Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) on “Solidification with Predominant Interface Resistance”.
1. Which of the following is used for the controlling of heat flow in the casting process?
a) Mechanical resistance
b) Biscuit
c) Chaplets
d) Thermal resistance
Answer: d
Clarification: In the casting process, the heat flow is generally controlled by the thermal resistance of the metal-mould interface. These interfaces can be of convex, concave or plane shape, depending on the heat flow rate required for the casting. These processes mainly include permanent mould casting and die casting.
2. Perfect wetting only occurs when there is no contact resistance phenomenon in the casting interface.
a) True
b) False
Answer: a
Clarification: The condition of no contact resistance exists only when the metal-mould contact is so close and tight that causes the occurrence of perfect wetting that is the casting gets soldered to the moulding surface. This causes the sticking problem and it becomes very difficult to extract casting from the mould cavity.
3. Superheating is the only main consideration for the existence of no contact resistance between the metal and mould.
a) True
b) False
Answer: b
Clarification: For no contact resistance between the metal and mould, the solidification process is considered by assuming that the thermal resistance at the interface is of over-riding importance. In this kind of cases, superheating is not considered for the temperature distribution in the casting solidification.
4. Good thermal conductivity of the solidified metal can lower down the surface temperature below than the freezing temperature.
a) True
b) False
Answer: a
Clarification: Good thermal conductivity of the solidified metal can provide considerable thermal resistance. Because of this, the surface temperature of the casting material can be lower down below than the freezing temperature of the casting, which results in the formation of sub-cooled solidified casting.
5. Surface irregularities of the solidifying casting can increase the temperature at the interface.
a) True
b) False
Answer: b
Clarification: The surface irregularities of the solidifying casting results in irregular contacts to establish between the die wall and the casting produced. This always results in a temperature drop across the interface. The extent of temperature drop or the degree of resistance mainly depends on the contact area, physical and thermal properties of the materials in contact.
6. The heat transfer through the interface of metal and mould is always considered as of convection mode.
a) True
b) False
Answer: b
Clarification: When the air gap forms at the interface, the heat transfer across the interface drops rapidly. The mode of heat transfer through the gap is considered to be due to both conduction and radiation, conduction is more dominant mode at the lower temperatures while radiation depends upon the surface temperatures and emissivities.
7. Conduction mode of heat transfer mainly depends upon the conductivity of the medium and the size of a gap at the interface.
a) True
b) False
Answer: a
Clarification: Conduction heat transfer depends on the conductivity of the medium and the gap size as well. Thus, the correct modeling of the heat transfer across the interface depends upon many factors like interface temperatures, surface characteristics, thermo-physical properties of the gap medium including coating material etc.
8. A gap of finite thickness at the interface of mould and metal always increases the heat flux.
a) True
b) False
Answer: b
Clarification: A gap of finite thickness separating the mould and casting interface always results in lowering of heat flux. It is the most important phenomena for controlling the solidification of the casting in the metallic mould. The time of its formation and distribution affects the temperature distribution in the casting.
9. For the complex geometries of the casting, the calculation is more difficult to obtain the air gap at the interface.
a) True
b) False
Answer: a
Clarification: In the process of casting different materials, due to complexities of material behavior, internal liquid metal pressure and the mould geometry make the calculation more difficult to obtain the air gap at the interface. To solve this, proper care is to take during the process of casting products.
10. The magnitude of the air gap at the interface of casting and mould continuously decreases with time.
a) True
b) False
Answer: b
Clarification: The constant air gap width at the interface cannot be assumed for simulation. The magnitude of the air gap continuously increases with time. The time of start of the air gap at the corner of the casting is always less than that for the middle, and the air gap never starts simultaneously at all points in the casting.