This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Types of Sintering”.
1. The _______ defines sintering as, “the thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles”.
a) ASME
b) ASTM
c) ISO
d) BIS
View Answer
Explanation: The International Standards Organization (ISO) defines sintering as, “the thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles”. The ASME, ASTM, and BIS are organization which provides us with specific standards for various materials.
2. ________ transforms compacted mechanical bonds between powder particles into metallurgical bonds.
a) Sintering
b) SHP
c) Mechanical alloying
d) Metal Injection Molding
View Answer
Explanation: Sintering is defined as the heating of loose or compacted aggregate of metal powders below the melting point of base metal to transform it to a denser material by interparticle bonding and thus, sintering transforms compacted mechanical bonds between powder particles into metallurgical bonds.
3. In _________, the densification is a result of atomic diffusion in solid-state.
a) Reaction sintering
b) Solid-state sintering
c) Activated sintering
d) Liquid phase sintering
View Answer
Explanation: In solid-state sintering, the densification is a result of atomic diffusion in the solid-state. Solid-state sintering is a very common process of agglomeration for most of the metals and alloys.
4. _________ refers to those processes where solid and liquid coexist during the entire or part of the sintering process.
a) Spark plasma sintering
b) SHS
c) Activated sintering
d) Liquid phase sintering
View Answer
Explanation: Liquid phase sintering, refers to those processes where solid and liquid coexist during the entire or part of the sintering process. It is one of the most common method of achieving rapid densification in the powder compact. This has proved valuable in the production of W-Cu, W-Ag, cemented carbide type, and cermet materials.
5. Doping of Ni in tungsten compacts is an example of _________
a) Activated sintering
b) Reaction sintering
c) Microwave sintering
d) Gas plasma sintering
View Answer
Explanation: The doping of Ni in tungsten compacts is an example of activated sintering. In activated sintering, the addition of a small amount of an alloying element called ‘doping’ increases the rate of densification to a great extent compared to the undoped compacts.
6. Which of the following characteristic the ‘doping’ additive should not possess?
a) Segregate at the grain boundary
b) Formation of low melting point phase
c) High atomic weight
d) High solubility
View Answer
Explanation: To promote activated sintering, any additive must form a low-melting phase during sintering and also must segregate at the grain boundaries. The additive must also have a very high solubility for the base metal to promote the rapid diffusion of base metal through the additive phase.
7. The presence of nickel at grain boundaries greatly increases the grain boundary lattice-diffusion of tungsten.
a) True
b) False
View Answer
Explanation: Nickel, which is added as a solution of its salts, gets reduced during sintering and forms a coating of a few monolayer on the surface of the W particles. By diffusing over the W particles, Ni accumulates at the necks of the particles from where the nickel penetrates the grain boundaries. This presence of nickel at the grain boundaries greatly increases the grain boundaryself-diffusion of tungsten. Thus, the segregation of the additive at the interparticle boundary creates a short circuit diffusion path for the rapid diffusion of the base metal resulting in enhanced densification.
8. _______ takes place when two or more components chemically react during the sintering process to form the final product.
a) Rate controlled sintering
b) Reaction sintering
c) Microwave sintering
d) Spark plasma sintering
View Answer
Explanation: Reaction sintering takes place when two or more components chemically react during the sintering process to form the final product. It is a special form of rapid sintering technique to process high-temperature materials, resulting from the chemical reaction between the individual constituents, giving very good bonding.
9. SiN is prepared by _______
a) Reaction sintering
b) Hot forging
c) Solid-state sintering
d) Liquid phase sintering
View Answer
Explanation: Reaction sintering may also involve heating in a gas atmosphere, which reacts with the powder compact resulting in the formation of the desired compound. A typical example is the production of reaction bonded SiN made by sintering green compact of Si powders in N2 atmosphere at 1,523 to 1,673 K for time up-to 70 hours.
10. _________ has the advantage of producing a finer grain size compared to pressureless sintering.
a) Solid-state sintering
b) Microwave sintering
c) Liquid phase sintering
d) Reaction sintering
View Answer
Explanation: We can obtain a finer grain size in microwave sintering compared to the pressureless sintering because of faster heating rate and shorter dwell time at the selected temperature.
11. _______ is an exothermic combustion process that uses the heat generated during a reaction to sinter the material.
a) SHS
b) Gas plasma sintering
c) Spark plasma sintering
d) Rate controlled sintering
View Answer
Explanation: Self-propagating High-Temperature Synthesis (SHS) is an exothermic combustion process that uses the heat generated during a reaction to sinter the material. The process, although is very fast and economical, needs an additional densification step to make useful shapes and hence adds to cost and time.
12. _______ is a process similar to the hot uniaxial pressing.
a) Solid-state sintering
b) SHS
c) SPS
d) Activated sintering
View Answer
Explanation: Spark Plasma Sintering (SPS) is a process similar to hot uniaxial pressing in which a momentary DC pulse is generated in addition to joule heating, by producing a ‘spark’ between the powder particles. The temperature can reach up-to 5,000°C to 10,000°C, but only momentarily so that no melting is allowed to take place.
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