This set of Powder Metallurgy written test Questions & Answers focuses on “Characteristics of Atomized Product”.
1. Which of the following factors affect the particle size of atomized powder?
a) Nozzle configuration
b) Atomizing medium and nozzle configuration
c) Properties of metal, atomizing medium and nozzle configuration
d) Atomizing medium
View Answer
Explanation: The factors which affect the particle size of atomized powder include- (1) Metal properties (temperature, viscosity, and feed rate). (2) Properties of atomizing medium (medium pressure and volume). (3) Nozzle configurations (jet angle and jet length).
2. The empirical equation for average particle size (d) in μm is ______
a) d=\(A\frac{\sqrt{\sigma}}{v\sqrt{\rho}}+B\Big(\frac{\sqrt{\mu}}{\sigma \rho}\Big)^{0.45}(1000\frac{f_1}{f_a})^{1.5}\)
b) d=\(A\frac{\sqrt{\sigma}}{v\sqrt{\rho}}+B\Big(\frac{\sqrt{\mu}}{\sigma \rho}\Big)^{1.5}(1000\frac{f_1}{f_a})^{0.45}\)
c) d=\(A\frac{\sqrt{\sigma}}{v\sqrt{\rho}}+B\Big(\frac{\sqrt{\mu}}{\sigma \rho}\Big)^{0.45}(1000\frac{f_a}{f_1})^{1.5}\)
d) d=\(A\frac{\sqrt{\sigma}}{v\sqrt{\rho}}+B\Big(\frac{\sqrt{\mu}}{\sigma \rho}\Big)^{0.45}(10000\frac{f_1}{f_a})^{1.5}\)
View Answer
Explanation: The empirical relationship for calculating the average particle size (d) in μm is given by d=\(A\frac{\sqrt{\sigma}}{v\sqrt{\rho}}+B\Big(\frac{\sqrt{\mu}}{\sigma \rho}\Big)^{0.45}(1000\frac{f_1}{f_a})^{1.5}\) where ρ is the density of the liquid, σ is the surface tension of the liquid, μ is the coefficient of viscosity of liquid, ν is the relative velocity between air and liquid, f1 is the volumetric flowrate of liquid, fa is the volumetric flowrate of air, and A and B are constants.
3. Weber number is given by ______
a) W=\(\frac{V^2}{\sigma D\rho}\)
b) W=\(\frac{V^3D}{\sigma \rho}\)
c) W=\(\frac{V^2D}{\sigma \rho}\)
d) W=\(\frac{V^2\rho}{\sigma D}\)
View Answer
Explanation: The Weber number is dependent on the density and surface tension of the liquid metal, the velocity of the atomizing medium, and the melt stream diameter. It is given by the equation W=\(\frac{V^2D}{\sigma \rho}\) where ρ is the density of liquid in g/cm3, σ is the surface tension of the liquid in N/m, and V is the velocity of the atomizing medium at the point of impact with the metal stream in m/s.
4. The empirical relation given by Lubanska for particle size is _______
a) dav=D\(((\frac{\eta_e}{W_{\eta_g}})(1+\frac{f_1}{f_a}))^{0.5}\)
b) dav=kD\(((\frac{\eta_e}{W_{\eta_g}})(1+\frac{f_1}{f_a}))^{0.5}\)
c) dav=kD\(((\frac{\eta_e}{W_{\eta_g}})(1+\frac{f_1}{f_a}))^{1.5}\)
d) dav=kD\(((\frac{\eta_e}{W_{\eta_g}})(1+\frac{f_a}{f_1}))^{0.5}\)
View Answer
Explanation: The empirical relation developed by Lubanska for particle size is given by dav=kD\(((\frac{\eta_e}{W_{\eta_g}})(1+\frac{f_1}{f_a}))^{0.5}\) where dav is the mean particle size in mm, k is a constant, D is the diameter of the nozzle in mm, f1 is the mass flow rate of the metal stream in kg/min, fa is the mass flow rate of the atomizing medium in kg/min, ηe is the kinematic viscosity of the metal stream in m2/s, ηg is the kinematic viscosity of the atomizing medium in m2/s, and W is the Weber number of the metal stream.
5. The particle size decreases with _______
a) increasing surface tension of liquid
b) decreasing velocity of atomizing medium
c) decreasing surface tension of liquid
d) constant velocity of atomizing medium
View Answer
Explanation: The particle size of atomized powder particles decreases with (1) increasing velocity of the atomizing medium, and (2) decreasing surface tension of the liquid metal.
6. ______ apex angles lead to fine particles.
a) 89°
b) 0°
c) Lower
d) Higher
View Answer
Explanation: For a specific nozzle design, the average particle size is controlled by the pressure of the atomizing medium as well as the apex angle of the gas jets, and thus with higher apex angles we obtain fine particle size.
7. ______ is obtained in water atomization compared to gas atomization.
a) Coarse powder
b) Fine powder
c) Ultra coarse powder
d) Spherical powder
View Answer
Explanation: Particle shape of powders produced by atomization is influenced by the factor that determine the cooling rate. Thus, the cooling rates are higher in magnitude in the case of water atomization than in gas atomization which results in finer powder in case of water atomization.
8. Gas atomization results in _______
a) dendritic shape powder
b) irregular shape powder
c) spherical shape powder
d) fine powder
View Answer
Explanation: Gas atomization generally results in spherical particles because of the influence of the surface tension forces. The shape of the powder will be irregular in the case of water atomized powders due to high-cooling rates resulting in smaller dendritic spacing.
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