WO2006041491A1

WO2006041491A1 - Method of assembling feedstock for cold hearth refining

Info

Publication number: WO2006041491A1
Application number: PCT/US2004/033285
Authority: WO
Inventors: Allen E. Blackburn; Fatai Odebisi; Allen J. Poore; Nick F. Haberman; Dionne A. Dillon
Original assignee: Titanium Metals Corporation
Priority date: 2004-10-07
Filing date: 2004-10-07
Publication date: 2006-04-20

Abstract

A standard form of feedstock is provided for cold hearth melting and refining of metals, for example, titanium or titanium-aluminum alloys. The standard form is fabricated from sheet metal as an open top trough (202). The open top trough (202) is used as a container for smaller pieces of raw materials such as titanium casting heads and wafers that can have varying form, shape and size. Alloy components, for example, aluminum, may be introduced in to furnace in bulk form as long drawn wires or rods (204) together with the primary feedstock.

Description

METHOD OF ASSEMBLING FEEDSTOCK FOR COLD HEARTH REFINING

SPECIFICATION

BACKGROUND OF THE INVENTION

This invention relates to cold hearth melting and refining of reactive and refractory materials. In particular, this invention relates to improvements to the charge material or feedstock used in electron beam or plasma arc cold hearth melting and refining processes.

Cold heart refining processes are now commonly employed for the production of reactive and refractory metals such as tantalum, niobium, molybdenum, tungsten, vanadium, hafnium, zirconium, titanium and their alloys. In electron beam cold hearth refining (EBCHR) processes, electron beams are used to melt target raw metals that are placed under high vacuum in water-cooled hearths. Metallic and non- metallic impurities having vapor pressures higher than those of the base constituents selectively evaporate from the molten material. Thus, the base constituents of the target material are purified.

In conventional EBCHR apparatus, the hearths are usually elongated copper troughs or crucibles, which are water-cooled. One or more electron guns are configured to direct electron beams to the surface of the target material placed in a hearth to melt and form shallow surface pools of liquid metal in the hearth. The molten metal in the surface pools flows along the hearth and then overflows from the hearth into a water-cooled mold. The hearths usually have distinct melting regions and refining regions. The melting region is at the end where the target material is introduced and the refining region extends from the melting region toward the overflow mold. The shallow pools of liquid metal (i.e., molten material) in the two regions are connected by a narrow channel through a skull of solid material that separates the two regions. Advantageously in the refining process, high- and low- density inclusions are also removed from the molten material in addition to the selective evaporation of volatile impurities. High-density inclusions settle and collect in the skull, while lighter inclusions either are dissolved in the liquid metal or are held back by a dam or other physical barriers prior to flowing into the mold. Exemplary EBCHR apparatus are described in co-assigned Harker United States patent No. 4,932, 635 and United States patent No. 4,961, 776, and Harker et al. United States patent No. RE32,932, all of which are incorporated by reference in their entireties herein.

The EBHCR apparatus may be used to produce metals or metal alloys from metal compounds, ores, or scrap metal. The hearths are configured to receive a charge of target material (e.g., titanium scrap or lumps) from one end or side, for example, by gravity feed through a feed chute. The charge or feedstock may include loose metal particulate mixtures. For example, in the production or recovery of titanium or titanium alloys, the melting region of the hearth is supplied with lumps, briquettes, or pieces of titanium sponge or machine turnings of titanium alloy consisting of scrap from the manufacture of titanium alloy parts. Solid lumps or pieces of scrap metal may move in situ in the melting region making it necessary to continuously monitor and adjust the position of the electron beam used to melt the charge. Further, the loose or particulate gravity-fed feedstock can splatter or roll into unintended areas of the hearth when it is gravity fed into the hearth. For example, large solid fragments of the feedstock may roll or float down stream from the melting region into the refining region of the hearth so as to contaminate the refined metal in the casting region. Light solids, such as chopped tubing may also escape complete melting and float into the refining region, causing the same problem. For the production of alloys, the feedstock mixtures are blended from different materials in proportion to according to the desired alloy compositions to obtain a homogeneous mixture. For example, in the production titanium-aluminum alloys, aluminum pieces or pellets are added to the feed stock mixtures to produce the desired alloy composition. The amount of aluminum added also may take into consideration the loss of volatile aluminum from the molten material as it flows through the hearth. Harker US. patent No. 5, 222, 57 describes use of a condensing screen and vibrator arrangement for reclaiming and reusing some of the aluminum volatilized from the molten material as it flows through the hearth. In some refining operations, a bar or ingot of first-melt material may be used as the initial feed stock. The first-melt bar is remelted and further refined in the EBHCR apparatus. Alloy constituents may be added to the melt to reach the desired alloy compositions. Uneven movement settling or settling of the alloy constituents may result in a heterogeneous distribution of some residual elements. Consideration is now being given to ways of enhancing both the processes and apparatus to improve the overall efficiency of electron beam cold hearth refining systems. Particular attention is devoted to the manner in which raw materials are prepared or fed into the hearth.

SUMMARY OF THE INVENTION

Accordingly it is an object of the present invention to provide feedstock arrangements that overcome the disadvantages of the prior art.

This and other objects of the invention are attained by assembling feedstock or charge in a common standard form for introduction into the cold hearth refining furnace. The standard form of includes an elongated shell having at least three solid or rigid sides to hold smaller pieces of raw material. The shell, which serves as a container to mechanically hold the smaller pieces of raw material, may be fabricated from sheet metal in the form of an open top trough. The smaller pieces of raw material contained in the shell can have varying form, shape and size (e.g., titanium casting heads, scrap wafers, particulate and loose feed material, compensation material, wires and rods). A standard form of feedstock simplifies loading equipment and operations leading to more manufacturing efficiency. hi a further or alternate improvement, alloy components in bulk form are attached to the primary feed stock. The bulk form may advantageously replace small pieces or pellets that are conventionally used to introduce compensation material in the refining of refractory alloys. For example, commercially available aluminum (or aluminum alloy) long drawn wires or rods may be used instead of aluminum pellets as compensation material in the preparation of refined titanium - aluminum alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view illustrating a cold hearth melting and refining arrangement in accordance with the prior art; FIGS. 2a - 2e are photographs illustrating the assembly of a raw material charge in a standard rectangular form, in accordance with the principles of the present invention.

FIG. 3 is a photograph illustrating compensation material titanium - aluminum rods that strapped to a primary feedstock charge in accordance with the principles of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The present disclosure provides solutions for improved cold hearth refining operations. The disclosed solutions concern preparation of the raw material feedstock or charge for refining by cold hearth refining processes.

The invention is directed to improving the manufacturing efficiency of refining processes using cold hearth technology. Cold hearths can be obtained in various designs or configurations. A particular hearth configuration may be selected on considerations such as the type of material to the refined, throughput or other manufacturing parameters. Exemplary, cold hearths, which can be used for electron beam refining titanium alloys, are described in Harker United States patent No. 4,932,635 and United States patent No. 4,961,776. Further exemplary electron beam cold hearths, which can operate at intermediate pressures (vacuum), are described in Harker United States Patent No 5,100,463, and United States Patent No 5,222,547.

FIG. 1, which is reproduced from United States patent No. 5,222,547, shows an exemplary electron beam furnace 10 that may be used for electron beam refining of raw materials. Electron beam furnace 10 includes a housing or chamber 11 enclosing a cold hearth 12 in a low pressure ambient. Cold hearth 12 is cooled by internal water circulation conduits 13 to form a solid skull 14 of the material being refined. Pieces of solid raw material 15 to be refined are deposited in a melting region of the hearth. Raw material 15 deposited in the hearth is melted by an electron beam from an electron beam gun 17 that is scanned over a desired hearth area in a suitable manner to provide a pool of molten material 18 in the hearth. A chute 16 extends from a load port (not shown) at an inlet side of chamber 11 toward the melting region of the hearth. The load port may involve conventional mechanical techniques (e.g., gate valves, load locks or load chambers, etc.) to bridge the difference between the pressures exterior and interior to. chamber 11. Pieces of solid raw material 15 , which may b e materials such as titanium sponge or titanium alloy turnings and mixtures, are deposited in the hearth through chute 16. In alternate arrangements (not shown) the raw material supplied to the furnace may be in the form of a solid bar or electrode, which is introduced in the hearth using suitable bar feeding devices. The electron beam melts one end of the solid bar as the bar is being fed toward the beam. The bar feeding devices may be configured to allow the - bar to be fed into the hearth without depressurization of the chamber or stopping of melting process. hi conventional processes for producing refined titanium alloys of a desired composition (e.g., Ti-Al alloys), pieces 15 of solid raw material may include a mixture of titanium materials and a metered or measured quantity of aluminum pieces or pellets. The measured quantity of aluminum pellets may be designed to compensate for evaporative losses of aluminum from the melt in the refining process. hi accordance with the present invention, the introduction of the right amount of aluminum for compensation and composition control is facilitated by the use of a solid bulk form of aluminum (or aluminum - titanium alloy) with dimensions that are more convenient for handling than small pieces or pellets. The bulk compensation material may have convenient elongated shapes such as rods, bars, tubes, strips or wires. The solid bulk form of compensation material may have dimensions that are greater than the dimensions of the primary feedstock material (e.g., titanium casting heads). The aluminum and aluminum - titanium alloys are, for example, readily available commercially as long drawn wire and rods, which can be a few feet to several feet long. The use of aluminum in this form may obviate the need to weigh out or mix the right amount of aluminum pellets or pieces. Aluminum or aluminum - titanium alloy wires or rods of suitable diameter and length may be introduced in the melting region of the hearth at a suitable rate to compensate for volatile losses in the melting process and to obtain refined titanium alloys of a desired composition. In an exemplary implementation, commercially available Al-Ti alloy rods (Al 90 %° Ti 10 %) are used. Any suitable feeding mechanism may be used to introduce the wires or rods into the furnace.

It will be understood that this improved method of compensation of volatile or evaporative alloy elements is not limited to aluminum but is readily applicable to other alloy constituents of interest (e.g., chromium). The compensating materials may be elemental or in alloy form, and have any suitable elongated shape, including wires, rods, and bar shapes. The compensating materials may be introduced into the furnace separately (e.g., through a separate feed port) from the primary feedstock material (e.g., titanium materials). Alternatively the compensating materials may be introduced into the furnace together with the primary feed stock through a common feed port. In a preferred embodiment, where the primary feed stock used is a bar or ingot (e.g., a first remelt ingot), the compensating material wire or rod may be physically attached (e.g., by a metal strap) to the primary feedstock bar or ingot for joint introduction into the furnace. The end of the physically attached compensating material and the primary feedstock combination is then melted in a usual manner by the electron beam.

Another aspect of the present invention relates to the assembly of a common standard form of feedstock or charge for introduction into the furnace. The standard form of feedstock or charge may be prepared from raw materials that themselves can have varying form, shape and size (e.g., titanium casting heads, scrap wafers, particulate and loose feed material, compensation material, wires and rods). A standard form of feed stock simplifies loading operations and increases manufacturing efficiency. In a preferred embodiment, this standard form of feedstock has a three dimensional elongated shape (e.g., a longitudinal bar shape with a rectangular, round or U-shape transverse cross section). FIGS. 2a-2e show how an exemplary titanium- aluminum feedstock or charge 250 can be assembled. A metal shell 200 holds other raw material (e.g., titanium casting heads, scrap wafers, particulate and loose feed material, and optional compensation material such as aluminum pieces, wires and rods). Shell 200 may be shaped like an open top trough that has at least three solid sides (FIG. 2a). Such an open top trough may be fabricated by suitably bending titanium or titanium alloy sheet metal. Alternatively, the open trough may be fabricated by welding suitable sheet metal pieces. The ends of the trough may be at least partially closed by welded metal blocks or wafers 202 (FIG. 2b and 2c). Raw material pieces (e.g., titanium scrap casting heads and wafers) are placed in shell 200 (FIGS. 2b-2d). The raw material pieces placed in shell 200 are physically or mechanically confined by the walls of shell 200 without the need of permanent attachment (e.g., welding) of the pieces to each other. In an exemplary implementation, feedstock or charge 250 has a length of about 10 to 12 feet long and is about 2 to 3 feet high.

When the standard feedstock charge is used for preparation of refined titanium-aluminum alloys, a suitable number of aluminum-titanium rods 204 may be placed lengthwise in or on top of along shell 200. FIGS 2a-2e show aluminum- titanium rods 204 placed on top of the raw material pieces loaded in shell 200. Rods 204 may be held in place by titanium metal straps 206 that are spot welded to underlying raw material pieces. (FIG. 2d). Further, FIG. 2e shows an optional yoke assembly 208, which is welded to an end of shell 200. Yoke assembly 208 may be designed so that shell 200 can be conveniently pushed, lifted or otherwise moved by suitable mechanical power means. Shell 200 may be introduced in the furnace as a single unitary structure. Shell 200 may be melted gradually from one end by the electron beam in the furnace in a manner similar to manner in which conventional solid ingots are melted in electron beam refining processes.

The use of a standard form of feedstock or charge using shell 200 to mechanically hold raw material pieces is more economical than the prior art methods in which raw material pieces are individually welded together. Further the "bulk" form of the charge may be mechanically more convenient for systematic introduction into the refining apparatus.

Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.

Claims

CLAIMS:

1. An improvement in melting and refining of a refractory metal alloy that includes a relatively volatile alloy component in a cold hearth apparatus, the improvement comprising: obtaining the volatile alloy component in a solid bulk form; introducing a quantity of the volatile alloy component in a solid bulk form in the cold hearth apparatus for co-melting with a primary refractory metal alloy charge, wherein the quantity compensates for evaporative losses of the volatile alloy component during the melting and refining and whereby the refined refractory metal alloy has a target percentage of the volatile alloy component.

2. The method of claim 1 wherein the refractory metal alloy is titanium.

3. The method of claim 2 wherein the relatively volatile alloy component is aluminum.

4. The method of claim 3 wherein the solid bulk form comprises aluminum -titanium alloy.

5. The method of claim 4 wherein the aluminum - titanium alloy comprises about 90% aluminum.

6. The method of claim 1 wherein the solid bulk form has an elongated shape comprising any one of a wire, rod, tube, strip or bar shape.

7. The method of claim 3 wherein the target percentage of the volatile alloy component is about 6.5 % of aluminum.

8. The method of claim 1 wherein a dimension of the solid bulk form of the volatile alloy component is greater than the dimensions of titanium material pieces in the primary feedstock.

9. An improvement in melting and refining of metals in a cold hearth apparatus, the improvement comprising: introducing a charge in the cold hearth apparatus, wherein the charge comprises a standard form, wherein the standard form is prepared from raw material pieces having dimensions smaller than a length of the standard form, and wherein the raw material pieces are not welded together.

10. The improvement of claim 9 wherein the standard form comprises an elongated shell.

11. The improvement of claim 10 wherein the elongated shell comprises sheet metal.

12. The improvement of claim 10 wherein the elongated shell is about 10 feet long.

13. The improvement of claim 9 wherein a raw material is selected from the group of titanium casting heads, scrap wafers, particulate and loose feed material and any combination thereof.

14. The improvement of claim 9 wherein an additional raw material is an alloy component in the form of a rod.

15. The improvement of claim 14 wherein the rod comprises aluminum.

16. A standard charge for cold hearth refining of refractory metals, the charge comprising: pieces of raw material an elongated shell to hold the pieces of raw material.

17. The standard charge of claim 16 wherein the shell comprises sheet metal.

18. The standard charge of claim 16 wherein the shell is shaped as an open top trough.

19. The standard charge of claim 16 wherein the open top trough is about 10 feet long and about 2 to 3 feet high.

20. The standard charge of claim 16 wherein the raw material pieces comprise t titanium.