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Materials processing and joining

To make WC–Co cemented carbide, WC powder is mixed with Co powder, compacted and sintered. Which statement correctly describes this sintering?

  1. AA Co-rich liquid wets the WC grains, and rearrangement and solution–reprecipitation give almost full density (liquid-phase sintering)
  2. BBelow the melting point of Co (about 1495°C) no liquid forms, and the Co evaporates during sintering, leaving a porous body of WC alone
  3. CThe whole body is heated above the melting point of WC to melt it, then solidified in a mold as in casting to make it dense
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Answer: A. A Co-rich liquid wets the WC grains, and rearrangement and solution–reprecipitation give almost full density (liquid-phase sintering)

In the WC–Co system, a eutectic reaction between WC and Co produces a liquid phase at a temperature below the melting point of Co (about 1495°C). At the sintering temperature the Co-rich liquid wets the WC particles well, capillary forces rearrange and pack the particles, and small WC particles dissolve and reprecipitate onto larger ones, so the body densifies to nearly full density. This is liquid-phase sintering. After cooling, Co remains as the binder phase that holds the hard WC particles together and provides toughness. WC has an extremely high melting point, and melting and casting it is not practical, so it is made by powder metallurgy. Co is not lost by evaporation during sintering; it is indispensable as the binder phase.

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