Page 186 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 6.1 1 Low-melting Alloys
6.ll.l Lead

Lead (Pb, after plumbum, the root of the word “plumber”) has characteristic prop-
erties of high density, resistance to corrosion (by virtue of the stable lead-oxide layer
that forms to protect the surface), softness, low strength, ductility, and good work-
ability. Alloying it with various elements (such as antimony and tin) enhances its
desirable properties, making it suitable for piping, collapsible tubing, bearing alloys
(Babbitt), cable sheathing, foil (as thin as 0.01 mm), roofing, and lead-acid storage
batteries. Lead also is used for damping sound and vibrations, radiation shielding
against X-rays, ammunition, as weights, and in the chemical industry. Because it
creeps even at room temperature, the use of lead for load-bearing applications is
very limited.
The oldest known lead artifacts were made in about 3000 B.C. Lead pipes
made by the Romans and installed in the Roman baths in Bath, England, two mil-
lennia ago are still in use. Lead is also an alloying element in solders, steels, and cop-
per alloys; it promotes corrosion resistance and machinability. An additional use of
lead is as a solid lubricant for hot-metal-forming operations. Because of its toxicity,
however, environmental contamination by lead (causing lead poisoning) is a major
concern; major efforts are currently being made to replace lead with other elements
(such as lead-free solders, Section 32.3.1). The most important mineral source of
lead is galena (PbS); it is mined, smelted, and refined by chemical treatments.

6.l l.2 Zinc
Zinc (Zn), is bluish white in color and is the metal that is fourth most utilized indus-
trially, after iron, aluminum, and copper. Although its existence was known for
many centuries, zinc was not developed until the 18th century. It has three major
uses: (1) for galvanizing iron, steel sheet, and wire, (2) as an alloy in other metals,
and (3) as a material in castings.
In galvanizing, zinc serves as an anode and protects the steel (cathode) from
corrosive attack should the coating be scratched or punctured. Zinc also is used
as an alloying element; brass, for example, is an alloy of copper and zinc. Major
alloying elements in zinc-based alloys are aluminum, copper, and magnesium; they
impart strength and provide dimensional control during casting of the metal. Zinc-
based alloys are used extensively in die casting for making such products as fuel
pumps and grills for automobiles, components for household appliances such as
vacuum cleaners and washing machines, kitchen equipment, various machinery
parts, and photoengraving equipment. Another use for zinc is in superplastic alloys
A very fine grained 78% Zn-22% Al sheet is a common example of a superplastic
zinc alloy that can be formed by methods used for forming plastics or metals.

Production. A number of minerals containing zinc are found in nature. The prin-
cipal mineral source is zinc sulfide, also called zincblende. The ore first is roasted in
air and converted to zinc oxide. It then is reduced to zinc either electrolytically (with
the use of sulfuric acid) or by heating it in a furnace with coal (which causes the
molten zinc to separate).

6.l l.3 Tin
Although used in small amounts compared with iron, aluminum, or copper, tin
Sn, from the Latin stannum is an im ortant metal. The most extensive use of tin
p
a silver-white lustrous metal is as a rotective coatin on steel sheets tin lates
> P g