U.S. patent application number 09/998248 was filed with the patent office on 2002-05-30 for method for making positive grids and lead-acid and batteries using such grids.
This patent application is currently assigned to GNB Technologies, Inc.. Invention is credited to Rao, Purushothama.
Application Number | 20020064711 09/998248 |
Document ID | / |
Family ID | 23095043 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020064711 |
Kind Code |
A1 |
Rao, Purushothama |
May 30, 2002 |
Method for making positive grids and lead-acid and batteries using
such grids
Abstract
A method for making positive grids for lead-acid batteries from
calcium-tin-silver lead-based alloys comprises casting an alloy
strip and then rolling the strip at a temperature between about the
solvus temperature and the peritectic temperature of the alloy,
quenching the rolled strip, then, preferably, heat aging at a
temperature of 200.degree. F. to 500.degree. F., and fabricating
into the positive grid, such grids having enhanced mechanical and
high temperature corrosion resistance characteristics.
Inventors: |
Rao, Purushothama; (Aurora,
IL) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
TWO PRUDENTIAL PLAZA, SUITE 4900
180 NORTH STETSON AVENUE
CHICAGO
IL
60601-6780
US
|
Assignee: |
GNB Technologies, Inc.
Alpharetta
GA
|
Family ID: |
23095043 |
Appl. No.: |
09/998248 |
Filed: |
November 30, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09998248 |
Nov 30, 2001 |
|
|
|
09285624 |
Apr 3, 1999 |
|
|
|
Current U.S.
Class: |
429/245 ; 29/2;
429/242 |
Current CPC
Class: |
H01M 4/84 20130101; Y10T
29/49108 20150115; Y02E 60/10 20130101; Y10T 29/10 20150115; H01M
4/685 20130101 |
Class at
Publication: |
429/245 ; 29/2;
429/242 |
International
Class: |
H01M 004/68; H01M
004/74; H01M 004/66 |
Claims
I claim:
1. A lead-acid battery comprising positive and negative plates and
a separator positioned between said positive and negative plates,
said positive plates having a grid of an alloy comprising about
0.025% to 0.065% calcium, about 0.4% to 1.9% tin, about 0.02% to
0.045% silver, and the remainder lead, the percentages being by
weight of the alloy, said grid being of an alloy strip rolled at a
temperature from about the solvus temperature up to less than the
peritectic temperature of the alloy composition.
2. The battery of claim 9 wherein the rolled strip has been
heat-aged at a temperature of about 200.degree. F. to 500.degree.
F. for a time to increase the population of special grain
boundaries in the rolled strip.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATION
[0001] This is a divisional application of U.S. Nonprovisional
Patent Application No. 09/285,624, filed Apr. 3, 1999 the
disclosure of which is hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to lead-acid cells and
batteries, and, more particularly, to a method for making positive
grids using calcium-tin-silver lead-based alloys.
BACKGROUND OF THE INVENTION
[0003] Over the last 20 or so years, there has been substantial
interest in automotive-type, lead-acid batteries which require,
once in service, little, or more desirably, no further maintenance
throughout the expected life of the battery. This type of battery
is usually termed a "low maintenance" or "maintenance-free
battery." The terminology maintenance-free battery will be used
herein to include low maintenance batteries as well. This type of
battery was first commercially introduced in about 1972 and is
currently in widespread use.
[0004] It has been well recognized over the years that lead-acid
batteries are perishable products. Eventually, such batteries in
service will fail through one or more of several failure modes.
Among these failure modes are failure due to positive grid
corrosion and excessive water loss. The thrust of maintenance-free
batteries has been to provide a battery that would forestall the
failure during service for a period of time considered commensurate
with the expected service life of the battery, e.g., three to five
years or so.
[0005] To achieve this objective, the positive grids used initially
for maintenance-free batteries typically had thicknesses of about
60 to about 70 mils or so. The batteries were likewise configured
to provide an excess of the electrolyte over that needed to provide
the rated capacity of the battery. In that fashion, by filling the
electrolyte to a level above that of the top of the battery plates,
maintenance-free batteries contained, in effect, a reservoir of
electrolyte available to compensate for the water loss occurring
during the service life of the battery. In other words, while the
use of appropriate grid alloys will reduce water loss during the
service life of the battery, there will always be some water loss
in service.
[0006] The principal criteria for providing satisfactory positive
grids for starting, lighting, and ignition ("SLI") automotive
lead-acid batteries are stringent and are varied. In general, and
by way of a summary, suitable alloys must be capable of being cast
into satisfactory grids and must impart adequate mechanical
properties to the grid. Still further, the alloys must impart
satisfactory electrical performance to the battery in the intended
application. Satisfactory alloys thus must impart the desired
corrosion resistance, and avoid positive active material softening
that will result in a loss of capacity.
[0007] More particularly, and considering each of the criteria
previously summarized, suitable alloys in the first instance must
be capable of being cast into grids by the desired technique, i.e.,
the cast grids must be low in defects as is known (e.g., relative
freedom from voids, tears, microcracks and the like). Such casting
techniques range from conventional gravity casting ("book molds" or
the like) to continuous processes using expanded metal techniques
and to a variety of processes using alloy strips from which the
grids are made, e.g., by stamping or the like.
[0008] The resulting cast grids need to be strong enough to endure
processing into plates and assembly into batteries in
conventionally used equipment. Even further, suitable grids must
maintain satisfactory mechanical properties throughout the expected
service life. Any substantial loss in the desired mechanical
properties during service life can adversely impact upon the
battery performance as will be more fully discussed
hereinafter.
[0009] Considering now the electrochemical performance required,
the grid alloy for the positive plates must yield a battery having
adequate corrosion resistance. Yet, the use of a continuous direct
casting process, or other processes using grid alloy strips,
desirable from the standpoint of economics, ostensibly can
compromise corrosion resistance. Continuous processes thus orient
the grains in the grids, thereby making the intergranular path
shorter and more susceptible to corrosion attack and to early
failures. Casting a thick strip and then cold rolling or the like
to the grid thickness desired even further exacerbates the
problem.
[0010] Positive grid corrosion thus can be a primary mode of
failure of SLI lead-acid batteries, particularly at higher ambient
temperatures. When positive grid corrosion occurs, this lowers the
electrical conductivity of the battery itself. Battery failure
occurs when the corrosion-induced decrease in the conductivity of
the grid causes the discharge voltage to drop below a value
acceptable for a particular application.
[0011] A second failure mechanism, also associated with positive
grid corrosion, involves failure due to "grid growth." During the
service life of a lead-acid battery, the positive grid corrodes;
and the corrosion products form on the surface of the grid. In most
cases, the corrosion products form at the grain boundaries and grid
surface of the positive grid where the corrosion process has
penetrated the interior of the "wires" of the grid. These corrosion
products are generally much harder than the lead alloy forming the
grid and are less dense and thus occupy a larger volume. Due to the
stresses created by these conditions, the grid alloy moves or grows
to accommodate the bulky corrosion products. This physical
displacement of the grid causes an increase in the length and/or
width of the grid. The increase in size of the grid may be
non-uniform. A corrosion-induced change in the dimension of the
grid is generally called "grid growth" (or sometimes "creep").
[0012] When grid growth occurs, the movement and expansion of the
grid begins to break the electrical contact between the positive
active material and the grid itself. This movement and expansion
prevents the passage of electricity from some reaction sites to the
grid and thereby lowers the electrical discharge capacity of the
cell. As this grid growth continues, more of the positive active
material becomes electrically isolated from the grid and the
discharge capacity of the cell decays below that required for the
particular application. The mechanical properties of the alloy thus
are important to avoid undue creep during service life.
[0013] As is now appreciated, what has occurred in the last several
years is the substantial increase in the under-the-hood temperature
to which the battery is exposed in automobile service. Obviously,
the under-the-hood temperature is particularly high in the warmer
climates. One automobile manufacturer has perceived that the
temperature to which an SLI battery is exposed under-the-hood in
such warmer climates has risen from about 125.degree. F. to about
165.degree. F.-190.degree. F. in new automobiles.
[0014] The specific temperature increase which is involved is not
particularly important. What is important is that such
under-the-hood temperatures have in fact increased. The impact of
the under-the-hood vehicle service temperature increases on the
failure modes has been to substantially increase the occurrence of
premature battery failures. The incidence of premature battery
failures due to excessive positive grid corrosion has been
significant.
[0015] A breakthrough was achieved in utilizing the positive grid
alloys disclosed in U.S. Pat. No. 5,298,350 to Rao. Utilizing such
positive grid alloys provided batteries that exhibited substantial
improvements in service life and have effectively eliminated
premature positive grid corrosion at elevated temperatures as being
the primary mode of failure.
[0016] The subject Rao patent has spurred considerable interest in
the type of positive grid alloys utilized, i.e., calcium-tin-silver
lead-based alloys. Thus, substantial effort has been made to
investigate this type of alloy through testing of various
properties with varying levels of the alloying constituents.
[0017] The interest has also extended to utilizing this family of
alloys in sealed lead-acid cells and batteries (often termed
"VRLA," viz., valve-regulated lead-acid). Sealed lead-acid cells
and batteries are widely used in commerce today for various
applications. In one type of application, generally termed as
stationary applications, lead-acid cells and batteries are used,
for example, for load leveling, emergency lighting in commercial
buildings, as standby power for cable television systems, and in
uninterruptible power supplies. The uninterruptible power supply
may be used to back up electronic equipment, such as, for example,
telecommunication and computer systems, and even as a back up
energy source for entire manufacturing plants. When the principal
power supply to the electronic equipment or the like has been cut
off, such as during a power outage, the sealed cells (typically
many electrically connected together) provide a source of reserve
power to allow the telecommunication or computer system to remain
operational until the principal power system can be restored. The
uninterruptible power supply also will accommodate short, or
intermittent, losses in power, so that the function of electronic
equipment will not be impaired during a brief power outage.
[0018] In addition, there are many applications where sealed
lead-acid cells and batteries are used in what are termed motive
power applications. Such applications are thus electrical vehicles,
fork-lift trucks, and the like, where such cells and batteries are
used as the power source.
[0019] In many of these applications where sealed cells and
batteries are used, the size of such cells and batteries and the
necessary service life requirements necessitate that relatively
thick grids be utilized in relation to the thickness of grids
typically utilized for SLI applications. More particularly, grid
thicknesses of 0.1 inch or more are often required.
[0020] What has also occurred in the last several years are a
variety of processes which utilize alloy strips to make grids, and
often in a continuous, or semi-continuous, fashion. The
desirability of such continuous plate-making processes is to
achieve higher grid production rates as well as to improve plate
quality in comparison to the production and quality issues
associated with using conventional gravity cast techniques. One
process for making a directly cast alloy continuous strip from
molten lead alloys is commercially available (Cominco Ltd.,
Toronto, Canada). U.S. Pat. No. 5,462,109 to Vincze et al.
discloses a method for making a directly cast strip. This directly
cast strip can then be converted by known expanded metal
fabrication techniques to achieve a continuous source of an
expanded lead-alloy grid mesh strip suitable for conversion into
positive lead-acid battery plates. U.S. Pat. No. 5,434,025 to Rao
et al. discloses batteries and positive grids made from cast strips
which achieve high temperature corrosion resistance while utilizing
calcium-tin-silver lead-based alloys.
[0021] Other types of grid manufacturing processes involve first
casting a continuous length billet having a thickness in the range
of, for example, 0.25 to 1.0 inch. Such a billet is then
mechanically rolled continuously to a thickness reduction in the
range of 10-15:1. The finished roll strip may then be made into
grids by a variety of commercially available techniques. Such
techniques have often been termed "expanded metal" techniques,
which techniques typically involve slitting the strip and expanding
the slit strip, creating a grid mesh having diamond-shaped
openings, hence the reference to the "expanded metal" terminology.
Alternatively, die punching or any other technique proposed and/or
used to make grids from the rolled strip can be employed.
[0022] What has not been appreciated, it is believed, is the
substantial adverse effect upon the microstructure of the
thus-rolled strips and the concomitant effect upon the desired
corrosion resistance and grid growth characteristics of grids made
from such strips. More particularly, in the rolling process whereby
the alloy strip is created from the cast billet, the stability of
the microstructure may be lowered, both non-uniform and higher
rates of matrix recrystallization can result. Such results can
increase the susceptibility to intergranular corrosion. These
higher rates of matrix recrystallization in such rolled alloys may
well be due to the excess strain energy absorbed during rolling.
The recrystallization temperature of the alloy may thereby be
lowered due to the excess strain energy and the magnitude of
lattice defects present in the matrix, in turn, due to the heavy
structure deformation at the lower recrystallization
temperature.
[0023] The significance, at least in part, is that the
precipitation in the matrix in Pb--Ca--Sn and Pb--Ca--Sn--Ag alloys
will be non-uniform; and recrystallization may well result in
localized movement of large angle grain boundaries.
Recrystallization results in non-uniform grain growth in the
lead-rich matrix. Excessive grain boundary movements also result in
pulling adjacent precipitate particles together which could
coalesce to form agglomerates. This will tend to increase the
precipitate size, also increasing the interparticle spacing, both
of which will reduce the effectiveness of the precipitates in
matrix strengthening, thereby contributing to loss of ductility and
toughness. Precipitate coarsening could also lead to grain boundary
precipitation and accordingly make the alloy more susceptible to
catastrophic intergranular corrosion in battery life. Such matrix
recrystallization may also result in reduction in the creep rate of
these alloys which will, in turn, exhibit higher grid growth rates
in battery service so as to limit the useful service life.
[0024] In view of the production and quality improvement capable of
being achieved by making grids from cast strips, there exists a
clear need for methods capable of utilizing the substantial
benefits that can be achieved using the calcium-tin-silver
lead-based alloys, while not unduly limiting the potential
advantageous properties.
[0025] Accordingly, it is an object of the present invention to
provide a method for making positive grids and plates for a
lead-acid battery utilizing a rolled or wrought strip.
[0026] It is an additional object of the invention to provide
lead-acid cells and batteries utilizing positive grids made with
such a method.
[0027] Another object of this invention is to provide such method
in which the wrought strip produced, made utilizing
calcium-tin-silver lead-based alloys, is characterized by superior
microstructure stability, stable and uniformly dispersed
(PbAgSn).sub.3Ca-type precipitates, lower rates of matrix strain
hardening, strain energy and residual stresses, equiaxed and
honeycomb grain structure, and a matrix relatively resistant to
recrystallization and corrosion.
[0028] Other objects and advantages of the present invention can be
seen from the following description of the invention.
BRIEF SUMMARY OF THE INVENTION
[0029] In general, the method of the present invention involves
carefully controlling the rolling of the billet so as to provide
positive grids made from calcium-tin-silver lead-based alloys which
have highly beneficial properties. Indeed, such positive grids are
considered to be ideally suited for sealed lead-acid cells and
batteries that are intended for relatively long term service lives.
On the other hand, if desired for use in SLI lead-acid battery
applications and the like, the use of grids made using the methods
of the present invention should possess such inherently high
corrosion resistance that the grid thickness and weight can be
reduced, if desired, by anywhere from about 5% to 10% or so.
Reductions of this level provide a potential economic reward that
is considerable.
[0030] As will be discussed more particularly hereinafter, the
method of the present invention involves casting the billet, then
rolling at a controlled temperature which is above the solvus, and
is somewhat less than the peritectic, temperature for the defined
calcium concentration in the alloy, quenching the rolled strip to
preserve the supersaturated, lead-rich solid solution, and then
maintaining the rolled strip at selected temperatures until ready
for conversion into grids.
[0031] According to a more preferred embodiment of the present
invention, it has been found that subjecting the rolled strip to a
controlled artificial aging sequence can further enhance the
corrosion resistance of grids made using the present invention.
Thus, as will be discussed hereinafter, such a controlled
artificial aging sequence can be utilized to reduce intergranular
corrosion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a block diagram showing the method of the present
invention;
[0033] FIG. 2 is a lead-calcium phase diagram and illustrating the
rolling temperature most preferred in the method of the present
invention when the alloy contains 0.04 wt. % calcium;
[0034] FIG. 3 is a perspective view of a maintenance-free battery
of the present invention;
[0035] FIG. 4 is a cross-sectional view taken generally along line
4-4 of FIG. 3 and showing a battery grid made utilizing an alloy
composition in accordance with the present invention; and
[0036] FIG. 5 is a side elevational, partial sectional view of a
valve-regulated lead-acid (VRLA) cell showing the internal
configuration.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] As to the preferred alloy composition utilized, any desired
calcium-tin-silver lead-based alloy may be used which possesses the
characteristics desired for the particular application. In this
regard, the type of application may, at least in part, play a
principal part in the selection of a particular alloy composition.
For example, for sealed lead-acid cells and batteries, alloy
compositions should be selected that are considered to impart to
the resulting grid enhanced resistance to grid growth when
relatively large sealed cells and batteries are necessary.
Similarly, such compositions should be selected so as to minimize,
if not eliminate, any thermal runaway issues. Alloys intended for
SLI automotive batteries preferably contain calcium in the range of
0.035-0.065%, tin in the range of 0.5-1.5%, and silver in the range
of 0.02-0.045%. The calcium content and the silver content are
slightly higher and would be acceptable for automotive batteries as
they are expected to last three-five years.
[0038] More broadly, the alloy compositions for use in the present
invention include, based upon the weight of the resulting strip,
from about 0.025% to 0.065% calcium, from about 0.4% to 1.9% tin,
and from about 0.015% to 0.050% silver. If desired, aluminum can be
present in an amount effective to reduce the drossing of the
calcium from the resulting alloy. Suitable amounts are known, and
an illustrative range can vary from about 0.003% to about 0.03% by
weight.
[0039] Still further, in the most preferred embodiment, it is
desired to minimize the level of trace elements in the alloy
composition. Of course, other ingredients may be added to the alloy
if desired, provided the beneficial properties of the alloy are not
disturbed by the addition of such ingredients. Set forth below in
Table 1 is the most preferred maximum level for various trace
elements:
1 TABLE 1 Element Composition in Wt. % Copper 0.050 Bismuth 0.040
Sulfur 0.0010 Tellurium 0.00050 Nickel 0.00030 Iron 0.0020 Cadmium
0.0020 Zinc 0.0020
[0040] Copper is limited to a maximum concentration of 0.05%, as
higher levels might induce grain boundary embrittlement and
increase the rate of gassing and self-discharge. Bismuth content
should be maintained below 0.04% to minimize alloy drossing during
casting as well as to minimize adverse effects on corrosion
resistance. Tellurium and nickel impurity limits are set at 5 and 3
ppm levels to minimize the adverse effects of lowering hydrogen
over-voltage and thus increasing gassing and water loss in MF
batteries and sealed batteries. Iron content is held at 20 ppm to
minimize the adverse effects on the self-discharging rate in the
battery.
[0041] Further, addition of tin to the calcium-lead alloy family
tends to lower the equilibrium solubility of calcium to achieve
supersaturation. This should be kept in mind as the tin content in
the alloy composition used is increased. This is one of the
principal reasons why the preferred compositions utilize no more
than about 1.9 wt. % tin. However, depending upon the application
and requirements, it may be suitable to use alloys of this family
containing tin up to 2% by weight, perhaps even up to 3% by weight
or so.
[0042] The alloy preferably is prepared by blending the ingredients
at temperatures of about 800.degree. F. to about 950.degree. F.
(426.degree. C. to about 510.degree. C.) until a homogeneous
mixture is achieved, and allowing the ingredients to cool. The
particular manner in which the alloys of this invention are
prepared does not form a part of the present invention. Any desired
alloying technique that is normally used for making alloys of this
type will be acceptable in producing the starting Ca--Sn--Ag--Pb
alloy for further processing.
[0043] FIG. 1 is a process flow diagram showing the method of the
present invention. The particular calcium-tin-silver lead-based
alloy composition desired for the particular grids is first
selected. Thereafter, in accordance with one aspect of the present
invention, the solvus temperature for the selected alloy
composition is determined, which solvus temperature is then used to
determine the temperature at which rolling is carried out as will
be discussed hereinafter.
[0044] In general, the solvus temperatures determined should yield
a single-phase, lead-rich matrix and should at least minimize, if
not eliminate, any grain-boundary embrittlement due to liquid-film
formation at the grain boundaries of the cast billet. A
satisfactory solvus temperature may be approximated using the
Ca--Pb phase diagram, as shown in FIG. 2. The approximate solvus
temperature for the various levels of calcium can be extrapolated
from the phase diagrams as follows:
2 Equilibrium Solvus Calcium Content Temperature - F. 0.02 378 0.03
459 0.04 500 0.05 525 0.06 540 0.065 562
[0045] However, as used herein, the term "solvus temperature" means
the temperature determined by any of the following techniques which
more precisely determine the solvus temperature of the specific
alloy selected. These techniques comprise either determining the
presence of a single phase, using x-ray diffraction to determine
the desired lattice parameters or employing electrical resistivity
measurements to determine the solvus temperature from the lowest
value determined. More particularly, a series of cast tensile
samples or 0.25 inch rods (3-5 inches in length) can be cast from
the desired Ca--Sn--Ag--Pb alloy, keeping in mind the approximate
solvus temperature (e.g., with a calcium content of 0.04%, having
an approximate solvus temperature of 500.degree. F., the four
samples can use reheat temperatures of: 485.degree. F., 495.degree.
F., 505.degree. F., and 515.degree. F.). Each sample is then
reheated to one of the selected temperatures, held at the
particular temperature for one hour, and then quenched in ice
water. A metallurgical sample is then prepared immediately after
quenching (i.e., cross-sectioning, polishing and etching the sample
for microstructure examination). The solvus temperature will be
that reheat temperature where no new phases are detected, and only
the single phase, lead-rich solid solution is seen. The phase or
phases present can be determined by microscopic examination as is
known and used in the metallurgical field. Utilizing an optical
microscope at an amplification of 100.times. to 400.times. or so
will be adequate to allow the phase or phases present to be
observed.
[0046] Another suitable technique involves using x-ray diffraction
to determine the lattice parameters of the selected alloy using the
quenched rod or other sample. Constancy of the measured lattice
parameters indicate the presence of only a single phase, and the
reheat temperature that yielded this condition will be the solvus
temperature. An .alpha.-lead rich phase has a face-centered cubic
("FCC") structure, and the lattice parameter "a" of the FCC unit
cell is : a=4.9495 .ANG. (at 20.degree.-28.degree. C.). If the
sample reheat temperature corresponds to the solvus temperature,
then the lattice parameter determined will be 4.9495 .ANG.. If the
lattice parameter deviates more than .+-.5% from this value, the
sample has either a distorted .alpha.-lead rich crystal structure
or may also contain a second phase precipitate like Pb.sub.3Ca. The
lattice parameter "a" of the single phase .alpha.-lead rich solid
solution crystal lattice should have the same value of 4.9495 .ANG.
(.+-.5%), regardless of the calcium content in the alloy of
choice.
[0047] Still another useful technique involves determining the
electrical resistivity of the quenched rods or other samples,
immediately after quenching. Stable and low values of the
electrical resistivity indicate the absence of undesired
precipitates, and, hence, a single phase. The sample having the
lowest electrical resistivity value gives the solvus temperature
(i.e., the reheat temperature for that sample).
[0048] Next, as shown in FIG. 1, the alloy composition selected can
be provided in the quantities necessary to make the cast billet
thickness desired for the particular grid thickness. Suitable
equipment for casting billets are known and may be utilized. Then,
the alloy is cast into billets. As illustrative examples, the
dimensions of the billet may be from 0.4-1.0 inch in thickness and
have a width ranging from 2-5 inches up to 100 inches or so.
[0049] Pursuant to the present invention, the billet (preferably
continuously cast) is then rolled to provide the desired strip
thickness for the expanded grid at a preselected rolling
temperature maintained during the rolling step. The rolling
temperature selected should be, as a minimum, at least the solvus
temperature; but, as a maximum, should be somewhat less than the
peritectic temperature for this family of alloys (viz., about
600.degree. F. or so, depending upon the calcium concentration in
the alloy composition selected). The rolling temperature should be
maintained in the desired range; and cooling, using water or other
cooling medium, may be needed if the heat energy generated during
the rolling process so requires.
[0050] More particularly, as a minimum, it is preferred to utilize
a temperature at, or more preferably, somewhat above, the solvus
temperature for the particular alloy so that the precipitation of
any undesired phase is at least minimized, if not eliminated.
Stated, differently, the preferred method of the present invention
carries out the process of rolling the billet to provide the grid
strip desired having a single-phase domain of the alloy selected,
promoting the formation of a very supersaturated .alpha.-Pb rich
matrix containing all the intermetallic phases of the solute
element as completely soluble species.
[0051] Thus, what should be avoided in any event is sufficient
precipitate formation that affects either the desired mechanical
properties or the corrosion resistance. On the other hand, the
rolling temperature used should not be so high as to cause
undesired levels of elastic liquid phase present along the grain
boundaries. Thus, if the rolling temperature is too close to the
peritectic transformation temperature (i.e., about 320.degree. C.),
then it is possible to create a small quantity of a liquid phase.
This liquid phase will lead to liquid phase embrittlement, and the
cast billet can crack and fracture during rolling, and thus
fail.
[0052] The rolling process itself can be carried out by using any
of the conventionally known techniques, so long as the appropriate
rolling temperature is maintained, as has been described herein.
Similarly, the thickness of the grid strip obtained from the roll
cast billets will, of course, vary depending upon the requirements
of the particular type of battery and the specific application. For
example, the thickness of the grids may vary from about 0.020
inches to about 0.060 inches for SLI battery applications to
thicknesses of 0.1 inch or more for VRLA applications. The present
invention is particularly useful for making grids where the desired
grid thickness is at least 0.1 inch.
[0053] As may thus be appreciated, in view of the thickness of the
cast billets typically being at least 0.4 inch and the desired
thickness of the grids generally being much less than that, the
entire thickness of the strip after completion of the rolling step
will have been mechanically worked. This is preferred since this
should provide a rolled strip having a homogeneous microstructure
profile throughout, characterized by the presence only of the
desired .alpha.-lead rich solid solution phase. However, as may be
appreciated, and while all of the advantages of the preferred
embodiment of this invention will not be achieved, advantages will
result even when the reduction in thickness (i.e., the ratio
between the thickness of the cast billet and the thickness of the
rolled strip) is less than the 2:1-10:1 or more that is preferably
utilized. Thus, some benefits may be achieved even with such ratios
as low as 1:0.8 or so.
[0054] After the billet has been rolled to the desired strip
thickness, the resulting strip should be immediately quenched so as
to preserve the supersaturated solid solution. As an illustrative
example, suitable quenching can be achieved using circulating
cooling water having a temperature of, for example, from about
35.degree. F. to about 45.degree. F.
[0055] Optionally, the resulting grid strip can be kept in a
relatively cool environment, e.g., at a temperature of less than
about 60.degree. F., until the grid is made from the strip by the
desired technique so as to prevent age hardening during the
grid-making procedure. This optional step will maintain the strip
in a relatively soft, ductile condition in the non-aged state.
[0056] In accordance with yet another, and more preferred, aspect
of the present invention, the corrosion resistance that the rolled
strips will impart to positive grids made therefrom can be enhanced
by treating the rolled strips so as to increase the population
(i.e., concentration) of the special grain boundaries in such
strips. Thus, the rolled strips (even when controlled pursuant to
this invention) will contain numerous fragmented and highly
oriented grain boundaries as well as a small fraction of what are
termed "special grain boundaries."
[0057] These special grain boundaries comprise a mix of: (a) low
angle grain boundaries with about 15.degree. for atomic mismatch or
orientation difference, and (b) coincidence-site grain boundaries.
Such special grain boundaries have a lower grain boundary energy
than the so-called random high angle grain boundaries. As is well
known, these special grain boundaries are more resistant to
intergranular fracture and exhibit much superior corrosion
resistance in comparison to the characteristics of random high
angle grain boundaries which exhibit sensitivity to both
intergranular crack formation and accelerated corrosion. Even when
the strip is rolled and quenched according to this invention, the
majority of the grain boundaries will likely be random with a high
angle of atomic arrangement mismatch.
[0058] Thus, pursuant to the most preferred aspects of the present
invention, the quenched rolled strip is rapidly heated (i.e., in
less than about one hour or so) to a temperature in the range of
about 200.degree. F. to 500.degree. F. and maintained at this
temperature range for a time sufficient to allow the microstructure
to be evolve, at which point the small population of special grain
boundaries will have been increased at the expense of the large
random high angle grain boundaries in the rolled strip. While the
time necessary for this artificial heat aging will vary with the
temperature and the strip thickness used, the evolution of the
microstructure should occur in a few minutes up to about one hour
or so. This evolved microstructure can be verified by examining the
microstructure using scanning electron microscopy (SEM),
transmission electron microscopy or x-ray stereographic projection
of crystal planes. These techniques are known and are used for such
microstructure examinations.
[0059] Rolled strips having been subjected to such a controlled
artificial aging sequence will be characterized by a relatively
large fraction of special grain boundaries (in comparison to the
population prior to such treatment), a stable microstructure and an
equiaxed grain structure. These microstructural characteristics
will give rise to a rolled strip having outstanding mechanical
properties and positive grids having, in service, outstanding high
temperature corrosion resistance.
[0060] In making grids from the strip, this can be carried out
immediately after the quenching step, if desired, or after the
controlled heat aging sequence previously discussed. Alternatively,
the rolled strip can be made into grids by expanded metal, die
punching or other techniques after the strip has been fully
age-hardened, either at ambient temperatures or at a higher
predetermined temperature.
[0061] The preferred process flow in the wrought process of this
invention is that described invention is that described herein,
i.e., roll and quench, then, if used, artificially age the rolled
quenched strip, and then fabricate into the grid. This process flow
is efficient and economical.
[0062] However, it is within the scope of the present invention to
first fabricate the grids from the rolled and quenched strip, and
then carry out the artificial dying, on line or in a separate
operation. While less preferred, this optional process flow does
have the additional benefit of reducing, if not eliminating,
residual stresses introduced during the grid expansion or other
grid-fabricating stage.
[0063] The particular grid configuration and that of the lead-acid
cells or batteries in which such positive grids are used can be
varied as desired. Many configurations are known and may be
used.
[0064] As one illustrative example, FIGS. 3 and 4 show a
maintenance-free battery utilizing the positive grids having of the
present invention. Thus, a maintenance-free battery 10 is shown
which includes a container 12, a pair of side terminal posts 14 and
a cover 16 sealed to the container by any conventional means. The
container is divided into a plurality of cells, a portion of one
cell being shown in FIG. 4; and a battery element is disposed in
each of these cells. The battery element comprises a plurality of
electrodes and separators, one of the positive grids being shown
generally at 18. The negative grids are of identical or similar
construction but are formed from any desired antimony-free alloy.
The electrode illustrated includes a supporting grid structure 20
having an integral lug 22 and a layer of active material pasted
thereto; and a strap 24 joining the lugs 22 of the respective
positive and negative grids together.
[0065] Intercell connectors are shown generally at 26 and include a
"tombstone" 28 which forms a part of the strap 24. The strap 24 may
be fused to the grid lugs 22 in assembling the components into an
element as is known. The terminals 14 are similarly electrically
connected through separate straps 24 to the supporting grid
structure 20 during assembly, the base of the terminal forming a
part of the strap 24. Suitable manifold venting systems for
allowing evolved gases to escape in flooded electrolyte SLI
batteries are shown at 30. Many satisfactory venting systems are
well known. In addition, it is believed that all the present
maintenance-free batteries manufactured in the United States will
typically utilize flame retardant explosion-proof vent designs.
[0066] The particular design configurations of the battery may be
varied as desired for the intended application. The positive grids
described herein may be advantageously utilized in any type and
size of lead-acid automotive battery. For example, the battery
grids of the present invention may be advantageously used in dual
terminal batteries such as those shown in U.S. Pat. No. 4,645,725.
Similarly, while a battery having side terminals has been
exemplified, the battery of this invention could comprise a top
terminal battery.
[0067] The thickness of the positive grids can vary as is desired
for a particular service life and a particular desired rated
capacity, as previously noted. However, with any given thickness
positive grid, the batteries utilizing the grids of the present
invention will impart enhanced characteristics to the battery in
comparison to conventional maintenance-free batteries having
positive grids formed from previously used casting methods.
[0068] FIG. 5 illustrates a lead-acid VRLA cell in accordance with
the present invention. The cell 40 includes a container 42 having a
series of positive and negative plates with an absorbent separator
separating the plates. A positive plate shown generally at 44
comprises positive active material 46, partially broken away, to
show the positive grid structure 48. Strap 50 is connected to
terminal 52.
[0069] As previously discussed, the thickness of the plates will
vary depending upon the application to which the cell is intended.
An illustration of a useful range is from about 0.030 inch to about
0.300 inch, often 0.100 inch or more, but thinner or thicker plates
may also be used. It is desired that the service life of the cell
should be dictated by the thickness of the positive plates, as
opposed to factors such as electrolyte or water loss or other modes
of failure. If positive plate corrosion dictates the service life
of the cell, the service life may be more readily predicted than
for other modes of failure.
[0070] Preferably, the container is normally sealed from the
atmosphere in use to provide an efficient oxygen recombination
cycle as is known. The container should be able to withstand the
pressure of the gases released during charging of the cell.
Pressures inside the container may reach levels as high as, for
example, 0.5-5.0 or 10.0 psig. Release venting is provided by a low
pressure, a self-resealing relief valve, such as, for example, a
bunsen valve. An example of such valve is illustrated in U.S. Pat.
No. 4,401,730 to Szymborski et al.
[0071] An electrolyte is also included within the container.
Preferably, the electrolyte is absorbed within the separator and
the positive and negative active material. The electrolyte
typically is sulfuric acid having a specific gravity in the range
of about 1.240 to about 1.340, or even more, as is considered
appropriate for a particular application.
[0072] The illustrative VRLA cell shown in FIG. 5 is only
exemplary. The particular design and configuration of the VRLA
cells used can vary as desired. The specific configuration does not
form a part of the present invention.
[0073] Utilizing the method of the present invention should provide
alloy strips and positive grids characterized by very stable,
effective and uniform dispersion of precipitates in the matrix. In
the preferred embodiment, the precipitate particles in the strip
and grid will have a size principally in the range of from about 10
to about 100 nM. Utilizing the controlled rolling temperature,
enhanced by (when used) the artificial aging, introduces a uniform
dispersion of very fine (AgSnPb).sub.3Ca, Ag.sub.3Sn, and other
binary Ca--Ag and Ca--Sn precipitates within the grains and not at
the grain boundaries. Still further, and desirably, the fragmented
cast grains in the strip and grid will form equiaxed grain
structure, or nearly so. Very low levels of residual stresses will
be retained, and high microstructure stability and low
recrystallization attributes at battery service temperatures can be
achieved.
[0074] Additional attributes of the present invention provide
strips and expanded or punched grids with fine precipitates having
a very high level of crystal lattice compatibility and coherency
between the precipitates and the lead-rich matrix. Such
compatibility and coherency provide efficient and stable matrix
strengthening. Very uniform and fine precipitate particles
distribution in the matrix of the strip and grid will be provided.
Additionally, the preferred method will achieve minimal grain
structure orientation which will enhance the corrosion rate
stability.
[0075] Higher strength, toughness, ductility, corrosion resistance
and creep-rupture strength at both normal and higher service
temperatures will be achieved in grids made using the method of the
present invention. These characteristics are particularly suited
for use as positive grids in VRLA cells and batteries that are
intended for long-term duty service.
[0076] Still further, and importantly, grids made using the method
of the present invention should exhibit highly uniform modes of
corrosion penetration with minimal intergranular corrosion. These
desirable characteristics should allow a significant reduction in
the grid thickness and weight of many applications. It is thus
believed that the inherently higher corrosion resistance of strips
and grids made using the method of the present invention should
allow a reduction in grid thickness and weight in the range of from
about 5% to 10% or so. As previously noted, such reductions provide
a substantial economic benefit. For a given strip thickness, the
corrosion resistance of these specially-processed strips will be
far superior to the corrosion resistance that results from strips
made from rolling processes having no, or inappropriate, process
temperature control.
[0077] While particular embodiments of the invention have been
shown, it will of course be understood that the invention is not
limited thereto since modifications may be made by those skilled in
the art, particularly in light of the foregoing teachings. Thus,
while the present invention has been described in conjunction with
SLI batteries and VRLA cells, it should be appreciated that the
alloys disclosed herein may be used in any other lead-acid cells or
batteries including, for example, bipolar and the like. Still
further, while the present invention contemplates use, as a
minimum, of a temperature for rolling of at least about the solvus
temperature determined for the particular alloy, it should be
appreciated that some of the advantages of the present invention
can be achieved even when the rolling temperature is somewhat
lower, resulting in some level of undesirable precipitates. Such
less-than-desirable rolling temperatures particularly can be
tolerated when the controlled heat aging sequence described herein
is utilized. Even further, while the optimum and most preferred
embodiment of the present invention utilizes the controlled rolling
process together with the heat aging sequence, it should be
appreciated that using the heat aging sequence by itself may
provide adequate benefits for some applications, even when a
substantially less than desired rolling step has been used.
[0078] Further, the present invention has been principally
described in conjunction with strips made from quaternary
Ca--Sn--Ag--Pb alloys; and these alloys are preferred, now being in
widespread commercial use. However, and, while less preferred, the
present invention can certainly be utilized with ternary Ca--Sa--Pb
alloys. Such ternary alloys and useful compositions are known and
are being used commercially to make lead-acid battery grids with a
wrought process.
[0079] Still further, while the present invention has been
described in conjunction with a If process in which the cast strip
is thicker, typically much thicker, than the desired grid
thickness, it should be appreciated that the invention is not so
limited. Thus, various processes are known wherein the strip is
directly cast at the thickness desired for the grid.
[0080] One illustrative example is shown in U.S. Pat. No. 4,315,357
to Laurie et al. which illustrates, in general, a method and
apparatus for forming the expanded mesh strip necessary for making
a continuously cast grid. Equipment for making the directly cast
strip and processing into an expanded mesh strip is commercially
available (Cominco Ltd., Toronto, Canada).
[0081] The strips, and the resulting grids, are characterized by
highly columnar grid microstructures that would be indicative of
positive grids having relatively high susceptibility to high
temperature corrosion resistance. Yet, in accordance with U.S. Pat.
No. 5,434,025 to Rao et al., the use of appropriate Ca--Sn--Ag
lead-based positive grid alloys achieve surprising performance,
even with such columnar grid microstructures.
[0082] However, the high temperature corrosion resistance and
mechanical properties can be even further enhanced by utilizing the
process of the present invention, at least a controlled rolling
step and, preferably, also a controlled artificial aging step.
Thus, rather than directly casing the strip at the grid thickness,
the strip is directly cast at a thickness greater than the desired
grid thickness so that a controlled temperature rolling step can be
carried out to reduce the thickness by at least 20%, based upon the
directly cast thickness. Preferably, the thickness can be reduced
up to 100% or so in order to insure that the strip is mechanically
worked throughout. A limiting factor will be the thickness at which
strips of satisfactory quality can be directly cast, thicknesses in
excess of 0.1 inch or so being more difficult to satisfactorily
produce. Accordingly, the use of such directly cast strips is
preferably utilized for positive grids having a desired thickness
of 0.025-0.06 inch.
[0083] It will be preferable to utilize a controlled artificial
aging step, as well, pursuant to this invention. In this fashion,
use of the present invention with such directly cast strips will
break up and fragment the columnar grid structure, providing a more
equiaxed grain structure characterized by an idealized precipitate
formation, as previously described.
* * * * *