US3549353A

US3549353A - Method and apparatus for melting reactive materials

Info

Publication number: US3549353A
Application number: US603841A
Authority: US
Inventors: Henley Frank Sterling; Wilbert Ridd George
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1966-01-28
Filing date: 1966-12-22
Publication date: 1970-12-22
Anticipated expiration: 1987-12-22
Also published as: DE1558202A1; NL6701463A; FR1509531A; GB1122944A

Description

1970 A H. F. STERLING EIAL 3,549,353

METHOD AND APPARATUS FOR MELTING REACTIVE MATERIALS Filed Dec. 22, 1966 2 Sheets-Sheet 1 D 2 ii 3 5 c H g 5 g H 0 ii 0 L :1 /4 I I F/ I? 5 l2 l3 M'VE/WDRS) H. F. Sterling-W.R.Georgc 40-1 BY 161 4 w h K7 10PM Y Dec. 22, 1970 H. F. STERLING EIAL 3,549,353

METHOD AND APPARATUS FOR MELTING REACTIVE MATERIALS Filed Dec. 22, 1966 .2 Sheets-Sheet 2 Inventor. H.F.Ster1ing-W.R.George 40-1 A Home y United States Patent O 3,549,353 METHOD AND APPARATUS FOR MELTlNG REACTIVE MATERIALS Henley Frank Sterling and Wilbert Ridd George, London, England, assignors to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed Dec. 22, 1966, Ser. No. 603,841 Claims priority, applicatgog great Britain, Jan. 28, 1966,

66 Int. c1. C221: 61/04 US. Cl. 7584.1 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF INVENTION The term metal, whenever used in the following specification and in the claims includes alloys besides the individual metals, and the term reactive is taken to mean reactive at temperatures of the order of the melting point and above with refractory material.

Examples of materials which are reactive when molten or when raised to a high temperature are such metals as nickel, titanium, zirconium, iron, chromium, molybdenum, tungsten and uranium, and some semiconductors of which silicon is a conspicuous example.

Because of their relatively high melting points, their resistance to corrosion, their comparatively low densities, and their high strength, certain metals are useful for making machine parts of various kinds. For example, titanium, zirconium, chromium based alloys and nickel based alloys have been found useful for forming compressor blades for turbines and jet engines.

The melting points of metals such as nickel, titanium, zirconium, chromium, molybdenum and tungsten or metals having like or similar characteristics, as well as some of the carbides, oxides and other particular compounds of such metals, are high enough to make melting in conventional refractory crucible-type furnaces without contamination extremely difficult and generally unsatisfactory because of the tendency of the metals to react chemically and physically with the refractory material with consequent contamination of the metal and change in its mechanical and chemical properties. Moreover, at elevated temperatures, metals are readily reactive with the oxygen and nitrogen of the air. The handling of metals at temperatures from about 1300 C. to above 3000 C., when such metals are melted in the conventional crucible type of furnace presents problems of such dilficulty as to preclude the use of such crucibles at these temperatures, if reasonable purity and repeatability of composition is required.

For the melting of titanium and its alloys or zirconium, an arc, struck within a water-cooled copper vessel has been used. However, an arc can introduce contamination from the electrodes into the metal.

SUMMARY OF INVENTION The present invention provides apparatus and a method for melting reactive material comprising:

(a) placing the material in a container formed from two or more members having hollow walls and being See made of a metal having a thermal conductivity of not less than 0.49 gram calories per cm. per cm. per deg. C. per sec. and a specific electrical resistance of not more than 2.665 microhms per cm. at 0 C., said members being electrically insulated from one another and so positioned with respect to each other that a cavity is formed within said container;

(b) circulating a cooling fluid through said walls and melting the said material by means of eddy currents induced therein solely by means of high-frequency electric currents flowing in the walls of the said container; and

(c) on completion of melting, allowing the said material to cool within said cavity of the container such that a solid form of said material is produced, or extracting the material therefrom.

DESCRIPTION OF DRAWINGS The foregoing and other features of the invention will be evident from the following description of an embodiment thereof taken in conjunction with the accompanying drawings in which:

FIG. 1 shows in cross section an apparatus suitable for melting and casting reactive materials;

FIG. 2 shows in perspective a container formed from two identical members and suitable for melting and casting reactive materials;

FIG. 3 is a sectional plan view of part of the apparatus shown in FIG. 1, taken at the plane indicated by the numerals 33;

FIG. 4 shows (in sectional plan view) a formation of container members alternative to that shown in FIG. 3; and

FIG. 5 is a diagrammatical representation of the highfrequency current flow within said container members which form an inductance or high-frequency heating work coil.

DESCRIPTION OF EMBODIMENTS With reference to the drawings, the container to hold material to be melted is formed from two hollow container members 10 having a molded inner portion 11 and constructed from copper, silver or gold; preferably copper with silver plate. Each container member has an inlet tube 12 extending through most of the length of the inner cavity, and an outlet tube 13, said tubes providing means whereby a cooling fluid may be continuously circulated.

The container members are held within a further container 14, for example a silica tube, the inlet and outlet tubes 12 and 13 extending through apertures in a base plate 15 of brass and being hard soldered to the said base plate, such that the container members are rigidly supported within the tube 14 to be normally as near as possible to each other without contact. The inner portions of the respective container members form a cavity 16 within which material may be melted and thence cast. Contact between the container members 10 may be avoided by placing between the members a thin sheet of insulating material 28 (FIG. 3) for example mica. Alternatively, each member may be constructed with a longitudinal fin 17 (FIG. 4) having apertures through which screws 29 of insulating material, for example, nylon may pass. The screws extend into corresponding apertures in the body of the other container member, thereby iniulating all parts of one container member from the ot er.

The preferred shape of the container members 10 is shown in FIG. 3. Light and heat from the melt within the cavity 16 are prevented from reaching the insulating material, which would otherwise be susceptible to decomposition. With reference to the construction of the coni n un tainer members as shown in FIG. 4, heat and light from the melt will impinge on the walls of the tube 14. If the latter were made of glass and not quartz, the heat developed would produce a crack in the glass. Therefore, it is desirable to construct the container members in the form shown in FIG. 3 if there is danger that the outer container will deform or crack when subjected to heat from the melt.

The tube 14 is held between the base plate 15 and an upper plate 18, for example brass, by means of two or more fiber glass rods 19, each passing through apertures in each plate and having screw threads at each end. By tightening the nuts on the screw threads, the tube 14 is held firmly in position. Gas tight seats 21 are provided at the interface between the edges of the tube 14 and the plates such that a particular gas pressure or degree of vacuum may be constantly maintained within the region defined by the tube 14 and

plates

15 and 18.

Radio-frequency or high-frequency current is supplied to the inside surface of the container by means of inductive coupling from a primary coil in the form of a hollow helical copper coil 22 heat insulated from and encircling the tube 14. Coil 22 may cover an area corresponding to the length of the cavity formed by the container members. Alternatively, it may cover only a small part of the length and be capable of being moved vertically during processing.

The coil 22 should be as close as possible to the container for effective coupling, a practical spacing being less than Ms of an inch, indicated by x in FIGS. 3 and 4.

An inlet pipe 23 is provided in the base plate 15 and an outlet pipe 24 in the top plate 18 such that, if required, a continuous gas flow may be provided through the tube 14.

A means for holding the material to be melted and for transporting said material to the cavity 16 is provided and is illustrated as a hopper 25 in FIG. 1.

As an example of the use of the above-described apparatus, the casting of a high-purity nickel alloy will be described. High-purity nickel alloy is fed via the hopper 25 into the cavity 16 formed by the container members 10. Air within the region defined by the tube 14 and upper plate 18 and base plate 15 is evacuated and the region flushed with argon. A steady flow of argon is maintained throughout the operation of melting and casting, by means of the valves 26 controlling the inlet and outlet pipes 23 and 24, respectively. Water is circulated within the container members 10 and through the helical copper tubing 22. The latter is coupled to a 15 kw., 400 kc. induction generator. As the power is increased, the nickel alloy is heated by eddy currents induced therein by radio-frequency currents flowing in the walls of the copper tubing. The relationships among the primary radio-frequency current 28, the secondary radio-frequency current 27, and the tertiary radio-frequency current 29 are shown in FIG. 5. The molten nickel is held within the cavity 16 by forces of surface tension and by the influence of the radio-frequency field. The cavity may be filled up to a desired level throughout the operation, or the hopper may be arranged for continuous feeding when the cavity is bottomless for continuous casting if desired; if this be the case, means such as a valve or a further evacuated chamber may be provided at the hopper so that gas flow Within the tube 14 is uninterrupted. On completion of melting, the radiofrequency power is switched oif and the molten alloy allowed to cool and solidify in the shape defined by the cavity 16.

When the material to be melted has a resistivity too high for radio-frequency heating to be eflective, it may be necessary to preheat the material to reduce the resistivity to a level suitable for melting by radio-frequency heating.

As hereinbefore stated, the difiiculties in melting reactive materials for casting in most conventional refractory crucibles are mainly those of contamination by impurities.

4 For example, in certain cases an average of over 75% of castings produced from nickel-based alloys by melting in zirconia-type refractory crucibles at temperatures in the order of 1600 C. have impurity contamination, or gross solid inclusions from the disintegration of the zirconia crucible material.

The present invention has been found particularly suitable for the melting and subsequent casting of reactive materials. By utilizing an essentially cold, non-wetting container, of high electrical and thermal conductivity, the container itself being an inductance, contamination is avoided. The metals, copper, silver, gold and aluminum are particularly suitable for the material of the container, because each of these metals has high electrical and thermal conductivity. Silver is preferred because its electrical and thermal conductivity is the highest and because it can be most readily polished to reflect heat optically onto the charge of material being melted. Alternatively, other metals having a high conductivity may be used in place of silver and may be silver-plated.

Although the invention has been described in terms of a container formed from two members, it will be evident that, if necessary, a greater number of members may be used. The optimum number of members for any particular application will be determined partly by the impedance requirements of the radio-frequency power source and partly by the provision of an adequate circulation of the cooling fluid, and will usually lie between two and eight.

The invention may be applied to melting of materials capable of responding to radio frequency induction, Whether preheating is necessary or not. For example, the invention may be applied to crystal pulling and singlecrystal growth. When this type of container is placed in a suitable radio-frequency field supplied by an induction heater, the container itself forms part of the work-coil inductance and therefore supplies energy for melting the charge. Moreover, these containers may be made in many forms depending on the particular application.

While the principles of the invention have been described with reference to a specific embodiment and particular modifications thereof, it is to be clearly understood that this description is by way of example only and should not be construed as a limitation on the scope of the invention, which is defined by the appended claims.

What we claim is:

1. A method of melting material capable of being heated by radio-frequency induction comprising the steps of:

(a) placing the material in a container formed from a plurality of members, each of said members being hollow and made of a metal having a thermal conductivity of not less than 0.49 gram calories per cm. per cm. per deg. C. per sec. and a specific electrical resistivity of not more than 2.665 microhms per cm. at 0 C., said members being electrically insulated from one another and so positioned in close proximity with respect to each other that a cavity is formed within said container;

(b) circulating a cooling fluid through said walls;

(0) melting the said material by means of eddy currents induced therein by means of radio-frequency electric currents flowing in the walls of the said con tainer; and

(d) on completion of the melting step, discontinuing said currents and allowing the said material to cool;

2. A method according to claim 1 wherein said container is surrounded by a medium having less than atmospheric pressure.

3. A method according to claim 1 wherein said container is surrounded by a medium of inert gas.

4. A method according to claim 1 wherein said material is continuously supplied to said container.

5. A method according to claim 1 wherein said material is chosen from the group comprising nickel, titanium,

zirconium, chromium, molybdenum, tungsten and uranium, and alloys and heat-stable compounds thereof.

6. A method according to claim 1 wherein said material is a semiconductive material.

7. A method according to claim 6 wherein said material is extracted from said cavity while molten to produce a monocrystal.

8. A method according to claim 1 wherein said container is formed from metals selected from the group comprising copper, silver, gold and aluminum.

9. An apparatus for melting material capable of being heated by radio-frequency induction including a container, formed from a plurality of members, each of said members being hollow, said members each being electrically conductive and insulated from one another and so positioned in close proximity with respect to each other that a cavity is formed therebetween, said container being electrically insulated from but inductively coupled to the coil of a radio-frequency current supply such that said container members act as secondary windings of a transformer of which said coil is the primary winding.

10. An apparatus according to claim 9, and further including means for supplying said material to said cavity.

References Cited UNITED STATES PATENTS 4/1933 Northrup 13-26 8/1933 Chesnut l3--26