Powder Metallurgy

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POWDER METALLURGY
Powder metallurgy is an art of producing metal powder and using them to make serviceable object. It is one of the important manufacturing processes. The different steps in production of part by powder metallurgy are as follows:1. Powder making 2. Mixing and blending 3. Compaction 4. Sintering 5. Finishing

Advantages of Powder metallurgy process
The powder metallurgy process has some distinct advantages over other manufacturing processes. They are as follows. 1. Articles of intricate shapes can be manufactured easily. 2. Machining operation is almost eliminated. If carried out, it is nominal. Very good dimensional accuracy and surface finish is possible. 3. Phase rule does not apply to powder metallurgy. Metals and alloys can be mixed into each other in any proportion. 4. Articles of any desired porosity can be manufactured. Lubricants can be impregnated into porous bearing under pressure resulting in components which are self lubricating. 5. 100 % yield. Clean and quiet operation. 6. Super hard cutting tool pieces, tungsten filament can only be manufactured by powder metallurgy.

Methods of Powder making
The performance of metal powder during processing and properties of product are highly dependent upon the characteristic of metal powder that are used. The most important characteristics are i) ii) iii) Purity Chemical composition Particle size Metal powders used in powder metallurgy have size in the range of 4 to 200 microns.  Particles size distribution is also an important factor because it determines packing density.  Particle shape also determines packing density and flowability of powder.  Apparent density of powder is defined as weight of loosely heaped quantity of powder necessary to fill the given die cavity completely  Flow rate is defined as the rate at which a metal powder will flow under gravity, from a container through an orifice, both having specific shape and size. Flow rate measures the ability of powder to be transferred. Flow rate is an important characteristic because the die must be fed rapidly with powder to achieve high rate of production. There is definite relation between particular method of powder production and desired property of powder metallurgy product. 

Powder making process
There are many mechanical and chemical methods of powder production such as 1. Automisation 2. Reduction of oxides 3. Electrolytic deposition 4. Crushing 5. Milling 6. Condensation of metal vapour 7. Hydride and carbonyl process

Automization
It is the method most frequently used for metals having low melting points such as lead, antimony, cadmium, aluminium etc. in this process molten metal is directed through an orifice and as it immerges, a high pressure stream of compressed air impinges on it, causing it to atomize into fine particles. Generally spherical shaped particles are produced by this method. A wide range of particle size distribution may be obtained by varying the temperature of metal, pressure and temperature of atomizing gas, rate of flow of metal through an orifice.

Reduction of oxide
The reduction of compounds of metal usually 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. In this process oxides of metals are reduced with carbon monoxide and hydrogen at a temperature below the melting point of the metal in the controlled atmosphere. The reduced powder is subsequently ground . If the oxide powder is graded before reduction, a high degree of size uniformity is maintained in a reduced powder. The particles produced are sponge like in structure and are ideal for moulding. This is the only practical method available for producing powders of refractory metals, such as Tungsten and molybdenum. It is also economical method of producing powders of iron, nickel, cobalt, copper, etc.

Electrolysis
Electrolysis is most suitable method for production of extremely pure powders of copper and iron. This process is essentially an adaptation of electroplating. By regulation of current density, temperature, circulation of electrolyte and proper choice of electrolyte ; powder may be deposited from electrode. The deposit may be soft, spongy substance which subsequently ground to powder. The shape of the powder obtained by this method is generally dendritic. The powder has low apparent density. But dendritic structure tends to give good moulding properties because of interlocking of particles during compacting.

Crushing
Various ferrous and non-ferrous alloys can be heat treated in order to obtain sufficiently brittle material, which can then be crushed into powder form. Equipments used for crushing may involve jaw crushers, gyratory crushers etc. equipments such as ball wheels, disk wheels are used for this purpose. This milling operation is employed after crushing to produce finer grains of powder.

Condensation of metal powder
This technique can be applied in case of metals such as zinc, cadmium, magnesium which can be boiled and the vapors are condensed in the powder form. A rod of metal is fed into high temperature flakes .The vaporised droplets of metal are allowed to condense onto a cool surface of material to which they will not adhere. This method is not suitable for mass production of powder.

Hydride Process
This process is used to produce powders of niobium, tantalum and zirconium. These metals when combine with hydrogen forms hydride which are stable at room temperature. When these hydrides are heated to 350 degree centigrade, hydrogen and pure metal get dissociated. Nickel, iron has tendency to combine with CO to from carbonyl. When the carbonyl vapour is decomposed, spherical pure metal particles are obtained. Nickel and iron powder can be produce by this method.

POWDER BLENDING AND MIXING.
It is also an important operation. When two or more types of powders are used, these powders should be thoroughly mixed into each other in order to obtain homogenous mixture. When the powder is not mixed properly, segregation is likely to occur.

Compacting
It is the most important operation in powder metallurgy technique. Mostly compacting is done cold but in some applications compacts are hot pressed. The purpose of compacting is to consolidate the powder into desired shape

and as closely as possible to final dimensions, taking into account any dimensional changes that result from sintering. Compacting also impart desired level of porosity and adequate strength for handling. Compacting techniques can be classified into two types

1. Pressure techniques - such as
i. ii. iii. iv. v. Die compaction Iso static compaction H.E.R.F. ( High energy rate forming ) Forging Extrusion etc.

2. Pressure less techniques such as slip casting

Pressure techniques :Die compaction

Fig:- Die compaction process

Die compaction

Complicated powder metal parts are manufactured by this process. The dies used in this process are made of hardened ground lapped tool steel. Punches are also made up of same steel but somewhat softer. They are perfectly aligned and are very closely fitted.

Sequence of operation
1. Filling a die cavity with a definite volume of powder. 2. Application of required pressure for the movement of upper and lower punches – Pressure employed is commonly ranged between 19 to 50 tons/inch 2. 3. Ejection of green compact by lower punch. For application of pressure mechanical or hydraulic presses may be employed. Mechanical presses are available with press reading of 10 to15 tons and speeds of 6 to 150 strokes per minuits. High production rate, flexibility and economy can be obtained with such presses. Hydraulic presses have higher pressure rating up to 5000 tons but lower stroke speeds – generally 20 stroke/min. These presses are used for higher pressure applications.

Iso static compaction
In this case pressure is applied simultaneously and equally in all directions. The powder is placed in a rubber mould which is immersed in a fluid bath within a pressure vessel so that the fluid may be placed under high pressure. Since the pressure is applied uniformly, it is possible to obtain a very uniform green density and high degree of uniformity in properties. This method of compaction is extensively used for ceramic materials rather than metals.

High energy rate forming
H.E.R. technique may either be mechanical, pneumatic, explosive or spark discharge method applied in a closed die. Advantages :- i) Short time and the high pressure that can be attained. ii) It is also possible to use low grade and very cheap powder.

Iii) Some parts due to increased strength of compact may be used without subsequent sintering. Disadvantages:-i) High punch and die wear ii) High cost

Forging and extrusion
These techniques are used only to a limited extent. In either case the powder is canned or placed in some kind of metal container. The sealed container is heated and then forged or extruded. After forging or extrusion, container material is removed either mechanically or chemically. Both the techniques yield the compact of extremely high density and usually do not require sintering.

Vibratory compaction
Vibratory compaction is a method in which pressure and vibrations are applied simultaneously to a mass of powder in a rigid die. Compared with ordinary die compaction, this method allows use of much lower pressure to achieve a given level of densification. But it is problematic to design the equipments to apply the vibrations to tooling and presses.

Pressure less technique
Slip casting
It is widely used for ceramics and only to a limited extent to metals. The process consists of first preparing a slip containing the powder in liquid vehicle (one type of organic compound) and additives to prevent particle setting. The slip is then placed in mould made of fluid absorbing material such as plaster of paries to form the slip casting. After removal from the mould the slip casting is dried and sintered. This technique is attractive for materials that are relatively in compensable by conventional die compaction. But the process does not lend itself to high production rates because of long time required for liquid to be removed through the porosity of mould. Molybdenum parts are generally compacted this way.

Gravity compaction

In this case the die is filled with loose powder which is then sintered in the die itself. The die is usually made of an inert material such as graphite. Since the pressure is not used, parts are generally porous. Commercially thios method is used for productions of filters.

Sintering
Sintering process follows compaction and is carried out at a temperature below the highest melting point constituents. The maximum sintering temperature recommended for different jobs are as follows:1000oC:- Brasses, Bronzes 1200oC:- Iron parts 1600oC:- For metals like platinum, tungsten etc. In some cases temperature is high enough to form a liquid constituent such as in case of manufacture of cemented carbides, where the sintering is done above the melting point of binder metal. Furnace used for sintering can be electric resistance type or gas/oil fired but for accurate temperature control electric furnaces are preferred. Bonding between the particles is greatly affected by surface films. The formation of undesirable surface films such as oxides must be avoided. This can be avoided by keeping the atmosphere in the furnace either neutral or reducing. Sintering is essentially a process of bonding solid bodies by atomic forces. Sintering forces tend to decrease with temperature but obstructions to sintering such as incomplete surface contact, presence of surface film, lack of plasticity all decrease more rapidly with increase in temperature. Thus elevated temperature tend to favor the sintering process. The longer the time of heating or higher the temperature, greater will be the bonding between the particles and greater will be the resulting tensile stress. But from economical stand point time required for sintering should be minimum possible. Sintering can be explained as follows:-

1. Original point contact

2. Neck growth – Short exposure to high temperature. This stage results in high Increase in strength and hardness

Hot pressing
This method consists of applying pressure and temperature simultaneously. Moulding and sintering takes place at the same time which results in higher densities and greater productions. The advantages of hot pressing as compared with cold compaction and sintering are:i) ii) iii) Reduction in gas content and shrinkage effect. High strength, hardness and density. The principal disadvantage is high cost of dies to stand under high pressure and high temperature. This method is used for production of very hard cemented carbide parts.

Powder metallurgy applications
1. Cutting tools:One of the outstanding uses of powder metallurgy is combination of hard materials in metallic matrix. It serves as basis for cemented carbide products. In the production of cemented carbide cutting tools, a suitable mixture of carbides of tungsten (W), tantalum (Ta), titanium (Ti) with cobalt as a binder is compacted and pre sintered. In this pre sintered condition, the material can be cut, machined and ground to the final dimension. The compact is then subjected to high temperature sintering operation during which liquid cobalt bind hard carbide particles into solid piece. Cemented carbide tools are noted for high compression strength, red hardness and wear resistance. Since they are brittle, they are employed as brazed on tips to a steel tool.

Flow chart

Other examples in this classification are diamond impregnated grinding wheels, drill core bits.

Friction material
Metal - non metal combinations have found wide use in the manufacture of friction material such as clutch facings, brake linings etc. these material contain metallic matrix of copper and bronze for heat conductivity, lead and graphite to form a smoothly engaging lining during operation. They may contain silica for frictional purposes.

Electrical contact materials:Theses are manufactured to produce component with properties which cannot be obtained by other known methods. The combinations of properties obtained are:     High electrical conductivity Low thermal conductivity High wear resistance Low contact resistance Low vapor pressure

The wide ranges of products manufactured are W-Cu, W-Ag, Mo-Ag etc. None of these combination of above alloys can be manufactured by melting.

Self lubricated bearings
Articles’ manufactured by powder metallurgy can be given any desired degree of porosity. By controlling the particle size and distribution and pressure during compaction,% porosity can be controlled within narrow range. If high porosity is required, the powder is mixed with a substance which produces foam. Foams exerts a pressure on individual particles during sintering and tries to separate them. Ultimately foam producing material burns during sintering, leaving porous components.

High porosity is required in porous oil impregnated bearings which are self lubricating. These bearings are made of bronze powder with controlled porosity after sintering. The cores are subsequently filled with oil. In operation, the load on bearing and increased heat setup by moving part within the bearing force oil out of cores to provide automatic and uniform lubrication.

Filters
These require high degree of porosity/ these also require higher mechanical strength and resistance to mechanical as well as thermal shocks. These are used in chemical and automobile industries.

Flow chart for manufacture of filters:Carefully graded powder ------ Mixing ------ Mould filling --------Sintering in mould (No pressure) ( Non oxidizing atm.)

Porous metal filter

Magnets Small alhico permanent magnets containing Al, Ni, Co powder can be made by powder metallurgy. They can also be cast. The cast alloy is difficult to machine and finishing to the dimensions must be done by tedious grinding. Therefore these magnets, magnetic articles, bimetallic strips are manufactured by powder metallurgy technique.

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