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mica composites
Segregation of mica particles in centrifugal and static castings of aluminium/mica composites D. NATH and P.K. ROHATGI
Mica particles were dispersed in molten AI - 4% Cu - 1.5% Mg alloys and then poured into rotating permanent cast iron moulds to obtain hollow cylindrical castings. The centrifugally cast hollow cylinders showed t w o distinct z o n e s - (1) an outer zone free from mica and (2) an inner zone, about 12 mm to 22 mm thick - where most of the mica particles had segregated. It was possible to segregate up to 9% mica near the inner periphery by centrifuging whereas it is difficult to obtain more than three percent in static castings. The mica particles near the inner periphery can serve as good solid lubricants when the cylindrical castings are used as bearings even after considerable machining, whereas the mica-free outer zone having nearly the same strength as the matrix alloy would serve as a good backing material. Effects of pouring temperature and particle size on the distribution of mica particles in the hollow cylindrical castings were studied. The mica-free zone width increased with an increase in the particle size of mica and the pouring temperature of the melt. Limited experiments on static casting are reported to establish that mica segregation is mainly due to density difference, and not to rejection by the solidifying front. Theoretical calculations give computed values of mica free zone widths which are in good agreement with those observed during centrifugal casting. Mica is considered to be a good solid lubricant because of its layered structure. Metal matrices 1-s containing dispersed mica particles have been shown to be good antifraction materials. The conventional method of making such metal/ ceramic-particulate composites is by powder metallurgical techniques, but recently, it has been possible to produce cast aluminium alloy/mica-particle composites by dispersing mica particles in molten aluminium alloys and then casting in permanent moulds, s This method is simpler than the powder metallurgy technique, and is capable of producing larger parts and more complex shapes. However, one limitation of casting aluminium alloy/mica composites s is the inability to produce static castings containing more than about three percent mica. As the quantity of mica particles in the liquid alloy is increased, the fluidity of the melt decreases and dispersion becomes more difficult above 3% mica. The mechanical strength of the matrix alloy would also decrease to a low value above a certain percentage of mica. To make use of the antifriction properties endowed by the mica particles and retain the strength of the component, it is desirable to restrict the mica particle dispersion to those regions which will be subjected to wear.
About 3 kg of A1 - 4% Cu alloy was melted in an oil fired furnace in a 'super salamander' crucible and superheated to around 800°C. The molten metal was then held at 700°C in an electrical resistance furnace and degassed with nitrogen (10 litres/min) for 2 minutes. The mica powder* was produced by milling muscovite mica sheets obtained from Mysore Micanite Industries, Bangalore. About 10 sheets (5 mm thick x 120 mm x 80 mm) were put together
Krishnan e t al 6 have shown that granular graphite can be segregated near the inner periphery of centrifugally cast graphite/aluminium alloy composites. There is no reported
* the composition of the mica powder was, SiO 2 - 4 8 . 1 2 w e i g h t %, A I 2 0 3 - 33.06%; K 2 0 - 7.1%; CuO - 1.62%; MgO - 1.13%; N a 2 0 - 0.75%; L i 2 0 - 0.33%; 4.78% was lost on ignition
work on the centrifugal casting of mica dispersed alumininm alloys. If mica particles can be segregated in a zone near the inner periphery of such centrifugally cast alloys one can produce cylinders which are ideal for bearing applications, s It is important that the mica-rich zone is thick enough to be machined and still leave a mica-rich face. The outer mica-free zone will act as a high-strength back-up material for the 'low-friction' inner surface. This paper describes the results of centrifugal casting of A1 - 4% Cu - 1.5% Mg alloy containing flaked mica particles dispersed in rotating permanent moulds. EXPERIMENTAL PROCEDURE
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COMPOSITES. APRI L 1981
Funnel for pouring the melt Nletal mould.
Cover plota sand packing
7
I' ° /
'
Turn table To belt drive Fig. 1
i \Mould cavity
180mm
mould
Schematic diagram of centrifugal casting unit
1) Metal was poured into a sand mould which had a water cooled copper chill at its bottom. This resulted in maximum heat dissipation being towards the bottom of the casting, in the opposite direction to the buoyancy force. That is, the buoyancy effect and the movement of the macroscopic solidifying front are in the same direction. 2) A heavy copper chill (25 kg) was kept on the top of a bottom poured sand mould during solidification. In this case, the solidifying front will move against the buoyancy effect. Solid castings were taken out of the mould, machined and sectioned horizontally and vertically to study the distribution of mica in radial and longitudinal directions. Macrophotographs of the sectioned surfaces were taken. Drillings taken from different parts of the castings were chemically analysed to determine the readial distribution of mica. RESULTS AND DISCUSSION
and held in a vice with their cleavage planes kept vertical and the cutting edge of the milling cutter perpendicular to the edge of the mica cleavage planes. This mica powder (preheated to 200°C for 12 hours and then to 700°C for 30 minutes) was added along with magnesium pieces (about 6 mm a) to the surface of the melt in a vortex created by stirring the liquid metal with a mechanical stirrer, s After adding the powder, the metal was stirred for half a minute and was then degassed by bubbling nitrogen into the melt at a rate slower than that used for degassing before dispersion. The degassing was accompanied by slow stirring of the melt for one minute. After this the crucible was taken out of the furnace - the melt being hand stirred with a graphite rod - and emptied into a rotating permanent mould. A vertical centrifugal casting machine shown in Fig. 1 was used for making cylindrical castings. The speed of rotation of the mould was kept at 150 rpm during pouring. This was increased to 680 rpm just after the pouring was completed and was kept constant until the solidification was completed. Two separate static casting experiments were done in which the directions of maximum heat dissipation (or of movement of macroscopic freezing fronts) were at different angles to the directions of the buoyancy effect (due to density difference) acting on the particles during solidification.
Macrophotographs of radial sections of centrifugally cast mica dispersed A1 - 4% Cu - 1.5% Mg castings containing different sizes of mica particles, and poured at different temperatures are shown in Fig. 2. In all the cases, the mica particles are segregated near the inner periphery of the cylindrical castings, leaving a mica-free outer ring. A typical macrophotograph of longitudinal section of centrifugally cast mica (120/am) dispersed aluminium alloy casting (poured at 760°C) where the bottom portion had not developed into a hollow cylinder is shown in Fig. 3. The density difference between mica powder (2.7 g/cm a) and aluminium alloy melt (2.77 g/cm a) has resulted in the segregation of the particles near the inner periphery. Fig. 4 shows a variation in thickness of the radial mica-free zone as a function of pouring temperature for different particle sizes. Fig. 5 shows the effect of particle size on the thickness of the radial mica-free zone at 720°C. From Fig. 4 it is clear that the higher the pouring temperature (within the range investigated), the greater is the thickness of micafree zone obtained in the casting. It is also clear from Fig. 4 that for the same pouring temperature, finer particle sizes give thinner mica-free zones. At higher pouring temperatures, more time is available for the particles to move and segregate before the metal solidifies resulting in a thicker mica-free zone. Likewise when the particles are fine their velocity in the liquid is expected to be lower leading to a narrower mica-free zone.
Fig. 2 Macrophotographs of radial sections of centrifugally cast mica dispersed AI -- 4% Cu -- 1.5% Mg alloy casting made by dispersing 2% mica particles of different sizes and poured at different temperatures (X 0.55) a) Size of particle: 40 p,m, pouring temperature: 690~C, b) Size of particle: 40/~m, pouring temperature: 728°C, c) Size of particle: 65/~m, pouring temperature: 680°C
C O M P O S I T E S . APR I L 1981
125
Chemical analysis as given in Fig. 6 indicates that in a typical centrifugal casting, the mica concentration increases from 0.07% at about 10 mm away from the outer periphery of the cylindrical casting, to above 8.50% at a distance of 24 mm from the outer periphery. This casting was poured at 760°C and contains 2 percent of 120/am size mica particles. Similar results were also obtained by Krishnan e t al 6 during -centrifugal casting of graphite-dispersed aluminium alloys in sand moulds except that the thicknesses of the mica-free zones obtained in the present investigation were smaller. This could be due to the following reasons: 1) smaller density difference between mica and AI - 4% Cu - 1.5% Mg alloy than that between graphite and the aluminium melt; 2) mica particles are flake shaped whereas the graphite particles are granules which are expected to move faster in liquid; 3) greater increases in melt viscosity are expected with the addition of flake shaped particles than with the addition of spheroidal particles, other conditions being same; 4) the permanent moulds used in the present investigation give higher freezing rates and hence less time for particle movement compared to the work with graphite where sand moulds were used. The mechanism of mica segregation at the inner periphery of the casting during centrifugal casting may be as follows: The molten aluminium alloy containing suspended mica particles poured into a rotating mould forms a paraboloid of revolution or cylinder due to centrifugal forces. The liquid freezes while it is in this tbrm due to the heat extracted by the mould. Due to the action of centrifugal force, the aluminium alloy melt which is heavier than mica particles moves farther away from the axis of
Fig. 3 Macrophotograph of longitudinal section of centrifugally cast mica dispersed aluminium alloy (X 1.00). Size of particle dispersed: 120/zm, Pouring temperature: 760°C 20 •- o - , , -
1 2 o l.u"n
10.0 /
65p, m - ~
40p.m
9.01lO i
8 o ~5
.5 o
I I 700 750 800 Pouring temper~tur~ (*C) Fig. 4 Thickness of radial mica-free zone as a function of pouring temperature 2O
E
650
o
~o
E
r~
5 I 20
I I I I 0 40 60 80 100 Particle size (l~m) Fig. 5 Effect of particle size of mica on thickness of radial mica-free zone at a pouring temperature of 720°C
126
120
10 15 20 25 Distance from inner periphery (ram)
Fig. 6 Variation of mica concentration with the distance from inner periphery in centrifugally cast mica dispersed aluminium alloy casting. Average particle size: 120/~m, pouring temperature: 760°C, speed: 680 rpm
C O M P O S I T E S . A P R I L 1981
rotation of the mould and mica particles move towards the centre of the casting while the alloy remains molten. This process continues in the zone which remains molten or partially molten even after solidification starts. On a microscopic scale it is observed that mica particles are generally present in the last freezing cell boundaries and they are apparently being rejected by the advancing solid/liquid front. Since in the case of permanent mould centrifugal casting much of the freezing is from the outer periphery to the inner periphery (freezing will also start at the inner periphery but at a much slower rate) it is possible that this also contributes to the segregation of mica near the inner periphery. However, the contribution of a moving dendritic interface to gross segregation of mica in a rotating permanent mould is not easy to compute and it was estimated by static casting experiments. The longitudinal sections of samples made by both of the static casting methods showed that all the particles had floated to the top portion of the casting. The shrinkage in the centre of the casting is an indication of some freezing from the top. This indicates that the segregation due to buoyancy prevails over any effect of rejection of particles by the freezing interface. This may be due to the fact that the freezing fronts under the present experimental conditions are dendritic and the mica particles rejected by the advancing solid front get into the inter-dendritic liquid where they are able to move along the direction of buoyancy force. It was seen that mica could segregate right up to the very top of the casting where the freezing front was moving downwards. The above observations suggest that the movement of mica particles in alloy melts during centrifugal casting is mainly due to centrifugal force. The extent to which the particle can move under centrifugal force in the casting is estimated in the following section. THEORETICAL AlVA L YSIS
After pouring, the liquid in the rotating mould takes the shape of a cylinder containing a cavity in the shape of a paraboloid of revolution and freezing begins at the mould wall as well as at the inner periphery. In permanent moulds the freezing rate at the mould wall will be much greater than that at the inner periphery. In the liquid cylinder initially, mica would be expected to be uniformly distributed across the cross section. Under the influence of centrifugal force the aluminium alloy melt which is heavier than the mica particles should move farther away from the axis of rotation of the mould, and the lighter mica particle should be displaced towards the inner periphery of the cylindrical castings. The mica particles initially accelerate from zero to terminal velocity, and then they can be expected to move with a constant velocity provided the viscosity of the melt remains unchanged. The distance travelled by the particles of various sizes during their acceleration to terminal velocity are calculated according to the following equation 6 :
etla _ 1 + x 1--x
where x is that fraction of terminal velocity which is reached in time tx. Here the fraction of terminal velocity is assumed to be equal to 0.95. Vm is the terminal velocity in cm/sec which is calculated by Stoke's law: Vm-9 A
2 (A - A') r2a kt
= density of mica particle = 2.7 gm/cm 3
A' = density of A1 - 4% Cu - 1.5% Mg alloy = 2.77 gm/cm 3 a
= acceleration whichis equal to 21681 cm/sec 2 at 680 rpm
/a
= viscosity in poise = 0.045
r
= radius of the particle in cms
The Stoke's law given above can be used to calculate the terminal velocity of spherical particle. The terminal velocity of non-spherical plate shaped particle such as mica will be different to that of the spherical particle. This also depends on the thickness to diameter ratio of the plate shaped particles. To calculate the terminal velocity of mica particle Stoke's law has to be modified. To calculate the rate of floatation of a circular plate 7 shaped particle of diameter ra Stoke's law can be applied using an effective spheroidal diameter r2 (the rate of floatation of a plate of diameter rl being the same as that of a sphere of diameter r2). The values of rl/r2 are7 3.05 and 2.16 respectively for thickness to diameter ratios of 1:20 and 1:10. The calculated values of r2 can be substituted in Stoke's equation to calculate the floatation ratios of the plate shaped particles. The terminal velocities of mica particles at different pouting temperatures as calculated from Stoke's law are given in Table 1. This table also shows that the distances travelled by the particles during the pre terminal-velocity phase are quite small compared to the thickness of the casting which is 2.5 cm in this case. Hence the distance travelled by a single particle from the outer periphery of the casting to the inner periphery can be approximated to the time for solidification multiplied by the terminal velocity of the particles. The time for solidification of the casting was calculated using the formula: t = S(H+CAT) h( TM - To)
where t
= time of solidification
H
= heat of fusion of pure aluminium
S =A.Vmln
h
= resistance to heat transfer at the mould/metal interface
where
C
= specific heat of alloy
S
= thickness of casting
2 + e tlA + e -tlA
A-
AVm 2(A - A~).a
COMPOSITES
. A P R I L 1981
TM = solidus temperature
127
Table 1.
Terminal velocity and distance travelled by 120/am mica particle across 2.5 cm thick centrifugal casting
Pouring temp. (°C)
Solidification time (seconds)
Distance to be travelled before attaining terminal velocity (cm)
Terminal velocity (cm/sec)
Calculated mica free zone (cm)
Actual mica free zone (cm)
680
47
6.23 x 10- 7
0.0274
1.29
0.9
720
50
7.25 x 10. 7
0.0290
1.45
1.05
760
53
9.42 x 10-7
0.0326
1.72
1.32
melts results in segregation of most of the mica particles in a mica-rich zone near the inner periphery of a cylindrical casting.
To = ambient temperature AT = (pouring temperature of alloy - TM). The assumption here is that the particles can move even when the alloy is in the mushy zone These calculations have been made assuming the viscosity of the melt containing the particles in suspension remains constant and is equal to the viscosity at pouring temperature without any suspended particles. According to this the velocity of the particle remains constant and the total freezing time is available for the movement of the particle. However, in reality the velocity of particle will not remain constant because of the following: (1) Centrifugal force acting on mica particles will decrease as the particles move towards the inner periphery. As a result, the velocity of the particles decreases progressively as they move towards the inner periphery. (2) Velocity of mica particles is reduced due to increasing concentration of particles as they are moving towards the inner periphery due to bridging of these particles against each other. (3) With time of centrifuging the temperature of the metal decreases and the viscosity will increase. This will offer a greater resistance to the movement of particles. (4) Viscosity of the melt increases due to the presence of particles in suspension in it and the viscosity further increases with increased segregation of the suspended particles travelling towards the inner periphery. In reality the movement will be arrested at a temperature somewhat above the solidus, thereby reducing the actual movement of particles. Due to the reasons given above the average velocity of the mica particles will actually be much lower and the actual distance travelled by the particle will be less than the calculated values in Table 1. The experimental values of mica-free zone width obtained for mica dispersed aluminium alloy containing 120/am mica particles in suspension and poured at 680°C, 720°C and 760°C were 0.90 cm, 1.05 cms and 1.32 cms respectively. These values are of the same order of magnitude as those in Table 1 but are slightly lower since the average effective viscosity of the composite melts during centrifuging is expected to be higher than that of suspension free melts. Nonetheless the experimental values confirm that the movement of the mica particles is mainly due to centrifugal forces. CONCLUSIONS
1. Centrifugal casting of mica dispersed aluminlurn alloy
128
2. The thickness of the radial mica-rich zone decreases with increasing pouring temperature and increasing mica particle size. However, under the range of conditions investigated the minimum thickness observed was 1.2 cms, which is adequate to leave a mica-rich bearing surface even after considerable machining. 3. Centrifugal casting of mica (120/am) dispersed aluminium alloy when poured at 760°C can result in mica concentrations up to about 9% at the inner periphery of the casting whereas it is difficult to make static castings containing more than 3% mica. 4. The calculated values of mica-free zone widths (using effective diameters in Stoke's Law for centrifugal flow) are of the same order of magnitude as the observed values. A CKNO WL ED GEMEN TS
The authors are grateful to Mr T.P. Murali for his editorial commerrts, to Dr R.M. Pillai and Mr M.K. Surappa for their comments on the heat transfer analysis. They are also grateful to Mr R. Babu for his line-drawings and to Mr B. Damodaran for typing the manuscript. REFERENCES 1
2 3
Afanas'ev, V.F. e t a l F i z K h i m M e k h a m M a t 5 N o 6 ( 1 9 7 0 )
p 680 Vishnevskii,V.B. and Afanas'ev, V.F. Izobert Prom Obvaztsky Tovaruye Znaski (1968) 45 No 5 p 107 Belitsky,M.E. Fiz Khim Mekham Mat 2 No 6 (1966) p 702
4
Rollet, Jean 'Self lubricating brushings' Compagnie
5
industrialle Des Metaux Eleetroniques Fr 2031, (CLF. 16 C B 32 f, C 10 m) (20 Nov. 1970) Appl (3 Feb 1969) p 4 Nath, Deo. Ph D Thesis (Department of Metallurgy, I I Se Bangalore, 1977)
6
Krishnan, B.P., Shetty, H.IL and Rohatgi, P.K. A F S Transactions 84 (1976) p 73
7
Gaudin,A.M. 'Principles of Mineral Dressing, Tata' (McGraw Hill Publishing Co Ltd, New Delhi, 1977) p 176
AUTHORS
Deo Nath is a lecturer at the Institute of Technology in Varanasi. P.K. Rohatgi is a Professor in the Metallurgy Department of the Institute of Science in Bangalore. Inquiries should be directed initially to Professor Rohatgi at the following address: Regional Research ,Laboratory, Trivandrum 695019, Kerala, India, of which he is a Director.
COMPOSITES . APR I L 1981
Mica particles were dispersed in molten AI - 4% Cu - 1.5% Mg alloys and then poured into rotating permanent cast iron moulds to obtain hollow cylindrical castings. The centrifugally cast hollow cylinders showed t w o distinct z o n e s - (1) an outer zone free from mica and (2) an inner zone, about 12 mm to 22 mm thick - where most of the mica particles had segregated. It was possible to segregate up to 9% mica near the inner periphery by centrifuging whereas it is difficult to obtain more than three percent in static castings. The mica particles near the inner periphery can serve as good solid lubricants when the cylindrical castings are used as bearings even after considerable machining, whereas the mica-free outer zone having nearly the same strength as the matrix alloy would serve as a good backing material. Effects of pouring temperature and particle size on the distribution of mica particles in the hollow cylindrical castings were studied. The mica-free zone width increased with an increase in the particle size of mica and the pouring temperature of the melt. Limited experiments on static casting are reported to establish that mica segregation is mainly due to density difference, and not to rejection by the solidifying front. Theoretical calculations give computed values of mica free zone widths which are in good agreement with those observed during centrifugal casting. Mica is considered to be a good solid lubricant because of its layered structure. Metal matrices 1-s containing dispersed mica particles have been shown to be good antifraction materials. The conventional method of making such metal/ ceramic-particulate composites is by powder metallurgical techniques, but recently, it has been possible to produce cast aluminium alloy/mica-particle composites by dispersing mica particles in molten aluminium alloys and then casting in permanent moulds, s This method is simpler than the powder metallurgy technique, and is capable of producing larger parts and more complex shapes. However, one limitation of casting aluminium alloy/mica composites s is the inability to produce static castings containing more than about three percent mica. As the quantity of mica particles in the liquid alloy is increased, the fluidity of the melt decreases and dispersion becomes more difficult above 3% mica. The mechanical strength of the matrix alloy would also decrease to a low value above a certain percentage of mica. To make use of the antifriction properties endowed by the mica particles and retain the strength of the component, it is desirable to restrict the mica particle dispersion to those regions which will be subjected to wear.
About 3 kg of A1 - 4% Cu alloy was melted in an oil fired furnace in a 'super salamander' crucible and superheated to around 800°C. The molten metal was then held at 700°C in an electrical resistance furnace and degassed with nitrogen (10 litres/min) for 2 minutes. The mica powder* was produced by milling muscovite mica sheets obtained from Mysore Micanite Industries, Bangalore. About 10 sheets (5 mm thick x 120 mm x 80 mm) were put together
Krishnan e t al 6 have shown that granular graphite can be segregated near the inner periphery of centrifugally cast graphite/aluminium alloy composites. There is no reported
* the composition of the mica powder was, SiO 2 - 4 8 . 1 2 w e i g h t %, A I 2 0 3 - 33.06%; K 2 0 - 7.1%; CuO - 1.62%; MgO - 1.13%; N a 2 0 - 0.75%; L i 2 0 - 0.33%; 4.78% was lost on ignition
work on the centrifugal casting of mica dispersed alumininm alloys. If mica particles can be segregated in a zone near the inner periphery of such centrifugally cast alloys one can produce cylinders which are ideal for bearing applications, s It is important that the mica-rich zone is thick enough to be machined and still leave a mica-rich face. The outer mica-free zone will act as a high-strength back-up material for the 'low-friction' inner surface. This paper describes the results of centrifugal casting of A1 - 4% Cu - 1.5% Mg alloy containing flaked mica particles dispersed in rotating permanent moulds. EXPERIMENTAL PROCEDURE
0010-4361/81/020124-05 ~02.00 © 1981 IPC BusinessPress Ltd
124
COMPOSITES. APRI L 1981
Funnel for pouring the melt Nletal mould.
Cover plota sand packing
7
I' ° /
'
Turn table To belt drive Fig. 1
i \Mould cavity
180mm
mould
Schematic diagram of centrifugal casting unit
1) Metal was poured into a sand mould which had a water cooled copper chill at its bottom. This resulted in maximum heat dissipation being towards the bottom of the casting, in the opposite direction to the buoyancy force. That is, the buoyancy effect and the movement of the macroscopic solidifying front are in the same direction. 2) A heavy copper chill (25 kg) was kept on the top of a bottom poured sand mould during solidification. In this case, the solidifying front will move against the buoyancy effect. Solid castings were taken out of the mould, machined and sectioned horizontally and vertically to study the distribution of mica in radial and longitudinal directions. Macrophotographs of the sectioned surfaces were taken. Drillings taken from different parts of the castings were chemically analysed to determine the readial distribution of mica. RESULTS AND DISCUSSION
and held in a vice with their cleavage planes kept vertical and the cutting edge of the milling cutter perpendicular to the edge of the mica cleavage planes. This mica powder (preheated to 200°C for 12 hours and then to 700°C for 30 minutes) was added along with magnesium pieces (about 6 mm a) to the surface of the melt in a vortex created by stirring the liquid metal with a mechanical stirrer, s After adding the powder, the metal was stirred for half a minute and was then degassed by bubbling nitrogen into the melt at a rate slower than that used for degassing before dispersion. The degassing was accompanied by slow stirring of the melt for one minute. After this the crucible was taken out of the furnace - the melt being hand stirred with a graphite rod - and emptied into a rotating permanent mould. A vertical centrifugal casting machine shown in Fig. 1 was used for making cylindrical castings. The speed of rotation of the mould was kept at 150 rpm during pouring. This was increased to 680 rpm just after the pouring was completed and was kept constant until the solidification was completed. Two separate static casting experiments were done in which the directions of maximum heat dissipation (or of movement of macroscopic freezing fronts) were at different angles to the directions of the buoyancy effect (due to density difference) acting on the particles during solidification.
Macrophotographs of radial sections of centrifugally cast mica dispersed A1 - 4% Cu - 1.5% Mg castings containing different sizes of mica particles, and poured at different temperatures are shown in Fig. 2. In all the cases, the mica particles are segregated near the inner periphery of the cylindrical castings, leaving a mica-free outer ring. A typical macrophotograph of longitudinal section of centrifugally cast mica (120/am) dispersed aluminium alloy casting (poured at 760°C) where the bottom portion had not developed into a hollow cylinder is shown in Fig. 3. The density difference between mica powder (2.7 g/cm a) and aluminium alloy melt (2.77 g/cm a) has resulted in the segregation of the particles near the inner periphery. Fig. 4 shows a variation in thickness of the radial mica-free zone as a function of pouring temperature for different particle sizes. Fig. 5 shows the effect of particle size on the thickness of the radial mica-free zone at 720°C. From Fig. 4 it is clear that the higher the pouring temperature (within the range investigated), the greater is the thickness of micafree zone obtained in the casting. It is also clear from Fig. 4 that for the same pouring temperature, finer particle sizes give thinner mica-free zones. At higher pouring temperatures, more time is available for the particles to move and segregate before the metal solidifies resulting in a thicker mica-free zone. Likewise when the particles are fine their velocity in the liquid is expected to be lower leading to a narrower mica-free zone.
Fig. 2 Macrophotographs of radial sections of centrifugally cast mica dispersed AI -- 4% Cu -- 1.5% Mg alloy casting made by dispersing 2% mica particles of different sizes and poured at different temperatures (X 0.55) a) Size of particle: 40 p,m, pouring temperature: 690~C, b) Size of particle: 40/~m, pouring temperature: 728°C, c) Size of particle: 65/~m, pouring temperature: 680°C
C O M P O S I T E S . APR I L 1981
125
Chemical analysis as given in Fig. 6 indicates that in a typical centrifugal casting, the mica concentration increases from 0.07% at about 10 mm away from the outer periphery of the cylindrical casting, to above 8.50% at a distance of 24 mm from the outer periphery. This casting was poured at 760°C and contains 2 percent of 120/am size mica particles. Similar results were also obtained by Krishnan e t al 6 during -centrifugal casting of graphite-dispersed aluminium alloys in sand moulds except that the thicknesses of the mica-free zones obtained in the present investigation were smaller. This could be due to the following reasons: 1) smaller density difference between mica and AI - 4% Cu - 1.5% Mg alloy than that between graphite and the aluminium melt; 2) mica particles are flake shaped whereas the graphite particles are granules which are expected to move faster in liquid; 3) greater increases in melt viscosity are expected with the addition of flake shaped particles than with the addition of spheroidal particles, other conditions being same; 4) the permanent moulds used in the present investigation give higher freezing rates and hence less time for particle movement compared to the work with graphite where sand moulds were used. The mechanism of mica segregation at the inner periphery of the casting during centrifugal casting may be as follows: The molten aluminium alloy containing suspended mica particles poured into a rotating mould forms a paraboloid of revolution or cylinder due to centrifugal forces. The liquid freezes while it is in this tbrm due to the heat extracted by the mould. Due to the action of centrifugal force, the aluminium alloy melt which is heavier than mica particles moves farther away from the axis of
Fig. 3 Macrophotograph of longitudinal section of centrifugally cast mica dispersed aluminium alloy (X 1.00). Size of particle dispersed: 120/zm, Pouring temperature: 760°C 20 •- o - , , -
1 2 o l.u"n
10.0 /
65p, m - ~
40p.m
9.01lO i
8 o ~5
.5 o
I I 700 750 800 Pouring temper~tur~ (*C) Fig. 4 Thickness of radial mica-free zone as a function of pouring temperature 2O
E
650
o
~o
E
r~
5 I 20
I I I I 0 40 60 80 100 Particle size (l~m) Fig. 5 Effect of particle size of mica on thickness of radial mica-free zone at a pouring temperature of 720°C
126
120
10 15 20 25 Distance from inner periphery (ram)
Fig. 6 Variation of mica concentration with the distance from inner periphery in centrifugally cast mica dispersed aluminium alloy casting. Average particle size: 120/~m, pouring temperature: 760°C, speed: 680 rpm
C O M P O S I T E S . A P R I L 1981
rotation of the mould and mica particles move towards the centre of the casting while the alloy remains molten. This process continues in the zone which remains molten or partially molten even after solidification starts. On a microscopic scale it is observed that mica particles are generally present in the last freezing cell boundaries and they are apparently being rejected by the advancing solid/liquid front. Since in the case of permanent mould centrifugal casting much of the freezing is from the outer periphery to the inner periphery (freezing will also start at the inner periphery but at a much slower rate) it is possible that this also contributes to the segregation of mica near the inner periphery. However, the contribution of a moving dendritic interface to gross segregation of mica in a rotating permanent mould is not easy to compute and it was estimated by static casting experiments. The longitudinal sections of samples made by both of the static casting methods showed that all the particles had floated to the top portion of the casting. The shrinkage in the centre of the casting is an indication of some freezing from the top. This indicates that the segregation due to buoyancy prevails over any effect of rejection of particles by the freezing interface. This may be due to the fact that the freezing fronts under the present experimental conditions are dendritic and the mica particles rejected by the advancing solid front get into the inter-dendritic liquid where they are able to move along the direction of buoyancy force. It was seen that mica could segregate right up to the very top of the casting where the freezing front was moving downwards. The above observations suggest that the movement of mica particles in alloy melts during centrifugal casting is mainly due to centrifugal force. The extent to which the particle can move under centrifugal force in the casting is estimated in the following section. THEORETICAL AlVA L YSIS
After pouring, the liquid in the rotating mould takes the shape of a cylinder containing a cavity in the shape of a paraboloid of revolution and freezing begins at the mould wall as well as at the inner periphery. In permanent moulds the freezing rate at the mould wall will be much greater than that at the inner periphery. In the liquid cylinder initially, mica would be expected to be uniformly distributed across the cross section. Under the influence of centrifugal force the aluminium alloy melt which is heavier than the mica particles should move farther away from the axis of rotation of the mould, and the lighter mica particle should be displaced towards the inner periphery of the cylindrical castings. The mica particles initially accelerate from zero to terminal velocity, and then they can be expected to move with a constant velocity provided the viscosity of the melt remains unchanged. The distance travelled by the particles of various sizes during their acceleration to terminal velocity are calculated according to the following equation 6 :
etla _ 1 + x 1--x
where x is that fraction of terminal velocity which is reached in time tx. Here the fraction of terminal velocity is assumed to be equal to 0.95. Vm is the terminal velocity in cm/sec which is calculated by Stoke's law: Vm-9 A
2 (A - A') r2a kt
= density of mica particle = 2.7 gm/cm 3
A' = density of A1 - 4% Cu - 1.5% Mg alloy = 2.77 gm/cm 3 a
= acceleration whichis equal to 21681 cm/sec 2 at 680 rpm
/a
= viscosity in poise = 0.045
r
= radius of the particle in cms
The Stoke's law given above can be used to calculate the terminal velocity of spherical particle. The terminal velocity of non-spherical plate shaped particle such as mica will be different to that of the spherical particle. This also depends on the thickness to diameter ratio of the plate shaped particles. To calculate the terminal velocity of mica particle Stoke's law has to be modified. To calculate the rate of floatation of a circular plate 7 shaped particle of diameter ra Stoke's law can be applied using an effective spheroidal diameter r2 (the rate of floatation of a plate of diameter rl being the same as that of a sphere of diameter r2). The values of rl/r2 are7 3.05 and 2.16 respectively for thickness to diameter ratios of 1:20 and 1:10. The calculated values of r2 can be substituted in Stoke's equation to calculate the floatation ratios of the plate shaped particles. The terminal velocities of mica particles at different pouting temperatures as calculated from Stoke's law are given in Table 1. This table also shows that the distances travelled by the particles during the pre terminal-velocity phase are quite small compared to the thickness of the casting which is 2.5 cm in this case. Hence the distance travelled by a single particle from the outer periphery of the casting to the inner periphery can be approximated to the time for solidification multiplied by the terminal velocity of the particles. The time for solidification of the casting was calculated using the formula: t = S(H+CAT) h( TM - To)
where t
= time of solidification
H
= heat of fusion of pure aluminium
S =A.Vmln
h
= resistance to heat transfer at the mould/metal interface
where
C
= specific heat of alloy
S
= thickness of casting
2 + e tlA + e -tlA
A-
AVm 2(A - A~).a
COMPOSITES
. A P R I L 1981
TM = solidus temperature
127
Table 1.
Terminal velocity and distance travelled by 120/am mica particle across 2.5 cm thick centrifugal casting
Pouring temp. (°C)
Solidification time (seconds)
Distance to be travelled before attaining terminal velocity (cm)
Terminal velocity (cm/sec)
Calculated mica free zone (cm)
Actual mica free zone (cm)
680
47
6.23 x 10- 7
0.0274
1.29
0.9
720
50
7.25 x 10. 7
0.0290
1.45
1.05
760
53
9.42 x 10-7
0.0326
1.72
1.32
melts results in segregation of most of the mica particles in a mica-rich zone near the inner periphery of a cylindrical casting.
To = ambient temperature AT = (pouring temperature of alloy - TM). The assumption here is that the particles can move even when the alloy is in the mushy zone These calculations have been made assuming the viscosity of the melt containing the particles in suspension remains constant and is equal to the viscosity at pouring temperature without any suspended particles. According to this the velocity of the particle remains constant and the total freezing time is available for the movement of the particle. However, in reality the velocity of particle will not remain constant because of the following: (1) Centrifugal force acting on mica particles will decrease as the particles move towards the inner periphery. As a result, the velocity of the particles decreases progressively as they move towards the inner periphery. (2) Velocity of mica particles is reduced due to increasing concentration of particles as they are moving towards the inner periphery due to bridging of these particles against each other. (3) With time of centrifuging the temperature of the metal decreases and the viscosity will increase. This will offer a greater resistance to the movement of particles. (4) Viscosity of the melt increases due to the presence of particles in suspension in it and the viscosity further increases with increased segregation of the suspended particles travelling towards the inner periphery. In reality the movement will be arrested at a temperature somewhat above the solidus, thereby reducing the actual movement of particles. Due to the reasons given above the average velocity of the mica particles will actually be much lower and the actual distance travelled by the particle will be less than the calculated values in Table 1. The experimental values of mica-free zone width obtained for mica dispersed aluminium alloy containing 120/am mica particles in suspension and poured at 680°C, 720°C and 760°C were 0.90 cm, 1.05 cms and 1.32 cms respectively. These values are of the same order of magnitude as those in Table 1 but are slightly lower since the average effective viscosity of the composite melts during centrifuging is expected to be higher than that of suspension free melts. Nonetheless the experimental values confirm that the movement of the mica particles is mainly due to centrifugal forces. CONCLUSIONS
1. Centrifugal casting of mica dispersed aluminlurn alloy
128
2. The thickness of the radial mica-rich zone decreases with increasing pouring temperature and increasing mica particle size. However, under the range of conditions investigated the minimum thickness observed was 1.2 cms, which is adequate to leave a mica-rich bearing surface even after considerable machining. 3. Centrifugal casting of mica (120/am) dispersed aluminium alloy when poured at 760°C can result in mica concentrations up to about 9% at the inner periphery of the casting whereas it is difficult to make static castings containing more than 3% mica. 4. The calculated values of mica-free zone widths (using effective diameters in Stoke's Law for centrifugal flow) are of the same order of magnitude as the observed values. A CKNO WL ED GEMEN TS
The authors are grateful to Mr T.P. Murali for his editorial commerrts, to Dr R.M. Pillai and Mr M.K. Surappa for their comments on the heat transfer analysis. They are also grateful to Mr R. Babu for his line-drawings and to Mr B. Damodaran for typing the manuscript. REFERENCES 1
2 3
Afanas'ev, V.F. e t a l F i z K h i m M e k h a m M a t 5 N o 6 ( 1 9 7 0 )
p 680 Vishnevskii,V.B. and Afanas'ev, V.F. Izobert Prom Obvaztsky Tovaruye Znaski (1968) 45 No 5 p 107 Belitsky,M.E. Fiz Khim Mekham Mat 2 No 6 (1966) p 702
4
Rollet, Jean 'Self lubricating brushings' Compagnie
5
industrialle Des Metaux Eleetroniques Fr 2031, (CLF. 16 C B 32 f, C 10 m) (20 Nov. 1970) Appl (3 Feb 1969) p 4 Nath, Deo. Ph D Thesis (Department of Metallurgy, I I Se Bangalore, 1977)
6
Krishnan, B.P., Shetty, H.IL and Rohatgi, P.K. A F S Transactions 84 (1976) p 73
7
Gaudin,A.M. 'Principles of Mineral Dressing, Tata' (McGraw Hill Publishing Co Ltd, New Delhi, 1977) p 176
AUTHORS
Deo Nath is a lecturer at the Institute of Technology in Varanasi. P.K. Rohatgi is a Professor in the Metallurgy Department of the Institute of Science in Bangalore. Inquiries should be directed initially to Professor Rohatgi at the following address: Regional Research ,Laboratory, Trivandrum 695019, Kerala, India, of which he is a Director.
COMPOSITES . APR I L 1981