Q. 1. A 200 mm dia particle requires approximately 4 s to solidify during atomization and travels 1 m during this time. What would be the distance traveled by a 40 mm dia particle of the same material before solidification under the same conditions?
Q. 2. Make an approximate calculation to determine possible combinations of maximum working distance and gun discharge velocity that would yield porosity-free coatings (note: to obtain zero porosity at maximum working distance, the droplets should just reach their solidification temperature at the substrate). Given: average droplet diameter = 10 mm, ambient gas temperature = 1100 C, the initial temperature of the droplet at gun exit is 1800 C, density of Ti = 4510 kg.m-3, melting point of Ti = 1660 C, specific heat of Ti = 0.5378 kJ/kg.K, heat transfer coefficient at droplet/gas interface = 500 J/m2.s.K.
Q. 3. The impact velocity of liquid metal droplets on a cast iron substrate during thermal spraying is 400 m/s. Fifty percent of the impact energy is converted to mechanical stress (the other 50% is dissipated as heat). Make a rough calculation to check if the droplet impact would cause any surface pitting of the substrate. Given: density of Ti=4.51 g/cc, yield strength of cast iron = 29 MPa (48 ksi). [Hint: the energy density (i.e., energy per unit volume) has the same dimensions as stress!!].
Q. 4. At what depth will pressure in a straight cylindrical powder compact of 10 mm in become (i) zero, and (ii) one-half of the punch pressure? Assume single-action compaction. Given: radial-to-axial stress ratio=0.5, friction coefficient=0.3.
Q. 5 Pure iron powder is to be compacted and sintered into a complex shape, the bulk volume of which is not easily measured. The following weights have been experimentally determined for the sintered product:
Weight in air = 95.00 g
Weight suspended in water = 82.65 g
Weight saturated with water = 96.80 g
Calculate: (a) open pore volume, (b) true volume, (c) apparent volume and closed pore volume, and (d) percentage of theoretical density. The true density of iron is 7.87 g/cc.
Q. 6. Use the following data obtained from a dry pressing experiment on aluminum powder to calculate (a) the green density in kg/m3, (b) compaction pressure in MPa, and (c) percent porosity in the green compacts. Assume the bars are cylindrical and were compressed along the cylinder axis. The theoretical density of Al is 2.4 g/cc (1N=0.2248 lbf).
Load (lb) Weight of bar (g) Height of bar (cm) bar dia (cm)
25,000 20 5.1 2.01
30,000 22 4.9 2.02
35,000 25 5.0 2.01