SPECIAL TUNGSTEN FORMS AIND QUALITIES - Globular Tungsten Powder - Coarse Tungsten Powder

Good flowing tungsten powder can be obtained by:

Reduction of W03 0r blue oxide at high temperature, high humidity, and alkali metal doping, and subsequent classification;

Disintegration and classification of presintered parts;

Fine tungsten powder +0.2% Ni is compacted and sintered at 1840 0C for 3 hours. Afterward, the Ni is dissolved by nitric acid.

Please visit our web site www.tungsten-powder.com

SPECIAL TUNGSTEN FORMS AIND QUALITIES - Globular Tungsten Powder

SPECIAL TUNGSTEN FORMS AIND QUALITIES

Globular Tungsten Powder

This substance can be produce d from fine powder in a plasma jet (150-200 µm). Monodisperse single crystal powder (containing small amounts of Ni) can be obtained by acid leaching of sintered W:Ni-Cu parts.

Please visit our website www.tungsten-powder.com

Plasma Spraying

Tungsten layers can be sprayed with suitable guns on different base materials. Easily flowing tungsten powder is used as spraying material Ar or Ar-N2 mixtures are preferred as protective atmosphere in closed chambers. Besides coatings, tungsten parts can be produced by spraying tungsten layers on an adequately formed nucleus until the desired thickness is reached.

One may distinguish between three basic types of substrates:

Expendable substrates are removed after spraying by selective chemical etching, melting, etc;

Reusable substrates are used for mass production;

Permanent substrates become an integral part of the article.

Density ranges from 50—95% of the theoretical value. Higher density is attained by uniform substrate temperature and Iow deposition rate. Density and strength might be increased by post-sinterning in hydrogen. Low-pressure plasma spraying is mainly used for producing intricate shapes, which otherwise cannot be manufactured.

Electron-Bean Zone Melting

Electron-beam zone melting is the preferred method to produce larger, single crystals of tungsten. A polycrystalline tungsten rod is mounted in a vertical position in a stainless steel bell jar and connected as anode. An electron gun (cathode) traverses along the rod by means of a screw drive mechanism. On melting in a high vacuum, a narrow, liquid zone is formed and held between the two solid rod parts by surface tension forces. As the zone slowly moves through the rod several times (zone floating), impurities are concentrated on both the starting and terminal end of the rod. Since the molten zone is quite narrow and the temperature gradient in its vicinity rather steep, nucleation starts essentially at one point and a single crystal is the result. The concentration of many impurity elements can be decreased considerably by zone melting. Al, Ca, Co, Cu, Fe, Mg, Mn, Ni, Si, Na, Ti, Ta, Re, Mo, and Nb concentrations can be reduced below 1 pg/g Interstitials remain at the following pg/g levels: O₂ 2, N₂5, C10, and H1.

Single crystals produced by this technique represent the most ductile tungsten currently available. They are free of grain boundaries, which cause brittleness.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-3. Fabrication of Tungsten -3.3 Bonding of Tungsten-02

Welding. Tungsten possesses only moderate welding properties. Welding must be carried out under controlled weld atmospheres, preferably in a dry box, since any contamination by oxygen will reduce the ductility of the joint. Before welding, the metal has to be degreased and pickled, commonly with a mixture of nitric acid and hydrofluoric acid (90/10 vol%). The weld seams have a coarse-grained structure in the hot-fusion zone, owing to recrystallization, and can therefore withstand only low mechanical stresses. Tungsten can be welded by tungsten inert gas (TIG) welding, laser beam welding (for thin parts), plasma welding, friction welding and electron beam welding, the latter being the preferred fusion-welding method. Tungsten can be welded to W-Re and Mo-Re alloys. W-Re alloys (in particular W-26Re) are recommended as filler metal for welding tungsten. Tungsten can be diffusion welded to tungsten and other metals. Despite the high melting point of tungsten, temperatures of 1300-2000℃ and pressures of 2—20N•mm^-2 give satisfactory joints. Intermediate layers of Ni, Pt, Rh, Ru, and Pd accelerate the diffusion process. Tungsten wires can be spot-welded to other metals under a protective gas blanket.

PLease visit our website www.tungsten-powder.com to get more information.

POWDER METALLURGY-3. Fabrication of Tungsten -3.3 Bonding of Tungsten

3.3 Bonding of Tungsten

Mechanical Joining. Mechanical joining, such as riveting, tacking, or lacing, comprises the simplest methods of joining tungsten, provided the joint need not be impermeable to liquids and/or gases. Tungsten and molybdenum parts can be used. Mechanical fastening is used for constructional parts, such as heating elements, containers, large shields, etc.

Brazing. The parts to be joined must be free from grease, oils, oxides, or other impurities. As tungsten is very sensitive to oxidation, brazing must be carried out under a protective gas, hydrogen, or in vacuum. If maximum strength of the joint is required, tungsten has to be brazed below its recrystallization temperature. Typical brazing metals and temperatures are; Rh (1970℃), Ni (1430℃), or AgCu20Pd15 (700-900℃). More derailed information is available. Brazing of tungsten to ceramics, graphite, and silicon has gained importance for the fabrication of refractory-metal composites.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-3. Fabrication of Tungsten -3.2 Shaping-02

Tungsten can be drilled, turned, milled, planed and ground. However, machining operations require experience and close adherence to optimum conditions. Preheating to about 200℃ and the use of cemented carbide tools are recommended. Complex shapes and holes can be formed by spark erosion; the tungsten workpiece forms the anode and the working electrode the cathode.

Tungsten (pure W, doped W, W-Re) is commercially available as wrought products (sintered bars, billets, forgings, rods, pins, disks, cylinders, tubes, sheet, strips, ribbons, wires) as well as in the form of a broad variety of shaped parts(crucibles, springs, heating elements, rivets, etc.).

Cold-rolled sheet is supplied in thicknesses from 0.025 to 0.4mm (up to 500mmxl000mm). Hot-rolled tungsten is available with thicknesses from 0.4 to 8.0mm (maximum lengths, 1500mm). Tungsten wires are manufactured from about I mm down to 5 µm.

Please visit our website www.tungsten-powder.com