Plasma Atomization Gas Atomization • The most commonly used method of powder production. • Relatively new technique that • Similar to gas atomisation but • Elemental feedstock is melted under an air produces high quality and water is employed as the or inert gas, or under vacuum. extremely spherical powders. atomising medium. • The chamber is then backfilled with gas to • Wire feedstock is fed into a • Used mostly for unreactive force molten alloy through a nozzle where plasma torch that with the aid materials such as steels,. high velocity air, N, He or Ar gas of gases atomised the powder. • impinges onto the flowing melt and breaks • Size ranges from 0 – 200 • It produces irregular shaped it up. micron. • particles. • Powder is mostly spherical, with some • Limited to alloys that can be asymmetric particles and satellites present. formed into a wire feedstock. • 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.
EIGA (Electrode Induction melting Gas Atomisation) Centrifugal atomisation • A simple process that is not in wide-spread use. • A good compromise between Gas Atomised • Works with all alloys but is most economic with reactive alloys and Plasma Atomisation. like Ti. Feedstock, in the form of bar, is rotated and melted • Generates powder that is more spherical by an induction coil. and has lower entrapped gas porosity • A film of molten metal flows downwards into a gas stream than Gas Atomisation but not to the quality for atomisation. of Plasma Atomisation . • Therefore, material does not come in contact with either • However, is cheaper than Plasma Atomisation. crucible or electrode during process. • Best suited to larger batch sizes of less • Powder size is 0 to 500 micron and morphologyis similar to gas reactive low melting temperature alloys, atomised. Process is cheap, clean, good for small batches but can also make Nickel superalloys . and produces small diameter powder .
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 954 o C, 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
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