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Effects of geometry and position of the metal delivery tube on ultrasonic gas atomization
Volume 5, number
MATERIALS
10
EFFECTS OF GEOMETRY AND POSITION ON ULTRASONIC GAS ATOMIZATION M.K. VEISTINEN
‘, E.J. LAVERNIA
September
1987
OF THE METAL DELIVERY TUBE
2, M. ABINANTE
Department of Materials Science and Engineering, Cambridge, MA 02139, USA Received
LETTERS
Massachusetts
and N.J. GRANT
Institute of Technolog.v,
I7 June 1987
The effects of the geometry and the position of the metal delivery tube on the pressure condition in the gas-metal interaction zone were studied. The measurements were performed under conditions which simulated ultrasonic gas atomization experiments, but at low gas atomization pressures (50-200 psig, 345-l 380 kPa). Low gas atomization pressures are used in spray atomization and deposition processes such as liquid dynamic compaction (LDC), and Osprey. Depending on the experimental conditions, either underpressure or overpressure in the metal delivery tube was detected. The magnitude of the underpressures and overpressures was found to increase with the gas atomization pressure; the maximum pressure differences with respect to the atomization tank pressure were about 3 psi (2 1 kPa) when argon was used as an atomization gas with a pressure of 200 psig (1380 kPa). Underpressure or overpressure effects of such magnitude have a large effect on the metal flow rate during gas atomization. The large pressure differences measured in the present work result from using an atomizer with gas jet diameters larger than those used in previous investigations. Using a large ultrasonic atomizer, argon gas flow rates of about 5 kg/min can be obtained for atomization pressures of 200 psig (I 380 kPa)
1. Introduction
The benefits associated with the rapid solidification of metals and alloys have been documented extensively [ l-31. Of the numerous rapid solidification techniques that are available today, gas atomization is the one most widely used, because it offers a large degree of processing flexibility with the potential for high tonnage production. The cooling rate obtained during gas atomization depends, among other factors, on the droplet size. The droplet size can be readily decreased, and correspondingly cooling rate increased, by increasing the ratio of gas to metal flow rates [ 4-61. In turn, the gas to metal flow ratio can be increased by increasing the gas atomization pressure, and/or increasing the gas exit area. The metal flow rate depends on the metallostatic pressure head in the crucible and on the area of the ’ Permanent address: Outokumpu Inc., Toolonkatu 4, P.O. Box 280,OOlOO Helsinki, Finland. ’ Formerly MIT., Presently Department of Mechanical Engineering, University of California, Irvine, CA 92717, USA.
0167-577x/87/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
metal delivery tube. Hence, during experiments in which the melt is not pressurized, as the metallostatic pressure head decreases, the metal flow rate decreases and the ratio of gas to metal flow rate increases. Additionally, it has also been reported [ 6,7] that the metal flow rate will depend on the relative position of the metal delivery tube in the gas atomizer. For certain atomization conditions, a lowpressure zone is formed at the exit of the metal delivery tube that effectively aspirates the metal and increases the metal flow rate. Alternatively, for other atomization conditions a high-pressure zone can form that effectively reduces the metal flow and in some cases even blocks it completely. Furthermore, the magnitude of the underpressure ( - ) or overpressure ( + ) at the tip of the metal delivery tube will also depend on the gas atomization pressure [ 6-8 1. However, care must be exercized when selecting the position of the metal delivery tube with respect to the gas jets, because variations in the position of the metal delivery tube have a strong effect on the atomization efficiency. When the position of the metal delivery tube is such that the atomization gas B.V.
373
Volume 5, number 10
MATERIALS LETTERS
September 1987
partly impacts the periphery of the tube instead of disintegrating the -moTtenmetal stream, large amounts of energy are wasted, and the resulting atomization efficiency is low. However, if the position of the metal delivery tube is such that the flow rate of the metal is not affected by the conditions in the atomization zone (unconfined gas atomization), the atomization efficiency will also be low, and the resulting powder distribution coarse [ 81. The purpose of this investigation was to study the various phenomena that affect the pressure distribution at the exit of the metal delivery tube during ultrasonic gas atomization with particular emphasis on the effect of the geometry, position and dimensions of the metal delivery tube. For this investigation, a large ultrasonic atomizer was used, which allows high gas flow rates at relatively low atomization pressures. This atomizer design is particularly relevant for spray atomization and deposition processes such as liquid dynamic compaction (LDC), where fine droplet sizes at low gas atomization pressures are desirable [ 91.
2. Experimental The simulated gas atomization experiments (without melt) were done using the same atomization tank and gas delivery system as those used in actual experiments. A detailed description of the experimental apparatus can be found in ref. [ lo]. The atomizer used in this study was an ultrasonic gas atomizer with a total gas-exit area of about 57 mm2. The gas atomization pressure was varied between 50 psig (345 kPa) and 200 psig (1380 kPa), with either nitrogen or argon used as the atomization gas. The gas flow rate was measured to be 5 kg/min for argon at an atomization pressure of 200 psig (1380 kPa). The pressure change with respect to ambient pressure at the tip of the metal delivery tube was measured with an Omega’s amplified voltage output type pressure transducer PX 304-050 A for which the pressure range is O-50 psia (O-345 kPa). The accuracy of the transducer (linearity, hysteresis and repeatability) was ?0.5% of the full scale at 75°F (24°C) where the measurements were performed. During the measurements the pressure port was placed inside the metal delivery tube (see fig. 1a). 374
Fig. 1. (a) Pressure transducer and pressure gauge used in the experiments. (b) The various geometries of the metal delivery tubes used.
The geometries of the metal delivery tubes used for the measurements are shown in fig. lb. The length of the delivery tubes was varied between 27 and 35 mm with an outer diameter of 15 mm and an inner diameter of 7 mm. Furthermore, for some measurements the inner diameter was varied with ceramic inserts (inner diameters of 3 and 5 mm), which are also used in the actual atomization experiments. The metal delivery tubes were designed to fit exactly in the central opening of the atomizer. The zero-position of the delivery tubes was chosen so that the geometrical extensions of the nozzles just touch the edge of the metal delivery tube (see fig. 2). Accordingly, the position of the metal delivery tube is given in millimeters below ( - ) or above ( + ) the zero-position. The lowest position for the delivery tube was about 8 mm (0.3 15”) under the zero-position and the highest position was about 6 mm (0.236”) above the zero-position. It is important to note that when the metal delivery tube is at zero-
Volume 5, number
MATERIALS
10
Outer
$15mm,
Inside
47mm
LETTERS
September
1987
50 loo 150200 250
P,(psi)
AP t
0. id
I’ . 1’ ,’ A’\*, 1
: ci !i :I :,”
AP (psi)
1 3 2
Fig. 2. Schematic drawing of the zero-position of the delivery tube with respect to the gas jets, for two geometries of the metal delivery tube.
or even above it a small portion of the atomization gas impacts the tube because the gas flow from the nozzles is divergent. Particularly this phenomenon is found for the tapered delivery tubes (e.g. tube no. 5 in fig. lb) because at the zero-position the tip of the metal delivery tube is at lower position than for the rectangular delivery tube (tube no. 1 in fig. 1b) . The position of the delivery tubes was varied by introducing spacers with a thickness of 0.5 mm (0.02”) and the exact position of the tube was measured using a micrometer. The pressure changes with respect to ambient pressure were detected with the digital meter connected to the pressure transducer (see fig. la); the gas flow was maintained in the measurements until the maximum value (overpressure) or the minimum value (underpressure) could be detected for a particular position and atomization pressure. position
3. Results and discussion The results of pressure measurements made with the rectangular delivery tube (tube no. 1 in fig. 1b, without ceramic insert) using nitrogen as an atomization gas ire shown in fig. 3. It can be seen that for low positions of the delivery tube the atomization gas causes an underpressure (-) at the tip of the metal delivery tube. The underpressure increases as the atomization pressure is increased reaching an underpressure of 2.8 psi for an atomization pressure of 200 psig (1380 kPa). The underpressure results
vf l-----20
25-n
-
ii 0, 0 ,I(: ._________. , - 0 ;
. __.____.’
,I’ .
.w.
= 200
psig,
N,
oooo
= 150
psig,
Ne
xxxxxx = 100
psig,
Ne
Fig. 3. Pressure measurements obtained with a rectangular metal delivery nozzle (no. I in fig. 1b), using nitrogen gas at various pressures.
from a low-pressure zone which forms at the tip of the delivery tube as the gas, which flows concentrically around the tube, sucks the air being inside the metal delivery tube. Correspondingly, during the actual gas atomization experiments the metal flow rate is increased by this aspiration effect. For example for aluminum alloys a 1 psi underpressure is equivalent to about 25 cm (10”) increase in metallic pressure head. Hence, a 2-3 psi underpressure can increase the metal flow rate dramatically over that for the case with zero underpressure (i.e. free-falling metal). The only benefit to be gained from such a large underpressure is that since the metal flow rate will be primarily controlled by suction effect, it will remain approximately constant for the duration of the experiment. Therefore the variation in gas to metal flow ratio from the beginning to the end of the experiment can be neglected. From fig. 3 it can be seen that as the delivery tube is moved upwards the underpressure decreases steeply. Probably for these positions of the metal delivery tube the atomization gas impacts the edge of the tube and forms turbulent eddies which intrud375
Volume 5, number 10
MATERIALS LETTERS
AP
(PSI) +
. . r--a**4.” IX xx
0.5 s
-
15
- 0 30
:o
-
-
x, 0 I
0
00
. . . .
‘din-
XxXxXx
=din_5mm,
Ar,
200
boo00
‘din-
Ar,
150
q OOO
qdi,-7mm,
Ne,
200
3mm,
7mm,
Ar,
200
T
Fig. 4. Pressure measurements obtained with rectangular metal delivery tube (no. 1 in fig. lb), for various inside diameters of the delivery tube, two atomization pressures, and for nitrogen and argon.
ing into the metal delivery tube decrease the underpressure. At and above the zero-position the underpressure may even change into an overpressure. For high positions of the metal delivery tube where the atomization gas completely misses the tube a small underpressure could be detected. Obviously the decrease of this underpressure at still higher positions of the delivery tube is due to the experimental arrangement where the distance between the pressure transducer and atomization gas jets increases. The results shown in fig. 3 were not essentially different when argon, instead of nitrogen, was used as the atomization gas. However, the maximum value of the underpressure recorded was higher for the experiments performed with nitrogen. The results of the pressure measurements with the rectangular delivery tube using ceramic inserts are presented in fig. 4. When compared to fig. 3, it can be seen that below the zero-position there is again an underpressure at the tip of the metal delivery tube. The maximum underpressure, however, decreases as 376
September 1987
the inner diameter is decreased. Above the zeroposition the underpressure changes into an overpressure; this was not the case for the experiments performed without the ceramic inserts (fig. 3). The presence of a strong overpressure (for insert with the diameter of 3 mm) at these positions was verified with water atomization. While it was possible to atomize the water when the delivery tube was placed 1 mm below the zero-position the water did not flow down from the crucible when the delivery tube was raised 0.5 mm because of the overpressure. Probably, one of the reasons for the overpressure readings when the ceramic inserts were used, can be attributed to the additional turbulence that forms when the inserts protrudes from the metal delivery tube. For example, it was found that when the ceramic insert was extended 2 mm (0.08”) from the tip of the metal delivery tube, the maximum underpressure decreased about 0.5 psi (for nitrogen with a pressure of 100 psig). Furthermore, with an extension of 4 mm, an overpressure of about 1.5 psi (10 kPa) was detected for the delivery tube position of 1 mm above the zero-position and using argon as an atomization gas with a pressure of 100 psig (690 kPa). From fig. 4 it can be seen that optimization of the position of the delivery tube in order to decrease the metal flow rate is not a simple task due to the sharp changes in pressure with position. However, these results were used to minimize the droplet size and maximize the cooling rate during spray deposition of an iron-neodymium-boron alloy resulting in a high coercivity product [ 111. The results of the pressure measurements with the tapered metal delivery tube (no. 5 in fig. 1b, without ceramic insert) are shown in fig. 5. It can be seen that for the range of positions of the delivery tube studied, overpressures were measured. A maximum overpressure of 3 psi (20 kPa) was measured for an experiment with a nitrogen gas using atomization pressure of 190 psig (1270 kPa). The maximum overpressures were measured for a delivery tube positioned 4 mm above the zero-position. The decrease in overpressure as the delivery tube was raised further is a result of the lack of interaction between the gas jets and the metal delivery tube. Such a large overpressure measured will prevent the flow of molten aluminum (reverse flow condition) when the metallic pressure head is less than 75 cm (30”)
Volume 5. number
MATERIALS
10
AP (PSI)
September
LETTERS
1987
AP (PSI) IO
25
8765432,l
*
Oo( 2345678y(mm)
05
IO
-
15
+,.‘-•
20 “l
0765432
101
--m
b
232567
y(mm)
= =
.WO
=
q
mmm = 0000 = l
190 150 90 50 150 100
psi, Nz psi, Np PSI, N2 psi, N2 psi, Ar psi, Ar
Fig. 5. Pressure measurements obtained with tapered metal delivery tubes (no. 5 in fig. 1b, no ceramic insert), using nitrogen and argon at various atomization pressures.
in height. Even an overpressure of 0.5 psi is usually too severe for the usual conditions encountered in gas atomization. Hence, unless the melt is not pressurized in order to compensate for the over-pressure this tapered geometry is not appropriate for the melt delivery tube. On the other hand, by using overpressure and pressurized crucible it would be possible to achieve very small metal flow rates and, additionally, it could be adjusted exactly. The disadvantage in the case of low metal flow rate would, however, be the fact that the melt freezes easily in the delivery tube. Furthermore, the results from fig. 5 show that the overpressure may be higher when nitrogen, instead of argon, is used as an atomization gas. This result is not consistent with the measurements of Backmark et al. [ 121, who found that argon causes higher overpressures but lower underpressures than nitrogen. The reason for this discrepancy may be the lower
i e0.m l
**a
I=
80mm
1=
5 3 mm
xxxxxx 1 = 3.2 mm
25 *** oooo xxxxx
l
4 I’
0000
1=
25mm
l mmm
1=
0
t Fig. 6. Pressure measurements obtained with tapered metal delivery tubes (no. 2 in fig. lb), for variations in the length of the tapered section using argon gas at 150 psig. The inner diameter of the delivery tube used was 7 mm.
atomization pressure and different geometry of the metal delivery tube used in the present work as compared to those used by Backmark et al. The higher underpressures measured when using nitrogen as the atomizing gas are not unexpected. During ultrasonic gas atomization the volumetric gas flow rate (as well as velocity of gas), have been reported to be higher for nitrogen than for argon [ 3,121. In the case of overpressure the situation is more complicated because also the mass flow rate, which in ultrasonic gas atomization is higher for argon than for nitrogen, is expected to affect the value of the overpressure in addition to the velocity of gas. The results of the pressure measurements with a tapered metal delivery tube (e.g. tube no. 2 in fig. lb) are shown in fig. 6 for variations in the length of the tapered section of the tube. From fig. 6 it is seen that as the length of the tapered section decreases, the overpressure decreases gradually and eventually changes into an underpressure. The results 377
Volume 5, number 10
MATERIALS LETTERS
of the pressure distribution with a delivery tube having a tapered section of 2.5 mm (0.1”) long are attractive for minimization of metal flow rate because only an underpressure of 0.2 psi is detected at zeroposition. However, as with the previous case, when a ceramic insert was placed inside the delivery tube, an overpressure was measured for positions above the zero-point position. For a 200 psig argon gas atomization pressure and an insert with 3 mm inner diameter, the maximum underpressure measured was 2.3 psi at the position of 3 mm below the zero-point. For the same conditions, the underpressure was 0.7 psi for the zero-point position. Similar to the results shown in figs. 3 and 5, the pressure gradient was very steep near the zero-position. Also, the results of fig. 6 show that as the length of the tapered section was decreased, and the geometry of the tapered delivery tube approached that of the rectangular delivery tube (tube no. 1 in fig. 1b), the aspiration effect became more dominant. This is consistent with the results shown in figs. 3-5. Furthermore the effect of the angle of the tapered section in the delivery tube (e.g. tubes no. 3 and 4 in fig. 1b) was studied (see fig. 7). The results from fig. 7 show that when the angle was increased from 45” (tube no. 5 in fig. lb) to 90”, the position for the maximum overpressure was shifted to a lower position. This is due to the fact that an increase in the angle and a decrease in the length of the tapered part enables the atomization gas to miss the tip of the delivery tube at lower positions. If the angle is increased sufficiently this effect should disappear as demonstrated in fig. 7. Furthermore, the increase in the angle changes the geometry of the delivery tube into one approaching that of the rectangular tube, and consequently the maximum overpressure decreases and underpressure increases. According to these results tapered metal delivery tubes do not offer any advantage in comparison with the rectangular ones when a minimum metal flow rate is desired. The situation would be different if the melt were pressurized and the atomization unit instrumented so that the high overpressure caused by the tapered metal delivery tubes could be used to adjust the metal flow rate. It should be noted that the above results were obtained only in simulated gas atomization experiments because the tank was open during the meas378
September 1987
AP
(psi) 2.0
+
I.0
0.5
0
LLJ I)
so-
120-
I)
2)
90” = . . . .
2) 120’ = XXX~XX 3) 45” = 0000 4) rec.t = a.** din 7 mm q
Fig. 7. The effect of the angle of the tapered section of the delivery tube (no. 3 and no. 4 in fig. 1b ) on the pressure, for argon gas at 150 psig.
urements and no melt was present. However, the results from water atomization, and LDC experiments [ 111 indicate that the simulated conditions discussed here match very closely those present during actual atomization and LDC experiments. The applicability of the results, however, should be limited to the gas pressure ranges used in this work. It has been reported that when high atomization pressures are used even small changes in experimental conditions (such as closing the tank or using a substrate for spray forming), may have a strong effect on the pressure condition of the metal delivery tube [131.
4. Summary and conclusions
In the present work the effects of the geometry and position of the metal delivery tube, as well as the type of atomizing gas, on the pressure distribution at the
Volume 5. number
10
MATERIALS
exit of the metal delivery tube during ultrasonic gas atomization were studied. Optimization of the geometry of the delivery tube to minimize the pressure (and hence the metal flow rate) at the exit of the delivery tube was found difficult. For the geometries used in this work there was either a high overpressure or steep pressure gradients for positions of the delivery tube which otherwise would be optimum during gas atomization. A small overpressure at the exit of the delivery tube can be tolerated during atomization but an additional metallic head equal to the overpressure is needed in the crucible. In turn, the steep pressure gradient makes the machining of the metal delivery tube difficult and, additionally, even small changes in the position of the tube can cause unexpected problems during atomization experiments. In the present study the overpressures and underpressures were found to increase with the gas atomization pressure. This phenomenon makes optimization of gas atomization even more difficult because the high atomization pressures are needed, when line powders are to be produced. The maximum underpressure and overpressure values measured in this work were about 3 psi even though the maximum atomization pressure was only 200 psig (1380 kPa).
LETTERS
September
1987
[ I] Y.M. Kim, W.M. Griffith and F.H. Froes, J. Met. 37, No. 8 (1985) 27. [2] N.J. Grant, J. Met. 35, No. 1 (1983) 20. [3] E.J. Lavernia, G. Rai and N.J. Grant, J. Mat. Sci. Eng. 79 (1985) 211. [4] R.A. Ricks and T.W. Clyne, J. Mat. Sci. Letters 4 (1985) 814. [ 51 American Society for Metals Handbook, Vol. 7. Powder metallurgy (I 986) pp. 25-5 1. [6] M.J. Couper and R.G. Singer, in: Proceedings of the 5th International Conference on Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (North-Holland, Amsterdam, 1985) p. 1737. [7] J.D. Ayers and I.E. Anderson, J. Met. 37, No. 8 (1985) 16. [ 81 J. Baram, M.K. Veistinen, E.J. Lavernia and N.J. Grant, to be submitted for publication. [9] E.J. Lavernia, M.K. Veistinen and N.J. Grant, work in progress. IO] W. Wang, Advanced Aluminum Alloys made from Ultrasonically Atomized Particulates, Sc.D. Thesis, M.I.T., Cambridge, Massachusetts, February 1982. 11 ] M.K. Veistinen, Y. Hara, E.J. Lavemia, R.C. O’Handley and N.J. Grant, 1987 MRS Spring Meeting, Anaheim, CA, April 21-25, 1987. [12 ] U. Backmark, N. Backstrom and L. Amberg, Production of Metal Powder by Ultrasonic Gas Atomization, Report, Swedish Institute for Metals Research (1985). Conference on Rapidly Quenched I13 ] J. Baram, International Metals, Montreal, Canada, August 3-7. 1987.
379
MATERIALS
10
EFFECTS OF GEOMETRY AND POSITION ON ULTRASONIC GAS ATOMIZATION M.K. VEISTINEN
‘, E.J. LAVERNIA
September
1987
OF THE METAL DELIVERY TUBE
2, M. ABINANTE
Department of Materials Science and Engineering, Cambridge, MA 02139, USA Received
LETTERS
Massachusetts
and N.J. GRANT
Institute of Technolog.v,
I7 June 1987
The effects of the geometry and the position of the metal delivery tube on the pressure condition in the gas-metal interaction zone were studied. The measurements were performed under conditions which simulated ultrasonic gas atomization experiments, but at low gas atomization pressures (50-200 psig, 345-l 380 kPa). Low gas atomization pressures are used in spray atomization and deposition processes such as liquid dynamic compaction (LDC), and Osprey. Depending on the experimental conditions, either underpressure or overpressure in the metal delivery tube was detected. The magnitude of the underpressures and overpressures was found to increase with the gas atomization pressure; the maximum pressure differences with respect to the atomization tank pressure were about 3 psi (2 1 kPa) when argon was used as an atomization gas with a pressure of 200 psig (1380 kPa). Underpressure or overpressure effects of such magnitude have a large effect on the metal flow rate during gas atomization. The large pressure differences measured in the present work result from using an atomizer with gas jet diameters larger than those used in previous investigations. Using a large ultrasonic atomizer, argon gas flow rates of about 5 kg/min can be obtained for atomization pressures of 200 psig (I 380 kPa)
1. Introduction
The benefits associated with the rapid solidification of metals and alloys have been documented extensively [ l-31. Of the numerous rapid solidification techniques that are available today, gas atomization is the one most widely used, because it offers a large degree of processing flexibility with the potential for high tonnage production. The cooling rate obtained during gas atomization depends, among other factors, on the droplet size. The droplet size can be readily decreased, and correspondingly cooling rate increased, by increasing the ratio of gas to metal flow rates [ 4-61. In turn, the gas to metal flow ratio can be increased by increasing the gas atomization pressure, and/or increasing the gas exit area. The metal flow rate depends on the metallostatic pressure head in the crucible and on the area of the ’ Permanent address: Outokumpu Inc., Toolonkatu 4, P.O. Box 280,OOlOO Helsinki, Finland. ’ Formerly MIT., Presently Department of Mechanical Engineering, University of California, Irvine, CA 92717, USA.
0167-577x/87/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
metal delivery tube. Hence, during experiments in which the melt is not pressurized, as the metallostatic pressure head decreases, the metal flow rate decreases and the ratio of gas to metal flow rate increases. Additionally, it has also been reported [ 6,7] that the metal flow rate will depend on the relative position of the metal delivery tube in the gas atomizer. For certain atomization conditions, a lowpressure zone is formed at the exit of the metal delivery tube that effectively aspirates the metal and increases the metal flow rate. Alternatively, for other atomization conditions a high-pressure zone can form that effectively reduces the metal flow and in some cases even blocks it completely. Furthermore, the magnitude of the underpressure ( - ) or overpressure ( + ) at the tip of the metal delivery tube will also depend on the gas atomization pressure [ 6-8 1. However, care must be exercized when selecting the position of the metal delivery tube with respect to the gas jets, because variations in the position of the metal delivery tube have a strong effect on the atomization efficiency. When the position of the metal delivery tube is such that the atomization gas B.V.
373
Volume 5, number 10
MATERIALS LETTERS
September 1987
partly impacts the periphery of the tube instead of disintegrating the -moTtenmetal stream, large amounts of energy are wasted, and the resulting atomization efficiency is low. However, if the position of the metal delivery tube is such that the flow rate of the metal is not affected by the conditions in the atomization zone (unconfined gas atomization), the atomization efficiency will also be low, and the resulting powder distribution coarse [ 81. The purpose of this investigation was to study the various phenomena that affect the pressure distribution at the exit of the metal delivery tube during ultrasonic gas atomization with particular emphasis on the effect of the geometry, position and dimensions of the metal delivery tube. For this investigation, a large ultrasonic atomizer was used, which allows high gas flow rates at relatively low atomization pressures. This atomizer design is particularly relevant for spray atomization and deposition processes such as liquid dynamic compaction (LDC), where fine droplet sizes at low gas atomization pressures are desirable [ 91.
2. Experimental The simulated gas atomization experiments (without melt) were done using the same atomization tank and gas delivery system as those used in actual experiments. A detailed description of the experimental apparatus can be found in ref. [ lo]. The atomizer used in this study was an ultrasonic gas atomizer with a total gas-exit area of about 57 mm2. The gas atomization pressure was varied between 50 psig (345 kPa) and 200 psig (1380 kPa), with either nitrogen or argon used as the atomization gas. The gas flow rate was measured to be 5 kg/min for argon at an atomization pressure of 200 psig (1380 kPa). The pressure change with respect to ambient pressure at the tip of the metal delivery tube was measured with an Omega’s amplified voltage output type pressure transducer PX 304-050 A for which the pressure range is O-50 psia (O-345 kPa). The accuracy of the transducer (linearity, hysteresis and repeatability) was ?0.5% of the full scale at 75°F (24°C) where the measurements were performed. During the measurements the pressure port was placed inside the metal delivery tube (see fig. 1a). 374
Fig. 1. (a) Pressure transducer and pressure gauge used in the experiments. (b) The various geometries of the metal delivery tubes used.
The geometries of the metal delivery tubes used for the measurements are shown in fig. lb. The length of the delivery tubes was varied between 27 and 35 mm with an outer diameter of 15 mm and an inner diameter of 7 mm. Furthermore, for some measurements the inner diameter was varied with ceramic inserts (inner diameters of 3 and 5 mm), which are also used in the actual atomization experiments. The metal delivery tubes were designed to fit exactly in the central opening of the atomizer. The zero-position of the delivery tubes was chosen so that the geometrical extensions of the nozzles just touch the edge of the metal delivery tube (see fig. 2). Accordingly, the position of the metal delivery tube is given in millimeters below ( - ) or above ( + ) the zero-position. The lowest position for the delivery tube was about 8 mm (0.3 15”) under the zero-position and the highest position was about 6 mm (0.236”) above the zero-position. It is important to note that when the metal delivery tube is at zero-
Volume 5, number
MATERIALS
10
Outer
$15mm,
Inside
47mm
LETTERS
September
1987
50 loo 150200 250
P,(psi)
AP t
0. id
I’ . 1’ ,’ A’\*, 1
: ci !i :I :,”
AP (psi)
1 3 2
Fig. 2. Schematic drawing of the zero-position of the delivery tube with respect to the gas jets, for two geometries of the metal delivery tube.
or even above it a small portion of the atomization gas impacts the tube because the gas flow from the nozzles is divergent. Particularly this phenomenon is found for the tapered delivery tubes (e.g. tube no. 5 in fig. lb) because at the zero-position the tip of the metal delivery tube is at lower position than for the rectangular delivery tube (tube no. 1 in fig. 1b) . The position of the delivery tubes was varied by introducing spacers with a thickness of 0.5 mm (0.02”) and the exact position of the tube was measured using a micrometer. The pressure changes with respect to ambient pressure were detected with the digital meter connected to the pressure transducer (see fig. la); the gas flow was maintained in the measurements until the maximum value (overpressure) or the minimum value (underpressure) could be detected for a particular position and atomization pressure. position
3. Results and discussion The results of pressure measurements made with the rectangular delivery tube (tube no. 1 in fig. 1b, without ceramic insert) using nitrogen as an atomization gas ire shown in fig. 3. It can be seen that for low positions of the delivery tube the atomization gas causes an underpressure (-) at the tip of the metal delivery tube. The underpressure increases as the atomization pressure is increased reaching an underpressure of 2.8 psi for an atomization pressure of 200 psig (1380 kPa). The underpressure results
vf l-----20
25-n
-
ii 0, 0 ,I(: ._________. , - 0 ;
. __.____.’
,I’ .
.w.
= 200
psig,
N,
oooo
= 150
psig,
Ne
xxxxxx = 100
psig,
Ne
Fig. 3. Pressure measurements obtained with a rectangular metal delivery nozzle (no. I in fig. 1b), using nitrogen gas at various pressures.
from a low-pressure zone which forms at the tip of the delivery tube as the gas, which flows concentrically around the tube, sucks the air being inside the metal delivery tube. Correspondingly, during the actual gas atomization experiments the metal flow rate is increased by this aspiration effect. For example for aluminum alloys a 1 psi underpressure is equivalent to about 25 cm (10”) increase in metallic pressure head. Hence, a 2-3 psi underpressure can increase the metal flow rate dramatically over that for the case with zero underpressure (i.e. free-falling metal). The only benefit to be gained from such a large underpressure is that since the metal flow rate will be primarily controlled by suction effect, it will remain approximately constant for the duration of the experiment. Therefore the variation in gas to metal flow ratio from the beginning to the end of the experiment can be neglected. From fig. 3 it can be seen that as the delivery tube is moved upwards the underpressure decreases steeply. Probably for these positions of the metal delivery tube the atomization gas impacts the edge of the tube and forms turbulent eddies which intrud375
Volume 5, number 10
MATERIALS LETTERS
AP
(PSI) +
. . r--a**4.” IX xx
0.5 s
-
15
- 0 30
:o
-
-
x, 0 I
0
00
. . . .
‘din-
XxXxXx
=din_5mm,
Ar,
200
boo00
‘din-
Ar,
150
q OOO
qdi,-7mm,
Ne,
200
3mm,
7mm,
Ar,
200
T
Fig. 4. Pressure measurements obtained with rectangular metal delivery tube (no. 1 in fig. lb), for various inside diameters of the delivery tube, two atomization pressures, and for nitrogen and argon.
ing into the metal delivery tube decrease the underpressure. At and above the zero-position the underpressure may even change into an overpressure. For high positions of the metal delivery tube where the atomization gas completely misses the tube a small underpressure could be detected. Obviously the decrease of this underpressure at still higher positions of the delivery tube is due to the experimental arrangement where the distance between the pressure transducer and atomization gas jets increases. The results shown in fig. 3 were not essentially different when argon, instead of nitrogen, was used as the atomization gas. However, the maximum value of the underpressure recorded was higher for the experiments performed with nitrogen. The results of the pressure measurements with the rectangular delivery tube using ceramic inserts are presented in fig. 4. When compared to fig. 3, it can be seen that below the zero-position there is again an underpressure at the tip of the metal delivery tube. The maximum underpressure, however, decreases as 376
September 1987
the inner diameter is decreased. Above the zeroposition the underpressure changes into an overpressure; this was not the case for the experiments performed without the ceramic inserts (fig. 3). The presence of a strong overpressure (for insert with the diameter of 3 mm) at these positions was verified with water atomization. While it was possible to atomize the water when the delivery tube was placed 1 mm below the zero-position the water did not flow down from the crucible when the delivery tube was raised 0.5 mm because of the overpressure. Probably, one of the reasons for the overpressure readings when the ceramic inserts were used, can be attributed to the additional turbulence that forms when the inserts protrudes from the metal delivery tube. For example, it was found that when the ceramic insert was extended 2 mm (0.08”) from the tip of the metal delivery tube, the maximum underpressure decreased about 0.5 psi (for nitrogen with a pressure of 100 psig). Furthermore, with an extension of 4 mm, an overpressure of about 1.5 psi (10 kPa) was detected for the delivery tube position of 1 mm above the zero-position and using argon as an atomization gas with a pressure of 100 psig (690 kPa). From fig. 4 it can be seen that optimization of the position of the delivery tube in order to decrease the metal flow rate is not a simple task due to the sharp changes in pressure with position. However, these results were used to minimize the droplet size and maximize the cooling rate during spray deposition of an iron-neodymium-boron alloy resulting in a high coercivity product [ 111. The results of the pressure measurements with the tapered metal delivery tube (no. 5 in fig. 1b, without ceramic insert) are shown in fig. 5. It can be seen that for the range of positions of the delivery tube studied, overpressures were measured. A maximum overpressure of 3 psi (20 kPa) was measured for an experiment with a nitrogen gas using atomization pressure of 190 psig (1270 kPa). The maximum overpressures were measured for a delivery tube positioned 4 mm above the zero-position. The decrease in overpressure as the delivery tube was raised further is a result of the lack of interaction between the gas jets and the metal delivery tube. Such a large overpressure measured will prevent the flow of molten aluminum (reverse flow condition) when the metallic pressure head is less than 75 cm (30”)
Volume 5. number
MATERIALS
10
AP (PSI)
September
LETTERS
1987
AP (PSI) IO
25
8765432,l
*
Oo( 2345678y(mm)
05
IO
-
15
+,.‘-•
20 “l
0765432
101
--m
b
232567
y(mm)
= =
.WO
=
q
mmm = 0000 = l
190 150 90 50 150 100
psi, Nz psi, Np PSI, N2 psi, N2 psi, Ar psi, Ar
Fig. 5. Pressure measurements obtained with tapered metal delivery tubes (no. 5 in fig. 1b, no ceramic insert), using nitrogen and argon at various atomization pressures.
in height. Even an overpressure of 0.5 psi is usually too severe for the usual conditions encountered in gas atomization. Hence, unless the melt is not pressurized in order to compensate for the over-pressure this tapered geometry is not appropriate for the melt delivery tube. On the other hand, by using overpressure and pressurized crucible it would be possible to achieve very small metal flow rates and, additionally, it could be adjusted exactly. The disadvantage in the case of low metal flow rate would, however, be the fact that the melt freezes easily in the delivery tube. Furthermore, the results from fig. 5 show that the overpressure may be higher when nitrogen, instead of argon, is used as an atomization gas. This result is not consistent with the measurements of Backmark et al. [ 121, who found that argon causes higher overpressures but lower underpressures than nitrogen. The reason for this discrepancy may be the lower
i e0.m l
**a
I=
80mm
1=
5 3 mm
xxxxxx 1 = 3.2 mm
25 *** oooo xxxxx
l
4 I’
0000
1=
25mm
l mmm
1=
0
t Fig. 6. Pressure measurements obtained with tapered metal delivery tubes (no. 2 in fig. lb), for variations in the length of the tapered section using argon gas at 150 psig. The inner diameter of the delivery tube used was 7 mm.
atomization pressure and different geometry of the metal delivery tube used in the present work as compared to those used by Backmark et al. The higher underpressures measured when using nitrogen as the atomizing gas are not unexpected. During ultrasonic gas atomization the volumetric gas flow rate (as well as velocity of gas), have been reported to be higher for nitrogen than for argon [ 3,121. In the case of overpressure the situation is more complicated because also the mass flow rate, which in ultrasonic gas atomization is higher for argon than for nitrogen, is expected to affect the value of the overpressure in addition to the velocity of gas. The results of the pressure measurements with a tapered metal delivery tube (e.g. tube no. 2 in fig. lb) are shown in fig. 6 for variations in the length of the tapered section of the tube. From fig. 6 it is seen that as the length of the tapered section decreases, the overpressure decreases gradually and eventually changes into an underpressure. The results 377
Volume 5, number 10
MATERIALS LETTERS
of the pressure distribution with a delivery tube having a tapered section of 2.5 mm (0.1”) long are attractive for minimization of metal flow rate because only an underpressure of 0.2 psi is detected at zeroposition. However, as with the previous case, when a ceramic insert was placed inside the delivery tube, an overpressure was measured for positions above the zero-point position. For a 200 psig argon gas atomization pressure and an insert with 3 mm inner diameter, the maximum underpressure measured was 2.3 psi at the position of 3 mm below the zero-point. For the same conditions, the underpressure was 0.7 psi for the zero-point position. Similar to the results shown in figs. 3 and 5, the pressure gradient was very steep near the zero-position. Also, the results of fig. 6 show that as the length of the tapered section was decreased, and the geometry of the tapered delivery tube approached that of the rectangular delivery tube (tube no. 1 in fig. 1b), the aspiration effect became more dominant. This is consistent with the results shown in figs. 3-5. Furthermore the effect of the angle of the tapered section in the delivery tube (e.g. tubes no. 3 and 4 in fig. 1b) was studied (see fig. 7). The results from fig. 7 show that when the angle was increased from 45” (tube no. 5 in fig. lb) to 90”, the position for the maximum overpressure was shifted to a lower position. This is due to the fact that an increase in the angle and a decrease in the length of the tapered part enables the atomization gas to miss the tip of the delivery tube at lower positions. If the angle is increased sufficiently this effect should disappear as demonstrated in fig. 7. Furthermore, the increase in the angle changes the geometry of the delivery tube into one approaching that of the rectangular tube, and consequently the maximum overpressure decreases and underpressure increases. According to these results tapered metal delivery tubes do not offer any advantage in comparison with the rectangular ones when a minimum metal flow rate is desired. The situation would be different if the melt were pressurized and the atomization unit instrumented so that the high overpressure caused by the tapered metal delivery tubes could be used to adjust the metal flow rate. It should be noted that the above results were obtained only in simulated gas atomization experiments because the tank was open during the meas378
September 1987
AP
(psi) 2.0
+
I.0
0.5
0
LLJ I)
so-
120-
I)
2)
90” = . . . .
2) 120’ = XXX~XX 3) 45” = 0000 4) rec.t = a.** din 7 mm q
Fig. 7. The effect of the angle of the tapered section of the delivery tube (no. 3 and no. 4 in fig. 1b ) on the pressure, for argon gas at 150 psig.
urements and no melt was present. However, the results from water atomization, and LDC experiments [ 111 indicate that the simulated conditions discussed here match very closely those present during actual atomization and LDC experiments. The applicability of the results, however, should be limited to the gas pressure ranges used in this work. It has been reported that when high atomization pressures are used even small changes in experimental conditions (such as closing the tank or using a substrate for spray forming), may have a strong effect on the pressure condition of the metal delivery tube [131.
4. Summary and conclusions
In the present work the effects of the geometry and position of the metal delivery tube, as well as the type of atomizing gas, on the pressure distribution at the
Volume 5. number
10
MATERIALS
exit of the metal delivery tube during ultrasonic gas atomization were studied. Optimization of the geometry of the delivery tube to minimize the pressure (and hence the metal flow rate) at the exit of the delivery tube was found difficult. For the geometries used in this work there was either a high overpressure or steep pressure gradients for positions of the delivery tube which otherwise would be optimum during gas atomization. A small overpressure at the exit of the delivery tube can be tolerated during atomization but an additional metallic head equal to the overpressure is needed in the crucible. In turn, the steep pressure gradient makes the machining of the metal delivery tube difficult and, additionally, even small changes in the position of the tube can cause unexpected problems during atomization experiments. In the present study the overpressures and underpressures were found to increase with the gas atomization pressure. This phenomenon makes optimization of gas atomization even more difficult because the high atomization pressures are needed, when line powders are to be produced. The maximum underpressure and overpressure values measured in this work were about 3 psi even though the maximum atomization pressure was only 200 psig (1380 kPa).
LETTERS
September
1987
[ I] Y.M. Kim, W.M. Griffith and F.H. Froes, J. Met. 37, No. 8 (1985) 27. [2] N.J. Grant, J. Met. 35, No. 1 (1983) 20. [3] E.J. Lavernia, G. Rai and N.J. Grant, J. Mat. Sci. Eng. 79 (1985) 211. [4] R.A. Ricks and T.W. Clyne, J. Mat. Sci. Letters 4 (1985) 814. [ 51 American Society for Metals Handbook, Vol. 7. Powder metallurgy (I 986) pp. 25-5 1. [6] M.J. Couper and R.G. Singer, in: Proceedings of the 5th International Conference on Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (North-Holland, Amsterdam, 1985) p. 1737. [7] J.D. Ayers and I.E. Anderson, J. Met. 37, No. 8 (1985) 16. [ 81 J. Baram, M.K. Veistinen, E.J. Lavernia and N.J. Grant, to be submitted for publication. [9] E.J. Lavernia, M.K. Veistinen and N.J. Grant, work in progress. IO] W. Wang, Advanced Aluminum Alloys made from Ultrasonically Atomized Particulates, Sc.D. Thesis, M.I.T., Cambridge, Massachusetts, February 1982. 11 ] M.K. Veistinen, Y. Hara, E.J. Lavemia, R.C. O’Handley and N.J. Grant, 1987 MRS Spring Meeting, Anaheim, CA, April 21-25, 1987. [12 ] U. Backmark, N. Backstrom and L. Amberg, Production of Metal Powder by Ultrasonic Gas Atomization, Report, Swedish Institute for Metals Research (1985). Conference on Rapidly Quenched I13 ] J. Baram, International Metals, Montreal, Canada, August 3-7. 1987.
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