Process for preparing hollow metallic bodies

A method of preparing hollow metallic bodies by contacting flat droplets of a molten metal with a liquid capable of readily gasifying at a temperature lower than the melting point of said metal. Upon contact the liquid gasifies under the flat droplets and the gas pushes up the center of the flat droplets. The edges of the droplets fall because the vapor near the edges goes around the edges. As the middle of the droplets rises the surface tension of the droplets brings the edges together thereby forming the hollow metallic body.

1. Field of the Invention: 
This invention relates to a process for the preparation of hollow metallic 
bodies having a thin wall. 
The hollow metallic bodies formed by the process of this invention can take 
typically the following three structures, namely (1) a honeycomb sandwich 
structure, (2) a syntactic foam structure and (3) a structure in which 
voids in hollow metallic bodies are filled with a foamed plastic material. 
These structures have a light weight and a high structural strength and are 
excellent in such properties as shock-absorption, heat-insulating and 
sound characteristics. 
2. Description of the Prior Art: 
In the prior art techniques of preparing hollow metallic bodies, there is 
known a process comprising forming a thin metal layer on a core by 
electroplating and removing the core (see, for example, U.S. Pat. No. 
3,135,044) and a process comprising coating a thin metal layer composed of 
metal particles or the like on a core and then removing the core (see, for 
example, U.S. Pat. No. 3,674,461). Any of these known conventional 
processes, however, inevitably include the step of forming a thin metal 
layer on a core and the step of removing the core while retaining the 
shape of the thin metal layer, and various complicated means must be 
adopted for performing each of these two steps. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to provide a process for preparing 
hollow metallic bodies having a thin wall. 
Another object of this invention is to provide a process in which seamless 
hollow metallic bodies can be instantaneously formed without employing a 
core. 
All metals including alloys which can be melted can be used in this 
invention. 
According to this invention, meltable metals such as, for example, iron, 
iron alloys, aluminum, aluminum alloys, copper, copper alloys, nickel and 
nickel alloys, are melted to form molten metal droplets having a suitable 
size, and they are contacted with a liquid capable of readily vaporizing 
or gasifying by evaporation or decomposition at a temperature lower than 
the melting point of the metal, such as water, an organic solvent and an 
aqueous solution of an inorganic or organic substance, whereby hollow 
metallic bodies can be formed. The molten metal droplets vaporize the 
above-mentioned liquid (hereinafter referred to merely as "liquid") upon 
contact therewith and the generated gas is instantaneously surrounded by 
the molten metal droplets to form interior voids. Then, the molten metal 
droplets are cooled and solidified and thus, the desired hollow metallic 
bodies are formed. 
In this invention, the starting molten metal droplets can be formed by the 
following methods, namely (1) a method comprising heating a rod-like metal 
body or granular metal body and melting it gradually, (2) a method 
comprising casting a molten metal into a vessel having perforated openings 
and dispersing the molten metal through said perforated openings by moving 
the vessel or providing a difference of pressure between the inside and 
outside of the vessel, and (3) a method comprising letting a molten metal 
fall and strike an object to thereby disperse the molten metal. 
The molten metal droplets thus formed in the gas are allowed to fall in a 
liquid while they are in the molten state, thereby contacting the liquid, 
or the molten metal droplets formed by dispersing a molten metal into a 
liquid, contact the liquid and flat metal droplets are formed. These flat 
metal droplets then contact the liquid and the hollow metallic bodies are 
formed when the liquid vaporizes and the flat droplets close around the 
vaporized liquid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hollow metallic bodies can be formed by contacting molten flat metal 
droplets with a liquid. The contact of the molten flat metal droplets with 
the liquid can be simply accomplished by letting the molten metal droplets 
fall into the liquid or by forming the molten flat metal droplets in the 
liquid. 
When the molten metal droplets are placed in the liquid, a gas or vapor is 
generated by the contact between the molten metal droplets and the liquid. 
In order to facilitate the surrounding of the generated gas by the molten 
metal droplets, it is desired that the molten metal droplets have a flat 
form or film-like form, and it is essential that they be at a temperature 
sufficient to convert such flat or film-like form into a hollow granule. 
For facilitating the surrounding of the generated gas, it is also desired 
that the velocity of the molten metal droplets in the liquid be low and 
that a liquid stream which will violently move the molten metal droplets 
not be present. 
When hollow metallic bodies are prepared by forming molten metal droplets 
in a gas and letting them fall in a liquid, if the velocity of intrusion 
of the molten metal droplets through the liquid surface is reduced to a 
low level, it is possible to obtain a desired flat or film shape in the 
molten metal droplets. If the velocity of the molten metal droplets in the 
gas is high, the velocity is reduced by allowing the bodies to collide 
against a barrier plate and then letting them fall into the liquid, 
whereby the intrusion velocity is effectively reduced and a desired flat 
shape can be obtained very conveniently. This barrier plate is disposed to 
direct the flying molten metal droplets downward upon collision 
thereagainst. A plate with a surface on which a liquid flows or a plate 
having a mechanical movement such as vibration, rotation or the like, can 
for example be used as the barrier plate. Adhesion of the molten metal 
droplets on the surface of the barrier plate is prevented by the liquid 
flowing on the surface of the plate or by the mechanical movement of the 
plate. 
When molten metal droplets are formed by dispersing a molten metal in a 
liquid by application of external force, the resulting molten metal 
droplets can have a desired flat shape or film-like shape. In this case, 
however, liquid streams are formed in the liquid by the external force 
applied for dispersing the molten metal. It is possible to alter the 
liquid streams by provision of a barrier wall in the liquid. An assembly 
of rods or plates arranged in a fretwork form can be used as the barrier 
wall. 
In preparing hollow metallic bodies within a liquid, it is very effective 
to provide in the liquid a plate disposed horizontally or at a slight 
angle to the horizontal plane, so that the molten metal droplets are 
allowed to collide against said plate and thus their movement is suddenly 
stopped and their shape flattened. Adhesion of the molten metal droplets 
to the plate can be prevented by vibrating or rotating the plate. 
Molten metal droplets which are to be flattened can be obtained by a method 
comprising heating a rod-like metal body or granular metal body and 
melting it gradually. In this method, an electric arc, a plasma arc or a 
gas flame can be used as a heating means. The size of the resulting molten 
metal droplets can be determined by adjusting the grain size in the case 
of a granular metal body or by imparting such movement as vibration or 
rotation to a rod-like metal body. 
Another method for forming molten metal droplets to be flattened comprises 
pouring a molten metal into a vessel having perforated openings on the 
side or bottom thereof, and dispersing the molten metal through said 
perforated openings by moving said vessel or by providing a difference of 
pressure between the inside and outside of the vessel. In this technique, 
in order to move the vessel, such movement as rotation, vibration and 
linear movement is given to the vessel and the molten metal is dispersed 
by an inertial force generated by such movement, thereby obtaining the 
molten metal droplets. Further, dispersion of the molten metal can also be 
accomplished by applying a back pressure to the molten metal in the vessel 
or providing vacuum on the outside of the perforated openings of the 
vessel. When the pressure difference is brought about between the outside 
and inside of the vessel in such manner, the molten metal is dispersed 
through the perforated openings and molten metal droplets which are to be 
flattened are formed. 
Still another method for forming molten metal droplets to be flattened 
comprises applying an external force to a molten metal stream. In this 
case, a mechanical inertial force caused by allowing the molten metal 
stream to collide against a rotor, a fluid force caused by allowing a 
fluid to impinge against the molten metal stream to disperse the molten 
metal, or an electric force such as an electromagnetic force can be 
utilized for formation of molten metal droplets. 
As the rotor for dispersing the molten metal stream, there can be employed, 
for example (1) a columnar rotor which may optionally have a rough 
surface, (2) a disc-like rotor which may optionally have a rough surface, 
(3) a conical rotor which may optionally have a rough surface, or (4) a 
bowl-like rotor. 
Hollow metallic bodies prepared according to the process of the invention 
have a globular shape or a shape resembling a sphere, and they are 
generally free of seams such as welding seams in weldings. In general, all 
of meltable metals including alloys can be formed into hollow metallic 
bodies according to the process of this invention. Hollow bodies referred 
herein are composed of a thin wall, and the ratio of the diameter to the 
thickness is at least 5, preferably from 20 to 100, in these hollow 
bodies. 
Some of the thin wall, of which the hollow bodies are composed have no 
openings, while a great majority of it have small openings therein. 
The hollow metallic bodies prepared by the above techniques are recovered 
from the liquid, and then dried. When the liquid is left in the interiors 
of the hollow metallic bodies, they are gradually heated and dried to 
remove the liquid through small openings therein. When the hollow metallic 
bodies are composed of an iron-type alloy, hollow bodies having excellent 
ductility or strength characteristics can be obtained by subjecting them 
to a heat treatment such as annealing and decarburizing. Further, the 
oxidized surface of the hollow bodies can be reduced by heating them in a 
reducing gas atmosphere. 
The process of this invention will now be illustrated more detailedly by 
reference to the accompanying drawing. 
FIGS. 1 and 2 illustrate one embodiment of the apparatus for use in 
practicing the process of this invention. A consumable electrode 18 and a 
non-consumable electrode 20 are disposed in a closed tank 26 facing each 
other, and each electrode is connected to an electric source 22 by wiring 
and an electric arc 14 is generated between the electrodes. The consumable 
electrode 18 is heated by this electric arc 14 and melts. The molten metal 
is gradually allowed to fall into a liquid 15 below the electrodes. The 
distance between electrode 20 and the surface of liquid 15 is less than 
800 mm and preferably in the range of about 50 mm to about 300 mm. If the 
liquid 15 is water, for example, its temperature is maintained below 
60.degree. C and preferably in the range of about 10.degree. C to about 
40.degree. C. The consumable electrode 18 which is to be melted is fed by 
a feed roll 50 rotated and driven by a motor 54, and a certain distance is 
maintained between the consumable electrode 18 and the non-consumable 
electrode 20 so that an electric arc 14 is continuously formed. The 
non-consumable electrode 20 is composed of a substance, the consumption of 
which is much smaller than that of the consumable electrode 18, such as 
tungsten and the like. The non-consumable electrode 20 is disposed as a 
fixed electrode, both ends of which are supported by insulators 24. 
The consumable electrode 18 is supported by a movable member 36 capable of 
reciprocating movement in the horizontal direction, and the electrode 18 
is moved reciprocatingly with respect to the non-consumable electrode 20. 
The upper and lower ends of the movable member 36 are supported by a 
fixing section 40 through bearings 38. The mobile member 36 is connected 
to a motor 48 through a connecting rod 42 and a crank shaft 44, whereby 
the rotary movement of the motor 48 can readily be converted to a linear 
movement of the mobile member 36. 
The inside of the tank 26 is shielded from the outer atmosphere, and a 
shield gas is introduced into the tank 26 from an inlet pipe 32 and the 
interior of the tank 26 above the level of a liquid 15 is filled with the 
shield gas. The movable member 36 is connected to the tank 26 by means of 
a flexible joint 52 to shield the assembly from the outer atmosphere. This 
shield gas is used in the tank to stabilize formation of the electric arc 
14. The liquid 15 is circulated by means of an inlet pipe 28 and an 
overflow pipe 30 to maintain the liquid level constant and recover the 
hollow metallic bodies floating thereon. 
Molten metal droplets 11 are formed from metal rods 18. The droplets 11 
which separate from the rods 18 have spherical or irregular shapes. The 
droplets 11 are carried toward the electrode 20 by the flow of electric 
arc 14, and impinge against the electrode 20. Upon impinging against the 
electrode 20, the droplets are flattened instantly. While the flattened 
droplets separate from the electrode 20 and fall downward, their surface 
area becomes small due to surface tension, their shape becomes small due 
to surface tension, and thus their shape becomes spherical, flat or 
irregular. The droplets 11 which are formed in the electric arc 14 have a 
high temperature at about 2000.degree. C and high liquidity. When the 
droplets 11 fall at a low speed and strike the surface of liquid 16, the 
droplets are spread and flattened instantaneously. 
When the droplet is flattened and falls downwardly in the liquid 15, it is 
surrounded by vapor, generated from the liquid 15. The vapor tries to rise 
upwardly in the form of bubbles, while the droplet tries to move downward 
in the liquid due to gravity. When the bubbles of vapor which are 
generated under the droplet try to rise upwardly, they are obstructed by 
the flattened droplet, and the droplet is thereby pushed upwardly by the 
rising bubbles. When the bubbles of vapor which are generated on the 
droplet separate from the droplet and rise upwardly, the droplet is also 
drawn upwardly by the rising bubbles. The bubbles which are generated 
under the edge part of the flattened droplet can pass around the outside 
of the flattened droplet, however, the bubbles which are generated at the 
center of the flattened droplet are held under the droplet and thus the 
central part of the flattened droplet is pushed upwardly by the rising 
bubbles beneath it. 
The droplet, which remains in a molten state, is rounded by the action of 
surface tension. Namely, the edge part of droplet which is flattened moves 
toward the center of droplet. Thus, as the central part of droplet moves 
upwardly by the action of rising bubbles and the edge part of droplet 
moves toward the center of droplet, the flattened droplet surrounds the 
bubbles and/or the liquid thereby forming a hollow body. The changing of 
the flattened droplet into the hollow body occurs in a very short period 
of time. 
The size of molten metal droplets 11 formed by the melting of the 
consumable electrode 18 can be controlled by changing the velocity of the 
reciprocating movement of the consumable electrode 18. Further, the 
falling velocity of the molten metal droplets 11 into the liquid 15 
positioned below can be reduced by the reciprocating movement of the 
consumable electrode. Thus hollow metallic bodies having a thin wall can 
be formed in the liquid 15. The size of the hollow metallic bodies is 
generally in the range of from 0.1 mm to about 8 mm. 
In FIG. 3 illustrating another embodiment of the apparatus for use in 
practicing the process of this invention, molten metal 58 in a tundish 56 
is allowed to fall from a bottom opening 60 of the tundish 56 in the form 
of a molten metal stream 9. The temperature of the molten metal 58 is 
maintained in the range of from about 50.degree. C to about 500.degree. C 
above the melting point of the metal and the diameter of columnar stream 9 
is generally between 4 mm and 10 mm. A liquid 16 is introduced into the 
tank 70 from an inlet pipe 74, flows over a weir 72 and is discharged from 
a discharge pipe 78. The temperature of liquid 16 is maintained at a low 
temperature. If for example water is used, it is maintained below 
60.degree. C and preferably in the range of about 10.degree. C to about 
40.degree. C. A columnar rotor 62, in the liquid 16, is rotated in the 
direction indicated by the arrow about a horizontally disposed shaft 64 
which is the rotation center. The rotor 62 generally rotates at a speed of 
between 400-1200 rpm and has a diameter of between 50-200 mm. The molten 
metal stream falling into the liquid 16 impinges against the columnar 
rotor 62 and is dispersed into the liquid 16 to form molten metal 
droplets. The distance between opening 60 and the surface of liquid 16 is 
maintained below 1 m and preferably in the range from 0.4 m to about 0.6 
m, while the distance between the surface of liquid 16 and rotor 62 is 
less than 150 mm and preferably less than about 50 mm. 
The columnar stream 9 of molten metal arrives at the top of rotary rotor 62 
without dispersion, and is then dispersed by the centrifugal force caused 
by rotation of rotor 62. Before the dispersion, the columnar stream 9 of 
molten metal impinges upon the surface of rotor 62 and moves along the 
surface of rotor 62 a very short distance, whereby the columnar stream is 
spread and flattened to form a flat stream of molten metal upon the 
surface of rotor 62. The flat stream is then dispersed into droplets 
having a flat form. Thus, the droplets 12 which are formed and ejected 
from the rotor 62 are substantially flat droplets. After the droplet is 
flattened, it remains in the molten state and it is rounded by the action 
of surface tension. When the flattened droplet is rounded in the liquid 
16, the droplet surrounds a little liquid and/or vapor to form a hollow 
body. 
Liquid streams generated by rotation of the columnar rotor 62 often cause 
the acceleration of the molten flat metal droplets and have undesired 
effects on formation of hollow metallic bodies. Accordingly, in order to 
reduce the liquid streams, a barrier wall composed of inclined plates 66 
is disposed in front of the columnar rotor 62 at a distance of between 
10-200 mm and preferably in the range from about 10 mm to about 50 mm. In 
addition, a flexible scraper 68 is provided to reduce the liquid streams. 
When molten metal droplets 12 are formed by dispersing the molten metal 
stream 9 by means of the above-mentioned columnar rotor 62, molten metal 
droplets traveling through the liquid in a direction approximating the 
horizontal direction can readily be made to have a flat shape or filmy 
shape, which can then be easily made hollow and granulated to form hollow 
metallic bodies, but molten metal droplets accelerated downwardly along 
the surface of the columnar rotor 62 by its rotation fall in the liquid in 
a spherical form or string-like form. When a bed plate 76 of the tank 70 
is disposed in the vicinity of the columnar rotor 62, the molten metal 
droplets falling in the spherical form or string-like form can be stopped 
by the bed plate 76 and their form can be converted to a desired flat or 
filmy shape. In this case, a small vibration in an oblique direction is 
given to the bed plate 76 to advance it smoothly to the weir 72, so that 
it prevents adherence of the molten metal droplets, facilitates formation 
of granular hollow metallic bodies and prevents agglomeration of the 
molten metal droplets. The small vibrations can be obtained by supporting 
the tank 70 by springs 82 and mounting a vibrating motor 80 obliquely on 
the bed plate 76. The movement of the molten metal droplets 12 on the bed 
plate 76 is identical with the movement of a substance fed on a 
conventional vibrating feeder. 
An example of the rotor to be used for dispersing a molten metal stream in 
the apparatus shown in FIG. 3, is illustrated in FIG. 4. The rotor 84 has 
a columnar shape and has nail-like projections 86 on the surface thereof. 
When this rotor 84 is attached to the apparatus shown in FIG. 3, the 
direction of flow of the molten metal droplets in the liquid is almost 
horizontal, and the amount of the molten metal accelerated downwardly is 
reduced substantially, with the result that the intended hollow metallic 
bodies are effectively formed. 
Another embodiment of the apparatus for use in practicing the process of 
this invention is illustrated in FIGS. 5 and 6. In reference to FIGS. 5 
and 6, a molten metal 59 placed in a tundish 57, falls from a bottom 
opening 61 of the tundish 57 in the form of a molten metal stream and is 
introduced into a pot 90. The pot 90 is rotated by a motor 96, and due to 
the centrifugal force caused by the rotation of the pot 90, the molten 
metal in the pot 90 is dispersed through perforated openings 92 and 
expelled from the pot in the form of molten metal droplets of a spherical 
or elongated shape. The pot 90 is heated by a flame 102 of a gas burner 
100 to prevent solidification of the molten metal and clogging of the 
perforated openings 92 and to maintain the temperature of the molten metal 
between the melting point and 500.degree. C above the melting point. With 
aluminum and aluminum alloys, the temperature of the molten metal is 
between the melting point and 200.degree. C above the melting point. A 
barrier plate composed of a conical plate 98 is disposed around the 
periphery of the rotary pot 90. The angle of the barrier 98 with the 
vertical is less than 45.degree. and preferably between 5.degree. and 
20.degree.. Fluid is carried to nozzle opening 106 by pipe 104. A fluid 
stream 108 which is discharged from nozzle openings 106 and a spray pipe 
104 uniformly flows on the plate 98. Water curtain 108 which is at about 
room temperature is used for preventing the adhesion of droplets 13 to the 
barrier 98. Further, since the water 108 is vaporized by heat of droplets 
13 impinging against the barrier 98, the droplets 13 are substantially 
prevented from direct contact with the metal surface of barrier 98. Due to 
the interference of vapor which has a low thermal conductivity, the 
droplets 13 do not solidify as might occur by direct contact between the 
droplets and the cooled metal surface. Water curtain 108 which has a 
thickness of less than 3 mm and preferably between about 0.2-1 mm, must 
flow slowly. The head of water 108 discharged from nozzle openings 106 of 
the spray pipe 104 is generally below 200 mm in an aqueous column, and 
preferably below about 50 mm in an aqueous column. The velocity of water 
curtain 108 which was calculated is thus below about 140 cm/sec, 
preferably about 70 cm/sec. 
Molten metal droplets 12 expelled from the perforated openings 92 of the 
rotary pot 90 impinge against the conical plate 98 acting as the barrier 
plate, and the velocity of the molten metal droplets 13 is reduced and the 
molten metal droplets 13 are allowed to fall downwardly into liquid 17 
without adhering to the barrier 98 due to the fluid 108 flowing downwardly 
on the surface of the barrier 98. The distance between the point of impact 
of a droplet 13 on barrier 98 and the surface of liquid 17 is less than 
800 mm and preferably about 100-500 mm. 
The tank in which the liquid 17 is contained is divided into a fixed 
section 114 and vibrating sections 110 disposed on both side ends. The 
vibrating and fixed sections 110 and 114 are connected and sealed by 
flexible sheets 116. Two terminal vibrating sections 110 are supported by 
spring 124 and they repeat oblique vibrations propagated in directions 
opposite to each other, which are caused by the vibrating movement of 
vibrating motors 80 attached obliquely on the bed plate of the tank. The 
small vibration of each vibrating section is absorbed by the flexible 
sheets 116 and it is not transmitted to the fixed section 114. The liquid 
17 is introduced into the tank from inlet pipes 120, flows over weirs 118 
and is withdrawn from discharge pipes 122. The temperature of liquid 17 is 
maintained at a low level. When water is used, it is maintained below 
60.degree. C and preferably in the range of about 10.degree. C to about 
40.degree. C. 
Molten metal droplets 12 coming out of the rotary pot 90 impinge against 
the barrier 98 and the droplets are flattened by impact on the barrier 98. 
The flattened droplets become small instantaneously due to the surface 
tension. A large majority of droplets maintain their flat shape, and then 
fall downward into liquid 17. A few of the droplets are made into hollow 
bodies. The droplets which fall into the liquid 17 remain in a molten 
state and are flattened by striking the surface of liquid in the manner 
described above in the first embodiment. 
Two horizontal plates 112 attached to the vibrating sections 110 are 
disposed in the liquid, and from these plates 112 small vibrations are 
continuously, obliquely propagated. The falling molten metal droplets 13 
which pass through the liquid 17 are further flattened by the plates 112 
to give the desired flat or filmy shape, and they are formed into hollow 
metallic bodies without adhering to the plates 112 because of the 
vibration thereof. When the droplets are flattened on the pipe 112 
disposed in the liquid 17, a film-like liquid is left between the 
flattened droplet and plate 112, and it is vaporized instantly by the heat 
of droplet. Due to the expansion of liquid between the droplet and plate 
112 caused by changing liquid into gas, the droplet is pushed upwardly at 
its center. The edge part of droplet is not affected by the expansion of 
liquid. After the droplet is flattened on the plate 112, it begins to 
contract itself by the action of surface tension. Namely, the edge part of 
droplet which is flattened moves toward the center of droplet. Since the 
edge part of the droplet moves toward the center and along the plate 112, 
and the central part of droplet moves upwardly, the edge part of droplet 
can easily move to the base center of the droplet. Thus, when the 
flattened droplet contracts itself, it surrounds the vapor and/or a little 
liquid which exist between the flattened droplet and plate 112, thereby 
forming a hollow body. The formation of the hollow body is performed in a 
very short time. In a large aluminum droplet, having a weight of 5 g, for 
example, time for changing the flattened droplet into the hollow body is 
about 0.03 seconds. 
The molten metal droplets which thus arrive at the plates 112 are moved 
toward weirs 118 by the small vibration of the plates 112, and the melt 
adhesion to another molten metal droplets which arrive at the plates 112 
at the later stage is thus prevented. 
In the same manner as described above, hollow bodies can be formed on the 
barrier 98 on which liquid curtain 108 flows. But the quantity of hollow 
bodies which are formed on the barrier 98 is very small. When a hollow 
body which is formed still has a high liquidity, vapor which exists in the 
hollow body can easily escape from the body by the action of the floating 
power of the vapor, or by the break of the liquid metal shell caused by 
expansion of vapor. Namely, vapor which is enclosed in a droplet having a 
high liquidity breaks the wall and then passes through the break in the 
wall. It can be seen that as the vapor passes through the break in the 
wall of the droplet or the droplet fails to surround the vapor, hollow 
bodies are not formed on the barrier 98. 
A bowl-like rotor 126 to be used for dispersing the molten metal in the 
apparatus shown in FIG. 5 is illustrated in FIG. 7. The bowl-like rotor 
126 is composed of a body 128, a holder 130 and a support plate 132. When 
this bowl-like rotor 126 is employed, the molten metal stream collides 
against the body 128 of the rotor 126, and is dispersed by a centrifugal 
force generated by rotation to form molten metal droplets. Then, the so 
formed molten metal droplets are allowed to fall into the liquid, and the 
intended hollow metallic bodies are formed. 
In all of the embodiments disclosed, the flattened droplets which contact 
with the liquid are rounded by the surface tension, and then surround 
vapor and/or liquid, thereby forming the hollow body. In order to form 
hollow bodies, the droplets must have a proper liquidity. When the 
liquidity of droplets is too high, vapor only passes through the droplets. 
And when the liquidity of droplets is too low, the droplets cannot change 
their forms. Therefore, in molten aluminum which has a high liquidity, for 
example, the temperature of molten metal 59 in the tundish 57 must be 
maintained to below 200.degree. C above the melting point of metal, 
preferably below 50.degree. C above the melting point. In molten copper, 
the temperature of molten metal 59 in the tundish 57 must be maintained to 
between 100.degree. C and 500.degree. C above the melting point, 
preferably between 200.degree. C and 400.degree. C above the melting 
point. Further in molten nickel, the temperature of droplets is maintained 
to high levels by heating them in an electric arc. 
This invention will now be illustrated more detailedly by reference to 
Examples. 
EXAMPLE I 
An apparatus such as shown in FIGS. 1 and 2 was employed. A nickel rod 
having a diameter of 4 mm was disposed as a consumable electrode facing a 
non-consumable electrode composed of carbon, and an electric arc was 
generated between both the electrodes. The nickel electrode was 
reciprocated at a stroke of 15 mm and a frequency of 120 cycles per 
minute. Argon gas filled the space in a closed tank above the water in the 
lower portion of the tank. The water was continuously circulated and the 
water level was so adjusted that the distance between the nickel rod and 
water level was always maintained at 75 mm. The nickel rod was heated and 
melted by the electric arc, and the dispersed melt of molten nickel 
droplets of spherical shape having a diameter of 0.7-4mm fell on the water 
surface which was at room temperature and introduced therein to form 
hollow bodies of nickel. The resulting hollow bodies had a spherical shape 
with a diameter of 2 to 8 mm and the bulk density of the product could be 
reduced to 0.4 g/cm.sup.3 in this Example. 
EXAMPLE II 
Procedures of Example 1 were repeated by employing a rod of nickel-iron 
alloy instead of the nickel rod. The Ni-Fe alloy used had a composition of 
0.7% of C and 45% of Ni, the balance being Fe. As a result, hollow bodies 
of the Ni-Fe alloy having a spherical shape of a diameter of 1 to 10 mm, 
and the bulk density of the product could be reduced to 0.35 g/cm.sup.3 in 
this Example. 
EXAMPLE III 
An apparatus such as shown in FIG. 3 was employed. Cast iron was melted at 
1550.degree. C. and charged in a tundish, and a molten iron stream having 
a diameter of 5 mm was allowed to fall from a bottom opening of the 
tundish into a tank. The distance between the opening and surface of the 
water was 500 mm. The tank was filled with water at room temperature, and 
a columnar rotor of an outer diameter of 180 mm having 12 nail-like 
projections, such as shown in FIG. 4, was driven and rotated at a rate of 
800 rpm in the water. The rotor was 20 mm below the surface of the water. 
A vibrating motor giving a vertical vibration was mounted on the bed plate 
of the tank with an inclination of 45.degree. to the horizontal plane, 
whereby the bed plate of the tank was vibrated in the direction of 
45.degree. at a frequency of 30 cycles per second at an amplitude of about 
3 mm. The molten iron stream was dispersed in water by means of the rotor 
to form hollow bodies of cast iron having a spherical shape of a diameter 
of 0.3 to 1 mm. The bulk density of the product could be reduced to 0.5 
g/cm.sup.3 in this Example. The composition of the hollow bodies was 4.2% 
of C and 2.0% of Si, the balance being iron. 
The hollow bodies of cast iron were wrapped with powder of ferrous oxide 
and heated at 1000.degree. C for 1 hour to effect decarburization, and 
then reduction was carried out by heating them in a hydrogen gas at 
800.degree. C for 30 minutes. The composition of the resulting hollow 
bodies was 0.1% of C and 2.0% of Si, the balance being Fe. By these post 
heat treatments, iron type hollow bodies rich in ductility and having a 
metallic gloss on the surface could be obtained. 
EXAMPLE IV 
An apparatus such as shown in FIG. 5 was employed. Aluminum was heated at 
700.degree. C and molten aluminum was charged in a tundish. The aluminum 
melt was allowed to fall from a bottom opening of the tundish in the form 
of a molten aluminum stream having a diameter of 5 mm. The molten aluminum 
stream was thus introduced into a pot having an outer diameter of 60 mm 
which was heated by a gas flame to about 700.degree. C. The pot had 
perforated openings of a diameter of 5 mm on the side wall thereof. Water 
was placed in a tank, and vibrating sections disposed on both ends of the 
tank were vibrated at a frequency of 30 cycles per second and an amplitude 
of about 2 mm. The vibrations were propagated in the directions opposite 
to each other at an angle of 45.degree. with respect to the horizonal 
plane. The pot was rotated at about 620 rpm and the aluminum melt was 
expelled through the perforated openings by the centrifugal force and 
impinged against to a conical plate at a distance of about 120 mm, 
following which it fell into the water below. A water curtain at room 
temperature and of about 1 mm in thickness flowed over the barrier which 
was at an angle of 1.5.degree. with the vertical. 
The droplets which passed through the water and struck a plate 20 mm below 
the surface, were further flattened and then formed the hollow metal 
bodies. The so formed hollow bodies were moved toward weirs while being 
prevented from adhering to newly formed bodies by virtue of the small 
vibration. The resulting hollow bodies of aluminum had a spherical or 
substantially spherical form of a diameter of 3 to 8 mm. In this Example, 
the bulk density of the product could be reduced to 0.3 g/cm.sup.3. 
EXAMPLE V 
A rotor shown in FIG. 7 was attached to the apparatus of FIG. 5 instead of 
the pot shown in FIG. 5, and employed in this Example. Copper was melted 
at 1300.degree. C and was charged into a tundish and allowed to fall from 
a bottom opening of the tundish in the form of a molten copper stream of a 
diameter of 5 mm onto a bowl-like rotor rotating at a rate of 800 rpm. The 
molten copper stream was dispersed by rotation of said bowl-like rotor to 
form molten bodies of copper. In the same manner as described in Example 
III, the molten bodies of copper were made hollow and granulated. The 
resulting hollow bodies of copper had a spherical form having a diameter 
of 3 to 6 mm. In this Example, the bulk density of the product could be 
reduced to 0.6 g/cm.sup.3.