Friday, 7 October 2016

Powder Manufacture in P/M - Atomization & other Techniques

Powder Manufacture
The manufacturing of the material powder is the first step in powder metallurgy processing route that It involves making, characterising, and treating the powder which have a strong influence on the quality of the end product. Different techniques of powder making are: Atomising Process
In this process the molten metal is forced through an orifice into a stream of high velocity air, steam or inert gas . This causes rapid cooling and disintegration into very fine powder particles and the use of this process is limited to metals with relatively low melting point.
Atomization This uses high pressure fluid jets to break up a molten metal stream into very fine droplets, which then solidify into fine particles .High quality powders of Al, brass, iron, stainless steel, tool steel, superalloys are produced commercially Types: water atomization, gas atomization, soluble gas or vacuum atomization, centrifugal atomization, rotating disk atomization, ultrarapid solidification process, ultrasonic atomization

Mechanism of atomization:

In conventional (gas or water) atomization, a liquid metal is produced by pouring molten metal through a tundish with a nozzle at its base. The stream of liquid is then broken into droplets by the impingement of high pressure gas or water. This disintegration of liquid stream is shown in fig. This has five stages
i) Formation of wavy surface of the liquid due to small disturbances
ii) Wave fragmentation and ligament formation
iii) Disintegration of ligament into fine droplets
iv) Further breakdown of fragments into fine particles
v) Collision and coalescence of particles.

The interaction between jets and liquid metal stream begins with the creation of small disturbances at liquid surfaces, which grow into shearing forces that fragment the liquid into ligaments. The broken ligaments are further made to fine particles because of high energy in impacting jet.
• Lower surface tension of molten metal, high cooling rate => formation of irregular surface => like in water atomization
• High surface tension, low cooling rates => spherical shape formation => like in inert gas atomization
• The liquid metal stream velocity, v = A [2g (Pi – Pg)]0.5 where Pi – injection pressure of the liquid, Pg – pressure of atomizing medium, – density of the liquid
Types of atomization
Atomization of molten metal can be done in different ways depending upon the factors like economy and required powder characteristics. At present, water or gas atomizing medium can be used to disintegrate a molten metal stream. The various types of atomization techniques used are,
1. 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.
2. 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.
3. 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. Hydrogen and argon mixture can also be used.
4. Centrifugal atomization: In this method, one end of the metal bar is heated and melted by bringing it into contact with a non-consumable tungsten electrode, while rotating it longitudinally at very high speeds. The centrifugal force created causes the metal drops to be thrown off outwards. This will then be solidified as spherical shaped particles inside an evacuated chamber. Titanium powder can be made using this technique
5. Rotating disk atomization: Impinging of a stream of molten metal on to the surface of rapidly spinning disk. This causes mechanical atomization of metal stream and causes the droplets to be thrown off the edges of the disk. The particles are spherical in shape and their size decreases with increasing disk speed.
6. Ultra rapid solidification processes: A solidification rate of 1000C/s is achieved in this process. This results in enhanced chemical homogeneity, formation of metastable crystalline phases, amorphous materials.

Atomization Unit

Melting and superheating facility: Standard melting furnaces can be used for producing the liquid metal. This is usually done by air melting, inert gas or vacuum induction melting. Complex alloys that are susceptible to contamination are melted in vacuum induction furnaces. The metal is transferred to a tundish, which serves as reservoir for molten metal.
Atomization chamber: An atomization nozzle system is necessary. The nozzle that controls the size and shape of the metal stream if fixed at the bottom of the atomizing chamber. In order to avoid oxidation of powders, the tank is purged with inert gas like nitrogen.

Powder collection tank:

The powders are collected in tank. It could be dry collection or wet collection. In dry collection, the powder particles solidify before reaching the bottom of the tank. In wet collection, powder particles collected in the bottom of the water tank. The tank has to be cooled extremely if used for large scale production.
During operation, the atomization unit is kept evacuated to 10-3 mm of Hg, tested for leak and filled with argon gas.

Atomizing nozzles

• Function is to control the flow and the pattern of atomizing medium to provide for efficient disintegration of powders
• For a given nozzle design, the average particle size is controlled by the pressure of the atomizing medium and also by the apex angle between the axes of the gas jets
• Higher apex angle lead to smaller particle size
• Apex angle for water atomization is smaller than for gas atomization
• Nozzle design: i) annular type, ii) discrete jet type;
i) free falling, ii) confined design
• In free falling, molten metal comes in contact with atomizing medium after some distance. Here free falling of metal is seen. This is mainly used in water atomization.
• In confined design used with annular nozzle, atomization occurs at the exit of the nozzle. Gas atomization is used generally for this. This has higher efficiency than free falling type. One has to be cautious that “freeze up” of metal in the nozzle has to be avoided.

Atomizing mediums
• The selection of the atomizing jet medium is based mainly on the reactivity of the metal and the cost of the medium
• Air and water are inexpensive, but are reactive in nature
• Inert gases like Ni, Ar, He can be used but are expensive and hence have to be recycled
• Pumping of cold gas along with the atomizing jet => this will increase the solidification rate
• recently, synthetic oils are used instead of gas or water => this yields high cooling rate & lower oxygen content compared to water atomized powders
• Oil atomization is suitable for high carbon steel, high speed steels, bearing steals, steel containing high quantities of carbide forming elements like Cr, Molybdenum
• This method is not good for powders of low carbon steels.

Important atomization processes
Inert gas atomization
- Production of high grade metal powders with spherical shape, high bulk density, flowability along with low oxygen content and high purity
- Eg. Ni based super alloys
- Controlling parameters: (1) viscosity, surface tension, temperature, flow rate of molten metal; (2) flow rate, velocity, viscosity of atomizing medium; (3) jet angle, jet distance of the atomizing system; (4) nature of quenching media
Water atomization
- Water jet is used instead of inert gas
- Fit for high volume and low cost production
- Powders of average size from 150 micron to 400 micron; cooling rates from 103 to 105 K/s. Rapid extraction of heat results in irregular particle shape => less time to spheroidize in comparison to gas atomization
- Water pressure of 70 MPa for fine powders in 10 micron range
- important parameters:
1. Water pressure: Increase water pressure => size decrease => increased impact
2. water jet thickness: increase thickness => finer particles => volume of atomizing medium increases
3. Angle of water impingement with molten metal & distance of jet travel

Atomization process parameters
1. Effect of pressure of metal head: r = a + b√h; r – rate of atomization
2. Effect of atomizing medium pressure: r = a√p + b; Increase in air pressure increases the fineness of powder up to a limit, after which no increase is seen
3. Molten metal temperature: As temperature increases, both surface tension and viscosity decrease; so available energy can efficiently disintegrate the metal stream producing fine powders than at lower temperature; Temperature effect on particle shape is dependent on particle temperature at the instant of formation and time interval between formation of the particle and its Solidification; Temperature increase will reduce surface tension and hence formation of spherical particle is minimal; however spherical particles can still be formed if the disintegrated particles remain as liquid for longer times.
4. Orifice area: negligible effect
5. Molten metal properties:
Iron and Cu powder => fine spherical size; Pb, Sn => irregular shape powder;
Al powders => irregular shape even at high surface tension (oxidation effect)
Gaseous Reduction
This process consists of grinding the metallic oxides to a fine state and subsequently, reducing it by hydrogen or carbon monoxide. This method is employed for metals such as iron, tungsten, copper, etc. Electrolysis Process
In this process the conditions of electrode position are controlled in such a way that a soft spongy deposit is formed, which is subsequently pulverised to form the metallic powder. The particle size can be varied over a wide range by varying the electrolyte compositions and the electrical parameters. Carbonyl Process
This process is based upon the fact that a number of metals can react with carbon monoxide to form carbonyls such as iron carbonyl can be made by passing carbon monoxide over heated iron at 50 – 200 bar pressure. The resulting carbonyl is then decomposed by heating it to a temperature of 200 – 3000 Stamp and Ball mills C yielding powder of high purity, however, at higher cost. These are mechanical methods which produce a relatively coarse powder. Ball mill is employed for brittle materials whereas stamps are used for ductile material. Granulation Process
This process consists in the formation of an oxide film in individual particles when a bath of metal is stirred in contact with air. Mechanical Alloying
In this method, powders of two or more pure metals are mixed in a ball mill. Under the impact of the hard balls, the powders are repeatedly fractured and welded together by forming alloy under diffusion.
Other methods
The other less commonly used methods to form metallic powder are by (i) precipitation from a chemical solution, (ii) production of fine metals by machining, and (iii) vapour condensation
Some other Techniques for Powder Manufactures are 1. milling During milling, impact, attrition, shear and compression forces are acted upon particles. During impact, striking of one powder particle against another occurs. Attrition refers to the production of wear debris due to the rubbing action between two particles. Shear refers to cutting of particles resulting in fracture. The particles are broken into fine particles by squeezing action in compression force type. Main objective of milling: Particle size reduction (main purpose), shape change, agglomeration (joining of particles together), solid state alloying, mechanical or solid state mixing, modification of material properties. Mechanism of milling: Changes in the morphology of powder particles during milling results in the following events. 1. Microforging, 2. Fracture, 3. Agglomeration, 4. Deagglomeration Micro forging => Individual particles or group of particles are impacted repeatedly so that they flatten with very less change in mass Fracture => Individual particles deform and cracks initiate and propagate resulting in fracture Agglomeration => Mechanical interlocking due to atomic bonding or vander Waals forces Deagglomeration => Breaking of agglomerates The different powder characteristics influenced by milling are shape, size, texture, particle size distribution, crystalline size, chemical composition, hardness, density, flow ability, compressibility, sinterability, sintered density Milling equipment: The equipments are generally classified as crushers & mills Crushing => for making ceramic materials, oxides of metals; Grinding => for reactive metals such as titanium, zirconium, niobium, tantalum
Vibratory ball mill • Finer powder particles need longer periods for grinding • In this case, vibratory ball mill is better => here high amount of energy is imparted to the particles and milling is accelerated by vibrating the container •During operation, 80% of the container is filled with grinding bodies and the starting material. Here vibratory motion is obtained by an eccentric shaft that is mounted on a frame inside the mill. The rotation of eccentric shaft causes the drum of the vibrating mill to oscillate. • In general, vibration frequency is equal to 1500 to 3000 oscillations/min. The amplitude of oscillations is 2 to 3 mm. The grinding bodies are made of steel or carbide balls, that are 10-20 mm in diameter. The mass of the balls is 8-10 times the charged particles. Final particle size is of the order of 5-100 microns Attrition mill: IN this case, the charge is ground to fine size by the action of a vertical shaft with side arms attached to it. The ball to charge ratio may be 5:1, 10:1, 15:1. This method is more efficient in achieving fine particle size. Rod mills: Horizontal rods are used instead of balls to grind. Granularity of the discharge material is 40-100 mm. The mill speed varies from 12 to 30 rpm. Planetary mill: High energy mill widely used for producing metal, alloy, and composite powders. Fluid energy grinding or Jet milling: The basic principle of fluid energy mill is to induce particles to collide against each other at high velocity, causing them to fracture into fine particles • Multiple collisions enhance the reduction process and therefore, multiple jet arrangements are normally incorporated in the mill design. The fluid used is either air about 0.7 MPa or stream at 2 MPa. In the case of volatile materials, protective atmosphere of nitrogen and carbon-di-oxide is used. • The pressurized fluid is introduced into the grinding zone through specially designed nozzles which convert the applied pressure to kinetic energy. Also materials to be powdered are introduced simultaneously into the turbulent zone. • The velocity of fluid coming out from the nozzles is directly proportional to the square root of the absolute temperature of the fluid entering the nozzle. Hence it is preferable to raise the temperature of fluid to the maximum possible level without affecting the feed material. • If further powdering is required, large size particles are separated from the rest centrifugal forces and re-circulated into the turbulent zone for size reduction. Fine particles are taken to the exit by viscous drag of the exhaust gases to be carried away for collection. • This Jet milling process can create powders of average particle size less than 5 m Machining: Mg, Be, Ag, solder, dental alloy are specifically made by machining; Turning and chips thus formed during machining are subsequently crushed or ground into powders Shotting: Fine stream of molten metal is poured through a vibratory screen into air or protective gas medium. When the molten metals falls through screen, it disintegrates and solidifies as spherical particles. These particles get oxidized. The particles thus obtained depends on pore size of screen, temperature, gas used, frequency of vibration. Metal produced by the method are Cu, Brass, Al, Zn, Sn, Pb, Ni. (this method is like making Boondhi) Graining: Same as shotting except that the falling material through sieve is collected in water; Powders of cadmium, Bismuth, antimony are produced. 2. Physical methods Electrolytic deposition • In this method, the processing conditions are so chosen that metals of high purity are precipitated from aqueous solution on the cathode of an electrolytic cell. This method is mainly used for producing copper, iron powders. This method is also used for producing zinc, tin, nickel, cadmium, antimony, silver, lead, beryllium powders. • Copper powder => Solution containing copper sulphate and sulphuric acid; crude copper as anode • Reaction: at anode: Cu -> Cu+ + e-; at cathode: Cu+ + e- ->Cu • Iron powder => anode is low carbon steel; cathode is stainless steel. The iron powder deposits are subsequently pulverized by milling in hammer mill. The milled powders are annealed in hydrogen atmosphere to make them soft • Mg powder => electrodeposition from a purified magnesium sulphate electrolyte using insoluble lead anodes and stainless steel cathodes • Powders of thorium, tantalum, vanadium => fused salt electrolysis is carried out at a temperature below melting point of the metal. Here deposition will occur in the form of small crystals with dendritic shape. In this method, final deposition occurs in three ways, 1. A hard brittle layer of pure metal which is subsequently milled to obtain powder (eg. iron powder) 2. A soft, spongy substance which is loosely adherent and easily removed by scrubbing 3. A direct powder deposit from the electrolyte that collects at the bottom of the cell Factors promoting powder deposits are, high current density, low metal concentration, pH of the bath, low temperature, high viscosity, circulation of electrolyte to avoid of convection Advantages: Powders of high purity with excellent sinterability Wide range of powder quality can be produced by altering bath composition Disadvantages: Time consuming process; Pollution of work place because of toxic chemicals; Waste disposal is another issue; Cost involved in oxidation of powders and hence they should be washed thoroughly.

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