Note: Descriptions are shown in the official language in which they were submitted.
A75-3t
108;~493
BACKGRO~ND OF THE INVENTION
This invention relates to a new high strength lead alloy.
While an alloy according to the invention finds particular
utility as a grid and/or top lead material for use in lead-
acid storage batteries, certain properties of the alloy make
it attractive for use in other applications as well where a
high strength lead is desired.
Lead-acid storage batteries typically emploj~lead alloys
containing antimony as a primary constituant due to the effect
of antimony on the physical properties of the lead. Antimony
is used to increase the strength and/or other physical prop-
erties of lead, facilitating various aspects of battery manu-
facture. In thef case of lead-acid battery grids, this is
particllarly important in order for the grids to withstand
normal ~.andling during battery manufacturing and service.
Recently the battery industry began producing batteries
which require little or no maintenance such as adding of water
to maintain the electrolyte level ~uring the service life of
a battery. In such batteries it~-is the pract~e to either
seal the battery or use vent plugs for the filling ports which
i are not easily removed by the ultimate battery user. Since
the purpose of such batteries is to eliminate the need for
filling, a lead alloy system must be selected in which the
supply of electrolyte will not be significantly diminished
over the intended life of the battery. The presence of antimony
typically causes excessive gas generation in lead-acid batteries,
` especially during periods of charging or overcharging, which
. '' ~
, -- 1
` A75-36
108;~493
ultimately depletes the quantity of electrolyte. In addition,
excessive gassing is unacceptable in reduced or no-maintenance
batteries if they are of the completely sealed type. Conven-
tional alloys for this type of battery contain calcium in
place of antimony. Calcium alloys reduce gas generation.
Examples of lead-calcium alloys are seen in the following U.S.
Patents: 3,920,473 issued November 18, 1975 to Sims; 3,881,953
issued May 6, 1975 to Turowski; 3,287,165 issued November 22,
1966 to Jensen; 2,794,707 issued June 4, 1957 to Walsh; 2,159,124
0 issued May 20, 1937 to Betterton et al; and 1,703,212 issued
February 26, 1929 to Shoemaker.
A disadvantage of the lead-calcium system is that its
alloys ~o not generally have mechanical properties comparable
to lead-antimony alloys. Since the battery industry is con-
tinually striving to make battery grids with smaller cross-
sections than previously used, the strength of the lead-calcium
grid alloys becomes a limiting factor in grid design. Lead-
calcium systems also have a propensity to grow mechanically
or creep as is well known in the art. If creep becomes ex-
cessive within a battery, adjacent parts may short out thereby
disabling the battery or seriously reducing its capacity. Creep
!~ may become excessive with minor alloy compositional changes.
A further disadvantage of conventional alloys lies in the
, ~ limitations on the re-use of lead scrap due to decreased pot
stability. For example, melting of the scrap results in ex-
- cessive drossing with a resultant loss of the calcium in the
1:
alloy. Adjustments must therefore be made in the re-melted
; - 2 -
A75-36
10824~t3
scrap to bring the alloy composition back into a desired range
prior to casting. Furthermore drossing of lead-calcium increases
during mechanical agitation of the molten alloy both initially
and for re-melted scrap thereby necessitating protective measures
such as the use of shielded pots.
Strontium has been proposed for use in lead alloys by
others. For example, U.S. Patent 1,158,672 issued November 2,
l91S to Frary et al, discloses a lead alloy containing a plural-
ity of alkaline earths including calcium, barium, strontium and
r 10 magnesium for use in bullets. U.S Patent 2,013,487 issued
June 7, 1934 to Canfield et al, discloses a lead-strontium-tin
allov for use in lead-acid battery grids, and U.S. Patent 2,040,078
issued May 12, 1936 to Canfield et al shows a lead-strontium alloy.
Finally U.S. Patent 2,170,650 issued August 22, 1939 to Bouton
et al shows a lead-calcium-barium-strontium alloy. These
' patents do not show lead alloys which have all the advantages
} displayed by applicant's invention however, such as dross pro-
t tection, pot stability on re-melt and high strength.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an im-
prove-l lead alloy for use in lead-acid storage batteries and
other applications where its physical and/or electrochemical
properties are advantageous.
A further object of the invention is to provide a lead
i, ~
alloy which may be re-melted without substantial change in its
composition.
Other objects and advantages of the present invention will
,~
-- 3 --
A75-36
10824~3
be apparent from the description of preferred embodiments.
The invention basically comprises-a lead based strontium
alloy containing 0.01 - 2 weight % strontium, 0.1 - 5.0 weight %
tin, 0.005 - 0.1 weight % aluminum, 0 - 0.25 weig~t % of copper
and the balance lead.
DESCRIPTION OF THE PREEl~RRED EMBODIMENTS
-
The alloy according to the invention may be made by con-
ventional smelting procedures. In the laboratory, research
quantities of the desired alloys were prepared from master
alloys which are higher in the desired constituants than the
final composition. A tin-aluminum master alloy was made by
dissolving the aluminum in either pure tin or a lead-tin binary
alloy. Copper master alloys were prepared by dissolving ele-
mental ~opper in corroding ~rade lead~;r lead-tin-aluminum
ternary alloys. A lead-strontium binary alloy was purchased
but conventional smelting techniques may be used to-produce
it. Specific compositions were made by adding proper amounts
of the respective master alloys to corroding grade lead at
I temperatures up to 550 C. When alloying was done in air,
1~ 20 excessive oxidation was avoided by adding tin-aluminum master
alloys first followed by the strontium master alloys. The
order of addition is not important if a gas shield is used
to protect the pot.
Table 1 illustrates the mechanical properties of typical lead-
calcium and lead-antimony alloys for use in lead-acid battery
grids. In the table Sy represents the yield strength in kilo-
~ -~ grams/mm' at .2% offset. Su represents the ultimate tensile
,~:
~ .
- 4 - -
.
5- s~
~1082493
strength of the alloy in kilograms/mm2 and El represents the
percentage of elongation at ultimate strength. The balance
of the alloys comprises lead. The lead which makes up the
balance of the alloy in Table 1 and in all succeeding examples,
comprises primary or secondary corroding grade lead having trace
impurities as is commercially available in the industry. Nat-
urally, while pure lead may be the most desirable, it is not
economically justifiable for use in lead-acid storage battery
grids as those skilled in the art will understand.
Table 2 sets forth various compositions of lead alloys
made according to the invention. Again, the same criteria as
used in Table 1 are set forth. It will be noted that in the
majority of cases the lead-strontium alloys are equal to or
exceed the strength of the calcium alloy. An examination of
the Table 2 will also indicate that after aging 24 hours and
14 days the strontium alloys are generally superior to the cal-
cium alloy and approach the strength of the lead-antimony alloy
shown ln Table 1.
We have determined that an alloy made according to the
invention lies within the following range of compositions by
weight percent:
Strontium 0.01% - 2.0%
Tin 0.1% - 5.0%
Aluminum 0.005% - 0.1%
Copper 0 - 0.25Z
Lead balanc~
The strontium level has a maximum of 2 weight percent
because greater q~antIties result in unreasonably high liquidus
temperatures. Also with respect to lead acid batteries, higher
-- 5 --
A75-36
- 1082493
percentages of strontium cause excessive precipitation of
intermetallic compounds resulting in poor corrosion properties.
At greater percentages of strontium increased drossing results
during air fir-ng,and the alloy would also have an excessively
high cost. A minimum of .01% strontium is required to impart
the desired strength to the alloy.
With regard to tin, 5% is an upper limit since there is
no gain in mechanical behavior over 5%. Additionally, accel-
erated drossing results at temperatures employed in battery
grid casting and greater amounts render the alloy economically
unfeasible. Below .1% tin, age hardening takes excessively
long and dross protection is reduced.
Aluminum should be present in an amount of at least .005%
; since l~sser amounts will not afford proper dross protection.
At quantities greater than .1% no additional benefits are
realized and potential processing difficulties arise due to
the presence of primary aluminum in the alloy.
Copper may be added to accelerate the age hardening
properties of the alloy. An upper limit of .25% provides
the maximum at wllich copper affects the age hardening and
also affects he liquidus temperature.
The preferred range of compositions of the alloy is set
forth as follows in weight percent:
Strontium Tin Aluminum Copper Lead
,~ 0.05% - 0.3% 0.25% - 1.0% 0.01% - 0.1% 0.005% - 0.1% Balance
Alloys made according to the invention have exhibited a
number of advantages over lead-calcium alloy. Such alloys
.
-- 6
A75-36
108~493
have very low dross generation and excellent compositional
stability, eliminating the need for frequent pot analysis
and adjustment. They generally exhibit higher yield and creep
strength combined with good ductility which improves processing
as well as vibration resistance under severe service conditions.
The alloys typically age harden rapidly allowing further pro-
cessing of the grids such as pasting within 16 hours. The
alloys typically can be cast at lower temperatures than lead-
calcium alloys thereby reducing the frequency of mold coating
and the alloy scrap may be directly re-melted and used for
casting grids or battery straps or other parts. Electrochemical
corros'on testing of the alloys indicates that they are superior
to antimontial lead alloys and at least equal to calcium-lead
alloys. Finally an alloy according to the invention is particu-
larly well suited for cast-on-strap designs as described in U.S.
Patent 3,087,005 issued April 23, 1963 to Sabatino et al.
While the alloy described is suitable for use in lead-acid
batteries, other uses may occur to those skilled in the art.
Accordingly, the scope of the invention is not to be limited by
the foregoing description, but is to be taken solely by an
interpretation of the claims which f~llow.
,
.
A7 5-36
108Z493
_~ ,~
U'~ U~
cn
,~
V~
oo o
U~
C~ ~
,~ o
~ ~ C~
Ull
~ U~
u~ . .
C~l
~.
o oo
,_
,,
J
gl ,~
O
.
~ o
:~ ~ o
~1
, ~ ~
.__
U~
. i ~o~ I O
~) o ~ ~'
,': ~
o
~: ~ ~ CQI ~ U~l ~ .
1~ o
,~
t~ Dl t~
~` ~
-- 8 --
A7'i~ s~
8z493
U~ o ~ o o~ o U~
--I C~l Lr) C`~ N C' ~ O C~ --I ~ 00
~ I I I I ........... I I I I I
U~
~1 ~ O 1~ ~ C~ U~ O ~ o~ ~ `J
c.nl II I I ........... I I I I
~ l O O 00 1~ --I C N
:~1 1~ ~ O 00 1~ 0 `J 1~ 11~ 00 N
,_1 C~l 1~ ~ N 01~ O U~ `J CO 1~ 0 1` N
~ I .. I ........... 1 ~ 1 1,
D ~ ~1 0 N 1~ 1~ ~ N
.
,:
U~
C~ oo 1~ U~ ~ O O ~ O ~ O
O tn .. ...........
~ ~ 1~ ~ c ~ oo ~ 1~ ~ ~ o o ~ 00 o c~
u~l ----
C~ ~ ~ ~ O ~ t~
~1 cs o oo u~
E~
I
o ~ o o
~) ~ ~ ~ ~ C~l N C~ I ~ N C~l
.
o ~ ~J ~ o o~ c~l ~ N ~ ~ N ~)
00 0~ O I O` 00 00 ~ I O N N 1~ 0~ C' I I I --I
cnl ~-
~, .
~1o o o ln
J ~
~ ~-~ ~ O O O O O--~ O I o o o
H t~
i o ~
b.l'
; ~ 3
J O --1 1 ~ l N
:
9 _
