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.[1]
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).
