed exterior to the mold, and consists of
dendritic fragments in suspension in the residual liquid. The
transfer of this mixture into a metal die causes the remaining
liquid to freeze quickly, giving a fine structure, but surrounded
by relatively coarse and separated dendrites, often in the form of
degenerate dendrites, rosettes, or nodules.
[0010] However, all molds made from metal possess a significant
economic disadvantage. Because the casting must freeze before it
can be removed from the mold, multiple metal molds must be used to
achieve high productivity. The need for multiple molds in permanent
mold casting increases the cost of tooling and typically results in
costs for tooling that are at least five (5) times more than those
associated with sand molds.
[0011] Another common feature of the internal structure of
conventional shaped castings, well known and well understood
throughout the casting industry, is that those regions of larger
geometric modulus (i.e., regions having a larger ratio of volume to
cooling area) generally have a coarser structure. Such regions of
the casting typically have significantly lower mechanical
properties. Further, such regions commonly exhibit shrinkage
cavities or pores because they are more easily isolated from feed
metal at a late stage of freezing. Such regions are often seen, for
instance, at hot spots formed by an isolated boss on a relatively
thin plate, or in the hot spot that is found at the T-junction
between two similar sections. Complicated castings are often full
of such features, resisting the attainment of any degree of
uniformity of properties. This problem greatly complicates the work
of the designer of the product. For instance, the thickening of a
section intended to increase its strength will lower properties and
in the worst instance may even lead to defects, and so is, often to
some indeterminate degree, counter productive.
[0012] In the locations of the casting where solidification is
slow, not only is (i) the structure coarse, typified by a coarse
DAS, but (ii) porosity is also present, and (iii) for those Al
alloys that commonly suffer iron as an impurity, large plate-like
crystals of iron-rich phases can form. All these factors are
greatly damaging to the ductility of the alloy.
[0013] Extremely fine cast microstructures have been produced as
laboratory curiosities in various scientific studies (for instance,
the research paper by G. S. Reddy and J. A. Sekhar in Acta
Metallurgica, 1989, vol. 37, Number 5, pp. 1509-1519 and the
research paper by L. Snugovsky, J. F. Major, D. D. Perovic and J.
W. Rutter; "Silicon segregation in aluminium casting alloy"
Materials Science and Technology 2000 16 125-128.).
[0014] However, in contrast to such laboratory studies, the
invention described in this application provides the unique
conditions in which the solidification microstructures described
herein are produced routinely by a production process that can be
operated to produce one-off or volume-produced castings that are
shaped in three-dimensions.
[0015] It is less generally known that the interior of castings can
experience accelerated freezing as a result of a geometrical effect
in shaped castings. The early solidification near the skin of the
casting occurs substantially unidirectionally, and typically varies
at a rate that decreases parabolically with time; i.e. the
solidification rate reduces in speed as the thickness of the
solidified layer increases. In contrast, the volume of remaining
liquid in the center of a casting dwindles with time, and
experiences increasing heat extraction from additional directions,
so that the speed of freezing can be greatly increased. This effect
is well described by one of the inventors. See Castings, John
Campbell, pp. 125-126 (2.sup.nd Edition, 2003), published by
Butterworth Heinemann, Oxford, UK, the entire disclosure of which
is incorporated herein by reference. The behavior explains the
so-called reverse chill effect in cast iron castings, in which the
center of the casting, seemingly inexplicably, sometimes exhibits a
white, apparently chilled, structure in contrast to the outer
regions of the casting that remain grey, signifying a slow cooling
rate.
[0016] As a result, it is desirable to develop a casting process
and related apparatus that have the advantage of rapid
solidification of metal molds, while also having the lower costs,
high productively and reclaim-ability associated with sand
molds.
[0017] It is also desirable to provide an aggregate molded shaped
casting exhibiting a region of fine solidification microstructure
over extensive regions of the casting, so as to promote
substantially uniform properties akin to those of forgings.
(Because of the relative insensitivity of properties to the
variations in cooling rate at the high cooling rates used in this
application, the variations that are discussed later, for instance
in FIG. 4, do not significantly affect properties, conferring
substantial uniformity of properties in the cast product). In
particular, it is desirable to provide an aggregate molded shaped
casting having a fine solidification microstructure that is finer
than a structure produced by a conventional aggregate casting
method, and possibly even finer than that produced by a permanent
mold.
[0018] It is further desirable to provide an aggregate molded
shaped casting having a fine solidification microstructure region
that is substantially continuous from a distal point of the casting
to the feeder or riser.
SUMMARY
[0019] The disclosure provides, in various embodiments, a shaped
metal casting formed in an aggregate mold by an ablation casting
process, the casting comprising a fine solidification
microstructure that is finer than the microstructure of a casting
of a similar metal having a similar weight or section thickness
that is produced by a conventional aggregate casting process,
wherein the fine microstructure comprises one or more of grains,
dendrites, eutectic phases, or combinations thereof.
[0020] The disclosure also provides, in various embodiments, a
metal casting exhibiting a cast microstructure, the microstructure
comprising a first region located adjacent a surface of the metal
casting, the first region comprising a coarse solidification
microstructure; and a second region located internal to the first
region, the second region comprising a fine solidification
microstructure.
[0021] In a further aspect, the disclosure provides, in various
embodiments, a metal casting formed from a eutectic-containing
alloy, the casting comprising a dual solidification microstructure
region, wherein the dual solidification microstructure region
comprises (i) one or more regions containing coarse dendrites; and
(ii) one or more regions containing fine eutectic.
[0022] Additionally, the disclosure provides a shaped metal casting
made in a mold that is at least a partially aggregate mold, the
casting comprising a dual solidification microstructure region,
wherein the dual solidification microstructure region comprises at
least one coarse solidification microstructure portion having a
grain size and/or dendrite arm and/or eutectic spacing in the range
commonly to be expected in a conventional aggregate or metal mold;
and at least one fine solidification microstructure having a grain
size and/or dendrite arm and/or eutectic spacing of less than one
third of the conventional spacing for that portion of the
casting.
[0023] In still another aspect, the disclosure provides a shaped
metal casting made in an aggregate mold, the casting comprising a
dual solidification microstructure region, wherein the dual
solidification microstructure region comprises: at least one coarse
solidification microstructure portion having a dendrite arm spacing
in the range of about 50 to about 200 micrometers; and at least one
fine solidification microstructure portion having a dendrite arm
spacing of less than about 15 micrometers.
[0024] In another aspect, the disclosure provides a shaped metal
casting formed in an aggregate mold by an ablation casting process,
the casting comprising a fine solidification microstructure having
a dendrite arm spacing that is finer than the dendrite arm spacing
of a casting having a similar metal of a similar weight or section
thickness that is produced by a conventional aggregate molded or
permanent molded casting process.
[0025] Further, the disclosure provides a shaped metal casting with
substantial soundness and with high and substantially uniform
properties, to some extent resembling those features normally
associated with forgings.
[0026] Other features and advantages of castings in accordance with
the disclosure are further understood in view of the drawings,
detailed description, examples, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a cooling curve for a solid solution alloy that
undergoes dendritic solidification;
[0028] FIG. 1A is a graph showing the relationship between dendrite
arm spacing and freezing or solidification rate;
[0029] FIG. 2 is a micrograph (at .times.200) showing the
microstructure of a solid solution alloy cast by conventional
casting methods;
[0030] FIG. 3 is a micrograph of a solid solution alloy comprising
a fine solidification microstructure region (Dual DAS structure)
produced by ablation;
[0031] FIGS. 4A-4E are schematic representations of metal castings
comprising fine solidification microstructure regions;
[0032] FIG. 5 is a cooling curve of a mixed dendrite and eutectic
alloy;
[0033] FIG. 6 is a micrograph (at .times.200) depicting the
microstructure of a 356 alloy showing coarse eutectic silicon
particles cast by conventional methods;
[0034] FIG. 7 is a micrograph (at .times.200) depicting an
ablation-frozen A356 alloy having regions of coarse plus fine
dendritic material (dual DAS structure) and fine eutectic
material;
[0035] FIG. 8 is a micrograph (at .times.200) of a portion of an
ablation-frozen A356 alloy showing uniform coarse DAS and uniform
fine eutectic phase;
[0036] FIG. 9 is a micrograph (at .times.200) of an ablation-frozen
A356 alloy having fine eutectic regions, but which are somewhat
coarsened after a solution heat treatment;
[0037] FIG. 10 is a micrograph (at .times.200) of an
ablation-frozen A356 alloy exhibiting coarse dendritic
microstructure and mainly fine eutectic microstructure, but
containing a trace of coarse eutectic phase;
[0038] FIG. 11 is a detail of the micrograph (at .times.1000) of
FIG. 10 at higher magnification;
[0039] FIG. 12 depicts the solidification rate of various portions
of a dendritic alloy formed in accordance with an exemplary
embodiment;
[0040] FIG. 13 is a graph depicting the cooling rate of various
sections of an exemplary embodiment casting;
[0041] FIG. 14 is a table showing the mechanical properties of
various exemplary embodiment castings;
[0042] FIG. 15 is a bar graph and table comparing the mechanical
properties of metal castings formed by various casting methods;
[0043] FIG. 16 is a chart showing the relationship between dendrite
cell size and solidification rate for aluminum alloys;
[0044] FIG. 17a is a micrograph (at .times.100) of a conventional
cast permanent mold microstructure of a 2.75'' diameter bar of an
aluminum alloy;
[0045] FIG. 17b is a micrograph (at .times.100) of such a
microstructure made with the ablation process for the same alloy at
the start of the ablated part (first part in); and,
[0046] FIG. 17c is a micrograph (at .times.100) of the ablated last
part.
[0047] The figures are merely for the purpose of illustrating
various embodiments of a development in accordance with the present
disclosure and are not intended to be limiting embodiments of the
development.
DETAILED DESCRIPTION
[0048] The disclosure relates to an aggregate molded, shaped
casting comprising at least a fine solidification microstructure
region. An aggregate molded shaped metal casting in accordance with
the disclosure includes a solidification microstructure region that
is finer than the solidification microstructure obtained by
conventional aggregate molding methods. In some embodiments, an
aggregate molded shaped metal casting in accordance with the
disclosure has a solidification microstructure that is
substantially free of shrinkage porosity.
[0049] The type or nature of the solidification microstructure will
vary and depend on the metal and/or metal alloys undergoing
solidification. Various microstructures include dendrites, eutectic
phases, grains, and the like. In one embodiment, for example, a
shaped casting may comprise only a single type of microstructure.
Further, an alloy may exhibit a solidification microstructure
comprising one or more different microstructures. For example, in
one embodiment, a shaped casting may exhibit a microstructure
comprising a combination of dendrites and grains. In another
embodiment, a shaped casting may exhibit a combination of dendrites
and eutectic phases. In still another embodiment, a shaped casting
may exhibit a combination of dendrites, eutectic phases, and
grains. These embodiments are not limiting embodiments, as other
combinations and/or other microstructures may be possible.
[0050] As used herein, eutectic alloys includes any alloy that
forms eutectic phases, including hypo-eutectic, near-eutectic or
hyper-eutectic alloys.
[0051] An aggregate molded, shaped metal casting in accordance with
the present disclosure may be formed by a method such as that
described in U.S. application Ser. No. 10/614,601 that was filed on
Jul. 7, 2003, and the entire disclosure of which is incorporated
herein by reference. Generally, application Ser. No. 10/614,601
discloses a process for the rapid cooling and solidification of
aggregate molded shaped castings. The method also provides for the
removal of the mold. The process described in application Ser. No.
10/614,601 is referred to herein in as "ablation."
[0052] Upon solidifying during an ablation process, a metal casting
exhibits a fine solidification microstructure that is finer than
the microstructure of a similar metal having a similar weight or
section thickness that is produced by a conventional aggregate
casting process. The fineness of a microstructure may be defined in
terms of the size or spacing exhibited by a particular type of
microstructure. For example, grains exhibit a grain size, dendrites
exhibit a dendrite arm spacing, and eutectic phases exhibit a
eutectic spacing.
[0053] With reference to FIG. 1, a cooling or solidification curve
for a solid solution alloy is shown. Solid solution type alloys
form only grains and/or dendrites during solidification. The
cooling curve shows the cooling of a solid solution alloy with
time, from the pouring temperature (T.sub.p) through the liquidus
temperature (T.sub.L) to the solidus temperature (T.sub.S), which
is the point at which solidification is complete.
[0054] Cooling of a solid solution type alloy in a conventional
aggregate mold is represented by the time/temperature profile
"abcdef". The total time for cooling using conventional methods is
in the range of from minutes to hours and is, of course,
particularly dependent on the thickness of the casting, and the
rate at which heat can transfer into the mold. The rate of cooling
slows at the liquidus temperature (T.sub.L) as a result of the
latent heat evolution during the formation of dendrites.
Solidification is complete at the solidus temperature (T.sub.S) and
the rate of fall of temperature increases once the evolution of
latent heat has subsided.
[0055] At point "e" on FIG. 1, the application of any rapid cooling
is too late to have any effect on the solidification
microstructure. Thus, for example, the cooling profile "el" would
not have any effect on the solidification microstructure and is not
a part of this patent application. Cooling profiles such as "el,"
however, are commonly utilized in the casting industry where
castings are taken from a metal die and quenched directly into
water.
[0056] The solidified structure of solid solution alloys typically
consists almost entirely of dendrites that are outlined by a
negligible thickness of residual inter-dendritic material. The
secondary dendrite arm spacing (often referred to more simply in
this application as dendrite arm spacing, or DAS) is dependent on
the freezing time, i.e. the solidification time, which is the time
that a fixed location in the casting exists at a temperature
between the liquidus T.sub.L and the solidus temperature T.sub.S of
the alloy. In terms of the period of time in FIG. 1a, it is seen to
be t.sub.s=t.sub.e-t.sub.c.
[0057] FIG. 1a shows the approximate logarithmic relation between
the local DAS and the local t.sub.s in many common Al alloys. This
figure illustrates that to reduce DAS by a factor of 10, t.sub.s is
required to be reduced by a factor of approximately 1000.
(Unfortunately such a relationship has not been investigated for
grains and eutectic spacing, so that a clear, quantitative
description of the refinement of these other features of the
solidification structure of some alloys cannot easily be made. Thus
the quantitative predictions of refinement of structure by ablation
described in this application concentrate on DAS. However, it is to
be understood that similar but unquantified refinements are
paralleled in grain size and eutectic spacing) Thus very large
increases in cooling rate are required to substantially affect the
fineness of the solidification microstructure.
[0058] While the quantitative relationship exemplified in FIG. 1 is
described with respect to dendrites and dendrites arm spacing,
similar parallel refinements are expected in grain size and/or
eutectic spacing in alloys comprising grains and/or eutectic
phases.
[0059] For conventional aggregate casting methods, for a small
aluminum alloy casting weighing a few pounds or kilograms, the
local solidification time is typically of the order of about 1,000
seconds. These conventional aggregate casting methods produce
castings having a DAS of around 100 micrometers, and sometimes in
the range of from about 50 micrometers to about 200 micrometers. As
used herein, a solidification microstructure having a DAS of
greater than 50 micrometers is referred to as "coarse"
microstructure. FIG. 2, is a micrograph depicting the coarse
microstructure of a solid solution cast alloy A206 (a nominal
Al-4.5 wt % Cu alloy) that was cast by conventional methods.
[0060] A casting in accordance with the present disclosure
comprises the presence of fine solidification microstructure in at
least a portion of the casting. That is, a casting may comprise
from greater than 0% to up to 100% of fine solidification
microstructure. In one embodiment, the casting is substantially
free of any coarse solidification microstructure and comprises up
to 100% fine solidification microstructure that is continuous
throughout the casting. In another embodiment, a casting comprises
a first region adjacent the surface of the casting that comprises
up to 100% coarse solidification microstructure, and a second
region internal to the first region wherein the second region
comprises up to 100% fine solidification microstructure. In still
another embodiment, a casting in accordance with the present
disclosure comprises a continuous or at least a substantially
continuous, region of fine solidification microstructure extending
from a distal end of the casting to a proximal end thereof, i.e.,
the end adjacent the feeder or riser.
[0061] In other embodiments, a casting comprises a dual
solidification microstructure region intermediate a coarse
solidification microstructure region and a fine solidification
microstructure region. As used herein, a dual microstructure region
is a region that includes areas of coarse microstructure having one
or more areas of fine microstructure interspersed therein. In still
another embodiment, the dual solidification microstructure in a
casting is substantially continuous throughout from a distal end of
the casting to the feeder.
[0062] The dendrite arm spacing is generally dependent on the time
over which solidification occurs. As shown in FIG. 1A, the log/log
relationship of dendrite arm spacing to freezing time is linear
and, for aluminum alloys, for example, has a slope of approximately
1/3. The graph shows there is approximately a factor of 5 spacing
reduction for each factor of 100 in freezing time. Thus, a
particular casting section in a conventional mold may experience
solidification in 1000 seconds giving a corresponding DAS of 100
micrometers. In the ablation casting techniques described in U.S.
application Ser. No. 10/614,601, the same casting section would
result in a local solidification time of only approximately 10
seconds, giving a dendrite arm spacing of about 20 micrometers. The
relationship between spacing and freezing time remains constant
over all experimental times. FIG. 3 is a micrograph showing regions
of both fine microstructure and regions of coarse
microstructure.
[0063] With reference to FIGS. 4A-4E, various exemplary embodiments
of aggregate molded, shaped metal castings comprising a fine
solidification microstructure region are shown.
[0064] With reference to FIG. 4A, an embodiment is shown in which a
casting comprises a small percentage of fine solidification
microstructure. The casting in FIG. 4A represents a situation in
which the rapid cooling process, such as ablation, is applied late
in the casting process. In the casting in FIG. 4A, some
solidification has occurred prior to rapid cooling (e.g.,
ablation), and portions of the casting, such as those having a
small geometric modulus (volume to cooling area ratio)
conventionally freeze. Dual solidification microstructure regions
occur where some portions have solidified prior to ablation but
other portions remain liquid at the time ablation begins. Even in
an example such as FIG. 4A where a portion of the casting has
solidified prior to a rapid cooling process, and thus constituting
a far from optimum application of this invention, the presence of
even a small amount of fine solidification microstructure is
advantageous. Specifically, in conventional processes, the small
percentage of residual liquid in metal alloys that remains is
particularly difficult to freeze without creating defects such as
shrinkage porosity. Applying a rapid cooling technique such as
ablation, however, converts these regions from defective zones to
fine structure zones. The presence of even small zones of fine
structure provide good mechanical properties compared to those
defective zones of a conventionally solidified casting. Although
the trapped region of residual liquid illustrated in FIG. 4A will
be expected to demonstrate some shrinkage porosity, ablation
freezing of this region will reduce the extent of the shrinkage,
and will replace it with a corresponding region of strong, sound
material. The solidification microstructure profile of FIG. 4A
would be expected where the rapid cooling step is applied rather
late, closer to, but before, point "e" on the graph of FIG. 1.
[0065] In another embodiment, an aggregate molded shaped casting
comprises a fine solidification microstructure region and a dual
solidification microstructure region wherein the dual
solidification microstructure region is substantially continuous
from a distal end of the casting to a proximal end of the casting.
The embodiment of FIG. 4B is an example of an embodiment of a
casting having a substantially continuous dual solidification
microstructure region. The solidification microstructure profile of
FIG. 4B is a profile that would be expected from following the
cooling profile "abcdjk" in FIG. 1. In FIG. 4B, the freezing point
arrives at and passes the point at which the natural freezing of
the constricted section reaches the center of the section. The
casting is frozen by a rapid cooling procedure, such as ablation,
from the more distant parts of the casting up to the center point.
If the fine structure zone produced by rapid freezing is terminated
on reaching the modulus constriction (as it does in the embodiment
in FIG. 4B) the casting will freeze soundly up to this point. Even
though the dual solidification microstructure is absent in the
local region of the constriction in the embodiment in FIG. 4B, the
rapid local solidification time throughout the remainder of the
casting creates a substantially continuous zone of fine and sound
alloy, free from shrinkage defects, throughout the remainder of the
casting. Thus, by driving the solidification directionally from
distal regions to those proximal to the feeder, the more distant
portions of the casting exhibit fine solidification microstructure
and mechanical soundness that would not have been achieved by
conventional methods.
[0066] In the embodiment in FIG. 4C, the casting includes a greater
percentage of fine solidification microstructure, and the region of
dual solidification microstructure is continuous through the
constricted region of the casting. Such a desirable solidification
microstructure may be achieved by applying a rapid cooling
procedure, such as ablation, at a time earlier than that of FIG.
4B.
[0067] In another embodiment, such as that of FIG. 4D, a metal
casting comprises a desirable fine solidification microstructure
region that is substantially continuous from the distal ends of the
casting to the feeder. Such a solidification microstructure may be
achieved by applying a rapid cooling procedure early in the cooling
profile. In this embodiment, the casting may also include regions
of dual solidification microstructure and coarse solidification
microstructure. The solidification microstructure profile of FIGS.
4C and 4D would be expected from applying a rapid cooling at an
early time, such that the narrowest part of the casting would
follow a path starting at a point between c and d on the cooling
profile in FIG. 1.
[0068] In still another embodiment, such as the embodiment of FIG.
4E, the entire solidification microstructure comprises a fine
solidification microstructure. Such a desirable structure might be
achieved by applying a rapid cooling method at Point "b" in the
cooling curve of FIG. 1 and following the profile "abghi." This
would occur if no freezing occurs due to loss of heat to the mold
and the freezing occurs totally unidirectionally and at a high
rate. Such a structure, however, is not easily achieved and has yet
to be experimentally achieved by the inventors. Difficulties in
achieving this solidification microstructure arise from the
impingement of the liquid coolant directly on the surface of the
still liquid casting. A casting comprising 100% fine solidification
microstructure may be achievable under certain conditions such as
using a highly insulating mold, and applying a highly directional
solidification process.
[0069] An aggregate molded, shaped metal casting may comprise from
about 1 to about 100% fine solidification microstructure. Even a
small amount of solidification microstructure is desirable for
enhancing the mechanical properties of a casting. This is
especially the case where the creation of small amounts of fine
solidification microstructure, denoting as it does in this
invention the action of directional solidification, and thus
optimal feeding, prevents defects such as shrinkage porosity from
occurring in the casting.
[0070] Castings in accordance with the present disclosure that
include a fine solidification microstructure region may be formed
from any solid solution alloy that solidifies dendritically. These
include both ferrous materials and non-ferrous materials. The
dendrite arm spacing of both the coarse and fine solidification
microstructure regions will vary depending on the metal that is
used. With respect to aluminum alloys, coarse solidification
microstructure regions typically have a dendrite arm spacing of
greater than about 50 micrometers. In some embodiments the coarse
solidification microstructure has a dendrite arm spacing of from
about 50 to about 200 micrometers. Also in aluminum alloys, the
fine solidification microstructure has a dendrite arm spacing of
less than about 15 micrometers, and, in some embodiments, is from
about 5 to about 15 micrometers.
[0071] Alloys in which solidification occurs partly by dendritic
solidification and partly by eutectic solidification, as is typical
of many Al--Si alloys, exemplified by Al-7Si-0.4Mg (A356) alloy,
may also exhibit fine dendritic and/or fine eutectic
microstructure. The conventional cooling curve for mixed
dendrite/eutectic alloys is illustrated in FIG. 5 as curve "a-h."
Starting at the pouring temperature (T.sub.P) at point "a" the
liquid alloy cools to the liquidus temperature (T.sub.L), at point
"c," which is the point at which dendrites start to form. The
dendrite growth is complete at point "e," which is the eutectic
temperature (T.sub.E). The fall in temperature is arrested, forming
a plateau until the completion of the eutectic solidification at
point "g". At this point the casting is completely frozen, and
further cooling to room temperature follows "gh." A second example
alloy of an Al--Si alloy that benefits powerfully from the
application of this invention is the widely used A319 alloy. This
alloy also contains some copper. The alloy differs somewhat from
A356 in having a eutectic formation region "eg" that is not
isothermal, the horizontal plateau "eg" of FIG. 5 being replaced by
a steady downward slope. However, the same principles apply
precisely.
[0072] This slow conventional cooling in, for instance, a silica
sand mold, results in a structure of dendrites, having a DAS in the
region of 200 down to 50 micrometers. The dendrites are surrounded
by eutectic, which is characterized by a spacing in the region of
20 down to 2 micrometers. This is denoted the conventional or
"coarse" eutectic microstructure for purposes of our description
(FIG. 6).
[0073] If it were possible to subject the alloy to total cooling by
ablation, the cooling profile would follow the path "bijkl"' so
that the whole solidification microstructure would consist of fine
dendrites and very fine eutectic. However, as discussed above,
although this structure is not easily obtained, and has yet to be
tested by the inventors, it may be achievable in special
conditions. These conditions might include circumstances in which
the mold is highly insulating, and the freezing is highly
directional. Castings having excellent mechanical properties are,
nevertheless, achievable without resort to these special
conditions.
[0074] In general, a more practical cooling profile is illustrated
by the cooling curve "abedmno." In this situation the prior cooling
from "cd" creates coarse dendrites to strengthen the casting in its
hot, partially solidified, and therefore weak state, prior to the
application of the coolant. The subsequent dendrites and the
eutectic are both then subjected to rapid cooling, so that the fine
solidification microstructure includes both fine dendrites (DAS in
the region of 30 down to 5 micrometers) and fine eutectic (not
resolvable at 1000.times. magnification, since its spacing is
around only 1 micrometer).
[0075] The two regions of the consequently dual microstructure form
visually highly distinct areas when viewed under the microscope.
FIG. 7 shows a structure in which the ablation was applied in time
to freeze some dendritic material, followed by the rapid freezing
of all of the eutectic. The eutectic is so fine that it is not
resolvable in this image, but appears as a uniform light grey phase
(in this case the alloy had no refining action of the additions of
chemical modifiers such as Na or Sr). Additionally, because all of
the eutectic freezes along the path "mn," the whole of the eutectic
phase, between both the coarse and the fine dendrites, is seen to
be uniformly fine in FIG. 7.
[0076] The uniform and extremely fine eutectic is a common feature
of ablated solidification microstructures and is unique to ablation
cooled alloys that have received no benefit from chemical
modification by Na or Sr as an aid to refine the microstructure. It
is seen in FIG. 8, in which some finely distributed dross and
associated pores can also be seen in the structure. The mechanical
properties of the castings appear to be remarkable insensitive to
most defects of this variety and size. FIG. 9 illustrates a similar
fine eutectic after a solution heat treatment. In FIG. 9, the
eutectic has coarsened somewhat to reduce its interfacial energy as
is common for two-phase structures submitted to high temperature
treatment.
[0077] In principle, although not normally desirable, it would be
possible to allow all of the dendrites to freeze with the coarse
structure, making only a late application of the ablation cooling,
such as, for instance, at point "f" in FIG. 5. In this case some of
the eutectic would have frozen with a coarse eutectic structure.
Such a structure is shown in FIG. 10. The final regions of the
eutectic that freezes with the benefit of ablation cooling adopt
the extremely fine structure and are generally free from porosity
and exhibit only fine iron-rich phases, which are generally too
small to be seen in FIG. 10. At higher magnification, a few
iron-rich phases can be seen, as shown in FIG. 11.
[0078] For mixed dendrite/eutectic alloys, most of the benefits of
ablation are enjoyed by those structures seen in FIGS. 10 and 11
because progressive solidification of the residual liquid will
still be effective to feed the casting directionally. The alloy in
FIG. 10, for example, has been heat treated, therefore to some
extent coarsening all of the silicon particles in the eutectic
phase.
[0079] In practice, however, it is desirable, and easily achieved,
for some coarse dendritic structure to be formed prior to the
application of the ablation (starting point "d" in FIG. 5). The
usual resulting dendritic microstructure is therefore dual in the
sense described above and includes relatively uniformly fine
eutectic.
[0080] As before, if coolant is applied extremely late, for
instance following path "gq" in FIG. 5, the action of ablation
cannot influence the solidification microstructure of the casting
because, of course, the casting has fully solidified prior to any
application of a coolant. Such cooling of a casting does not form
part of this patent application, and falls into the casting
production regime well known to those skilled in the art.
[0081] For conventionally cooled castings (those that adopt the
cooling path ending in "h" or "q") the last regions of the casting
to solidify often contain porosity. In addition, such castings when
made in a typical aluminum alloy such as A356 alloy, often contain
thin platelets of beta-iron precipitates that further impair
properties.
[0082] Ablation-cooled castings, including both dendritic castings
and dendritic/eutectic castings, comprising a fine solidification
microstructure are generally free of defects that are often found
in castings formed by conventional casting methods. In one
embodiment, a casting comprising a fine solidification
microstructure portion is substantially free from porosity. The
rapid freezing and directional feeding created by ablation reduces
both gas and shrinkage porosity. In another embodiment, a casting
comprising a fine solidification microstructure portion is
substantially free of large damaging iron-rich platelets. In other
embodiments, a casting is substantially free of both porosity and
iron-rich platelets. Without being bound to any particular theory,
the reduction in size of the iron-rich platelets may be the result
of the more rapid quench of the liquid alloy. The reduction of
porosity also benefits from this speed. In addition, it is
significantly aided by the naturally progressive action of the
ablation process, in which the cooling action of water (or other
fluid) is moved steadily along the length of the casting to drive
the solidification in a highly directional mode towards the source
of feed metal. Furthermore, the maintenance of a relatively narrow
pasty zone by the imposition of a high temperature gradient in this
way is highly effective in assisting the feeding of the
casting.
[0083] The substantial reduction or elimination of shrinkage
porosity is significant, and may be restated as follows. Shrinkage
porosity would normally be expected in regions of the casting such
as an unfed hot spot. In principle, however, these regions can be
fed if the freezing process is carried out directionally. The water
or other cooling fluid is applied to ablate the mold and cool and
cause solidification in the casting progress systematically,
creating a uniquely strong directional temperature gradient. Thus,
those regions that would have been isolated from feed liquid in a
conventional casting are easily and automatically fed to soundness,
or greatly improved soundness, when the benefits of the invention
are correctly applied.
[0084] For this reason, alloys that cannot normally be cast as
shaped castings because of hot-shortness problems, such as the
wrought alloys 6061 and 7075, etc., or with long freezing range
such as alloys 7075 and 852, can easily and beneficially be cast
into a shaped form via ablation techniques. In addition, the
ablated castings are characterized by a solidification
microstructure that is immediately identifiable as being unique in
a shaped casting.
[0085] An aggregate molded shaped metal casting comprising a fine
solidification microstructure region is further described with
reference to the following examples. The examples are merely for
the purpose of illustrating potential embodiments of a shaped metal
casting having a fine solidification microstructure region and are
not intended to be limiting embodiments thereof.
EXAMPLES
[0086] In one example of the application of the technique described
in this application, a single test bar was molded of diameter 20 mm
and length 200 mm furnished with a small conical pouring basin at
one end that was filled to act as a feeder. The mold material was
silica sand bonded with a water-soluble inorganic binder as
described in our co-pending application Ser. No. 10/614,601.
[0087] Thermocouples were inserted into the mold cavity at the base
of the feeder, and at the base of the cavity. Two additional
thermocouples were located at equal intervals along the axis. These
four thermocouples were labeled TC1 (riser), TC2 (top midsection),
TC3 (bottom midsection) and TC4 (bottom).
[0088] An aluminum alloy 6061 at a temperature of 730.degree. C.
(1350.degree. F.) was poured into the cavity, arranged with its
axis vertical. Within approximately 10 seconds, water at 20.degree.
C. (68.degree. F.) was then applied from nozzles directed at the
base of the mold so as to start the ablation of the mold from the
base upward. The rate of upward progression of the ablation front
was approximately 25 mm/s.
[0089] Cooling traces of the four thermocouples were recorded as
seen in FIG. 12. The thermocouple TC4 is seen to cool rapidly,
signaling the freezing and cooling to below the boiling point of
water in only about 2 seconds. At this time the thermocouple
immediately above, TC3, still records that the metal is still
molten, and that cooling has only just begun. This pattern is
repeated successively up the mold. (The jump in temperature for TC2
records the unintentional momentary loss of cooling water in this
experiment). The thermal traces confirm that the temperature
gradient caused by the application of ablation was sufficient to
freeze the melt and cool it to near ambient temperatures within a
distance of less than the spacing between thermocouples (50 mm).
Furthermore, the effect was easily and accurately sustainable for
the length of an average automotive casting.
[0090] In a second example an automotive knuckle casting was made
in alloy A356. Many knuckle castings have a reputation for being
difficult to cast because of their complexity, having heavy
sections distant from points where feeders can be added. This
casting was no exception. The casting was filled with metal at
750.degree. C. (1385.degree. F.) on a tilt pouring station, taking
8 seconds to fill, resulting in an excellent surface finish. The
feeder (riser) was located at the far end of the casting from the
pouring cup and down-sprue. Thermocouples in the sprue and feeder
are illustrated in FIG. 13. It is seen that freezing took place in
the sprue, being the first to ablate, in approximately 20 seconds.
Freezing was then caused to progress across the casting, arriving
at the feeder approximately 90 seconds later, at which point the
feeder is caused to freeze at a similar time of only about 20
seconds.
[0091] To achieve ablation for this casting, three banks of water
spray nozzles were employed, with water pressure at approximately
0.7 bar (approximately 10 psi) and temperature approximately
40.degree. C. (approximately 100.degree. F.).
[0092] The casting was found to be completely sound, and with
properties exceeding specification.
[0093] In a third example a control arm casting, a
steering/suspension component of an automobile, was molded in the
mold material as specified for Example 1. The mold was poured with
A356 alloy of approximately composition Al-7Si-0.35Mg-0.2Fe at
approximately 700.degree. C. (approximately 1400.degree. F.).
Ablation cooling of this mold produced a casting that was
subsequently cut up and machined to produce tensile test bars that
were subject to a T6 heat treatment. The solution treatment was
538.degree. C. (1000.degree. F.) for 0.5 hours, water quench at
26.degree. C. (78.degree. F.) and age at 182.degree. C.
(360.degree. F.) for 2.5 hours. Four test bars were cut from each
of three castings numbered 45, 46 and 47. The bars were subjected
to tensile testing and the results are listed, together with
averages, in FIG. 14. (The one low extension value of 9% was
attributed to a large oxide inclusion since the control of the melt
quality was known to be less than optimum on this occasion.) The
results are compared with those from competitive casting processes
in FIG. 15. The properties are clearly attractive, exceeding those
of all current best competitive processes.
[0094] It should be appreciated that certain potential process
conditions could result in a conventional microstructure from an
ablated mold when using the ablation casting process. As an
example, an 852 aluminum alloy (Al-6Sn-2Cu-1Ni-0.75Mg alloy) is
known as a long range freezing alloy, wherein the eutectic freezes
approximately at the melting point of tin (232 C, 610 F). This
alloy was ablated using the ablation casting process. The mold was
symmetrical, allowing the identical mold halves to be produced from
a single sided pattern and then assembled. The metal was poured at
or near 700 C (1275 F). The casting section thickness was
approximately 75 mm (3 inch) in diameter. The pouring of the mold
by gravity was achieved in 10 seconds. The mold was then left to
sit for a period of nearly 180 seconds, to achieve a mostly
solidified alpha phase. After this period of normal solidification
rate being controlled by the molding aggregate (in this case silica
sand), the mold was ablated.
[0095] The ablation conditions were as follows. Water pressure used
for ablation was approximately 1 bar (15 psi). The spray volume was
limited by the spray nozzles. However, the water volume is nearly
insignificant as the pressure controls the water impingement
against the as cast surface. This procedure captured a conventional
but uniform cooling rate that yielded similar properties to that
produced by a metal mold (i.e., a permanent mold, see FIG. 16). In
this connection, FIG. 16 shows the spectrum of various casting
processes and the relationship between dendrite cell size and
solidification rate for aluminum alloys. A conventional permanent
mold microstructure is illustrated in FIG. 17a. FIGS. 17b and c
show the same alloy but now created using the ablation process. In
all three of FIGS. 17a-c, the same magnification, 100.times., was
employed. The final eutectic structure was closely similar to that
produced by a permanent mold. Although such a conventional
microstructure can be achieved in the ablation process, in some
conditions, those microstructures unique to ablation, including
extremely fine phases possibly of dendrites, but more often of
extremely fine eutectic, can also be observed.
[0096] To use the ablation process to capture a conventional
microstructure (such as that resulting from a permanent mold),
several variables have significant potential. An important
parameter is the mold aggregate itself. In addition, the volume,
pressure and temperature of the cooling medium used in removing the
mold while simultaneously causing solidification of the metal are,
of course, also important. During their advance along the length of
the casting, the dwell time of the cooling sprays upon the casting
can be beneficially adjusted to allow for the local surface to
volume ratio (the geometrical modulus). The rate of dissolution of
the mold binder can be reduced to slow the rate of ablation of the
mold, and so reduce the rate of thermal extraction. This could
limit the rate so as to produce a conventional microstructure.
Furthermore, the water pressure can be varied. At first, a higher
pressure can be used to remove the aggregate of the mold. Then, the
pressure can be reduced to create a cooling rate that would be akin
to that of a conventional metal mold process.
[0097] Although a conventional microstructure may be attained in
this way, there are significant additional benefits offered by the
ablation process that make the ablation process uniquely desirable.
First, there is improved control of the residual stress within the
casting since the final solidification occurs with sufficient
liquid ahead of the final solidification front to allow for a
complete accommodation of thermal strains. Second, the porosity in
the casting is significantly reduced (practically to zero in the
majority of cases) because of the excellent directional
solidification. This occurs as a result of the high applied
temperature gradient that ablation uniquely achieves.
[0098] Aggregate molded or shaped castings having time
solidification microstructure have been described with reference to
the present disclosure and various exemplary embodiments. It will
be appreciated that variations or modifications may be within the
capabilities of a person skilled in the art and that the present
application and claims are intended to encompass such
modifications. It is intended that the development be construed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims and the equivalents
thereof.
* * * * *