What is sintering?

Sintering may be defined as a thermal process that induces diffusion bonds. The sintering process bonds together tangent metal surfaces at their points of contact without the addition of any filler metal or bonding agent. 

A suitable environment of high temperature and isostatic pressure causes this bonding to occur via  surface and volume molecular diffusion, vaporation and recondensation, grain growth and recrystallization, deformation and plastic flow.