Lead-calcium alloys mechanical properties

Rea.ctivity ofLea.d—Ca.lcium Alloys. Precise control of the calcium content is required to control the grain stmcture, corrosion resistance, and mechanical properties of lead—calcium alloys. Calcium reacts readily with air and other elements such as antimony, arsenic, and sulfur to produce oxides or intermetaUic compounds (see Calciumand calciumalloys). In these reactions, calcium is lost and suspended soHds reduce fluidity and castibiUty. The very thin grids that are required for automotive batteries are difficult to cast from lead—calcium alloys. 

Lead—copper alloys are specified because of superior mechanical properties, creep resistance, corrosion resistance, and high temperature stabiUty compared to pure lead. The mechanical properties of lead—copper alloys are compared to pure lead, and to lead—antimony and lead—calcium alloys in Tables 4 and 5. 

Tia is also used as an ahoyiag element ia lead—antimony alloys to improve fluidity and to prevent drossiag, ia lead—calcium alloys to improve mechanical properties and enhance electrochemical performance, ia lead—arsenic alloys to maintain a stable composition, and as an additive to low melting alloys. 

As the grain boundaries move through the casting, they are trapped on defects and impurities. This can result in boundaries that are more prone to corrosion. It has been suggested [25] that heat treatment of the alloy can redissolve the PbsCa particles near the boundary and, thereby, reduce the rate of corrosion of the alloy. Certainly, the mechanical properties, creep rate, and corrosion rates of lead-calcium alloys are... 

Table 2.1. Mechanical properties of binary lead-calcium alloys.

At low tin contents, the calcium initially precipitates rapidly as PbsCa in the same manner as in binary lead-calcium alloys. These alloys have a fine grain structure and quickly reach high hardness as discontinuous precipitation predominates. At higher tin contents, the mode of precipitation changes to a mixed discontinuous precipitation of PbsCa followed by a continuous precipitation reaction of Pb cSn Ca. These reactions lead to overageing of the precipitates and a decrease in mechanical properties. 

In modern maintenance-free batteries, the lead alloy often contains calcium or other alkaline earth elements to make the lead stiffer. Calcium is added to the lead in the range of 0.03 to 0.20 wt% depending on the different battery manufacturers. The current trend is to lower the calcium level to 0.03% to 0.05% for better corrosion resistance. Tin has been added to the lead-calcium alloy to enhance the mechanical and corrosion-resistance properties in the range of 0.25 to 2 wt%. Some battery manufacturers have substituted strontium for calcium. The other type of lead alloy is a quaternary alloy that contains lead, calcium, tin, and aluminum. Aluminum is used to stabilize the dressing loss of calcium while molten. 

Lead—Calcium-Tin Alloys. Tin additions to lead—calcium and lead—calcium—aluminum alloys enhances the mechanical (8) and electrochemical properties (12). Tin additions reduce the rate of aging compared to lead—calcium binary alloys. The positive grid alloys for maintenance-free lead—calcium batteries contain 0.3—1.2 wt % tin and also aluminum. 

Cast lead—calcium—tin alloys usually contain 0.06—0.11 wt % calcium and 0.3 wt % tin. These have excellent fluidity, harden rapidly, have a fine grain stmcture, and are resistant to corrosion. Table 4 Hsts the mechanical properties of cast lead—calcium—tin alloys and other alloys. 

Wrought lead—calcium—tin alloys contain more tin, have higher mechanical strength, exhibit greater stabiUty, and are more creep resistant than the cast alloys. RoUed lead—calcium—tin alloy strip is used to produce automotive battery grids in a continuous process (13). Table 5 Hsts the mechanical properties of roUed lead—calcium—tin alloys, compared with lead—copper and roUed lead—antimony (6 wt %) alloys. 

Wrought lead—calcium—tin anodes have replaced many cast lead—calcium anodes (14). Superior mechanical properties, uniform grain stmcture, low corrosion rates, and lack of casting defects result in increased life for wrought lead—calcium—tin anodes compared to other lead alloy anodes. 

Mechanical properties of cast lead—calcium tin alloys... 

The mechanical properties and ageing response of cast lead alcium-tin alloys have been described by several authors [44,74,75]. The effect of calcium at 0.5 and... 

Table 2.3. Mechanical properties of lead-calcium-tin alloys with 1.5wt.% Sn.

Even higher tin contents (up to 2wt.%) have been reported [89] to provide reduction in the rate of corrosion and growth of positive lead-calcium grids in VRLA batteries employed in standby service at elevated temperatures. The beneficial effects of high tin on positive-grid corrosion in VRLA batteries have recently been confirmed [90]. It is proposed that the improved corrosion resistance is due to the large number of fine precipitate particles and better accommodation of the stresses of corrosion by the high mechanical properties of the alloys. 

Silver imparts little improvement to the mechanical properties of lead-calcium tin-(silver) alloys which are fully aged (Table 2.6), but gives lower elongation in both cast and rolled alloys. There is no effect on the microstructure at 0.03 wt.% Ag [51]. Significant improvement in creep resistance has been observed in alloys which contain silver [99]. Significant increases in cycle-life and capacity have been described... 

Table 2.6. Comparison of mechanical properties of lead-calcium-tin-silver alloys.

Treatment of steel with calcium enhances, for example, the mechanical properties such as formability, drawing, impact, tensile, machinability, resistance to cracking and tearing and leads to an improved surface and internal cleanliness calcium alloys are used to deoxidize magnesium, to strengthen electrodes, and to produce special aluminum alloys, etc. 

The changes in mechanical properties, YS, UTS and creep resistance (CR), of fully aged cast lead—calcium—tin alloys with two different Sn concentrations (0.5 wt% Sn or 1.5 wt% Sn) as a function of Ca content are presented in Fig. 4.36. Data from Ref. [66] were used to plot these dependences. The alloys with low-Ca content (0.02 and 0.03 wt% Ca) have low mechanical properties which improve with an increase of the Ca content up to the peiitectic concentration, pass through a maximum and decrease thereafter. All three measured parameters (YS, UTS and CR) have higher values when the alloys contain 1.5 wt% Sn (higher r value) than at the lower 0.5 wt% Sn level. Tin accelerates the precipitation reaction to completion. This holds for the whole range of calcium concentrations but proceeds at different rates. 

Mechanical properties of cast lead—calcium—tin alloys with 0.5 or 1.5 wt% Sn contents. Data from... 

Mechanical properties of wrought (rolled) lead—calcium—tin alloys with three different Ca concentrations. Data from ref [18] are used. 

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