US3857696A - Melting and casting of transitional metals and alloys - Google Patents

Abstract

A method of melting and casting a transitional metal or an alloy thereof uses an unclad thin-walled graphite crucible having a bottom opening therein in which is positioned a fusible metallic member, usually alike the charge metal in composition, to obturate the opening; charge metal is positioned in the crucible and is rapidly heated by electrical induction to a temperature above its melting temperature without substantially melting the obturating member, and thereby provides a pool of molten metal flowing onto the member to melt a pour hole therethrough through which the molten charge discharges from the crucible.

Description

United States Patent [191 Aldersley et al.

MELTING AND CASTING OF TRANSITIONAL METALS AND ALLOYS Inventors: Kenneth Aldersley, Reading;

Gordon William Telford, Tadley, both of England United Kingdom Atomic Energy Authority, London, England Filed: May 18, 1973 Appl. No.: 361,490

Assignee:

Foreign Application Priority Data May 23, 1972 Great Britain 2 4184/72 U.S. Cl. 75/10 R, 75/65 R Int. Cl C22d 7/04 Field of Search 75/10, 84; 164/80, 51

References Cited UNITED STATES PATENTS 8/1966 Sump 75/10 R Dec. 31, 1974 3,484,840 12/1969 Spoth 164/80 3,504,093 3/1970 Persson 75/10 R 3,598,168 8/1971 Clark 164/51 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Larson, Taylor and Hinds [5 7] ABSTRACT A method of melting and casting a transitional metal or an alloy thereof uses an unclad thin-walled graphite crucible having a bottom opening therein in which is positioned a fusible metallic member, usually alike the charge metal in composition, to obturate the opening; charge metal is positioned in the crucible and is rapidly heated by electrical induction to a temperature above its melting temperature without substantially melting the obturating member, and thereby provides a pool of molten metal flowing onto the member to melt a pour hole therethrough through which the molten charge discharges from the crucible.

10 Claims, 1 Drawing Figure MELTING AND CASTING OF TRANSITIONAL METALS AND ALLOYS BACKGROUND OF THE INVENTION This invention relates to the melting and casting of transitional metals and alloys thereof having high melting temperatures and, in particular, to the melting and casting of titanium and titanium alloys.

US. Pat. No. 3,484,840 discloses a method of melting titanium alloys which comprises providing a refractory ceramic crucible having a bottom opening therein, positioning an electrically conductivegraphite sleeve within said crucible, positioning a titanium alloy disc at the base of the sleeve and extending completely across said bottom opening, positioning a charge of the alloy to be melted within said sleeve and in spaced relation to said disc, and rapidly inductively heating such charge above its melting point without similarly melting the disc, to thereby provide a pool of molten metal flowing onto the disc and thereby melt an opening through the disc.

The specification explains the disadvantages of skull melting in water-cooled crucible arc-melting units, namely the need for specialized and high cost equipment, the difficulty in achieving temperature control whence the variation in pouring temperatures can cause casting defects'and the hazard of arc-melting using water-cooled crucibles. The specification states that the above-cited method utilises equipment which can be installed and operated by investment casting foundries without substantial capital expenditure, that the method provides a much bettercontrol of the pouring temperature and that the cost of melting and pouring titanium, using the method, is significantly reduced.

It has now been unexpectedly established that a container vessel in the form of a refractory ceramic crucible is unnecessary in the melting of titanium and its alloys and that a graphite crucible thin enough for the inductive power loss therein to be low is capable of holding the molten charge without bursting, even though not supported within a ceramic casing.

Accordingly it is an object of the present invention to provide an improved inductive heating method for melting and casting titanium and other transitional metals and the alloys thereof having high melting temperatures.

SUMMARY OF THE INVENTION According to the invention a method of melting and casting a transitional metal or an alloy thereof comprises providing an unclad thin-walled graphite crucible having a bottom opening therein, positioning a fusible metallic member substantially non-deleterious to the molten charge metal at the bottom of said crucible to obturate said opening, positioning a charge of said metal or alloy to be melted within said crucible above said member and rapidly heating said charge by electrical induction to a temperature above its melting temperature without substantially melting said member to thereby provide a pool of molten metal flowing onto the member and thereby melt a pour hole through the member through which the molten charge discharges from the crucible.

The graphite crucible may have a wall thickness substantially less than the electrical reference depth a where a (1/2 7r) 1/ (p/uj) p being the resistivity of graphite (1300 microhm/cm), p. the permeability of graphite (l, for a non-ferrous material) and f the frequency of the electrical supply. In practice, it is found that the wall thickness may be less than one-fifth the electrical reference depth and preferably is less than one-tenth.

The crucible wall should have a thickness such that the crucible is mechanically strong enough to hold the charge metal yet be such that energy generated by the induction coil is absorbed mainly in the metal charge to be melted, graphite being, electrically, a susceptor material.

The molten charge metal exerts its maximum lateral thrust at a position close to the bottom of the crucible: to provide support for the crucible in this region it is preferred to seat the crucible in a refractory supporting cup the walls of which form a sliding fit with said crucible and extend to embrace the crucible over said re gion, said cup having a bottom opening enabling molten charge metal to flow from the crucible. Preferably the supporting cup is made of graphite thereby functioning as a heat sink for heat transmitted through the lower wall of the crucible.

To reduce carbon pick-up by the molten metal from the crucible wall the crucible may be lined with a ceramic material subsequently inert to the molten metal. Suitable materials for use with titanium and titanium alloys include tungsten, yttria-stabilised zirconia, and yttriaor calcium-stabilised zirconia mixed with titanium hydride. The material may be present as a coating applied by plasma spraying. Alternatively, freestanding liners fabricated from these materials may be inserted into the crucible to be in contact with and to protect the crucible wall.

The fusible metallic member may be made from said metal or an alloy thereof.

Transitional metals reactive to air in the molten state, such as Ti and its alloys, are preferably inductively heated in vacuo.

The invention also provides apparatus for melting and casting a transitional metal or an alloy thereof comprising an unclad thin-walled graphite crucible having an opening in the base thereof for the discharge of molten charge metal there-through, and in which a fusible metallic member is locatable to obturate the opening, and an electrical inductor heating coildisposed without said crucible and locatable in relation thereto such that a charge of said metal or alloy, containable within the crucible above said obturating member, is within the field of said coil.

The fusible metallic member may be made from metal or an alloy thereof.

The inductor heating coil may include, in addition to a plurality of coil turns providing the magnetic flux for melting, at least one reverse wound turn so spaced from said plurality of turns to be positionable in relation to the rim of the crucible to substantiallycurtain metal splash-over during meltdown.

By the use of electric induction heating rather than heating by an electric arc the stirring effect in the molten metal produced by induction heating ensures that, immediately prior to the melting of the metallic member, the whole of the molten charge is at a substantially uniform temperature. As a result of this stirring action the whole of a molten charge, immediately prior to casting, can not only be raised to a uniform temperature for efficient casting but can also be made to discharge from the crucible at a substantially constant temperature in successive melts. A temperature variation of about 25C in the casting-temperature, in successive melts, has been found possible.

DESCRIPTION OF THE DRAWING mately to scale and has an unclad thin-walled graphite crucible 1 located within an inductor coil 2. The crucible is supported by a shallow cup 3 provided with support legs 4. The cup 3 has a bottom opening 12 within which is located a spout 11 supported by shoulders at its uppermost end. The lower part of the crucible is-a sliding fit within the walls of the cup: when a charge of metal 6 is rendered molten by high rateinductive heating the molten charge exerts its maximum lateral thrust against the supporting crucible a short distance from the bottom of the crucible; at greater distances from the bottom the metal tends to retract inwards from the crucible wall. The cup 3 provides lateral support for the crucible over this short distance from the bottom and being made of graphite also acts as a localised heat sink. To curtain susception in the cup wall three vertical slots are cut through the wall (not shown) the slots being cut at intervals of 120: one'slot is extended to cut through the base of the cup and forms a radial slot therein; this has been sufficientto curtain susception in the base.

The'crucible 1 has an opening 7 in its base 8 for the discharge of the molten charge from the crucible. The opening 7 is obturated by a metallic member 9 which, in this embodiment, is in the form of a stepped plug seated in the opening. The plug 9 is surrounded by an annular insert 10 made of graphite: the insert is slotted through vertically (slot not shown) to curtain susception and provides a false bottom with the plug 9 positioned centrally therein. The spout 11 is also made of graphite. It will be apparent that the crucible 1 serves to short-circuit the slots cut through the cup wall 5: to obviate this effect thin coatings of alumina are painted or sprayed on the inside of the cup wall and/or on the outside of the crucible where it slides into the cup.

The inductor coil 12 has 14 turns and, spaced above the crucible 2 reverse wound turns 13. The reverse wound turns create a magnetic field in opposition to that created by the lower 14 turns and serve to curtain metal splash-over which can otherwise occur when HF power for high rate heating is applied to the coil 2. The reverse wound turns thus allow the requisite power to be applied without causing an unstable melt.

EXAMPLES OF THE PRESENT PROCESS The apparatus of the present embodiment has been used for melting Ti and Ti alloys. Short bars of metal 2% ins dia X 3 ins were inserted into the 2.6 in dia crucible to rest on the false bottom 10, as shown.

The electrical measurements for a typical melt were as follows (the charge metal was 90% Ti 6% Al 4% alloy) Inductance of 14 turn coil 10.85 p.H

Capacity Alter- Alter- Field in nator nator KW KVAR Current Circuit Volts Amps (amps) KVA Empty coil 360.0 350 26 -7 1.90 TO Melt 380.0 350 I46 50 8 2.15

The wall thickness of the crucible l was within the range 0.0500.070 ins and the base thickness also in the range 0.050-0.070 ins. By using a crucible having these dimensions the crucible is electrically and mechanically thinz" a large proportion of the electrical power is absorbed by the charge. Moreover, a thin crucible can better i) withstand the thermal shock due to the rapid heat rate developed when power is applied and ii) promotea high rate of heat loss by radiation 'thus effectively cooling the inner crucible surface. If the inner crucible surface can be kept chilled by high radiant heat loss, the rate of reaction between the crucible and a melt can be substantially reduced.

Thus the requirements for a crucible as used in the present invention are i) a capability to hold the charge ii) low power consumption and iii) high radiant heat loss. It will be apparent that the need for a crucible of sufficient strength to hold the charge is the limiting factor.

In practising the present invention high heating rates and short heat times are preferably used to reduce carbon pick-up. Carbon pick-up can be further reduced by lining, or providing liners for, the inner surface of the crucible. The crucible can be lined with (for Ti and Ti alloys) tungsten, yttria-stabilised zirconia, or yttriaor calcium-stabilised zirconia with titanium hydride, and these materials can be applied by plasma spraying: alternatively free-standing liners in these materials can be produced by plasma spraying and the liners slipped inside the crucible.

Crucibles for use in the present invention can be made, for example, by i) machining from dense graphite masses, ii) machining from porous graphite and densifying the inner surface with pyrolitic graphite to impede metal penetration and confer additional strength, iii) deposition of pyrolitic graphite onto a mandrel and iv) spraying graphite into carbon fibre or cloth wound ona mandrel and consolidating the inner surface of the crucible so formed with pyrolitic graphite. Lined crucibles canbe made by deposition of liner material onto a mandrel followed by a graphite backing produced by,

for example, spraying or by deposition of pyrolitic graphite.

For apparatus of larger size than that of the embodiment to wall thickness of the crucible may be increased within limits: to retain low power absorption with thicker-walled crucibles the frequency of the HF power supply has to be lowered.

The following Table collates heat times with the carbon content and hardness values for charge metal and cast metal when melted in unlined and lined crucibles. The charge metal was a Ti alloy Ti 6% Al 4% V).

Carbon Content Hardness. VPN

7a Heat Charge Charge Crucible Structure Time metal cast metal metal cast metal Mins Dense Graphite 2.05 0045 0.230 330 345-380 Graphite (-3071 porisity) lined 2.00 0.045 0089-022 330 345-376 with Pyrolitic graphite Dense graphite Liner: Yttria 2.00 0.045 0040-0048 330 272.363 stub. Zirconia Dense Graphite Liner: Yttriu 2.50 0045 0042-0060 330 309-350 stab. Zirconia Dense Graphite Liner: ytrria 2.40 0.045 0042-0067 330 300-355 stab. Zirconia Ti In melting Ti and Ti alloys the apparatus of the present invention is held in vacuo: U.S. Pat. No. 3,484,840 draws attention to the problem of contamination in the melting and casting of titanium and mentions the embrittling effect of hydrogen, oxygen, carbon and nitrogen; accordingly air should be excluded from the melt environment so far as is practicable.

Although the above embodiment of the invention is concerned with titanium and its alloys the invention has application to high-melting transitional metals in general. Uranium and uranium alloys can, for example, with advantage be melted and cast according to the invention. It is believed that, like Ti, the reactive nature of many of these metals when molten is curtailed by the high radiant heat loss from the thin-walled crucibles of the invention and that by inducing chill at the metalcrucible interface in this manner the rate of reaction in the region of this interface is substantially reduced: lined or unlined crucibles can be-used according to the reactive habit of individual metals.

What we claim is:

l. A method of melting and pouring as a molten charge a transitional metal or an alloy thereof comprising providing an unclad, thin-walled graphite crucible having a bottom opening therein, positioning a fusible metallic member substantially non-deleterious to the molten charge metal at the bottom of said crucible to obturate said opening, positioning a charge of said metal or alloy to be melted within said crucible above said member and rapidly heating said charge by electrical induction to a temperature above its melting temperature without substantially melting said member to thereby provide a pool of molten metal flowing onto the member and thereby melt a pour hole through the member through which the molten charge discharges from the crucible.

2. A method as claimed in claim 1 wherein the graphite crucible has a wall thickness substantially less than the electrical reference depth, a, as herein before defined. V W m V 3. A method as claimed in claim 2 wherein the crucible wall thickness is less than one-fifth the electrical e ttsqsi tt h-w 4. A method as claimed in claim 3 wherein the crucible wall thickness is less than one-tenth the electrical ef r n slq hi wall ofthe crucible is lined with a ceramic material subt atielly. i rttethsmateriali y 8. A method as claimed in claim 7 wherein, in the melting of Ti and Ti alloys, the ceramic material is chosen from tungsten, yttria-stabilised zirconia and yttriaor calcium-stabilised zirconia mixed with titau hyd de.

9. A method as claimed in claim 7 wherein the crucible lining is provided by plasma spraying a coating of' the ceramic material onto the inner wall of the crucible.

10. A method as claimed in claim 7 wherein the crucible lining is provided by inserting a free-standing liner, fabricated from the ceramic material, into the crucible.

Claims (10)

1. A METHOD OF MELTING AND POURING AS A MOLTEN CHARGE A TRANSITIONAL METAL OR AN ALLOY THEREOF COMPRISING PROVIDING AN UNCLAD, THIN-WALLED GRAPHITE CRUCIBLE HAVING A BOTTOM OPENING THEREIN, POSITIONING A FUSIBLE METALLIC MEMBER SUBSTANTIALLY NON-DELETERIOUS TO THE MOLTEN CHARGE METAL AT THE BOTTOM OF SAID CRUCIBLE TO OBTURATE SAID OPENING, POSITIONING A CHARGE OF SAID METAL OR ALLOY TO BE MELTED WITHIN SAID CRUCIBLE ABOVE SAID MEMBER AND RAPIDLY HEATING SAID CHARGE BY ELECTRICAL INDUCTION TO A TEMPERATURE ABOVE ITS MELTING TEOPERATURE WITHOUT SUBSTANTIALLY MELTING SAID MEMBER TO THEREBY PROVIDE A POOL OF MOLTEN METAL FLOWING ONTO THE MEMBER AND THEREBY MELT A POUR HOLE THROUGH THE MEMBER THROUGH WHICH THE MOLTEN CHARGE DISCHARGES FROM THE CRUCIBLE.

2. A method as claimed in claim 1 wherein the graphite crucible has a wall thickness substantially less than the electrical reference depth, Alpha , as herein before defined.

3. A method as claimed in claim 2 wherein the crucible wall thickness is less than one-fifth the electrical reference depth.

4. A method as claimed in claim 3 wherein the crucible wall thickness is less than one-tenth the electrical reference depth.

5. A method as claimed in claim 2 including seating the crucible in a refractory supporting cup the inner wall of which forms a slidable fit with the crucible outer wall and extends to embrace the lower region of the crucible.

6. A method as claimed in claim 5 in which the supporting cup is a graphite cup.

7. A method as claimed in claim 5 wherein the inner wall of the crucible is lined with a ceramic material substantially inert to the molten metal.

8. A method as claimed in claim 7 wherein, in the melting of Ti and Ti alloys, the ceramic material is chosen from tungsten, yttria-stabilised zirconia and yttria- or calcium-stabilised zirconia mixed with titanium hydride.

9. A method as claimed in claim 7 wherein the crucible lining is provided by plasma spraying a coating of the ceramic material onto the inner wall of the crucible.

10. A method as claimed in claim 7 wherein the crucible lining is provided by inserting a free-standing liner, fabricated from the ceramic material, into the crucible.

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