Powder metallurgy is a forming and fabrication technique
consisting of three major processing stages. First, the primary material is
physically powdered, divided into
many small individual particles. Next, the powder is injected into a mold or passed through a die to
produce a weakly cohesive structure (via cold welding) very near the dimensions of the
object ultimately to be manufactured. Finally, the end part is formed by
applying pressure, high temperature, long setting times (during which
self-welding occurs), or any combination thereof.
Two main techniques used to
form and consolidate the powder are Sintering
and Metal Injection Molding.
History and capabilities
The history of powder
metallurgy and the art of metals and ceramics
sintering are intimately related. Sintering
involves the production of a hard solid metal or ceramic piece from a starting
powder. There is evidence that iron powders were fused
into hard objects as early as 1200 B.C. In these early manufacturing operations,
iron was extracted by hand from metal sponge following reduction and was then
reintroduced as a powder for final melting or sintering.
A much wider range of products
can be obtained from powder processes than from direct alloying
of fused materials. In melting operations the "phase rule" applies to
all pure and combined elements and strictly dictates the distribution of liquid
and solid phases which can exist for specific
compositions. In addition, whole body melting of starting materials is required
for alloying, thus imposing unwelcome chemical, thermal, and containment
constraints on manufacturing. Unfortunately, the handling of aluminium/iron
powders poses major problems. Other substances that are especially reactive
with atmospheric oxygen, such as tin, are sinterable in
special atmospheres or with temporary coatings.
In powder metallurgy or
ceramics it is possible to fabricate components which otherwise would decompose
or disintegrate. All considerations of solid-liquid phase changes can be
ignored, so powder processes are more flexible than casting,
extrusion, or forging
techniques. Controllable characteristics of products prepared using various
powder technologies include mechanical, magnetic, and other unconventional
properties of such materials as porous solids, aggregates, and intermetallic
compounds. Competitive characteristics of manufacturing processing (e.g., tool
wear, complexity, or vendor options) also may be closely regulated.
Powder Metallurgy products are
today used in a wide range of industries, from automotive and aerospace
applications to power tools and household appliances. Each year the
international PM awards highlight the developing capabilities of the
technology
Powder production techniques
Any fusible material can be
atomized. Several techniques have been developed which permit large production
rates of powdered particles, often with considerable control over the size
ranges of the final grain population. Powders may be prepared by comminution, grinding, chemical reactions, or
electrolytic deposition. Several of the melting and mechanical procedures are
clearly adaptable to operations in space or on the Moon.
Powders of the elements Ti, V, Th, Nb, Ta, Ca, and
U have been produced by high-temperature reduction of the corresponding nitrides
and carbides. Fe, Ni, U, and Be submicrometre powders
are obtained by reducing metallic oxalates
and formates. Exceedingly fine particles also have
been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame,
simultaneously atomizing and comminuting the material. On Earth various
chemical- and flame-associated powdering processes are adopted in part to
prevent serious degradation of particle surfaces by atmospheric oxygen.
Atomization
Atomization is accomplished by
forcing a molten metal stream through an orifice at moderate pressures. A gas
is introduced into the metal stream just before it leaves the nozzle, serving
to create turbulence as the entrained gas expands (due to heating) and exits
into a large collection volume exterior to the orifice. The collection volume
is filled with gas to promote further turbulence of the molten metal jet. On
Earth, air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are
annealed, which helps reduce the oxide and carbon content. The water atomized
particles are smaller, cleaner, and nonporous and have a greater breadth of
size, which allows better compacting.
Simple atomization techniques
are available in which liquid metal is forced through an orifice at a
sufficiently high velocity to ensure turbulent flow. The usual performance
index used is the Reynolds number R = fvd/n,
where f = fluid density, v = velocity of the exit stream, d = diameter of the
opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at
higher velocities the stream becomes turbulent and breaks into droplets.
Pumping energy is applied to droplet formation with very low efficiency (on the
order of 1%) and control over the size distribution of the metal particles
produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry,
multiple impinging streams, or molten-metal injection into ambient gas are all
available to increase atomization efficiency, produce finer grains, and to
narrow the particle size distribution. Unfortunately, it is difficult to eject
metals through orifices smaller than a few millimeters in diameter, which in
practice limits the minimum size of powder grains to approximately 10 μm.
Atomization also produces a wide spectrum of particle sizes, necessitating
downstream classification by screening and remelting a significant fraction of
the grain boundary.
Centrifugal disintegration
Centrifugal disintegration of
molten particles offers one way around these problems. Extensive experience is
available with iron, steel, and aluminium. Metal to be powdered is formed into
a rod which is introduced into a chamber through a rapidly rotating spindle.
Opposite the spindle tip is an electrode from which an arc is established which
heats the metal rod. As the tip material fuses, the rapid rod rotation throws
off tiny melt droplets which solidify before hitting the chamber walls. A
circulating gas sweeps particles from the chamber. Similar techniques could be
employed in space or on the Moon. The chamber wall could be rotated to force
new powders into remote collection vessels (DeCarmo, 1979), and the electrode
could be replaced by a solar mirror focused at the end of the rod.
An alternative approach
capable of producing a very narrow distribution of grain sizes but with low
throughput consists of a rapidly spinning bowl heated to well above the melting
point of the material to be powdered. Liquid metal, introduced onto the surface
of the basin near the center at flow rates adjusted to permit a thin metal film
to skim evenly up the walls and over the edge, breaks into droplets, each
approximately the thickness of the film.
Other techniques
Another powder-production
technique involves a thin jet of liquid metal intersected by high-speed streams
of atomized water which break the jet into drops and cool the powder before it
reaches the bottom of the bin. In subsequent operations the powder is dried.
This is called water atomisation. The advantage is that metal solidifies faster
than by gas atomization since thermal conductivity of water is some magnitudes
higher. The solidification rate is inversely proportional to the particle size.
As a consequence, one can obtain smaller particles by water atomisation. The
smaller the particles, the more homogeneous the micro structure will be. Notice
that particles will have a more irregular shape and the particle size
distribution will be wider. In addition, some surface contamination can occur
by oxidation skin formation. Powder can be reduced by some kind of
pre-consolidation treatment as annealing.
Finally, mills are now
available which can impart enormous rotational torques on powders, on the order
of 2.0×107 rpm. Such forces cause grains to disintegrate into yet
finer particles.
Powder pressing
Although many products such as
pills and tablets for medical use are cold-pressed directly from powdered
materials, normally the resulting compact is only strong enough to allow
subsequent heating and sintering. Release of the compact from its mold is
usually accompanied by small volume increase called "spring-back."
In the typical powder pressing
process a powder compaction press is employed with tools and dies. Normally, a
die cavity that is closed on one end (vertical die, bottom end closed by a
punch tool) is filled with powder. The powder is then compacted into a shape
and then ejected from the die cavity. Various components can be formed with the
powder compaction process. Some examples of these parts are bearings, bushings,
gears, pistons, levers, and brackets. When pressing these shapes, very good
dimensional and weight control are maintained. In a number of these
applications the parts may require very little additional work for their
intended use; making for very cost efficient manufacturing.
In some pressing operations
(such as hot isostatic pressing)
compact formation and sintering occur simultaneously. This procedure, together
with explosion-driven compressive techniques, is used extensively in the
production of high-temperature and high-strength parts such as turbine blades
for jet engines. In most applications of powder metallurgy the compact is
hot-pressed, heated to a temperature above which the materials cannot remain
work-hardened. Hot pressing lowers the pressures required to reduce porosity
and speeds welding and grain deformation processes. Also it permits better
dimensional control of the product, lessened sensitivity to physical
characteristics of starting materials, and allows powder to be driven to higher
densities than with cold pressing, resulting in higher strength. Negative
aspects of hot pressing include shorter die life, slower throughput because of
powder heating, and the frequent necessity for protective atmospheres during
forming and cooling stages.
Sintering
Solid State Sintering is the
process of taking metal in the form of a powder and placing it into a mold or
die. Once compacted into the mold the material is placed under a high heat for
a long period of time. Under heat, bonding takes place between the porous
aggregate particles and once cooled the powder has bonded to form a solid
piece.
Sintering can be considered to
proceed in three stages. During the first, neck growth proceeds rapidly but
powder particles remain discrete. During the second, most densification occurs,
the structure recrystallizes and particles diffuse into each other. During the
third, isolated pores tend to become spheroidal and densification continues at
a much lower rate. The words Solid State in Solid State Sintering simply refer
to the state the material is in when it bonds, solid meaning the material was
not turned molten to bond together as alloys are formed.[2]
One recently developed
technique for high-speed sintering involves passing high electrical current
through a powder to preferentially heat the asperities.
Most of the energy serves to melt that portion of the compact where migration
is desirable for densification; comparatively little energy is absorbed by the
bulk materials and forming machinery. Naturally, this technique is not
applicable to electrically insulating powders.
Continuous powder processing
The phrase "continuous
process" should be used only to describe modes of manufacturing which
could be extended indefinitely in time. Normally, however, the term refers to
processes whose products are much longer in one physical dimension than in the
other two. Compression, rolling, and extrusion are the most common examples.
In a simple compression
process, powder flows from a bin onto a two-walled channel and is repeatedly
compressed vertically by a horizontally stationary punch. After stripping the
compress from the conveyor the compact is introduced into a sintering furnace.
An even easier approach is to spray powder onto a moving belt and sinter it
without compression. Good methods for stripping cold-pressed materials from
moving belts are hard to find. One alternative that avoids the belt-stripping
difficulty altogether is the manufacture of metal sheets using opposed
hydraulic rams, although weakness lines across the sheet may arise during
successive press operations.
Powders can also be rolled to
produce sheets. The powdered metal is fed into a two-high rolling mill and is
compacted into strip at up to 100 feet per minute. [3] The
strip is then sintered and subjected to another rolling and sintering.[4]
Rolling is commonly used to produce sheet metal for electrical and electronic
components as well as coins. [5]Considerable work also
has been done on rolling multiple layers of different materials simultaneously
into sheets.
Extrusion processes are of two
general types. In one type, the powder is mixed with a binder or plasticizer at
room temperature; in the other, the powder is extruded at elevated temperatures
without fortification. Extrusions with binders are used extensively in the
preparation of tungsten-carbide composites. Tubes, complex sections, and spiral
drill shapes are manufactured in extended lengths and diameters varying from
0.5-300 mm. Hard metal wires of 0.1 mm diameter have been drawn from powder
stock. At the opposite extreme, large extrusions on a tonnage basis may be
feasible.
There appears to be no
limitation to the variety of metals and alloys that can be extruded, provided
the temperatures and pressures involved are within the capabilities of die
materials. Extrusion lengths may range from 3-30 m and diameters from 0.2–1 m.
Modern presses are largely automatic and operate at high speeds (on the order of
m/s).
Extrusion Temperatures Of
Common Metals And Alloys
|
Metals and alloys
|
Temperature of extrusion, K
|
°C
|
|
Aluminium and alloys
|
673-773
|
400-500
|
|
Magnesium and alloys
|
573-673
|
300-400
|
|
1073-1153
|
800-880
|
|
|
923-1123
|
650-850
|
|
|
Nickel
brasses
|
1023-1173
|
750-900
|
|
Cupro-nickel
|
1173-1273
|
900-1000
|
|
1383-1433
|
1110-1160
|
|
|
1373-1403
|
1100-1130
|
|
|
1443-1473
|
1170-1200
|
|
|
1323-1523
|
1050-1250
|
Special products
Many special products are
possible with powder metallurgy technology. A nonexhaustive list includes Al2O3
whiskers coated with very thin oxide layers for improved refractories; iron
compacts with Al2O3 coatings for improved
high-temperature creep strength; light bulb
filaments made with powder technology; linings for friction brakes; metal
glasses for high-strength films and ribbons; heat shields
for spacecraft reentry into Earth's atmosphere; electrical contacts for
handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.
Extremely thin films and tiny
spheres exhibit high strength. One application of this observation is to coat
brittle materials in whisker form with a submicrometre film of much softer
metal (e.g., cobalt-coated tungsten). The surface
strain of the thin layer places the harder metal under compression, so that
when the entire composite is sintered the rupture strength increases markedly.
With this method, strengths on the order of 2.8 GPa versus 550 MPa have been
observed for, respectively, coated (25% Co) and uncoated tungsten carbides. It
is interesting to consider whether similarly strong materials could be
manufactured from aluminium films stretched thin over glass fibers (materials
relatively abundant in space).
