In the second of his series on making lead-acid batteries, Consultant Mike McDonagh takes a critical look on the process of fabricating battery plates.
There is a school of thought which believes that our modern world’s dependency on technology and energy relies upon methods of torturing nature into submission to get what we need. If that is true, then the lead-acid battery has to be the most perfect format for nature to gain her revenge.
It is difficult to understand how an almost inert material with one of the highest physical densities and so low down in the electrochemical series could be considered as a suitable material for making an electrochemical source of energy. However it is still here and despite every possible effort to replace it with more likely candidates it remains the most used rechargeable system in the world. In this article I intend to examine lead and its properties with relation to its manufacture and its use as a conductor for the active material. Specifically, the processes of gravity casting and pressure die mold casting with relation to the properties of the alloys, additives and the optimisation of the processes to maximise quality and production efficiency.
So why should lead be used as an electrode material? Looking at its chemical and physical properties it is difficult to see why lead should have been chosen at all let alone have endured for as long as it has and in so many applications. Table 1 summarises the chemical and physical properties of lead. Actually, the use of lead in batteries is something of a misnomer; other than its use in Planté cells it is the alloys of lead which are used extensively in both positive and negative electrodes. The decision as to which alloys are used depends upon the application and the production methods. Table 2 lists the alloy, the application and the reasons for using it.
The primary reason for using alloys rather than the pure metal is mainly to provide a stronger frame or support for the active material during the manufacturing process. The alloying additions are chiefly antimony and calcium which increase the UTS and hardness of the alloy, which facilitates the mechanical handling of the frame through the subsequent processes, i.e. adding the active material and assembly of the plates into the cell or battery.
However, with diversification of battery uses and the opening of new markets, the properties of the conducting material have had to be modified in order to enable the lead-acid battery to be used in those applications. For example, no one would consider using a lead antimony grid alloy in a VRLA application due to the detrimental effect of lowering the hydrogen over-voltage and creating excessive gassing.
As can be seen in Table 2 there are two categories of lead alloy, those using antimony and those using calcium as the main hardening agent. In general these alloys are designed for use in the final application. The manufacturing issues have then been resolved largely by further alloy modifications and by designing machinery and processes to handle their differing mechanical and chemical properties. Regarding the use of the alloys, lead antimony has been the most widely used hardening agent due to its compatibility with lead and its effect not only on the mechanical properties but also on the fluidity, enabling grid manufacture via gravity and pressure die casting.
However, increasing demands for maintenance-free and sealed batteries has resulted in batteries which require minimal gas evolution and water loss. The consequential effect of antimony in lowering the hydrogen over-voltage on lead results in water loss due to gas evolution.
In response to this, antimony concentrations in the grid have been reduced to levels where gas evolution is minimised, and as a further improvement, lead calcium has been introduced as an alternative to antimony. Lead calcium alloys are used mainly in applications where gas evolution is minimised, particularly in VRLA batteries.
Lead calcium alloys are used widely in automotive and VRLA batteries due to their higher hydrogen over-voltage potential. However, lead calcium alloys do not have the same mechanical strength or fluidity as lead antimony and therefore require additions of secondary elements to improve both of those properties. This equally applies to reduced antimony alloys which also benefit from the addition of secondary elements, notably tin, arsenic and selenium.
There are consequences from using these alloys, which affect the manufacturing process as well as battery performance in service. As previously mentioned, antimony affects the hydrogen over-voltage on lead and makes the loss of water more pronounced during the charging cycle of the battery in service. However, the benefits for fluidity during the casting processes, the mechanical strength required for handling and the resistance to growth in service, mean that antimony is still widely used, albeit at minimum levels for particular applications.
It is possible to gravity cast pure lead and this is still done for some applications such as Planté and submarine batteries. The overwhelming majority of cases utilise the benefit of an alloying agent such as antimony or calcium to enable the casting to have sufficient mechanical integrity to withstand subsequent processing.
However, these alloys have very different characteristics in relation to their fluidity and solidification behaviour during the casting process. It is the use of the two prime alloys, their secondary elements and the effects of their chemical and metallurgical properties on the manufacturing process which will be discussed in this article.
The common elemental additions to lead alloys and their effects are listed in Table 3. The use of tin, arsenic and selenium is common to both types of alloy and has the same basis for its addition.
Both calcium and antimony have the same effect of increasing the hardness and mechanical strength of the alloy. This is achieved by precipitation of the element as a second phase within the grain structure. This occurs at room temperature from a matrix which is supersaturated with the solute element.
The supersaturation is created by rapid cooling of the alloy in the casting mould from a higher temperature where the antimony or calcium is in a liquid solution. Once trapped in place, the atoms migrate to lower energy positions in the solid lattice, helped by the internal stress energy caused by the mismatch between their physical and chemical properties (Hulme Rothery rules). This happens over a period of time and is the basis of the phenomena called age hardening. This process can be enhanced by the addition of secondary elements and also by adjusting the casting and ageing conditions
Most of the secondary elements are common to both lead calcium and lead antimony. They also have similar functions. Broadly they can be classified into three groups: age hardening, grain refining and fluidity. There are also other effects on the battery performance which will be discussed in a future article.
- Age hardening: this is usually arsenic which enhances the precipitation of the second phase in both lead calcium and lead antimony alloys during the age hardening process. The most likely mechanism is that of Cottrell atmospheres wherein the arsenic molecule or atom introduces strain into the solid metal lattice. Particles of second phase alloy then migrate to these positions to relieve the strain energy. Provided that there are sufficient sites, which is dependent upon the concentration of arsenic, this increases the final hardness, reduces the time to achieve peak hardness and ensures a uniform distribution
- Grain refiners: selenium, copper and sulphur are the most common. Their effect is to precipitate out of solution during the casting process as the alloy nears its freezing temperature. This provides sites for nucleation of the grains as the alloy freezes. The more sites for nucleation, the smaller and more finely dispersed the grain structure. The principle feature is that they must have very limited solubility in lead and therefore precipitate out of solution as the alloy is cast. Temperature control of the whole process is vital to ensuring that the effects of these constituents are maximised.
- Fluidity: mostly tin is added, which affects the surface tension of lead, reducing the contact angle with most materials, particularly the cork used as a mould spray in casting.
In addition to these groups, there is another use for secondary elements. Aluminium is now commonly added to lead calcium alloys, which prevents oxidation and loss of alloying constituents, particularly calcium, in the lead melting pot.
To some extent the processes used to fabricate the grid structures are linked to the alloy type. Lead antimony alloys are mostly gravity cast or pressure die cast. I include the continuous casting method as gravity casting although there are sufficient differences between this and book mould casting to treat them separately. Lead calcium alloys are processed by book mould casting and also from cast and rolled strip, a process which is enabled by the solidification characteristics of the lead calcium alloy. For the purposes of this article, only the book mould gravity casting process will be considered. Pressure die casting, continuous casting and various strip methods of grid production will be dealt with in a separate article.
Book mould gravity casting
This method relies on transferring molten lead alloy from a pot via a feed pipe and a ladle dispenser into a split mould. The temperatures at each stage of this process are critical for several reasons:
- Lead pot – this holds the ingredients in solution, as can be seen from solubility limits. A difference of just 15°C can greatly reduce the amount of calcium or sulphur in the lead alloy before it reaches the casting part of the process. Likewise 15°C too high can cause precipitation of secondary compounds such as Pb3Ca in the lead feed pipe, the pump or the ladle dispense valve, if they are at a lower temperature.
- Lead feed pipe – If the temperature is lower than that of the pot, then this will create precipitation of compounds within the pipe due to their limited solubility. This will build up as internal dross and eventually block the pipe. Removal and cleaning is both time consuming and harmful to those who do the job.
- Ladle – too low a temperature will cause precipitation of alloy components, not only causing potential blockages and false temperature readings from the immersed thermocouple; it can also reduce the concentration of vital hardening components such as calcium and arsenic.
Apart from grain refiners such as sulphur, copper and selenium, other additives designed to improve mechanical properties or reduce oxidation can be present in the alloy. Within the casting process, use of sparingly soluble secondary elements can cause operational problems if incorrect process temperatures or inappropriate casting equipment are used.
Once the lead has been poured into the mould it has to fill the grid impression which has been machined into the mould block, solidify sufficiently to be ejected, then be transferred to a trim die and stacked ready for the age hardening process and subsequent operations. The process must allow the lead alloy to flow into the mould impression but also to cool rapidly enough to allow reasonable casting rates. To achieve this an insulation is used for coating the steel or iron mould face which has the property of reducing the rate of heat transfer from the molten metal to the mould surface. The rate of heat transfer is critical and a proprietary coating of cork powder is sprayed onto the mould surface. The rate of heat transfer is expressed as follows:
Q is the heat transfer with time t (rate of transfer)
K is the thermal conductivity
A is the area
Th and Tc are the hot and cold surfaces, in this case the cork surface and the steel mould surface respectively
d is the distance or thickness of the coating
It is immediately obvious that the transfer rate is dictated by the thermal conductivity and the thickness of the cork layer. For comparison, Q values for steel and iron are around 50 to 80W/m.°C compared to cork of 0.04 to 0.08 W/m.°C .
It is also evident that the thickness of the cork layer is equally important. Further work is often carried out by the operator in order to equalise the cooling rate between the thin and thick sections of the mould impression (frame and wires).This is the process of marking. For this, the operator, after applying the spray, will use a soft tool to scrape away some of the cork in the thicker sections. This effectively will reduce the thickness of the cork and increase the rate of heat transfer for that section (Figure 1). Experience rather than calculation is used and great care should be taken to ensure that the correct casting thicknesses are maintained.
Figure 2 shows an example of marking where the cork is completely removed and the lead alloy is in direct contact with the mould face. This will give dimensional and weight problems at the pasting as well as the casting stages. Companies such as Wirtz have used their experience to incorporate these features into their mould designs and have spraying techniques and materials specifically designed for their equipment, which simplifies this operation and gives better reproducibility.
The two principle alloying additions to lead are antimony and calcium. They have very different solidification characteristics which require different casting conditions through all stages of the casting process from the pot through to the cast grid handling. These conditions will be discussed with relation to the type of alloy which these elements form with lead.
Gravity casting of lead antimony alloys
The properties relevant to casting can be understood by examination of the phase diagram for PbSb. Figure 3 shows the binary phase equilibrium diagram for lead antimony. The real situation is of course more complex due to the addition of other elements and the non-equilibrium conditions of the casting process. However, it will be adequate to explain the cooling, flow and solidification processes which occur during the casting process.
The phase diagram shows the different phases or alloys which are present for a particular concentration level and a particular temperature. The shape is a classic representation of a eutectic system. The lead rich end on the left is pure lead and the antimony rich end is on the right. The alpha and beta phases are the solid solutions of each metal in its alloying partner. In the case of the lead rich phase (alpha) this occurs in the solid state at a temperature of 280°C.
Concentrating on the lead rich end where all the antimony alloys used in lead-acid battery manufacture are found, it is also evident that there is a solid and liquid equilibrium phase (alpha + liquid) which starts at decreasing temperatures as the level of antimony decreases. In other words as the alloy cools from temperatures above 300°C, there is a period where solid and liquid are in equilibrium.
The transition from solid to liquid is not a singular transition it occurs gradually through a range of solid to liquid ratios with liquid being happy to coexist with solid during this transition. At the final point when remaining liquid transforms to solid this remaining liquid is at the eutectic composition of 11.1 wt% Sb.
So what does this mean for the casting process? If you follow the arrow drawn on the diagram and imagine you can look at the lead being poured into a transparent mould, and also that lead was not impervious to most forms of radiation, so that you could see the nucleation and growth of the solid alpha phase as it is swirled round inside the mould impression, you would see the following:
- Just poured into the mould, the temperature of the alloy is 450°C, so the alloy is liquid and flows around the main frame and into the wires. During this time (about 2 seconds or less) the initial precipitate grains nucleate just a few microns in size, and a couple of millimetres apart.
- Down to 310°C the nucleating grains grow rapidly in a dendritic, (tree like) spiky, structure.
- At 260°C these spiky dendrites would form a tumbling mass jamming themselves in tight crevices and corners of the mould impression and restricting flow.
- The final phase around 247°C would be the solidification of the eutectic liquid which would solidify around these grains and in some areas, where a thin wire cross section meets a thicker frame cross section, the remaining liquid in the thicker part would be pulled into the thinner wire as it shrinks away from the frame due to more rapid cooling of the thinner section.
The scenario above, which all takes place in a couple of seconds or less for an average automotive grid, explains why there are characteristic defects with casting low antimony alloys. In particular the removal of remaining eutectic liquid from thicker sections due to shrinkage effects may result in internal voids or depressions in the frame due to atmospheric pressure. Remaining low melting point liquid can also result in cracks being formed as the casting is ejected from the mould.
Using an alloy of less than 2.75% Sb would seem to be an effective answer to the problem of the second phase liquid. Unfortunately, under the casting conditions of battery grids the rapid chilling is a form of supercooling in which there is insufficient time for the antimony solute, rejected from the alpha phase, to diffuse into the lead to form more alpha phase. The result is a higher level of eutectic phase than would be predicted from equilibrium conditions.
Even at levels less than 2% there is substantial eutectic. The effects of this on the electrochemical properties will be discussed in a later article. During the manufacturing process it can give rise to the defects listed above.
Modern low-antimony alloys are formulated to reduce this problem by including grain refiners which reduce the size of the individual dendrite grains by providing more nucleating sites for the initial precipitation, and also by changing the surface energy of the nucleating particle with respect to the liquid lead. In effect, the shape becomes rounder and less spiky.
The net effect is to improve macro flow due to the smaller, rounder nucleating particles, and to allow after flow of the remaining eutectic liquid in the final stages of the solidification process to prevent voids forming in areas of differing cross section. Typical grain refiners and their specified levels in the incoming alloy are listed in Table 3. By their very nature they are only sparingly soluble in lead so the concentrations are low.
It is worth noting that selenium has become the most favoured grain refining agent due to its stability in the lead pot. Sulphur for example is more difficult to maintain due to its ease of oxidation, particularly in constant flow lead returns.
Gravity casting lead calcium
The main difference between lead antimony and lead calcium is the method of solidification. The phase equilibrium diagram for the lead calcium binary system is a peritectic, illustrated in Figure 4. This shows the first one percent addition of calcium at the lead-rich end of the diagram.
Typical values of calcium are from 0.065 to 0.12wt %
It is immediately obvious (to a metallurgist at least) that there is no low melting point liquid formed from a eutectic mixture as with lead antimony, and apart from a small area above 0.08wt% Ca there is no solid/liquid equilibrium. You can also see that the temperature range for the solid solution alpha phase is higher than with the lead antimony system, the alpha phase existing completely as a solid below the solid liquid transition temperature, with no liquid in equilibrium. In plain terms, for lead calcium alloys, the flow and solidification characteristics when filling a grid mould are very different when compared with lead antimony alloys.
Once again taking a point of single concentration, 0.08 wt% in this case, and mapping the phase changes as the alloy cools, we have the following situation:
- At a pour temperature of 500°C the alloy is completely liquid and swirls round the frame and back fills into the wires in less than a couple of seconds
- During the filling process, the alloy begins to nucleate solid particles a few microns in size at the lead surface in contact with the cooler mould face. These are swept up in the flow and form a thickening sludge, which fills the mould cavity and solidifies
- The mould is filled before the solidification temperature at around 321°C. The alloy is relying entirely on superheat introduced in the ladle, to keep the alloy temperature far enough above its melting point, to maintain sufficient fluidity to fill the mould The nucleating grains have grown in size and are roughly spherical and sometimes directional, emanating from the grid/mould interface. There is no after-flow of low melting point liquid between the grains, which are packed together, separated only by grain boundaries.
Considering the differences between the alloys it is necessary to define the processes of casting each alloy. The chief difference to the operator are the temperature settings. Table 4 gives guidelines for the temperatures required for each stage of casting from the lead pot to the mould. The actual temperatures used in casting may vary from those indicated depending upon the design of the casting and the equipment used, but in general it is evident that the use of grain refiners requires higher pot temperatures and that the feed-lines and ladles must be kept at correspondingly higher temperatures. Too high temperatures in the lead pot can increase the production of dross and increase the rate of secondary element loss through oxidation. The temperature is a compromise between maximising the concentration of secondary elements in solution due to the higher solubility at higher temperatures , and minimising the loss of those elements through surface oxidation.
Because of the different melting and solidifying temperatures, calcium alloys must be poured at a higher temperature, and the mould temperature must be kept higher to prevent solidification before the casting impression is completely filled. Additional thickness of mould spray is also important in order to ensure adequate fluidity of the alloy during the filling process.
Other considerations for the casting equipment are the fact that lead calcium castings are considerably softer than lead antimony in the ‘as cast’ state. Use of more ejection pins will be necessary and the transport of the casting after ejection from the mould should be controlled. Likewise the feeding of the casting into the guillotine should use powered rollers.
Modern casting alloys are a complex mixture of additives which all play a distinctive role in the casting process and/or determining the electrochemical properties of the battery in service. Dross is formed by the oxidation of the surface of the lead exposed to the atmosphere. There are important rules to follow:
- Agitation of the surface should be kept to a minimum
- There should be an overflow return system from the ladle
- The ladle should be kept in an inert or reducing atmosphere (low gas flame)
- The dross should not be removed more than once per shift
- The dross should be stirred back into the lead mass before removal
- Only powder should be removed. Sticky metallic residues contain most of the secondary elements
Removal of dross is a critical factor, as secondary elements which assist in grain refining and enhance the performance of the lead-acid battery can be removed, or drastically reduced in concentration if the procedure is incorrect. Calcium also is easily oxidised at the lead surface. Once CaO is formed it is almost impossible to reduce this back to Ca and return it by dissolution into the lead alloy.
Most operators prefer to see a clean surface as the returned trim from the castings slide easily into the shiny molten lead surface. This keeps the return chute clear and prevents blockages from jamming up the moving parts of the machine. The amount of dross must be recorded and weighed and the concentration of key elements in the pot such as calcium and tin must be monitored each day. This will enable the manufacturer not only to control the losses but also to calculate and monitor the amount of secondary elements being lost through drossing procedures.
In summary, there are three critical aspects to getting the best from the standard alloy specifications in terms of ease of manufacturing and in service performance.
- Retaining the primary alloying elements, calcium and antimony, throughout the casting process to maximise their concentration in the grid.
- Ensuring that secondary alloying elements used as grain refiners or age hardening agents are retained in the melting pot.
- Temperatures at each stage of casting, from the melting pot, through delivery pipes to the ladle and then the mould, are set to minimise precipitation of secondary elements or compounds within the lead transport system of the casting machine.
To ensure trouble free processes, good mechanical and chemical properties of the grid and economic production, it is necessary to keep firm control of temperatures at each stage of casting. This should be combined with defined drossing procedures and correct cork spraying methods. Investment in calibrated temperature measuring equipment and improved tools for removing dross is highly recommended and will have an immediate impact not only on the quality but also the cost of the process.
In the next article, the high volume continuous methods of grid production and pressure die casting will be examined along with age hardening processes. Room temperature transformation within the grain structures of lead calcium tin alloys will be discussed with relation to alloy content and processing methods. An introductory discussion of the corrosion of the common lead alloys relating to their mechanical properties, grain structure and in-service conditions will also be included.