Die stress and internal friction during quasi-static and dynamic powder compaction

Abstract

Three commercial iron powders have been studied in terms of their particle morphology and frictional characteristics. The frictional characteristics were measured in shear cell experiments at normal applied loads of up to 4.7 MPa and deduced from measurements of axial stress and radial stress in an instrumented die used in quasi-static and dynamic compaction experiments at axial stresses of up to 300 MPa. Particle shape factor and specific surface area did not greatly affect the coefficient of internal friction. For the range of powder parameters considered, it was most affected by particle size with the smaller particle samples having the higher friction. There was good qualitative agreement between the coefficient of internal friction deduced from shear cell and die pressing experiments. Despite evidence of the development of cohesion in the powders in the die pressing experiments, the linearity of the axial stress-radial stress relations suggested that the powder response could be characterised in terms of a constant value of the coefficient of internal friction. In dynamic compaction, appreciably lower radial stresses are developed. This is consistent with a higher value of internal friction under these loading conditions.