ZINC AND ZINC ALLOYS
Zinc, one of the least expensive nonferrous metals, is produced from
sulfide, silicate, or carbonate ores by a process involving concentration
and roasting followed either by thermal reduction in a zinc-lead blast
furnace or by leaching out the oxide with sulfuric acid and electrolyzing
the solution after purification. Zinc distilled from blast-furnace production
typically contains Pb, Cd, and Fe impurities that may be eliminated
by fractional redistillation to produce zinc of 99.991 percent purity.
Metal of equal purity can be directly produced by the electrolytic process.
Zinc ingot range from cast balls weighing a fraction of a pound to a
1.1-ton [1 metric ton (t)] ‘‘jumbo’’ blocks. Over 20 percent of zinc
metal produced each year is from recycled scrap.
The three standard grades of zinc available in the United States are
described in ASTM specification B-6 (see Table 6.4.52). Special high
grade is overwhelmingly the most commercially important and is used
in all applications except as a coating for steel articles galvanized after
fabrication. In other applications, notably die casting, the higher levels
of impurities present in grades other than special high grade can have
harmful effects on corrosion resistance, dimensional stability, and formability.
Special high grade is also used as the starting point for brasses
containing zinc.
Galvanized Coatings
Protective coatings for steel constitute the largest use of zinc and rely
upon the galvanic or sacrificial property of zinc relative to steel. Lead
can be added to produce the solidified surface pattern called spangle
preferred on unpainted articles. The addition of aluminum in amounts of
5 and 55 percent results in coatings with improved corrosion resistance
termed Galfan and Galvalume, respectively.
Zinc Die Castings
Zinc alloys are particularly well suited for making die castings since the
melting point is reasonably low, resulting in long die life even with
ordinary steels. High accuracies and good surface finish are possible.

Alloys currently used for die castings in the United States are covered
by ASTM Specifications B-86 and B-669. Nominal compositions and
typical properties of these compositions are given in Table 6.4.53. The
low limits of impurities are necessary to avoid disintegration of the
casting by intergranular corrosion under moist atmospheric conditions.
The presence of magnesium or nickel prevents this if the impurities are
not higher than the specification values. The mechanical properties
shown in Table 6.4.53 are from die-cast test pieces of 0.25-in (6.4-mm)
section thickness. Zinc die castings can be produced with section thicknesses
as low as 0.03 in (0.75 mm) so that considerable variations in
properties from those listed must be expected. These die-casting alloys
generally increase in strength with increasing aluminum content. The 8,
12, and 27 percent alloys can also be gravity-cast by other means.
Increasing copper content results in growth of dimensions at elevated
temperatures. A measurement of the expansion of the die casting after
exposure to water vapor at 203°F (95°C) for 10 days is a suitable index
of not only stability but also freedom from susceptibility to intergranular
corrosion. When held at room temperature, the copper-containing
alloys begin to shrink immediately after removal from the dies; total
shrinkage will be approximately two-thirds complete in 5 weeks. The
maximum extent of this shrinkage is about 0.001 in/in (10 mm/cm). The
copper-free alloys do not exhibit this effect. Stabilization may be effected
by heating the alloys for 3 to 6 h at 203°F (95°C), followed by air
cooling to room temperature.
Wrought Zinc
Zinc rolled in the form of sheet, strip, or plate of various thicknesses is
used extensively for automobile trim, dry-cell battery cans, fuses, and
plumbing applications. Compositions of standard alloys are shown in
Table 6.4.54. In addition, a comparatively new series of alloys containing
titanium, exhibiting increased strength and creep resistance together
with low thermal expansion, has become popular in Europe. A typical
analysis is 0.5 to 0.8 percent Cu, 0.08 to 0.16 percent Ti, and maximum
values of 0.2 percent Pb, 0.015 percent Fe, 0.01 percent Cd, 0.01 percent
Mn, and 0.02 percent Cr. All alloys are produced by hot rolling
followed by cold rolling when some stiffness and temper are required.
Deep-drawing or forming operations are carried out on the softer
grades, while limited forming to produce architectural items, plumbing,
and automotive trim can be carried out using the harder grades.
Alloys containing 0.65 to 1.025 percent Cu are significantly stronger
than unalloyed zinc and can be work-hardened. The addition of 0.01
percent Mg allows design stresses up to 10,000 lb/in2 (69 MPa). The
Zn-Cu-Ti alloy is much stronger, with a typical tensile strength of
25,000 lb/in2 (172 MPa).
Rolled zinc may be easily formed by all standard techniques. The
deformation behavior of rolled zinc and its alloys varies with direction;
its crystal structure renders it nonisotropic. In spite of this, it can be
formed into parts similar to those made of copper, aluminum, and brass,
and usually with the same tools, provided the temperature is not below
70°F (21°C). More severe operations can best be performed at temperatures
up to 125°F (52°C). When a cupping operation is performed, a
take-in of 40 percent on the first draw is usual. Warm, soapy water is
widely used as a lubricant. The soft grades are self-annealing at room
temperature, but harder grades respond to deformation better if they are
annealed between operations. The copper-free zincs are annealed at
212°F (100°C) and the zinc-copper alloys at 440°C (220°C). Welding is possible with a wire of composition similar to the base metals. Soldering
with typical tin-lead alloys is exceptionally easy. The impact extrusion
process is widely used for producing battery cups and similar
articles.
Effect of Temperature
Properties of zinc and zinc alloys are very sensitive to temperature.
Creep resistance decreases with increasing temperature, and this must be
considered in designing articles to withstand continuous load. When
steel screws are used to fasten zinc die castings, maximum long-term
clamping load will be reached if an engagement length 4 times the
diameter of the screw is used along with cut (rather than rolled) threads
on the fasteners.

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