1. Material science & metallurgy
Powder metallurgy
Guided by: S. P. Joshi sir

2. Prepared by: 140080119020: Aakash B. Patel 15MES302: Krunal k. Rana
15MES303: Vishnu Thanki

3. Introduction
Powder metallurgy may be define as the art of producing metal powders and using them to make serviceable objects.
Powder metallurgy principles were used far back in 3000 B.C. by the Egyptions to make iron parts.
The use of gold, silver, copper, brass & in powders for ornaments was common during the middle age.
Recently, materials with mechanical properties far better than those of conventional materials have been developed by improving heat-treatment, powder composition and processing methods to achieve higher densities.
So powder metallurgy has become a manufacturing technique to produce considerably complex shaped components to exact dimensions at high rates of production with extremely low costs.

4. Basic process of powder metallurgy
The four basic operation of the powder metallurgy technique are:
Manufacture of powder
Mixing or blending powder particles
Compacting and,
Sintering

5. Manufacture of metal powder
The size of powder particles normally required in powder metallurgy ranges from 1 to 100 µm; preferably within 10 to 20 µm. For the manufacture of these sizes of powder particles there are various methods are following:
Atomization
Reduction of oxides
Electrolytic deposition

6. Atomization
Atomization is the method most frequently used for metals having low melting points, such as tin, lead, zinc, cadmium, and aluminum.
As shown in fig. liquid metal is forced through a small orifice, a stream of compressed air cause the metal to disintegrate and solidity into finely divided particles.
Atomized products are generally in the form of the sphere-shaped particles.

7. A wide range of particle-size distribution may be obtained by varying the temperature of the metal, pressure and temperature of the atomizing gas, rate of flow of metal through the orifice, and the design of the orifice and nozzle.
The main advantages of atomization process is its flexibility.
When the method is used for production of powder of alloyed metals, it offers high production rate (as high as 18 to 20 tons per hour).

8. Reduction of compounds
The reduction of compounds of the metals (usually an oxide) provides a convenient, economical, and flexible method of producing powders.
The largest volume of metallurgical powder is made by the process of oxide reduction.
These compounds are heated with reducing agents such as carbon, coal gas, blast furnace gas, carbon monoxide (CO), hydrogen (H2) or dissociated ammonia, in an atmosphere-controlled furnace.
The particles produced by oxide reduction are spongelike in structure and are ideal for molding.
The shape is generally jagged and irregular and the particles are porous.
This is only practical method available for producing powders of the refractory metals such as tungsten and molybdenum.
Oxide reduction is also an economical method of producing powders of iron, nickel, cobalt, and copper.

9. Electrolytic deposition
The method of electrolytic deposition is most suitable for the production of extremely pure powder of principally copper and iron.
Electrolysis is similar to electroplating.
By regulation of current density, temperature, circulation of electrolyte, and proper choice of electrolyte, the powder may be directly deposited from the electrolyte.
The deposit at the bottom of anode in the tank is periodically removed during the process.

10. The deposit may be a soft spongy substance which is subsequently ground to powder, or the deposit may be a hard, brittle metal.
Powders obtained from hard, brittle eletrodeposits are generally not suitable for molding purpose.
The shape of electrolytic powder is generally dendritic, although the resulting powder has low density, the dendritic structure tends to give good molding properties because of interlocking of the particles during compacting.

11. Compacting
The most important operation next to mixing & blending is compacting or pressing of the powder particles in powder metallurgy.
Most compacting is done cold, although there are some application for which compacts are hot-pressed
The purpose of compacting is to consolidate the powder into the desired shape and as closely as possible to final dimensions.
Compacting technique may be classified into two type:
Pressure technique such as die, roll pressing, extrusion, high energy rate forming, vibratory compaction
Pressureless technique such as slip casting, continuous pressureless compaction.

12. Die compaction Die compaction is most widely used method.
This uses special mechanical or hydraulic presses including feed hopper, shaping dies, upper punch & lower punch. During the operation, the die cavity is filled with powder particles through a feed hopper with definite quantity.
Next, the required pressure is applied by moving the upper & lower punches towards each other, and finally, ejecting the “green compact” by raising the lower punch further after moving the upper punch upwards.

13. Mechanical presses can provide pressure ranging from 100 kN to 400 kN for producing sintered-bronze bearings. Higher capacity (1 MN to 5 MN) is used for other metals.
Hydraulic presses can provide even higher pressure, but slower stroke speeds limit their uses to move complicated powder metal parts requiring very high pressure. Mechanical presses have advantages of high speed production rates, flexibility in design, simplicity, economy of operation and relatively low investment cost.
Dies are usually made of hardened, ground and lapped tool steels.
The die punches are made of die steel heat treated to be slightly softer than the die as they can easily be replaced compared to dies.

14. Extrusion technique
Extrusion technique has been used only to a limited extent, as it does not give such an efficient control as that given by die or roll pressing methods.
Fig. shows the method in which powder is ‘canned’ or placed in some metal container.
The sealed container is heated or evacuated and then extruded.
After extruding, the container material is removed either mechanically or chemically.
This method provides compacts with extremely high density and usually do not required sintering.
Forging technique is similar for the extrusion method as explained above.

15. High energy rate forming
This method may be either mechanical, pneumatic, explosive-discharge or spark-discharge methods.
the advantages of these methods is the short time & high pressures that can be applied.
Even low grade & cheap powder can also be used. Some parts may be used due to increased strength of green compacts without sintering.
Disadvantages include high punch & die wear, limited tolerance control and high cost.

16. Vibratory compaction
Vibratory compaction requires application of pressure & vibration simultaneously to the powder mass in a rigid die.
Compared to ordinary die compaction, this method allows the use of much lower pressures to achieve given level of density.
A major limitation is the design of equipment which can apply vibration to tooling & presses practically.

17. Pressureless compacting
Slip casting
Slop casting is widely used for ceramics but only to a limited extent for metals. The process consists of preparing a ‘slip’ containing the powder suspended by a liquid & additives to prevent settling down of particles.
Then the slip is placed in mold made of fluid absorbing material to form the slip casting. After removal from the mould, slip casting is dried & sintered.
This method is mainly useful for relatively incompressible materials for conventional die compaction. The process definitely takes long time for the liquid to be removal from the porous mold.

18. Continuous pressureless compaction
Continuous pressureless compaction is used to produce porous sheet for electrodes of Ni-Cd batteries. The powder may be applied in the form of a slurry to be coated on a metal screen or solid metal sheet to produce unusual composites.

19. Sintering
The sintering process is usually carried out at a temperature below the highest melting constituent. In some cases the temperature is high enough to form a liquid constituent, such as in the manufacture of cemented carbides, where sintering is done above the melting point of the binder metal. In other cases, no melting of any constituent takes place.
Sintering furnaces may be either the electric-resistance type or gas or oil-fired type. Close control of temperature is necessary to minimize variation in final dimensions. The very uniform and accurate temperature of the electric furnace makes it most suitable for this type of work.
Sintering is essentially a process of bending solid particles by atomic diffusion. Mechanism of bonding particles during sintering can be divided into three phases describe as under

20. Diffusion of atoms between adjacent particle surface at the point of contact which leads to development of grain boundaries among themselves
Next, the newly formed bonded-areas grow in size by diffusion of atoms across the grain boundaries
The growing size of bonded-areas results into formation of pore within the grain of larger particles by volume diffusion of atoms. Finally, with time, the pores are eliminated completely and a single union of solid mass is produced after sintering is over.
In general, a sintering process requires a careful technical control in terms of time, temperature, heating rate & cooling rate for consistent properties t be achieved in final product of powder metallurgy.
Sintering process can be classified into two groups as:
Solid phase sintering,
Liquid phase sintering

22. Solid phase sintering
In this type of sintering, neither of the compacted metals melt but grain growth & diffusion take place across the cold-welded surface which are adjacent.
This leads to adequate cementing bonding of the particles into a cellular type of structure as shown in fig.
Sintering of pure tungsten can be done by this method.

24. Liquid phase sintering
In this type of sintering, one of the metals melts which penetrates in between the particles of the other metals, thus alloying results and finally continuous metallic bond is established as explained in mechanism of sintering.
Production of sintered bronze and cemented carbide tips is typically done by this method

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