Powder metallurgy (P/M) technology provides a useful means
of fabricating net-shape components that enables machining to
be minimized, thereby reducing costs. Aluminum P/M alloys can
therefore compete with conventional aluminum casting alloys,
as well as with other materials, for cost-critical
applications. In addition, P/M technology can be used to
refine microstructures compared with those made by
conventional ingot metallurgy (I/M), which often results in
improved mechanical and corrosion properties. Consequently,
the usefulness of aluminum alloys for high-technology
applications, such as those in aircraft and aerospace
structures, is extended.
Aluminum alloys have numerous technical advantages that have
enabled them to be one of the dominant structural material
families of the 20th century. Aluminum has low density (2.71
g/cm3) compared with competitive metallic alloy systems. It
also has good inherent corrosion resistance because of the
continuous, protective oxide film that forms very quickly
in the air, and good workability that enables aluminum and
its alloys to be economically rolled, extruded, or forged
into useful shapes.
Major alloying additions to aluminum such as copper,
magnesium, zinc, and lithium - alone, or in various
combinations - enable aluminum alloys to attain high
strength. Designers of aircraft and aerospace systems
generally like to use aluminum alloys because they are
reliable, reasonably isotropic and low in cost compared to
more exotic materials such as organic composites.
Aluminum alloys do have limitations compared with competitive
materials. For example, Young`s modulus of aluminum (about 70
GPa, or 10 x 106 psi) is significantly lower than that of
ferrous alloys (about 210 GPa, or 30 x 106 psi) and titanium
alloys (about 112 GPa, or 16 x 106 psi). This lower modulus
is almost exactly offset by the density advantage of aluminum
compared to iron- and titanium-base alloys. Nevertheless,
designers could exploit higher-modulus aluminum alloys in
many stiffness-critical applications.
Aluminum P/M Processing
There are several steps in aluminum P/M technology that can
be combined in various ways, but they will be conveniently
described in three general steps:
- Powder production
- Powder processing (optional)
- Degassing and consolidation.
Powder can be made by various RS processes including
atomization, splat quenching to form particulates, and melt
spinning to form ribbon. Alternatively, powder can be made by
non-RS processes such as by chemical reactions including
precipitation or by machining bulk material.
Powder-processing operations are optional and include
mechanical attrition (for example, ball milling) to modify
powder shape and size or to introduce strengthening features,
or comminution such as that used to cut melt-spun ribbon into
powder flakes for subsequent handling.
Aluminum has a high affinity for moisture, and aluminum
powders readily adsorb water. The elevated temperatures
generally required to consolidate aluminum powder causes
the water of hydration to react and form hydrogen, which
can result in porosity in the final product, or under
confined conditions can cause an explosion. Consequently
aluminum powder must be degassed prior to consolidation.
This is often performed immediately prior to consolidation
at essentially the same temperature as that for consolidation
to reduce fabrication costs. Consolidation may involve
forming a billet that can be subsequently rolled, extruded,
or forged conventionally or the powder may be consolidated
during hot working directly to finished-product form.
Atomization is the most widely used process to produce
aluminum powder. Aluminum is melted, alloyed, and sprayed
through a nozzle to form a stream of very fine panicles that
are rapidly cooled, most often by an expanding gas.
Splat cooling is a process that enables cooling rates even
greater than those obtained in atomization. Aluminum is
melted and alloyed, and liquid droplets are sprayed or
dropped against a chilled surface of high thermal conductivity
- for example, a copper wheel that is water cooled
internally. The resultant splat particulate is removed from
the rotating wheel to allow subsequent droplets to contact
the bare, chilled surface. Cooling rates of 105 K/s are
typical, with rates up to 109 K/s reported.
Melt-spinning techniques are somewhat similar to splat
cooling. The molten aluminum alloy rapidly impinges a cooled,
rotating wheel, producing rapidly solidified product that is
often in ribbon form. The leading commercial melt-spinning
process is the planar flow casting (PFC) process developed
by M.C. Narisimhan and co-workers at Allied-Signal Inc.
The liquid stream contacts a rotating wheel at a carefully
controlled distance to form a thin, rapidly solidified ribbon
and also to reduce oxidation. The ribbon could be used for
specialty applications in its PFC form but is most often
comminuted into flake powder for subsequent degassing and
Mechanical Attrition Process
Mechanical attrition processes often involve ball milling in
various machines and environments. Such processes can be used
to control powder size and distribution to facilitate flow or
subsequent consolidation, introduce strengthening features
from powder surfaces, and enable intermetallics or ceramic
particles to be finely dispersed. The two leading mechanical
attrition processes today - mechanical alloying in the United
States and reaction milling in Europe - are improvements on
sinter-aluminium-pulver (SAP) technology developed by Irmann
SAP Technology. In 1946, Irmann and co-workers were preparing
rod specimens for spectrographic analysis by hot pressing
mixtures of pure aluminum and other metal powders. They
noticed the unexpectedly high hardness of the resulting rods.
Mechanical property evaluations revealed that the hot-pressed
material had strength approaching that of aluminum structural
alloys. Based on microstructural evaluations, Irmann
attributed the high strength of the hot-pressed compacts to
the breakdown of the surface oxide film on the powder
panicles during hot pressing. Irmann performed mechanical
tests at elevated temperatures and showed that these alloys
not only had surprisingly high elevated-temperature strength,
but also retained much of their room-temperature strength
after elevated-temperature exposure.
The mechanical alloying process is a high-energy ball-milling
process that employs a stirred ball mill called an attritor,
a shaken ball mill, or a conventional rotating ball mill.
Elemental powders may be milled with aluminum powders to
effect solid-solution strengthening or to disperse
intermetallics. The dispersed oxides, carbides, and/or
intermetalics create effective dislocation sources during
the milling process and suppress dislocation annihilation
during subsequent working operations, which result in greatly
increased dislocation density.
Thus, mechanical alloying enables the effective
superimposition of numerous strengthening mechanisms,
- Oxide dispersion
- Carbide dispersion
- Fine grain size
- High dislocation density and substructure
- Solid-solution strengthening.
Consequently, high strength can be obtained without reliance
on precipitation strengthening, which may introduce problems
such as corrosion and SCC susceptibility, and propensity for
planar slip. Nevertheless, the aforementioned five
strengthening contributions can be augmented by precipitation
strengthening as well as intermetailic dispersion
Powder Degassing and Consolidation
Can Vacuum Degassing. This is perhaps the most widely
used technique for aluminum degassing because it is relatively
non-capital intensive. Powder is encapsulated in a can,
usually aluminum alloys 3003 or 6061. A spacer is often
useful to increase packing and to avoid safety problems when
the can is welded shut. Packing densities are typically 60%
of theoretical density when utilizing this method on
mechanically alloyed powders. Care must be used to allow a
clear path for evolved gases through the spacer to prevent
pressure buildup and explosion.
Dipurative Degassing. Roberts and co-workers at Kaiser
Aluminum & Chemical Corporation have developed an improved
degassing method called "dipurative" degassing. In this
technique, the vacuum-degassed ponder, which is often canned,
is backfilled with a dipurative gas (that is, one that
effectively removes water of hydration) such as extra-dry
nitrogen, and then re-evacuated. Several backfills and
evacuations can be performed resulting in lower hydrogen
content. In addition, the degassing can often be performed
at lower temperatures to reduce microstructural coarsening.
Vacuum Degassing in a Reusable Chamber. The cost of
canning and de-canning adversely affects the competitiveness
of aluminum P/M alloys. This cost can be alleviated somewhat
by using a reusable chamber for vacuum hot pressing. The
powder or CIPed compact can be placed in the chamber and
vacuum degassed immediately prior to compaction in the same
chamber. Alternatively, the powder can be "open tray"
degassed, that is degassed in an unconfined fashion, prior
to loading into the chamber.
Direct Powder Forming. One of the most cost-effective
means of powder consolidation is direct powder forming.
Degassed powder, or powder that has been manufactured with
great care to avoid contact with ambient air, can be
consolidated directly during the hot-forming operation.
Hot Isostatic Pressing (HIP). In HIP degassed and
encapsulated powder is subjected to hydrostatic pressure in
a HIP apparatus. Can vacuum degassing is often used as the
precursor step to HIP. Furthermore, net-shape encapsulation
of degassed powder can be used to produce certain near
net-shape parts. Relatively high HIP pressures (~200 MPa)
are often preferred. Unfortunately, the oxide layer on the
powder particle surfaces is not sufficiently broken up for
optimum mechanical properties. A subsequent hot-working
operation that introduces shear-stress components is often
necessary to improve ductility and toughness.
Rapid Omnidirectional Consolidation. Engineers at
Kelsey-Hayes Company have developed a technique to use
existing commercial forging equipment to consolidate powders
in several alloy systems. Called rapid omnidirectional
consolidation (ROC), it is a lower-cost alternative to HIP.