This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Types of Variables in Sintering”.
1. Which of following is the correct set of material variables in sintering?
a) Particle structure and sintering atmosphere
b) Particle composition, particle shape, green density and sintering temperature
c) Green density and sintering time
d) Green density and particle size
Explanation: The major variables in sintering include process variables and material variables. The process variables include sintering atmosphere, sintering time and sintering temperature whereas the material variables include particle composition, particle shape, particle size, particle structure, and green density.
2. ________ particle size leads to increased sintering.
Explanation: Finer particle size leads to increased sintering. Similar-sized particles have a greater pore/solid interfacial area producing a greater driving force for sintering. Small grain size promotes grain boundary diffusion and a large interparticle contact area to volume diffusion.
3. A _______ in green density signifies a greater driving force for sintering.
Explanation: The decrease in green density signifies an increasing amount of internal surface area. Although the % change in density ↑ with decreasing green density, the absolute value of the sintered density remains highest for the higher green density material.
4. Heterogenous nature of powders improve sintering kinetics.
Explanation: Heterogenous nature of powders (e.g. alloy powders or powder mix) promotes enhanced diffusion and improves sintering kinetics. Dispersed phases within the matrix may promote sintering by inhibiting grain boundary motion.
5. ________ sphericity and ________ macro or micro-surface roughness promote sintering.
a) Decreasing, decreasing
b) Increasing, decreasing
c) Decreasing, increasing
d) Increasing, increasing
Explanation: Any factor that lead to greater and intimate contact between particles and increased internal surface area promote sintering and these factors include decreasing sphericity and increasing macro or micro-surface roughness.
6. The degree of sintering _______ with increasing time, though the effect is small in comparison to the effect of temperature.
Explanation: The degree of sintering increases with increasing time, though the effect is small in comparison to the effect of temperature. The loss of driving force with increasing time at any temperature is one of the reasons why it is so difficult to remove all porosity by sintering. Longer times may also result in grain coarsening, which affects the mechanical properties adversely.
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