This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Directional Solidification”.
1. Which of the following materials has superior strength and rigidity at high elevated temperature?
a) Aluminium-based alloys
b) Copper-based alloys
c) Zinc-based alloys
d) Nickel-based alloys
Explanation: In the high temperature technology, the need for materials with superior strength, ductility and rigidity has been growing. Nickel-based alloys are the most widely used materials at the high elevated temperature, and super-alloys of it, are the greatest in the mechanical properties.
2. Directional solidification allows components to be produced from the melt in a single stage process.
Explanation: In the production of composite castings by directional solidification of eutectics, this allows the components are to be produced from the melt in a single stage process, by which there is intrinsic thermodynamic stability and also the chemical compatibility between the matrix and reinforcement of the casted composite component.
3. It is not easy to control the material at the microstructure level in the directional solidification.
Explanation: In directional solidification, material can be controlled over a wide range from well aligned to finely dispersed particles or fibers at microstructure level by the adjusting the various solidification conditions, in which several eutectic types are analyzed as potential materials for high temperature applications.
4. It is not possible to replace super-alloys by the directionally solidified high temperature eutectics.
Explanation: Directional solidification of binary or pseudo-binary eutectics is able to produce well aligned regular structures consisting of fibrous constituents. Such structures may offer a substantial hike in high-temperature strength and creep resistance. So easily, there can be a replacement of super-alloys by the directionally solidified eutectics.
5. Oxidation resistance should be the prime requirement for the high temperature structural applications.
Explanation: A primary requirement for high temperature structural applications is an adequate oxidation resistance at temperature more than 1000 ⁰C. So, attention must be focused on those alloys which have a high concentration of aluminium and silicon, however, these elements produce protective oxide scales of alumina and silica respectively to protect the material.
6. To achieve useful strength of the material at temperatures of 1000-1600 ⁰C, what should be the melting point temperature of the material?
a) 1000 ⁰C
b) 1300 ⁰C
c) 1600 ⁰C
d) More than 1600 ⁰C
Explanation: To achieve useful strengths at the temperatures of 1000-1600 ⁰C, melting points in excess of 1600 ⁰C are required. Most of the materials retain significant strength to approximately 0.5-0.6 of their absolute melting temperature, but for high strength, high melting point temperature is preferred.
7. Which of the following shows the relationship between the growth rate and inter-phase spacing?
a) λ2R = constant
b) λR2 = constant
c) λ/R = constant
d) λ⁴R = constant
Explanation: The relationship between the growth rate (R) and inter-phase spacing (λ) for the eutectic structure is given by: λ2R = constant, where the constant is related to the magnitudes of the liquid-us slopes at the eutectic temperature and the composition difference between the two phases.
8. Which of the following furnaces is used for the directional solidification of cast ingots of eutectic?
a) Pneumatic zone furnace
b) Cupola furnace
c) Hydraulic zone furnace
d) Optical floating zone furnace
Explanation: The drop cast ingots are directionally solidified using an optical floating zone furnace, in which radiation is emitted from a xenon lamp and focused on the specimens enclosed in a quartz tube which is basically evacuated first and back filled with the argon gas. And then drop-cast rods are used as the feed material for the performing of other operations.
9. Which of the following methods is used for the cutting of directionally solidified rods or castings?
a) Thermal cutting
b) Electro-discharge machining
c) Water jet cutting
d) Weld cutting
Explanation: The directionally solidified castings are cut transversely and longitudinally by the application of electro-discharge machining. In this method, material is removed from the work-piece by discharging of current between the electrodes, results in an electric voltage. This method is also known as spark machining.
10. In the process of directional solidification, weight always remains constant for all the materials.
Explanation: According to the conservation of mass, weight remains constant until occurring of any reaction in the process. So, in some cases of materials, weight loss can occur by evaporation when the vapour pressure is very high. For example, vapour pressure of chromium is relatively high, about ten times the vapour pressure of silicon at the processing temperatures.
11. Directional solidification always ends at the sprue part and starts from the end of the casting.
Explanation: Directional solidification is always started from the farthest end of the casting and finishes at the sprue part of the casting system. Another term is the progressive solidification, in which solidification starts from the walls of the casting that is also known as parallel solidification.
12. Progressive solidification is better than the directional solidification and it is independent to the geometry of mould.
Explanation: Solidification is greatly affected by the geometrical shape of the mould cavity; variation in geometry causes the change in heat flow which leads to altering of solidification. In the condition of progressive solidification, there is always a chance of shrinkage defects. So it cannot be better than directional solidification.
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