Singh R. Introduction to Basic Manufacturing Processes and Workshop Technology
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458 Introduction to Basic Manufacturing Processes and Workshop Technology
POWDER METALLURGY
25.1 INTRODUCTION
Powder metallurgy is used for manufacturing products or articles from powdered metals
by placing these powders in molds and are compacting the same using heavy compressive
force. Typical examples of such article or products are grinding wheels, filament wire, magnets,
welding rods, tungsten carbide cutting tools, self-lubricating bearings electrical contacts and
turbines blades having high temperature strength. The manufacture of parts by powder
metallurgy process involves the manufacture of powders, blending, compacting, profiteering,
sintering and a number of secondary operations such as sizing, coining, machining,
impregnation, infiltration, plating, and heat treatment. The compressed articles are then
heated to temperatures much below their melting points to bind the particles together and
improve their strength and other properties. Few non-metallic materials can also be added
to the metallic powders to provide adequate bond or impart some the needed properties. The
products made through this process are very costly on account of the high cost of metal
powders as well as of the dies used. The powders of almost all metals and a large quantity
of alloys, and nonmetals may be used. The application of powder metallurgy process is
economically feasible only for high mass production. Parts made by powder metallurgy process
exhibit properties, which cannot be produced by conventional methods. Simple shaped parts
can be made to size with high precision without waste, and completely or almost ready for
installation.
25.2 POWDER METALLURGY PROCESS
The powder metallurgy process consists of the following basic steps:
1. Formation of metallic powders.
2. Mixing or blending of the metallic powders in required proportions.
3. Compressing and compacting the powders into desired shapes and sizes in form of
articles.
4. Sintering the compacted articles in a controlled furnace atmosphere.
5. Subjecting the sintered articles to secondary processing if needed so.
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Powder Metallurgy 459
25.2.1 Production of Metal Powders
Metallic powders possessing different properties can be produced easily. The most
commonly used powders are copper-base and iron-base materials. But titanium, chromium,
nickel, and stainless steel metal powders are also used. In the majority of powders, the size
of the particle varies from several microns to 0.5 mm. The most common particle size of
powders falls into a range of 10 to 40 microns. The chemical and physical properties of metals
depend upon the size and shape of the powder particles. There are various methods of
manufacturing powders.The commonly used powder making processes are given as under.
1. Atomization
2. Chemical reduction
3. Electrolytic process
4. Crushing
5. Milling
6. Condensation of metal vapors
7. Hydride and carbonyl processes.
The above mentioned metallic powder making techniques are discussed briefly as under.
1. Atomization
In this process, the molten metal is forced through an orifice and as it emerges, a high
pressure stream of gas or liquid impinges on it causing it to atomize into fine particles. The
inert gas is then employed in order to improve the purity of the powder. It is used mostly
for low melting point metals such as tin, zinc, lead, aluminium, cadmium etc., because of the
corrosive action of the metal on the orifice (or nozzle) at high temperatures. Alloy powders
are also produced by this method.
2. Chemical Reduction Process
In this process, the compounds of metals such as iron oxides are reduced with CO or H
2
at temperatures below the melting point of the metal in an atmosphere controlled furnace.
The reduced product is then crushed and ground. Iron powder is produced in this way
Fe
3
O
4
+ 4C = 3Fe + 4CO
Fe
3
O
4
+ 4CO = 3Fe + 4CO
2
Copper powder is also produced by the same procedure by heating copper oxide in a
stream of hydrogen.
Cu
2
+ H
2
= 2Cu + H
2
O
Powders of W, Mo, Ni and CO can easily be produced or manufactured by reduction
process because it is convenient, economical and flexible technique and perhaps the largest
volume of metallurgy powders is made by the process of oxide reduction.
3. Electrolytic Process
Electrolysis process is quite similar to electroplating and is principally employed for the
production of extremely pure, powders of copper and iron. For making copper powder, copper
plates are placed as anodes in a tank of electrolyte, whereas, aluminium plates are placed in
to the electrolyte to act as cathodes. High amperage produces a powdery deposit of anode
metal on the cathodes. After a definite time period, the cathode plates are taken out from the
460 Introduction to Basic Manufacturing Processes and Workshop Technology
tank, rinsed to remove electrolyte and are then dried. The copper deposited on the cathode
plates is then scraped off and pulverized to produce copper powder of the desired grain size.
The electrolytic powder is quite resistant to oxidation.
4. Crushing Process
The crushing process requires equipments such as stamps, crushers or gyratory crushes.
Various ferrous and non-ferrous alloys can be heat-treated in order to obtain a sufficiently
brittle material which can be easily crushed into powder form.
5. Milling Process
The milling process is commonly used for production of metallic powder. It is carried out
by using equipments such as ball mill, impact mill, eddy mill, disk mill, vortex mill, etc.
Milling and grinding process can easily be employed for brittle, tougher, malleable, ductile and
harder metals to pulverize them. A ball mill is a horizontal barrel shaped container holding
a quantity of balls, which, being free to tumble about as the container rotates, crush and
abrade any powder particles that are introduced into the container. Generally, a large mass
to be powdered, first of all, goes through heavy crushing machines, then through crushing
rolls and finally through a ball mill to produce successively finer grades of powder.
6. Condensation of Metal Powders
This process can be applied in case of metals, such as Zn, Cd and Mg, which can be boiled
and the vapors are condensed in a powder form. Generally a rod of metal say Zn is fed into
a high temperature flame and vaporized droplets of metal are then allowed to condense on
to a cool surface of a material to which they will not adhere. This method is not highly
suitable for large scale production of powder.
7. Hydride and Carbonyl Processes
High hardness oriented metals such as tantalum, niobium and zirconium are made to
combine with hydrogen form hydrides that are stable at room temperature, but to begin to
dissociate into hydrogen and the pure metal when heated to about 350°C. Similarly nickel and
iron can be made to combine with CO to form volatile carbonyls. The carbonyl vapor is then
decomposed in a cooled chamber so that almost spherical particles of very pure metals are
deposited.
25.2.2 Characteristic of Metal Powders
The performance of powder metallurgical parts is totally dependent upon the characteristics
of metal powders. Most important characteristics of metal powders are powder particle size,
size distribution, particle shape, purity, chemical composition, flow characteristics and particle
microstructure. Some of the important properties are discussed as under.
Powder particle size and size distribution
Particle size of metal powder is expressed by the diameter for spherical shaped particles
and by the average diameter for non-spherical particle as determined by sieving method or
microscopic examination. Metal powders used in powder metallurgy usually vary in size from
20 to 200 microns. Particle size influences density/porosity of the compact, mold strength,
permeability, flow and mixing characteristics, dimensional stability, etc. Particle size distribution
is specified in terms of a sieve analysis i.e. the amount of powder passing through 20 or 40
mesh sieves.
Powder Metallurgy 461
Particle shape
There are various shapes of metal powders namely spherical, sub-rounded, rounded,
angular, sub-angular, flakes etc. Particles shape influences the packing and flow characteristics
of the powders.
Chemical composition
Chemical composition of metallic powder implies the type and percentage of alloying
elements and impurities. It usually determines the particle hardness and compressibility. The
chemical composition of a powder can be determined by chemical analysis methods.
Particle microstructure
Particle microstructure reveals various phases, inclusions and internal porosity.
Apparent density
Apparent density is defined as the weight, of a loosely heated quantity of powder necessary
to fill a given die cavity completely.
Flow characteristics
Flow-ability of metal powders is most important in cases where moulds have to be filled
quickly. Metal powders with good flow characteristics fill a mould cavity uniformly.
25.2.3 Mixing or Blending of Metallic Powders
After the formation of metallic powders, proper mixing or blending of powders is the
first step in the forming of powder metal parts. The mixing is being carried out either wet
or dry using an efficient mixer to produce a homogeneous mixture.
25.2.4 Compacting of Powder
Compacting is the technique of converting loose powder in to compact accurately defined
shape and size. This is carried out at room temperature in a die on press machine. The press
used for compacting may be either mechanically or hydraulically operated. The die consists
of a cavity of the shape of the desired part. Metal powder is poured in the die cavity and
pressure is applied using punches, which usually work from the top and bottom of the die as
shown in Fig 25.1. Dies are usually made of high grade steel, but sometimes carbide dies are
used for long production runs. In compacting process, the pressure applied should be uniform
and applied simultaneously from above and below. The pressure applied should be high
enough to produce cold welding of the powder. Cold welding imparts a green strength, which
holds the parts together and allows them to be handled.
The metal parts obtained after compacting are not strong and dense. To improve these
properties, the parts should be sintered.
25.2.5 Sintering
Sintering is the process of heating of compacted products in a furnace to below the
melting point of at least one of the major constituents under a controlled atmosphere. The
sintering temperature and time vary with the following factors-
1. Type of metal powder
2. Compressive load used, and
3. Strength requirements of the finished parts.
462 Introduction to Basic Manufacturing Processes and Workshop Technology
Metal powder
Die
Lower punch
Upper punch
Die cavit
y
Compact
Die
Fig. 25.1 Powder metallurgy die setup
In the sintering furnace, the metal parts are gradually heated and soaked at the required
temperature. During this gradually heating process, powders bond themselves into coherent
bodies. Sintering results in strengthening of fragile green compacts produced by the pressing
operation. It also increases electrical conductivity, density and ductility of the powder metal
parts.
25.2.6 Secondary Operations
Some powder metal parts may be used in the sintered condition while in some other
cases additional secondary operations have to be performed to get the desired surface finish,
close tolerance etc.. The secondary operations may be of following types-
1. Annealing.
2. Repressing for greater density or closer dimensional control.
3. Machining.
4. Polishing.
5. Rolling, forging or drawing.
6. Surface treatments to protect against corrosion.
7. In some cases infiltration is needed to provide increased strength, hardness, density
obtainable by straight sintering.
8. The procedures for plating powdered metal parts are quite different from those used
for wrought or cast metal parts. In powdered metal parts, porosity must be eliminated
before the part is plated. After the porosity has been eliminated regular plating
procedures can be used.
25.3 ADVANTAGES OF POWDER METALLURGY
1. The processes of powder metallurgy are quite and clean.
2. Articles of any intricate or complicated shape can be manufactured.
Powder Metallurgy 463
3. The dimensional accuracy and surface finish obtainable are much better for many
applications and hence machining can be eliminated.
4. Unlike casting, press forming machining, no material is being wasted as scrap and
the process makes utilizes full raw material
5. Hard to process materials such as diamond can be converted into usable components
and tools through this process.
6. High production rates can be easily achieved.
7. The phase diagram constraints, which do not allow an alloy formation between
mutually insoluble constituents in liquid state, such as in case of copper and lead
are removed in this process and mixtures of such metal powders can be easily
processed and shaped through this process.
8. This process facilitates production of many such parts, which cannot be produced
through other methods, such as sintered carbides and self-lubricating bearings.
9. The process enables an effective control over several properties such as purity,
density, porosity, particle size, etc., in the parts produced through this process.
10. The components produced by this process are highly pure and bears longer life.
11. It enables production of parts from such alloys, which possess poor cast ability.
12. It is possible to ensure uniformity of composition, since exact proportions of
constituent metal powders can be used.
13. The preparation and processing of powdered iron and nonferrous parts made in this
way exhibit good properties, which cannot be produced in any other way.
14. Simple shaped parts can be made to size with 100 micron accuracy without waste
15. Porous parts can be produced that could not be made in any other way.
16. Parts with wide variations in compositions and materials can be produced.
17. Structure and properties can be controlled more closely than in other fabricating
processes.
18. Highly qualified or skilled labor is not required. in powder metallurgy process
19. Super-hard cutting tool bits, which are impossible to produce by other manufacturing
processes, can be easily manufactured using this process.
20. Components shapes obtained possess excellent reproducibility.
21. Control of grain size, relatively much uniform structure and defect such voids and
blowholes in structure can be eliminated.
25.4 LIMITATIONS OF POWDER METALLURGY
1. Powder metallurgy process is not economical for small-scale production.
2. The cost of tool and die of powder metallurgical set-up is relatively high
3. The size of products as compared to casting is limited because of the requirement
of large presses and expensive tools which would be required for compacting.
4. Metal powders are expensive and in some cases difficult to store without some
deterioration.
464 Introduction to Basic Manufacturing Processes and Workshop Technology
5. Intricate or complex shapes produced by casting cannot be made by powder
metallurgy because metallic powders lack the ability to flow to the extent of molten
metals.
6. Articles made by powder metallurgy in most cases do not have as good physical
properties as wrought or cast parts.
7. It may be difficult sometimes to obtain particular alloy powders
8. Parts pressed from the top tend to be less dense at the bottom.
9. A completely deep structure cannot be produced through this process.
10. The process is not found economical for small-scale production.
11. It is not easy to convert brass, bronze and a numbers of steels into powdered form.
25.5 APPLICATIONS OF POWDER METALLURGY
The powder metallurgy process has provided a practical solution to the problem of
producing refractory metals, which have now become the basis of making heat-resistant
materials and cutting tools of extreme hardness. Another very important and useful item
of the products made from powdered metals is porous self-lubricating bearing. In short,
modern technology is inconceivable without powder metallurgy products, the various fields
of application of which expand every year. Some of the powder metal products are given as
under.
1. Porous products such as bearings and filters.
2. Tungsten carbide, gauges, wire drawing dies, wire-guides, stamping and blanking
tools, stones, hammers, rock drilling bits, etc.
3. Various machine parts are produced from tungsten powder. Highly heat and wear
resistant cutting tools from tungsten carbide powders with titanium carbide, powders
are used for and die manufacturing.
4. Refractory parts such as components made out of tungsten, tantalum and
molybdenum are used in electric bulbs, radio valves, oscillator valves, X-ray tubes
in the form of filament, cathode, anode, control grids, electric contact points etc.
5. Products of complex shapes that require considerable machining when made by
other processes namely toothed components such as gears.
6. Components used in automotive part assembly such as electrical contacts, crankshaft
drive or camshaft sprocket, piston rings and rocker shaft brackets, door, mechanisms,
connecting rods and brake linings, clutch facings, welding rods, etc.
7. Products where the combined properties of two metals or metals and non-metals
are desired such as non-porous bearings, electric motor brushes, etc.
8. Porous metal bearings made which are later impregnated with lubricants. Copper
and graphite powders are used for manufacturing automobile parts and brushes.
9. The combinations of metals and ceramics, which are bonded by similar process as
metal powders, are called cermets. They combine in them useful properties of high
refractoriness of ceramics and toughness of metals. They are produced in two forms
namely oxides based and carbide based.
Powder Metallurgy 465
25.6 QUESTIONS
1. What do you under stand by powder metallurgy? What are the main stages of powder
metallurgy process?
2. What important considerations should be made in designing the powder metallurgy products?
3. Explain the objectives of powder compaction and list important products of powder metallurgy.
4. Describe briefly the methods by which powders suitable for powder metallurgy can be
produced. Also enumerate the main characteristics of metal powder.
5. Describe the atomization process of making powder in detail.
6. What are the effects of sintering on the powder compact produced by pressing?
7. Describe the process of blending, compacting and sintering in detail.
8. What are the effects of sintering on the powder compact produced by pressing?
9. Name the products of powder metallurgy.
10. List the advantages, dis-advantages and applications of powder metallurgy process.
11. It is necessary to use lubricants in the press compaction of powders? Explain.
12. What are ‘cermets’? Give a few examples of useful applications
13. State briefly the process of making a powder metallurgy product having improved properties.
14. What are cemented carbides? How are they processed?
466 Introduction to Basic Manufacturing Processes and Workshop Technology
INSPECTION AND QUALITY CONTROL
26.1 INTRODUCTION
Inspection or checking of components or products with required specifications is very
minutely related with quality control. It is generally an accepted fact that no two things can
ever be exactly same. It also holds true with manufactured parts. Therefore certain variations
or deviations in dimensions and other product specifications are accepted. However, only few
produced articles or parts may be rejected if the deviations go beyond the specified quality
standards. Therefore it becomes essential to detect errors so that the manufacturing of faulty
product does not go uncorrected. The philosophy of inspection is only preventive and not
remedial. In other words the inspection of products is measuring or checking its quality in
terms geometrical tolerances of other specified feature of needed design. Generally, there are
three basic areas of inspection namely receiving inspection, in-process inspection and final
inspection. In the receiving inspection, inspections are performed on all incoming materials
and purchased parts. In the in-process inspection the products are inspected as they are in
processed in stages from starting station to finished station. In the final inspection, all
finished products or parts are inspected finally prior to delivering them to the customer.
The main motive of manufacturing is to process engineering materials and produce
desired and useful components or products to the specified shape, size and finish. The
specifications for the shapes, sizes and finishes on the products are furnished by the
manufacturing operations through specified process plan using part drawings or manufacturing
drawings. These specifications basically termed as called quality characteristics. The quality
of manufactured product is depend always upon the process capability of controlling
manufacturing functions which may lead to a certain amount of variation as a result of chance
and some cause. Also some chance or cause is inherent in any particular scheme of production
and inspection. The reasons for variation outside this stable system should be discovered and
corrected to minimize wastage and finally to improve quality.
26.2 TOLERANCES ON PARTS
It is difficult to manufacture any product or component to its exact size. Tolerance on
the parts is therefore the amount of variation in size tolerated to cover reasonable imperfections
in workmanship and it varies with different grades of work. Tolerance on a dimension can
also be specified as the difference between the maximum limit of size and the minimum limit
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Inspection and Quality Control 467
of size. It is equal to the algebraic difference between the upper and lower deviations and has
an absolute value without sign. Its value is a function of the basic size and is designated by
a number symbol, called the grade. There are two basic ways of specifying the working
tolerance: (1) bilateral and (2) unilateral, tolerances. Bilateral tolerances are used where the
parts vary in both directions from the desired or nominal size. Unilateral tolerances are used
where it is important for the dimension to vary in only one direction. Components produced
will fall close to the desired dimension but can vary in only one direction. An example is the
drilled hole, as the drill is made close to the normal hole size, it is seldom possible to drill
a hole undersize. All drilled holes therefore should carry only a plus tolerance. Since the
tolerance is allowed on one side of the nominal size, the system is said to be unilateral. Closer
the tolerance higher is the cost of product.
26.3 INTERCHANGEABILITY
The dimensions of mating parts are generally controlled to have a proper fitting of
matching parts for its optimal functional requirement. Providing dimensions on components
or parts is the job of a product designer. Interchangeability of the parts is, therefore, a major
pre-requisite for economic production, operation and maintenance of machinery mechanism
and instruments. It is therefore very much possible to interchange spare parts in various
machines, tractors, motor cars, machines tools, airplanes and many, others so that they can
be dismantled for replacement of parts in service conditions in the field, and also in many
local workshops with least possible loss of time. In order to produce interchangeable or
identical parts, the components of interchangeable system and the various terms related with
inter-changeability of the mating parts should be understood clearly by the product designing,
manufacturing and product inspecting staff working in industries.
26.3.1 Size
It expresses the numerical value of a length in a particular unit on the part. The basic
size of a part is its nominal dimension from where all variations are generally made. It is
determined by the part designer from its functional requirements to meet the specified
objective. The other term used with respect to a part is the nominal size. The nominal size
is the size of the part specified in the drawing as a matter of convenience. It is used primarily
for the purpose of identification of a component and is never used in the precision measurement
of parts. A rigid attitude towards the maintenance of a basic size of the part may increase the
manufacturing cost and a little variation in dimension is accepted resulting in a size, which
is different from the basic size of the part. This is called the actual size. The actual size of
a dimension or part is its measured size. An actual size of a ready part will, therefore, always
deviate from one specified in the drawing, i.e. from the nominal or basic size of the part.
Whereas the difference between the basic size and actual size must not exceed from a certain
limit otherwise it will interfere with the interchangeability of mating components during
assembly or sub assembly of parts.
26.3.2 Limits of Size
Limits of size are two extreme permissible sizes between which the actual size exists.
The maximum limit for a dimension is the largest permissible size, while the minimum limit
for a dimension is the smallest permissible size. The maximum or minimum sizes represented
by tolerances are called the limits. The basic sizes deviation and tolerances for both shaft and
hole are illustrated in Fig.26.1.