This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Mechanism of Solidification”.
1. Which of the following characteristics is mainly considered for the solidification of castings?
a) Appearance
b) Crystal structure
c) Moulding capacity
d) Surface finish
View Answer
Explanation: A clear understanding of the mechanism of solidification and cooling of molten metal and alloys is very essential for the production of successful castings. During solidification, crystal structure of the casting material is mainly considered along with its composition at different parts of the casting.
2. A proper care should be taken while cooling or solidification of castings to avoid defect formation.
a) True
b) False
View Answer
Explanation: A proper care is must for the solidification of castings. Unless a proper care is taken, defects like shrinkage cavity, hot tear, misrun and cold shuts are occurred in the castings. To avoid defects, a proper technique should be implemented with clear understanding of material used for the formation of castings.
3. In the case of pure metals, liquid metal should be cooled above their freezing points.
a) True
b) False
View Answer
Explanation: Liquid metal always needs to be cooled below their freezing points before the solidification starts. This is because the energy is required to create surfaces for new crystals and the degree of super-cooling necessary is also reduced by the presence of particles which serve as the initial nuclei for crystal growth.
4. Which of the following types of crystal are formed near the mould face during solidification?
a) Big crystals with random orientation
b) Small crystals with systematic orientation
c) Small crystals with random orientation
d) Big crystals with systematic orientation
View Answer
Explanation: When a liquid metal is poured into a mould, the mould face itself acts as the nucleus for crystal growth, and when the conductivity of the mould is high, that causes the formations of randomly oriented small crystals which grow near the mould surface. Subsequently, a temperature gradient results within the casting.
5. As the solidification progresses gradually inwards, small crystals with systematic orientation are formed.
a) True
b) False
View Answer
Explanation: After the starting of solidification of casting material, temperature gradient is set up and by progressing inwards, long columnar crystals are formed with their axes perpendicular to the mould face. This orientation of crystal growth is basically desirable from the point of strength of the castings.
6. An alloy, unlike a pure metal, does not have a sharply defined freezing temperature.
a) True
b) False
View Answer
Explanation: An alloy, unlike a pure metal, does not have a sharply defined freezing temperature. Due to this, the solidification of an alloy takes place over a range of temperature. This makes the separate out of the solids parts at different temperatures which possess varying compositions in the material.
7. Which of the following factors is independent to the direction of crystal growth in an alloy?
a) Thermal gradient
b) Composition gradient
c) Variation of solidus temperature
d) Material aesthetics
View Answer
Explanation: The direction of crystal growth in an alloy depends on many factors, such as the thermal gradient within the mould, the variation of solidus temperature with composition and the composition gradient within the casting. And material aesthetics basically deals with the appearance of the casting, which is independent to the direction of crystal growth in an alloy.
8. The difference in the progress of crystallization at different points in the casting material causes fast and abrupt growth of crystals.
a) True
b) False
View Answer
Explanation: The difference in the progress of crystallization at the different points in the casting material does not make fast growth of crystals, even this makes the hampering of crystal growth which basically restricts or retards the columnar growth of crystals starting from the moulding surface during casting process.
9. A dendritic crystal growth structure is formed by the difference in progress of crystallization at different points in the casting material.
a) True
b) False
View Answer
Explanation: A dendritic crystal growth structure is produced when there is difference in the progress of crystallization that shows completion of crystallization at any point before with respect to another point. This is mainly caused by very small thermal gradient with a very high concentration difference. Dendritic structure formed is looks like as shown in below figure.
10. The presence of solid crystals at solid-liquid interface makes feeding of molten metal more easily.
a) True
b) False
View Answer
Explanation: The presence of solid crystals ahead of the solid-liquid interface does not make easy feeding, even this makes more difficult to feed the liquid metal. This basically implies a greater risk of having voids within the casting which is normally referred to as centre-line shrinkage defect.
11. Which of the following parts can be used to have uniform crystallization at all over the points in the casting material?
a) Chaplets
b) Biscuit
c) Chills
d) Core
View Answer
Explanation: To avoid the problem of difference in progress of crystallization, a large thermal gradient within the mould can be produced by having chills in the casting system. Chills generally are cooled metal block with the high thermal conductivity; this is normally placed at the mould’s end.
12. Rapid heat extraction from the casting causes narrowing of liquid-solid zone which sweeps itself across the molten metal.
a) True
b) False
View Answer
Explanation: In solidification of casting material, it is commonly starts from the centre-line of the mould before the solidification is complete even at the mould face. In the chilled mould, due to the rapid heat extraction from the casting, a narrow like liquid-solid zone sweeps across the molten metal.
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