Processing of Rare Metals - degruyter.com

High Temperature Materials and Processes, Vol 9, Nos. 2-4,1990

Processing of Rare Metals

F. Schreiber

FH München, 8000 Munich, Federal Republic of Germany

CONTENTS Page

ABSTRACT 93 1. BASICS OF METALLURGY 94

1.1. Refractory Metals 94 1.2. Titanium and Zirconium 96

2. STARTING MATERIALS 97 2.1. Tungsten and Molybdenum 97 2.2. Tantalum and Niobium 98 2.3. Titanium and Zirconium 98

3. SINTERING, MELTING AND CASTING 99 3.1. Tungsten and Molybdenum 99 3.2. Tantalum and Niobium 102 3.3. Titanium and Zirconium 111

4. FABRICATION OF MILL PRODUCTS 115 4.1. Tungsten and Molybdenum 115 4.2. Tantalum and Niobium 116 4.3. Titanium and Zirconium 118

5. MACHINING AND WELDING 120 5.1. Tungsten and Molybdenum 120 5.2. Tantalum and Niobium 121 5.3. Titanium and Zirconium 122 REFERENCES 123

Abstract

This article, "Processing of Rare Met-als", concentrates on the materials:

- titanium and zirconium of the IVB-group

- tantalum and niobium of the VB-group - tungsten and molybdenum of the VIB-

group

of the periodic system. Rare metals belong to the group of re-

active metals. They represent the transition metals and show characteristic properties. A high melting point and the tendency to dissolve small atomic non-metals and enter into stable compounds with small atomic non-metals at a higher temperature are an impediment to production and processing.

The solution to these problems in the production of tungsten and molybdenum through sintering, in the case of tantalum and niobium through Electron Beam Melting (EB) and in the case of titanium and zirconium through Vacuum Arc Melting (VAR) was the prerequisite for subsequent processing. The noncutting shaping is carried out in a similar way to that of other cold-ductile metal materials.

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Vol. 9, Nos. 2-4, 1990

All heat treatments must be carried out in high vacuum or under specific gas atmo-sphere. Welding is only possible under in-ert gas, or in high vacuum.

1. Basics of Metallurgy /1,2/

1.1. Refractory Metals

Refractory metals are those materials which have a melting point exceeding 2000 °C (Fig. 1). From the elements men-tioned above, it concerns the rare metals


tantalum and niobium, tungsten and molybdenum. These metals have a cubic body-centered crystal structure, high binding energy, high density, low linear thermal expansion coefficients, low mobil-ity of the atoms in the crystal, and, there-fore, high recrystallization temperatures and high-temperature strength (Table 1).

The mechanical properties of pure metals are influenced by the kind of start-ing material used, production and pro-cessing. The main factors of influence are the purity and the microstructure. Impuri-ties have an unfavourable effect and there-

3500

number of d-electrons in the complementary-shell

0 0 , 2 5 5 ? 7 * ,0 l = M C _ r C 4 b 7 β 10 η

ι ι ι ι ι ι ι ι ι ι ι I : —L(GrcphH,5ublm) \ A / , 2rC

3000

Os ToBj, NbB2

Ί π in iv ν

A-group Β-group VI vu

Fig. 1: Melting point of metals and posit ion in the periodic table 121

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F. Schreiber High Temperature Materials and Processes

TABLE 1. Physical and Mechanical Properties of Refractory Metals /2,12/

Tungsten Molybdenum Tantalum Niobium

Melting point °C 3410 2620 3000 2470

Density g/cm3 19.3 10.2 16.6 8.57

Linear thermal expansion co-efficient 10"6

4.3 5.3 6.5 7.2

Recrystallization temperature °C (0.5 hour, 50% def.)

1400 1200 1300 900

Tensile strength N/mm2 average at 800 °C 500 400 170 140

1200 "C 350 90 120 65 1600 °C 120 30 45 30

fore have to be kept low. This holds true for the selection of the raw materials as well as for the production and subsequent processing. The further influence through the microstructure with grain size and grain quality depends on sintering and melting, respectively, processing and heat treatment. The mechanical properties can be additionally improved with alloying, in fact through micro-alloying with small quantities of additions and through solid solution with greater quantities of addi-tions. One possibility to increase the strength further is precipitation hardening through second phases.

Tungsten and molybdenum differ from tantalum and niobium in that the impuri-ties in the microstructure are in different proportions. Among the results are some prerequisites for the production and pro-cessing. In the case of tungsten and molyb-denum, the undesired impurities concen-trate themselves on the grain boundaries

and reduce the grain boundary strength. When shaping, this reduction leads to anisotropy of the mechanical properties, which are dependent on the direction of processing. Another effect of the impuri-ties at the grain boundaries is that plastic deformation is not possible unless a certain "brittle-ductile" transition temperature has been reached. In the case of tantalum and niobium, the undesired local content of impurities, such as oxygen, can be reduced by the high solubility of oxygen. The con-centration is still clearly below a critical limit. This is also the reason why tantalum and niobium can be welded very well, whereas the weldability of tungsten and molybdenum is only possible to a limited extent.

The production, processing and applica-tion of the refractory metals can be divided into two groups under consideration of the aforementioned influences:

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Vol. 9, Nos. 2-4,1990

- tungsten and molybdenum are prefer-ably sintered in hydrogen atmosphere. The non-cutting shaping is carried out above the brittle-ductile transition tem-perature. The necessary heat treat-ments for this procedure before, during and after the forming are, as a general rule, carried out under hydrogen in high-temperature furnaces. The appli-cation at high temperatures is prefer-ably under hydrogen, reducing gases, inert gases or in vacuum.

- tantalum and niobium must be sintered and melted, respectively, in high-vac-uum. The non-cutting forming is carried out in normal atmosphere at room temperature. Again, the intermediate heat treatments are to be carried out in high vacuum. The application at high temperatures is preferably in high vac-uum or in inert gases.

1.2. Titanium and Zirconium

Titanium and zirconium belong to the reactive metals with a melting point below 2000 °C (Fig. 1). These metals have a hexagonally close-packed crystalline struc-ture, an allotrope transformation, low den-sities, low linear thermal expansion coeffi-cients, low thermal conductivity and, for metals, comparatively high specific electric resistances (Table 2).

The mechanical properties of pure metals are influenced by the starting mate-rial, production and processing. The purity and the microstructure can be regarded here as main influences as well. Impurities are unfavourable to the mechanical prop-erties and therefore they should be kept low. The structural influence results from the grain size and the grain quality, which are dependent on melting and subsequent processing. Alloying can improve the me-chanical properties.

Titanium alloys are divided into alpha-, alpha+beta- and beta- alloys according to


TABLE 2. Physical Properties of Reactive Metals 711/

Titanium Zirconium

Melting point °C 1668 1852

ο Density g/cm 4.5 6.5

Linear thermal expansion coefficient 10"6

9.5 5.8

Allotrope transus °C

882 865

the corresponding phases. Alpha-phases have a hexagonally close-packed crystalline structure, beta-phases have a body-cen-tered cubic structure. In the case of pure ti-tanium, the alpha-beta-transition is carried out at approximately 882 °C. The beta-phase is then retained until the melting point of 1668 °C is attained. The physical and mechanical properties can be altered by selective additions of alloying elements.

- Certain alloying additions, notably alu-minium, stabilize the alpha-phase, in which the beta-transition temperature is increased.

- Most of the alloying additions, such as chromium, niobium, copper, iron, man-ganese, molybdenum, tantalum, and vanadium stabilize the beta-phase by lowering the beta-transition tempera-ture. The beta-phase may be stable for example until room temperature is reached.

- Some elements, e.g. tin and zirconium, behave as neutral solutes in titanium. Although they have little effect on the beta-transition temperature, they strengthen the alpha-phase.

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Some of the remarkable features of al-pha-alloys are good high-temperature strength, weldability and excellent oxida-tion resistance. They cannot be heat-treated to develop better mechanical properties and mostly show a lower cold forming. Alpha-beta alloys have a higher strength compared to alpha-alloys and they can be significantly strengthened by heat treatment. The strengthening by precipita-tion hardening is caused by a heat treat-ment in the alpha-beta range of tempera-ture with subsequent quenching and ageing at a somewhat lower temperature. The quick cooling suppresses the beta-alpha transformation. The subsequent evacua-tion leads to the precipitation of fine al-pha-particles. Beta-alloys basically consist of metastable beta-phases. Beta-alloys can be strengthened by heat treatment. Their stability is limited and therefore the good high-temperature strength cannot be fully utilized.

As with titanium alloys, the alpha- and beta-phase, respectively, can be stabilized in zirconium by additions. Most of the al-loying additions have only a limited solu-bility and exist as intermetallic compounds or as a second phase.

- Some examples of alpha-stabilizing el-ements are aluminium, tin, hafnium, ni-trogen, oxygen. They increase the beta-transition temperature.

- Some more typical beta-stabilizers are iron, chromium, niobium, tantalum, vanadium, tungsten and titanium. They lower the beta-transition temperature.

- The elements carbon, silicon and phos-phorus have a very low solubility in zir-conium and readily form stable inter-metallic compounds.

Alloying elements are added to zirco-nium in order to increase strength and cor-rosion resistance. The quantity of alloy ad-ditions is limited by the solubility. Once this

solubility limit is exceeded, intermetallic compounds or second phases are formed, which tend to lower ductility and reduce corrosion resistance.

Besides the main influence through the microstructure on the mechanical proper-ties, reference has still to be made to the effects of impurities. Titanium and zirco-nium are strongly reactive at higher tem-peratures and form stable compounds with small non-metal elements. The so-called metalloids embrittle the metal and must therefore be kept below the admissible limit in production and processing. Conse-quently, melting is carried out in a vacuum. Heat treatment is preferably carried out in electric heating furnaces under inert gas or in vacuum. Alternatively, gas and oil-burning furnaces can be used, if air surplus is supplied and the product is not directly exposed to the flames.

2. Starting Materials

2.1. Tungsten and Molybdenum /2,3/

The annual world production of tung-sten is estimated to be approximately 50,000 t. The largest deposits are situated in the People's Republic of China, others in Korea, Burma, Austria, Malaysia, Thai-land and the USA. The major consumer is the hard metal industry (approximately 50%), followed by the steel industry (approximately 25%). The metallic pro-portion amounts to about 15%.

The annual world production of molyb-denum is estimated to be approximately 130,0001. The largest deposits of molybde-num-bearing ore are located in the USA, the Canadian province of British Columbia and Chile. The major consumer is the steel industry with approximately 83%, whilst chemicals and lubricants have a share of about 9%, super-alloys about 3% and molybdenum and alloys about 4%. Tung-sten- and molybdenum-oxide are reduced to metallic powder in hydrogen at a tem-

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Vol. 9, Nos. 2-4,1990 Processing of Rare Metals

perature around 1000 °C mainly in the last stage. The recycling of milling scrap is an-other source of supply.

2.2. Tantalum and Niobium /4-6/

The world annual production of tanta-lum is estimated to be more than 1,000 t. Major producers are Thailand, Malaysia, Canada, Brazil, Nigeria, Zaire and Mozambique. Tantalum fluoride is sepa-rated from niobium fluoride by solvent ex-traction. Grey tantalum powder is usually obtained by metallic sodium reduction of tantalum fluoride. Alternatively, in the USA salt bath electrolysis can be used for the reduction of tantalum.

Recycling is another important source of supply. Scraps accumulate in the fabri-cation of mill products of ingots, bars, rods, wires, sheets, tubes and shaped parts. This new metal scrap is made available to the material recycling for melting in a cleaned, concentrated and separated form. The scrap from the fabrication of mill products on average amounts to about 30% of the ingot weight. In tube manufacturing the quantity of scrap is greater, whereas it is distinctly lower in rod production. Another additional economical source of supply is the scrap of anode production in the man-ufacture of electrolytic capacitors. The old or secondary scrap has not yet gained sig-nificance, due to the long life-time of the product and the expenditures incurred in recycling.

One important application of tantalum is its use in the production of tantalum electrolytic capacitors (approximately 55%). Further consumers are chemical en-gineering and high-temperature vacuum engineering companies. Tantalum as an alloying element is used in superalloys and in steel.

The world annual production of nio-bium is estimated to be about 18,000 t. Major producers are Brazil with about 82%, Canada (approximately 11%) and

the USSR (about 5%). Concentrated nio-bium oxide is the most important starting material for the production of highly puri-fied iron niobium, further niobium alloys, niobium metal and niobium carbide. The metallic granulate of niobium may be ob-tained by a carbothermic reduction. An-other alternative is reduction via an alu-minothermic reduction (CBMM proce-dure).

The major consumer of niobium as an alloying element is the steel industry with about 85%, with about 5% allotted to su-peralloys. Another consumer is the hard metal industry. Niobium and niobium al-loys are used in chemical engineering as electrode supports for high-pressure sodium lamps, in aerospace engineering and for superconducting magnets and cav-ity material for accelerators.

2.3. Titanium and Zirconium /7,8/

The world annual production of tita-nium is estimated to be more than 140,000 t. The concentrated titanium dioxide (native titanium dioxide) is first chlorinated and the purified gaseous TiCl4 thus ob-tained is conveyed to the steel reactor, which is filled with magnesium bars (or, al-ternatively, with sodium) and inert gas. At about 700 °C there is an exothermic reac-tion into titanium sponge, which is subse-quently disintegrated, washed, cleaned, dried and mixed.

Here again, recycling is another source of supply. The potential is almost exclu-sively restricted to metals. Scrap accumu-lates in the production of ingots, sheets, plates, bars, rods, tubes and shaped parts, and is made available as new scrap to the material recycling. The scrap from the fab-rication of mill products amounts to about one third of the ingot weight. At least four-fifths of it may be used again after the preparation of scrap, which corresponds to about 25% of the ingot weight. In the pro-duction of machined parts from pure tita-

98


nium or titanium alloys, the accumulation of scrap is exceptionally high compared to other metals. For example, in massive workpieces of machined parts about 70 to 90% of the semi-product are worked off; in individual cases in the aerospace industry even up to 95%. The recycling of scrap from the fabrication of mill products and the machining of workpieces refers to pure titanium and titanium alloys and con-tributes to a decisive decrease in the cost of production.

The major consumer of titanium is the aerospace industry, with more than 50%. Other consumers are the chemical, cellu-lose and textile industries, as well as paper mills. Titanium is also used in desalination plants to produce potable water from sea water, in galvanization and in sports equipment.

As with titanium dioxide, zirconium dioxide is first chlorinated and subse-quently reduced with magnesium or sodium. For the production of pure zirco-nium it is necessary to separate the hafnium dioxide contained therein in an amount of about 3% via an intermediate stage by solvent extraction.

The major consumer of zirconium is the reactor engineering industry. Other users are in chemical and electrical engineering, and in metallurgy, where zirconium is used as an alloying element.

3. Sintering, Melting and Casting


Survey Powder metallurgical consolidation of

metallic powder to compact metal takes place exclusively in tungsten and predomi-nates in molybdenum. In powder metal-lurgy, the melting process is avoided due to the high melting temperature.

The processing procedure is divided into the following steps:

1. Preparation of powder 2. Pressing of powder 3. Sintering of pressed compacts 4. Fabrication of mill products (cf. Sect.

4.1).

The procedure is mainly characterized by the three variables of powder, pressing and sintering.

The powder characteristics depend on

- the chemical analysis and purity - the particle size, grain distribution, par-

ticle structure - the surface quality and activity.

The following parameters for pressing have to be determined:

- the range of pressure - the method of pressing by linear press-

ing, isostatic pressing or hot isostatic pressing

- size and shape of pressed compacts - facilitated pressing with lubricant addi-

tions.

The sintering processes are determined by

- the temperature - the time - the atmosphere with inert gas or vac-

uum - the direct or indirect method of sinter-

ing.

Starting powder The customary refractory metals and

their alloys are shown in Table 3. The compaction of these metallic powders de-pends among other things on the particle shape and surface quality. Spongy and dendritic powders produce stable-edged pressed compacts, whereas spherical and very fine powders produce a high filling charge. The particle size and the size dis-tribution have the same significance. Tung-

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Vol. 9, Νos. 2-4,1990 Processing of Rare Metals

TABLE 3. Reactive metals and alloys /2,3/

Pure metals Solid solution

Alloys

Oxide disperse Carbide-oxide disperse and solid solution

Molybdenum MoW MoRe MoCo

Mo-Zr02 Mo-(Ti,Zr) C(O)

Tungsten WRe WMo WTa

W-Th02

Tantalum TaW (Nb)

Niobium NbZr NbTi

sten, molybdenum and carbide powder have a particle size of < 10 μ-m.

In practice, the packing density is an important characteristic. The following characteristics are found in tungsten:

packing density ρχ = 3.3 pressed density p2 = 13.5 filling factor (p^pj) F = 4.1 theoretical density plh = 19.3

The following characteristics are found in molybdenum:

packing density p2 = 1.3 pressed density p2 = 7.1 filling factor (p^pj) F = 5.5 theoretical density pth = 10.22

Another variable is the flow behavior of the powders. The filling of the molds should be carried out in as short a time as possible. Fine powders, like molybdenum, tungsten and carbide powder, trickle badly and are granulated, if necessary. For better processing, the following technological measures have to be taken into account:

- homogenization of the powder charge by blending

- classification by sieving or air separation - free-flowability by granulation - precompression and crushing to in-

crease the packing density - improvement of moldability with lubri-

cant additions.

The blending is preferably carried out in ribbon blenders or in ball mills with si-multaneous crushing. Smaller quantities of additions are supplied and mixed and stirred in the form of solutions. The solvent is vaporized prior to the subsequent treat-ment.

Pressing of powder

In refractory metals, of the many possi-bilities, only linear pressing is carried out by hydraulic or mechanical means. Isostatic or hydrostatic pressing is also employed. Through linear pressing (Fig. 2a), stable-edged and accurate pressed compacts in the form of rods, plates and rounds are produced. The difference of pressure be-tween upper stamp and bottom stamp may be reduced by lubrication (Fig. 2b). For smaller shaped parts dies are used which are, for example, incorporated in a hy-draulic linear press (Fig. 3). Isostatic pressing is used for large pressed compacts

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Stamp

I I I ! M M I I I ! M M I I I ! 1 I I I

MM,

tttt Die

Displacement of stamp

Fig. 2: Die press and distribution of pressure 72,3/

Pressure

and in mass production of mill products. A rubber bag or a plastic container is filled with powder and subjected to hydrostatic pressure from all sides up to approximately 2000 bar to receive pressed compacts (Fig. 4). The densities obtained after the press-ing amount to about 60% of the theoretical density (Fig. 5).

Sintering The sintering of molybdenum and tung-

sten is almost exclusively achieved by indi-rect heating. Direct resistance heating in the current passage is nowadays hardly ever used. The sintering equipment in indi-rect heating consists of tungsten heating elements in the heating zone which are surrounded by heat shields of tungsten and molybdenum to protect the exterior water-cooled chamber. The interior chamber is flooded with hydrogen as an inert gas. The pressed compacts are sintered at tempera-tures between 2000 and 2800 °C for several hours. As a rule, sintering densities of about 95% of the theoretical density are obtained. At first it is not possible to re-ceive dense and porous-free compacts by sintering (Fig. 5). In order to obtain com-

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Vol 9, Nos. 2-4,1990

Ί !

c m

"1 ι I I I I

! Ι - Θ I

Pressure-control led process ing

Fig. 3: Linear press for compacting /2,3/


pletely dense material, further multistage deformations are necessary (Fig. 6), as de-scribed in Sect. 4.1. Mention has still to be made that in special cases molybdenum can be pressed to consumable electrodes for melting processes in the vacuum arc furnace.

Section 3.2. describes how tungsten and molybdenum have been consolidated in individual cases by means of "Electron Beam Melting".

3.2. Tantalum and Niobium /9,10/

Survey

The last three decades have witnessed a rapid expansion in the use of Electron Beam (EB) Technology for melting and remelting, especially of reactive and refrac-tory metals. The process is also being in-creasingly used for melting of superalloys. The present state of the application of EB-melting in the western world is shown in Table 4. It has become evident that the major field of application is the production of ingots of reactive and refractory metals. For the production of superclean reactive and refractory metals, the EB-process

TABLE 4. Application of Electron Beam Melting 191

Metals

W, Mo

Ta, Nb

Hf, V

Zr.Ti

Western Europe

1300

360

Installed EB-power in KW USA Japan Africa, South

America, People's Republic of China

4200

300

2400

600

1500

1200

450

2400

Fe, Ni-Alloys 500 750

102

Fig. 4: Isostatic press for compacting molybdenum powder into compacts weighing up to 5,000 kg. Photography: Metallwerk Plansee GmbH, Austria.


α ο

ο <D _C

(Λ c Ol ο

10 20 30 4 0 % 50

Deformation

Fig. 5: Density increase and density variability during processing of refractory metals 12,3/

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Vol 9, Nos. 2-4, 1990 Processing of Rare Metals

Fig. 6: Forging of a large molybdenum rod. Photography: Metallwerk Plansee GmbH, Austria

plays a dominant role and, in some cases, the EB-process cannot be replaced at pre-sent by any other process. The reasons why the EB-process occupies this dominant role are:

- Melting under high vacuum and in wa-ter-cooled copper crucible or copper mold

- Melting with any desired rate and con-dition to remove volatile trace elements

- Melting to remove inclusions - Control of ingot solidification for the

desired primary structure - High flexibility in size and quality of the

feedstock material - Automation of the process.

The high flexibility of this source of heat has led to several technological develop-ments of the process. From these, two pro-cesses, namely Drip Melting with vertical or horizontal electrode feeding and Cold Hearth Melting, are the most commercially used.

Drip Melting

The first commercial use of the EB-heating under high vacuum was in the form of drip melting for the production of duc-tile tantalum and niobium ingots (Fig. 7).

With EB-melting the ingot diameter does not necessarily have to be bigger than that of the feedstock material, compared

104


1 GUN

2 ELECTRODE

3 VACUUM CHAMBER

U WATER COOLED MOLD

5 RETRACTABLE INGOT

Fig. 7: Schematic of EB-drip melting process with vertical electrode feeding.

to Vacuum Arc Melting (VAR); Sec. 3.3. The drip melting can be carried out either with vertical or horizontal feeding of the barstock. For the first melting process, powder, granulate and solid materials are pressed to form electrodes (feedstock ma-terial) which are melted in vacuum. The material is then purified by repeated remelting under vacuum. In western coun-tries, the drip melting is generally applied for melting and refining of reactive and re-fractory metals with design capacity be-tween 20 and 1500KW and a vacuum be-

tween 10"6 and 10"1 mbar. Fig. 7 shows the vertical arrangement of tubular guns to serve as sources of energy (Fig. 8). Two or more guns are frequently combined to form a multiple gun system with integrated control. The gun produces a sharply bun-dled electron-beam with a round cross-sec-tion. The electron-beam, accelerated in a high voltage potential, is focused onto the metal to be melted. The electrode rotates and has a vertical feed. The material drips down into a water-cooled mold. The solidi-fied ingot is then withdrawn downwards.

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Vol 9, Nos. 2-4,1990 Processing of Rare Metals

Fig. 8: EB-guns KRS 600. Photography: Leyboid AG, Federal Republic of Germany.

The volatilization of impurities can be con-trolled both by regulating the superheating of the molten pool in the mold and by ad-justing the rate of feed of the melting elec-trode. Nearly all the energy is converted into heat. Part of this energy is used to melt the electrode, part is used to keep the pool

of metal liquid and the rest is lost to the surroundings. The whole volume of the furnace is under vacuum in order to

- satisfy the particular requirements of the special metals,

- keep the energy losses in the electron-beam low, and

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KW

Fig. 9: Power requirements for various metals and ingot sizes

- to minimize wear and tear to the elec-tron guns.

The design capacity of the electron-beam furnace is largely determined by the melting point of the material to be melted and the diameter of electrode and mold. Fig. 9 summarizes the power requirements and Fig. 10 shows a multifunctional EB-furnace for refractory and reactive metals. Some typical process data and results achieved for refractory metals are shown in Table 5. The effects of starting materials on the melting is demonstrated in Table 6. AJuminothermically-reduced (ATR) bars

with dimensions of 90 χ 60 χ 600 mm are used with a high content of iron, alu-minium, tantalum, nitrogen, oxygen and hardness of 170 BH (Brinell Hardness). After the first, second and third melting, the hardness was reduced to 55 BH. The content of iron and aluminium achieved 10 ppm, nitrogen 30 ppm and oxygen 130 ppm. Only tantalum, a refractory metal, does not change the content of 1450 ppm. Particularly striking is the observation that with the ATR bars a melting loss of 26% occurred after the first melting. The melt-ing loss with granulate was only about 2% (Table 5). The high loss can be explained

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Vol, 9, Nos. 2-4,1990 Processing of Rare Metals

TABLE 5. Process data and results of EB-drip melting /9/

Furnace Material Ingot Ingot Melt EB Operat. Melt Mater Analysis Brinell Type diam. weight rate power vacuum energy yield C Ο Ν Η Hardness

mm kg kg/h kW mbar kWh/kg % ppmppm ppm ppm kg/mm

ES Tungsten 704 100 30 10 2/12/100 1. Melt 40 19 20 98 5 10"5 4.9 95.5 45 115 11 1

2. Melt 60 37 22 119 8 10"6 5.4 97.5 10 8 5 1 200

ES Niobium 804 500 330 40 2/50/400 1. Melt 140 210 25 218 5 10'5 8.7 98.7 22 190 90 13

2. Melt 140 206 28 201 8 10-6 7.2 98.1 6 111 52 8 66

Molybdenum 200 750 60 10 1. Melt 180 415 84 245 5 10"5 2.9 98.3 40 90 10 4 2. Melt 180 408 125 290 8 10-6 2.3 98.5 10 12 11 2 140

Tantalum 32 650 25 10 1. Melt 160 535 74 276 5 10"5 3.7 93.2 8 45 17 5 2. Melt 160 523 80 371 8 10'6 4.6 98.5 6 15 13 2 69

TABLE 6. Effect of starting materials I Al

Niobium Impurity Content, ppm Melt

# 1 # 3

E l e m e n t ATR Ingot Melt 1 mm sheet, 1 h/1050°(

Mo <10 <10 <10 Fe 400 - 1000 80 < 10 Ni 10 <10 <10 Al 0.5 - 1.4% 300 < 10 Si >1000 250 50 Ti 10 <10 <10 Zr ND ND ND Ta 1450 1450 1450 W 40 40 40 Η 50 20 8 14 Ν 400 - 1000 95 30 80 Ο 2.3 - 4.5% 1800 130 180 C 70 40

Hardness (Ingot: HB 5/250 117 51 64 Sheet: HV 10)

ND = None Detected (< < 10 ppm)

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Fig. 10: Multifunctional EB-furnace for refractory and reactive metals. Photography: Leybold AG, Federal Republic of Germany

by a combination of the evaporation of the very high oxygen content, the evaporation of a large proportion of the aluminium and the high energy density and slow feed.

For the reasons mentioned above, the impurities of the starting material should not exceed certain values, e.g.

carbon oxygen nitrogen hydrogen

< < < <

800 ppm 1500 ppm 300 ppm 50 ppm

109


in order to manage with only 2 melts. Con-formity with relevant standards is ensured by strict quality control.

Continuous Flow Cold Hearth Melting /10/

Another versatile application of the source of EB-heat is the melting in a water-cooled copper hearth with a continuous melt flow into a water-cooled crucible. This is a process including several steps; melt-ing, refining and solidification are sepa-rated from each other (Fig. 11). With a complete separation of melting and refin-ing from the solidification of the melt, it is

possible to refine the melt without dis-turbing the solidification. As the solidifica-tion of the melt takes place on a separate water-cooled mold, different types of in-gots - rounds, squares, polygonals - can be produced. Although melting and refining of reactive and refractory metals in EB-furnace is a common practice to obtain op-timum results, problems of evaporation arise during melting of superalloys and ti-tanium-aluminium-alloys. Investigations have shown that the evaporation losses can only be held at an acceptable level by heavily decreasing the residence time of the melt in the hearth by using a high metal flow-rate.

Fig. 11: Schematic of EB-Cold Hearth Melting 191

110


3.3. Titanium and Zirconium /9,11/

Survey

The vacuum arc furnace (VAR), like the EB-furnace, has found its application in melting reactive metals. The emphasis, however, lies in the melting of titanium and zirconium, whereas the EB-furnace is pri-marily used for the more gas-sensitive metals such as niobium and tantalum. The primary feature of vacuum arc melting and remelting is the continuous melting of a consumable electrode by means of a dc arc under vacuum.

The molten material solidifies in a wa-ter-cooled or sodium liquid metal-cooled copper mold. The basic design of a VAR-furnace has remained largely unchanged over the past years. Significant advances have been made in the field of control and regulation of the process in order to achieve a fully automatic melting proce-dure. This has had a decisively positive in-fluence on the metallurgical properties of

the products. The manufacture of homo-geneous ingots with a minimum of segrega-tion requires controlled and reproducible melting parameters. Of these, the melting current density has the largest influence on the melting, bath geometry and conditions of solidification. The advantages of melting with VAR-process are:

- Removal of dissolved gases, such as hy-drogen

- Minimizing the content of undesirable trace elements with high vapor pressure

- Improvement of oxide cleanliness - Achievement of directional solidifica-

tion of the ingot from bottom to top in order to avoid macrosegregation and to minimize microsegregation.

Three different versions of VAR-pro-cesses are used:

- Melting of consumable electrodes (Fig. 12)

1 Consumable electrode

2 Vacuum chamber

3 Mold (water-cooled)

U Ingot

Fig. 12: Remelting of consumable electrodes /9,16/

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Vol. 9, Nos. 2-4,1990 Processing of Rare Metals

- Melting of electrodes and subsequent casting procedures (Fig. 13)

- Melting with nonconsumable electrodes (Fig. 14).

Melting of consumable electrodes: Starting material for the manufacturing of consumable electrodes is sponge with specified quality. The necessary purity of the sponge is laid down in the material specifications for the qualities to be manu-factured of titanium and zirconium. In the case of titanium, for example, special at-tention must be paid to the impurities of iron and oxygen, which are admissible to a maximum. At first, granules are weighed out and thoroughly mixed. If an alloy is to be produced, alloying elements are blended with the granules at this stage and the contents of the mixer are divided into a number of equal portions which are

pressed to semi-octagonal compacts. These compacts are welded together and form the electrode for the melting process with a length of 3 - 5 m. A vacuum plasma-weld-ing unit is used to assemble a titanium sponge electrode. In the next stage the electrode is loaded vertically into the fur-nace (Fig. 15), which is cooled by liquid sodium-potassium. The heat generated by melting the electrode in VAR results from the electric arc between the consumable electrode to the liquid pool on the top of the ingot (Fig. 12). The pressure in the ves-sel is usually in the order of 10~3 to 10"2

mbar. The melting process is carried out twice to ensure a maximum homogeneity. The first-melt ingot is used as the electrode for the second melt to obtain the final in-got. Conformity with relevant standards is ensured by strict quality control at all stages of manufacture.

1 Consumable electrode 2 Vacuum chamber 3 Casting mold (movable) U Rotary table 5 Lock valve

6 Charging lock chamber 7 Discharging lock chamber

Fig. 13: Melt ing of e lec t rodes a n d subsequent cast ing p rocedures / 9 , 1 6 /

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1 Nonconsumable

2 Material feeding

3 Vacuum chamber

U Mold (water-cooled)

5 Ingot

Fig. 14: Melting with nonconsumable electrodes /9,16/

Skull-Melting of Electrodes and Casting

The skull-melting process (Fig. 13) was developed above all for the melting of re-active metals like titanium and zirconium. To eliminate any reactions for ceramic-free master melt of crucible material, a solid coat of the melted reactive metal is at the bottom, which is called the skull. In this way, a high purity is achieved when melting the consumable electrode by means of VAR and subsequent casting. When the

crucible has reached the desired melting quantity, the electrode is automatically withdrawn and is subsequently poured into a movable casting mold. The operation of the whole melting process is fully auto-matic and, for example, can be filled until 1,000 kg of the crucible capacity is reached (Fig. 16). The pouring is done by compact casting in the case of titanium and zirco-nium, notably for titanium as precision casting utilising the lost wax process.

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VoL 9. Nos. 2-4,1990 Processing of Rare Metals

Fig. 15: Vacuum-Arc (VAFO-furnace for Ti-ingots, 1000 mm dia. ingots. Photography: Leybold AG, Federal Republic of Germany

Liquid titanium reacts with almost any known materials, so that it is difficult to find suitable molding materials. In castings, embrittled surface layers of approx. 0.1 to 0.3 mm may occur. Removal is possible by means of pickling or blasting. Inherent de-

fects, i.e., micro-cavities and porosity, may be removed by means of hot isostatic pressing (HIP). The HIP-process is, for ex-ample, carried out in titanium at 1000 bar and 920 °C with a residence time of 2 hours.

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Fig. 16: Skull-Melting-VAR. Charge 300 kg. Photography: Leybold AG, Federal Republic of Germany.

4. Fabrication of Mill Products


The refractory metals are used in the form of mill products without porosity. It is not yet possible to obtain products without

porosity by sintering (Fig. 5). The remain-ing porosity amounts to approximately 3 -10%. In order to obtain sheets, bars and wires and other mill products with a den-sity of 100%, a multiple deformation pro-cess becomes necessary.

The non-cutting formation is carried out

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with the same mechanical equipment as is customarily used for other metals. A sum-mary of the equipment used for different deformation processes is shown in Table 7. Reshaping by forging (Fig. 6), swaging, rolling, drawing, extruding and others be-long to these deformation processes. The same fundamental principles hold true for the deformation behavior of other metals. In the case of deformation below the re-crystallization temperature, strain harden-ing may occur resulting in three times the

TABLE 7. Equipment for fabrication of mill products for molybdenum and tungsten 12,3/

Deformation Equipment

FORGING: forging of bars drop forging hammer-forging

SWAGING: swaging of rods

DRAWING: drawing of rods drawing of wire

ROLLING: rolling to sheet

rolling to ribbon

Forging press Screw-press forging machine

Swaging machine, continuous or one-sided

Drawing machine Drawing mill

Two-high-mill Four-high-mill Four-high-mill Four-high-mill with ribbon folder Multiple-roll-mills

FABRICATION OF SHAPED PARTS: flow turning Spinning lathe spinning Spinning lathe deep drawing Deep-drawing press

EXTRUDING: extrusion of rods extrusion of tubes

Extrusion press

strength of the unworked material. This in-crease of strain hardening restricts further deformation. Such strain hardening may be partially or completely reversed by heat treatment. The processes in heat treatment can above all be described by the terms stress-relief and recrystallization. Stress-relief means interactions with thermal en-ergy occurring below the recrystallization temperature, leading to the recovery of the yield strength. The grain boundaries do not change. A completely recrystallized mi-crostructure is achieved by means of re-crystallization annealing. A prerequisite for a homogeneous and fine-grain size mi-crostructure is a high degree of deforma-tion with shapings of more than 70%. The recrystallization and the grain growth are strongly influenced by the addition of al-loying elements. The results is a distinctly higher recrystallization temperature. In case of non-cutting forming, heat treat-ments must be carried out before, during and, if necessary, after the processing due to the strain hardening. In spite of inter-mediate anneals strong stress of the equipment and tools may occur due to the high temperature strength of tungsten and molybdenum. Table 8 shows the objects and the kinds of heat treatment used, which must principally be carried out un-der hydrogen in high temperature fur-naces. The choice of heat treatment tem-peratures and times depends above all on the dimensions of the product, degree of deformation, further processing and the properties required. The preheating tem-peratures are between 1200 °C and 1700 °C. Recrystallization temperatures depend above all on the degree of defor-mation and the product, and range be-tween 900 and 1400°C. Stress relief is car-ried out at lower temperatures.

4.2. Tantalum and Niobium /1,4,10/

It is preferable to use ingots as starting material for the fabrication of mill prod-

116

F. Schreiber

TABLE 8. Heat treatment of tungsten and molybdenum 12,3/

Heat treatment Object in hydrogen

Preheating for working recommended tempera-ture range for working is above the transition tem-perature brittle-ductile and below the recrystal-Iization temperature, lower deformation re-sistance.

Recrystallization

Stress-relieving anneal

Diffusion anneal

Purification anneal

The structure is altered due to formation of new grains which reduces strength and hardness.

Recovery of yield strength, reducing the residual stresses at temperatures before recrystallization.

Uniform distribution of elements in metals.

Removal of surface impurities, usually oxide layers.

ucts, the production of which has been de-scribed in Sect. 3.2. Fig. 17 schematizes the steps of processing in manufacturing mill products from ingots. The mechanical equipment used corresponds to those listed in Table 7. Tantalum and niobium are very ductile and the work is normally carried out at room temperature. Larger deformation steps, such as forging, are preferably performed at somewhat ele-vated temperatures up to about 600 °C in air. Prior to an intermediate heat treat-ment, the surface scale of oxides and ni-

Iiigh Temperature Materials and Processes

trides formed during hot rolling or forging must be removed to avoid oxygen and ni-trogen diffusing into the bulk.

Rolling at room temperature is similar to that of the other cold-ductile metals. Plate and thick sheet are normally rolled on two-high mills, thin sheet and strip are usually rolled on four-high mills and foils on multiple-roll mills. Foils are generally produced by package rolling. Intermediate and final annealing of sheet products is ef-fected with cleaned surfaces.

In particular, for tantalum and niobium, wire drawing is carried out in the pre-oxi-dised condition to avoid cold welding in the die. Because of this oxidation treatment, the wire must be replated after every few drawing operations.

Seamless tube production at room tem-perature begins with forged and rolled bar, which is machined and bored to hollows. The extruded tube hollows are reduced by multiple-stage processes, using tube-drawing units. Similarly to other mill prod-ucts, tubes are finished by annealing. They are inspected by pressure testing, ultra-sonic testing and normal visual, dimen-sional, mechanical and chemical tests.

Welded tube is manufactured by tube forming from strip and seam welding. Tube forming and welding line shape the flat strip into a cylinder before argon-arc welding in order to weld the two edges to-gether to form a seam. Additionally, the welded tubes are reduced by tube-drawing units. Facilities are included for strip in-spection and testing of the final welded tube product.

All heat treatments mentioned above must be carried out in high vacuum or un-der a pure inert gas atmosphere. Even rel-atively small quantities of dissolved gases affect the mechanical properties. The work hardening induced by deformation is re-lieved by heat treating between 950 and 1200 °C in high vacuum or under pure inert gas atmosphere to form a completely re-crystallized microstructure.

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Vol 9. Nos. 2-4,1990 Processing of Rare Metals

PRESS ING ROLLING

BILLET ROLLING MACHINING OF HOLLOWS SWAGING

HEAT TREATMENT DRAWING DEEP DRAWING OF TUBE BLANKS

Fig. 17: Process steps for tantalum and niobium miii products

4.3. Titanium and Zirconium /1,11,13/

Ingots are preferably used as starting materia] for the fabrication of mill prod-ucts, the production of which has been de-scribed in Sect. 3.3. Fig. 18 schematizes the steps in manufacturing mill products from ingots. The same fundamental principles as for other metals hold true for the plastic deformation behavior and the necessary heat treatments, see Sect. 4.1.

Initial ingot breakdown is carried out on the forging press with two giant manipula-tors which hold the workpiece. Most ingots are forged into billet or slab for further processing.

Sheet is normally manufactured by a single-sheet hand-rolling process. Rolled plate with a thickness of 8 - 15 mm is sheared to rolling blanks and electrically preheated prior to hot rolling through a four-high mill. Cold rolling is carried out also on a four-high mill. At intermediate and final stages, sheets are descaled and pickled with caustic or acid.

The production of bar or rod is started with electric preheating prior to hot rolling through a roughing and cogging mill. Sub-sequent passes take place on a cross-coun-try mill. Necessary facilities are descaling and pickling rod straightening, annealing

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Ingot

I Forging and pressing

into billet or slab

I Machining

f ι Hot rolling to Hot rolling to Machining of

plate bar hoi

Hot rolling to Drawing to sheet wire

t

ows

Cold rolling

Extrusion to tube hollows

I Reducing/Pilger

Heat treatment Heat treatment Heat treatment

i Sheet Bar Tube Strip Wire

Fig. 18: Process steps for titanium mill products.

and centreless grinding. Wire is produced by a conventional drawing route with inter-stage anneals.

Seamless tube production begins with forged billet, which is machined, bored to hollows and extruded to tube hollows. Through one or multiple-stage processes

the tube hollows are reduced to size. Tubes are finished by annealing and polishing and are inspected by pressure testing, ultra-sonic testing, dimensional, visual, mechani-cal and chemical tests. The choice of heat treatment temperatures and times depends principally on the dimensions of the prod-

119


uct, the degree of deformation, further processing and the required properties. Thus, the temperatures for forging and rolling in the case of titanium and titanium alloys range between 950 and 750°C. The recrystallization temperatures depend on the degree of deformation and the material used and are approximately between 650 and 800°C. The stress-relief annealing is carried out at lower temperatures - be-tween 450 and 600°C.

5. Machining and Welding


Machining The recommended conditions for ma-

chining are shown in Table 9.

Welding Tungsten can only be welded under cer-

tain conditions. The welds have a coarsely grained structure and, in consequence, are low in strength. The parts to be welded should be ground as finely as possible in the weld areas, should not contain cracks and should be purified by pickling with acid prior to the welding. TIG-welding in dry boxes with inert gas is possible. Electron beam welding with narrow welds and heating zones is still more favorable. It is advisable to preheat the parts to approxi-mately 700 to 800 °C prior to welding. Since melt welding is restricted in its use, diffusion welding has gained special impor-tance. In spite of the high melting point of tungsten, temperatures of 1300 to 2000°C

TABLE 9. Recommended conditions for machining of tungsten and molybdenum /3,12/

Machining Tungsten Molybdenum

Drilling: dry/chlorinated cutting oils emulsion tools cemented carbide HSS/cemented carbide cutting speed 20 - 25 m/min 10 -15/150 m/min

Turning: dry, 200 "C, if necessary emulsion tools cemented carbide HSS/cemented carbide side rake approx. 250 20° clearance angle 8° - 10 ' 7 - 1 0 ' nose radius 0.5 mm cutting speed 30 - 50 m/min 30-40/100-150 m/min

Milling: dry emulsion tools cemented carbide HSS/cemented carbide side rake 10° 10' clearance angle 8 ' 15' nose radius 0.3 mm cutting speed 20 - 30 m/min 20-25/80-120 m/min

Grinding: water-soluble emulsion water-soluble emulsion wheels silicon carbide silicon carbide particle size 60 -120 60 -120 structure average average binder ceramic ceramic grinding speed 1800 m/min 1800 m/min

120


and pressures of 2 to 20N/mm2 suffice to obtain suitable weldings. Diffusion welding is carried out either under vacuum or in pure hydrogen.

TZM, a molybdenum alloy containing 0.5% of titanium and 0.08% of zirconium, is easier to weld compared to other molyb-denum alloys. In this case, it also holds true that, prior to welding, a cleaned prepara-tion without cracks is the best prerequisite for good welding. Since the ductility of the welds is strongly reduced by traces of oxy-gen and nitrogen, the welding is carried out under inert gas with the TIG procedure. For this purpose dry boxes are widely used. Another alternative is electron beam welding in vacuum with low energy gain. Diffusion welding is also used in special cases.

5.2. Tantalum and Niobium /10.12/

Machining The recommended conditions for ma-

chining are shown in Table 10.

Welding Tantalum and niobium are sensitive to

oxygen and nitrogen at higher tempera-tures. The absorption of gas embrittles the workpiece. If, however, the diffusion of undesired gases is avoided, tantalum and niobium are very easy to weld. In this case, too, a clean, smooth preparation of weld is an important prerequisite. TIG-welding is the principal welding procedure used, in order to save costs. Welding is carried out in chambers, which are evacuated and

TABLE lO.Recommended conditions for machining of tantalum and niobium /10,12/

Machining Tantalum Niobium

Drilling: cooling with chlorinated hydrocarbons cooling with chlorinated hydrocarbons tools HSS HSS cutting speed 15 m/min 15 m/min

Turning·. cooling with chlorinated hydrocarbons cooling with chlorinated hydrocarbons tools HSS/cemented carbide cemented carbide side rake 28°-30° 30 ° - 3 5 ° clearance angle 8° -30° 7° nose radius 0.5 mm sharp cutting speed 15-18/50-70 m/min 50 m/min

Milling: cooling with chlorinated hydrocarbons cooling with chlorinated hydrocarbons tools HSS HSS/cemented carbide side rake 45° 20 - 30° clearance angle 20° 20° cutting speed 15 m/min 20/40-80 m/min

Grinding: wheels silicon carbide silicon carbide particle size 120 120 grinding speed 1800 m/min 1800 m/min

121

VoL 9, Nos. 2-4,1990 Processing of Rare Metals

flooded with high-grade argon of 99.996% and are subjected to a slight excess pres-sure during welding. In the case of large equipment, the welding is carried out with local covering above and below the weld to prevent the hot areas of the welds from contact with air, which would lead to em· brittlement. The argon should flow calmly and nonvortically and should act as a pro-tective jacket until quenching is reached below 250°C. Electron beam welding un-der vacuum is excellent as far as quality is concerned. To save cost, the EB-procedure is used only in exceptional cases. In special cases, if welding is not possible due to the workpiece, diffusion welding is used. The criteria for the application of claddings are cost-saving, stress through vacuum and

good heat transmission. In loose linings of containers, for example, the underpressure would lead to indentations. This is why preference is given to cladded units. In this case, explosion cladding is a suitable and reliable procedure.

5.3. Titanium and Zirconium / l l , 13,14,15/

Machining Recommended conditions for machin-

ing are shown in Table 11.

Welding Titanium and zirconium show a reactive

behavior similar to that of tantalum and niobium. Almost the same observations

TABLE 11.Recommended conditions for machining of titanium and zirconium /11,13-15/

Machining Titanium (Ti-Alloys) Zirconium

Drilling·. emulsion or 5% soluble N a N 0 2 emulsion or 5% soluble N a N 0 2

tools HSS with Co HSS cutting speed 8 -15 m/min 10 - 20 m/min

Turning: emulsion or 5% soluble NaN0 2 emulsion or 5% soluble N a N 0 2

tools HSS + Co/cemented carbide HSS/cemented carbide side rake 5° -1070" -5° 15° clearance angle 878" 10° nose radius 0.2-0.8/0.8-1.5 mm 0.8 mm cutting speed 30-50/80-100 m/min 25 - 40 m/min

(10-20)/(40-70)

Milling·. emulsion or 5% soluble NaN0 2 emulsion or 5% soluble N a N 0 2

tools HSS + Co/cemented carbide HSS/cemented carbide side rake 076° -10° clearance angle 10°-12710°-12° cutting speed 20 - 40 / 40 - 60 m/min

(10-20)/(15-35)

Grinding: emulsion or 5% soluble NaN0 2 emulsion or 5% soluble N a N 0 2

wheels silicon carbide silicon carbide particle size 320 grinding speed 240 - 720 m/min 1000 m/min

122


can be made as to weldability, preparation of seam weld, welding process and equip-ment.

References 1. Kieffer, R., Jangg, G. and Ettmayer, P. Son-

dermetalle, Springer Verlag, Heidel-berg/Wien/New York (1971).

2. Benesovsky, F. (Hrsg.) Pulvermetallurgie und Sinterwerkstoffe, Metallwerk Plansee GmbH, Reutte (1982).

3. Eck, R. and Lugscheider, E. Molybdän, Wol-fram und ihre Legierungen, in Lehrgang Son-derwerkstoffe, Technische Akademie Esslin-gen, Germany (1982).

4. Schreiber, F. and Schulze, K. Production and purification of niobium and niobium alloys, in Int. Symposium Niobium 81, San Francisco (1981).

5. Eggert, P., Kamphausen, Ο. et al. Unter-suchungen über Angebot und Nachfrage min-eralischer Rohstoffe "Tantal", Deutsches In-stitut für Wirtschaftsforschung (DIW), Berlin (1982).

6. Eggert, P. et al. Untersuchungen über Ange-bot und Nachfrage mineralischer Rohstoffe "Niob", Deutsches Institut für Wirtschafts-forschung (DIW), Berlin (1982).

7. Eggert, P., Kamphausen, O. et al. Unter-suchungen über Angebot und Nachfrage min-eralischer Rohstoffe "Titan", Deutsches Insti-

tut für Wirtschaftsforschung (DIW), Berlin (1980).

8. Biehler, W., Boos, W. et al., Untersuchungen ü Angebot und Nachfrage mineralischer Rohstoffe "Industrieminerale", Seite 715, Zirkonium, Prognos AG, Basel (1986).

9. Hauff, A. General Review of Secondary Met-allurgy Including Subsequent Refining Pro-cess like ESR, VAR, EB and Plasma, Leybold AG, Hanau, Federal Republic of Germany (1988).

10. Schreiber, F. Verarbeitung und Anwendung von Tantal, Niob und Vanadin, RADEX-RUNDSCHAU, Heft 1/2, Osterreich (1983).

11. Rüdinger, K , Titan, Zirkonium, Hafnium, Lehrgang Sonderwerkstoffe, Technische Akademie Esslingen, Federal Republic of Germany (1982).

12. Wolfram, Molybdän, Tantal, Niob, Druckschriften Metallwerk Plansee AG, Reutte, Osterreich (1988).

13. IMI Titanium, Properties and Applications, GB (1988).

14. Titan und Sonderlegierungen, Zirkonium, TISTO, Federal Republic of Germany (1988).

15. RMI Titanium-Basic Design, Zapp-Werk-stofftechnik, Federal Republic of Germany (1988).

16. Vacuum-Arc, Melting and Casting. Druckschrift Leybold AG, Federal Republic of Germany (1988).

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