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Mechanical atomization of molten metals
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Mechanical atomization of molten metals A.J. A l l e r a n d A. L o s a d a ( C h e m i s t r y D e p a r t m e n t , U n i v e r s i t y o f Le~n, Spain)
D e v e l o p m e n t s in m o l t e n m e t a l a t o m i z a t i o n h a v e f o l l o w e d the r e q u i r e m e n t s of clearly identifiable m a r k e t s for a d v a n c e d alloy compositions. Consequently, new requirements are leading to d e t a i l e d d i f f e r e n c e s in t h e a t o m i z a t i o n p r o c e s s e s . The foil o w i n g article outlines s o m e o f the d e v e l o p m e n t s w h i c h h a v e t a k e n p l a c e o v e r r e c e n t y e a r s in the a t o m i z a t i o n p r o c e s s e s f o r the m o l t e n m e t a l s , a n d w h i c h can r e p l a c e w a t e r ~ g a s a n d rotary atomization.
he d e v e l o p m e n t of a d v a n c e d m a t e r ials to be used in e x t r e m e c o n d i t i o n s forces r e s e a r c h into new m a n u f a c t u r ing technologies. Power m e t a l l u r g y (PM) is becoming an a d e q u a t e a l t e r n a t i v e to t h e m o r e c o n v e n t i o n a l p r o d u c t i o n m e t h o d s of casting techniques, due f u n d a m e n t a l l y to t h e r e d u c e d costs in PM technology. In t h e two-fluid a t o m i z a t i o n p r o c e s s e s (1), as well as in t h e centrifugal a t o m i z a t i o n ones (2), the energy yield with r e s p e c t to the s e p a r a t i o n w o r k is relatively small and, consequently, an economic d i s a d v a n t a g e . There is therefore, an increasing d e m a n d for a m o r e a p p r o p r i a t e p r o c e s s for t h e p r o d u c t i o n of m e t a l powders. The m a i n p u r p o s e of this article is to describe briefly all a l t e r n a t i v e m e t h o d s of m o l t e n m e t a l a t o m i z a t i o n . A t o m i z a t i o n techniques in this category do not involve r o t a t i o n a n d / o r i m p a c t of t h e m e l t by gas or ~ a t e r , a n d all of t h e m a r e c h a r a c t e r i z e d by t h e fact t h a t t h e energy used in t h e blasting of the liquid m e t a l s t r e a m is a p o t e n t i a l energy (1).
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In this t e c h n i q u e a s t r e a m of m o l t e n m e t a l is fed between the inwardly r o t a t i n g rolls (3-5). Upon contact, the alloy s t a r t s to
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FIGURE 1." Schematic diagram of roller atomization.
solidify being p a r t i a l l y hot rolled forming thin metallic foils (Figure 1). W a t e r is used as t h e first cooling agent, a n d a secondary. cooling is p e r f o r m e d when t h e p a r t i c u l a t e s are collected in an a l u m i n i u m bin. This m e t h o d is c a p a b l e of converting a wide range of m e t a l s (?d, Cu, Sn, Pb) into flakes, acicular, irregular, or s p h e r i c a l p a r t i c l e s (6). Mean p a r t i c l e size (220 pro) is inversely p r o p o r t i o n a l to t h e r o t a t i o n a l speed. Typical v a r i a b l e s are: roll r o t a t i n g s p e e d (200 r p s ) , roll gap (60 # m ) , s t r e a m diam e t e r (2.4 m m ) , s u p e r h e a t (100°C), a n d solidification rate (105°C/s). Roller a t o m i z a t i o n m e c h a n i s m is s i m i l a r to ca~-itation in oil l u b r i c a t e d bearings (3), in which liquid ( m o l t e n m e t a l in roller atomization) cavitates a small distance d o ~ n s t r e a m from t h e roll n i p p r o d u c i n g p e r f o r a t i o n s a n d s t r e a m e r s . A t high roller s p e e d s t h e m o l t e n m e t a l flows a r o u n d t h e p o i n t s of cavitation forming t h i n t h r e a d s of metal, which b r e a k down b e c a u s e of the m o l t e n m e t a l j e t instabilities.
Ultrasonic atomization
The d i r e c t c o n t a c t of liquid m e t a l with s o m e p a r t s of t h e e q u i p m e n t has been overcome by a t o m i z i n g t h e l i q u i d m e t a l s t r e a m bet~veen t~vo vertical n o n - c o n t a c t e d ultrasonic r e s o n a t o r plates. The alloy is induction heated in a rare earth ( t h e r m o d y n a m i c a l l y o r kinetically s t a b l e in c o n t a c t with molten alloy) o x i d e c o a t e d z i r c o n i a crucible u n d e r a inert gas atmoM P R J a n u a r y 1991 4 5
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FIGURE 2: Ultrasonic gas atomization.
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I Col lectionI chamber sphere, and the molten metal is compressed through an unheated tundish falling into a stationary ultrasonic field. Molten metal passes this zone very fast e x p a n d i n g immediately after compression and 'explodes' into small droplets (Figure 2). In ultrasonic atomization (Figure 3), a thin layer of a molten metal covering the surface of a resonator forms stationary capillary-waves from a certain limiting value of the elongation-amplitude (7). This totally non-contaminating disintegration method is dependent on melting temperature. The pneumoacoustic method based on the Hartmann oscillator is performed in the ultrasonic range (100 KHz), vibrating a freely pendulous vibrator, the end of which hits against a molten metal alloy producing very fine metallic powders. The vibrator can V,~rticol Vibrator
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be placed vertically or horizontally (8-i0). Recently, the ~'ibrator has been changed to an u!tr-a~3nic die (generator) of gas without direct c o n t a c t with the molten metal stream, and atomizing and solidibing it in a one-step process. L~qtrasonic gas atomization (dD' gas a t o m i z a t i o n ) has been developed using similar atomizers to the usual gas-oil burners. This technique enables improved control of the atomizing gas jets compared to conventional methods, resulting a close control of the characteristics of the atomized powders. The characteristic working frequency, is between 60 000 and 120 000 cps, with high velociW pulses of gas up to Mach 2. The multiple ducts that deliver the energy pulse to the metal stream function as Hartmann shock wave tubes. The atomization equipment comprises an ultrasound generator, vibrator (halfwavelength compound transducer, stepped horn amplitude transformer, bending resonator), atomization vessel, and furnace for melt supply. A typical generator is a big circular die with the following the dimensions: high 2.5 cm (1 in.), OD 3.8-5 cm (1.5-2 in.), and ID 1.3-1.4 cm (1/2-3/4 in). A circular multiple die next to the external upper zone feeds the gas at pressures of 15-30 atmospheres into 20-30 tubes which vibrate the gas at a characteristic frequency onto a molten metal flow through the generator ID. A great number of alloys and amorphous metallic (Cu6oZr4o), powders can be manufactured (6, 11), but melting point must be necessarily below 1000°C. Advantages of this technique compared to gas atomization are the following (12) (a) low velocities for ejected droplets, (b) reduced dimensions of the atomization vessel (c) no atomization media is consumed, (d) facilitates the set-up of hermetically tight units, (e) decreased production cost, (f) high energy efficiency, (g) a very narrow size range (1 up to 50/~m), (h) very repetitive particle shapes, (i) spherical particles, (j) very fine powders (up to 1 #m), (k) average particle sizes are in the range 4-10 #m with a yields in excess for 90% at these small particle diameters, (1) measured dendritic arm spacings are consistent ~ith a particle cooling rate in the range 107-109 K/s, but generally cooling rates in the range 10-105 K/s are obtained (comparabtes to gas atomization).
Vacuum atomization IIII II III
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FIGURE 3: Principle of capillary-wave atomization (d, = ~.,/4).
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In this process, also called 'gas soluble atomization', the potential energy necessary to produce the atomization is stored into the molten metal, obtaining a high increase in the efficiency. The principle of
FIGUF~E 4: Vacuum atomization process:
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this process (Figure 4) is b a s e d on t h e fact t h a t when a m o l t e n m e t a l s u p e r s a t u r e d with a gas ( u s u a l l y hydrogen) u n d e r p r e s s u r e is s u d d e n l y e x p o s e d to v a c u u m , t h e g a s e x p a n d s a n d c a u s e s a t o m i z a t i o n of t h e metal. Therefore, this p r o c e s s is a degasific a t i o n of a m o l t e n m e t a l s t r e a m by a v a c u u m nozzle. The d i s i n t e g r a t i o n a g e n t of t h e s t r e a m is d i s s o l v e d n o r m a l l y in a c o n c e n t r a t i o n of 1-3 ppm. The e q u i p m e n t for this process comp r i s e s two vertical c h a m b e r s c o n n e c t e d by a pipe, a lower section of v a c u u m i n d u c t i o n m e l t i n g and a u p p e r t a n k to collect t h e powder. Vacuum i n d u c t i o n m e l t i n g of t h e m e t a l is followed by a p r e s s u r i z a t i o n of t h e i s o l a t e d m e l t c h a m b e r with a m i x t u r e of i n e r t a n d s o l u b l e gas u n d e r c o n t r o l l e d p r e s s u r e (13). This p r e s s u r i z e d system is e x p o s e d to the e v a c u a t e d collection chamber, t h e s t o r e d energy of the gas forces t h e m o l t e n m e t a l t h r o u g h a ceramic t r a n s f e r t u b e (electrically h e a t e d ) causing its fragm e n t a t i o n a n d a t o m i z a t i o n . The d r o p s p r o d u c e d fall into t h e u p p e r t a n k where t h e y solidify a n d cool. The p o w d e r s prod u c e d a r e m a i n t a i n e d u n d e r v a c u u m or i n e r t a t m o s p h e r e . Both t h e a t o m i z a t i o n a n d the v a c u u m c h a m b e r a r e w a t e r quenched. The possibility of s e p a r a t e l y m a n i p u l a ting t h e p r e s s u r e in t h e a t o m i z a t i o n a n d collection c h a m b e r s and, varying gas mixtures p e r m i t s control of t h e p o w d e r characteristics. Various alloy (A1, Ti, superalloys, Cu, Ni, Co, Mg, Fe, Mo, Mo-Cr) systems have been m a n u f a c t u r e d (6, 14, 15). C h a r a c t e r Rotating Eiec%rode
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istics of the process are: (a) very clean p o w d e r s (low oxH'gen c o n t e n t ) with s o m e pr~)tuberances, (b) high densiD~ c o m p a c t s are o b t a i n e d by h o t i s o s t a t i c p r o c e s s i n g (HIP) ( t a p d e n s i w of powders: 69% of the t h e o r e t i c a l d e n s i w ) , (c) p a r t i c l e s h a p e c a n n o t be controlled, (d) p a r t i c l e s below 100 !tin have preferent!y s p h e r i c a l shape, (e) c o a r s e r p a r t i c l e s h a v e p r e f e r e n t l y l a m i n a r shape, (f) p a r t i c l e size a n d p a r t i c l e size d i s t r i b u t i o n c a n n o t be c o n t r o l l e d b u t the l a t t e r is n a r r o w e r c o m p a r e d to inert gas and r o t a r y m e t h o d s , (g) hydrogen e x c e s s m a y be e l i m i n a t e d , ( h ) s o m e c e r a m i c inclusions p r o c e e d i n g of t h e crucible m a t e r ial of i n d u c t i o n h~rnace c o n t a m i n a t e t h e powder.
Vibrating electrode This p r o c e s s p r o d u c e s h i g h - p u r i t y m e t a l p o w d e r s by t h e v i b r a t i o n of a c o n s u m a b l e e l e c t r o d e (16). The electrode, forming a r e s o n a n t rod ~ i t h a fixed a n d free end, is c o n t i n u o u s l y moved between rollers t o w a r d a slowly r o t a t i n g c o p p e r disk (Figure 5). A t o m i z a t i o n t a k e s p l a c e in the arc formed between t h e w a t e r cooled d i s k a n d the v i b r a t i n g end of t h e electrode. S p h e r i c a l p a r t i c l e s a r e formed, whilst p a r t i c l e size a n d p a r t i c l e size d i s t r i b u t i o n can be c o n t r o l l e d by changing t h e length of the r e s o n a n t rod. The n a r r o w e s t p a r t i c l e size d i s t r i b u t i o n is o b t a i n e d at the r e s o n a n t frequency of t h e electrode. At a given frequency of oscillation both the mean particle diameter and p a r t i c l e size range d e c r e a s e with d e c r e a s i n g wire d i a m e t e r . Very clean p o w d e r s (oxygen c o n t e n t 4 0 . 1 % ) of mild steel, CrNi steel, a n d t u n g s t e n alloy systems can be o b t a i n e d .
Melt drop technique The p r i n c i p l e of this p r o c e s s is based on t h e d i s i n t e g r a t i o n of liquid m e t a l s t r e a m s with t h e help of t h e gravity, the surface tension of the m e l t a n d t h e horizontal v i b r a t i o n s of a v a r i a b l e frequency a p p l i e d to t h e crucible. The e q u i p m e n t consists of a crucible w h o s e b o t t o m c o n t a i n s a nozzle with m a n y holes of equal size from which h q u i d m e t a l s t r e a m s e m e r g e and e x p a n d into a v a c u u m o r inert gas chamber. For small d i a m e t e r nozzles, a b a c k up p r e s s u r e of hydrogen is used over t h e m e l t (Figure 6). The crucible is v i b r a t e d by a v a r i a b l e s p e e d direct c u r r e n t m o t o r a t a v i b r a t i o n frequent3" of 7-25 cps, a n d 10 m m a m p l i t u d e . The m e l t d r o p a t o m i z a t i o n m e c h a n i s m is a n a l o g o u s to the d i s i n t e g r a t i o n of a j e t formed from a c a p i l l a r y tube, in which the j e t d i s r u p t s small d r o p l e t s u n d e r t h e a c t i o n of the surface tension. Disintegration t a k e s MPR January 1991 47
FIGURE 6: Atomization by the melt-drop
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place when a capillary wave is applied to the jet. Particle s h a p e and size (coarse or granules up to 1.5 ram) can be controlled by modifying the following factors (17, 18): (a) nozzle diameter, (b) melt s t r e a m velocity, (c) specific gravity of the melt, (d) particle cooling rate, and (e) surface tension of the liquid. Cooling rate is modified by the drop height so controlling structure and shape of particle. Imposing longitudinal oscillations (50-1000 Hz) to melt the particle size distributions are very narrow, and the absolute particle size also decreases with increasing frequency. The melt drop technique has been applied to A1, Be, Cu, Pb, mild steel and superallo)~.
Concluding remarks ......
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The choice of a powder production process is a difficult selection, because there are different variables (material, process, economics) in c o m p e t i t i o n . However, the following f a c t o r s m u s t be t a k e n into account when a powder production method is chosen (a) start material characteristics (chemical reactivity, fusion point, display form, transformation possibilities, etc.), (b) economic factors such as starting material as manufactured powders, (c) advantages and disadvantages from each production method, (d) installation and maintenance costs, (e) annual production of powder, (f) automation possibilities, (g) future applications of manufactured powder, (h) secondary t r e a t m e n t s of p o w d e r ( t r e a t m e n t number and type). The following powder treatments are generally, but not necessarily used: mixing, friction, sieving, striking out the ceramic particles, by both a centrifugal process for the smallest size particles or a method based on the electrical property differences for the coarser particles, and vacuum degasification (firstly at 10 a mbar and later at 10 ~ mbar). In conclusion, powder production methods have rapidly improved during recent years. Nevertheless, methods currently in 48 MPR January 1991
use are the two-fluid atomization, but an increased need for high qualiw powders ~Jides the development towards other new special techniques for powder production. In spite of efforts performed in order to understand the mechanisms governing the atomization process (19), it is still necessa~" for further contributions with the aim of standardizing the influence of the process variables in the manufactured materials. It is clear that each technique offers advantages and limitations which must be carefully studied when chosing a production method.
References (1). A.J. Aller and A. Losada J. of Powder & Bulk Solids Technology, 1990, In press. (2). AJ. Aller and A. Losada Metal Powder Repart, 1990, Vol. 45, No. 1, p. 51-55. (3). A.R.E. Singer and A.D. Roche, Modern Developments in PM, Vol 9, 1977, p.127. (4). P.tC Dolmalavage, and N.J. Grant, Metal Powder Report, 1983, Vol. 38, No. 10, p.555562. (5). J. Ishihara et al, J. of Japan Institute of Metals, 1983, No 6, p.527-532. (6). A~I. Aller, Aluminium, 1984, Vol. 60, No. 5, p.357-361. (7). R. Ruthardt and E.G. Lierke, Modern Developments in PM, Vol 12, 1980, p.105111. (8). K.K. Sankaran et al, Paper presented at the 1983 AIME Annual Meeting, Atlanta, USA, 6-10 March, 1983. (9). Sh.M. Sheikhaliev and S.I. Popel, Poro s h k o v a y a Metallurgiya, 1984, No. 10, p. 18-24. (10). S.M.L., Sastry, Metal Powder Report, 1984, Vol. 39, No. 9, p.537-538. (11). P. Domalavage et al, Materials Science and Engineering, 1983, Vol. 57, No. L1-L2. (12). D.H. Ro and H. Sunwood, Paper presented at the 1983 Annual PM Conferene, New Orleans, USA, 1-4 May. (13). C.W. Fox, Paper presented at the 1983 AIME Annual Meeting, Atlanta, USA, 610 March. (14). A.J. Aller and L Deban, J. of Powder& Bulk Solids Technology, 1984, Vol. 8, No. 1, P.l-13. (15). AJ. Aller, J. of Pawder & Bulk Solids Technology, 1984, Vol. 8, No. 3, P.1-20. (16). G. Matei et al, Modern Developments in PM, Vol. 9, 1977, p.153. (17). F. Aldinger et al, Modern Developments in PM, Vol. 9, 1977, p.141. (18). E.M. Luygur, Productivity and Powder Metallurgy, MPR P u b l i s h i n g Services, Shrewsbury, UK, 1980, p.33. (19). A.J. Aller and A. Losada Powder Metallurgy Internatonal, 1989, Vol. 21, No. 5, p.15-19.
S P E C ~
F E A T U ~
Mechanical atomization of molten metals A.J. A l l e r a n d A. L o s a d a ( C h e m i s t r y D e p a r t m e n t , U n i v e r s i t y o f Le~n, Spain)
D e v e l o p m e n t s in m o l t e n m e t a l a t o m i z a t i o n h a v e f o l l o w e d the r e q u i r e m e n t s of clearly identifiable m a r k e t s for a d v a n c e d alloy compositions. Consequently, new requirements are leading to d e t a i l e d d i f f e r e n c e s in t h e a t o m i z a t i o n p r o c e s s e s . The foil o w i n g article outlines s o m e o f the d e v e l o p m e n t s w h i c h h a v e t a k e n p l a c e o v e r r e c e n t y e a r s in the a t o m i z a t i o n p r o c e s s e s f o r the m o l t e n m e t a l s , a n d w h i c h can r e p l a c e w a t e r ~ g a s a n d rotary atomization.
he d e v e l o p m e n t of a d v a n c e d m a t e r ials to be used in e x t r e m e c o n d i t i o n s forces r e s e a r c h into new m a n u f a c t u r ing technologies. Power m e t a l l u r g y (PM) is becoming an a d e q u a t e a l t e r n a t i v e to t h e m o r e c o n v e n t i o n a l p r o d u c t i o n m e t h o d s of casting techniques, due f u n d a m e n t a l l y to t h e r e d u c e d costs in PM technology. In t h e two-fluid a t o m i z a t i o n p r o c e s s e s (1), as well as in t h e centrifugal a t o m i z a t i o n ones (2), the energy yield with r e s p e c t to the s e p a r a t i o n w o r k is relatively small and, consequently, an economic d i s a d v a n t a g e . There is therefore, an increasing d e m a n d for a m o r e a p p r o p r i a t e p r o c e s s for t h e p r o d u c t i o n of m e t a l powders. The m a i n p u r p o s e of this article is to describe briefly all a l t e r n a t i v e m e t h o d s of m o l t e n m e t a l a t o m i z a t i o n . A t o m i z a t i o n techniques in this category do not involve r o t a t i o n a n d / o r i m p a c t of t h e m e l t by gas or ~ a t e r , a n d all of t h e m a r e c h a r a c t e r i z e d by t h e fact t h a t t h e energy used in t h e blasting of the liquid m e t a l s t r e a m is a p o t e n t i a l energy (1).
T
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In this t e c h n i q u e a s t r e a m of m o l t e n m e t a l is fed between the inwardly r o t a t i n g rolls (3-5). Upon contact, the alloy s t a r t s to
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FIGURE 1." Schematic diagram of roller atomization.
solidify being p a r t i a l l y hot rolled forming thin metallic foils (Figure 1). W a t e r is used as t h e first cooling agent, a n d a secondary. cooling is p e r f o r m e d when t h e p a r t i c u l a t e s are collected in an a l u m i n i u m bin. This m e t h o d is c a p a b l e of converting a wide range of m e t a l s (?d, Cu, Sn, Pb) into flakes, acicular, irregular, or s p h e r i c a l p a r t i c l e s (6). Mean p a r t i c l e size (220 pro) is inversely p r o p o r t i o n a l to t h e r o t a t i o n a l speed. Typical v a r i a b l e s are: roll r o t a t i n g s p e e d (200 r p s ) , roll gap (60 # m ) , s t r e a m diam e t e r (2.4 m m ) , s u p e r h e a t (100°C), a n d solidification rate (105°C/s). Roller a t o m i z a t i o n m e c h a n i s m is s i m i l a r to ca~-itation in oil l u b r i c a t e d bearings (3), in which liquid ( m o l t e n m e t a l in roller atomization) cavitates a small distance d o ~ n s t r e a m from t h e roll n i p p r o d u c i n g p e r f o r a t i o n s a n d s t r e a m e r s . A t high roller s p e e d s t h e m o l t e n m e t a l flows a r o u n d t h e p o i n t s of cavitation forming t h i n t h r e a d s of metal, which b r e a k down b e c a u s e of the m o l t e n m e t a l j e t instabilities.
Ultrasonic atomization
The d i r e c t c o n t a c t of liquid m e t a l with s o m e p a r t s of t h e e q u i p m e n t has been overcome by a t o m i z i n g t h e l i q u i d m e t a l s t r e a m bet~veen t~vo vertical n o n - c o n t a c t e d ultrasonic r e s o n a t o r plates. The alloy is induction heated in a rare earth ( t h e r m o d y n a m i c a l l y o r kinetically s t a b l e in c o n t a c t with molten alloy) o x i d e c o a t e d z i r c o n i a crucible u n d e r a inert gas atmoM P R J a n u a r y 1991 4 5
': P M
SPECIAL
FF.ATURE
FIGURE 2: Ultrasonic gas atomization.
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I Col lectionI chamber sphere, and the molten metal is compressed through an unheated tundish falling into a stationary ultrasonic field. Molten metal passes this zone very fast e x p a n d i n g immediately after compression and 'explodes' into small droplets (Figure 2). In ultrasonic atomization (Figure 3), a thin layer of a molten metal covering the surface of a resonator forms stationary capillary-waves from a certain limiting value of the elongation-amplitude (7). This totally non-contaminating disintegration method is dependent on melting temperature. The pneumoacoustic method based on the Hartmann oscillator is performed in the ultrasonic range (100 KHz), vibrating a freely pendulous vibrator, the end of which hits against a molten metal alloy producing very fine metallic powders. The vibrator can V,~rticol Vibrator
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be placed vertically or horizontally (8-i0). Recently, the ~'ibrator has been changed to an u!tr-a~3nic die (generator) of gas without direct c o n t a c t with the molten metal stream, and atomizing and solidibing it in a one-step process. L~qtrasonic gas atomization (dD' gas a t o m i z a t i o n ) has been developed using similar atomizers to the usual gas-oil burners. This technique enables improved control of the atomizing gas jets compared to conventional methods, resulting a close control of the characteristics of the atomized powders. The characteristic working frequency, is between 60 000 and 120 000 cps, with high velociW pulses of gas up to Mach 2. The multiple ducts that deliver the energy pulse to the metal stream function as Hartmann shock wave tubes. The atomization equipment comprises an ultrasound generator, vibrator (halfwavelength compound transducer, stepped horn amplitude transformer, bending resonator), atomization vessel, and furnace for melt supply. A typical generator is a big circular die with the following the dimensions: high 2.5 cm (1 in.), OD 3.8-5 cm (1.5-2 in.), and ID 1.3-1.4 cm (1/2-3/4 in). A circular multiple die next to the external upper zone feeds the gas at pressures of 15-30 atmospheres into 20-30 tubes which vibrate the gas at a characteristic frequency onto a molten metal flow through the generator ID. A great number of alloys and amorphous metallic (Cu6oZr4o), powders can be manufactured (6, 11), but melting point must be necessarily below 1000°C. Advantages of this technique compared to gas atomization are the following (12) (a) low velocities for ejected droplets, (b) reduced dimensions of the atomization vessel (c) no atomization media is consumed, (d) facilitates the set-up of hermetically tight units, (e) decreased production cost, (f) high energy efficiency, (g) a very narrow size range (1 up to 50/~m), (h) very repetitive particle shapes, (i) spherical particles, (j) very fine powders (up to 1 #m), (k) average particle sizes are in the range 4-10 #m with a yields in excess for 90% at these small particle diameters, (1) measured dendritic arm spacings are consistent ~ith a particle cooling rate in the range 107-109 K/s, but generally cooling rates in the range 10-105 K/s are obtained (comparabtes to gas atomization).
Vacuum atomization IIII II III
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46
M P R J a n u a r y 1991
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In this process, also called 'gas soluble atomization', the potential energy necessary to produce the atomization is stored into the molten metal, obtaining a high increase in the efficiency. The principle of
FIGUF~E 4: Vacuum atomization process:
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this process (Figure 4) is b a s e d on t h e fact t h a t when a m o l t e n m e t a l s u p e r s a t u r e d with a gas ( u s u a l l y hydrogen) u n d e r p r e s s u r e is s u d d e n l y e x p o s e d to v a c u u m , t h e g a s e x p a n d s a n d c a u s e s a t o m i z a t i o n of t h e metal. Therefore, this p r o c e s s is a degasific a t i o n of a m o l t e n m e t a l s t r e a m by a v a c u u m nozzle. The d i s i n t e g r a t i o n a g e n t of t h e s t r e a m is d i s s o l v e d n o r m a l l y in a c o n c e n t r a t i o n of 1-3 ppm. The e q u i p m e n t for this process comp r i s e s two vertical c h a m b e r s c o n n e c t e d by a pipe, a lower section of v a c u u m i n d u c t i o n m e l t i n g and a u p p e r t a n k to collect t h e powder. Vacuum i n d u c t i o n m e l t i n g of t h e m e t a l is followed by a p r e s s u r i z a t i o n of t h e i s o l a t e d m e l t c h a m b e r with a m i x t u r e of i n e r t a n d s o l u b l e gas u n d e r c o n t r o l l e d p r e s s u r e (13). This p r e s s u r i z e d system is e x p o s e d to the e v a c u a t e d collection chamber, t h e s t o r e d energy of the gas forces t h e m o l t e n m e t a l t h r o u g h a ceramic t r a n s f e r t u b e (electrically h e a t e d ) causing its fragm e n t a t i o n a n d a t o m i z a t i o n . The d r o p s p r o d u c e d fall into t h e u p p e r t a n k where t h e y solidify a n d cool. The p o w d e r s prod u c e d a r e m a i n t a i n e d u n d e r v a c u u m or i n e r t a t m o s p h e r e . Both t h e a t o m i z a t i o n a n d the v a c u u m c h a m b e r a r e w a t e r quenched. The possibility of s e p a r a t e l y m a n i p u l a ting t h e p r e s s u r e in t h e a t o m i z a t i o n a n d collection c h a m b e r s and, varying gas mixtures p e r m i t s control of t h e p o w d e r characteristics. Various alloy (A1, Ti, superalloys, Cu, Ni, Co, Mg, Fe, Mo, Mo-Cr) systems have been m a n u f a c t u r e d (6, 14, 15). C h a r a c t e r Rotating Eiec%rode
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istics of the process are: (a) very clean p o w d e r s (low oxH'gen c o n t e n t ) with s o m e pr~)tuberances, (b) high densiD~ c o m p a c t s are o b t a i n e d by h o t i s o s t a t i c p r o c e s s i n g (HIP) ( t a p d e n s i w of powders: 69% of the t h e o r e t i c a l d e n s i w ) , (c) p a r t i c l e s h a p e c a n n o t be controlled, (d) p a r t i c l e s below 100 !tin have preferent!y s p h e r i c a l shape, (e) c o a r s e r p a r t i c l e s h a v e p r e f e r e n t l y l a m i n a r shape, (f) p a r t i c l e size a n d p a r t i c l e size d i s t r i b u t i o n c a n n o t be c o n t r o l l e d b u t the l a t t e r is n a r r o w e r c o m p a r e d to inert gas and r o t a r y m e t h o d s , (g) hydrogen e x c e s s m a y be e l i m i n a t e d , ( h ) s o m e c e r a m i c inclusions p r o c e e d i n g of t h e crucible m a t e r ial of i n d u c t i o n h~rnace c o n t a m i n a t e t h e powder.
Vibrating electrode This p r o c e s s p r o d u c e s h i g h - p u r i t y m e t a l p o w d e r s by t h e v i b r a t i o n of a c o n s u m a b l e e l e c t r o d e (16). The electrode, forming a r e s o n a n t rod ~ i t h a fixed a n d free end, is c o n t i n u o u s l y moved between rollers t o w a r d a slowly r o t a t i n g c o p p e r disk (Figure 5). A t o m i z a t i o n t a k e s p l a c e in the arc formed between t h e w a t e r cooled d i s k a n d the v i b r a t i n g end of t h e electrode. S p h e r i c a l p a r t i c l e s a r e formed, whilst p a r t i c l e size a n d p a r t i c l e size d i s t r i b u t i o n can be c o n t r o l l e d by changing t h e length of the r e s o n a n t rod. The n a r r o w e s t p a r t i c l e size d i s t r i b u t i o n is o b t a i n e d at the r e s o n a n t frequency of t h e electrode. At a given frequency of oscillation both the mean particle diameter and p a r t i c l e size range d e c r e a s e with d e c r e a s i n g wire d i a m e t e r . Very clean p o w d e r s (oxygen c o n t e n t 4 0 . 1 % ) of mild steel, CrNi steel, a n d t u n g s t e n alloy systems can be o b t a i n e d .
Melt drop technique The p r i n c i p l e of this p r o c e s s is based on t h e d i s i n t e g r a t i o n of liquid m e t a l s t r e a m s with t h e help of t h e gravity, the surface tension of the m e l t a n d t h e horizontal v i b r a t i o n s of a v a r i a b l e frequency a p p l i e d to t h e crucible. The e q u i p m e n t consists of a crucible w h o s e b o t t o m c o n t a i n s a nozzle with m a n y holes of equal size from which h q u i d m e t a l s t r e a m s e m e r g e and e x p a n d into a v a c u u m o r inert gas chamber. For small d i a m e t e r nozzles, a b a c k up p r e s s u r e of hydrogen is used over t h e m e l t (Figure 6). The crucible is v i b r a t e d by a v a r i a b l e s p e e d direct c u r r e n t m o t o r a t a v i b r a t i o n frequent3" of 7-25 cps, a n d 10 m m a m p l i t u d e . The m e l t d r o p a t o m i z a t i o n m e c h a n i s m is a n a l o g o u s to the d i s i n t e g r a t i o n of a j e t formed from a c a p i l l a r y tube, in which the j e t d i s r u p t s small d r o p l e t s u n d e r t h e a c t i o n of the surface tension. Disintegration t a k e s MPR January 1991 47
FIGURE 6: Atomization by the melt-drop
technique.
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place when a capillary wave is applied to the jet. Particle s h a p e and size (coarse or granules up to 1.5 ram) can be controlled by modifying the following factors (17, 18): (a) nozzle diameter, (b) melt s t r e a m velocity, (c) specific gravity of the melt, (d) particle cooling rate, and (e) surface tension of the liquid. Cooling rate is modified by the drop height so controlling structure and shape of particle. Imposing longitudinal oscillations (50-1000 Hz) to melt the particle size distributions are very narrow, and the absolute particle size also decreases with increasing frequency. The melt drop technique has been applied to A1, Be, Cu, Pb, mild steel and superallo)~.
Concluding remarks ......
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The choice of a powder production process is a difficult selection, because there are different variables (material, process, economics) in c o m p e t i t i o n . However, the following f a c t o r s m u s t be t a k e n into account when a powder production method is chosen (a) start material characteristics (chemical reactivity, fusion point, display form, transformation possibilities, etc.), (b) economic factors such as starting material as manufactured powders, (c) advantages and disadvantages from each production method, (d) installation and maintenance costs, (e) annual production of powder, (f) automation possibilities, (g) future applications of manufactured powder, (h) secondary t r e a t m e n t s of p o w d e r ( t r e a t m e n t number and type). The following powder treatments are generally, but not necessarily used: mixing, friction, sieving, striking out the ceramic particles, by both a centrifugal process for the smallest size particles or a method based on the electrical property differences for the coarser particles, and vacuum degasification (firstly at 10 a mbar and later at 10 ~ mbar). In conclusion, powder production methods have rapidly improved during recent years. Nevertheless, methods currently in 48 MPR January 1991
use are the two-fluid atomization, but an increased need for high qualiw powders ~Jides the development towards other new special techniques for powder production. In spite of efforts performed in order to understand the mechanisms governing the atomization process (19), it is still necessa~" for further contributions with the aim of standardizing the influence of the process variables in the manufactured materials. It is clear that each technique offers advantages and limitations which must be carefully studied when chosing a production method.
References (1). A.J. Aller and A. Losada J. of Powder & Bulk Solids Technology, 1990, In press. (2). AJ. Aller and A. Losada Metal Powder Repart, 1990, Vol. 45, No. 1, p. 51-55. (3). A.R.E. Singer and A.D. Roche, Modern Developments in PM, Vol 9, 1977, p.127. (4). P.tC Dolmalavage, and N.J. Grant, Metal Powder Report, 1983, Vol. 38, No. 10, p.555562. (5). J. Ishihara et al, J. of Japan Institute of Metals, 1983, No 6, p.527-532. (6). A~I. Aller, Aluminium, 1984, Vol. 60, No. 5, p.357-361. (7). R. Ruthardt and E.G. Lierke, Modern Developments in PM, Vol 12, 1980, p.105111. (8). K.K. Sankaran et al, Paper presented at the 1983 AIME Annual Meeting, Atlanta, USA, 6-10 March, 1983. (9). Sh.M. Sheikhaliev and S.I. Popel, Poro s h k o v a y a Metallurgiya, 1984, No. 10, p. 18-24. (10). S.M.L., Sastry, Metal Powder Report, 1984, Vol. 39, No. 9, p.537-538. (11). P. Domalavage et al, Materials Science and Engineering, 1983, Vol. 57, No. L1-L2. (12). D.H. Ro and H. Sunwood, Paper presented at the 1983 Annual PM Conferene, New Orleans, USA, 1-4 May. (13). C.W. Fox, Paper presented at the 1983 AIME Annual Meeting, Atlanta, USA, 610 March. (14). A.J. Aller and L Deban, J. of Powder& Bulk Solids Technology, 1984, Vol. 8, No. 1, P.l-13. (15). AJ. Aller, J. of Pawder & Bulk Solids Technology, 1984, Vol. 8, No. 3, P.1-20. (16). G. Matei et al, Modern Developments in PM, Vol. 9, 1977, p.153. (17). F. Aldinger et al, Modern Developments in PM, Vol. 9, 1977, p.141. (18). E.M. Luygur, Productivity and Powder Metallurgy, MPR P u b l i s h i n g Services, Shrewsbury, UK, 1980, p.33. (19). A.J. Aller and A. Losada Powder Metallurgy Internatonal, 1989, Vol. 21, No. 5, p.15-19.