Page 82 - Shigley's Mechanical Engineering Design

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Materials 57
Titanium
Titanium and its alloys are similar in strength to moderate-strength steel but weigh half
as much as steel. The material exhibits very good resistence to corrosion, has low ther-
mal conductivity, is nonmagnetic, and has high-temperature strength. Its modulus of
elasticity is between those of steel and aluminum at 16.5 Mpsi (114 GPa). Because of
its many advantages over steel and aluminum, applications include: aerospace and mil-
itary aircraft structures and components, marine hardware, chemical tanks and process-
ing equipment, fluid handling systems, and human internal replacement devices. The
disadvantages of titanium are its high cost compared to steel and aluminum and the dif-
ficulty of machining it.

Copper-Base Alloys
When copper is alloyed with zinc, it is usually called brass. If it is alloyed with another
element, it is often called bronze. Sometimes the other element is specified too, as, for ex-
ample, tin bronze or phosphor bronze. There are hundreds of variations in each category.
Brass with 5 to 15 Percent Zinc
The low-zinc brasses are easy to cold work, especially those with the higher zinc con-
tent. They are ductile but often hard to machine. The corrosion resistance is good. Alloys
included in this group are gilding brass (5 percent Zn), commercial bronze (10 percent Zn),
and red brass (15 percent Zn). Gilding brass is used mostly for jewelry and articles to
be gold-plated; it has the same ductility as copper but greater strength, accompanied by
poor machining characteristics. Commercial bronze is used for jewelry and for forgings
and stampings, because of its ductility. Its machining properties are poor, but it has
excellent cold-working properties. Red brass has good corrosion resistance as well as
high-temperature strength. Because of this it is used a great deal in the form of tubing or
piping to carry hot water in such applications as radiators or condensers.
Brass with 20 to 36 Percent Zinc
Included in the intermediate-zinc group are low brass (20 percent Zn), cartridge brass
(30 percent Zn), and yellow brass (35 percent Zn). Since zinc is cheaper than copper,
these alloys cost less than those with more copper and less zinc. They also have better
machinability and slightly greater strength; this is offset, however, by poor corrosion
resistance and the possibility of cracking at points of residual stresses. Low brass is very
similar to red brass and is used for articles requiring deep-drawing operations. Of the
copper-zinc alloys, cartridge brass has the best combination of ductility and strength.
Cartridge cases were originally manufactured entirely by cold working; the process
consisted of a series of deep draws, each draw being followed by an anneal to place the
material in condition for the next draw, hence the name cartridge brass. Although the
hot-working ability of yellow brass is poor, it can be used in practically any other fab-
ricating process and is therefore employed in a large variety of products.
When small amounts of lead are added to the brasses, their machinability is greatly
improved and there is some improvement in their abilities to be hot-worked. The
addition of lead impairs both the cold-working and welding properties. In this group are
1
low-leaded brass (32 percent Zn, 1 percent Pb), high-leaded brass (34 percent Zn,
2 2
1
2 percent Pb), and free-cutting brass (35 percent Zn, 3 percent Pb). The low-leaded
2
brass is not only easy to machine but has good cold-working properties. It is used for
various screw-machine parts. High-leaded brass, sometimes called engraver’s brass, is
used for instrument, lock, and watch parts. Free-cutting brass is also used for screw-
machine parts and has good corrosion resistance with excellent mechanical properties.