Page 187 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications
used in making containers (tin cans) for food and for various other products. The
low shear strength of the tin coatings on steel sheets improves deep drawability
(Section 16.7.1) and performance in general pressworking. Unlike galvanized
steels, if this coating is punctured or destroyed, the steel corrodes because the tin is
cathodic.
Unalloyed tin is used in such applications as a lining material for water distil-
lation plants and as a molten layer of metal in the production of float glass plate
(Section 18.3.1). Tin-based alloys (also called white metals) generally contain cop-
per, antimony, and lead. The alloying elements impart hardness, strength, and corro-
sion resistance. Tin is an alloying element for dental alloys and for bronze
(copper-tin alloy), titanium, and zirconium alloys. Tin-lead alloys are common
soldering materials, with a wide range of compositions and melting points.
Because of their low friction coefficients (which result from low shear strength
and low adhesion), some tin alloys are used as journal-bearing materials. These
alloys are known as babbitts (after I. Babbitt, 1799-1862) and contain tin, copper,
and antimony. Pewter, an alloy of tin, copper, and antimony, was developed in the
15th century and has been used for tableware, hollowware, and decorative artifacts.
Organ pipes are made of tin alloys. The most important tin mineral is cassiterite (tin
oxide), which is of low grade. The ore is mined, concentrated by various techniques,
smelted, refined, and cast into ingots for further processing.

6.l2 Precious Metals

The most important precious (costly) metals, also called noble metals, are the
following:

° Gold (Au, from the Latin aurum) is soft and ductile and has good corrosion
resistance at any temperature. Typical applications include jewelry, coinage,
reflectors, gold leaf for decorative purposes, dental work, electroplating, and
electrical contacts and terminals.
° Silver (Ag, from the Latin argentum) is ductile and has the highest electrical and
thermal conductivity of any metal (Table 3.2). However, it develops an oxide
film that affects its surface characteristics and appearance. Typical applications
for silver include tableware, jewelry, coinage, electroplating, electrical contacts,
solders, bearing linings, and food and chemical equipment. Sterling silz/er is an
alloy of silver and 7.5% copper.
° Platinum (Pt) is a soft, ductile, grayish-white metal that has good corrosion resist-
ance even at elevated temperatures. Platinum alloys are used as electrical contacts;
for spark-plug electrodes; as catalysts for automobile pollution-control devices;
in filaments and nozzles; in dies for extruding glass fibers (Section 18.3.4), in
thermocouples; and in jewelry and dental work.

6.13 Shape-memory Alloys (Smart Materials)

Shape-memory alloys are unique in that, after being plastically deformed into vari-
ous shapes at room temperature, they return to their original shape upon heating.
For example, a piece of straight wire made of such material can be wound into the
shape of a helical spring; when heated with a match, the spring uncoils and returns