Method of producing a metallic member having a unidirectionally solidified structure

A metallic material is passed through a heating mold having a melting zone to produce a metallic member having a unidirectionally solidified structure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a method of producing a metallic member having a 
unidirectionally solidified structure and in the form of a wire, bar or 
sheet, or of any other desired cross sectional shape. More particularly, 
it is concerned with a method which produces a metallic member having a 
unidirectionally solidified structure by passing through a heating mold 
having a melting zone a metallic material prepared by casting or plastic 
working and having a length which is considerably greater than its 
diameter or thickness, such as a metal wire, bar or sheet. 
2. Description of the Prior Art: 
With its rapid growth, the electronic industry has incessantly been calling 
for still smaller and more precise machines and devices. They require 
metallic materials which are still smaller in thickness or diameter, and 
which are still better in quality. More specifically, they require thinner 
wires, sheets and foils formed from a material having a unidirectionally 
solidified structure, and which is free from any cavity or blowhole, and 
any grain boundary where impurities are likely to gather. 
It is generally known that if a metallic material is subjected to cold 
working, such as rolling or drawing, it hardens and is eventually likely 
to fracture or crack at a primary crystal grain boundary which is formed 
in the casting process. Therefore, it is highly desirable to use a 
metallic material having a structure free from any such grain boundary in 
order to make a wire, sheet or foil which is extremely small in diameter 
or thickness. 
There is known a method which is called zone melting, and which has 
primarily been developed for refining a metal. This method employs an 
elongated refractory vessel having a horizontal groove and called a boat. 
A metal ingot in the form of a bar is placed in the boat. Heat is applied 
from the outside of the boat by, for example, a resistance heater, gas 
burner or high-frequency induction coil to melt the ingot locally, and the 
melted portion of the metal is moved in one direction. This method not 
only refines the ingot, but can also produce an ingot having a 
unidirectionally solidified structure. The length of the ingot which this 
method can produce is, however, limited by the boat. It is incapable of 
making continuously an elongated ingot having a desired cross sectional 
shape and a unidirectionally solidified structure. 
In order to produce an ingot in the form of a bar having a unidirectionally 
solidified structure, such as for a cast magnet, it has often been the 
case to place a metal bar vertically, heat it by a zone melting furnace 
having a resistance heater not contacting the bar, or a high-frequency 
induction coil to melt it locally, and move the melted portion of the bar 
upwardly or downwardly. This method does not employ any mold, but relies 
upon the surface tension of the molten metal to maintain the shape of the 
melted bar surface. The solidifed metal fails to retain the original 
smooth surface of the bar and has an uneven surface which has to be ground 
or polished before the bar can be used for making any product. 
SUMMARY OF THE INVENTlON 
It is an object of this invention to provide a method which can very easily 
overcome the drawbacks of the prior art as hereinabove pointed out, and 
make a metallic product having a unidirectionally solidified structure 
which imparts a high degree of workability and/or good magnetic properties 
thereto, while removing therefrom any cavities, blowholes and other 
internal defects that have been formed during the casting of an ingot. 
According to this invention, a metallic material is continuously or 
intermittently fed into a heating mold having a melting zone through one 
end thereof and melted in the melting zone. The melted material is caused 
to solidify within the mold having an inner wall surface heated to a 
temperature which is higher than that of the metallic material contacting 
it, so that no nucleation of any new crystal may take place on the inner 
wall surface of the mold from its melting zone and the other or outlet end 
thereof. Thus, a metallic member having a unidirectionally solidified 
structure can be easily produced through the outlet end of the mold. The 
term "outlet end" of the mold not only means its outer extremity, but also 
covers the area where the solidified metal leaves the inner wall surface 
of the mold as in case of using diversified mold. 
It is desirable that there should not be any friction between the inner 
wall surface of the mold extending from its melting zone to its outlet end 
and the solidified metal. It is possible to avoid any such friction 
causing scratches on the solidified metal if the mold is so heated that 
the inner wall surface of the mold adjacent to its outlet end may be held 
at a temperature higher than the solidifying temperature of the metal to 
form on the metal surface a thin film of molten metal which is allowed to 
solidify upon leaving the outlet end of the mold. This method facilitates 
the production of a metallic member having a unidirectionally solidified 
structure and a mirror surface. 
The melting zone is located adjacent to the outlet of the mold. The mold is 
so heated that its inner wall surface adjacent to its outlet end may have 
a temperature higher than the solidifying temperature of the metal, but 
very close thereto, while the metal is forcibly cooled outwardly of the 
outlet end of the mold so as to have a convexly projecting solidified end. 
The molten metal does not solidify on the inner wall surface of the mold 
at the outlet end thereof, but solidifies preferentially in its central 
portion. Therefore, no impurity or gas is trapped in the center of the 
metal. As shown in FIG. 1, the partly solidified metal S is covered by a 
very thin film F of molten metal M and finishes solidifying immediately 
after leaving the mold 1. Therefore, the solidified metal is not brought 
into frictional contact with the inner wall surface of the mold. This 
greatly facilitates the continuous production of a metallic member having 
a smooth and beautiful surface. 
According to this invention, it is possible to produce a metallic member 
having a unidirectionally solidified structure continuously only if a 
polycrystalline metallic material prepared by casting or plastic 
deformation working and having a structure not regularly oriented is 
passed through a heating mold having a melting zone.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, there is shown by way of example an apparatus which 
can be employed for producing a metallic member having a unidirectionally 
solidified structure continuously in accordance with the method of this 
invention. The apparatus comprises a heating mold 1 formed from graphite 
or refractories and an electric resistance heater or high-frequency 
induction coil 2 surrounding the mold 1. The heater or coil 2 is 
surrounded by a heat insulating material 3. The apparatus is adapted to 
treat an elongated metallic material 4 having a structure composed of a 
multiplicity of irregularly directed crystals, as shown in FIG. 2, or a 
plurality of like metallic materials 4 joined longitudinally one after 
another. A plurality of pairs of pinch rolls 5 and 6 are provided for 
conveying the metallic material or materials 4 through the mold 1. A guide 
7 is provided for preventing the vibration of the metallic material or 
materials 4. The guide 7 is formed from graphite or refractories. A spray 
device 8 is provided for jetting out a cooling agent, such as water, mist, 
air or gas, against a metallic member leaving the mold 1. Thermocouples 9 
and 10 are provided for measuring the temperature of the mold 1. 
An electric current is supplied to the heater 2 to heat the metallic 
material 4 in the mold 1 and melt it in a melting zone 11, as shown in 
FIG. 3. Then, the pinch rolls 5 and 6 are rotated to move the metallic 
material in the direction of an arrow in FIG. 3. If the inner wall surface 
of the mold 1 at the outlet end thereof is heated to a temperature which 
is higher than, but very close to, the solidifying temperature of the 
metallic material, the metal which has been melted in the mold 1 begins to 
solidify at the outlet end of the mold 1 and forms a metallic member 12 
having a unidirectionally solidified structure. The metallic member 12 is 
drawn by the pinch rolls 6 in the direction of an arrow in FIG. 3. Insofar 
as the temperature of the inner wall surface of the mold at the outlet end 
thereof is higher than, but very close to, the solidifying temperature of 
the metallic material, there is a delay in the solidification of the metal 
forming a surface layer of the metallic member, and if the member has a 
small cross sectional area, it has a rounded solidifying end projecting 
into the mold 1. Therefore, it is possible to prevent any solute 
segregation or gas confinement in the center of the metallic member 12. 
The metallic member 12 has a film 13 of molten metal on its surface when 
leaving the mold 1 and this molten metal is allowed to solidify 
immediately after it has left the mold. Therefore, the metallic member 12 
has a very smooth and beautiful surface. 
According to this invention, the mold can be disposed horizontally or 
vertically, or at any appropriate angle therebetween so that the metallic 
material 4 may be moved horizontally or vertically, or at an angle 
therebetween. If the material is, for example, an alloy which is liable to 
segregation in the melting zone 11 due to a difference in specific gravity 
between its constituents, it is useful to use a vertically disposed mold 
as shown at 1' in FIG. 4 or at 1" in FIG. 5 to pass the metallic material 
4 downwardly or upwardly through the mold. This ensures the production of 
a metallic member 12 which is free from any substantial segregation. The 
apparatus shown in FIG. 4 further includes a receptacle 14 for receiving 
any dropping cooling agent in the event it is a liquid. 
If the metallic material is passed through the mold having between its 
melting zone and its outlet end an inner wall surface maintained at a 
temperature which is higher than that of the material, it is possible to 
prevent completely the nucleation of any new crystal on the inner wall 
surface of the mold. Therefore, the number of the crystals composing the 
metallic member formed by the solidification of the melted material 
decreases as a result of the growth competition of those crystals, and 
there is eventually formed a metallic member which is composed of a single 
crystal. Thus, the method of this invention is not only suitable for 
producing a metallic member having a unidirectionally solidified 
structure, but also facilitates the production of a metallic member 
composed of a single crystal. 
Referring to FIG. 6, there is shown by way of example a method which 
facilitates the continuous production of a metallic member composed of a 
specifically oriented single crystal. A metallic material 4 is brought 
into contact with a dummy member 16 carrying a single seed crystal 15 at 
one end thereof facing the material 4. The mold 1 is heated to start the 
melting and solidification of the material 4 and the material 4 is moved 
in the direction of an arrow in FIG. 6. 
When the method of this invention is carried out, it is advisable to ensure 
the intimate contact of the metallic material with the inner wall surface 
of the mold so that there may not be formed therebetween any clearance 
causing the molten material to leak out. 
The mold may be formed from a heat resistant metal not reacting with the 
molten material, such as graphite or stainless steel, if the material to 
be treated is a low-melting one, such as tin or a lead alloy. If it is a 
high-melting material, such as aluminum, copper or an iron alloy, the mold 
may be formed from silicon carbide or nitride, boron nitride, alumina, 
magnesia, zirconia, or any other refractories not reacting with the molten 
oxide of the metal composing the material to be treated. 
In order to prevent the oxidation of the melted metallic material when 
producing metallic member having a high melting point, it is effective to 
employ in the mold a protective gas atmosphere which can, for example, be 
formed by introducing an inert gas, such as argon or nitrogen, or a 
reducing gas, such as hydrogen or carbon monoxide. 
If the metallic material has an oxide film on its surface, it is likely to 
corrode the inner wall surface of the mold in its melting zone. Therefore, 
it is advisable to remove any such oxide film by employing a gas to reduce 
the oxide or grinding the surface of the material immediately before it is 
fed into the mold. 
If the material is particularly easily oxidizable, it is effective to keep 
it at a sufficiently low temperature not to cause any oxidation thereof, 
or hold it in an inert or reducing gas atmosphere so that the heat of the 
mold may not cause any oxidation of the material before it is fed into the 
mold. 
The resistance heater may, for example, be of nichrome wire or silicon 
carbide if the material to be treated is a low-melting temperature metal, 
such as tin, zinc or lead, or aluminum. If it is a high-melting 
temperature metal, the heater may be formed from, for example, tantalum, 
tungsten, molybdenum, platinum or silicon carbide. The resistance heater 
can be replaced by a high-frequency induction coil, as hereinbefore 
stated. 
The method of this invention is primarily capable of producing a metallic 
member having a structure which differs from that of the material fed into 
the mold, but a cross sectional shape which is identical to that of the 
material. According to this invention, however, it is also possible to 
produce continuously a metallic member having any desired cross sectional 
shape differing from that of the material if the outlet end of the mold 
adjacent to which the melting zone is established is formed with an 
appropriate cross sectional shape, and if the speed at which the material 
is moved at the inlet end of the mold and the speed at which the member 
formed therefrom is moved at the outlet end of the mold are so established 
that the member leaving the mold may always be of the same volume as that 
of the material entering it. 
As the inner wall surface of the mold is held at a temperature which is 
higher than the solidifying temperature of the molten material, the 
metallic member being formed therefrom has a rounded solidifying end, 
since the molten material close to the inner wall surface of the mold does 
not solidify. Therefore, it is possible to produce a metallic member of 
high quality which is free from any fine cavity, blowhole or macroscopic 
segregation that may have existed in the material entering the mold. 
The method of this invention is an epoch-making one for producing a 
material which is required to have a unidirectionally solidified 
structure, such as a magnetic material. It can not only improve the 
crystal structure of a metallic material prepared by the plastic working 
of a conventionally cast ingot if the material is merely passed through a 
mold having a melting zone. It is also a very effective method for the 
preliminary treatment of a metallic material which is used for making a 
very thin metal wire or foil. The production of a very thin wire or foil 
has hitherto required a complicated process including a repeated cycle of 
ingot annealing and plastic working and consuming a large amount of 
energy. This invention enables a drastic reduction in the work for any 
such heat treatment, since it can easily convert a metallic material 
prepared from a conventionally cast ingot to a member having a 
unidirectionally solidified structure and hence a high degree of 
workability. For example, the inventor of this invention has found that a 
sheet of tin produced by the method of this invention and having a 
thickness of 6 mm and a width of 10 mm can be worked to a thickness of 8 
microns without requiring any heat treatment, and it has also been able to 
work a 20 mm dia. rod of an aluminum alloy containing 1% of silicon into a 
fine wire having a diameter of 30 microns without heat treating it at all. 
The invention will now be described more specifically with reference to 
several examples thereof. 
EXAMPLE 1 
A 6 mm dia. rod of 99.9% purity tin having a circular cross section and a 
unidirectionally solidified structure was produced by employing a 
horizontally disposed hollow heating mold, as will hereunder be described. 
The mold was a graphite tube having an inside diameter of 6 mm, an outside 
diameter of 12 mm and a length of 100 mm and surrounded by a heater 
comprising a coil of nichrome wire. An alumel-chromel thermocouple was 
provided in the vicinity of the outlet end of the mold, and another 
alumel-chromel thermocouple in its melting zone, for measuring the 
temperature of the inner wall surface of the mold. 
A rod of polycrystalline tin prepared by plastic working and having a 
diameter of 6 mm was placed through the mold by pinch rolls. An electric 
current was supplied to the heater to heat the mold, while water was being 
sprayed onto the surface of the rod at a rate of 100 cc per minute by a 
spray cooling device disposed at a distance of 20 mm from the outlet end 
of the mold. The electric current was so controlled that the mold might 
have a temperature of 240.degree. C. in the vicinity of its outlet end and 
a temperature of 260.degree. C. in the vicinity of its inner wall surface 
in the melting zone. The pinch rolls were rotated to pass the rod through 
the mold at a speed of 100 mm per minute. Thus, it was possible to produce 
continuously a rod or member of tin having a unidirectionally solidified 
structure, a shape which was identical to that of the original rod, and a 
very beautiful mirror surface. 
Although a multiplicity of crystals not specifically oriented had been 
found in the original rod, the rod leaving the mold was found to be 
composed of ten longitudinally oriented crystals at a distance of 200 mm 
from its solidifying end and only a single crystal at a distance of 500 mm 
therefrom. 
EXAMPLE 2 
A 6 mm dia. rod of a Pb-5% Sn alloy having a circular cross section and a 
unidirectionally solidified structure was produced by employing a 
vertically disposed hollow heating mold through which the material was 
passed downwardly. The mold was a graphite tube having an inside diameter 
of 6 mm, an outside diameter of 12 mm and a length of 100 mm and 
surrounded by a heater comprising a coil of nichrome wire. An 
alumel-chromel thermocouple was provided in the vicinity of the outlet end 
of the mold, and another alumel-chromel thermocouple in its melting zone, 
for measuring the temperature of the inner wall surface of the mold. 
The starting material was a rod of a polycrystalline Pb-5% Sn alloy 
prepared by plastic working and having a diameter of 6 mm. It was placed 
through the mold by pinch rolls. An electric current was supplied to the 
heater to heat the mold, while water was being sprayed onto the surface of 
the rod at a rate of 100 cc per minute by a spray cooling device disposed 
at a distance of about 20 mm from the outlet end of the mold. The electric 
current was so controlled that the mold might have a temperature of 
320.degree. C. in the vicinity of its outlet end and a temperature of 
350.degree. C. in the vicinity of its inner wall surface in the melting 
zone. The pinch rolls were rotated to pass the rod downwardly through the 
mold at a speed of 100 mm per minute. Thus, it was possible to produce 
continuously a rod or member of the Pb-5% Sn alloy having a 
unidirectionally solidified structure, a shape which was identical to that 
of the original rod, and a very beautiful mirror surface. 
Although a multiplicity of crystals not specifically oriented had been 
found in the original rod, the rod leaving the mold was found to be 
composed of eight longitudinally oriented crystals at a distance of 200 mm 
from its solidifying end and only a single crystal at a distance of 700 mm 
therefrom. 
EXAMPLE 3 
A 4 mm thick, 20 mm wide sheet of 99.9% purity zinc having a 
unidirectionally solidified structure was produced by employing a 
horizontally disposed hollow heating mold. The mold was a graphite mold 
having an inside height of 4 mm, an inside width of 20 mm, an inside 
length of 100 mm and a wall thickness of 3 mm, and surrounded by a heater 
comprising a coil of nichrome wire. An alumel-chromel thermocouple was 
provided in the vicinity of the outlet end of the mold, and another 
alumel-chromel thermocouple in its melting zone, for measuring the 
temperature of the inner wall surface of the mold. 
A polycrystalline zinc sheet prepared by plastic working and having a 
thickness of 4 mm and a width of 20 mm was placed through the mold by 
pinch rolls. The mold was heated, while water was being sprayed onto the 
surface of the sheet at a rate of 60 cc per minute by a spray cooling 
device disposed at a distance of 40 mm from the outlet end of the mold. 
The electric current supplied to the heater was so controlled that the 
mold might have a temperature of 425.degree. C. in the vicinity of its 
outlet end and a temperature of 450.degree. C. in the vicinity of its 
inner wall surface in the melting zone. The pinch rolls were driven to 
move the sheet through the mold at a speed of 100 mm per minute. As a 
result, there was continuously produced a sheet of zinc having a 
unidirectionally solidified structure, a shape which was identical to that 
of the original sheet, and a very beautiful mirror surface. 
Although a multiplicity of crystals not oriented in any specific pattern 
had been found in the original sheet, the sheet leaving the mold was found 
to be composed of 15 longitudinally oriented crystals at a distance of 200 
mm from its solidifying end and only a single crystal at a distance of 
1000 mm therefrom. 
EXAMPLE 4 
An 8 mm dia. rod of an Al-1% Si alloy having a circular cross section and a 
unidirectionally oriented structure was produced by employing a 
horizontally disposed hollow heating mold. The mold was a silicon carbide 
tube having an inside diameter of 8 mm, an outside diameter of 12 mm and a 
length of 200 mm and surrounded by a coil of nichrome wire defining a 
heater. An alumel-chromel thermocouple was provided in the vicinity of the 
outlet end of the mold, and another alumel-chromel thermocouples in its 
melting zone, for measuring the temperature of the inner wall surface of 
the mold. 
A rod of a polycrystalline Al-1% Si alloy prepared by plastic working and 
having a diameter of 8 mm was placed through the mold by pinch rolls. An 
electric current was supplied to the heater to heat the mold, while water 
was being sprayed onto the surface of the rod at a rate of 500 cc per 
minute by a spray cooling device disposed at a distance of 40 mm from the 
outlet end of the mold. The electric current was so controlled that the 
mold might have a temperature of 700.degree. C. in the vicinity of its 
outlet end and a temperature of 750.degree. C. in the vicinity of its 
inner wall surface in the melting zone. The pinch rolls were driven to 
move the rod through the mold at a speed of 100 mm per minute. As a 
result, there was continuously produced a rod or member of the alloy 
having a unidirectionally solidified structure, a shape which was 
identical to that of the original rod, and a very beautiful mirror 
surface. 
Although a multiplicity of crystals not oriented in any particular pattern 
had been found in the original rod, the rod leaving the mold was found to 
be composed of 15 longitudinally oriented crystals at a distance of 300 mm 
from its solidifying end and only a single crystal at a distance of 1000 
mm. 
EXAMPLE 5 
A 6 mm dia. rod of copper having a circular cross section and a 
unidirectionally solidified structure was produced by employing a 
horizontally disposed hollow heating mold. The mold was a graphite tube 
having an inside diameter of 6 mm, an outside diameter of 12 mm and a 
length of 200 mm and surrounded by a coil of nichrome wire defining a 
heater. An alumel-chromel thermocouple was provided in the vicinity of the 
outlet end of the mold, and another alumel-chromel thermocouple in its 
melting zone, for measuring the temperature of the inner wall surface of 
the mold. 
A polycrystalline copper rod prepared by plastic working and having a 
diameter of 6 mm was placed by pinch rolls through the mold disposed in a 
protective nitrogen gas atmosphere. An electric current was supplied to 
the heater to heat the mold, while water was being sprayed onto the 
surface of the rod at a rate of 100 cc per minute by a spray cooling 
device disposed at a distance of 50 mm from the outlet end of the mold. 
The electric current was so controlled that the mold might have a 
temperature of 1100.degree. C. in the vicinity of its outlet end and a 
temperature of 1180.degree. C. in the vicinity of its inner wall surface 
in the melting zone. The pinch rolls were driven to move the rod through 
the mold at a speed of 100 mm per minute. As a result, there was 
continuously produced a rod of copper having a unidirectionally solidified 
structure, a shape which was identical to that of the original rod, and a 
very beautiful mirror surface. 
Although a multiplicity of crystals not oriented in any particular pattern 
had been found in the original rod, the rod leaving the mold was found to 
be composed of ten longitudinally oriented crystals at a distance of about 
100 mm from its solidifying end and only a single crystal at a distance of 
500 mm therefrom.