Introduction

CONSOLIDATION of metal powders generally involves sintering, whereby an assembly of loose or compacted particles metallurgically bond into a coherent body at elevated temperatures. During the process, sintering powder particles form strong bonds and alloy any admixed elements. Temperatures below the melting point of the major constituent are usually used, although liquid phase sintering is the most common technique in production.

Although sintering has been the subject of extensive experimental and theoretical work, there is no universally applicable theory of sintering. Much is not understood about the process, and no single applicable explanation or understanding of the process has emerged. Sintering generally consists of six distinct, but not necessarily sequential, stages:

• Initial particle bonding

• Pore rounding

• Pore channel closure

• Densification and pore shrinkage

• Pore coarsening

Material transport mechanisms involved include surface diffusion, volume diffusion, evaporation and condensation, and grain boundary diffusion. Several factors affect sintering. However, the most significant are time and temperature, with temperature being the most important single variable. Particle size, compact porosity, and prealloying also affect the sintering process.

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