Influence of the degree of green part elongation and strain hardening of powder material on compaction with punch rotation

Abstract

The rheological constitutive equations of plastic yielding of irreversibly compressible materials with normality flow rule behavior predict reduce of the medium pressure with increase of the shear strain intensity. In contrast to the classical elasticity theory or linear-viscous isotropic fluid flow, this effect is associated with the fact that the specific power of strain internal forces during plastic flow is not the sum of two terms describing the shape distortion and the volume change respectively. Moreover, the loading surface convexity as a required thermodynamic condition with the normality plastic flow rule results in reducing of the medium pressure with increase of the shear strain intensity. In powder metallurgy practice this phenomena is interesting because of the shear strain development due to mutual rotation of the die and punch which allow us to reduce pressure value during powder compaction needed for obtaining the certain level of density. At the same time, the hardening of the solid phase material of a porous body due to additional shear strain leads to an increase in working pressure. Also similarly to the torsion under pressure for compact materials, the degree of green part elongation should have an important influence on the compaction. For more elonga¬te green part the shear strain may not be distributed more or less equally through the green part volume but be localized in a certain narrow area which is the surface of plastic flow localization with velocity field spike observation. This work has been devoted to the investigation of the above-mentioned factors using numerical simulation with the finite element method. In terms of powder metallurgy technologies, conducted numerical simulation gives an answer about the influence of such technological parameters as the angular velocity of cylindrical punch rotation, the degree of cylindrical green part elongation and the degree of strain hardening of powder material on the working pressure of compaction.