Mill products of aluminum vary from foil thinner than tissue paper to plate 6 in.
thick, and from wrist-thick stranded electrical power transmission conductor to
magnet wire finer than human hair.
Ingot, from which all other aluminum products are made, is generally supplied in
one of seven commercial forms.
Unalloyed Ingot. Unalloyed aluminum ingot is furnished in sizes
ranging from small-notched bars weighing a pound or less to large ingots weighing
a ton or more. Unalloyed ingot may vary from about 98 to 99.999%
Al; 99.5% Al is the most common grade.
Electrical conductor (EC) ingot and rotor ingot for motors are special grades in
which impurities objectionable for these applications are controlled.
Rich Alloy Ingot. Although unalloyed ingot is often used as
produced, it is more often necessary to alloy it with other elements such as
chromium, copper, iron, magnesium, manganese, nickel, silicon, titanium, and zinc.
To accomplish this easily in production, rich alloy ingot, or "hardener",
is employed; hardeners contain from less than 1% to as much as 50% of alloying
Casting Alloy Ingot. Large quantities of scrap are consumed in
the production of casting alloy ingot. When the impurity limitations on such
alloys permit the introduction of large amounts of scrap, the resulting product
is referred to as secondary casting alloy ingot. When the composition limits
restrict the use of scrap, the product is designated as primary or virgin casting
alloy ingot. There are many areas of overlap between primary and secondary.
Extrusion ingot. Extrusion ingot is usually furnished in
cylindrical form, both solid and hollow. It varies in outside diameter from
75 to 800 mm. This product is most often made by the direct chill (DC) casting
process or by some modification thereof. It is sawed to lengths varying, conforming
to the requirements of the press in which it will be extruded. With certain alloys,
extrusion ingots are often soaked or preheated before shipment to the extruder.
This treatment, sometimes called homogenizing, permits higher extrusion speeds,
improved surface finish on the extrusions, and longer die life. When extrusions
of the highest quality are required, as in strong alloy aircraft parts, extrusion
billets may be scalped before shipment to remove surface liquation. Hollow ingots
are used to extrude tube and other hollow shapes. These ingots are normally cast
to the required dimensions in the direct chill casting machine, but manufacturing
limitations often require machining of the inside diameter.
Forging Ingot. Although most small forgings are produced from
rolled or extruded stock, cast ingots are often used for large forgings. To
prevent irregularities in the cast surface from affecting the quality of the
forgings, these ingots are always scalped before shipment. Forging ingot is
generally supplied in cylindrical form.
Hot Metal. Molten metal is sometimes transferred directly in
insulated ladles from the smelter to the customer’s plant, occasionally at
distances up to several hundred miles.
In order of commercial importance, the three major forms of aluminum castings are:
die, permanent mold, and sand.
Die casting. Die-casting is inherently suited to large-quantity
production of both ornamental and structural aluminum parts. If the quantity
permits investment in a die, if wall thickness can be reduced as much as the
casting process permits, and if objectionable undercuts are not present in the
design, aluminum castings can usually be produced by this process at a lower cost
than by any other method.
Permanent Mold Castings. In the permanent mold process, cast
iron molds and cores are generally used; less frequently, steel and inlays of
other metals are employed. Molten aluminum is poured into the mold cavity under a
normal gravity head. In special cases, a small amount of pressure is applied to
the mold through the application of vacuum, by pumping the molten metal, or by
centrifugal force. In the semi permanent mold process, cores of dry sand or other
expendable material are employed, overcoming many of the design limitations imposed
by metal cores. Permanent mold castings are metallurgically superior to die or
sand castings, having greater soundness, pressure tightness, higher strength,
greater speed of production, and thinner walls.
Sand Castings. The sand casting process is the most versatile
method of producing a cast aluminum shape and is characterized by universal
adaptability. It is employed to produce small quantities of identical castings,
parts requiring intricate coring, and very large castings. Modern high-speed
molding equipment and methods produce sand castings relatively cheaply.
Other Cast Forms. Direct chill castings -- both solid and hollow,
having round, rectangular or odd-shaped sections -- are used because of their
outstanding mechanical properties after heat treatment and their sound structure.
Mechanical properties approach those of wrought products except that the
elongation is lower. Cast tool and jig plate and large sizes of cast bus bar are
commonly made by this process. When thin walls and close dimensional tolerances
are required, and if the quantity does not warrant die-casting, shell mold
casting, plaster mold casting, or investment casting are often employed. The
last two processes are also used where difficult undercuts and intricate coring
are required. The centrifugal casting process is used for the production of large
aluminum alloy tubes and rings.
Forgings and Pressings
Aluminum alloy forgings and pressings are produced commercially in conventional
hammer equipment, on hydraulic and mechanical presses, in ring rollers, and on
upsetters. They may be classified as die forgings, hand forgings, and rolled
Die Forgings. Depending upon the amount of machining necessary
to obtain a finished part, aluminum die forgings and pressings are categorized
as (a) blocker, requiring the most machining; (b) conventional, providing a good
balance between die cost and machining cost; or (c) precision forgings and
pressings. Precision forgings are of advantage where it is desirable to obtain
thin web sections and thin, accurate ribs with a minimum of machining. Draft
angles are controlled to less than 10 if desired; corner fillets can be held
to a minimum.
Hand forgings are produced by working aluminum stock between
flat dies or other simple tools that shape the piece roughly to the required
contour. Prototypes are frequently made as hand forgings to reduce delivery time.
Slabs as large as 7 in. thick by 120 in. wide and 450 in. long, weighing over
35,000 lb, are available in the form of hand forgings.
Rolled Ring Forgings. Precision ring rolling equipment is
available to produce rolled rings in a wide range of diameters up to 150 in.,
in any wall thickness and alloy. Larger rings can be fabricated by forging over