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Information on tungsten: sources, properties and uses

Powder Metallurgy

Powder Metallurgy - A Flexible Manufacturing Process

Cemented carbides are produced by Powder Metallurgy (PM).  The respective powders (WC, Co, but also other metallic carbides or carbonitrides as well as Fe and Ni) are at first ball milled or attritor milled to form a powder mix. Then, a part is formed by different shaping technologies. In the case of large lot sizes and comparatively simple geometries, as with cutting inserts or mills, the part is formed by die pressing to its final shape (direct forming). Cylindrical shaped parts or parts with large length-to-diameter ratio are formed by extrusion.  Plastfiers (eg waxes) are added prior to extrusion to render a smooth flow of the powder mix through the die. Small parts with complex geometries can be shaped by PIM (Powder Injection Moulding).  In this case the plastified material is pressed into a mould, which is subsequently opened to remove the shaped part (indirect forming). 

Large parts, such as rolls, hobs, anvils or rotary cutters for the hygiene industry are mainly produced by cold isostatic pressing (CIP).  A “green” (isostatically pressed) block is formed at first and subsequently machined to the desired shape, either in the pressed or presintered stage, to improve the strength of the still porous part (40 to 50 vol%) for the shaping.

After shaping, the materials undergo a thermal treatment, called sintering, to form a dense, near-pore free body (residual porosity commonly below 0.02 vol%).  Sintering is done either in vacuum or under hydrogen.  Pressure-aided sintering (sinterhip) has become a standard technology to produce defect-free materials of outstanding strength.

During sintering, the body shrinks as a result of pore elimination (lateral shrinkage between 17 and 24%) but retains its shape. The better is the manufacturing process (milling, granulation, pressing, sintering), the more the part can be sintered to its final geometry (near-net-shape technology), and the less material has to be removed by subsequent precision grinding by diamond tools.

The result of sintering is a material with varying shape and composition (depending on the respective formulation for subsequent application), outstanding strength properties, high hardness, and high modulus of elasticity at a still considerable toughness level.

© 2011 International Tungsten Industry Association
Copyright images courtesy of: CERATIZIT (dentist end mill)