Aluminum Forging Processes: An Overview
By H. James Henning | Published August 17, 2007
This overview covers all aspects of forging aluminum alloys, from equipment needed to heating
for forging and heat treatment, from trimming and machining to design considerations and
Of all structural metals and alloys, the aluminum alloys are the most readily
forged to precise intricate shapes. There are a number of reasons for this:
aluminum alloys are ductile; they can be forged with dies heated essentially to
the same temperatures as the workpiece; they do not develop scale during
heating; and they require lower forging pressures. The specific forging
processes are similar, in some ways, for aluminum and steel … and very
different in others.
Most aluminum-alloy bars are supplied as rounds or rectangular shapes
suitable for forging. Most bars are extruded from billet or ingot, though at least
three producers offer continuously cast, fine-grain bar suitable for direct
heating and subsequent forging.
Forging temperatures are very low, being below incandescent temperatures.
Thus, temperature controls and furnace construction for aluminum are
different from those used with ferrous materials. Radiation type (infra-red)
sensors are increasingly used for measuring billet surface temperatures. Most
indirect-fired, recirculating air furnaces are controlled by internal
Aluminum does not scale, so there are fewer problems from furnace
atmospheres. However, under some heating conditions, "blister" formation is
a problem that is traceable to absorbed hydrogen from moisture in the various
heating furnaces. Most of these blisters appear during or after solution heat
There are few differences in the forging process outlines for aluminum and
steel: Bars are sawed or sheared into slugs, sawing being preferred for bars
larger than 2-in. diameter. These pieces are heated in different types of
indirect-fired ovens, like furnaces. The pieces are preformed, blocked, and
finish forged in progressions that are similar to steels, and the final forgings
usually are cold trimmed — but sometimes hot trimmed.
There are significant differences in the design of flash for aluminum versus
steel forgings, it usually being much thinner. Also, the shrinkage allowances
are less for aluminum than for steels. (0.126 in. / ft for aluminum versus 0.375
in. / ft for steels).
Indirect-fired or electric resistance-type furnaces equipped with internal fans
are often preferred for aluminum. These are almost always captive to the
lower temperature regimes for aluminum. This means new furnaces rather
than modifications for the steel forger contemplating forging aluminum.
Grain size in forgings is highly dependant on the forging reductions and final
temperatures but does not show up until after solution heat treatment where
serious grain growth can occur if not controlled properly during forging. Unlike
steel, there is no grain refinement coming from the heat treatment.
Overheating is a serious event and must be avoided entirely. This means
controls are even more critical than for steels.
Induction heating has some risks, due to a lack of good measurement
systems (although IR systems are improving.) Contact temperature
measuring is preferred in most cases.
Aluminum alloys can be forged in almost all presses and impact equipment,
with a few exceptions. Hammer forging of 7075 or 7050 grades must be more
carefully performed because of their lower melting temperatures and a
tendency for temperature build-up to occur in the forgings during the typically
faster deformation rates.
In presses, it is a relatively easy task to heat the dies and support tooling to
temperatures close to the forging temperatures (typically 725° - 900°F,
depending on alloy.) Most die steels will withstand preheating to temperatures
up to about 850°F before they begin to soften. Thus, die temperatures for
aluminum forgings typically are controlled in the range of 650° - 800°F for
near-net forging, and a broader range is acceptable for conventional forgings.
Hydraulic presses often are preferred for aluminum because they are most
versatile and slow forging speeds are of little concern, compared to what
would be true for steel forgings.
Metal flow in aluminum forging
Aluminum forgings tend to develop surface metal smearing, or "scrubbing,"
wherein the metal in some areas actually pressure welds to the die steels,
and then partially re-welds onto the forging. This occurs especially on die
surfaces that are uni-directionally polished in the direction of the predominant
metal flow. The obvious solution is to lubricate the blanks and the dies, and
especially to polish the dies in the same direction as the metal flows over the
This "scrubbing" can result in blisters because some lubricant or other
contaminants enters into pockets between the loose metal and the base
metal. These "scrub" blisters become obvious on reheating. If the dies are
tempered at 600°- 800°F after polishing in air or in a steam atmosphere, they
form an oxide that is very resistant to the scrubbing. Once re-polished,
however, the dies again become prone to scrubbing defects.
It is common also to forge aluminum in more than one heating, with a pickling
operation and some surface polishing between blocker forging and finish
forging, as for example with larger forgings.. This is when the conditions
described above are removed by polishing.
The closer the die temperatures match the metal temperature, the better is die
filling in deep cavity dies. This is especially true for hydraulic press forging.
Trimming of aluminum forgings — Small aluminum forgings can be cold
trimmed in much the same way as steel forgings. Larger forgings can be hot
trimmed or band-saw trimmed, depending on size and configuration. Again, a
key problem is with pressure welding of aluminum to the hard edges of the
trim dies. Water quenching the forgings immediately after forging improves
cold trimming but excessive delays can cause natural aging, thus re-
Heat treatment of aluminum forgings — Heat treatment of aluminum
forgings consists of a solution treatment followed by quenching in water and
then an artificial bake-aging cycle for 8-16 hours at temperatures between
250° and 350°F, depending on the alloy. Again, the furnaces are more like
those used for tempering steels for the solution cycles and simple, low-
temperature ovens for the aging cycles.
Metallurgical quality issues — Ultrasonic inspection methods
(longitudinal wave) generally are used to determine internal soundness of
aluminum forgings. Coarse grain sizes can cause variations in penetration
and defect resolution. Most harmful internal defects can be detected with
these methods. Some near-surface defects are not readily detected, however
and must be checked with a shear wave testing program. Surface defects can
be detected visually after a suitable etch inspection, followed by suitable
Blisters are obvious in patterns of small "bumps" on the surface of forgings.
These must be removed by polishing, making certain not to compromise
dimensional integrity. As a general rule, it is best to determine the extent of
blistering first and, if it is excessive, the full lot of affected forgings could be
subject to rejection. There are three notable blister types:
• Scrub blisters, where the surface layers are separated with lubricant-black
underneath the blister. Such blisters usually are seen in a repeating pattern.
• Hydrogen blisters, where excessive hydrogen is absorbed during the heating
and heat treating cycles; metal usually is white underneath the blister. Blisters
are not located in a repeating pattern, but random, usually on thinner sections.
• Burned metal blisters, where the metal overheats. Again, these usually are
located in a pattern associated with the largest reductions. The underlying
metal usually appears to be gray.
These last two types of blisters are sufficient cause for rejection and should
not be polished out. Such parts should be rejected at the forge plant.
Dimensional quality issues — Aluminum forgings are produced in a
variety of die shapes, including no draft and flashless configurations. This is
partly because the dies are readily heated to temperatures that are similar to
the aluminum blank material. Aluminum shrinks less than steel and does not
undergo the transformation-related expansions that steels do. This means
that the parts will likely be very similar to the forging dies. When the dies are
correct dimensionally, the forgings are correct. Also, since dies wear very
slowly, they last for many forgings without dimensional changes caused by
tool wear. Often, tools will experience wear out as a result of excessive
polishing to remove pressure welded aluminum from the die steels. This is
why a good parting compound applied to the preforms minimizes this cause of
Distortion during subsequent machining — This problem, discussed
earlier, requires further comment. It has not been fully resolved for large
forgings, nor for some smaller forgings that have very thin sections, before
heat treatment. The practice of cold re-striking has done much to ensure
dimensional stability for most small forgings.
After heat treating at the corresponding thermal cycles for each alloy grade,
the forgings are ready for machining. Some variations include heat treatments
of T652, or T651, where the forgings are cold deformed with tensile or
compressive stresses and with very light reductions. These treatments tend to
reduce distortion and/or stress-corrosion cracking in service. Any of these
processes for impression die forging is that they require further steps and
handling — and thus add costs.
Residual stress — A significant problem with heat-treatable aluminum
forgings is the residual stresses imposed by quenching, and sometimes by
straightening just after quenching. The aging temperatures usually are too low
to adequately remove the residual stresses, which, in some cases, exceed
30,000 psi. Residual stresses often cause warpage during machining and
promote stress corrosion. Partially successful approaches to these problems
include: (1) quenching into boiling water to minimize residual stresses, (2)
cold coining or peening to lower the residual stresses, and (3) reverse
quenching to cooling solution-heat-treated forgings slowly to extremely low
temperatures after quenching, then reheating them rapidly by steam, hot
water, hot dies, etc. The theory behind these methods is that residual stresses
can be minimized by imposing stresses essentially equal but opposite to the
original quenching stresses.
Lubrication — Lubricants used for aluminum-alloy forgings vary from
kerosene to oil-graphite suspensions, to complex, proprietary commercial
compounds. Forging companies frequently mix many of these with one or
more oils, and some companies permit individual press operators to modify
the mixtures slightly for ease of application. Hence, it is difficult to apply a
precise rating to the many possible lubricants.
In general, the graphite suspensions applied by spraying are preferred for
press forging, while water-soluble soaps applied by swabs are frequently used
for hammer forging. Another technique consists of dipping aluminum forging
blanks into caustic solution (10% NaOH) to produce a porous conversion
coating on the surface. Then, the forging blanks are dipped into a slightly
acidic colloidal graphite dispersion and dried prior to forging. This technique
eliminates many of the seizing and galling problems but adds the problem of
corrosion if the forgings are not cleaned soon after forging. The result is a
copper-rich surface on the copper-containing alloys like 2014 or 2025. This
serves as an excellent lubricant as well.
Blisters — Aluminum alloys are susceptible to blister or void formation near
the forging surface. These defects usually are observed after solution heat
treatment. There are at least three types of blisters found on forged aluminum.
When the blisters are opened with a pointed tool, the inside surfaces appear
to be gray, dark brown or black, or white. These colors generally identify
blisters caused by, respectively, (1) incipient melting, (2) improper lubrication,
or (3) dissolved gases. The first type is observed where solution heat-treating
temperatures are too high. The second type of blisters form when inadequate
lubrication causes the surface metal to seize and shear away from the
subsurface metal. Lubricants trapped below the surface volatilize during heat
treatment, causing the blisters. These blisters are most frequently found on
edges of blades and portions of other forgings characterized by rapid metal
The third type of blister is not fully understood. The most popular theory holds
that atomic hydrogen, accumulated during melting, fabricating, and heat
treating concentrates in lattice imperfections, and then changes to the less-
soluble molecular form. Further, it is believed that the blisters form when the
trapped gas expands during the solution-annealing cycle. Whatever the true
origin for the gas-type blisters, they can be prevented by applying commercial,
water-soluble corrosion inhibitors to the forgings before heat treatment.
Another theory about these last blisters is that any serious exposure to
products of combustion in the heating furnaces for forging or heat treating can
promote hydrogen pick-up. This is one reason that furnaces heated with
electric elements, induction, or in indirect gas firing, are preferred.
Forging design — In principle, aluminum alloys can be forged to any shape
consistent with limits set by die design. In the past decade, for example,
forging companies developed a no-draft precision forging that requires little to
no preassembly machining. Tooling costs for such forgings are considerably
higher than for more conventional designs, and so precision forgings should
be specified only where the tooling costs can be amortized over large
production quantities. A frequently quoted rule is that a production order
should total 500 forgings or more for precision forging to be economical. This
figure will vary according to the balance between final machining-cost
reductions and tooling-cost increases. Most forging companies will provide
estimates of both conventional and precision designs so that a direct cost
comparison can be made.
The U.S. Air Force's heavy-press program made possible the closed-die
forging of aluminum-alloy parts up to 20 ft long. There are at least three
presses in the U.S. with capacities of 50,000 tons or greater. Vertical ribs 1/8
inch thick with zero draft have been forged successfully on aluminum parts
weighing over 50 lb. These same parts have thin webs ranging from 3/16 to
1/4 in. thick. Maximum limits on forging size depend a great deal on the
precision requirements and final techniques, but, as a rule, the maximum
forging size (plan area) is in the neighborhood of 3,500 in2
for aluminum die
Small forgings may vary down to a few ounces, which may be forged in
platters or multiple impressions per die. A critical factor in designing small
forgings is the grain-flow patterns.
For example, a horseshoe shape can be forged in platters, but if the grain-
flow pattern is not bent to follow the part contour, there could be losses in
ductility. This is because aluminum forgings are generally dependent on grain
direction for maximum ductility. To explain further, elongations well over 15%
can be typical for longitudinal grain-flow patterns while the corresponding
transverse elongations might be as low as 5-6%.
Because aluminum alloys are not sensitive to scale formation or
contamination, excess stock allowances are necessary only to obtain
dimensional accuracy and provide for localized deviations. Often, precision
forgings are forged to final size and require no excess stock allowances.
Metallurgical design factors — Because some alloys (notably 6061 and
2014) are subject to excessive grain growth, the problems of grain-size
control require carefullly balanced reductions, forging temperatures, and die
temperatures. Frequently, close-tolerance forgings receive only small
reductions during the final stages of forging at comparatively low
temperatures, and are likely to exhibit abnormal grain growth upon
subsequent solution heat treatment. This is rarely a problem with forgings
more conventional in design.
H. James Henning is a consultant to the forging industry. In his column in
every issue of Forging, Ask Jim, he answers forgers' technical and
operational questions. See p. 40 for his latest entry.