Advanced Powder Metallurgy Processing Techniques Questions and Answers – Mechanism and Process Parameters

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This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Mechanism and Process Parameters”.

1. Which force fragments the liquid into ligaments in atomization process?
a) Tensile force
b) Shear force
c) Compressive force
d) Torque

Explanation: The interaction between the jets and the liquid metal stream begins with the creation of small disturbances at liquid surface, which grow into shearing forces that fragment the liquid into ligaments. The energy of the impinging jet is so high that the ligaments further disintegrate into droplets of very fine size.

2. Under which set of conditions is the irregular particle shape obtained?
a) Lower surface tension
b) Lower surface tension and low cooling rates
c) Higher surface tension and low cooling rates
d) Lower surface tension and high cooling rates

Explanation: Lower surface tension of the molten metal and high-cooling rate favor the formation of irregular shaped particles, as in water atomization (same set of conditions prevail here).

3. Under which set of conditions is the spherical particle shape obtained?
a) Lower surface tension
b) Lower surface tension and low cooling rates
c) Higher surface tension and low cooling rates
d) Lower surface tension and high cooling rates

Explanation: Higher surface tension of the molten metal and low-cooling rate favor the formation of spherically-shaped particles, as in inert gas atomization (same set of conditions prevail here).

4. The velocity of the liquid metal stream is given by the equation _________
a) v=A$$\sqrt{2g(P_g-P_i)\rho}$$
b) v=A$$\sqrt{2g(P_i-P_g)\rho}$$
c) v=A$$\sqrt[3]{2g(P_i-P_g)\rho}$$
d) v=A$$\sqrt[3]{2g(P_g-P_i)\rho}$$

Explanation: The liquid metal stream has a velocity v, given by the equation=A$$\sqrt{2g(P_i-P_g)\rho}$$ where A is a geometric constant, g is the acceleration due to gravity, Pi is the injection pressure of the liquid metal, Pg is the pressure of the atomizing medium, and ρ is the density of the liquid.

5. The shearing forces involved in atomization depends on the ________
a) Reynolds number
b) Temperature
c) Pressure
d) Biot’s number

Explanation: The shearing forces that lead to the formation of ligaments from the liquid metal depends on the Reynolds number, which in turn is related to size and velocity of the stream, density, and viscosity of the liquid metal.

6. Which of the following is a process parameter for atomization process?
a) Orifice volume
b) Pressure of atomizing medium
c) Temperature of atomizing medium
d) Atomizing medium properties

Explanation: The major process parameters for atomization process are- pressure of metal head, pressure of atomizing medium, metal temperature, orifice area, and molten metal properties.

7. The effect of pressure of metal head is given ________
a) r=a+b$$\sqrt[3]{h}$$
b) r=a+b$$\sqrt{h}$$
c) r=a-b$$\sqrt[3]{h}$$
d) r=a-b$$\sqrt{h}$$

Explanation: The effect of the pressure of the metal head on the rate of atomization is given by the relationship r=a+b$$\sqrt{h}$$ where r is the rate of atomization, a and b are the constants, and h is the metal head.

8. The fraction of coarse powder will increase as the quantity of metal available for atomization increases.
a) True
b) False

Explanation: Generally, the energy required for atomization depends on the final particle size desired. Since the energy available for disintegration is a constant, any increase in the quantity of the metal available for atomization will result in a decrease in the available energy per unit weight of metal. Thus, the fractions of coarse powders will increase.

9. Which of the following set of conditions leads to maximum fineness?
a) Increase in air pressure and increase in flow of metal
b) Decrease in air pressure and increase in flow of metal
c) Increase in air pressure and decrease in flow of metal
d) Decrease in air pressure and decrease in flow of metal

Explanation: The increase in air pressure increases the fineness, which is possible only when an increase in air pressure (which increases the available energy and hence the fineness of the particles) and the increase in the flow of metal (which causes decrease in available energy) balance each other, thereby producing maximum fineness.

10. The effect of pressure of atomizing medium is given ________
a) r=a+b$$\sqrt[3]{p}$$
b) r=a-b$$\sqrt{p}$$
c) r=a-b$$\sqrt[3]{h}$$
d) r=a+b$$\sqrt{p}$$

Explanation: The effect of the pressure of the atomizing medium on the rate of atomization is given by the relationship r=a+b$$\sqrt{p}$$ where r is the rate of atomization, a and b are the constants, and p is the air pressure.

11. The rate of atomization ______ with increase in temperature.
a) Increases
b) Decreases
c) Zero
d) Constant

Explanation: The rate of atomization declines steadily with increase in temperature. This is because the volume of the metal flowing through the nozzle is important and not the mass of the metal in the case of annular nozzles. Thus, with increase in temperature even though the volume of metal flowing through the nozzle remains constant, the mass of the metal will be less compared to that at lower temperatures.

12. What is the effect of increased temperature on the particle size?
a) Independent
b) Decreases
c) Increases
d) Increases first and then decreases

Explanation: When the temperature of the particle size increases, the surfacetension forces as well as the viscosity decreases. So that the available energy can more methodically break apart the metal stream producing fine particle sizes than at lower temperature.

13. The particle shape of water atomized copper becomes more irregular upon addition of _____
a) Silicon
b) Lithium
c) Tantalum
d) Potassium

Explanation: The additions which lower surface tension of the liquid metal can promote irregular particle shapes. Thus, the particle shape of water-atomized copper powder becomes more irregular by addition of small amounts of elements such as Mg, Ti, Li or Ca.

14. The rate of atomization and orifice area have a _______ relation.
a) Logarithmic
b) Inverse
c) Exponential
d) Linear

Explanation: The rate of atomization varies linearly with the orifice area. The only exception is for low-orifice diameter where wall friction reduces the velocity of liquid flowing through the orifice.

15. Which of the following depicts the stages of molten metal disintegration?
a) Wave formation → ligament formation → powder
b) Wave formation → ligament formation → fine droplets → breakdown to smaller particles → powder
c) Wave formation → ligament formation → fine droplets → breakdown to smaller particles → collision & coalescence of particles
d) Wave formation → ligament formation → fine droplets → collision & coalescence of particles

Explanation: The disintegration of molten metal stream consists of five stages:
i) Wave formation
ii) Ligament formation
iii) Fine droplets
iv) Breakdown to smaller particle
v) Collision & coalescence of particles.

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