Gas Atomization The most commonly used method of powder - - PowerPoint PPT Presentation

Gas Atomization

Plasma Atomization

• Relatively new technique that

produces high quality and extremely spherical powders.

• Wire feedstock is fed into a

plasma torch that with the aid

• f gases atomised the powder.
• Size ranges from 0 – 200

micron.

• Limited to alloys that can be

formed into a wire feedstock.

• The most commonly used method of

powder production.

• Elemental feedstock is melted under an air
• r inert gas, or under vacuum.
• The chamber is then backfilled with gas to

force molten alloy through a nozzle where high velocity air, N, He or Ar gas

• impinges onto the flowing melt and breaks

it up.

• Powder is mostly spherical, with some

asymmetric particles and satellites present.

• Heat sizes range from 5kg to 3000kg. Size

ranges from 0 to 500 micron.

• Yield within20-150 micron range varies

from 10 to 50% of total.

• Mostly used for Ni, Co and Fe alloys, also

available for Ti and Al alloys.

• Similar to gas atomisation but

water is employed as the atomising medium.

• Used mostly for unreactive

materials such as steels,.

• It produces irregular shaped
• particles.

Centrifugal atomisation

• A simple process that is not in wide-spread

use.

• A good compromise between Gas Atomised

and Plasma Atomisation.

• Generates powder that is more spherical

and has lower entrapped gas porosity than Gas Atomisation but not to the quality

• f Plasma Atomisation .
• However, is cheaper than Plasma Atomisation.
• Best suited to larger batch sizes of less

reactive low melting temperature alloys, but can also make Nickel superalloys.

• Works with all alloys but is most economic with reactive alloys

like Ti. Feedstock, in the form of bar, is rotated and melted by an induction coil.

• A film of molten metal flows downwards into a gas stream

for atomisation.

• Therefore, material does not come in contact with either

crucible or electrode during process.

• Powder size is 0 to 500 micron and morphologyis similar to gas
• atomised. Process is cheap, clean, good for small batches

and produces small diameter powder.

EIGA (Electrode Induction melting Gas Atomisation)

Atomization process parameters

• Effect of pressure of metal head: r = a + b√h; r – rate of atomization
• 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

• 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.

• Orifice area: negligible effect
• 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)

Effect of Oxygen Content on Powder Size

Summary of Powder particles

Gatorizing Process of Getting Fine Grained Superalloys from PM Ingots

• Gatorizing is an isothermal forging process utilizing slow strain rates to promote superplastic deformation of

very fine-grained material at low flow stresses.

• Near-net-shape forgings can be made with lower input weights and reduced machining costs.
• This process is extensively used in the forging of powder metal superalloys because of the very uniform and fine

grain size achieved in the extruded billet as well as the high cost associated with the PM product.

• However it is not typically used in the forging of cast and wrought processed alloys because they do not offer the

very fine-grained structure needed for the Gatorizing process.

• There had been much work to obtain fine-grained billet during conversion from ingot.
• Studies with alloys such as 718, 901, or A286 succeeded in producing fine-grained billet by using precipitating

phases to restrict grain growth and among these processes, the "delta processed 718 was capable of yielding average grain size of ASTM 11 in the billet

• As-received microstructure of fine-grained

billet

• In the full-size Gatorizing experiment, tooling used to produce a

contoured disk from conventional PM billet was selected for the forging trials.

• The die was about 60 cm in dia. and 8 cm in thickness at the rim.
• A two-step Gatorizing (isothermal forging) process consisting of an

initial flat pancake operation to convert the microstructure to a finer grain size for improved superplasticity in the second forging step was defined.

• The second step was a contoured-forging operation to demonstrate

near-net-shape capability.

• Two step forging processes are routinely used in producing PM

superalloy forgings.

The microstructure following initial pancake forging shown for two forging Conditions at 954oC, 0.3 per min. strain rate

Final Forged 718 Alloy

Hot isostatic pressing

• Ideal method for consolidation of powders of nickel and cobalt base super alloys, tool

steels, maraging steels, titanium alloys, refractory metal powders, cermets.

• HIP is the application of pressure at elevated temperatures to obtain net or near net

shape parts from metal, ceramic, cermet powders.

• HIP unit consists of a pressure vessel, high temperature furnace, pressurizing system,

controls and auxiliary systems (material handling, vacuum pumps, metering pumps).

• The pressure vessel is made of low alloy steel. Its function is to heat the powders while

applying uniform gas pressure on all the sides. Furnaces are of radiation or convection type heating furnaces with graphite or molybdenum heating elements.

• The furnace heats the powder part, while pressurizing medium (a gas) is used to apply a

high pressure during the process. Generally, argon, nitrogen, helium or even air is used as pressurizing medium.

• The pressurizing gas, usually argon, is let into the vessel and then a compressor is used

to increase the pressure to the desired level. The furnace is then started and both temperature and pressure are increased to a required value

Principal of Mechanical alloying Schematic of MA Components

Microstructures of powder-processed Ni-based superalloys containing elevated levels of refractory alloying additions

Investment Casting Process