Method for making positive grids and lead-acid and batteries using such grids

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° F. to 500° F., and fabricating into the positive grid, such grids having enhanced mechanical and high temperature corrosion resistance characteristics.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. 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. 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 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. 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. The alloy preferably is prepared by blending the ingredients at temperatures of about 800° F. to about 950° F. (426° C. to about 510° 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. 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. 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 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° F., the four samples can use reheat temperatures of: 485° F., 495° F., 505° F., and 515° 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× to 400× or so will be adequate to allow the phase or phases present to be observed. 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 &agr;-lead rich phase has a face-centered cubic (“FCC”) structure, and the lattice parameter “a” of the FCC unit cell is : a&equals;4.9495 &angst; (at 20°-28° C.). If the sample reheat temperature corresponds to the solvus temperature, then the lattice parameter determined will be 4.9495 &angst;. If the lattice parameter deviates more than ±5% from this value, the sample has either a distorted &agr;-lead rich crystal structure or may also contain a second phase precipitate like Pb 3 Ca. The lattice parameter “a” of the single phase &agr;-lead rich solid solution crystal lattice should have the same value of 4.9495 &angst; (±5%), regardless of the calcium content in the alloy of choice. 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). 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. 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° 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. 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 &agr;-Pb rich matrix containing all the intermetallic phases of the solute element as completely soluble species. 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° 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. 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. 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 &agr;-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. 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° F. to about 45° F. Optionally, the resulting grid strip can be kept in a relatively cool environment, e.g., at a temperature of less than about 60° 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. 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.” These special grain boundaries comprise a mix of: (a) low angle grain boundaries with about 15° 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. 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° F. to 500° 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 . 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. 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. 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. 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. 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) 3 Ca, Ag 3 Sn, 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. 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. 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. 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. 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. 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. 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. 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). 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. 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. 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.