Aluminum forging processes an overviewDocument Transcript
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 thermocouples.
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 treatment.
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 die surface.
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-hardening.
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
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 tool wear-out.
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
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
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
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
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 flow.
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 forgings.
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
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.