This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Sintering – Infiltration”.
1. ______ process involves preparing a porous metallic body and filing its pores with a molten metal having a lower melting point.
a) Liquid phase sintering
Explanation: Infiltration process involves preparing a porous metallic body or skeleton metal with high melting point and subsequently filing its pores with a molten metal having a lower melting point.
2. A continuous network of infiltrant metal is a must.
Explanation: A continuous network of a ductile infiltrant metal is must for the process of infiltration to be fruitful because it gives the composite much greater resistance to thermal and mechanical stresses.
3. How many stages occur in the process of infiltration?
Explanation: Generally, two stages occur in the process of infiltration: (1) infiltration proper- filling of pores of the skeleton with the molten infiltrant metal by capillary action; (2) structural changes occur during the soaking period.
4. Which of the following is not a characteristic of infiltrant?
a) Melting point lower than the skeleton metal
b) High surface tension
c) Good or perfect wetting
d) Must not alloy or react with the skeleton metal
Explanation: The essential requirements in choosing a suitable infiltrant are: (1) Its melting point must be lower than the skeleton metal; (2) Low surface tension so as to fill small interstices; (3) There should be ‘good or perfect wetting’ of the skeleton (solid) by the infiltrant (liquid); and (4) It must not alloy or react with the skeleton metal.
5. For infiltration, the porosity in the skeleton should be ________
Explanation: The porosity in the porous skeleton should be interconnected since the infiltrant cannot penetrate in closed-off pores in the skeleton and the skeleton material should be insoluble in the liquid infiltrant.
6. __________ are among the most important P/M parts produced by infiltration.
a) Electrical contacts
c) Sintered carbides
d) Porous P/M parts
Explanation: Electrical contacts are among the most important P/M parts produced by infiltration. They are most commonly compound materials with a tungsten skeleton infiltrated with copper, which are also used as resistance welding electrodes. Combinations of W-Ag, Mo-Ag are other examples of infiltrated materials.
7. Which of the following changes does not improve the process of infiltration?
a) Lowering the interfacial tension between skeleton and infiltrant
b) Removing dissolved or adsorbed elements in the skeleton
c) Increasing surface energy of the infiltrant
d) Increasing the interfacial tension between skeleton and infiltrant
Explanation: It has been observed that the process of infiltration can be improved by the addition of properly selected detergents by removing dissolved or adsorbed elements in the skeleton, by increasing the surface energy of the infiltrant and by lowering the interfacial tension between skeleton and infiltrant.
8. A _____ wetting angle promotes infiltration.
Explanation: The dihedral angle or wetting angle at the intersection of a boundary of two grains of the solid phase and the liquid phase plays an important role in infiltration. A low wetting angle, therefore, promotes infiltration (i.e. ready wetting of the solid by the liquid). With a dihedral angle= 0°, the infiltrant will penetrate along the grain boundaries and thus destroy the skeleton. Moreover, large dihedral angles do not lead to effective wetting.
9. The infiltration process is used to prepare ____ kinds of material.
a) Single phase materials
b) Duplex materials
c) Single phase, and duplex materials
d) Multicomponent materials
Explanation: The process of infiltration is used to prepare two kinds of material. (1) single phase material which are rarely made and (2) Duplex materials, which are of further two types: (I) consists of those metals which exhibit complete insolubility at room temperature and infiltration temperature (e.g. W-Ag, Mo-Cu) , and (II) consists of those exhibiting partial solubility of skeleton metal in the infiltrant metal at infiltration temperature (e.g. Cu- infiltrated Fe).
10. The process of infiltration can be depicted by the following diagram.
Explanation: The figure truly represents the process of infiltration, (a) sintered part with an infiltrant slug on top, (b) infiltration in progress, and (c) infiltrated part.
11. Erosion of the contact surfaces of the skeleton metal can be reduced by ______ method.
a) Bridge method
b) Oil method
c) Flyover method
d) Compensation method
Explanation: Erosion of the contact surfaces of the skeleton metal can be reduced by bridge method in which a porous metal part is known as “bridge” is kept between the skeleton and the infiltrant. The infiltrant saturates the skeleton metal after crossing the bridge.
12. Why alloy infiltrant is used for the production of copper-infiltrated ferrous base materials?
b) Reduce erosion of contact surfaces of the skeleton
c) Material saving
d) Variety production
Explanation: In infiltration erosion of contact surfaces of the skeleton metal takes place because of the dissolution of the surface layers of the skeleton metal in molten infiltrant thereby causing an undesirable pitting on the surface of the sintered component. This can be avoided by alloying the infiltrant with one or more elements in which the skeleton metal does not dissolve. Thus, for production of copper-infiltrated ferrous base materials, alloy infiltrant consists of Cu containing 5% Mn and 5%Fe is used.
13. Why infiltrated pump cylinder blocks are popular in use?
a) Mechanical strength
b) Resistance to thermal stress
c) Pressure- tightness
Explanation: The infiltrated parts become impermeable due to the absence of interconnected porosity and hence they can be used where pressure tightness is desired. One such example is the infiltrated pump cylinder blocks.
Sanfoundry Global Education & Learning Series – Powder Metallurgy.
To practice all areas of Powder Metallurgy, here is complete set of 1000+ Multiple Choice Questions and Answers.