WO1999005732A1

WO1999005732A1 - Method for the continuous manufacture of positive battery grids and positive grids obtained by said method

Info

Publication number: WO1999005732A1
Application number: PCT/FR1998/001643
Authority: WO
Inventors: Bertrand Madelin; Luc Albert
Original assignee: Metaleurop S.A.
Priority date: 1997-07-25
Filing date: 1998-07-24
Publication date: 1999-02-04
Also published as: KR20010022181A; FR2766622A1; JP2001511587A; BR9811290A; AU8868198A; CA2297768A1; FR2766622B1; EP0996987A1; PL338604A1

Abstract

The invention concerns a method for the continuous manufacture of acid lead positive battery grids, comprising the following steps: a) preparing a lead alloy containing calcium, at a relative concentration ranging between 0.04 wt.% and 0.12 wt.%, of tin at a relative concentration less than 3 wt.%, aluminium, at a relative concentration ranging between 0.001 wt.% and 0.035 wt.%, and barium, at a relative concentration ranging between 0.02 wt.% and 0.1 wt.%; b) carrying out a continuous casting of said lead alloy; c) mechanically treating said continuous alloy casting to form said positive grids.

Description

PROCESS FOR THE CONTINUOUS MANUFACTURE OF POSITIVE BATTERY GRIDS, AND POSITIVE GRID OBTAINED ACCORDING TO SUCH

PROCESS

The present invention relates generally to grids of lead-acid accumulators for batteries and more particularly to a process for the continuous production of positive grids.

Recent developments in the design of motor vehicles have led to changes in the ventilation conditions of the engine compartments. Consequently, the temperature prevailing under the hood of an engine compartment of a motor vehicle has increased considerably and is currently around 50 ° C to 55 ° C. Among the engine components that suffer from this temperature rise, the so-called "maintenance-free" or "gas recombination" starter battery is particularly affected.

Indeed, the passage of the temperature from 30 ° C to 55 ° C, considerably increases the kinetics of the transformation processes in the solid state of the alloys in particular of those constituting the positive grids of the batteries of starting batteries, and therefore the kinetics corrosion of said grids.

In addition, in stationary batteries used for energy storage in hot regions, and in battery packs mounted very compactly against each other, the positive grids of accumulators are subjected to intense mechanical stresses. and chemical (corrosion phenomenon).

Thus, we are looking for alloys for positive grids of increasingly efficient accumulators, in particular with good corrosion resistance and constant high mechanical properties over time.

For the batteries described above, it is therefore more and more excluded to use positive grids called antimony, and it is the alloys of lead - calcium - tin which have been used for several years to make such grids.

The content of tin in the lead alloy conditions both the corrosion resistance of the alloy and the conductivity of the interface between the positive metallic grid and the active material (lead oxide) which it supports. This last characteristic is particularly sought after in the case of starter batteries which, throughout their life, experience repeated deeper charge-discharge cycles.

Calcium gives lead very interesting mechanical properties.

The solubility of calcium in the lead matrix is however conditioned by the tin content of the alloy. The higher the tin content, the more insoluble the calcium at low temperatures. For such lead alloys with well-known structural hardening, faced with a high tin content, the calcium content must be fixed in a low concentration range if one wishes to avoid the premature appearance of the phenomenon of aging. Monitoring, which develops at the grain boundaries, is in the form of precipitates (Pbι- _X Sn _κ ) ₃ Ca, and causes a reduction in the mechanical properties of the affected area which is subject to accelerated corrosion.

The tin content being fixed, it then completely determines the range of calcium contents. Manufacturers of lead - calcium - tin alloys also add a small amount of aluminum, at a relative concentration by weight of between 0.001% and 0.035%, to prevent oxidation of the molten alloy. The current trend in the lead-acid battery manufacturing industries is to favor the production of positive grids using continuous development processes. These continuous manufacturing development processes are more productive and provide consistent quality grids.

The principle of these continuous manufacturing processes of positive grids is to produce a blank from a continuous casting of alloy then, to subject this blank to a more or less significant mechanical deformation, either during cutting and of an expansion of the blank either during rolling to produce the positive grids at the desired thickness.

The term “blank” is understood here to mean the result of the solidification by cooling of the alloy casting.

The choice of the relatively low calcium content, imposed by the tin content, of the lead - calcium - tin alloy used for the production of positive accumulator grids, results in obtaining a grain size. several hundred microns in the alloy castings carried out in a first phase of a continuous manufacturing process of grids, of the aforementioned type.

It can then be seen that on a thickness of the casting blank of 1 millimeter, two or even three grains have developed in the thickness. When such an alloy casting blank is subsequently subjected to mechanical treatment, either during cutting - expansion of the casting - either during a rolling of the latter, the large grain cast alloy localizes the mechanical deformation on a few rare grain boundaries. As a result, it has a lower resistance to corrosion and reduced mechanical characteristics.

Indeed, in the same way as the rise in temperature, work hardening (effect of the mechanical stress of the alloy) is a motor of the aging process and beyond the aging of the microstructure of said alloy.

Thus, when such positive lead alloy grids produced according to a known continuous manufacturing process, are subjected to the corrosion conditions, they develop penetrating corrosion at the grain boundaries of the alloy constituting them. Under the combined effect of the stresses generated by very voluminous corrosion products and the stresses coming from the positive active material (lead oxide), the size of the positive grids thus produced increases, progressively deteriorating the electrical contact between the grids and the active ingredient and which can eventually lead to a short circuit in the battery. When the blank from the lead-calcium-tin alloy casting is subsequently laminated, it is then observed that the casting structure is overturned. The grains of this structure are mostly refined and oriented in the rolling direction. This then results in a much better resistance to corrosion.

However, on examination of the rolling microstructure, it is observed that the alloy consists of an alternation of layers with fine grains having undergone a significant work hardening (recrystallization zone) and coarse-grained layers, signatures of the casting microstructure.

The subsequent expansion of this laminated alloy casting blank then affects these different areas differently and locally causes overaging sites.

It has also been observed a dramatic drop in mechanical properties and corrosion resistance, after having aged, six months at 60 ° C, positive grids made of a lead alloy

(of composition: Ca = 0.075%, Sn = 1.2%, Al = 0.03%) resulting from such a continuous manufacturing process.

Thus, it is clear that the mechanical treatment of the cast blank of a lead - calcium - tin alloy according to a continuous manufacturing process of positive grids, leads to an altered microstructure or carrying sites of premature aging, such grids positive thus obtained with lower corrosion resistance and reduced mechanical characteristics.

In order to overcome the aforementioned drawbacks, the present invention provides a new process for the continuous production of positive grids of lead-acid accumulators, which makes it possible to obtain positive grids having a high hardness, an ability to retain high mechanical properties. level, constant over time, and good corrosion resistance. More particularly, the continuous manufacturing process according to the invention comprises the following steps: a) a lead alloy is prepared which comprises calcium, at a relative concentration by weight of between 0.04% and 0.12%, tin at a relative concentration by weight of less than 3%, aluminum , at a relative concentration by weight between 0.001% and 0.035%, and barium, at a relative concentration by weight between 0.02% and 0.1%, b) A blank is produced from a continuous casting of said lead alloy, c) mechanically treating said blank to form said positive grids.

The fine analysis of the microstructure of the alloy constituting said positive grids obtained according to the process according to the invention, made it possible to note that tin is rejected in the intercellular space in the state of precipitate of the Sn ₃ type Bao, 9Cao, ι. Such a precipitate makes it possible to stabilize the microstructure of the alloy. As a result, the choice of tin and calcium concentrations for the alloy is freed. Higher calcium contents can be used because in such a lead-calcium-tin alloy supplemented with barium, the high calcium contents do not induce an acceleration of the aging of the alloy. The grain boundaries remain remarkably fine during aging tests at 60 ° C over several thousand hours for example.

It is then possible according to the invention to adequately combine the respective relative concentrations by weight of tin (for corrosion resistance), calcium (for the fineness of the grain) and barium to obtain an alloy casting blank. having a desired fineness of grain, that is to say of small size and extending uniformly in all directions. Thus according to the invention, positive grids of small thickness, of about one millimeter, are produced, in which many grains in the thickness are crystallized, such grids then being less sensitive to cracking corrosion.

In addition, the microstructure of the preform resulting from the continuous casting of lead alloy produced according to the invention, supports without deteriorating over time (aging), the mechanical treatment for the final shaping of said positive grids. In fact, an absence of transformation of the casting microstructure has been observed under the combined effect of temperature, work hardening and time, which gives the positive grids an improved corrosion resistance. The corrosion process of said positive grids develops uniformly on the grains of the alloy instead of preferentially attacking the grain boundaries. At identical weight loss, the positive grids obtained according to the process according to the invention, which contain barium, retain their mechanical characteristics and their dimensions better than the positive grids of the prior art which are free of barium.

According to one embodiment of the method according to the invention, in step a) a lead alloy is prepared whose relative concentration by weight of calcium is between 0.05% and 0.11%, the relative concentration by weight tin is between 0.5% and 2.5% and the relative concentration by weight of aluminum is between 0.0016% and 0.025%.

Preferably, in accordance with the method according to the invention, in step a) a lead alloy is prepared which comprises a relative concentration by weight of calcium equal to approximately 0.11% and a relative concentration by weight of barium equal to approximately 0.084%.

Such an alloy has an even finer grain structure which is stable over time. According to one embodiment of the method according to the invention, in step b) an alloy strip is formed, the thickness of which corresponds to the thickness of the grids targeted and in step c) said strip d is deployed. alloy in a series of positive grids. According to a variant of the method according to the present invention, in step b) thick grids are formed and in step c) the grids are laminated to the desired thickness.

According to another variant of the process according to the invention, in step b) a thick alloy strip is formed and in step c) said alloy strip is laminated to the desired thickness and then said strip is deployed laminated into a series of positive grids.

The present invention also provides positive grids of lead-acid accumulators manufactured according to the aforementioned process.

Other aspects and advantages of the present invention will appear better on reading the following detailed description, made with reference to the appended drawings, in which:

FIG. 1 shows the alloy microstructure of a positive grid of accumulator obtained according to a continuous manufacturing process of the state of the art, seen in transverse and longitudinal sections,

FIG. 2 shows the alloy microstructure of a positive grid seen in cross section and horizontal, obtained according to the continuous manufacturing process in accordance with the present invention, FIG. 3 illustrates the corrosion behavior of two alloys, a first free of barium and a second comprising barium (prepared during step a) of the process according to the invention),

FIG. 4 shows two microstructures of two positive lead alloy grids with and without barium, obtained by continuous casting of an alloy strip, rolling and expansion of the laminated alloy strip,

FIG. 5 shows the alloy microstructure of a positive grid obtained according to a preferred variant of the method according to the invention. The alloys are prepared as follows:

Commercial alloys of secondary fusion are used as basic alloys. The relative composition by weight of these alloys is indicated in Tables I and II. So-called soft lead (primary) is added when the content of the elements contained in commercial alloys has to be diluted. Its composition is given in Table III. We also add master alloys Pb-19% Sn, Pb-0.4% Ba and Pb-0.14% Ca-0.13% Ba depending on the case. The materials described with reference to Figures 1, 2, 3 were poured to form a strip of the desired thickness and then deployed. The molten metal was kept at 400 ° C and the temperature of the casting drum was set at 40 ° C. The casting speed was 15 m / min and the thickness of the strip was 1mm.

The materials described in Figures 4 and 5 were cast in a strip 7 mm thick and a speed of 40 ° C / s cooling from a molten alloy at 450 ° C.

Figure 1 shows cross and horizontal sections of a positive grid of a lead alloy containing in relative concentration by weight, 0.069% calcium, 1.24% tin and 0.0016% aluminum. This grid is obtained according to a process of the prior art continuously from a lead alloy free of barium, by carrying out a continuous casting of a strip of said alloy to the desired thickness and expansion of the strip in the form of a series of grids.

The sections shown were made on an un aged material. It can be seen that the reagent for revealing the alloy microstructure of this grid preferentially attacked many grains, a sign of premature aging of the structure. It can also be noted that in the angle of the two strands, the microstructure of the gate alloy is typical of a material recrystallized under the effect of mechanical work undergone. This area of the grid is much more sensitive to revelation and will therefore also be more prone to corrosion when the grid is in service. Figure 2 shows cross sections (microstructure from above) and horizontal (microstructure from below) of an alloy of a positive grid of accumulator produced according to the method according to the invention, and more particularly made of an alloy of lead which contains in relative concentration by weight, 0.071% of calcium, 1.22% of tin and 0.002% of aluminum as well as 0.04% of barium. The grid is obtained more particularly by the formation of a continuous casting of said aforementioned lead alloy to form a strip of the desired thickness. Then we deployed in the form of a series of grids, the alloy strip obtained during continuous casting.

It can be seen in FIG. 2 that the alloy microstructure of the grid has completely preserved its casting structure, a sign of the complete absence of aging at the end of the mechanical treatment of the casting. Such an alloy will then exhibit increased corrosion resistance as well as good mechanical properties which are constant over time. FIG. 3 shows alloy corrosion facies of two positive grids, the first positive grid (top facies) is obtained according to a process of the state of the art and consists of a lead - calcium - tin alloy - barium-free aluminum (Ca = 0.05%, Sn = 0.65%, Al = 0.004%) and the second grid (facies below) is obtained according to the process according to the invention from an alloy of lead comprising the same relative concentrations by weight of calcium, tin and aluminum and an addition of barium at a relative concentration by weight of 0.028%.

After five days, at 50 ° C., at a potential of 1.45 volts relative to the mercury sulphate electrode, the corrosion facies of the barium-free alloy constituting the positive grid of the prior art is clearly of the intergranular type whereas for the second positive grid obtained according to the process according to the invention, the corrosion of the alloy is uniform. Note, with regard to the grid obtained according to the method according to the invention, the refinement of the grain size of the alloy.

Figure 4 shows the change in microstructure between a positive grid of a lead alloy

(containing in relative concentration by weight Ca = 0.076%, Sn = 0.7% and Al = 0.025%) without barium (top microstructure), which has conventionally been continuously cast, rolled and deployed and a positive grid of lead alloy obtained according to the invention (microstructure of below), by carrying out a blank for continuous casting of lead alloy (Ca = 0.076%, Sn = 0.7% and Al = 0.025%) added with barium to form a thick strip, rolling the thick strip to the desired thickness and expansion of this strip in the form of a series of grids. Note the very refining effect of the contribution of 0.08% barium on the microstructure of the alloy which constitutes the positive grid obtained according to the invention.

FIG. 5 illustrates that a grain structure of lead alloy (containing a relative concentration by weight of tin equal to approximately 0.7%) even finer and stable over time (aging of 10,000 hours at 60 ° ), is obtained according to the invention under the combined effect of the choice of a high calcium content Ca = 0.11% (relative concentration by weight) and the addition of barium Ba = 0.0843% relative concentration in weight).

Examples

Example 1.

The mechanical performances of the materials of Figures 1 and 2 were characterized by measuring the Vickers hardness under 2 kg, either directly after grid expansion, or after a few thousand hours at 60 ° C. Table IV Hardness versus time and addition of barium

There is not only a reduction in the hardness of the material not doped with barium, but also the higher level of mechanical characteristics of the latter without alteration over time. Example 2.

The materials, shown in FIG. 3, were followed for their mechanical performance just after their development and after having subjected them to an aging stage of 192 hours at 60 ° C. and corrosion tests of 120 hours at 50 ° C. The hardness measurement is carried out using a microdurometer (load of 25 grams), under a microscope, in order to measure the hardness of the over-aged areas.

Table V Measurement of the microhardness of an alloy doped or not with barium

There is an astonishing conservation of the performances of the alloy with barium and of the microstructure which does not suffer from aging.

Example 3.

7mm thick blanks were cast and then rolled with a hardening factor of 68%. By way of comparison, an industrial casting and rolling alloy according to the state of the art is also described. Table VI below gives the evolution of the hardness over time at 60 ° C.

Alloy A Ca = 0.073%, Sn = l. 23%, A1 = 0.0091%

Alloy B Ca = 0.09%, Sn = 1.27%, Al = 0.025%, Ba = 0.026% (laboratory material)

Alloy C Ca = 0.076%, Sn = l. 19%, A1 = 0.0167% (industrial material)

Table VI Evolution of hardness as a function of time

It is observed that the casting and rolling conditions carried out in the laboratory are well corroborated by the results of the alloy C. The addition of barium to a blank intended to be hardened by rolling does not undergo any alteration in its microstructural characteristics. under the combined effect of temperature _and 1 'ecrouissement applied.

Table I Composition of one of the basic commercial allies

Table II Composition of one of the basic commercial allies

Table III Composition of the so-called soft lomb of first fusion

Claims

1. A process for the continuous production of positive grids of acid lead accumulators, characterized in that it comprises the following stages: a) a lead alloy is prepared which comprises calcium, at a relative concentration by weight of between

0.04% and 0.12%, tin at a relative concentration by weight of less than 3%, aluminum, at a relative concentration by weight of between 0.001% and

0.035%, and barium, at a relative concentration by weight of between 0.02% and 0.1%, b) a blank is produced from a continuous casting of said lead alloy, c) said blank is mechanically treated to form said positive grids.

2. Method according to claim 1, characterized in that in step a) a lead alloy is prepared whose relative concentration by weight of calcium is between 0.05% and 0.11%, the relative concentration in weight of tin is between 0.5% and 2.5% and the relative concentration by weight of aluminum is between 0.0016% and 0.025%.

3. Method according to one of claims 1 or 2, characterized in that in step a) a lead alloy is prepared which comprises a relative concentration by weight of calcium equal to approximately 0.11% and a relative concentration by weight of barium equal to approximately 0.084%.

4. Method according to one of claims 1 to 3, characterized in that in step b) an alloy strip is formed whose thickness corresponds to the thickness of target grid and in step c) said alloy strip is deployed in a series of positive grids.

5. Method according to one of claims 1 to 3, characterized in that in step b) forming thick grids and in step c) laminating said grids to the desired thickness.

6. Method according to one of claims 1 to 3, characterized in that in step b) forming a thick alloy strip and in step c) laminating said alloy strip to the thickness desired then deploying said laminated strip in a series of positive grids.

7. Positive lead acid battery, characterized in that it is produced according to the method according to one of claims 1 to 6.