Powder Properties, Compacting and Sintering

Book chapter, 2009

Metal Powder represents a statistical set of particles with different geometry and mechanical properties. The change from powder to compact under the pressing pressure is interpreted by different overlaying mechanisms:At low pressures the densification occurs by particle ranslations and rotations giving a higher packing coordination; after the particle rearrangement the elastic and plastic deformation of particles starts through their contacts which results in formation of small strain hardened zones. As the pressure increases, the contact area between the particles increases and particles undergo extensive plastic deformation (stage of strain-hardening). The porosity elimination obtained at the pressing pressure p is the result of interactions between the instantaneous geometric properties and instantaneous plastic deformation of particles. Further densification and strengthening is achieved by sintering. The parts are heated in a controlled atmosphere to a sintering temperature (for iron-based alloys this is usually 1100-1150°C; in some cases also up to 1300°C) and held for between 15 and 60 minutes, depending on the application. The main mechanisms of material transport during sintering are surface diffusion, grain boundary and volume diffusion.