Tungsten Powder Metallurgy - 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.
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.
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.
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.
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