Powder
Metallurgy Manufacturing Techniques
1. INTRODUCTION
Among the various metalworking
technologies, powder metallurgy (P/M) is the most diverse manufacturing
approach. One attraction of P/M is the ability to fabricate high quality,
complex parts to close tolerances in an economical manner. In essence, P/M
takes a metal powder with specific attributes of size, shape, and packing, and
then converts it into a strong, precise, high performance shape. Key steps
include the shaping or compaction of the powder and the subsequent thermal
bonding of the particles by sintering. The process effectively uses automated
operations with low relative energy consumption, high material utilization, and
low capital costs. These characteristics make P/M well aligned with current
concerns about productivity, energy, and raw materials. Consequently, the field
is experiencing growth and replacing traditional metal-forming operations.
Further, powder metallurgy is a flexible manufacturing process capable of
delivering a wide range of new materials, microstructures, and properties. That
creates several unique niche applications for P/M such as wear resistant
composites.
The applications of P/M are quite
extensive. Consider the use of metal powders in the fabrication of tungsten
lamp filaments, dental restorations, oil-less bearings, automotive transmission
gears, armour piercing projectiles, electrical contacts, nuclear power fuel
elements, orthopaedic implants, business machines, high-temperature filters,
aircraft brake pads, rechargeable batteries, and jet engine components.
Furthermore, metal powders find uses in such products as paint pigments, porous
concretes, printed circuit boards, enriched flour, explosives, welding
electrodes, rocket fuels, printing inks, brazing compounds, and catalysts.
Worldwide market share for P/M parts are given in the following pie chart.
Automotive industry is leading with more than 75% of the share. American cars
contain more than 16 kg of P/M parts while European cars have 7 kg and
Distribution of P/M parts in a car
Typical iron and copper based P/M parts
2. POWDER PRODUCTION
Almost all materials can be made into powder,
but the method selected for fabricating a powder depends on specific material
properties. Knowing how a powder is made provides a good basis for estimating
and understanding some of its characteristics. The four main categories of
fabrication techniques are based on mechanical comminution, electrolytic
deposition, chemical reactions, and liquid metal atomisation. In addition to
these main approaches, several specialty techniques are used for selected
materials. Atomisation processes produce more than 60% of all powders consumed
in the industry.
Mechanical Comminution
Brittle materials such as inter-metallic compounds, ferro-alloys,
ferro-chromium, ferro-silicon, etc. are crushed mechanically in ball mills.
However, milling is not useful for many ductile metals since they do not easily
fracture. Instead, the ductile particles cold-weld together and form larger
particles. Milling is currently used for production of metallic flakes from
ductile metals like aluminium. In this case a lubricant is used for elimination
of adhesion and cold welding.
Electrolytic
Deposition
By choosing suitable conditions; composition and strength of the
electrolyte, temperature, current density, etc., many metals can be deposited
in a spongy or powdery state. Extensive further processing; washing, drying,
reducing, annealing and crushing may be required. Copper is the main metal to
be produced in this way, but chromium and manganese powders are also produced
by electrolysis. In these cases, however, a dense and normally brittle deposit
is formed and requires to be crushed to powder. Electrolytic iron was at one
time produced on a substantial scale but it has been largely superseded by
powders made by less costly processes. Electrolytic powders are very pure.
Chemical
Reactions
This has been for long the most widely used method for the production of
iron powder. Selected ore is crushed, mixed with carbon, and passed through a
continuous furnace where reaction takes place leaving a cake of sponge iron
which is then further treated by crushing, separation of non-metallic material,
and sieving to produce powder. Since no refining operation is involved, the
purity of the powder is dependent on that of the raw materials. The irregular
sponge-like particles are soft, and readily compressible, and give compacts of
good green strength. Refractory metals are normally made by hydrogen reduction
of oxides, and the same process can be used for copper.
Atomisation
In this process molten metal is broken up into small droplets and
rapidly frozen before the drops come into contact with each other or with a
solid surface. The principal method is to disintegrate a thin stream of liquid
metal by subjecting it to the impact of high energy jets of gas or liquid. Air,
nitrogen and argon are commonly used gases, and water is the liquid most widely
used. By varying the several parameters; design and configurations of the
jets, pressure and volume of the atomising fluid, thickness of the stream of
metal, etc., it is possible to control the particle size distribution over a
wide range. The particle shape is determined largely by the rate of
solidification and varies from spherical, if a low heat capacity gas is
employed, to highly irregular if water is used. In principle, the technique is
applicable to all metals that can be melted, and is commercially used for the
production of iron, tool steels, alloy steels, copper, brass, bronze and the
low-melting-point metals, such as aluminium, tin, lead, zinc, cadmium. The
readily oxidising metals, for example chromium-bearing alloys, are being
atomised on an increasing scale by means of inert gas, especially argon.
Atomisation is particularly useful for the production of alloys in powder form,
since the constituent metals are fully alloyed in the molten state. Thus each
powder particle has the same chemical composition.
In addition, there are several other atomisation processes that are
finding increasing application, an important one being centrifugal atomisation
in which droplets of molten metal are discharged from a rotating source. There
are basically two types of centrifugal atomisation processes: in one a cup of
molten metal is rotated on a vertical axis at a speed sufficient to throw off
droplets of molten metal, or a stream of metal is allowed to fall on a rotating
disc or cone; in the other, a bar of the metal is rotated at high speed and the
free end is progressively melted, e.g. by an electron beam or plasma arc. This
latter process is called the Rotating Electrode Process (REP), and the bar may
be rotated either on a horizontal or on a vertical axis. A special advantage of
these processes is that they can be carried out in a sealed vessel in a
controlled atmosphere, even vacuum, and thus produce clean powders of highly
reactive metals.
Other Processes
Thermal decomposition of a chemical compound is used in some cases, a
notable one being nickel carbonyl. This Carbonyl Process was originally
developed as a means of refining nickel, crude metal being caused selectively
to react with carbon monoxide under pressure to form the carbonyl which is
gaseous at the reaction temperature and which decomposes on raising the
temperature and lowering the pressure. The same process is used for iron, and
carbonyl iron powder finds small-scale application where its very high purity
is useful. Recently, demand for very fine powders for the injection moulding
process has given a considerable impetus to the carbonyl process. Typically the
particle size of carbonyl iron powder is 1 - 5 μm,
but, as in the case of nickel, it can be tailored to suit particular
requirements. Another case of thermal decomposition is platinum powder of which
is made from sponge produced by heating salt platinum ammonium chloride. In the
Sherritt-Gordon process, nickel powder is made by hydrogen reduction of a
solution of a nickel salt under pressure. Chemical precipitation of metal from
a solution of a soluble salt is used in other cases, e.g., silver, powder of
which is produced by adding a reducing agent to a solution of silver nitrate.
This is, of course, the same basic process as is used to produce black and
white photographs.
Powder Characteristics
The further processing and the final results achieved in the sintered
part are influenced by the characteristics of the powder: particle size, size
distribution, particle shape, and structure and surface condition. A very
important parameter is the apparent density of the powder, i.e., the mass of a
given volume of powder without any pressing or settling. The apparent density
is a function of particle shape and the degree of porosity of the particles.
The choice of powder characteristics is normally based on compromise, since
many of the factors are in direct opposition to each other. An increase in the
irregularity and porous texture of the powder particle, i.e., decrease in
apparent density, increases the reduction in volume that occurs on pressing and
thus the degree of cold-welding, which in turn gives greater green strength to
the compact. This increase in contacting surfaces also leads to more efficient
sintering. Compressibility, density achieved at a specified pressure, is also
an important powder property. Low compressibility powders require greater
pressure and consequently larger presses and stronger dies. The ease and
efficiency of packing the powder in the die depends to a large extent on a wide
particle size distribution. So that the voids created between large particles
can be progressively filled with those of smaller size.
The purity of the powder is critically important. Impurity levels, which can be
tolerated, depend to a large extent on the nature and state of combination of
the substances concerned. For example, the presence of combined carbon in iron
tends to harden the matrix so that increased pressures are required during
compaction. Free carbon, however, is often an advantage, acting as a lubricant
during the pressing operation. A thin oxide film coats most metal powder
particles, but in general these do not interfere with the process, since they
are ruptured during the pressing operation to provide clean and active metal
surfaces which are easily cold-welded. Their final reduction under the reducing
sintering atmosphere is essential for complete metal bonding and maximum
strength.
3. PRODUCTION OF P/M PARTS
The general sequence of operations involved in the P/M process is shown
schematically in the flow chart. The component powders are mixed, together with
lubricant, until a homogeneous mix is obtained. The mix is then loaded into a
die and compacted under pressure, after which the compact is sintered. An
exception is the process for making filter elements from spherical bronze
powder where no pressure is used; the powder being simply placed in a suitably
shaped mould in which it is sintered. There are various powder compaction
methods such as; uniaxial pressing, rolling, extrusion, injection moulding, isostatic pressing, etc. Selection of the method is
dependent to part geometry and number of production.
Mixing
The metal powder is mixed with lubricant and
optional alloying elements to form a homogenous blend. 0.5 - 1.5% lubricant is
normally added in the mix, and metallic stearate and waxes are commonly used
lubricants. The main function of the lubricant is to reduce friction between the
powder mass and the surfaces of the tool, die walls, core rods etc., along
which the powder must slide during compaction. This assists the achievement of
uniform density from top to bottom of the compact. Of equal importance is the
fact that the reduction of friction also makes it easier to eject the compact. As an alternative to pre-alloyed powders, alloying
elements can be added to the mix as powders. The high compressibility of a pure
iron powder is thus maintained and the solution-hardening effect of the
alloying elements is avoided. The most commonly used alloying element is
carbon, which is added as graphite powder.
Pressing
The most
common way of compacting is axial pressing in a steel or carbide die under
pressures of 300 - 800 MPa. It is possible to press parts with complicated
shape in a single operation and with high production rate, up
to 25 part/minute. The part achieves sufficient strength to be ejected
from the tool die and can be handled before sintering by interlocking and cold-welding
between the particles. Highly compressible iron powder can achieve a density of
7.3 g/cm3 (or 93%) of the theoretical density at a compacting pressure of 800
MPa.
Pressing
Steps
By utilising Warm
Compaction, which means that a powder mix with special lubricant and the tool
set are heated to 130°C and 150°C respectively, density can be increased by up
to 0.2 g/cm3, compared to conventional cold compaction. Warm
compacted components have a strength high enough to allow some machining
operations before sintering, which drastically decreases tool wear.
Sintering
Sintering is a heat treatment
wherein the pressed parts gain strength. The most common sintering temperature
range for iron-based alloys is 1100 - 1150°C. In some cases, higher sintering
temperatures up to 1250°C are employed. The time at temperature varies between
10 and 60 minutes, depending on the application. The most common type of
furnaces is the mesh belt furnace. Components are placed on a tray, or directly
on the mesh belt, which transports them through the furnace. Mesh belt furnaces
are limited to a temperature of maximum 1150°C. For higher temperatures,
walking beam or pusher-type furnaces are common. The sintering cost increases
significantly if sintering temperatures higher than 1150°C are used. An
atmosphere, which prevents oxidation, is necessary in the sintering furnace.
Dissociated ammonia, endothermic or nitrogen-based atmospheres are commonly
used. It is of great importance also to have a controlled carbon potential, in
order to ensure consistent mechanical properties and tolerances on the sintered
components.
A sintering operation consists of
de-waxing, sintering and cooling steps. In the de-waxing zone of the furnace,
the lubricant is burned off. During the sintering, several reactions take
place. Initially, the furnace atmosphere reduces oxides on the surface of the
particles, and an initial bond between particles in contact is formed. The main
mechanisms of sintering are surface and volume diffusion. By diffusion, a
coherent body of metal is formed, and the admixed alloying elements are
distributed into the iron. Driven by the force to minimise free energy, stated
by the laws of thermodynamics, rounding of pores takes place and small pores
disappear in favour of larger. In the cooling zone of the sintering furnace,
the parts are cooled under protective atmosphere in order to not oxidise in
contact with air. The cooling speed, especially in the range 850 - 500°C, also
affects the mechanical properties, due to phase transformations in the
material. During the sintering, a moderate dimensional change takes place. Most
materials shrink but some alloying elements, such as copper, cause growth. In
design of the press tool, the dimensions must be compensated for the
dimensional change.
Infiltration
The interconnected porosity is
filled with an alloy having a melting point lower than the sintering
temperature of the metal of which the component is made, e.g., copper-based
alloys infiltrate ferrous parts, usually during the sintering phase.
Infiltration makes the components impermeable and there is some increase in
mechanical properties, but at expense of dimensional accuracy. Infiltration
simplifies some heat treatments. For instance, it is easier to obtain a defined
case depth without interconnected porosity.
Oil
Impregnation
Sintered parts achieve greater
protection against corrosion by being impregnated by oil or other non-metallic
material. Self-lubricating bearings are manufactured by impregnating porous
sintered bearings with lubricants and these bearings can only be produced by
powder metallurgy.
Sizing
and Coining
Sizing and coining are additional
press operations after sintering. The main objective is to improve the
dimensional accuracy, but the surface finish is also normally improved. Quite
moderate pressures are normally required for sizing, since only a slight
plastic deformation is necessary. Coining has a double purpose. Not only is
dimensional accuracy improved, but the use of higher pressures also increases
the density of the part. Normally, a press tool specific to the task of sizing
or coining is used.
Double
Pressing
A second pressing operation serves
to decrease porosity for applications where density is crucial to achieve the
required mechanical or magnetic properties. By pre-sintering the pressed part
at temperatures of 700-800°C, the admixed lubricant is burned off and
re-crystallisation takes place. Once the work hardening and internal stresses
are removed, the material reacquires its ductility and therefore its capacity
for further densification. After re-pressing, the parts are sintered for the
second time.
Steam Treatment
Applicable only
to ferrous parts. By heating the
parts to a temperature of 550°C and exposing them to water vapour, a thin layer
of Fe3O4 is formed both on the outer surface and along
the interconnected porosity. Steam treatment induces considerable corrosion
resistance, increased hardness, increased resistance to compressive strength,
and improved wear resistance.
Machining
Although
a major attraction of producing sintered components is the ability to produce
complex shapes and close tolerances, limitations do exist. Therefore, machining
operations such as milling, drilling (e.g., holes perpendicular to the pressing
direction), threading and machining can be used to
achieve features not possible to obtain by pressing in rigid dies. Sintered
metals are generally less easy to machine than wrought alloy of similar
composition, so cutting speed and cutting tools should therefore be adjusted
for optimum results. To increase tool life, machinability-enhancing
additives such as MnS can be admixed with the powder.
After sintering, they remain evenly distributed in the structure and mechanical
properties are only marginally affected.
De-burring
This operation removes burrs
resulting from the compacting operation or machining step. The most common
method is tumbling, and sometimes a liquid medium with an abrasive powder is
employed.
Joining
Larger parts and very complex shapes
can be obtained by joining. Several techniques exist for joining, such as
diffusion bonding, sinter brazing and laser welding.
Heat
treatment
Phase transformation depends on the
composition and homogeneity of the alloy, not on its porosity, so all heat
treatments applicable to the cast and wrought alloys are applicable for
sintered materials as well. Hardening operations with quenching and tempering
substantially increase strength, and improve wear-resistance; but at the
expense of ductility. Surface hardening by carburising and carbonitriding is
extensively used on sintered parts.
Surface
plating
When needed, corrosion protection
can be obtained by plating. However, low-density parts must be impregnated
before plating, to prevent electrolyte from entering the pores and causing
subsequent corrosion.
4.
PROPERTIES OF P/M PARTS
The properties of sintered materials are
depending on a range of factors and can be optimised to fit special
requirements. Chemical composition and density of the component are important
factors to consider when the material is to be specified for a certain
application.
Chemical composition and mechanical properties
are commonly used factors to specify the properties of a sintered machine part.
Some examples are given below:
