powder metallurgy and micromachining notes

Module III

POWDER METALLURGY

Powder metallurgy (PM) is a metal working process for forming precision metal components from metal powders. The metal powder is first pressed into product shape at room temperature. This is followed by heating (sintering) that causes the powder particles to fuse together without melting. Strength and other properties are imparted to the components by sintering operations.The most commonly used metals in P/M are iron, copper, aluminum, tin, nickel, titanium and refractory metals. The parts produced by PM have adequate physical and mechanical properties while completely meeting the functional performance characteristics. The cost of producing a component of given shape and the required dimensional tolerances by PM is generally lower than the cost of casting or making it as a wrought product, because of extremely low scrap and the fewer processing steps. The cost advantage is the main reason for selecting PM as a process of production for high – volume component which needs to be produced exactly to, or close to, final dimensions. The rate of production of parts is quite high, a few hundreds to several thousands per hour. Parts can be produced which are impregnated with oil or plastic, or infiltrated with lower melting point metal. They can be electroplated, heat treated, and machined if necessary. Industrial applications of PM parts are several. These include self – lubricating bearings, porous metal filters and a wide range of engineered shapes, such as gears, cams, brackets, sprockets, etc.Basic steps of the Powder Metallurgy ProcessThe manufacturing of parts by powder metallurgy process involves the following steps:(a) Manufacturing of metal powders(b) Blending and mixing of powders(c) Compacting(d) Sintering(e) Secondary and Finishing operations

Department of Mechanical Department SSET 2014

Preparation of metal powdersPowders of almost all metals can be produced. Powder production processes are constantly being improved to meet the quality, cost and performance requirements of all types of applications. Which powder production process is used depends on the required production rate, the desired powder properties and the properties desired in the final part. There are various methods available for the production of powders, depending upon the type and nature of metal. Some of the important processes are:

Atomization Reduction methods (Chemical ) Electrolytic Deposition Carbonyls (Thermal decomposition) Crushing and Milling ( also called comminution) Shotting

Chemical and electrolytic methods are used to produce high purity powders. Mechanical milling is widely used for the production of hard metals and oxides.

Atomization In this process molten metal is broken up into small droplets and rapidly frozen before the drops come into contact with each other or with a solid surface. The principal method is to disintegrate a thin stream of molten metal by subjecting it to the impact of high energy jets of gas or liquid (shown in figure). Air, nitrogen and argon are commonly used gases, and water is the liquid most widely used.It is the dominant method for producing metal powders from aluminium, brass, iron, alloy steel, super-alloy, titanium alloy and other alloys etc.

Methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) vacuum atomization(d) centrifugal atomization (spinning disk or cup, rotating electrode methods)


Water atomization: High pressure water jets are used to bring about the disintegration of molten metal stream. Water jets are used mainly because of their higher viscosity and quenching ability. This is an inexpensive process and can be used for small or large scale production. But water should not chemically react with metals or alloys used.

Gas atomization: Here instead of water, high velocity argon, nitrogen and helium gas jets are used. The molten metal is disintegrated and collected as atomized powder in a water bath. Fluidized bed cooling is used when certain powder characteristics are required.

Vacuum atomization: In this method, when a molten metal supersaturated with a gas under pressure is suddenly exposed into vacuum, the gas coming from metal solution expands, causing atomization of the metal stream. This process gives very high purity powder. Usually hydrogen is used as gas.

Centrifugal atomization (disk or cup)Centrifugal force can be used to break up the liquid as it is removed from the periphery of spinning disk/cup. Rotating consumable electrode methodDue to the corrosion action on the orifice or nozzle at high temperature, another method is that an electric arc is struck between non-rotating, non-consumable

tungsten electrode and rotating consumable electrode(metal from which power is to be produced). The metal droplets from the rotating consumable electrode are thrown off, are collected and are finally crushed to the required powder size. The process is carried out in a chamber filled with inert gas (argon gas).


As the metal stream exits through the nozzle, it is struck by a high velocity stream of the atomizing medium (water, air, or an inert gas). The molten metal stream is disintegrated into fine droplets which solidify during their fall through the atomizing tank. Particles are collected at the bottom of the tank. Process controlling parameters determining size and shape of particles

1. Metal flow rate2. Pressure of the stream of air or gas.3. Temperature of the stream.

Atomization process steps The molten alloy is prepared in a furnace and then it is transferred to the tundish. The melt is poured from the tundish through the nozzle into the chamber. The water (air, gas) jets break the melt stream into fine droplets. The droplets solidify when they fall in the chamber. The powder is collected at the bottom of the chamber, removed and dried

Reduction method (Chemical methods)Pure metal is obtained by reducing its oxide with a suitable reducing gas at an elevated temperature below the melting point. Selected ore is crushed, mixed with reducing gas or solid (carbon monoxide, hydrogen etc) and passed through a continuous furnace where reaction takes place leaving a cake of sponge iron which is then further treated by crushing, separation of non-metallic material, and sieving to produce powder. Since no refining operation is involved, the purity of the powder is dependent on that of the raw materials. Fe3O4 + 4CO + (heat) → 3Fe + 4CO2

2CuO2 + 4H2 (heat) → 2Cu + 4H2OThis process is cheap and a large amount of powder is made by this method. This is a convenient and extremely flexible method for controlling the properties of size, shape and porosity. It is used in the manufacture of Fe, Cu, Ni, Mo and Co.

The resulting particles are of irregular shape and are quite porous and spongy. Readily compressible and have good green strength. Furnace temperature, amount of gas and its purity are the controlling factors.

Electrolysis method (Electrolytic Deposition)This is the reverse of electroplating. To produce iron, impure steel acts as anodes in tanks containing electrolyte. Sheets of stainless steel are placed in the tank acted as cathode. When DC current is passed through an electrolyte, pure iron gets deposited on cathode. The cathode plates are then removed and the electrolytic iron is stripped from them. Additional crushing

and milling is necessary. Used for making copper, iron, silver and tantalum powders.

Electrolytic powders are of high purity, soft spongy dendrite structure.


Processes parameters of electrolyte method of powder preparation (a) Electrolyte composition and strength (b) Current density(c) Temperature of electrolyte

Advantages of electrolysis(a) High degree of purity(b) Uniformity in characteristics(c) Excellent compacting and sintering property (high quality product)(d) Economical

Disadvantages (a) Time consuming(b) Unsuitable for alloy powder(c) Low production rate

Carbonyls method (decomposition method)The metal carbonyl process is used as a way of refining or making pure metal from ores. Metal reacts with carbon monoxide to form metal carbonyl gas, which can be decomposed back to pure metal at moderate temperatures with the recovery of carbon monoxide.Carbonyls can be obtained by passing carbon monoxide over spongy metal (iron or nickel) at specific temperature and pressure. Then decompose the metal compound by raising the temperature and lowering the pressure gives the purest metal.

M e (metal ) +n (CO ) → M e⋮(CO )n↓ (Decomposition)

M e( pure metal powder )

Carbonyls process1. Metal carbonyls are formed by letting impure or ore of iron or nikel react with

carbon monoxide2. Reaction products is decomposed to iron and nickel (pure powder form)

Fe+5 CO→Fe(CO )5↑(gas)

Fe(CO )5→Fe+5 CO (fine iron powder + carbon moxide)Carbonyls powder is spherical, fine and porous with an onion skin structure. Carbonyl powder has high purity (99.5%) and excellent sintering properties and flowability. Iron and nickel are produced in large quantities by the decomposition of the metal carbonylThis reaction can be controlled by changing temperature and pressure.Examples of carbonyls

Fe (CO)5 Ni (CO)4) W(CO)6


Comminution method (mechanical pulverization by crushing and milling)It is mechanical method of powder preparation involving breaking solid particles in pulverizing mills (ball, vibratory, hummer). This method is generally applied for the preparation of powders of brittle materials. Metal particles is mixed with ball mills and rotated or send through the rolling mill to pulverize the metal to form powder.

ShottingIn this method a fine stream of molten metal is passed through a vibrating screen into air or neutral atmosphere. Likewise the molten metal is disintegrated into a large number of droplets which solidify as spherical particles during its free fall. All metals can be shotted. The type size and properties of the resultant shot depends on:

a. Temperature of the molten metal and gas.b. Diameter of the holes.c. Frequency of vibration of the vibrating screen.

Mixing and BlendingA single powder may not fulfil all the requisite properties and hence, powders of different materials with wide range of mechanical properties are blended to form a final part.In this step more than one powder is mixed thoroughly with lubricants, adhesives and binders and blended to ensure their even distribution.1. Blending imparts uniformity in the shapes of the powder particles, 2. Mixing facilitates mixing of different powder particles to impart wide ranging physical

and mechanical properties,3. Lubricants can be added during the blending process to improve the flow characteristics

of the powder particles reducing friction between particles and dies,4. Binders can be added to the mixture of the powder particles to enhance the green

strength during the powder compaction processBlending: It is the process of mixing powder of the same chemical composition but different sizes. Different particle sizes are often blended to reduce porosity. Mixing: Process of combining powders of different chemistries (nickel and iron, zirconium alumina, wax, tungsten carbide) to improve the properties is called mixing. Mixing depends on the powder material, particle size, particle shape, surface conditions and environment conditions such as temperature and pressure.


Metal powder characteristics or Properties of fine powderCharacteristics/properties of metal powder

1. Size and shape of powder2. Powder distribution (homogenious)3. Purity of powder4. Composition of powder5. Porous nature of powder6. Surface area or aspect ratio7. Flow rate8. Density

All these properties of powder influences the following1. Green strength2. Compressibility 3. Mechanical property etc

Flow rate: It is the ability of powder to flow readily to fill the mould cavity. It is a very important property, since the minimum time of filling improves the production rates and economy. Very fine particles will flow just like a liquid. When such powder is pressed in a die, it will flow into complex die cavities. Flow rate or flow ability depends on the:

Shape of the powder particle Size of the powder particle Size distribution of the powder particle

Green strength: Green strength is used to describe the strength of the pressed powder after compacting, but before sintering. The green strength increases with the increase of compaction pressure and apparent density.

1. It helps to retain the sharp edges from damage during ejection and handling time2. To handle the part for quality measurements,3. To handle for sintering operations.

Apparent Density: Density of loose powder after filling the volume. It depends upon the particle shape, size and size distribution. The apparent density of irregularly shaped particles will be lower than that of spherical particles and fine particles. And Green density is the density of powder after the compacting process.

Compressibility and compression ratio: It is the measure of the powder’s ability to deform under applied pressure. It is also defined as the ratio of the volume of the powder poured into the die to the volume of the pressed compact. The compression ratio can be varied from 2 to 8, and the normally adopted value is 3.

compressibility=density after sin tering processdensity before compacting

Compressibility depending factors1. Size and shape of particles2. Porosity3. Lubricant4. Mechanical properties of metal powder


Particle shape: The particle shape depends on the method of powder manufacture. The various shapes are spherical, rounded, angular, dendrite, porous and irregular etc. The particle shape influences the flow characteristics of powders.The particle shape has an effect on packing of a powder and has an influence on its compacting and sintering properties and the mechanical strength of the sintered product. The irregularly shaped particles have reduced apparent density and flow rate, but good pressing and sintering properties. Whereas the spherical particles have maximum apparent density and flow rate, but reduced pressing properties and good sintering properties. Dendrite powders too have reduced apparent density and poor flow rates.

Particle size: It is expressed by the diameter for spherical shaped particles and by the average diameter for non-spherical particles. The particle size influences the control of porosity, compressibility and amount of shrinkage. It is determined by passing the powder through standard sieves or by microscopic measurement.

Particle size distribution: It is specified as the amount of powder passing through 100, 200 etc., mess sieves. It influences apparent density, compressibility, flow ability, final porosity and the strength of the part. Theoretically, powders containing variable particle size will result in greater density as a result of finer particles filling up the voids between large particles. But normally during mixing, the finer particles have the tendency to separate and segregate. Thus it is efficient to use uniform size particles and rely on the compacting pressure to get the required final density. The particle size distribution is important to the end user in several ways

Direct impact on the quality of finished product. Simple and easy filling a die. Distributions permit voids between larger particles to be filled with smaller particles. An surplus of fines has negative effects on flow characteristics

Surface Area/ aspect ratio: The specific surface of a powder is defined as the total surface area per unit weight. It indicates the area available for bonding. It depends on size, shape, density and surface conditions of the particle. High specific surface results in high sintering rate.

Purity: Metal powders should be free from impurities as the impurities reduce the life of dies and effect sintering process. The oxides and the gaseous impurities can be removed from the part during sintering by use of reducing atmosphere.


Compacting Blended powders are pressed in dies under high pressure to form them into the required shape. The work part after compaction is called a green compact or simply a green, the word green meaning not yet fully processed. The compaction is done to bring the finely divided particles of powder into close proximity while imparting the desired part configuration. The following methods are adopted for compacting:

1. Die Pressing2. Centrifugal compacting3. Injection moulding4. Extrusion5. Rolling6. Gravity sintering7. Slip casting8. Isostatic moulding9. Explosive moulding

Die Pressing: The metal powders are placed in a die cavity and compressed to form a component shaped to the contour of the die. The pressure used for producing green compact of the component vary from 80 Mpa to 1400 Mpa, depending upon the material and the characteristics of the powder used. Mechanical presses are used for compacting objects at low pressure. Hydraulic presses are for compacting objects at high pressure.

The basic components are: Hydraulic mechanism to apply

pressure

A die of adequate strength having a cavity of the desired shape and dimension.

Feeding devices for fill the die cavity.

Upper and lower punches to apply pressure, and to assist in the ejection of the green compact.

Control system to maintain the magnitude of pressure and rate of pressure application, speed of punches etc.

Single Action Die Compaction: Used to manufacture flat, thin parts such as washers, discs, thin rings etc. The lower punch is stationery during the application of pressure by the motion of the upper punch acting from the direction only on the powder placed in the cavity. After compression the punch is raised in order to eject the part from the die cavity. Advantages are:

Tools used are very simple Mechanical or hydraulic press may be employed.

Disadvantage: it is not suitable for manufacture of long parts because of non-uniform density distribution.Double Action Die Compaction: The powder is compacted simultaneously from opposite directions by both the top and bottom punches. Equal or different amounts of pressure may be


employed from each direction. The die table remains stationery and the upward movement of the lower punch, ejects the part out.This technique can be used for the manufacture of thin walled bushing and cylindrical bearings.

Centrifugal Compacting: The powder is swirled in a mould and packed uniformly with pressures up to 3 MPa. The uniform density is obtained as a result of centrifugal force. , acting on each particle of powder. This method is employed for heavy metals such as tungsten carbide and for materials that are relatively incompressible by conventional die compaction. The main drawback of this process is relatively slower process because it takes larger time for the fluid to be absorbed by the method.

Extrusion: This method is employed to produce the components with high density. Both cold and hot extrusion processes are for compacting specific materials. In cold extrusion, the metal powder is mixed with binder and this mixture is compressed into billet. The billet is charged


into a container and then forced through the die by means of ram. The cross-section of product depends on the opening of the die. The cross-section of products depends on the opening of the die. Cold extrusion process is used for cemented carbide drills and cutters.

In the hot extrusion, the powder is compacted into billet and is heated to extruding temperature in non-oxidising atmosphere. Extrusion is used for manufacturing furnace tubes, thermocouple components and heat exchanger tubes.

Injection molding is the method of compaction of ceramic powder fed and injected into a mold cavity by means of a screw rotating in cylinder. The method is similar to the plastic injection molding.

Rolling: This method is used for making continuous strips and rods having controlled porosity with uniform mechanical properties. In this method, the metal powder is fed between two rolls which compress and interlock the powder particles to form a sheet of sufficient strength as shown. It is then rerolled and heat treated if necessary. The metals that can be rolled are Cu, Brass, Bronze, Ni and Stainless steel.

Slip casting technique In this method, the powder is converted into slurry with water and poured into the mould made of plaster of paris. The liquid in the slurry is gradually absorbed by the mould leaving the solid compact within the mould. The mould may be vibrated to increase the density of the compact.

Steps in slip casting: Preparing assembled plaster mould, filling the mould,


absorption of water from the slip into the porous mould, removal of part from the mould, trimming of finished parts from the mould

Advantages of slip casting: Products that can not be produced by pressing operation can be made, no expensive equipment is required, works best with finest powder particlesDisadvantage: slow process, limited commercial applications Applications: tubes, boats, cones, turbine blades, rocket guidance fins; Hollow and multiple parts can be produced

Gravity Sintering: Gravity or “loose powder” sintering is used to make porous metal parts from powders that diffusion-bond easily (most production parts are made from bronze). In this process, no outside pressure is applied to shape the part. The appropriate material, graded for size, is poured into a mold cavity, which is a void in the shape of the finished part. These metal particles are then heated to their sintering temperature at which point a metallurgical bonding

takes place, and joining “necks” are formed at contact points.

Explosive Compacting: In this method, the pressure generated by an explosive is used to compact the metal powder. Metal powder is placed in water proof bags which are immersed in water container cylinder of high wall thickness. Due to sudden deterioration of the charge at the end of the cylinder, the pressure of the cylinder increase. This pressure is used to press the metal powder to form green compact.

Cold Isostatic pressing: (CIP) In this method the powdered material is contained in a tightly sealed flexible mould subjected to uniform pressure (65-650 Mpa) is applied simultaneously from all sides thereby achieving


uniform density and strength. After pressing, the compact is removed from chamber. The pressure-transmitting medium used is liquid such as water, oil mixture.

The powder is loaded in a shaped flexible envelope for the production of desired shape of the pressed part and tightly sealed against leakage. The flexible envelope is usually made from natural rubber, synthetic rubber, plastics, thin metal foils. Isostatic pressing is generally used to produce large PM parts to near-net shapes.

The flexible envelope should possess the following characteristics:1. Flexible mould2. It must be completely impervious to the pressurizing fluid.3. It must be easily sealed.4. It must be rigid enough to withstand the internal pressure

Advantages of cold iso-static pressing (comparing die compacting)1. Uniform and high density compact2. Higher dimensional accuracy- near net shaped product3. Better mechanical properties like ductility, strength, hardness etc4. More complex geometrical shapes can be made5. Higher green strength 6. Absence of lubricant7. Reduced friction

Disadvantages 1. Higher equipment cost2. Low productivity3. Dimensional control is less4. Flexible mould life is less

Application of Isostatic Pressing: Wide use in aviation defence, medical equipment to produce cutting tools, automotive cylinder liners, corrosion resistant components etc.

Hot isostatic pressing (HIP)Hot iso-static pressing is a compacting process where high temperature and pressure is applied simultaneously (3-dimensions) to produce a dense component. The pressure is


uniform in all directions (isostatic). At high temperatures, the hermetic container deforms plastically and the powder is compacted within it under pressure. No further sintering process is needed here as the combination of heat and pressure during the process is done. Metal or glass is used for making the hermetic container. The pressurizing medium is a gas (inert argon/helium) with a pressure 100 to 200 MPa and temperatures to 2200°C.

Hot isostatic pressing (CIP) is combining the compaction and sintering processes in PM production process. So it eliminates separate sintering.

Advantages 1. Little or no porosity2. Better surface finish3. Neat net shape product4. Improvements in mechanical and physical properties, fatigue, surface finish,

reliability 5. Fast delivery6. More uniform strength 7. Less pressure requirement

Disadvantages 1. Very expensive2. Protective environment is needed

SinteringSintering is a heat treatment process applied to a green compact (product after the compacting) in order to impart strength and integrity in a controlled atmosphere (reducing


atmosphere which protects oxidation of metal powders). Sintering increases the bond between the particles and therefore strengthens the powder metal compact. The temperature used for sintering is below the (0.6 to 0.8 times) melting point of powder material. The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece.Diffusion is due to various mass-transport mechanisms. These can be divided into surface transport and bulk transport mechanisms. In surface transport mechanisms, atoms move from the surface of one particle to the surface of another particle. In bulk transport mechanisms, atoms move from the particle interior to the surface.Sintering reduces the porosity and enhances properties such as strength, electrical conductivity, translucency and thermal conductivityAn example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice. Examples of pressure driven sintering are the pressing of loose snow together to a hard snowball by pressing.

The main driving force during the sintering process is the reduction of energy due to the reduced surface area. Powders with a greater surface area will have a higher driving force towards bonding and to reduce this potential energy.Stages of SinteringThis process is carried out a constant temperature and time is varied to obtain the desirable results. The four phases of sintering are:

1. Local bonding: Particles stick together and neck formation2. Initial stage: Neck growth3. Final stage: Pores are round up then finally closed

The time, temperature and the furnace atmosphere are the three critical factors that control the sintering process. Sintering process enhances the density of the final part by filling up the incipient holes and increasing the area of contact among the powder particles in the compact perform

. Property change during sintering


Microscopic scale the changes that occur during sintering of metallic powders. Types of sinteringBasically, sintering processes can be divided into two types: solid state sintering and liquid phase sintering. Solid state sintering occurs when the powder compact is densified wholly in a solid state at the sintering temperature. No liquid is present and atomic diffusion in the solid state produces joining of the particles and reduction of porosity. All densification is achieved through changes in particle shape, without particle rearrangement.Liquid phase sintering Liquid phase sintering is the process of adding an additive to the powder which will melt before the matrix phase. This also occurs when the powder contains a component, having the melting point lower, than the melting point of the base metal. For materials which are hard to sinter, liquid phase sintering is commonly used. Materials for which liquid phase sintering is common are Si3N4, WC, SiC, and more. Some liquid phase present in the powder compact will enhance sintering process.

The process of liquid phase sintering has three stages: Rearrangement – As the liquid melts capillary action will pull the liquid into pores

and also cause grains to rearrange into a more favorable packing arrangement. Solution-Precipitation –atoms will preferentially go into solution and then precipitate

in areas of lower chemical potential where particles are non close or in contact. Final Densification – liquid movement from efficiently packed regions into pores.

The changes occur in sintering1. Strength, hardness and fracture toughness2. Electrical and thermal conductivity3. Permeability to gases and liquids4. Average grain number, size, shape and distribution5. Average pore size and shape 6. Distribution of pore size and shape 7. Chemical composition and crystal structure


http://en.wikipedia.org/wiki/Silicon_carbide

Continuous sintering furnace

Pre-Sintering Sometimes product after the sintering process is rather difficult for secondary operation as product is hard and strong. And cost of operation also will be high as tool life is less.So the green compact is heated to a temperature well below the final sintering temperature and it will gain enough strength to be handled and machined without any difficulty. This process is necessary when holes are to be drilled in the end product. Pre-sintering in addition removes lubricants and binders added to the powder during blending operation. Pre-sintering can be avoided if no machining of the final product is desired.Sintering AtmosphereThe choice of furnace temperature depends on the characteristics of the material and the properties desired from the sintered product. Functions of the sintering atmosphere

1. It must prevent oxidation on the metal surface at the sintering temperature 2. It must avoid carburizing, decarburizing or nitriding conditions in certain metals.3. It must not contaminate the metal powder compact at the sintering temperature.

The atmosphere prevailing in various types of sintering furnaces are considered to be:1. Reducing atmosphere like dry H2 and CO 2. Neutral atmosphere3. Oxidizing atmosphere like O2 and air

Vacuum sintering is costly and therefore employed on a small scale in very special cases like research work.


Secondary and finishing operations Sometimes additional operations are carried out on sintered PM parts in order to further improve their properties or to impart special characteristics.1. Coining and sizing. These are high pressure compacting operations. Their main function

is to impart (a) greater dimensional accuracy to the sintered part, and (b) greater strength and better surface finish by further densification.

2. Forging. The sintered PM parts may be hot or cold forged to obtain exact shape, good surface finish, good dimensional tolerances, and a uniform and fine grain size. Forged PM parts are being increasingly used for such applications as highly stressed automotive, jet – engine and turbine components.

3. Impregnation. The inherent porosity of PM parts is utilized by impregnating them with a fluid like oil or grease. A typical application of this operation is for sintered bearings and bushings that are internally lubricated with upto 30% oil by volume by simply immersing them in heated oil. Such components have a continuous supply of lubricant by capillary action, during their use. Universal joint is typical grease – impregnated PM part.

4. Infiltration. The pores of sintered part are filled with some low melting point metal with the result that part's hardness and tensile strength are improved. A slug of metal to be impregnated is kept in close contact with the sintered component and together they are heated to the melting point of the slug. The molten metal infiltrates the pores by capillary action. When the process is complete, the component has greater density, hardness, and strength. Copper is often used for the infiltration of iron – base PM components. Lead has also been used for infiltration of components like bushes for which lower frictional characteristics are needed.

5. Heat Treatment. Sintered PM components may be heat treated for obtaining greater hardness or strength in them.

6. Machining. The sintered component may be machined by turning, milling, drilling, threading, grinding, etc. to obtain various geometric features.

7. Joining. PM parts can be welded by several conventional methods. Electric resistance welding is better suited than oxy- acetylene welding and arc welding because of oxidation of the interior porosity. Argon arc welding is suitable for stainless steel PM parts

Finishing process: Almost all the commonly used finishing method is applicable to PM parts. Some of such methods are plating, burnishing, coating, and colouring.

1. Plating. For improved appearance and resistance to wear and corrosion, the sintered compacts may be plated by electroplating or other plating processes. To avoid penetration and entrapment of plating solution in the pores of the part, an impregnation or infiltration treatment is often necessary before plating. Copper, zinc, nickel, chromium, and cadmium plating can be applied.

2. Burnishing. To work harden the surface or to improve the surface finish and dimensional accuracy, burnishing may be done on PM parts.

3. Coating. PM sintered parts are more susceptible to environmental degradation than cast and machined parts. This is because of inter – connected porosity in PM parts. Coatings fill in the pores and seal the entire reactive surface.

4. Colouring. Ferrous PM parts can be applied colour for protection against corrosion.


Advantages of powder metallurgyThe main advantages of P/M process over the conventional material processing methods such as casting, forging and rolling are the following

1. PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining which saves time and cost. It also reduces wastes by material about 97% of the starting powders are converted to product

2. Special alloys can be synthesized. Wide variety of materials(metals and non metals) and composition can be made possible to impart required properties like magnetic, and mechanical properties

3. PM parts can be made with a specified level of porosity, to produce porous metal parts. Examples: filters, oil impregnated bearings and gears

4. Parts can be produced from high melting point refractory metals with respectively less difficulty and at less cost.

5. Difficult to machine materials like carbide and tungsten can be made by this method 6. Wide property control is possible with the product with variation in composition and

further heat treatment7. Certain metals that are difficult to fabricate by other methods can be shaped by

powder metallurgy. Example: Tungsten filaments for incandescent lamp bulbs are made by PM

8. Certain alloy combinations made by PM cannot be produced in other ways9. Close dimensional control is possible when comparing with other process10. PM production methods can be automated for economical production for large

production volume.11. More eco-friendly process

Limitations and Disadvantages with PM Processing1. High tooling and equipment costs2. Metallic powders are expensive3. Problems in storing and handling metal powders

a. Examples: degradation over time, fire hazards with certain metals4. Limitations on part geometry because metal powders do not readily flow laterally in

the die during pressing5. Variations in density throughout part may be a problem, especially for complex

geometries6. Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally

divided state present fire hazard and risk of explosion. 7. Low melting point metal powders (such as of zinc, tin, cadmium) give thermal

difficulties during sintering operation, as most oxides of these metals cannot be reduced at temperatures below the melting point.

8. Powder metallurgy is not economical for small scale production.9. Articles produced by powder metallurgy process possess poor ductility


Application of powder metallurgyThere is a great variety of machine components that are produced from metal powders, many of these are put to use without any machining operation carried out on them. Following are some of the prominent PM Products. Self-Lubricating Bearing and Filters:

Porous bronze bearings are made by mixing copper and tin powder in correct proportions, cold pressed to the desired shape and then sintered. These bearings soak up considerable quantity of oil. Hence during service, these bearings produce a constant supply of lubricant to the surface due to capillary action. These are used where lubricating is not possible. Porous filters can be manufactured and are

used to remove, undesirable materials from liquids and gases.

Cutting Tools and Dies: Cemented carbide cutting tool inserts find extensive applications in machine shops. These are produced by PM from tungsten carbide powder mixed with cobalt binder. Machinery Parts: Several machinery parts including gears, bushes and bearings, sprockets, rotors are made from metal powders mixed with sufficient graphite to give to product the desired carbon content. Friction Materials: These are made by powder metallurgy. Clutch liners and Brake bands are the example of friction materials.Gears and Pump Rotors: Gears and pump rotor for automobile oil pumps are manufactured by powder metallurgy. Iron powder is mixed with graphite, compacted under a pressure of 40 kg/ cm and sintered in an electric furnace with an atmosphere and hydrocarbon gas. These are impregnated with oil.Refractory Materials: Metals with high melting points are termed as refractory metals. These basically include four metals tungsten, molybdenum, tantalum and niobium. Refractory metals as well as their alloys are manufactured by powder metallurgy. Magnets: Small magnets produced from different compositions of powders of iron, aluminum, nickel and cobalt has shown excellent performance, far superior to that cast. Electrical Parts: Several combinations such as copper – tungsten, cobalt – tungsten, silver – tungsten, copper-nickel, and silver – molybdenum have been used for production of these parts. These parts are required to have excellent electrical conductively, be wear resistant, and somewhat refractory.


Micro-machining Micro-Machining includes all cutting operations in which material is removed at the micron level. Present notion of the term ‘micro-machining’ has been used more commonly in the fabrication of micro-components in the size range of 1 to 500 μm. Micromachining is the most basic technology for the production of miniaturized parts and components.

Over the past several years there has been an increased interest in micro machining technology that has captured the imagination of every manufacturing and industry segment; from aerospace, medical appliance and the automotive world, the potential for product miniaturization continues to grow and while posing numerous technical challenges.

Advantages of micro-products (1-500µm size products)1. Increased function 2. Reduced material requirment3. Reduced power requirment4. Less space is needed5. Less handling and transportaion etc

Diamond turn machining Micromachining is the most basic technology for the production of miniaturized parts and components. Micro turning is one type of micromachining process which uses a solid tool and its material removal process is almost similar to conventional turning operation.Diamond is a transparent solid made mostly of one kind of atom, carbon. The advantages of using diamond cutting tools often include improved workpiece quality, increased productivity, and reduced costs. High hardness and wear resistance result in good surface finishes over long production runs, consistent control of dimensions for extended periods, and long tool life. High quality tools can be made of single crystal natural diamond. They are used to make high precision parts in metals, plastics, ceramics and a host of other materials.

The diamond tool is commonly used in micro-machining as it can withstand the micro hardening of the workpiece surface during micro-machining. Diamond only softens at 13500C and melts at 3027 0C, and is also the hardest material in the world. The high hardness is important for reducing wear rate and enable machinability of glass and ceramic materials.


Cutting edge radius of single point diamond tool can be sharpened down to 20nm. Its sharp edge can be retained without major wear. Micro-machining using diamond tool could be performed at high speeds and generally fine speeds to produce good surface finish such as mirror surfaces and high dimensional accuracy in non-ferrous alloys and abrasive non-metallic materials.

Material removal mechanism in micro-machiningIn nano and micromachining processes the actual material removal can be limited to the surface of the workpiece, i.e. only a few atoms or layers of atoms. Material removal rates in micro-milling are considerably lower than in conventional macro-scale machining.The mechanism for material removal involved plastic deformation, microfracture and dislodgement of grains.

Chip formation in micro cutting


cutting edge radius 5nmMagnetorheological nano-finishing processCommonly used traditional finishing processes are grinding, lapping and honing. All these processes use multipoint cutting edges in the form of abrasives, which may or may not be bonded, to perform cutting action. These processes have been in use from the earliest times because of their capability to produce smooth surface at close tolerances. Earlier there has been a limit on the fine size of abrasives (a few μm) but today, new advances in materials syntheses have enabled production of ultra fine abrasives in the nanometer range. The ultimate precision obtainable through finishing is when chip size approaches atomic size (0.3 nm). To finish surfaces in nanometer range, it is required to remove material in the form of atoms or molecules individually or in the groups. To name a few, these magnetic field assisted finishing processes include Magnetic Abrasive Finishing (MAF), Magnetic Float Polishing (MFP), Magnetorheological Finishing (MRF), and Magnetorheological Abrasive Flow Finishing (MRAFF).

Magnetorheological fluid (MR fluid)A magnetorheological (MR) fluid is a suspension of magnetically soft ferromagnetic particles in a carrier liquid. Typically, the particles are of the order of a few microns in diameter and their volume concentration is 30% to 40%. When exposed to a magnetic field, the viscosity and yield stress of the suspension increase several orders of magnitude. Under magnetic field, the particles line up, thickening the fluid and fluid to behave more like a solid. The term "magnetorheological" comes from this effect. The particles are tiny, measuring between 3 to 10 microns of carbonyl iron dispersed in a non-magnetic carrier medium like silicone oil, mineral oil or water.

Magnetic particles (µm /nm) size suspended within the carrier oil are distributed randomly

Under magnetic field, microscopic particles align themselves along the lines of magnetic flux


Characteristics of MR fluid Particle size, shape, density and distribution of carbonyls particles and properties of

carrier fluid are factors controlling MR fluid. In off condition (absence of magnetic filed) MR fluid appear similar to liquid paint

and exhibit viscosity 0.1 to 1 Pa sec Their viscosity changes significantly 105 to 106 times within a few milli seconds when

magnetic field is applied Particles held together by magnetic field, form chain which resist to a level of shear

stress. The change of viscosity is completely reversible when magnetic field is removed

Magnetorheologcal polishing fluid (MRP fluid)When the MR fluid is mixed with abrasives particles, we get a MRP fluid. So MRP fluid consists of carbonyls iron particles (CIP) and nano sized abrasive particles (SiC)MRP fluid consists of:

o Carbonyl iron powder 20%o Silicon carbide 20% (non magnetic abrasive particles)o Base fluid medium 60%o Additives

The flow behaviour of the MRP fluid exhibits a transition from liquid like structure to a gel like structure on the application of magnetic field. The rheological properties of MRP-fluid depend on carbonyl iron particle size (CIP), silicon carbide (SiC) particle size, their volume concentration, magnetic properties and magnetic field strength. The MR fluid temporarily stiffens and conforms to the surface of the component being finished. This allows geometries of almost any shape to be polished as easily as a spherical optic

Advantages of MRP fluid over the traditional methods (lapping)1. It does not load up as a grinding wheel2. It is flexible and adapts the shape of the part of the workpiece3. Carries heat and debris away from the polishing zone4. Processes are more controllable

Characteristic of MRP fluid1. Concentration of magnetic and abrasive particles2. Density and size of particles3. Yield stress under magnetic field4. Property of carrier fluid5. Low off-state viscosity6. Resistance to corrosion


7. Stability against sediments etc

Abrasive flow machining (AFM)A one way or two way flow of an abrasive media is extruded through a workpiece, smoothing and finishing rough surfaces. The process is particularly useful for difficult to reach internal passages, bends, cavities, and edges.

Magnetorheological Finishing (MRF) -MRP fluid is used All traditional finishing process are incapable of producing required surface finish of nanometer level. Magnetorheological finishing is one of the new process which can provide surface finish up to nano meter level. Magnetic abrasives are emerging as important finishing methods for metals and ceramics. For the polishing purposes, proper abrasive slurry (silicon carbide) is incorporated into the MR fluid (MRP), which is supplied to the narrow gap between the wheel and workpiece. When magnetic field is applied, the magnetic particles hold the abrasive particles together and act as a solid and relative movement is given between the work and abrasive slurry.

The low viscosity MR fluid is pumped through a shaping nozzle onto a vertical, rotating wheel. At the apex of the wheel, the fluid stiffens into a ribbon, under the influence of a dc magnetic field. The workpiece is placed into the ribbon and forms a converging gap. Shaping and smoothing are accomplished simultaneously as the rotating workpiece is moved through the ribbon under computer control. An electromagnet, located below the polishing wheel, has specially designed pole pieces that extend up to the underside of the apex of the wheel rim. These pole pieces exert a strong local magnetic field gradient over the upper side of the wheel. When the magnetorheological fluid passes through the magnetic field, it stiffens in milliseconds, then returns to its original fluid state as it leaves the field, again in milliseconds. This precisely controlled zone of magnetized fluid becomes the polishing tool. When an optical surface is placed into the fluid in this zone, the stiffened fluid ribbon is "squeezed" from its original thickness of about 2 mm, to about l mm. The "squeezing" results in significant shear stress and subsequent polishing pressure over that section of the optical surface. At the same instant, the MR fluid conforms to the local curvature of the part being polished.


Magnetorheological Abrasive Flow Finishing (MRAFF)Magnetorheological Abrasive Flow Finishing (MRAFF) is a precision finishing process developed for nanofinishing of complex internal geometries using smart magnetorheological polishing fluid. It is a homogenious mixture of carbonyls iron particles (CIP) and abrasive particles in a base medium (paraffin liquid). When the external magnetic field is applied, carbonyls iron particles (CIP) form a chain like structure with abrasives embedded in between. The magnetic force between iron particles holding abrasive grains provides the bonding strength which depends on iron concentration, magnetic field strength and particle size etc. The Magneto-Rheological Abrasive Flow Finishing is a polishing process that results from the sum of Abrasive Flow Finishing (AFF) and Magneto-Rheological Finishing (MRF). In other hands, it is a hybrid process developed to preserve the advantages of both processes.

The MRP fluid is extruded back and forth through the passage, material removal will takes place.

Structure formed with abrasives trapped and embedded between iron chains, in the presence

of finite magnetic field.


Module III (Questions)1. Explain the procedure of manufacturing parts by powder metallurgy.2. Explain the manufacturing of powder metallurgy components with suitable flowchart. 3. Describe, the steps involved in the production of powder metallurgy parts.****4. Discuss secondary operations in Powder Metallurgy.

**************************************5. What are the advantages of powder metallurgy offers?****6. What are advantages and limitations of powder metallurgy?***7. Discuss/Explain the applications of Powder Metallurgy. ****8. What are the main industrial uses of powder metallurgy?9. Explain the process capabilities of powder metallurgy 10. Explain why powder metallurgy has become highly competitive with casting, forging

and machining processes. **11. What are the design considerations for the powder metallurgy parts?***12. Describe the design considerations making powder metallurgy parts. How different

are these compared to casting and forging of metals.*******************************************

13. Describe briefly the methods by which powders suitable for powder metallurgy can he produced.******

14. Explain how metal powders are produced by atomization. ***15. Explain the various methods of powder production. Give the characteristics required

for metal powders16. Explain three methods of powder production with neat sketches and discuss their

influences on the properties of the final product.17. Explain the characteristics of metal powders required?****18. What are the desirable properties of metal powder? 19. What are the important physical characteristics of powder-metals20. Explain the effects of using fine powders and coarse powders respectively in making

P/M parts***************************************

21. What are some of the objectives of powder mixing or blending**22. Briefly explain blending of powders in powder metallurgy


23. List and discuss the material properties affecting blending in Powder metallurgy process.*************************

24. Explain the various compaction techniques used in Powder metallurgy. ****25. What are some of the objectives of the compacting operation?26. Why might double-action pressing be more attractive than compaction with a single

moving punch?27. In what ways might the final density of a P/M product be reported?28. What are some of the other methods that can produce high-density PIM products? 29. Why is there a density variation in compacting powders?30. With suitable sketches explain double compaction of parts out of powders. 31. Discuss cold and hot isostatic pressing process. 32. What is meant by Isostatic pressing? 33. What is isostatic compaction? For what product shapes might it be preferred? 34. What are the disadvantages of hot pressing? How can you overcome them 35. Describe the relative advantages and limitations of cold and hot isostatic pressing.** 36. Describe, with suitable sketches, hot isostatic pressing of metal powders.***37. What are some of the attractive properties of HlP products? 38. What are some of the major limitations-of HIP process and how does the sinter

process eliminate or minimize them?******************************************************

39. Differentiate between infiltration and "impregnation” with reference to powder metallurgy

40. What is the purpose of repressing, coining or sizing operations? 41. Why can the original compaction tooling not be used to shape the product during

repressing?42. What are pre alloyed and pre-coated powders? How are these powders manufactured?43. What are impregnations and infiltration processes in powder metallurgy?

*******************************************44. Why it is necessary to use lubricants in the press compaction of powders? State and

explain the advantages of porous and self-lubricating bearings over the standard sleeve bearings.

45. What are self lubricating bearings?**46. Why is pore size important in the manufacture of self lubricating bearing? How may

pore size be controlled?*********************************************

47. Define the following terms in relation to metal powders :a. Surface area, b. Compressibility,c. Apparent density and particle size distribution.

48. Write notes on a. Hot pressing,b. Impact compactingc. Powder rolling.**


***************************************************49. Explain pre-sintering and sintering in detail.***50. Differentiate between pre-sintering and sintering.51. What is meant by sintering of powder compacts?52. Give an account of sintering atmospheres**53. What are the effects of sintering on the powder compact produced by pressing54. Explain the mechanism of sintering of single and multi-phase materials. 55. Outline the advantages of pre-sintering and coining on the metal compacts.56. Give an account of sintering furnaces used in powder metallurgy industries.57. Should green compacts be brought up to the sintering temperature slowly or rapidly?

Explain the advantages and limitations of each.

Extra notes

PM comparing with casting and forging processa. Net shape manufacturingb. Controlled porosity productc. Higher melting point material product can be maded. Brittle and hard material e. High production rate with automationf. Higher finish and dimensional accuracyg. Less scraph. More ecofriedlyi. Higher mechanical and other properties

Powder productionMetal powder production techniques are used to manufacture a wide spectrum of metal powders designed to meet the requirements of a large variety of applications. Powders of almost all metals can be produced. Various powder production processes allow precise control of the chemical and physical characteristics of powders and permit the development of specific attributes for the desired applications. Powder production processes are constantly being improved to meet the quality, cost and performance requirements of all types of applications. Metal powders are produced by mechanical or chemical methods. The most commonly used methods include water and gas atomization, milling, mechanical alloying, electrolysis, and chemical reduction of oxides. Which powder production process is used depends on the required production rate, the desired powder properties and the properties desired in the final part. Chemical and electrolyic methods are used to produce high purity powders. Atomization is the most versatile method for producing metal powders. It is the dominant method for producing metal and pre-alloyed powders from aluminum, brass, iron, low- alloy steel, stainless steel, tool steel, superalloy, titanium alloy and other alloys. For the production of ultra fine or nano-powders, a growing market, gas phase reactions, spray drying or precipitation methods are used.


In the case of iron, sponge powder is produced from magnetite iron ore that is directly reduced at elevated temperatures to obtain sponge form. The material is then disintegrated into powder and annealed to obtain the desired properties.Sponge iron has a very high surface area and exhibits high green strength.It is used for low and medium density ferrous P/M parts.Advantages of PM

1. Eliminates or minimizes machining2. Eliminates or minimizes scrap losses3. Maintains close dimensional tolerances4. Permits a wide variety of alloy systems5. Produces good surface finishes6. Provides materials which may be heat-treated for increased strength or increased wear

resistance7. Provides controlled porosity for self-lubrication or filtration8. Facilitates manufacture of complex or unique shapes which would be impractical or

impossible with other metalworking processes9. Parts can be made to net or near net shape10. Suited to moderate -to high volume components production requirements11. Rapid solidification allows extension of solubility limits, production of novel phases,

and more refined microstructures than conventional metallurgical techniques12. Permits the production of metal-matrix composites13. Permits the production of nanostructured materials

Carbonyls process steps1. Impure metal or metal ore is heated in a furnace with carbon monoxide to form metal

carbonyls (gaseous state) under pressure.2. The metal carbonyl gas is condensed to a liquid at room temperature3. Then heated at atmospheric pressure to vaporize liquid carbonyls4. Decomposition of vapour carbonyls gives pure metal powder (at temperature at 200 to

3500C).Heat treating: The main purpose is to improve wear resistance rather than strength. The process of heating and cooling sintered parts is to improve

1. Wear Resistance2. Grain Structure3. Strength

The following heat treatment processes are used to the parts made by powder metallurgy:1. Stress relieving2. Carburising3. Nitriding 4. Induction Hardening

CarburizingSteel parts may be carburized using solid, liquid or gas medium at conventional temperature in a controlled atmosphere of high carbon potential. Arrangements for enriching the furnace


with carbon is done to, provide nascent carbon for the carburization of powder metallurgy parts. After heat treatment they are quenched and tempered to obtain high surface hardness. Gas carburizing is most widely used because gases penetrate much faster through the pores in the sintered parts. Thus deeper carbon penetration.

CarbonitridingThis process is similar to carburizing, in which heating of the sintered part takes place in a mixture of endothermic gas with propane and ammonia. Since carried out at low temperatures, care should be taken to prevent formation of carbide networks, which are dissolved at higher temperatures. Used to produce high hardness and hardenability of the case. Better movement of gas particles through the pores. Treatment time is less. It may be oil quenched to ensure high hardness of the surface.

NitridingThis method is used to increase the fatigue strength of the sintered parts. It also provides good wear resistance, increased hardness and low co-efficient of friction. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material.

Through HardeningThis treatment consists of heating the sintered components to 815ºC to 870ºC under controlled atmospheric condition for 15 to 30 minutes, followed by quenching in oil. However a more severe quenching medium is required here because of the lower thermal conductivity of porous materials. The combination of high strength and flexibility of properties has made the through hardening popular.

Induction HardeningThe part is heated to 50ºF to 100 ºF higher than through hardening in an induction coil for a very short time and then quenched in oil. Since heating cycle is short, little grain growth occurs and controlled atmosphere condition is not essential particularly for very dense parts.This is process is done where a hard, wear-resistant surface is required but the interior part must be left unhardened for close dimensional tolerances or subsequent machinery.

Particle size and shape


Ductile regime machining processEvery material however hard and brittle shows some degree of ductility at nano/ micron level. If the material removal is carried out nano level (ductile regime)plastic deformation of hard and brittle material will takes place. The process of machining brittle materials where the material is removed by plastic flow, thus leaving a crack free surface is known as ductile-regime machining. Ductile machining of brittle materials can produce surfaces of very high quality comparable with processes such as polishing, lapping etc.Brittle materials such as silicon, germanium, glass and ceramics are widely used in semiconductor, optical, micro-electronics and various other fields. The conventional machining processes such as single point turning and milling are not conducive to brittle materials as they produce discontinuous chips owing to brittle failure at the shear plane before any tangible plastic flow occurs.

Magnetic abrasive finishing process (MAF)

Ferromagnetic particles mixed with fine abrasive power (silicon carbide/aluminium oxide etc) subjected to magnetic field. The magnetic field act as a binder and it is similar to a wire brush.The magnetic abrasive grains are combined to each other magnetically between magnetic poles along a line of magnetic force, forming a flexible magnetic abrasive brush. MAF uses this magnetic abrasive brush for surface and edge finishing. The magnetic field retains the powder in the gap, and acts as a binder causing the powder to be pressed against the surface to be finished

Sintering process
