Note: Descriptions are shown in the official language in which they were submitted.
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GRAIN REFINED TIN BRASS
This invention relates to copper alloys having high strength, good
formability and relatively high electrical conductivity. More particularly,
grain refinement of a tin brass is obtained by a controlled addition of iron,
cobalt or other element that initiates a peritectic reaction during
solidification.__
Throughout this patent application, all percentages are given in
weight percent unless otherwise specified.
Commercial tin brasses are copper alloys containing from 0.35%-4%
tin, up to 0.35% phosphorous, from 49% to 96% copper and the balance zinc.
The alloys are designated by the Copper Development Association (CDA) as
copper alloys C40400 through C49080.
One commercial tin brass is a copper alloy designated as C42500.
The alloy has the composition 87%-90% of copper, 1.5%-3.0% of tin, a
maximum of 0.05% of iron, a maximum of 0.35% phosphorous and the
balance zinc. Among the products formed from this alloy are electrical switch
springs, terminals, connectors, fuse clips, pen clips and weather stripping.
The ASM Handbook specifies copper alloy C42500 as having a
nominal electrical conductivity of 28% IACS (International Annealed Copper
Standard where "pure" copper is assigned a conductivity value of 100% IACS
at 20°C) and a yield strength, dependent on temper, of between 45 ksi
and 92
ksi. The alloy is suitable for many electrical connector applications, however
the yield strength is lower than desired.
It is known to increase the yield strength of certain copper alloys
2 5 through controlled additions of iron. For example, European Patent Office
Publication EP 0769563A 1 entitled "Iron Modified Phosphor-Bronze" that
was published on April 23, 1997, discloses the addition of 1.65% - 4.0% of
iron to phosphor bronze. The alloy has an electrical conductivity in excess of
30% IACS and an ultimate tensile strength in excess of 95 ksi.
3 0 Japanese patent application number 57-68061 by Furukawa Metal
Industries Company, Ltd. discloses a copper alloy containing 0.5%-3.0%,
each, of zinc, tin and iron. It is disclosed that iron increases the strength
and
heat resistance of the alloy.
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While the benefit of an iron addition to phosphor-bronze is known,
iron causes problems for the alloy. The electrical conductivity of the alloy
is
degraded and processing of the alloy is impacted by the formation of
stringers.
Stringers form when the alloy contains more than a critical iron content,
which
content is dependent on the alloy composition. The stringers originate when- --
properitectic iron particles precipitate from liquid prior to solidification
and
elongate during mechanical deformation. Stringers are detrimental because
they affect the surface appearance of the alloy and can degrade the
formability
characteristics.
In high copper (in excess of 85% Cu) tin brasses, the maximum
permissible iron content, as an impurity, is typically 0.05%. This is because
iron is known to reduce electrical conductivity and, through the formation of
stringers, deteriorate the bend properties.
Other metallic additions to the alloy that induce the formation of a
peritectic phase during solidification may substitute for the iron, either in
whole or in part. One particular addition is cobalt, while other suitable
additions include vanadium, niobium, iridium and molybdenum.
There exists, therefore, a need for an iron modified tin brass alloy that
does not suffer from the stated disadvantages of reduced electrical
2 0 conductivity and stringer formation.
Accordingly, it is an object of the invention to provide a tin brass
alloy having increased strength. It is a feature of the invention that the
increased strength is achieved by an addition of controlled amounts of a
combination of iron and zinc. It is another feature of the invention that by
2 5 processing the alloy according to a specified sequence of steps, a fine
microstructure is retained in the wrought alloy.
Among the advantages of the alloy of the invention are that the yield
strength is increased without a degradation in electrical conductivity. The
microstructure of a refined as-cast alloy, grain size less than 100 microns,
and
3 0 a wrought alloy, grain size of about 5-20 microns, is fine grain. Still
another
advantage is that the electrical conductivity is about equal to that of copper
alloy C42500 with a significant increase in yield strength.
_... .-_._._~_.._...._., r . , fi
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In accordance with the invention, there is provided a copper alloy.
This alloy consists essentially of from 1 % to 4% by weight of tin, from 0.8%
to 4.0% by weight of iron, from an amount effective to enhance iron initiated
' grain refinement to 20% by weight of zinc, up to 0.4% by weight of
phosphorus and the remainder is copper, as well as inevitable impurities. The
grain refined alloy has an average as-cast grain size of less than 100 microns
and an average grain size after processing of between about 5 and 20 microns.
The above stated objects, features and advantages will become more
apparent from the specification and drawings that follow.
Figure 1 is a flow chart illustrating one method of processing the
alloy of the invention.
Figure 2 graphically illustrates the effect of iron content on the yield
strength.
Figure 3 graphically illustrates the effect of iron content on the
1 S ultimate tensile strength.
Figure 4 graphically illustrates the effect of tin content on the yield
strength.
Figure 5 graphically illustrates the effect of tin content on the
ultimate tensile strength.
2 0 Figure 6 graphically illustrates the effect of zinc content on the yield
strength.
Figure 7 graphically illustrates the effect of zinc content on the
ultimate tensile strength.
The copper alloys of the invention are an iron modified tin brass.
2 5 The alloys consist essentially of from 1 % to 4% of tin, from 0.8% to 4.0%
of
iron, from 5% to 20% of zinc, up to 0.4% of phosphorus and the remainder is
copper along with inevitable impurities. As cast, the grain refined alloy has
an
average crystalline grain size of less than 100 microns.
When the alloy is cast by direct chill casting, in preferred
3 0 embodiments, the tin content is from 1.5% to 2.5% and the iron content is
from 1.6% to 2.2%. 1.6% of iron has been found to be a critical minimum to
achieve as-cast grain refinement. Most preferably, the iron content is from
1.6% to 1.8%.
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Tin
Tin increases the strength of the alloys of the invention and also
increases the resistance of the alloys to stress relaxation.
The resistance to stress relaxation is recorded as percent stress
remaining after a strip sample is preloaded to 80% of the yield strength in a
. --
cantilever mode per ASTM (American Society for Testing and Materials)
specifications. The strip is heated to 125 °C for the specified number
of hours
and retested periodically. The properties were measured at up to 3000 hours at
125 °C. The higher the stress remaining, the better the utility of the
specified
composition for spring applications.
However, the beneficial increases in strength and resistance to stress
relaxation are offset by reduced electrical conductivity as shown in Table 1.
Further, tin makes the alloys more difficult to process, particularly during
hot
processing. When the tin content exceeds 2.5%, the cost of processing the
alloy may be prohibitive for certain commercial applications. When the tin
content is less than 1.5%, the alloy lacks adequate strength and resistance to
stress relaxation for spring applications.
Table 1
Composition Electrical ConductivityYield Strength
(%IACS) (ksi) (MPa)
88.5% Cu
9.5% Zn 26 75 517
2% Sn
0.2% P
87.6% Cu
9.5% Zn 21 83 572
2.9% Sn
0.2% P
94.8% Cu
5% Sn 17 102 703
I 0.2% P
2 0 Preferably, the tin content of the alloys of the invention is from about
1.2% to about 2.2% and most preferably from about I.4% to about 1.9%.
.._..._m...._~.._.v.__._... . , , . ,
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Iron
Iron refines the microstructure of the as-cast alloy and increases
strength. The refined microstructure is characterized by an average grain size
' of less than 100 microns. Preferably, the average grain size is from 30 to
90
microns and most preferably, from 40 to 70 microns. This refined
microstructure facilitates mechanical deformation at elevated temperatures,
such as rolling at 850°C.
When the iron content is less than about 1.6%, the grain refining
effect is reduced and coarse crystalline grains, with an average grain size on
the order of 600-2000 microns, develop. When the iron content exceeds 2.2%,
excessive amount of stringers develop during hot working.
The effective iron range, 1.6%-2.2%, differs from the iron range of
the alloys disclosed in EP 0769563A 1 that discloses that grain refinement was
not optimized until the iron content exceeded about 2%. The ability to refine
the grain structure at lower iron contents in the alloys of the present
invention
was unexpected and believed due to a phase equilibrium shift due to the
inclusion of zinc. To be effective, this phase shift interaction requires a
minimum zinc content of about 5%.
Large stringers, having a length in excess of about 200 microns, are
2 0 expected to form when the iron content exceeds about 2.2%. The large
stringers impact both the appearance of the alloy surface as well as the
properties, electrical and chemical, of the surface. The large stringers can
change the solderability and electro-platability of the alloy.
To rnaxinuze the grain refinement and strength increase attributable
2 5 to iron without the detrimental formation of stringers, the iron content
should
be maintained between about 1.6% and 2.2% and preferably, between about
1.6% and 1.8%.
Zinc
The addition of zinc to the alloys of the invention would be expected
30 to provide a moderate increase in strength with some decrease in electrical
conductivity. While, as shown in Table 2, this occurred, surprisingly, with a
minimum of 5% zinc present, the grain refining capability of the iron addition
was significantly enhanced, as illustrated in Table 3.
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Table 2
Composition Electrical Tensile Strength
Conductivity (ksi) (MPa)
(% IACS)
1.8 Sn
2.2 Fe 33 99 683
balance Cu
I.8 Sn
2.2 Fe 29 99 683
Zn
balance Cu
I .8 Sn
2.2 Fe 25 108 683
Zn
balance Cu
('Tensile strength measured following 70% cold reduction)
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Table 3
Composition Grain Size
1.9 Fe
1.8 Sn Coarse
._
0.04 P
balance Cu
5 Zn
1.9 Fe
1.8 Sn Medium
0.04 P
balance Cu
7.5 Zn
1.9 Fe
1.8 Sn Fine
0.04 P
balance Cu
10 Zn
1.9 Fe
1.8 Sn Fine
0.04 P
balance Cu
I S Zn
3.3 Co
1.8 Sn Fine
0.04 P Fine
balance Cu
Preferably, the zinc content is from that effective to enhance iron
initiated grain refinement to about 20%. More preferably, the zinc content is
from about 5% to about 15% and most preferably, the zinc content is from
about 8% to about 12%.
Peritectic Reaction for Cast Grain Refinement
It is believed that the grain refining effectiveness of the iron addition
is due to the iron separating from the melt first, during solidification, as
numerous, relatively fine, dendritically shaped particles of fcc (face
centered
cubic) gamma iron. With continued cooling, these properitectic iron particles
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effectively nucleate cast grains of the alloy via the peritectic
solidification
reaction:
Fe + L ~--~ Cu (alloy),
effectively raising the nucleation rate, in turn resulting in cast grain
refinement.
Other metallic elements that undergo a similar peritectic
decomposition reaction with elemental or intermetallic properitectic particles
in a tin brass may also be used, subject to the proviso that the peritectic
composition does not require such a large amount of the addition that the
desirable properties of the tin brass, such as processing capability,
conductivity or bend formability, are severely degraded.
Cobalt is a suitable substitute for either a portion, or all, of the iron as
shown in Table 4.
Table 4
Composition Grain Size
10 Zn
2.7 Co Coarse
1.8 Sn
0.04 P
balance Cu
10 Zn
3.0 Co Coarse
1.8 Sn
0.04 P
balance Cu
10 Zn
3.3 Co Fine
1.8 Sn
0.04 P
balance Cu
From Table 4, the cobalt content, when used as the primary grain
refiner, should be in excess of about 3.0%. Preferably, the cobalt content is
between about 3.2% and 4.4% and most preferably from between 3.2% and
3.6%. Excessive amounts of cobalt should be avoided because coarse
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stringers of excess properitectic cobalt particles may occur and degrade alloy
properties.
Cobalt may be added as a partial substitute for iron. Cobalt less
effectively refines the grain structure of the alloys of the invention and the
substitution should satisfy the equation:
Fe+MCo = iron ranges specified above.
M is between 0.45 and 0.65, and preferably from 0.5 to 0.6. Most
preferably, the substitution is in the higher range, about 0.6 for lower
contents
of cobalt and about 0.5 for higher contents of cobalt with an approximate
delineation between the lower contents and the higher contents being a 2%
cobalt.
Other suitable properitectic particle formers include iridium in an
amount of from about 10% to about 20% and preferably in an amount of from
about 11% to 15%; niobium in an amount of from about 0.01% to about 5%
and preferably in an amount of from about 0.1 % to about 1 %; vanadium in an
amount of from about 0.01% to about S% and preferably in an amount of from
about 0.1 % to about 1 %; and molybdenum in an amount of from about 0.5%
to about 5% and preferably in an amount of from about 1% to about 3%.
One or more of these other peritectic reaction initiators may
2 0 substitute, in whole or in part, for cobalt or iron.
Other additions
Phosphorous is added to the alloy for conventional reasons, to
prevent the formation of copper oxide or tin oxide precipitates and to promote
2 5 the formation of iron phosphides. Phosphorous causes problems with the
processing of the alloy, particularly with hot rolling. It is believed that
the
iron addition counters the detrimental impact of phosphorous. At least a
minimal amount of iron must be present to counteract the impact of the
phosphorous.
3 0 A suitable phosphorous content is any amount up to about 0.4%. A
preferred phosphorous content is from about 0.03% to 0.3%.
Other elements that remain in solution when the copper alloy
solidifies may be present in amounts of up to 20% and may substitute, at a 1:1
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atomic ratio, for either a portion, or all, of the zinc. The preferred ranges
of
these solid-state soluble elements are those specified for zinc. Among the
preferred elements are manganese and aluminum.
Less preferred are additions of elements that affect the properties of
the alloy. Although, less preferred, additions such as nickel, magnesium,
beryllium, silicon, zirconium, titanium, chromium and mixtures thereof may
be included.
For example, nickel additions severely reduce electrical conductivity.
As a result, the less preferred additions are preferably present in an amount
of
less than about 0.4% and most preferably, in an amount of less than about
0.2%. Most preferably, the sum of all less preferred alloying additions is
less
than about 0.5%.
Processing
The alloys of the invention are preferably processed according to the
flow chart illustrated in Figure 1. An ingot, being an alloy of a composition
specified herein, is cast 10 by a conventional process such direct chill
casting.
The alloy is hot rolled 12, at a temperature of from about 650°C
to about
950°C and preferably, at a temperature of between about 825°C
and 875°C.
Optionally, the alloy is heated 14 to maintain the desired hot roll 12
2 0 temperature.
The hot rolling reduction is, typically, by thickness, up to 98% and
preferably, from about 80% to about 95%. The hot rolling may be in a single
pass or in multiple passes, provided that the temperature of the ingot is
maintained at above 650°C.
2 5 After hot rolling 12, the alloy is, optionally, water quenched 16. The
bars are then mechanically milled to remove surface oxides and then cold
rolled 18 to a reduction of at least 60%, by thickness, from the gauge at
completion of the hot roll step 12, in either one or multiple passes.
Preferably,
the cold roll reduction 18 is from about 60%-90%.
3 0 The strip is then annealed 20 at a temperature between about 400°C
and about 600°C for a time of from about 0.5 hour to about 8 hours to
recrystallize the alloy. Preferably, this first recrystallization anneal is at
a
temperature between about S00°C and about 600°C for a time
between 3 and
,.f
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hours. These times are for bell annealing in an inert atmosphere such as
nitrogen or in a reducing atmosphere such as a mixture of hydrogen and
nitrogen.
The strip may also be strip annealed, such as for example, at a
5 temperature of from about 600°C to about 950°C for from 0.5
minute to 10 --
minutes.
The first recrystallization anneal 20 causes additional precipitates of
iron and iron phosphide to develop. These precipitates control the grain size
during this and subsequent anneals, add strength to the alloy via dispersion
hardening and increase electrical conductivity by drawing iron out of solution
from the copper matrix.
The bars are then cold rolled 22 a second time to a thickness
reduction of from about 30% to about 70% and preferably of from about 35%
to about 45%.
The strip is then given a second recrystallization anneal 24, utilizing
the same times and temperatures as the first recrystallization anneal. After
both the first and second recrystallization anneals, the average grain size is
between 3 and 20 microns. Preferably, the average grain size of the processed
alloy is from 5 to 10 microns.
2 0 The alloys are then cold rolled 26 to final gauge, typically on the
order of between 0.25 mm {0.010 inch) and 0.38 mm (0.015 inch). This final
cold roll imparts a spring temper comparable to that of copper alloy C51000.
The alloys are then relief annealed 28 to optimize resistance to stress
relaxation. One exemplary relief anneal is a bell anneal in an inert
atmosphere
2 5 at a temperature of between about 200 ° C and about 300 ° C
for from 1 to 4
hours. A second exemplary relief anneal is a strip anneal at a temperature of
from about 250°C to about 600°C for from about 0.5 minutes to
about 10
minutes.
Following the relief anneal 28, the copper alloy strip is formed into a
3 0 desired product such as a spring or an electrical connector.
The advantages of the alloys of the invention will become more
apparent from the examples that follow.
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EXAMPLES
Example 1
Copper alloys containing 10.5% zinc, 1.7% tin, 0.04% phosphorous,
between 0% and 2.3% iron and the balance copper were prepared according to
the process of Figure 1. Following the relief anneal 28, the yield strength
and-
the ultimate tensile strength of sample coupons, 51 mm (2 inch) gauge length,
were measured at room temperature (20°C).
The 0.2% offset yield strength and the tensile strength were measured
on a tension testing machine (manufactured by Tinius Olsen, Willow Grove,
PA).
As shown in Figure 2, increasing the iron from 0% to 1 % led to a
significant increase in yield strength. Further increases in the iron content
had
only a minimal effect on strength,but increased the likelihood of stringers.
Figure 3 graphically illustrates a similar relationship between the iron
content and the ultimate tensile strength.
Example 2
Copper alloys containing 10.4% zinc, 1.8% iron, 0.04% phosphorous,
between 1.8% and 4.0% tin and the balance copper were processed according
2 0 to Figure 1. Test coupons in the relief anneal condition 28, were
evaluated for
yield strength and ultimate tensile strength.
Figure 4 graphically illustrates that increasing the tin content leads to
an increase in yield strength. While Figure 5 graphically illustrates the same
effect from tin additions for the ultimate tensile strength.
2 S Since the strength increase is monatomic with the amount of tin while
the conductivity decreases, the tin content should be a trade-off between
desired strength and conductivity.
Example 3
3 0 Copper alloys containing 1.9% iron, 1.8% tin, 0.04% phosphorous,
between 0% and 15% zinc and the balance copper were processed according
to Figure 1. Test coupons in the relief anneal condition 28, were evaluated
for
yield strength and ultimate tensile strength.
,.r
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Figure 6 graphically illustrates that a zinc content of less than about
5% does not contribute to the strength of the alloy, and as discussed above,
does not enhance the grain refining capability of the iron. Above 5% zinc, the
alloy strength is increased, although a decrease in electrical conductivity is
experienced. --
Figure 7 graphically illustrates the same effect from zinc additions for
the ultimate tensile strength of the alloy.
Example 4
Table 5 illustrates a series of alloys processed according to Figure i.
Alloy A is an alloy of the type disclosed in EP 0769563A1. Alloys B and C
are in accordance with the present invention and alloy D is conventional
copper alloy C510. All properties were measured when the alloy was in a
spring temper following a 70% cold roll reduction in thickness.
Table 5
Alloy Composition Elec. Tensile Yield
Conduct. Strength Strength
%IACS (ksi) MPa (ksi) MPa
A 1.8 Sn 33% (99) 682 (96) 662
2.2 Fe
0.06 P
balance Cu
B 1.8 Sn 29% (99) 682 (94) 648
2.2 Fe
0.06 P
5.0 Zn
balance Cu
C 1.8 Sn 25% (108) 745 (101) 696
2.2 Fe
0.06 P
10.0 Zn
balance Cu
D 4.27 Sn 17% (102) 703 (96) 662
0.033 P
balance Cu
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Table 5 shows that the addition of 5% zinc did not increase the
strength of the alloy and slightly reduced electrical conductivity. A 10% zinc
addition had a favorable impact on the strength.
The benefit of the zinc addition is more apparent in view of Table 6
where the strength to rolling reduction is compared. ._
Table 6
Alloy % Red. YS TS MBR/t MBR/t
GW BW
A 25 552 (80) 572 (83) 1.0 1.3
C 25 579 (84) 607 (88) 0.8 1.6
A 33 572 (83) 593 (86) 1.0 1.3
C 33 614 (89) 648 (94) 0.9 2.1
A 58 662 (96) 683 (99) 1.7 3.9
C 60 662 (96) 703 (102) 1.6 6.4
A 70 690 (100) 717 (104) 1.9 6.3
C 70 696 ( 101 745 { 108)I .9 s 7
)
~~o Ked. = percent reduction Fn thickness at the final cold step (reference
numeral 26 in Figure 5).
YS = Yield strength in MPa and (ksi).
1 o TS = Tensile strength in MPa and (ksi).
MBR/t (GW) = Good way bends about a 180° radius of curvature.
MBR/t (BW) = Bad way bends about a 180° radius of curvature.
A further benefit of the zinc addition is the improved good way bends
achieved with alloy C. Bend formability was measured by bending a 12.7
mm (0.5 inch) wide strip 180° about a mandrel having a known radius of
curvature. The minimum mandrel about which the strip could be bent without
cracking or "orange peeling" is the bend formability value. The "good way"
bend is made in the plane of the sheet and perpendicular to the longitudinal
2 0 axis (rolling direction) during thickness reduction of the strip. "Bad
way" is
parallel to the longitudinal axis. Bend formability is recorded as MBR/t, the
,.t
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minimum bend radius at which cracking or orange peeling in not apparent,
divided by the thickness of the strip.
Usually, an increase in strength is accompanied by a decrease in bend
formability. However, with the alloys of the invention, an addition of 10%
zinc increases both the strength and the good way bends.
Example 5
Alloys of the compositions indicated in Table 7, with the balance
being copper, were processed according to Process 1. Table 7 shows the
effectiveness of cobalt as a partial substitute for iron in the tin brass
alloys of
the invention.
Table 7
Zn Sn Fe Co P As-castCR 22% CR 65%
(RA) (RA)
Grain YS/UTS/EL)YS/UTS/EL
Size
(MPa/MPa/%)(MPa/MPaI%)
(ksi/ksil%)(ksi/ksil%)
I
10.4 1.80 1.5 0.5 0.04 fine 572/60017696/745/4
(83/87/7)(101/108/4)
10.4 1.80 1.78--- 0.04 fine 558/586!117031795/2
(81/85/11)(102/108/2)
10.4 1.80 1.5 --- 0.04 coarse ---- ----
YS = yield strength
UTS = ultimate tensile strength
EL = elongation
CR = cold roll
RA = relief anneal
Table 8 illustrates the magnetic permeability of hot rolled plate when
formed from cobalt containing tin brass is higher than the magnetic
permeability of the same alloy when an equivalent amount of iron is present,
using O.bCo = Fe as the equivalency relationship.
2 5 Table 8
Zn Sn Fe Co P As-cast Magnetic
Grain Permeability
Size
(Hot Rolled
Plate)
10.2 1.87 2.02 --- 0.03 fine 1.05-1.10
10.5 1.80 --- 3.3 0.04 fine 1.2
While described particularly in terms of direct chill casting, the alloys
of the invention may be cast by other processes as well. Some of the
3 0 alternative processes have higher cooling rates such as spray casting and
strip
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casting. The higher cooling rates reduce the size of the properitectic iron
particles and are believed to shift the critical maximum iron content to a
higher value such as 4%.
It is apparent that there has been provided in accordance with the
invention an iron modified phosphor bronze that futly satisfies the objects, --
means and advantages set forth hereinabove. While the invention has been
described in combination with embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to those skilled
in
the art in light of the foregoing description. Accordingly, it is intended to
1 o embrace all such alternatives, modifications and variations as fall within
the
spirit and broad scope of the appended claims.
~ ,.r
