POWDER METALLURGY INTRODUCTION Powder metallurgy is the name given to the process by which fine...
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POWDER METALLURGY INTRODUCTION Powder metallurgy is the name given to the process by which fine powdered materials are blended, pressed into a desired shape (compacted), and then heated (sintered) in a controlled atmosphere to bond the contacting surfaces of the particles and establish desire properties.
POWDER METALLURGY INTRODUCTION Powder metallurgy is the name given to the process by which fine powdered materials are blended, pressed into a desired
POWDER METALLURGY INTRODUCTION Powder metallurgy is the name
given to the process by which fine powdered materials are blended,
pressed into a desired shape (compacted), and then heated
(sintered) in a controlled atmosphere to bond the contacting
surfaces of the particles and establish desire properties.
Slide 2
INTRODUCTION..... The process, commonly designated as P/M,
Readily lends it self to the mass production of small, intricate
parts of high precision, often eliminating the need for additional
machining or finishing. There is little material waste; unusual
materials or mixtures can be utilized; and controlled degrees of
porosity or permeability can be produced.
Slide 3
INTRODUCTION......... Major areas of application tend to be
those for which the P/M process has strong economical advantage or
where the desired properties and characteristics would be difficult
to obtain by other method.
Slide 4
INTRODUCTION......... Because of its level of manufacturing
maturity, powder metallurgy should actually be considered as a
possible means of manufacture for any part where the geometry and
production quality are appropriate.
Slide 5
BASIC PROCESS The powder metallurgy process generally consists
of four basic steps: (1) powder manufacture, (2) mixing or
blending, (3) compacting, and (4) sintering. Optional secondary
processing often follows to obtain special properties or enhanced
precision.
Slide 6
POWDER MANUFACTURE The properties of powder metallurgy products
are highly dependent on the characteristics of the starting powders
that are used. Some important properties and characteristics
include chemistry and purity, particle size distribution, particle
shape, and the surface texture of the particles. Several processes
can be used to produce powdered materials, with each imparting
distinct properties and characteristics to the powder and hence to
the final product.
Slide 7
Over 80% of all commercial; powder is produced by some form of
melt atomization, where a liquid is fragmented into molten droplets
which then solidify into particles, various forms of energy have
been used to form the droplets. Regardless of the process details,
atomization is an extremely useful means of producing prealloyed
powders. By starting with an alloyed melt or prealloyed electrode
each powder particle has the desired alloy composition.
Slide 8
Powders of aluminum alloys, cobalt based alloys, and various
low-alloy steel, nickel-based alloys (such as Monel), titanium
alloys, cobalt based alloys and various low-alloys steel have all
been commercially produced. The size and shape of the powder
particles can be varied depend on process features.
Slide 9
As the velocity and media of the atomizing jets or speed of
electrode rotation, the starting temperature of the liquid (which
affects the time that surface tension can act on the individual
droplets prior to solidification), and the environment provided for
cooling. When cooling is slow (such as in gas atomization) and
surface tension, irregular shapes tend to be produced.
Slide 10
Other methods of powder manufacture include chemical reduction
of particulate compounds (generally crushed oxides or ores)
electrolytic deposition from solution or fused salts, pulverization
or grinding of brittle materials (comminution), thermal
decomposition of hydrides or carbonyls, precipitation from
solution, and condensation of metal vapors.
Slide 11
Almost any metal, metal alloy, or nonmetal (ceramic, polymer,
or wax or graphite lubricant) can be converted into powder form by
one or more of the powder production methods. Some methods can
produce only elemental (often of high purity), while others can
produce prealloyed particles. Further operations, such as drying or
heat treatment, may be performed prior to further processing.
Slide 12
POWDER MIXING AND BLENDING It is rare that a single powder will
possess all of the characteristics desired in a given process and
product. Most likely, the starting material will be a mixture of
various grades or sizes of powder, or powders, of different
compositions, with additions, of lubricants or binders.
Slide 13
The final product chemistry is often obtained by combining pure
metal or non- metal powders, rather than using prealloyed material.
Sufficient diffusion must then occur during the sintering operation
to produce a uniform chemistry and structure in the final product.
Unique composites can also be produced such as the distribution of
metals and nonmetals in a single product such as tungsten carbide
cobalt matrix cutting tool for high- temperature service.
Slide 14
Some powders, such as graphic, can even play a dual role,
service as lubricant during compacting and a source of carbon as it
alloys with iron during sintering to produce steel. Lubricants such
as graphite or stearic acid improve the flow characteristics and
compressibility at the expense of reduced green strength. Binders
produce the reverse effect. Most lubricants or binders are not
wanted in the final product and are removed (volatilized or burned
off in the early stages of sintering, length holes that are reduced
in size or closed during subsequent heating
Slide 15
Blending or mixing operations can be done either dry or wet,
where water or other solvent is used to enhance particle mobility,
reduce dusting, and lessen explosion hazards. Large lots of powder
can be homogenized with respect to both chemistry and distribution
of components, sizes, and shapes. Quantities up to 16,000kg have
been blended in single lots to ensure uniform behavior processing
and the production of a consistent product.
Slide 16
COMPACTING One of the most critical steps in the P/M process is
compacting. Loose powder is compressed and densified into a shape
known as a green compact, usually at room temperature. High product
density and the uniformity of that density throughout the compact
are generally desire characteristics. In addition, the compacts
should possess sufficient green strength for in process handling
and transport to the sintering furnace.
Slide 17
Most compacting is done with mechanical pressed and rigid
tools, but hydraulic and hybrid (combinations of mechanical,
hydraulic, and pneumatic) presses can also be used.
Slide 18
SINTERING In the sintering operation, the pressed powder
compacts are heated in a controlled atmosphere environment to a
temperature below the melting point but high enough to permit solid
state diffusion, and held for sufficient time to permit bonding of
the particles. Most metals are sintered at temperatures of 70 to
80%.of their melting point, while certain refractory materials may
require temperatures near 90%. When the product is composed of more
than one material, the sintering temperature may even be above the
melting temperature of one or more components. The lower- melting
point materials then melt and flow into the voids between the
remaining particles, and the process becomes liquid phase
sintering.
Slide 19
Most sintering operations involve three stages, and many
sintering furnaces employ three corresponding zones. The first
operation, the burn off or purge, is designed to combust any air,
volatilize and remove lubricants or binders that would interfere
with good bonding, and slowly raise the temperature of the compacts
in a controlled manner. Rapid heating would produce high internal
pressure from air entrapped in close pores or volatilizing
lubricants, and would result in swelling or fracture of the
compacts. When the compacts contain appreciable quantities of
volatile materials, their removal creates additional porosity and
permeability within the pressed shape. The manufacture of products
such as metal filters is designed to take advantage of this
feature.
Slide 20
When the products are load bearing components, however, high
amounts of products of porosity are undesirable, and the amount of
volatilizing lubricant is kept to an optimized minimum. The second,
or high temperature stage is where the desired solid state
diffusion and bonding between the powder particles take place.
Atoms move, toward the points of contact between the particles; the
areas of contact become larger; and the part becomes a solid mass
with small pores of various sizes and shapes.
Slide 21
The mechanical bonds of compaction become metal lurgical bonds.
The time in this stage must be sufficient and final properties, and
usually varies from 10 minutes to several hours. Finally, a cooling
period is required to lower the temperature of the products while
retaining them in a controlled atmosphere. This feature serves to
prevent oxidation that would occur upon direct discharge into air
as well as possible thermal shock from rapid cooling. Both batch
and continuous furnaces are used for sintering.
Slide 22
Slide 23
All three stages of sintering must be conducted in the oxygen
free condition of a vacuum or protective atmosphere. This is
critical because the compacted shapes typically have 10 to 25%
residual porosity, and some of the internal voids are connected to
exposed surfaces. At elevated temperatures, rapid oxidation would
occur and significantly impair the quality of interparticle
bonding. Reducing atmospheres, commonly based on hydrogen,
dissociated ammonia, or cracked hydrocarbons, are preferred since
they can reduce any oxide already present on the particle surface
and combust harmful gases that are liberated during the sintering.
Inert gases cannot reduce existing oxides but will prevent the
formation of any additional contaminants. Vacuum sintering is
frequently employed with stainless steel, titanium, and the
refractory metals. Nitrogen atmosphere are also common.
Slide 24
During the sintering operation, a number of changes occur in
the compact. Metallurgical bonds form between the powder particles
as a result of solid-state atomic diffusion, and strength,
ductility, toughness, and electrical and thermal conductivities all
increase. If different chemistry powder were blended,
interdiffusion will promote the formation of alloys or
intermetallic phases. In addition, there will be a concurrent
increase in density and contraction in product dimensions. To meet
final tolerances, the dimensional shrinkage will have to be
compensated through the design of oversized compaction dies. During
sintering, not all of the porosity is removed, however.
Conventional pressed and sintered P/M products generally contain
between 5 and 25% residual porosity.
Slide 25
Sinter brazing is the process in which two or more separate
pieces are joined by brazing while they are also being sintered.
The individual pieces are compacted separately, and are assembled
with the braze metal positioned so it will flow into the joint.
When the assembly is heated for sintering, the braze metal melts
and flows between the joint surfaces to create the bond. As
sintering continues, much of the braze metal diffuses into the
surrounding metal, producing a final joint that is often stronger
than the materials being joined.
Slide 26
SECONDARY OPERATIONS Powder metallurgy products are often ready
to use when they emerge from the sintering furnace. Many P/M
products however, utilize one or more secondary operations to
provide enhanced precision, improved properties, or special
characteristics. During sintering, product dimensions shrink due to
densification. In addition, warping, or distortion may occur during
cooldown from elevated temperature. As a result, a second pressing
operation, known as repressing,coining, or sizing, may be required
to restore or improve dimension precision.
Slide 27
PROPERTIES OF P/M PRODUCTS Because the properties of powder
metallurgy products depend on so many variables type and size of
powder, amount and type of lubricant, pressing pressure, sintering
temperature and time, finishing treatments, and so on it is
difficult to provide generalized information. Products can range
all the way from low-density, highly porous parts with tensile
strength as low as 70 MPa up to high density pieces with tensile
strengths of 1250 MPa or more.
Slide 28
Most mechanical properties show a strong dependence on product
density, with the fracture limited properties of toughness,
ductility, and fatigue life being more sensitive than strength and
hardness. The voids in the P/M part act as stress concentrators and
assist in starting and propagating fractures. The yield strength of
P/M products made from the weaker metals is often equivalent to the
same material in wrought form. If higher strength materials are
used or the fracture related tensile strength is specified, the
properties of the P/M product tend to fall below those of wrought
equivalents by varying but usually substantial amounts.
Slide 29
When larger presses or processes such as P/M forging or hot
isostatic pressing are used to produce higher density, the strength
of P/M products a approaches that of the wrought material. If the
processing results in full density with fine grain size, P/M parts
can actually have properties that exceed their wrought or cast
equivalents. Since the mechanical properties of powder metallurgy
products are so dependent upon density, it is important that P/M
products be designed and materials selected so that the final
properties will be achieved with the anticipated amount of final
porosity.
Slide 30
Physical properties can also be affected by porosity. Corrosion
resistance tends to be reduced due to the presence of entrapment
pockets and fissures. Electrical, thermal, and magnetic properties
all vary with density, usually decreasing in the presence of pores.
The presence of porosity, however, increases the ability to damp
both sound and vibration, and many P/M parts have been designed to
take advantage of this feature.
Slide 31
POWDER METALLURGY PRODUCTS The products that are commonly
produced by powder metallurgy can generally be classified into six
groups. Porous or permeable products, such as bearings, filters,
and pressure or flow regulators. Oil impregnated bearings, made
from either iron or copper alloys, constitute a large volume of P/M
products. They are widely used in home appliance and automotive
applications since they require no lubrication P/M filters can be
with pores of almost any size, some as small as 0.0025 mm(0.0001 in
). Unlike many alternative filters, powder metallurgy filters can
withstand conditions of elevated temperature, high applied
stresses, and corrosive environments.
Slide 32
Products of complex shapes that would require considerable
machining when made by other processes. Because of the dimensional
accuracy and fine surface finish that are characteristics of the
P/M process, many parts require no further processing and others
require only a small amount of finish machining tolerances can
generally be held to within 0.1 mm (0.005 in ). Large numbers of
small gears are currently being made by the powder metallurgy
process. Other complex shapes, such as pawls, cams, and small
activating levers, can be made quite economically.
Slide 33
Product made from materials that are difficult to machine or
with high melting points. Some of the first modern uses of powder
metallurgy were the production of tungsten lamp filaments and
tungsten carbide cutting tools.
Slide 34
Products where the combined properties of two or more metals
(or metals and non metals) are the desired. This unique capability
of the powder metallurgy process is applied to a number of
products. In the electrical industry, copper and graphite are
frequently combined in applications like motor or generator brushes
where copper provides the current carrying capacity and graphite
provides lubrication.
Slide 35
Bearings have been made of graphite combined with iron or
copper, or from mixture of two metals, such as tin and copper,
where the harder material provides wear resistance and the softer
material deforms in a way that better distributes the wear the
load. Electrical contacts often combine copper or silver with
tungsten, nickel, or molybdenum. Here, the copper or silver
provides high conductivity, while the high melting temperature
material provides silver provides high conductivity, while the high
melting temperature material provides resistance to fusion when the
contacts experience arcing and subsequent closure.
Slide 36
Products where the powder metallurgy process produces clearly
superior properties. The development of processes that produce full
density has resulted in P/M products that are clearly superior to
those produces by competing techniques. In areas of critical
importance such as aerospace applications, the additional cost of
the processing may be justified by the enhancement of properties.
As another example, consider the production of P/M magnets. A
magnetic field can be used be used to align the particles prior to
sintering, resulting in a product with extremely high flux
density.
Slide 37
Products where the powder metallurgy process offers definite
economic advantage consideration of the process advantages, reveals
features that may make powder metallurgy the most economical among
two or more alternative ways to produce an equivalent part.
Slide 38
ADVANTAGES AND DISADVANTAGES OF POWDER METALLURGY Like all
other manufacturing processes, powder metallurgy has distinct
advantages and disadvantages that should be considered if the
technique is to be employed economically and successfully. Among
the important advantages are:-
Slide 39
Elimination or reduction of machining. The dimensional accuracy
and surface finish of P/M products are such that subsequent
machining operations can be totally eliminated for many
applications. If unusual dimensional accuracy is required, simple
coining or sizing operations can often give accuracies equivalent
to those of most production machining. High production rates. All
steps in the P/M process are simple and readily automated. Labor
requirements are low, and product uniformity and reproducibility
are among the highest in manufacturing.
Slide 40
Complex shapes can be produced. Subject to the limitations
discussed previously, complex shapes can be produced, such as
combination gears, cams, and internal keys. It is often possible to
produce parts by powder metallurgy that cannot be machined or cast
economically. Wide various in compositions are possible. Parts of
very high purity can be produced. Metals and ceramics can be
intimately mixed. Immiscible materials can be combined, and
solubility limits can be exceeded. In most cases the chemical
homogeneity of the product exceeds that of all competing
techniques.
Slide 41
Wide variation in properties are available. Products can be
range from lowdensity parts with controlled permeability to high
density parts with properties that equal or exceed those of
equivalent wrought counterparts. Damping of noise and vibration can
be tailored into a P/M product. Magnetic properties, were
properties, and others can all be designed to match the needs of a
specific application.
Slide 42
Scrap is eliminated or reduced. Powder metallurgy is the only
common manufacturing process in which no material is wasted. In
casting, machining, and press forming, the scrap can often exceed
50% of the starting material. This is particularly important where
expensive materials are involved and may make it possible to use
more costly materials without increasing the overall cost of the
product. An example of such a product would be the rare earth
magnets.
Slide 43
The major disadvantages of the powder metallurgy process are:-
Inferior strength properties. Because of residual porosity, powder
metallurgy parts generally have mechanical properties that are
inferior to wrought or cast products of the same material. Their
use may be limited when high stresses are involved. The required
strength and fracture resistance, however, can often be obtained by
using different materials of by employing alternate or secondary
processing techniques that are unique to powder metallurgy.
Slide 44
Relatively high tooling cost. Because of the high pressures and
severe abrasion involves in the process, the P/M dies must be made
of expensive materials and be relatively massive. Because of the
need for part specific tooling, production quantities of less than
10,000 identical parts are normally not practical.
Slide 45
High material cost. On a unit weight basis, powder metals are
considerably more expensive than wrought or cast stock. However,
the absence of scrap and the elimination of machining can often
offset the higher cost of the starting material. In addition,
powder metallurgy is usually employed for rather small parts where
the material cost per part is not very great.
Slide 46
Design limitations. The powder metallurgy process is simply not
feasible for many shapes. Parts must be able to eject from the die.
The thickness/diameter (or thickness/width) ratio is limited. Thin
vertical sections are difficult, and the overall size must be
within the capacity of available presses.
Slide 47
Health and safety hazards. Many metals, such as aluminum,
titanium, magnesium, and iron, are pyrophoric they can ignite or
explode when in particle form with large surface/volume ratios.
Fine particles can also remain airborne for long times and can be
inhaled by workers. To minimize the health and safety hazards. The
handling of metal powders frequently requires the use of inert
atmospheres, dry boxes, and hoods, as well as special cleanliness
of the working environment.