R.K. Dube

Changes in the longitudinal flow and apparent plastic poisson’s ratio of a porous metal strip during hot densification rolling

Analytical relationships between the longitudinal flow and the thickness deformation, and between the apparent plastic Poisson’s ratio and the relative density of a porous metal strip during hot rolling have been derived from geometrical considerations. Experimental data obtained from the hot rolling of porous copper strips of relative density of 0.4 have been shown to agree well with the results predicted from analytical relationships. Changes in the longitudinal flow and the apparent plastic Poisson’s ratio can be divided as taking place in three distinct stages of densification. Stage I,i.e., the densification up to the relative density of 0.70, involves the densification of the strip merely by decreasing the pore channel widths without closing them. Stage II,i.e., the densification from the relative density from 0.70 to that of 0.95, is associated with the patching of pore channels at some points, the number of which increases as the thickness deformation given to the strip increases. Stage III,i.e., the densification of the strip from the relative density of 0.95 and onward, involves the elimination of the newly formed closed porosity by shearing.

Analysis of cold densification rolling of a sintered porous metal strip

Abstract Cold densification rolling of a sintered porous metal strip has been analyzed by the upper-bound technique coupled with the plasticity theory of porous metals. The analysis considers plane-strain deformation and incorporates work-hardening effects during cold rolling. A stream function form which produces a kinematically admissible velocity field has been assumed in the upper-bound analysis. The total power for rolling, which is the sum of the internal power for plastic deformation, frictional power and shear discontinuity losses along the surface of velocity discontinuity, has been minimized to find the velocity and density distributions. The calculated results for density and strains have been verified experimentally for the multi-pass rolling of a sintered porous copper strip.

Densification and deformation behavior

The relationships between the longitudinal and thickness strains and between the apparent plastic Poissons ratio and the relative density achieved during hot rolling of porous nickel strips of initial relative densities 0.36, 0.48, and 0.67 are experimentally determined. The experimental data have been found to agree well with the results predicted from analytical relationships. Hot rolling of such porous metal strips can be divided into three distinct stages. In stage I, the powder particles rearrange and restack as the rolling progresses and the longitudinal elongation and apparent plastic Poissons ratio are negligibly low. The end of this stage is marked by the situation when no further densification without longitudinal flow is possible by hot rolling. In the present case, stage I continues until a relative density of 0.7 has been achieved in the strip, resulting into a predominantly orthorhombic pattern of particle arrangement. Stage II,i.e., densification from the relative density from 0.70 to 0.95, is associated with the continuous increase in interparticle contact area, longitudinal elongation, and apparent plastic Poissons ratio as the hot rolling progresses. In this stage, the interconnected porosity changes to the isolated one. In stage III,i.e., densification beyond the relative density of 0.95, pore fragmentation and collapsing of the opposite pore surfaces take place until all the porosity has been eliminated.