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

3.2 Shaping. Compared to ductile metals and alloys, the fabricability of tungsten is rather poor: Tungsten should always be heated before shaping. The temperature range for forming has a lower limit, set by the brittle-to-ductile transformation temperature, and an upper limit, set by the recrystallization temperature. This temperature is mainly dependent on the purity, the history of deformation, and heat treatment of the material. Highly deformed products, such as thin tungsten wires, ribbons, or foils, are ductile at room temperature.

Thin, strongly deformed sheet and foil have a pronounced structure in the longitudinal direction due to elongation of the grains during rolling. The bending properties long the direction of rolling are therefore different from those across it. Therefore, tungsten sheet should always be bent in a way such that the bending edge is perpendicular to the rolling direction. If bending in the longitudinal direction cannot be avoided, owing to the design, much higher bending temperatures are required. At high temperatures, tungsten sheet can be stamped, punched, and sheared. Sharp tools are essential to clean cutting action without sheet cracking or delamination. Tungsten cylinders and cones can be formed by spinning, flow turning, or forging. The use of stress-relieved tungsten is suggested for optimum fabrication results.

Please visit our website: www.tungsten-powder.com

POWDER METALLURGY-3. Fabrication of Tungsten -3.1 Fabrication of wrought P/M Tungsten-04

With a high degree of deformation, the grains become more fibrous due to the unidirectional nature of the processing (sheet rolling, rod extrusion, or swaging) and mechanical anisotropy develops. Strong crystallographic orientations and planar distributions of dispersed, insoluble phases (in the case of doped tungsten) are characteristic of the stage of heavily worked tungsten.

The grain size and grain structure of the final product have a great influence on the mechanical properties. They can be controlled by type of deformation, degree of deformation, and annealing processes both during and after machining.

Forming of tungsten is commonly carried out without protecting atmosphere. In air, tungsten is readily oxidized. Tungsten trioxide forms on the surfaces of the worked piece and, above 800℃, it volatilizes. The oxide layer acts as a protective layer against contamination from the working tools and is removed at certain stages of deformation by pickling and/or machining. Intermediate annealing and stress-relieving annealing is performed under hydrogen to avoid enhanced oxidation of the metal and sublimation of WO₃.

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

POWDER METALLURGY-3. Fabrication of Tungsten -3.1 Fabrication of wrought P/M Tungsten-03

With a high degree of deformation, the grains become more fibrous due to the unidirectional nature of the processing (sheet rolling, rod extrusion, or swaging) and mechanical anisotropy develops. Strong crystallographic orientations and planar distributions of dispersed, insoluble phases (in the case of doped tungsten) are characteristic of the stage of heavily worked tungsten.

The grain size and grain structure of the final product have a great influence on the mechanical properties. They can be controlled by type of deformation, degree of deformation, and annealing processes both during and after machining.

Forming of tungsten is commonly carried out without protecting atmosphere. In air, tungsten is readily oxidized. Tungsten trioxide forms on the surfaces of the worked piece and, above 800℃, it volatilizes. The oxide layer acts as a protective layer against contamination from the working tools and is removed at certain stages of deformation by pickling and/or machining. Intermediate annealing and stress-relieving annealing is performed under hydrogen to avoid enhanced oxidation of the metal and sublimation of WO₃.

please visit our website www.tungsten-powder.com

POWDER METALLURGY-3. Fabrication of Tungsten -02

The aim of primary working (ingot breakdown) is threefold: (1) to eliminate the residual porosity, (2) to convert the original massive form (sinter ingot) to the desired preshape (sheet, rod, tube), and (3) to refine the grain size in order to improve the formability during subsequent cold forming.

The first forming step is usually carried out at 1500-1700℃. As a result of the low specific heat and high thermal conductivity of tungsten, several reheating stages are necessary in the first stages of shaping, because the heat is lost rapidly at these temperatures and the ingot cools down rap idly. If the metal is worked at too low a temperature, cracks and splits will easily develop. As the forming process continues, the forming temperature is reduced progressively since the recrystallization temperature decreases as deformation proceeds. Tungsten is generally worked below its recrystallization temperature, because recrystallization is combined with grain boundary embrittlement. With increasing work hardening during deformation, both the hardness and strength of the worked part increase significantly, and intermediate stress relief annealings are necessary to minimize the hazards of cracking (laminating) and to avoid overstraining the working tool.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-3. Fabrication of Tungsten

3. Fabrication of Tungsten

3.1 Fabrication of wrought P/M Tungsten. With only some exceptions, tungsten is used in the form of pore-free p. reforms (“wrought” P/M tungsten). To obtain a completely dense material, as well as the desired shape and mechanical properties, a complex, multistage, hot and cold forming process is required. The most important forming techniques for tungsten are rolling (for rods and sheet products), round forging (for large diameter parts), swaging (for rods), forging (for large parts), drawing (for wires and tubes). Secondary forming processes include flat rolling of wires, flow turning, spinning, deep drawing and wire coiling. For a detailed description of the most important forming processes, we refer the reader to the book, Tungsten, by Yih and Wang.

In general, plastic forming of tungsten is difficult and needs experience. In the as sintered condition, tungsten is brittle except at quite high temperatures, because it is recrystallized (coarse grained) and not fully-dense. Unlike most metals, the low-temperature ductility of tungsten increases with progressive deformation, because embrittlement is due to grain boundary segregations of interstitial soluble elements, such as oxygen, carbon, and nitrogen. With the breakdown of the coarse microstructure during deformation, these impurities are distributed over a larger intergranular area, which makes the material more ductile and less sensitive to cracking during forming at lower temperatures.

Please visit www.tungsten-powder.com

POWDER METALLURGY-2. Sintering - 2.3 Indirect Sintering

2.3 Indirect Sintering.

The green compacts (no presintering necessary) are placed inside a cylindrical or basketlike heating element of the furnace (constructed of Mo or preferably W). In a radial direction to the outside, the furnace is adapted with radiation shields (inner shields made of W, out sheet made of Mo), which protect the furnace wall and concentrate the heat to the center. A vacuum system is necessary to empty the furnace prior to hydrogen flooding. Maximal dimension depends on furnace size. Compacts of and desired shape can be sintered. In order to achieve even shrinkage, the compacts have to be placed on green tungsten shims. A slow heating rate is essential; otherwise, surface densification will occur too early, not allowing the outgassing of the interior. Internal stress will be built up, resulting in cracks. This is particularly important for large parts, such as tungsten billets for forging or rolling. Holding times are essential. In practice, the sintering schedule is adjusted to the specific requirements. Proper furnace loading is also important, since the shielding of parts by others can lead to different densification rates and less uniformity.

Common sintering temperatures are between 2000 and 2700℃ (max). Sintering times of 8 to 24 hours, or even more, are common. Furnaces having uniform hot zones of up to 1.2m and 0.14m² are available. Weights in the range of a thousand kg can be sintered in one batch. For very high loads, deformation of the part can take place due to gravity.

Advantages are: no loss of material, no dimensional restrictions, and high capacity. Disadvantages are: longer heating times, less purification, Iower efficiency of heating, and higher maintenance costs.

Please visit our website www.tungsten-powder.com to get more details.

POWDER METALLURGY-2. Sintering - 2.2 Direct Sintering -02

The electric current is used to control the sintering progress and as an indicator of the prevailing sintering temperature. The current is kept low-at the beginning due to the higher resistance of the bar (less contact between the grains). It is then gradually raised up to 2700-3000℃. Proper hold points are important to allow the outgassing of impurities. This is particularly important for the sintering of NS-tungsten, because both Al and Si evaporate rapidly above 2150℃. If densification occurs too rapidly, excess dopants will be trapped and may cause difficulties during subsequent working or even bursting of the bar during sintering. Commonly, less than 10ppm Si and Al but 60 to 100ppm K remain in the ingot, the latter in the form of desire potassium bubbles.

Depending on the bar size, the heating up time is between 20 and 60 minutes and the holding time between 30 and 60 minutes. Typical bar dimensions are 15-25 mm *15-25mm*600-920mm. Typical weights are between 1.5 and 6kg. The density of sintered bars is between 88 and 96% of the theoretical value. It is always lower for NS-tungsten (88 to 92%) than for pure tungsten, due to the presence of trapped potassium.

Advantages compared to indirect sintering are short sintering times; temperature inside the bar is higher than on the surface, which favors the diffusion and evaporation of impurities; higher temperature results in a better cleaning effect; a simpler aggregate with comparatively low maintenance costs.

Disadvantages are the dimensional restrictions (only simple shapes can be sintered); low capacity; the loss of the “head” (clipped parts) which did not obtain the temperature due to the heat sink effect of the contacts.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-2. Sintering - 2.2 Direct Sintering

2.2 Direct Sintering. The tungsten bars are commonly mounted in pairs or several units in a vertical, free-hanging position in a water-cooled chamber. They are clamped at the top using water cooled copper contacts with tungsten inserts. The bottom clip is made of tungsten and must be able to move as the bar shrinks. An electric current of several thousand ampere is passed thro ugh it in order to heat it up to about 3000℃by Joule’s energy. Sintering is conducted under dry hydrogen. Several such sintering aggregates are commonly connected in series so that several bars can be sintered simultaneously.

In order to achieve an appreciable strength of the green compact, which is necessary for handling, it must be presintered. Presintering is done by heating to 1100-1300℃in muffle furnaces under hydrogen atmosphere. By reducing the oxide film covering the powder particles, the cohesive strength between them is increased.

Please visite our website www.tungsten-powder.com

POWDER METALLURGY-2. Sintering-2.1 General -07

The oxide dispersoids are stable at the high sintering temperatures and do not dissolve in the tungsten matrix. They pin the grain boundaries during the later stages of sintering, and in that way they significantly restrict grain coarsening. So for example, a rod containing 0.75% of thoria has a grain size of 5000 to 10,000 grains per square millimeter, as compared to1500 grains per square millimeter for a similar rod of pure tungsten.

NS-doped tungsten powder contains small inclusions of potassium aluminosilicates, which were incorporated during the reduction process. During sintering, the silicates dissociate thermally and submicron potassium bubbles form in the tungsten ingot. Similar to the oxides, these bubbles pin the-grain boundaries and inhibit grain coarsening during sintering. Since potassium is gaseous during sintering, the bubbles are under high pressure, which is balanced by the surface tension of’ the pore. They can be seen as small pores in the fracture surfaces of NS-doped tungsten besides the significantly coarser residual sintering pores, as characteristic for undoped tungsten. They constitute the starting point for subsequent formation of rows of bubbles during thermo-mechanical processing.

Up to the sixties, the only sintering method used in practice was direct sintering. Although still in use for the production of doped tungsten, it has lost its importance. From then on, mainly because of the increasing demand for Iarger parts and the higher capacity of the aggregates, indirect sintering furnaces were developed. This technique is used nowadays as the main route for producing pure tungsten.

You can visit our website www.tungsten-powder.com also

POWDER METALLURGY-2. Sintering-2.1 General -06

Since the ductility of tungsten is very sensitive to most of the impurities, purification is important. Therefore, special care must be taken so that, during sintering, evaporation can take place to the desired extent (i.e., as long as there is an open porosity). If the ingot densifies too quickly, impurities can be trapped. Due to the higher sintering temperature, direct sintering is more effective in cleaning than indirect sintering.

The sintering of tungsten can be enhanced by the addition of small amounts of alloying elements (0.5-1%), such as Ni or Pd. This phenomenon is called“activated sintering.” It is explained by the enhancement of grain-boundary diffusion in tungsten due to the presence of the respective element in the grain boundary, High densities (up to 99%) can be obtained even at 1100℃ (at this temperature, tungsten compacts are commonly presintered). However, since the alloys so produced are rather brittle, activated sintering has never been used in industry.

The sintering of doped tungsten is a peculiar case in sintering of tungsten. This includes dispersion-strengthened materials such as thoriated tungsten or tungsten with additions of CeO₂, La₂O₃, and ZrO₂ as well as NS (non-sag) tungsten used for lamp filaments.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-2. Sintering-2.1 General -05

Tungsten sintering, in practice, is always performed in reducing atmosphere which removes the oxygen coating of the powder particle surfaces. High-purity dry hydrogen is commonly used.  Under vacuum or in inert gas, sintering is retained by residual oxygen, and the desired density will not be achieved.

The high temperatures used for sintering also lead to a significant purification of the metal by volatilization of impurities. Interstitials, such as oxygen and carbon, cannot be removed completely and have to be held at a low level; otherwise, they significantly affect the workability of the parts. Metallic elements, such as Fe, Ni, Cr, Nb, etc., are evaporized but remain partly in the ingot, forming solid solutions. If Fe or Ni is present in the form of large, heterogeneous contaminations-(such as iron particles stemming from reduction boats), these can lead to local melt formation and subsequent formation of large voids, which do not close during sintering, Elements which have practically no solubility in tungsten, such as alkali or earth-alkali metals, are volatilized. Grain boundary diffusion and carrier distiIlation effects most likely play an important role in“cleaning” the grain boundaries.

Please visit our webstie www.tungsten-powder.com

POWDER METALLURGY-2. Sintering-2.1 General -04

Temperature is the most important parameter affecting densification. Higher sintering densities can be obtained much more rapidly by increasing the sintering temperature than by prolonging the sintering time. Below 1900℃ little densification occurs, unless very long sintering times are applied. For example, more than 50h are required to obtain 92% density for a tungsten powder of 4 µm particle size at 1800℃. At 2400℃, the time necessary to obtain high densities decreases to 1-2 hours and, at 3000℃, 20-30 minutes. The finer the starting tungsten powder, the more rapid the densification at a given temperature.

The influence of temperature and time on densification can be estimated by using so-called density diagrams, which are based on approximate sintering models. Nevertheless, empirical rate equations are used for industrial purposes to calculate e necessary sintering times at different temperatures.

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

POWDER METALLURGY-2. Sintering-2.1 General -03

Investigations have shown that the densification is controlled by grain boundary diffusion over most of the densification range, unless at very high densities it becomes controlled by lattice diffusion.

Since the motion of grain boundaries, necessary for grain growth, is impeded by the presence of pores, grain coarsening proceeds at a higher-rate above 97% density. Grain sizes of the as-sintered ingots are commonly in the range of 1 0 to 30µm.

Besides temperature and time, several other parameters influence densification, such as powder particle size, green density, sintering atmosphere, powder purity, compact size/weight, heating rate, thermal gradients, and the presence of insoluble phases such as oxides (Th02, La203, Ce02, 2r02) or metallic potassium (NS-tungsten).

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

POWDER METALLURGY-2. Sintering - Sintering Stages

Sintering is commonly regarded as taking place in three stages：

*During the early stage, necks are formed between individual particles and grow by diffusion, increasing the interparticle contact area. The powder aggregate shrinks，involving center to center approach of the particles. In this stage, the degree of densification is still low and the pore structure is open and fully connected.

*With increasing neck formation (intermediate stage), the necks become and lose their identity. The pores are assumed to be cylindrical. Their radii vary along their lengths and, with increasing shrinkage, the pore channels break up into small, still partly interconnected segments. During this stage (channel closure stage), pronounced densification occurs and significant grain growth occurs concurrent with shrinkage.

*Finally, in the last stage (isolated pore stage), the pore segments further break up into chains of discrete, isolated pores of more or less spherical symmetry. This stage occurs when about 90% of the theoretical density is achieved. The sintering density then approaches asymptotically the practical limit of 92-98%.

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

POWDER METALLURGY-2. Sintering-2.1 General

2. Sintering

2.1 General. ln order to increase the strength of the green compacts, they are subjected to heat treatment, which is called sintering. The main aim of sintering is densification order to provide the metal with the necessary physical and mechanical properties and a density which is suitable for subsequent thermomechanical processing. Sintering of tungsten is commonly carried out in a temperature range of 2000 up to 3050℃ under flowing hydrogen either by direct sintering  (self- resistance heating) or indirect sintering (resistance element heating systems). The density thereby obtained should be a minimum of 90% of the theoretical density, but is commonly in the range between 92 to 98%.

The main driving force for sintering is the lowering of free energy, which takes place when individual particles grow together, pores shrink, and the high surface area of the compact (i.e., its high excess surface energy) decreases. The decrease in surface area is accomplished by diffusional flow of matter into the pore volume under the action of capillary forces (surface tension force). Besides shrinkage, recovery (change of subgrain structures and strain relief), recrystallization (formation of strain free crystals low in dislocation density), and grain growth occur during sintering, also contributing to the minimization of free energy.

Please visti our website www.tungsten-powder.com to get more information.

POWDER METALLURGY-- Compacting-1.2 Cold Isostatic Pressing

1.2 Cold Isostatic Pressing. ln isostatic pressing, the powder is filled into filexible molds made of rubber or elastomers and subjected-to hydrostatic pressure. The pressure is commonly in the range of 200 t0 400 MPa. As a result of the uniform pressure, a much higher uniformity in density is achieved. Isostatic pressing has gained much importance during the last 30 years, because it offers several further important advantages compared to rigid die pressing:

*Lower pressure required for a certain green density.

*Higher strength of the compacts.

*More free choice in dimension (ratio of diameter to lengths).

*Parts with undercuts and reentrant angles can be pressed.

*Thin-walled tubes can be produced.

*Very Iarge parts can be compacted.

Less precise dimensional control and a much lower rate of production are the two main disadvantages.

Isostatic pressing is carried out by two different techniques: wetbag pressing or drybag pressing.  In wetbag pressing, the powder is filled into flexible molds which are sealed outside the pressure vessel. Several molds (either of the same shape or of different shape) are then immersed in a fluid, most commonly water, and pressure is applied isostatically. Wetbag pressing is the most common technique for producing forging or rolling performs, where an even and high green density is more important than dimensional control. Nevertheless, wetbag pressing is also used for more complex geometries and even near-net shape parts. The size of the part is limited by the size of the pressure vessel. Tungsten ingots of up to 1000kg are produced via wetbag pressing.

In drybag pressing, the elastomeric mold is fixed into the pressure vessel. The mold is filed with powder and sealed with a cover plate. Then pressure is applied between the mold and the vessel wall. After pressure release, the cover is removed and the part removed. Then the procedure starts again. Pressure Vessels with both top and bottom plate are in use, and allow more rapid removal of compact.

Drybag pressing is used for sample shapes, such as plugs, and high production rates (mass production). However, only one compact (with one specific shape) can be pressed at a time.

POWDER METALLURGY-- Compacting-1.1 Die Pressing -2

Typical compaction pressures are in the range of 200-400 MPa (2-4 t/cm²) but can reach 1000 MPa (using hardmetal dies and punches). The green density (compact density) is in the range of 55-65% of the theoretical density (75% at most) and it depends upon the applied pressure, particle size, size distribution, particle shape, and size of the compact. There are several theoretical equations relating green density and applied pressure, but in practice empirical relations are used.

The relationship between the average particle size and the green density as well as the compressive strength of compacts is shown in Fig. 5.31 for a const ant pressure. Although the green density increases as average particel size varies from 1 to 9µm, the compressive strength exhibits a maximum, between 3 and 61 µm. This maximum corresponds to the preferred particle size range for most tungsten compacts.



tungsten powder












Any inquiry of tungsten powder please feel free to contact us by 86 592 5129595

POWDER METALLURGY-- Compacting-1.1 Die Pressing

1.1 Die Pressing. Pressing of powders in rigid dies is carried out either in mechanical or hydraulic presses. The pressure is applied from the top, or from the top and bottom (double action presses). Die and punches are made of high-speed tool steel or (more rarely) hardmetal. Mechanical presses (pressing forces up to 1 MN) are used for small parts and high production rates. They allow a higher degree of dimensional precision and are well suited for process automation. Hydraulic presses are mainly used for simple preforms. Large presses with up to 30 MN (3000t) pressing force are used for pressing of plates which are to be rolled to sheet metal. The size and shape of the compact are limited by the capacity of the press and also by the geometry of the part.

Due to the friction between the powder and the die wall and the nature of the load distribution inside the die, the pressing density is not the same all over the compact. This is more pronounced for large parts and large part heights and can lead to crack formation and/or distortion of the pressed compact during sintering. Large and critical parts are therefore commonly pressed isostatically.

Please visit our website www.tungsten-powder.com

POWDER METALLURGY-- Compacting-1

1. Compacting

Tungsten powder is consolidated into a compact by two main routes: pressing in rigid dies (uniaxial pressing) an isostatic pressing in flexible molds (compaction under hydrostatic pressure).  Other techniques, such as powder rolling, cold extrusion, explosive compaction, slip casting, vibratory compaction, or metal injection molding, have gained no industrial importance.

Tungsten powder is not easy to compact due to its relatively high hardness and difficult deformation. Nevertheless, in most cases compaction is performed without lubricant to avoid any contamination by the additive. The resulting compacts are generaIIy sufliciently strong so that they can be handled without breaking. For machining the part, it “must be pre-sintered beforehand.

You visit www.tungsten-powder.com to get more details.

POWDER METALLURGY

Although the Wollaston process of producing platinum metal from platinum powder is regarded as the birth of modern powder metallurgy (1808-1815), it was the pioneering work of W.D.Coolidge on the production of ductile tungsten wires in 1909-1913 which led to its first commercial application. Over the years, only few changes were made in the industrial production of tungsten metal. Powder metallurgy is still today the main route in tungsten and tungsten alloy manufacture. Unlike other refractory metals, such as Zr, Hf, Nb, or Ta, melting technology has assumed no industrial importance for metal production, since the very high temperature, necessary for melting, and the resulting coarser microstructure of the “as cast” material make further processing both difficult and costly.

Powder metallurgy comprises two steps: compaction and sintering.

Please visti our website www.tungsten-powder.com

TUNGSTEN METAL POWDER PRODUCTION - Reduction of Doped Tungsten Oxides- 4

During sintering, the silicates dissociate thermally. The smaller Al, Si, and O atoms ,escape via different through the tungsten crystal lattice while the large K atoms remain in the form of potassium bubbles. It is this potassium that enables the formation of rows of smaller potassium bubbles during wire fabrication, and consequently the formation of a long-grained interlocking microstructure during subsequent filament operation, which is the key to the non-sag characteristics .

The grain-growth-enhancing effect of K makes it possible to conduct the reduction at a lower temperature than normally applied for W powder of 3-5 µm average grain size . The hydrogen gas has to be kept separated from hydrogen for reduction of undoped powder, because it is contaminated by K.

NS-W powders exhibit a characteristic morphology, exhibiting holes in the crystal surface, which form during HF-washing of the powders due to the dissolution of partly intergrown silicate phases.

please our website: www.tungsten-powder.com to get more information.