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2
F I G U R E 2 - 21
Tensile Test Stress-Strain Curves of Three Aluminum Alloys (From Fig. 5.17, p. 160, in N. E. Dowling, Mechanical Behavior of Materials, Prentice-Hall, Englewood Cliffs, N.J., 1993, with permission) Some aluminum alloys are hardenable by heat treatment and others by strain hardening or precipitation and aging. High-strength aluminum alloys are about 1.5 times harder than soft steel, and surface treatments such as hard anodizing can bring the surface to a condition harder than the hardest steel.
Aluminum is among the most easily worked of the engineering materials, though it tends to work harden. It casts, machines, welds,* and hot and cold forms† easily. It can also be extruded. Alloys are specially formulated for both sand and die casting as well as for wrought and extruded shapes and for forged parts.
WROUGHT-ALUMINUM ALLOYS are available in a wide variety of stock shapes such as I-beams, angles, channels, bars, strip, sheet, rounds, and tubes. Extrusion allows relatively inexpensive custom shapes as well. The Aluminum Association numbering system for alloys is shown in Table 2-6. The first digit indicates the principal alloying element and defines the series. Hardness is indicated by a suffix containing a letter and up to 3 numbers as defined in the table. The most commonly available and most-used aluminum alloys in machine-design applications are the 2000 and 6000 series.
The oldest aluminum alloy is 2024, which contains 4.5% copper, 1.5% magnesium, and 0.8% manganese. It is among the most machinable of the aluminum alloys and is
* The heat of welding causes
heat treatable. In the higher tempers, such as -T3 and -T4, it has a tensile strength ap-localized annealing, which can
proaching 70 kpsi (483 MPa), which also makes it one of the strongest of the alumi-remove the desirable strengthen-
ing effects of cold work or heat
num alloys. It also has high fatigue strength. However, it has poor weldability and treatment in any metal.
formability compared to the other aluminum alloys.
† Some aluminum alloys will cold-
The 6061 alloy contains 0.6% silicon, 0.27% copper, 1.0% manganese, and 0.2%
work when formed to the degree
chromium. It is widely used in structural applications because of its excellent that trying to bend them again
(without first annealing) will cause
weldability. Its strength is about 40 to 45 kpsi (276 to 310 MPa) in the higher tempers.
fractures. Some bicycle racers
It has lower fatigue strength than 2024 aluminum. It is easily machined and is a popu-prefer steel frames over aluminum
despite their added weight
lar alloy for extrusion, which is a hot-forming process.
because, once an aluminum frame
is bent in a fall, it cannot be
The 7000 series is called aircraft aluminum and is used mostly in airframes. These straightened without cracking.
Damaged steel tube frames can be
are the strongest alloys of aluminum with tensile strengths up to 98 kpsi (676 MPa) and straightened and reused.
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MACHINE DESIGN -
An Integrated Approach
Table 2-6
Aluminum Association Designations of Aluminum Alloys
A partial list - other alloys are available - consult the manufacturers 2
Series
Major Alloying Elements
Secondary Alloys
1xxx
Commercially pure (99%)
None
2xxx
Copper (Cu)
Mg, Mn, Si
3xxx
Manganese (Mn)
Mg, Cu
4xxx
Silicon (Si)
None
5xxx
Magnesium (Mg)
Mn, Cr
6xxx
Magnesium and Silicon
Cu, Mn
7xxx
Zinc (Zn)
Mg, Cu, Cr
Hardness Designations
xxxx-F
As fabricated
xxxx-O
Annealed
xxxx-Hyyy
Work hardened
xxxx-Tyyy
Thermal/age hardened
the highest fatigue strength of about 22 kpsi (152 MPa) @ 108 cycles. Some alloys are also available in an alclad form which bonds a thin layer of pure aluminum to one or both sides to improve corrosion resistance.
CAST-ALUMINUM ALLOYS are differently formulated than the wrought alloys.
Some of these are hardenable but their strength and ductility are less than those of the wrought alloys. Alloys are available for sand casting, die casting, or investment casting. See Appendix A for mechanical properties of wrought- and cast-aluminum alloys.
Titanium
Though discovered as an element in 1791, commercially produced titanium has been available only since the 1940s, so it is among the newest of engineering metals. Titanium can be the answer to an engineer’s prayer in some cases. It has an upper service-temperature limit of 1 200 to 1 400F (650 to 750C), weighs half as much as steel (0.16
lb/in3 {4 429 kg/m3}), and is as strong as a medium-strength steel (135 kpsi {930 MPa}
typical). Its Young’s modulus is 16 to 18 Mpsi (110 to 124 GPa), or about 60% that of steel. Its specific strength approaches that of the strongest alloy steels and exceeds that of medium-strength steels by a factor of 2. Its specific stiffness is greater than that of steel, making it as good or better in limiting deflections. It is also nonmagnetic.
Titanium is very corrosion resistant and is nontoxic, allowing its use in contact with acidic or alkaline foodstuffs and chemicals, and in the human body as replacement heart valves and hip joints, for example. Unfortunately, it is expensive compared to aluminum and steel. It finds much use in the aerospace industry, especially in military aircraft structures and in jet engines, where strength, light weight, and high temperature and corrosion resistance are all required.