Powder Metallurgy Question Paper on “Advantages and Atomization Unit – 2”.
1. The ________ and __________ influence the shape and size of the atomized particles.
a) Cooling conditions, surface energy
b) Cooling conditions, pressure
c) Cooling conditions, surface tension forces
d) Surface tension, volume
Answer: c
Clarification: The prevailing cooling conditions and surface tension forces on the surface of the droplets influence the shape and size of the atomized particles. Thus, fine particles are formed slightly below the point where the fluid jet strikes the liquid.
2. Which of the following factors control the average particle size, for a given nozzle design?
a) Pressure of the atomizing medium
b) Reflex angle between the axes of the gas jets
c) Pressure of the atomizing medium, reflex angle between the axes of the gas jets
d) Pressure of the atomizing medium, apex angle between the axes of the gas jets
Answer: d
Clarification: The average particle size of a nozzle design is controlled by the pressure of the atomizing medium as well as by the apex angle between the axes of the gas jets. Higher apex angles lead to smaller particle sizes.
3. The results of designs used for free fall atomization are difficult to reproduce by _________
a) True
b) False
Answer: a
Clarification: The apex angle for the water atomization is smaller than that for gas atomization. Thus, the results used for free-fall atomization for making spherical and fine powders are difficult to reproduce with water atomization. Moreover, water atomization is also limited to metal or alloys from which the surface oxide can be easily removed by mechanical or chemical method.
4. Which of the following are the types of nozzles used in atomization process?
a) Annular type
b) Discrete jet
c) Annular and discrete jet type
d) Concentric type
Answer: c
Clarification: The nozzles that are used in atomization process (both water and gas) are either annular type (around the metal stream) or discrete jet type. Nozzles are available in ‘free-fall’ or ‘confined’ designs.
5. _______ type of nozzle is used in water atomization.
a) Concentric
b) Free-fall
c) Multiple-jet
d) Annular
Answer: b
Clarification: The free-fall type of nozzle is used in water atomization. In free-fall atomization, the liquid metal issuing from the tundish falls freely under gravity to a certain distance and then comes into contact with the atomization medium and gets disintegrated.
6. ________ type of nozzle design is used frequently in gas atomization.
a) Confined
b) Free-fall
c) Multiple-jet
d) Concentric
Answer: a
Clarification: In confined design used with annular gas jets, atomization occurs at the exit of the tundish nozzle. This design is used in gas atomization systems. Confined atomization has higher efficiency however, ‘freeze-up’ of metal in the nozzle may occur if the design is not proper.
7. ________ gas is produced from fractional distillation of natural gas.
a) Nitrogen
b) Argon
c) Helium
d) Boron
Answer: c
Clarification: The helium gas is produced from fractional distillation of natural gas and it is the most expensive gas too. The nitrogen gas and the argon gas both are produced from the fractional distillation of air.
8. Oil atomization is not suitable for __________
a) bearing steel
b) high speed steel
c) high carbon steel
d) low-carbon steel
Answer: d
Clarification: Oil atomization is suitable for producing powders of high-carbon steel, high-speed steels, bearing steels, and for steels containing high quantities of carbide forming elements such as Cr and Mo. But it is unsuitable for producing powders of low carbon steels. A low temperature heat treatment in a protective atmosphere is necessary to remove the excess oil from the powders.
9. __________ is successfully used as an atomizing medium in place of water and gas.
a) air
b) synthetic oil
c) nitrogen
d) argon
Answer: b
Clarification: Synthetic oils have been successfully used as an atomizing medium in place of water or gas. Use of oil combines the advantages of a high-quenching rate and lower oxygen content as compared to water-atomized particles.
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