Exploration of die wall friction for powder compaction using a discrete finite element modelling technique

The paper presents a micromechanical discrete-element modelling technique to investigate the friction mechanisms that are present between the powder and tool set surfaces in die compaction. The technique includes particle kinematics and deformations to be accounted for together and has allowed a theoretical exploration into particle sliding over smooth surfaces and locking into the asperities of rough surfaces. A two-dimensional model has been developed to represent a shear-plate technique that is used to measure friction in compacted powder systems and in this study assumes that the consequent cylindrical rods are of identical diameter. In the case of smooth surfaces and with no friction coefficient between the tool set surfaces and the powder, the particles simply slide over the surface and are not subjected to gross deformations. The mechanical resistance that may be expressed as a friction coefficient is small under these circumstances. When fine powders are used (fine in comparison with the roughness of the surface), the particles form a sticking layer and the consequent friction coefficient due to particle deformation is large due to the need to shear the powder mass. It is also noted that harder powder particles lead to a lower friction level. When mechanical friction is included between the powder and tool set surfaces, the combination is offset by the friction coefficient that is used between the powder and tool set surfaces.