Although this third factor may be combined with one or both of the others, discussion
is simplified by considering each factor separately.
The only valid reason for choosing a material is that it performs required functions
at the lowest over-all cost. This result can be achieved in three general ways:
- First, the chosen material may be lower in first cost.
- Second, it may be more economical in the long run because of lower maintenance costs.
- Third, it may have special characteristics peculiarly suiting it to the application.
There are a few applications for which aluminum is more economical than practical
alternates simply on the basis of ingot price; electrical conductor is the outstanding
example. Copper is the only other general-purpose metal used for conductors, and
aluminum is less expensive even on a per-pound basis.
To make a realistic comparison of the basic cost of copper versus aluminum for
conducting electricity, the conductivity and density of the two materials must be
considered. Electrical conductor (EC) grade aluminum is rated at 63% IACS (International
Annealed Copper Standard). Combining this conductivity measure, which is on a volume
basis, with the densities of the two metals yields the result that 0.22 kg of aluminum
has the same conductive capability as 0.45 kg of copper. To complete the cost
comparison it is, of course, necessary to make allowances for fabricating both materials
into final form.
The complete analysis of cost usually leaves aluminum with a clear advantage when the
conductors are large, as for transmission or distribution lines. For smaller conductors,
such as house wiring, the saving in cost of the wire is generally not sufficient to
justify the effort required of the small user to learn to handle a new material. With
very fine wire, such as is employed for winding fractional horsepower motors for
household appliances, fabricating costs overshadow metal cost, and copper is normally
used. This example of electrical conductors illustrates one type of relationship of
metal cost to over-all cost of final product.
A similar situation exists in the competition between zinc and aluminum in die-castings.
Aluminum is cheaper on a volume basis, and this results in a price advantage for
aluminum in large castings. Smaller parts, and those that have very thin sections,
usually cost less in zinc because its lower casting temperature permits longer die
life, better lubrication, and thinner sections. Aluminum die-castings have the advantage
of lightweight, but zinc is more economical to chromium plate. Automotive usage of the
two materials reflects these influences.
Stainless steel is frequently in competition with aluminum for parts and structures
requiring resistance to weathering or other corrosive environments. The ingot price
advantage of aluminum is maintained in fabricated products such as sheet and plate.
There are a few applications where lead and aluminum are in competition; price favors
aluminum. Although these two metals are similar in their ability to withstand the effects
of time and atmosphere, they are otherwise so different that comparisons are difficult,
unless a specific application is considered.
Steel has the largest share of the metal market, largely because of its ability to
fulfill so many, varied requirements at the lowest cost. Aluminum is more economical
than steel only when one or more of its special characteristics can be utilized.
Cost Over Service Life
A most important factor contributing to the increasing use of aluminum for outdoor
structures is the low cost of keeping it presentable in appearance and sound
structurally, with minimum expense for field chipping, spot priming, and repainting.
Many highway accessory items, such as signs, railings, and lighting standards, are so
expensive to paint in the field that the cost of a single repainting equals the
additional price of aluminum. Sometimes the cost of shutting down equipment for
painting or other maintenance is an important consideration. Aluminum structural
alloys have been used in outdoor electric switching stations on this basis for nearly
The widespread use of aluminum for house siding, rain-carrying equipment, and other
items for residential construction stem from consumer appreciation of the ease of
A final consideration in the over-all cost of any piece of equipment or structure is
its residual or scrap value. Aluminum can be remelted with little loss due to oxidation,
and even rather complex structures such as aircraft justify the effort of reclaiming
them for the metal values contained. Good melting scrap is readily marketable, and this
factor should be considered in assessing the over-all cost of an aluminum structure.
Every application of aluminum involves consideration of its performance compared to
that of other materials. Lightweight is the basic reason for using aluminum in all
types of transportation equipment, as well as in moving and movable parts in general.
The list of applications predicated on lightness is almost endless, but other
capabilities are usually required, to the extent that it is sometimes difficult to
assign a top priority.
The universal use of aluminum for aircraft structures probably represents the most
exacting -- and at the same time the most economically rewarding -- use of the
highest-strength aluminum alloys where weight saving is the primary requirement. A
conservative figure for the value of a pound of "lightness" in a modern
long-range civil transport is $2500.
Resistance to weathering is equal in importance to light weight in number of applications
and volume of metal consumed. Those aluminum alloys that are especially formulated for
outdoor exposure are regularly used without paint or other protective finish. Numerous
installations have been exposed for 30 years with no loss of structural integrity, an
acceptable level of architectural appearance, and no maintenance other than the cleaning
effects of rain.
Closely associated with its resistance to weathering is the performance of aluminum in
combination with organic coatings. Properly applied paint coatings on aluminum exhibit
maximum adhesion, and local penetrations of the coating seldom expand. This
characteristic is largely responsible for the extensive application of aluminum
siding for residences.
High electrical conductivity was mentioned before compared with other metals. Even
when reason ruble conductor size and nonmagnetic characteristics are of no concern
(as in the third rail for electric railroads using direct current), aluminum can provide
a given current-carrying capacity at a lower per-foot cost than steel, copper, or any
High thermal conductivity of aluminum parallels its high electrical conductivity.
Heat exchange equipment, including cooking utensils, requires many other characteristics;
thus, thermal conductivity does not control the application to the same extent that
electrical conductivity does in its field. However, aluminum is used where the basic
function is conduction of heat; its other capabilities contribute to the final choice.
The chief function of the cast aluminum sole plate in a hand iron is the conducting of
heat from the heating element to the cloth. The lightweight is attractive to the
housewife, and manufacturing economy (the element is "cast-in", and no
protective finish is required) results in a favorable price.
The low modulus of aluminum causes complications in applications where space is strictly
limited and deflection also must be held to a minimum. Such conditions are rare, and they
are perhaps balanced by other situations where a large elastic deflection is desirable.
Aluminum diving boards utilize the large deflection resulting from low modulus, and
truck bodies for hauling rock also exploit the increased energy absorption in resisting
permanent denting. It is interesting to note that a considerable group of important
structural materials have nearly the same ratio of modulus of elasticity to density.
Steel, aluminum, magnesium, and titanium are close. Wood is not far from this range.
Copper, lead, and cast iron are much heavier in relation to modulus, whereas beryllium
is lighter by a factor of about 4 in relation to its modulus of elasticity.
Aluminum alloys, in general, are somewhat less permeable to magnetic fields than air
(paramagnetic). Uses resulting from this characteristic are not many, but permeability
is an important contributing characteristic in some applications.
Low-temperature applications of aluminum rely on the fact that aluminum does not
become brittle at temperatures at least as low as -423°F (-252°C, the boiling
point of hydrogen).
High reflectance for visible, ultraviolet and infrared radiation makes aluminum the
choice for many types of reflectors.
Aluminum is receptive to a number of unique finishes, including anodic oxide coatings,
chemical conversion coatings, and special finishes for reflectors involving a combination
of electrolytic or chemical brightening for high reflectance and an anodic coating for
long life even in outdoor exposure.
Some other characteristics of aluminum that do not warrant discussion here but should
be mentioned are: short half life of aluminum 28, low absorption for x-rays, high
affinity of oxygen, high resistance to sparking, and colorless corrosion products.
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