This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Solidification of Ingots and Castings”.
1. Which among the following is not a zone in solidified alloy ingots?
a) Columnar zone
b) Outer chill zone
c) Central equiaxed zone
d) Coverage zone
View Answer
Explanation: (i) An outer chill zone of equiaxed crystals, (ii) A columnar zone of elongated or column-like grains, and (iii) A central equiaxed zone. If you take in general these are the three different zones that can be distinguished in solidified alloy ingots.
2. Most engineering alloys begin by being poured or cast into a fireproof container or mould. If the as-cast pieces are permitted to retain their shape afterwards, or are reshaped by machining, they are called as _______
a) Metallurgy
b) Extrusion
c) Casting
d) Ingots
View Answer
Explanation: If the as-cast pieces are permitted to retain their shape afterwards, or are reshaped by machining, they are called castings. If they are later to be worked, e.g. by rolling, extrusion or forging, the pieces are called ingots, or blanks if they are relatively small.
3. In the chill zone of ingots, if the pouring temperature is low the whole of the liquid will be rapidly cooled below the liquidus temperature and the crystals swept into the melt may be able to continue to grow. This is known as______
a) Big Bang nucleation
b) Rapid quenching
c) Big Boss nucleation
d) Casting prep
View Answer
Explanation: The above mentioned is known as ‘big-bang’ nucleation since the liquid is immediately filled with a myriad of crystals. This produces an entirely equiaxed ingot structure, i.e. no columnar zone forms.
4. The chill zone grow dendritically in certain crystallographic directions. In the case of cubic metals it is given as______
a) <111>
b) <100>
c) <222>
d) <101>
View Answer
Explanation: Very soon after pouring the temperature gradient at the mould walls decreases and the crystals in the chill zone grow dendritically in certain crystallographic directions, e .g. <100> in the case of cubic metals. Those crystals with a <100> direction close to the direction of heat flows grow fastest and are able to outgrow less favourably oriented neighbours.
5. What happens to most metals on solidification?
a) Shrink
b) Expand
c) Elongate
d) No change
View Answer
Explanation: In general most of the metals shrink on solidification and these affects final ingot structure. There is a decrease in the level of the liquid remaining and this is a consequence of thicker outer shell of solid and with a narrow freezing range the mushy zone is also narrow.
6. The equiaxed zone consists of equiaxed grains. They are oriented randomly in which part the ingot?
a) Bottom edge
b) Centre
c) Right side
d) Top corner
View Answer
Explanation: If we take the equiaxed zone, the center of the ingot is found to contain the equiaxed grains which are just randomly oriented. The melted-off dendrite side-arms are assumed to be the origin of these grains.
7. There is approximately no temperature gradient perpendicular to the growth direction for a ______
a) Multidirectional solidification
b) Unidirectional solidification
c) Mixed solidification
d) Partial solidification
View Answer
Explanation: If we take the temperature gradient perpendicular to the growth direction it turn out to be zero in case of unidirectional solidification. The spacing in the dendrite arm is probably the one which reduces constitutional super cooling in the intervening liquid to a very low level.
8. If the lamellar distance k tends to infinity, the free energy change associated with the solidification of 1 mol of liquid is given by_____
a) ΔG(k) = ΔG(infinity) + 2Vm/k
b) ΔG(k) = ΔG(infinity)
c) ΔG(k) = ΔG(infinity) + c
d) ΔG(k) = ΔG(infinity) – 2Vm/k
View Answer
Explanation: The free energy change associated with the solidification of 1 mol of liquid is given by, ΔG(k) = ΔG(infinity) + 2Vmγ/k. where Vm is the molar volume of the eutectic and ΔG(infinity) is the free energy decrease for very large values of k. Since here k->infinity, the second term disappears.
9. Which among the following can be the reason for the disappearance of the primary dendrites?
a) Growth rate is raised above critical level
b) Under slow growth rate
c) Very minute temperature gradient
d) Forced Growth rate
View Answer
Explanation: If the growth rate is raised above a critical level this happens. The reason for the disappearance of the primary dendrites is that for a given growth velocity the eutectic is able to grow at a higher temperature than the dendrite tips.
10. In practice the maximum casting speed and billet cross-section are less for steel than for aluminium or copper.
a) True
b) False
View Answer
Explanation: In case of continuous casting, the isotherm distributions are affected by the conductivity of the solidifying metal and its speed of withdrawal from the mould. This means, for example, that the depth of the liquid pool in continuous casting is much greater for steel than for aluminium alloys under comparable conditions. This implies that in practice the maximum casting speed and billet cross-section are less for steel than for aluminium or copper.
11. In alloys with a wide freezing range the mushy zone can occupy the whole of the ingot. In this case no central pipe is formed.
a) True
b) False
View Answer
Explanation: It is commonly found in the alloys with a wide freezing range that ingot is completely occupied by the mushy zone hence there is no chance for the formation of central pipe. As a result in order to compensate for the shrinkage of the dendrites the liquid level gradually falls across the width of the ingot as liquid flows down.
12. What are the two different types of segregation that can be distinguished in solidified structures?
a) Competent and non-competent
b) Semi and complete
c) Macro and micro
d) Full and partial
View Answer
Explanation: Generally, there are two types of segregations which can be distinguished in the solidified structures. The first one is micro, that occurs on the scale of the secondary dendrite arm spacing and the second done is the macro which changes composition over comparable distance.
13. Which among the following is not an important variable in weld solidification or continuous casting?
a) Thickness of plate being welded
b) The rate of heat input
c) Speed of arc movement
d) External condition
View Answer
Explanation: Except the external conditions rest everything plays an important role in the weld solidification and continuous casting even the thermal conductivity of the metal being welded also plays a crucial role in the weld solidification.
14. Which among the following is not a factor that leads to macro-segregation in ingots?
a) Shrinkage due to solidification and thermal contraction
b) Density differences in the interdendritic liquid
c) Density differences between the solid and liquid
d) Expansion due to mild heating
View Answer
Explanation: (i) Shrinkage due to solidification and thermal contraction; (ii) density differences in the interdendritic liquid; (iii) density differences between the solid and liquid; and (iv) convection currents driven by temperature-induced density differences in the liquid. These are four factors that lead to macro segregation in ingots.
15. Shrinkage effects can give rise to what is known as inverse segregation. The effect is particularly marked in alloys with a_______
a) Wide freezing range
b) Short freezing range
c) Wide melting range
d) Short melting range
View Answer
Explanation: The effect is particularly marked in alloys with a wide freezing range. As the columnar dendrites thicken solute-rich liquid (assuming (partition coefficient),k < 1) must flow back between the dendrites to compensate for shrinkage and this raises the solute content of the outer parts of the ingot relative to the centre. Example Cu-Sn alloy.
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