In this article we will discuss about:- 1. Meaning of Powder Metallurgy 2. Necessity of Powder Metallurgy 3. Useful Guides for Good Results 4. Preparation 5. Properties 6. Fabrication Methods 7. Applications 8. Advantages 9. Disadvantages.
Contents:
- Meaning of Powder Metallurgy
- Necessity of Powder Metallurgy
- Useful Guides for Good Results in Powder Metallurgy
- Preparation of Powder Metallurgy
- Properties of Powder Metallurgy
- Fabrication Methods of Products from Powder Metallurgy
- Applications of Powder Metallurgy
- Advantages of Powder Metallurgy
- Disadvantages of Powder Metallurgy
1. Meaning
of Powder Metallurgy:
Powder Metallurgy is the process whereby metallic shapes are manufactured from metallic powders. The process involves the manufacture of metallic powders and the subsequent welding of these powders into a solid form of the required shape. In powder metallurgy, the metal or alloy is solid at the very start of manipulation and remains completely solid during the manipulation process.
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Powder metallurgy is becoming an increasingly important tool in the fabrication of many products. Powdered-metal techniques are invaluable in the manufacture of parts from refractory materials or materials which are extremely difficult to work or machine e.g., sintered carbides etc.
Often sintering is the only satisfactory method but in some cases alternative processes are also used. In such cases selection should be done properly, since sintered components are brittle than solid metal. Tooling costs are heavy, and as such it can’t be justified for small quantities.
Usually 20,000 parts are considered as the breakeven point between sintering and machining from solid metal. Sintered parts are normally made up of particles having a range of sizes like 50% between 100 and 150 mesh, 25% between 150 and 200 mesh, 25% between 200 and 300 mesh.
2. Necessity of Powder Metallurgy:
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Necessity of powder metallurgy arose because the processes of making alloys by melting were unsuitable for following cases:
(i) Difference in melting temperatures of two elements is so much that one would become gas before the other has melted.
(ii) Melting and solidification causes poor quality.
(iii) Melting causes loss of identity of the constituents: e.g. tungsten carbide on melting breaks down.
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(iv) Some metals do not form a liquid solution and thus can’t be allowed to produce special properties.
Today powder metallurgy is extensively used for the production of large quantities of small and medium size parts.
The basic operations involved in manufacture of parts using powder metallurgy are:
(a) Preparation, grading and blending of the powders.
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(b) Pressing or compacting of powders.
(c) Sintering of the compacted powders.
A part produced by powder metallurgy is often termed as sintered part.
3. Useful Guides for Good Results in Powder
Metallurgy:
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(i) The range of particle size and shape must be controlled to optimise compaction and sintering.
(ii) Mixed particle sizes give a higher apparent density.
(iii) For uniform shrinkage, higher mechanical properties and faster sintering, fine grain size with individual particles and fine powders must be used.
(iv) Higher compaction pressure is possible without particle fracture if powder is annealed.
(v) Above a certain compaction pressure density can’t be increased significantly.
(vi) Flow rate of powder into dies must be rapid for high mass production. Sometimes excess lubricant (stearates) may inhibit the flow.
(vii) Some chemical changes may occur on heating due to powder surfaces desorbing any previously adsorbed gases; and also due to burning off of certain organic lubricants.
(viii) Hot pressing the powder in heated dies removes work hardening limitation on compaction pressure and gives higher sintered density.
(ix) During sintering the particles lose their work-hardened structure from compaction and thus elastic strains are relieved. Repressing after sintering becomes helpful.
(x) Dimensional changes also occur during sintering due to shrinkage.
(xi) Phenomenon of alloying and phase changes also occur during sintering.
(xii) Uniaxial dies give non-uniform compacted density. Isostatic compaction gives uniform compaction.
In isostatic compaction, the powder is encapsulated in a flexible, impermeable mould which is submerged in a fluid and the fluid is pressurised. For higher compacted density, hot fluid may be used, (hot isostatic compaction).
(xiii) Open porosity can be filled with another metal with a lower melting point (infiltration).
(xiv) Porosity can be used to advantage in some cases by impregnating with oil so that oil expands and flows to lubricate the surface. Upon cooling, the oil returns to the open pores by capillary action.
(xv) Steps should be taken to avoid defects like soft agglomerates (low strength due to inadequate pressure during powder consolidation), hard agglomerates (local higher strength due to non-uniform attractive forces of the powders), organic inclusions, inorganic inclusions, large grains due to grain growth from heating after initial sintering, over firing (resulting in war page, lower strength, new and unwanted phases at high temperature, surface cracks due to binder being driven off).
4. Preparation of Powder Metallurgy:
i. Mechanical Pulverisation:
This method is applicable to brittle metals like antimony. Brittle metals and alloys can be powdered down to a size of 0.001 mm. Many varieties of mechanical pulverisers are in use; some of these have counter-rotating plates or rapidly moving hammers.
These methods carry out mechanical disintegration to the maximum fineness possible. Mechanical pulverisation is usually followed by ball milling. Other mechanical methods include machining and shotting.
Machining results in coarse particles which may be further milled and is principally used for magnesium powders. Shotting is the operation of powdering molten metal through a sieve or orifice and cooling it by dropping it into water. Spherical or pearl-shaped particles are obtained by this process. Shotting is mainly used for metals like aluminium, lead and zinc.
ii. Electrolytic Process:
This method is mostly used for the manufacture of copper, iron, tungsten, tantalum, silver, zinc and even tin powders. This process uses a salt of the metal. The conventional electrolysis process is carried out and copper is deposited as a fine powder to the required degree of fineness.
The powder is scrapped from the electrodes and made to collect in the bottom of the vat and periodically withdrawn when sufficient quantity is accumulated. The powder is washed and dried. The electrolytic powder is quite resistant to oxidation.
iii. Chemical Reduction:
This process is commonly employed for some of the metals like tungsten which have high melting points. For example, pulverised tungsten oxide is heated in a current of hydrogen; the hydrogen reduces the oxide to metallic tungsten powder. Pure iron is made from iron oxide or ores of iron such as magnetite and hematite.
The powder formed by this process has particles of a sponge like nature and it is ideal for cold pressing due to its softness and plasticity. Swedish sponge iron is made by heating iron ores in contact with charcoal at relatively low temperature and reducing the oxide to sponge powder. Other methods include condensation and precipitation. In condensation, vapours of low temperature metal such as zinc powder are condensed to powdered form.
In precipitation, carbonyl precipitation is used for producing iron and nickel powders which under suitable pressure and temperature conditions will react with carbonyl to produce carbonyl liquid. At the reduced pressure and elevated temperature, this carbonyl decomposes and results in the metal precipitation.
This precipitate is then dried and condensed to form powder. Precipitation is used to an increasing extent to produce fine particles and highly pure powders of copper and silver. The carbon reduction method is also widely used.
iv. Atomization:
In this process, the molten metal is forced through a small orifice and broken up by a stream of compressed air. Fine powder may be made by this process but it requires special nozzles and careful control of temperature and pressure etc. Oxidation can be prevented by use of an inert gas.
If air is used, little oxidation occurs, the particles get oxidised and form a thin protective coating upon the surface thus preventing excessive oxidation from occurring. This process is used for metals like tin, zinc, lead, aluminium, cadmium etc. which have low melting points. Alloy powders are also produced by this method. After powder is made, it is washed before grading.
5. Properties of Powder Metallurgy:
i. Particle Size and Size Distribution:
Particle size is one of the most important characteristics of any powder. The common method of measuring the size of any metal powder particles is to pass the powder through screens (sieves) having a definite number of meshes. In the screening method, the size of particles is measured by a square-mesh screen of standard size which will just retain the particles.
Since the shape of the particle is irregular, in general, size refers to the largest cross-sectional dimension and not to the diameter of an actual sphere or the edge of the real cube.
Their size is commonly reported by screening out the course, then the medium and then the fine particles and reporting as below:
66% – 100 to 200 mesh; 17% – 200 to 325 mesh; 17% – 325 mesh;
this means that 66% of the powder by weight will pass the 100 mesh screen but not the 200 mesh screen, and so on.
ii. Physical and Structural Characteristics:
The powder, as it is produced, is loose and there is considerable swell. When compacted in the die, it is reduced in volume as most of the voids are filled up due to interlocking of the grains. The compression ratio (the ratio of the density of the compact to the apparent density of the powder) varies between 2 : 1 and 3:1.
(Apparent density- The loading weight of a powder may be considered to be the mass of loosely heaped powder necessary to completely fill a given die cavity. In terms of weight/unit volume, this is referred to as the apparent density).
The structure of the powder greatly influences such characteristics as plasticity and ability to be cold pressed. It influences pressure required in pressing, strength of the final product, flow characteristics etc.
iii. Packing Factor:
The sintered properties in powder metallurgy depend upon how the powder particles are packed, i.e. the porosity, volume, density, packing factor etc. There could be internal voids in particles or external voids between particles or between particle and confining shape boundary (as shown in Fig. 7.1).
Open pores are spaces into which water can penetrate, whereas closed pores are spaces into which water cannot penetrate. True porosity is the volume of both open and closed pores divided by the total volume of confining shape boundaries.
The apparent porosity is the ratio of volume of open pores and total volume of boundaries of confining space. Bulk volume is defined as true volume of particles plus volume of open and closed pores. The apparent volume is defined as true volume of particles plus volume of closed pores only.
Similarly bulk density is rate of mass of particles and bulk volume; true density is ratio of mass of particles and true volume; and apparent density is the ratio of mass of particles and apparent volume or mass divide by sum of true volume and volume of closed pores.
Packing factor is also defined as true and apparent. True packing factor is ratio of true volume and bulk volume, or ratio of bulk density and true density. Apparent packing factor is the ratio of apparent volume and bulk volume, or ratio of bulk density and apparent density. For instance packing factor for a particle of radius ‘r’ contained in a cube of sides 2r would be 4/3 πr3/(2r)3 = π/6 = 0.524 and Porosity would be 1 – 0.524 = 47.6%.
Porosity may also be defined as 1-packing factor. For good results, we have to find ways and means of optimising or controlling the packing factor.
The three different ways in which porosity can be changed are:
(i) By changing shape of particles, for instance particles of cuboid or similar shape can be packed with less porosity than spheres,
(ii) By compression and impregnation, i.e. by excluding air pockets, and
(iii) By using particles of mixed size so that small particles can fill the spaces between large particles.
iv. Flow Characteristics:
Flow ability of powders is most important in cases where moulds have to be filled quickly. A measure of flow ability may be obtained by determining the weight of powder which will flow through a 0.8 mm diameter orifice in one minute.
Powders with good flow characteristics fill a mould cavity uniformly. Particle size distribution, particle shape and the coefficient of friction of the metal being handled exert considerable influence on flow characteristics.
v. Surface Condition:
Oxides may be present in the powder and they greatly weaken the final product by cold-pressing. Chemical properties are dependent upon the purity of the powder, amount of oxides permitted and the percentage of other elements allowed. Clean surface of particles is essential for attaining desired mechanical properties.
6. Fabrication Methods of Products from Powder Metallurgy:
i. Compacting of Powder (or Cold Pressing):
Pressure Less Forming:
This is used for highly porous parts. The powder is poured or vibrated into a mould which is then heated to sintering temperature. (Loose sintering). The part is then removed from the mould.
The press used for compacting may be either mechanically or hydraulically operated. It may be appreciated that powder under pressure does not behave like a liquid, i.e. pressure is not distributed equally throughout. This is because of friction between the granules themselves and between the granules and the tool. For this reason, two punches are commonly used to compact the metal powder in the die.
This results in a stronger and denser part. The punches are located at each end of the die and normally operate independently of each other (Ref. Fig. 7.2). In automatic presses one or a number of pressings are produced at each stroke of the machine and these are used for small parts which require low pressures.
During compacting, the voids between the particles are reduced. The particles also get deformed on account of the pressure and they key into each other and get locked; surface irregularities of the particles are flattened out. The pressures used are 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 density is also increased. Soft and malleable metal powders deform easily; so for tin, a pressure of 0.75 kg/sq mm is sufficient; whereas for tungsten and refractory material a pressure of 80 to 160 kg/sq mm may be necessary.
There is usually an optimum pressure for any particular metal or alloy which gives best results for a given expenditure of energy. Beyond this optimum, further increase in pressure does not give corresponding increase in properties like density, tensile strength, etc. In compacting process, the pressure applied should be even and applied simultaneously from above and below.
Rate of pressure application is also very important, as too rapid rate may result in entrapping of some air and it gives rise to certain defects. Sometimes waxes and soaps are used to lubricate powders and to produce a more even pressure distribution. These in particular help in reducing friction at the die walls. However green strength is reduced as cold welding is inhibited.
The compacts obtained by the above process are not strong and dense. To improve these properties, the components should be sintered. The strength of un-sintered components is called green strength.
Hot Pressing:
It is not possible to cold press very hard powders (like diamonds) satisfactorily. Hot pressing above recrystallization temperature of the metal is used which eliminates work hardening and produces accurately shaped high density parts. It is important to carry out hot pressing in vacuum or neutral or reducing atmosphere as otherwise the metal may be oxidised.
Selection of die material to suit temperatures encountered is important. Below 1000°C, metallic dies can be used and above 1000°C graphite dies are used. Advantages of graphite are that it is cheap, easily machinable, resistant to thermal shock and provides its own reducing atmosphere. Powders can be hot formed by rolling, forging or extruding also.
ii. Sintering:
Heating of powder compacts in a furnace to below the melting point of at least one of the major constituents under a controlled atmosphere is called sintering. In the sintering furnaces, the components are gradually heated and soaked at the required temperature which depends on the type of material.
During this cycle of heating operation, powders bond themselves into coherent bodies. The sintering temperature and time vary with the compressive load used, the type of powders and the strength requirements of the finished parts.
Few examples are:
Sintering results in strengthening of the fragile green compacts produced by the pressing operation. Apart from strengthening the component, sintering increases electrical conductivity, density and ductility. Since shrinkage increases with rise in sintering temperature, the sintering temperature selected is a compromise between strength and dimensional stability. The highest temperature which provides an acceptably small dimensional change is chosen.
When a mixture of different powders is to be sintered after pressing and the individual powder metallurgy in the compact have marked different melting points, the sintering temperature used is above the melting point of one of the component powders. The metal with a low melting point will become liquid.
When the solid phase of powder is soluble in the liquid metal, a marked diffusion of the solid metal through the liquid phase may occur, which will develop a good union between the particles and result in high density.
Most cold-pressed powder metallurgy shrink during sintering operation. In general, factors influencing shrinkage include particle size, pressure used, sintering temperature and time. It is possible to control the amount of shrinkage by carefully selecting the metal powder and determining the pressure, sintering temperature and time. The amount of shrinkage or volume change should be determined so as to allow for this change in the design of the dies used in the process of fabricating a given shape.
Full strength associated with a high degree of bonding is not acquired until active elevated temperature sintering is accomplished. Assuming that a compact has been made at room temperature in an unevaluated system, the first increase in adhesion between the powder particles on the application of heat will occur at around 150°C except for refractory metals (W, Mo, Ta etc.). At the same time there will be measurable increase in porosity of the components.
This behaviour is due to the removal of absorbed gases. On further heating, a temperature is reached at which sintering progresses very rapidly, the action tapering off with time at this temperature. The major structural and property changes in the component take place at this time (recrystallization).
They are:
a. Grain growth across the original inter-particle boundary.
b. Pores tend to diminish in size.
c. Spheroidisation of angular powder particles.
d. Trapped oxides tend to spheroidise.
Although the application of pressure during heating is not essential to the sintering operation, densities in sintering products are improved by hot pressing. Protective atmospheres are essential for successful sintering of the compacts.
The object of such an atmosphere is to protect the pressed compacts from oxidation which would prevent the successful welding together of the particles of metal powder. For reduction of oxides, hydrogen is commonly used. Water vapour should be removed from the hydrogen gas by activated alumina dryers before it enters the furnace.
Sintering can be carried out in either gas fired, oil fired or electrically heated type furnaces which may be either of batch or continuous type.
Sometimes sintered products are processed further to obtain better properties as follows:
a. Infiltration:
Porosity of sintered products can be improved by process of infiltration, in which the pores are filled with a lower melting point metal by capillary action, e.g. copper is used as infiltrant to strengthen the iron and steel by passing the compact with the infiltrant laid on top.
b. Sizing and Coining:
Accuracy and mechanical properties of sintered products are improved by cold working. Sizing produces better dimensional accuracy with significant increase in its density. Coining on the other hand significantly increases the density and at the same time also improves the mechanical properties, surface finish and dimensional accuracy.
c. Machining:
Sometimes sintered parts are finished by conventional machining when high degree of accuracy is required.
d. Heat Treatment:
A sintered part produced from a powdered heat treatable alloy or produced from a mixture of metals that responds to heat treatment, can be heat treated to improve various properties.
7. Applications
of Powder Metallurgy:
i. Porous Products:
Articles manufactured by powder metallurgy can be given any degree of desired porosity. By controlling the article size and distribution and the pressure during compaction, the percentage of porosity can be controlled within the narrow limits. High porosity is required in porous oil impregnated bearings which are self- lubricating, in high capacity electrical accumulator plates and for filtering media, etc.
ii. Babbitt Bearings for Automobiles:
Powder metallurgy techniques are employed for the manufacture of Babbitt connecting rods and main bearings for automobile engines. The process consists of applying a mixture of copper and nickel powders to a steel backing piece and passing the same in a sintering furnace. The copper will form a strong bond with the steel backing piece and nickel will alloy and form a composite porous bearing material with the copper.
The sintered bearings are tested mechanically for their worthiness by rolling them between chromium plated rollers. This test will ensure that under service conditions the matrix will not get crushed.
The air from the pores of the matrix is now removed and the Babbitt alloy made to flow into the pores at normal atmospheric pressure. The final thickness of the Babbitt does not exceed 0.05 mm. The powder used for the matrix is 45% nickel of (150µ) average particle size and 55% copper (100µ).
iii. Oil Pump Gears:
Gears for automobile oil pumps are now extensively manufactured by powder metallurgy. Iron powder is mixed with graphite, compacted under a pressure of about 40 kg/sq mm and sintered in an electric furnace with an atmosphere of hydrocarbon gas. Then these gears are impregnated with oil and the finishing machining operation is done.
Only about 0.02 to 0.1 mm material is removed by machining. By this process true involute gear form, correct to 0.02 mm is obtained. Oil absorbed in the pores during impregnation is available as a stored lubricating medium, seeping to the contact surface under operating pressure and being reabsorbed when the load is decreased.
iv. Cemented Carbides:
These are very important products of powder metallurgy and find wide application as cutting tools, wire drawing dies and deep drawing dies. These consist of carbides of tungsten, tantalum, titanium and molybdenum.
The actual proportion of the various used carbides in a specific grade depends upon application and these are carefully selected and blended to suit the purpose for which it L intended. Either cobalt or nickel is used as the bonding agent while sintering.
The first step in manufacturing is to reduce tungsten oxide and cobalt oxide powders by intimately mixing them with lamp black and heating the same in a current of hydrogen. The tungsten oxide and cobalt oxide are reduced in separate lots to metallic state.
The size of the metallic powders obtained is controlled by the heating time, temperature and the rate of the hydrogen flow. Tungsten powder is then ground, mixed with required lamp black and then the mixture is heated for several hours in a reducing atmosphere at a temperature of 1600°C. By this process tungsten is converted into tungsten carbide.
The next stage is to mix the tungsten carbide and cobalt powders in suitable proportions in a mixing machine. The cobalt should be evenly distributed in the tungsten carbide so that the bond is uniformly strong. There should not be much variation in the size of the powders.
The mixture is then compacted in alloy steel dies at a pressure of about 48 kg/sq mm, the sintering is done in two stages. The preliminary sintering is done at a temperature of about 900°C in a controlled atmosphere (Hydrogen). Cobalt fuses at the sintering temperature whereas the tungsten carbide remains intact. (Hence the products is known as cemented tungsten carbide).
These products are then shaped to the exact size by machining. The final sintering is done at a temperature of about 1300°C for 2 hours. Cooling is done gradually in the furnace itself. Tungsten carbide tools, usually called tipped tools are the most important cutting tools in the machine shops today.
v. Refractory Metal Composites:
To attain serviceable characteristics at temperatures higher than those for which the high temperature alloys are suited; refractory metal composites have been developed. These materials, being developed for service upto a possible 1300°C, consist of metals or alloys combined in various ways with ceramic oxides, carbides, nitrides, borides, and silicide’s. They are also called cermet’s. The most common refractory metals produced by powder metallurgy techniques are tungsten, molybdenum, tantalum and platinum.
vi. Tungsten Wires:
Wires for filaments in the lamp industry are made from pure tungsten powder which is pressed and sintered to form a bar of tungsten. This bar is swaged to about 2 mm or less in diameter and is then drawn through tungsten carbide dies to about 0.2 mm diameter.
At temperature of about 100°C the wire is again drawn through diamond dies to the finished diameter. The tensile strength of this tungsten wire reaches about 470 kg/sq mm, more than twice of the hardest steel and is the strongest material known so far.
vii. Diamond Impregnated Tools:
These are made from a mixture of iron powder and diamond dust. Diamond dust acts as cutting medium and iron powder acts as the bond. These tools are used for cutting porcelain and glass. Thin discs are also made for the cutting of tungsten carbide.
The mixture consisting of nearly 30% diamond dust is sintered at a temperature of about 1000°C under a pressure of at least 8 kg/sq mm. The tool is finally welded or brazed to a steel backing piece.
viii. Electrical Contract Materials:
Electrical contact materials are manufactured to produce components with properties which cannot be obtained by any other known method. One type of composite contact material combines the low contact resistance, current-carrying capacity and thermal conductivity of metals such as silver or copper with the hardness and compressive strength of refractory metals such as W, Mo or Ta.
These materials are used in such applications as circuit breakers, relays and resistance welding electrodes. The composite may be made by pressing and sintering the refractory component to form a porous skeleton of the desired shape and then impregnating it with molten copper or silver. If desired, the porous skeleton may be of some regular shape and final shape may be attained by forging, rolling or extending after impregnation.
Copper-graphite brushes are also made. They are composite contact materials used to give efficient, long life service in the transfer of electrical current between moving and stationary parts of electrical equipment. They are particularly well adapted to high current and low voltage applications. Graphite reduces friction and wear without affecting contact resistance.
None of the combination of these alloys can be made by melting.
ix. Magnetic Materials:
Magnetic materials fabricated from powders include those of permanent and soft magnetic characteristics. The Alnicos (Aluminium, Nickel, Cobalt and Iron alloys) are in the permanent magnet category. These alloys are difficult to cast and not machinable.
Typical applications of powder products having soft magnetic properties include pole piece for d.c. motors or generators and cores for high-frequency inductance. In the latter application, the powders are insulated, pressed and thermally treated but not fully sintered, iron-silicon or iron-nickel (perm alloys) are the common systems involved.
Other magnetic materials produced by powder metallurgy techniques include cobalt-platinum, iron-cobalt and also ferrites consisting of mixed oxides of iron and other metals. Magnadur is a typical hexagonal ferrite permanent magnet with high coercively. A magnesium-manganese ferrite has a square hysteresis loop and is used for computer storage elements.
In the soft magnetic field a manganese-zinc-ferrous cubic ferrite is an example of the type M.Fe204 and is used as a low eddy current loss ferrite in applications from low frequencies to micro-waves. These can be produced only be powder metallurgy techniques.
8. Advantages of Powder Metallurgy:
a. Articles of any intricate shape can be manufactured (but size is limited). Closely controlled tolerances of the order of ± 0.02 mm are possible.
b. Machining operation is almost eliminated. The parts have an excellent finish and high dimensional accuracy.
c. Metals and alloys in any proportion can be mixed together which are difficult and sometimes not possible by melting, e.g. tungsten (3,400°C) and copper (1,083°C).
d. There is overall economy as material wastage is negligible.
e. Metals and non-metals can be mixed in any proportion, e.g. graphite is added to copper for the manufacture of dynamo brushes.
f. Articles of any desired porosity can be manufactured. Lubricant can be impregnated into porous bearings and bushings under pressure resulting in components which are self-lubricating for life. The combinations of large particles and controlled porosity are used to produce filters.
g. Antifriction alloy strips made by powder metallurgy can be made to adhere on a strong alloy backing piece.
h. Super-hard cutting tools bits, and refractory materials which can never be manufactured by any other method, are made by powder metallurgy techniques (e.g., Sintered carbides, Stellite).
i. Tungsten which has a melting point of about 3300°C cannot be easily obtained in the molten state since only few special furnace refractories can withstand such high temperature. By powder metallurgy tungsten alloys are manufactured into desired shapes.
j. Diamond impregnated tools for cutting porcelain, glass and tungsten carbides are made possible only by powder metallurgy.
k. Components containing blind recesses with sharp corners (which are otherwise difficult using conventional machining operations) can be produced.
l. A wide range of parts with special electrical and magnetic properties can be produced.
m. It is a very rapid process and can produce parts with excellent dimensional accuracy and surface finish.
9. Disadvantages and Limitations
of Powder Metallurgy:
1. Metal powders are expensive and in some cases difficult to store without some deterioration. Further not all metals and alloys are suitable for powder metallurgy.
2. The equipment used for the operation are costly.
3. Pressure upto 100 tonnes capacity are used even for small products.
4. Dies used must be of high accuracy and capable of withstanding high temperature and pressure.
5. The product size limitation is determined by the capacity of presses, cost of dies and compression ratio of various powders.
6. Initial cost of preparing metallic powders is very high.
7. Since there is no flow of metal particles during compacting, therefore, it is difficult to obtain intricate shapes in the products.
8. Parts produced by powder metallurgy have poor ductility.
9. The shapes of components produced by this method are limited mainly by the fact that unlike liquids, metal powders cannot be made to flow in a horizontal plane of round corners. Drastic changes in section have to be avoided. Sharp or feathered edges should be avoided.
10. The process is limited to the production of relatively small components because of very high pressures required in the pressing operation.
11. Some shrinkage of the component usually occurs during the sintering process. When a component must possess a high surface finish or its dimensions must be held within close limits, a sizing or coining operation is performed after it has been sintered.
12. A completely dense product is not possible by this process/ However porosity can be reduced by hot pressing. Also strength and toughness of a part produced by powder metallurgy is lower than when casting or working is done.
13. Since powder is compacted vertically, components must be of a shape to facilitate their ejection from the die. Re-entrant shapes or undercuts and cross holes, therefore, can’t be produced.
14. Tooling costs for the pressing operation are high, and the production quantities must be in excess of 10,000 to make the process economical.