A plasticity theory is developed for a powdered metal compact. The yield criterion used for sintered powdered metals is modified for describing asymmetric behavior of powdered metal compacts in tension and in compression. Three parameters α, δ, and p are involved in the yield function. The two parameters, α and δ, are determined from the behavior of sintered powdered compacts as functions of relative density R. Green strength vs relative density R relationship for powdered metal compacts is used for defining completely the yield function with the effect of relative density. Predictions are made for compaction pressures as functions of relative density for unidirectional, bidirectional and isostatical compaction, and compared with experimental data for copper and iron powders. Also yield surfaces for various density levels are compared with the experimental data with copper powders.

Deformation characterization of powdered metals in compaction

This work introduces basic plasticity equations for the forging of sintered porous metals, presenting a model for a general state of stresses. In the simple uniaxial compression, parameters are semi-empirically determined as a function of the relative density. These parameters are introduced into equations derived for representing. Experimental results of cold forging of sintered preforms prove the validity of the present theory.

Theoretical and experimental analysis of plasticity equations for porous metals

Analysis of the kinematics of the process of free upsetting of a porous cylinder in the presence of contact friction

PM preform material is regarded as two-phase, consisting of a mixture of matrix metal and pores. During the upsetting stage of the powder-forging process, the metallic portion deforms in accordance with the ideal plastic Poisson's ratio (ν=0·5) while previously published experimental observations suggest that the porosity diminishes at a decreasing rate. This gives rise to the assumption that the total volume of the porosity decreases exponentially in the longitudinal direction for corresponding increases in radial strain, which leads to the concept of a ‘coefficient of consolidation’ C. The actual value of this coefficient varies between 0 and 0·5 depending upon the initial preform density, mode of deformation, and prevailing frictional conditions. Compatibility equations are derived which take into account the combined effects of ν and C and the relative proportions of solid metal to porosity. These equations are combined to provide a general equation connecting apparent plastic Poisson's ratio ...

T. J. Griffiths

Papers

Analysis of Consolidation During Powder Forging