POWDER METALLURGY
Content
POWDER METALLURGY
Powder metallurgy is the name given to the process by which fine powdered materials are blended, pressed into a desired shape, and then heated to bond surfaces.
Need of Powder Metallurgy:• Melting is unsuitable in some cases. (Large difference in Melting temperatures of the principle constituents of an alloy so much that one would vaporize before other melts) • Melting and solidification causes poor quality. • Melting causes loss of identity of the constituents. • Some metal do not form liquid solution.
Basic Steps In Powder Metallurgy
• • • • • Powder Production. Blending or Mixing. Powder Consolidation. Sintering. Finishing.
.
Powder Production
1 Mechanical pulverization
(a) roll crushing, • •
(b) ball mill,
(c) hammermilling.
• • •
Mechanical pulverisers: consists of two counter rotating discs of steel one of which is adjustable. The size of the pulverized particles depend upon the distance between the discs. Ball mills: A big cylindrical shell is filled with granulated materials along with hardened steel balls and the whole system is rotated in clockwise and anticlockwise directions for specific periods so that the granulated materials are disintegrated to required powder size. Rapid moving hammers: shell shaped containers are filled with 2/3rd materials. The shell has a inbuilt hammer which is rotated at high speeds to disintegrate the material to required size. Machining: some powders are prepared by means of machining process as this leads to very fine particles. Grinding: grinding removes the material in very small quantities and in very fine form which is ideal to be classified as powders.
2. Atomization
In this process, molten metal is made to pass through a nozzle and it is broken into small droplets after spray. It is then cooled artificially by high pressured water jet as shown in the sketch. This method is suitable for those metals having low melting point such as Lead, Tin and Zinc. 3 Electrolysis
In this process, the anode dissolves in the electrolyte and is intentionally made to deposit on Teflon coated cathode. This is done to enable easy scraping off the deposited anode. Method is inherently costly and very small amount of powder is obtained by this process. 4 Chemical reaction : Ores which are very easily pulverised are done so and are then heated to remove the oxides. This can be very effectively done by heating in an atmosphere of hydrogen or carbon monoxide. Although proper size cannot be monitored here, it is more than compensated by its highly economical nature. 5 Other methods : Some of the other methods incorporated for the preparation of powders are – 1. Granulation – Molten metal is converted into small particles by rapidly stirring the molten metal while it is cooling. 2. Condensation - Molten metal is boiled and then condensing the vapor. 3. Shooting – Molten metal is poured through orifice/ sieves and cooled by dropping in water. It must be noted that all the above methods are suited for low melting point materials such only. (Mg Zn and cadmium). 6. Precipitation (carbonyl decomposition):
In this method metal are made to combine with CO to form volatile carbonyl at suitable pressure and temperature carbonyl decomposes to get pure metals. (Fe Ni Mo W etc)
CHARACTERISTICS OF METAL POWDER:Flow rate:-a measure of the ease by which powder can be fed and distributed into the die, - significantly influenced by the cycle time and production rate. This determines the fineness of the particles. Nozzle is used to determine the flow rate. Density:-apparent density –ability to fill available space without the application of external pressure. It dependence on powder shape size and size distribution. True density means density after compactation. Density distribution should be uniform. Compressibility:-Effectiveness of applied pressure in raising density of the powder i.e. green strength. This again depends on the shape, size and size distribution of the powder particles. Particles should have the ability to be easily compressed.
Shape, size and size distribution :- It determines the amount of pressure required and the strength to be possessed by the final product. Spherical particles can be used when high permeability is desired. If high strength is required, we can select coarse particles. Shape depends on method of powder manufactured .-spherical, dendritic angular nodular irregular lamellar etc. Spherical powder have excellent sintering properties. Irregular particles are superior for practical moulding and achieving good green strength because they will interlock on compactation. Size distribution means quantity of each standard particle size in mixture. The particle shape, size and structure play an important role in the quality of the final product. They are chosen on the basis of various applications involved. Various particle sizes and shapes are shown below
Purity: - High purity is required for a better product. For maintaining this, the particles must be isolated from the atmospheric oxidation or any contamination.
MIXING AND BLENDING:If high purity particles are not being used, mixing and blending is essentially required. Powder particles are first graded with respect to their shape, size and purity. They are then weighed and mixed in proper proportions. Binders are added for the compactation of hard particles. Binders improve adhesiveness and increases strength. Lubricants like organic solvents and
wax may be added to increase the flowability and enhance the permeability of the particles. Sintering agents are added to accelerate the heating process. After this, the particles are manually mixed together taking care of the fact that the powder particles are uniformly distributed throughout the mass. Mixing may be accomplished in ball mill. It may be done either dry or wet. For wet water or solvent is added to the dry powder to reduce the possible hazard of dust and explotion. Mix to obtain uniformity Mix to obtain desired physical and mechanical properties Mix lubricants to improve flow characteristics Blend in air, inert gas (to avoid oxidation) or in liquids.
BRIQUETTING/COMPACTATION
Loose powder is compacted and densified into a shape, known as green compact. Most compacting is done with mechanical presses and rigid tools. Hydraulic and pneumatic presses are also used. Care should be taken at this stage so as to apply only the optimum pressure. Less pressure leads to poor quality products and undesired extra pressure leads to damage of the die without any significant enhancement in the property of the powder particles.
Friction problem in cold compaction:The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction. The pressure tapers off rapidly and density diminishes away from the punch. When the pressure is applied by only one punch, the maximum density occurs right below the punch surface and decreases away from the punch. For complex shapes, multiple punches should be used. The compacted mass should be able to be withdrawn from the die cavity without damage. If this is achieved, it is called as “green compact.
Compaction with a single moving punch, showing the resultant nonuniform density (shaded), highest where particle movement is the greatest
Density distribution obtained with a double-acting press and two moving punches. Note the increased uniformity compared to previous Figure. Thicker parts can be effectively compacted.
Changes noticed after compaction
The green strength of the compact is achieved due to: 1. Adhesion or even cold welding of some of the powders 2. Mechanical interlocking of particles 3. By addition of bonding agents which forms strong bonds with the powders. During the compaction voids between the particles are reduced. Surface irregularities of the particles are flattened out and density also increases. In this process the pressure applied should be even and applied simultaneously. Rate of pressure application is also very important as too rapid rate may result in entrapping of some air and it gives rise to sudden defects.
Other compaction methods
Although conventional process is most widely used, there are other processes of compaction which have their own benefits. 1. Pressure less compaction: - It is used for the manufacture of porous filters where relatively large gap is required for quick filtration. For this, the particles are shaken well in a die cavity without the application of pressure to get the required compactation, and sintered thereafter. 2. Cold Isostatic Pressing:- Cold isostatic pressing applies pressure from multiple directions for achieving greater uniformity of compaction (high-quality parts) and increased shape capability, compared to uniaxial pressing. The powder may be encapsulated in a flexible mold, which is then immersed in a pressurized gas or liquid. There are two methods of carrying out isostatic pressing. In wet-bag isostatic pressing, powder is encased in a rubber sheat that is immersed in a liquid which transmits the pressure uniformly to the powder. In dry-bag isostatic pressing, rather than immerse the tooling in a fluid, the tooling itself is built with internal channels into which high-pressure fluid is pumped.
Schematic illustration of cold isostatic pressing as applied to formation of a tube. The powder is enclosed in a flexible container around a solid core rod. Pressure is applied isostatically to the assembly inside a high-pressure chamber. 3. Hot-Isostatic Pressing Hot-isostatic pressing (HIP) combines powder compaction and sintering into a single operation. Gas-pressure squeezing the powder in a mould at high temperatures. Heated powders may need to be protected from harmful environments. Products emerge at full density with unifrom, isotropic properties. Near-net shapes are possible. Here the whole isostatic chamber is enclosed in a temperature controlled chamber (480-2000 C). Heating and pressing are carried out simultaneously. Moulds are made up of flexible ceramic because they can withstand high temperature.
4.Powder Rolling
5. Hot pressing It is not possible to cold press very hard powders like dimond satisfactorily. Hot pressing above the re-crystillization temperature of metal is used to eliminate work hardening and produce accurately shaped high density parts. It is important to carry out hot pressing in vacuum or neutral or reducing atmosphere as otherwise the metal maybe oxidized. The selection of die materials to suit the temperature encountered is important. Below 10000 C metallic dies can be used and above 10000 C graphite dies are used. Advantage of graphite is that: 1.It is cheap 2.Easily machinable 3. Resistance to thermal shocks 4. Provides its own reducing atmosphere Powders can be hot formed by rolling forging and extrusion also.
SINTERING
It is a process of heating under controlled atmosphere. The critical parameters are: sintering time, sintering temp, cooling rate, controlling atmosphere and sintering furnace used. The temperature and time should be properly monitored. Cooling should not be of sudden quenching type. Controlling atmosphere should be maintained throughout so as to prevent any contamination or oxidation. Nitrogen is effective for this purpose although methane is also used widely.
Sintering furnace which is an electrical furnace can be of batch type or continuous type. Various structural as well as property changes take place during sintering which enhance the quality of material and make it suitable for the desired use. Three stages of sintering Burn-off (purge)- combusts any air and removes lubricants or binders that would interfere with good bonding High-temperature- desired solid-state diffusion and bonding occurs Cooling period- lowers the temperature of the products in a controlled atmosphere.
CHARACTERISTICS ACCOMPLISED BY SINTERING: 1. Residual stresses within the particles caused by the compacting operation are relaxed allowing the particles to deform plastically. 2. Voids tend to become closed and this causes some shrinkage during sintering. 3. The impurities are forced out of each individual particle into small impurity voids within the sintered compact. 4. Individual crystals may be recrystalized with in the compact hence effect on crystalline structure.
SPARK DISCHARGE SINTERING
This process is variant of HIP in which a high energy electric discharge is produced during the compaction. .Due to high energy of the spark the diffusion bonding is instantaneous .It thus combines compacting and sintering. The metal powders are sintered to a dense part in 12 to 15 seconds. The greatest advantage is its ability to maintain dimensional accuracy of the part.
As shown in the sketch, this is as form of sintering wherein which a no. of sparks are focused on the powder particles. A small gap or clearance is left between the die and the punch. These sparks are at a temperature of over 1000 degree celsius. The sparks are not allowed to convert to an arc by interrupting the power supply. Moreover they are targeted on a very small area which leads to very fast sintering process. The cycle time is very less as the process is completed in a matter of seconds. The initial high cost though, has to be borne before making use of this type of sintering. Hence it is not suited for small scale production.
SECONDARY OPERATION 1. Sizing and coining- these operations increases the density and improves
dimensional tolerance of a part with better surface finish. Sizing operation consists of holding the part in a simple fixture so that an accurate tool can be forced through the hole slot or hallow feature of the part. Coining: coining is a closed dry forging in which there is no flash. It consists of repressing p/m part by use of high pressure in die. Increased density of 95% maybe obtained by mechanically reducing the voids. Densification, surface quality and precision of the part will improve. 2. Infiltration: Molten metal is used to fill the porosity in the sintered compact. It is carried out by immersing the part in molten metal. Capillary action fills the pores and vacuum aids the process. The melting point of liquid metal used should be as low as possible and below that of that of any of the powders in the compact. Silver infiltering tungsten and copper infiltering with iron part are most popular applications. 3. Impregnation: if the pores in a sintered compact are filled with oil, the operation is called as impregnation. The lubricants are added to the porous bearings gears and pump rotors etc. The parts such as bearings, gears are immersed in a tank of heated oil for a period of time to fill the pores. Application of vacuum aids the process. Porous components impregnated with lubricants in this manner do not need external lubrication during operation. Eg self lubricated bearing (porosity ranging from 25% to35%). As part of heat because of friction during operation the contained oil will expand and emerged on the bearing surface providing film of lubricant. Subsequent cooling (after operation) results in reabsorbing of oil into the pores of the part. 4. Heat treatment: Powder Metal parts are heat treated to improve grain structure, strength and hardness. Conventional heat treatment methods such as carburizing, nitriding, carbonitriding, induction hardening and precipitation hardening are used. A controlled atmosphere or vacuum furnace must be used.
APPLICATIONS OF POWDER METALLURGY
• • • • Porous product (any degree of porosity can be provided. Eg filters flow regulators and bearing parts) Babbitt bearings for automobiles eg engine main bearing. Oil pump gears Cemented carbides (for cutting tools, wire drawing dies & deep drawing dies)
• • • • •
Refractory metal composites. (jet engine nose cone crucible etc.) Diamond impregnated tools (eg glass cutter grinding wheel dressers etc.) Electrical contact materials (eg circuit breakers resistance-welding electrodes contact switches carbon bushes etc.) Magnetic materials (eg pole piece of generators motors transformer cores computer memories hard discs etc.) Tungsten filaments used in incandescence bulbs.
ADVANTAGES
1. Elimination or reduction in machining: Time for machining is completely eliminated or considerably reduced. Moreover, labour cost is reduced. 2. Possibility of production of complex shapes: The final product takes the shape of the die which can be easily given the desired shape. 3. High rate of production: The lead time is very less as the processes can be easily automated. In some cases, the sintering time takes a matter of seconds (spark discharge sintering) 4. Possibility of imparting wide range of properties: Various metallic and non-metallic properties can be very easily tailored into the final product by means of impregnation and infiltration. 5. Wide variations in compositions. 6. Scrap is eliminated or reduced.
LIMITATIONS
1. It is suitable for only a certain class of materials. 2. Very large parts cannot be manufactured. 3. Tooling cost is very high in most cases. 4. Inferior strength properties hence structural materials like beams are not compatible with this process.
5. Storage of powders is difficult as they can be very easily contaminated by atmospheric action. 6. Powders impose a safety and health hazard to the persons working on it.
Powder metallurgy is the name given to the process by which fine powdered materials are blended, pressed into a desired shape, and then heated to bond surfaces.
Need of Powder Metallurgy:• Melting is unsuitable in some cases. (Large difference in Melting temperatures of the principle constituents of an alloy so much that one would vaporize before other melts) • Melting and solidification causes poor quality. • Melting causes loss of identity of the constituents. • Some metal do not form liquid solution.
Basic Steps In Powder Metallurgy
• • • • • Powder Production. Blending or Mixing. Powder Consolidation. Sintering. Finishing.
.
Powder Production
1 Mechanical pulverization
(a) roll crushing, • •
(b) ball mill,
(c) hammermilling.
• • •
Mechanical pulverisers: consists of two counter rotating discs of steel one of which is adjustable. The size of the pulverized particles depend upon the distance between the discs. Ball mills: A big cylindrical shell is filled with granulated materials along with hardened steel balls and the whole system is rotated in clockwise and anticlockwise directions for specific periods so that the granulated materials are disintegrated to required powder size. Rapid moving hammers: shell shaped containers are filled with 2/3rd materials. The shell has a inbuilt hammer which is rotated at high speeds to disintegrate the material to required size. Machining: some powders are prepared by means of machining process as this leads to very fine particles. Grinding: grinding removes the material in very small quantities and in very fine form which is ideal to be classified as powders.
2. Atomization
In this process, molten metal is made to pass through a nozzle and it is broken into small droplets after spray. It is then cooled artificially by high pressured water jet as shown in the sketch. This method is suitable for those metals having low melting point such as Lead, Tin and Zinc. 3 Electrolysis
In this process, the anode dissolves in the electrolyte and is intentionally made to deposit on Teflon coated cathode. This is done to enable easy scraping off the deposited anode. Method is inherently costly and very small amount of powder is obtained by this process. 4 Chemical reaction : Ores which are very easily pulverised are done so and are then heated to remove the oxides. This can be very effectively done by heating in an atmosphere of hydrogen or carbon monoxide. Although proper size cannot be monitored here, it is more than compensated by its highly economical nature. 5 Other methods : Some of the other methods incorporated for the preparation of powders are – 1. Granulation – Molten metal is converted into small particles by rapidly stirring the molten metal while it is cooling. 2. Condensation - Molten metal is boiled and then condensing the vapor. 3. Shooting – Molten metal is poured through orifice/ sieves and cooled by dropping in water. It must be noted that all the above methods are suited for low melting point materials such only. (Mg Zn and cadmium). 6. Precipitation (carbonyl decomposition):
In this method metal are made to combine with CO to form volatile carbonyl at suitable pressure and temperature carbonyl decomposes to get pure metals. (Fe Ni Mo W etc)
CHARACTERISTICS OF METAL POWDER:Flow rate:-a measure of the ease by which powder can be fed and distributed into the die, - significantly influenced by the cycle time and production rate. This determines the fineness of the particles. Nozzle is used to determine the flow rate. Density:-apparent density –ability to fill available space without the application of external pressure. It dependence on powder shape size and size distribution. True density means density after compactation. Density distribution should be uniform. Compressibility:-Effectiveness of applied pressure in raising density of the powder i.e. green strength. This again depends on the shape, size and size distribution of the powder particles. Particles should have the ability to be easily compressed.
Shape, size and size distribution :- It determines the amount of pressure required and the strength to be possessed by the final product. Spherical particles can be used when high permeability is desired. If high strength is required, we can select coarse particles. Shape depends on method of powder manufactured .-spherical, dendritic angular nodular irregular lamellar etc. Spherical powder have excellent sintering properties. Irregular particles are superior for practical moulding and achieving good green strength because they will interlock on compactation. Size distribution means quantity of each standard particle size in mixture. The particle shape, size and structure play an important role in the quality of the final product. They are chosen on the basis of various applications involved. Various particle sizes and shapes are shown below
Purity: - High purity is required for a better product. For maintaining this, the particles must be isolated from the atmospheric oxidation or any contamination.
MIXING AND BLENDING:If high purity particles are not being used, mixing and blending is essentially required. Powder particles are first graded with respect to their shape, size and purity. They are then weighed and mixed in proper proportions. Binders are added for the compactation of hard particles. Binders improve adhesiveness and increases strength. Lubricants like organic solvents and
wax may be added to increase the flowability and enhance the permeability of the particles. Sintering agents are added to accelerate the heating process. After this, the particles are manually mixed together taking care of the fact that the powder particles are uniformly distributed throughout the mass. Mixing may be accomplished in ball mill. It may be done either dry or wet. For wet water or solvent is added to the dry powder to reduce the possible hazard of dust and explotion. Mix to obtain uniformity Mix to obtain desired physical and mechanical properties Mix lubricants to improve flow characteristics Blend in air, inert gas (to avoid oxidation) or in liquids.
BRIQUETTING/COMPACTATION
Loose powder is compacted and densified into a shape, known as green compact. Most compacting is done with mechanical presses and rigid tools. Hydraulic and pneumatic presses are also used. Care should be taken at this stage so as to apply only the optimum pressure. Less pressure leads to poor quality products and undesired extra pressure leads to damage of the die without any significant enhancement in the property of the powder particles.
Friction problem in cold compaction:The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction. The pressure tapers off rapidly and density diminishes away from the punch. When the pressure is applied by only one punch, the maximum density occurs right below the punch surface and decreases away from the punch. For complex shapes, multiple punches should be used. The compacted mass should be able to be withdrawn from the die cavity without damage. If this is achieved, it is called as “green compact.
Compaction with a single moving punch, showing the resultant nonuniform density (shaded), highest where particle movement is the greatest
Density distribution obtained with a double-acting press and two moving punches. Note the increased uniformity compared to previous Figure. Thicker parts can be effectively compacted.
Changes noticed after compaction
The green strength of the compact is achieved due to: 1. Adhesion or even cold welding of some of the powders 2. Mechanical interlocking of particles 3. By addition of bonding agents which forms strong bonds with the powders. During the compaction voids between the particles are reduced. Surface irregularities of the particles are flattened out and density also increases. In this process the pressure applied should be even and applied simultaneously. Rate of pressure application is also very important as too rapid rate may result in entrapping of some air and it gives rise to sudden defects.
Other compaction methods
Although conventional process is most widely used, there are other processes of compaction which have their own benefits. 1. Pressure less compaction: - It is used for the manufacture of porous filters where relatively large gap is required for quick filtration. For this, the particles are shaken well in a die cavity without the application of pressure to get the required compactation, and sintered thereafter. 2. Cold Isostatic Pressing:- Cold isostatic pressing applies pressure from multiple directions for achieving greater uniformity of compaction (high-quality parts) and increased shape capability, compared to uniaxial pressing. The powder may be encapsulated in a flexible mold, which is then immersed in a pressurized gas or liquid. There are two methods of carrying out isostatic pressing. In wet-bag isostatic pressing, powder is encased in a rubber sheat that is immersed in a liquid which transmits the pressure uniformly to the powder. In dry-bag isostatic pressing, rather than immerse the tooling in a fluid, the tooling itself is built with internal channels into which high-pressure fluid is pumped.
Schematic illustration of cold isostatic pressing as applied to formation of a tube. The powder is enclosed in a flexible container around a solid core rod. Pressure is applied isostatically to the assembly inside a high-pressure chamber. 3. Hot-Isostatic Pressing Hot-isostatic pressing (HIP) combines powder compaction and sintering into a single operation. Gas-pressure squeezing the powder in a mould at high temperatures. Heated powders may need to be protected from harmful environments. Products emerge at full density with unifrom, isotropic properties. Near-net shapes are possible. Here the whole isostatic chamber is enclosed in a temperature controlled chamber (480-2000 C). Heating and pressing are carried out simultaneously. Moulds are made up of flexible ceramic because they can withstand high temperature.
4.Powder Rolling
5. Hot pressing It is not possible to cold press very hard powders like dimond satisfactorily. Hot pressing above the re-crystillization temperature of metal is used to eliminate work hardening and produce accurately shaped high density parts. It is important to carry out hot pressing in vacuum or neutral or reducing atmosphere as otherwise the metal maybe oxidized. The selection of die materials to suit the temperature encountered is important. Below 10000 C metallic dies can be used and above 10000 C graphite dies are used. Advantage of graphite is that: 1.It is cheap 2.Easily machinable 3. Resistance to thermal shocks 4. Provides its own reducing atmosphere Powders can be hot formed by rolling forging and extrusion also.
SINTERING
It is a process of heating under controlled atmosphere. The critical parameters are: sintering time, sintering temp, cooling rate, controlling atmosphere and sintering furnace used. The temperature and time should be properly monitored. Cooling should not be of sudden quenching type. Controlling atmosphere should be maintained throughout so as to prevent any contamination or oxidation. Nitrogen is effective for this purpose although methane is also used widely.
Sintering furnace which is an electrical furnace can be of batch type or continuous type. Various structural as well as property changes take place during sintering which enhance the quality of material and make it suitable for the desired use. Three stages of sintering Burn-off (purge)- combusts any air and removes lubricants or binders that would interfere with good bonding High-temperature- desired solid-state diffusion and bonding occurs Cooling period- lowers the temperature of the products in a controlled atmosphere.
CHARACTERISTICS ACCOMPLISED BY SINTERING: 1. Residual stresses within the particles caused by the compacting operation are relaxed allowing the particles to deform plastically. 2. Voids tend to become closed and this causes some shrinkage during sintering. 3. The impurities are forced out of each individual particle into small impurity voids within the sintered compact. 4. Individual crystals may be recrystalized with in the compact hence effect on crystalline structure.
SPARK DISCHARGE SINTERING
This process is variant of HIP in which a high energy electric discharge is produced during the compaction. .Due to high energy of the spark the diffusion bonding is instantaneous .It thus combines compacting and sintering. The metal powders are sintered to a dense part in 12 to 15 seconds. The greatest advantage is its ability to maintain dimensional accuracy of the part.
As shown in the sketch, this is as form of sintering wherein which a no. of sparks are focused on the powder particles. A small gap or clearance is left between the die and the punch. These sparks are at a temperature of over 1000 degree celsius. The sparks are not allowed to convert to an arc by interrupting the power supply. Moreover they are targeted on a very small area which leads to very fast sintering process. The cycle time is very less as the process is completed in a matter of seconds. The initial high cost though, has to be borne before making use of this type of sintering. Hence it is not suited for small scale production.
SECONDARY OPERATION 1. Sizing and coining- these operations increases the density and improves
dimensional tolerance of a part with better surface finish. Sizing operation consists of holding the part in a simple fixture so that an accurate tool can be forced through the hole slot or hallow feature of the part. Coining: coining is a closed dry forging in which there is no flash. It consists of repressing p/m part by use of high pressure in die. Increased density of 95% maybe obtained by mechanically reducing the voids. Densification, surface quality and precision of the part will improve. 2. Infiltration: Molten metal is used to fill the porosity in the sintered compact. It is carried out by immersing the part in molten metal. Capillary action fills the pores and vacuum aids the process. The melting point of liquid metal used should be as low as possible and below that of that of any of the powders in the compact. Silver infiltering tungsten and copper infiltering with iron part are most popular applications. 3. Impregnation: if the pores in a sintered compact are filled with oil, the operation is called as impregnation. The lubricants are added to the porous bearings gears and pump rotors etc. The parts such as bearings, gears are immersed in a tank of heated oil for a period of time to fill the pores. Application of vacuum aids the process. Porous components impregnated with lubricants in this manner do not need external lubrication during operation. Eg self lubricated bearing (porosity ranging from 25% to35%). As part of heat because of friction during operation the contained oil will expand and emerged on the bearing surface providing film of lubricant. Subsequent cooling (after operation) results in reabsorbing of oil into the pores of the part. 4. Heat treatment: Powder Metal parts are heat treated to improve grain structure, strength and hardness. Conventional heat treatment methods such as carburizing, nitriding, carbonitriding, induction hardening and precipitation hardening are used. A controlled atmosphere or vacuum furnace must be used.
APPLICATIONS OF POWDER METALLURGY
• • • • Porous product (any degree of porosity can be provided. Eg filters flow regulators and bearing parts) Babbitt bearings for automobiles eg engine main bearing. Oil pump gears Cemented carbides (for cutting tools, wire drawing dies & deep drawing dies)
• • • • •
Refractory metal composites. (jet engine nose cone crucible etc.) Diamond impregnated tools (eg glass cutter grinding wheel dressers etc.) Electrical contact materials (eg circuit breakers resistance-welding electrodes contact switches carbon bushes etc.) Magnetic materials (eg pole piece of generators motors transformer cores computer memories hard discs etc.) Tungsten filaments used in incandescence bulbs.
ADVANTAGES
1. Elimination or reduction in machining: Time for machining is completely eliminated or considerably reduced. Moreover, labour cost is reduced. 2. Possibility of production of complex shapes: The final product takes the shape of the die which can be easily given the desired shape. 3. High rate of production: The lead time is very less as the processes can be easily automated. In some cases, the sintering time takes a matter of seconds (spark discharge sintering) 4. Possibility of imparting wide range of properties: Various metallic and non-metallic properties can be very easily tailored into the final product by means of impregnation and infiltration. 5. Wide variations in compositions. 6. Scrap is eliminated or reduced.
LIMITATIONS
1. It is suitable for only a certain class of materials. 2. Very large parts cannot be manufactured. 3. Tooling cost is very high in most cases. 4. Inferior strength properties hence structural materials like beams are not compatible with this process.
5. Storage of powders is difficult as they can be very easily contaminated by atmospheric action. 6. Powders impose a safety and health hazard to the persons working on it.
