Cold extrusion forging of sintered porous materials

Abstract

Experiments have confirmed the validity of an arbitrary division of the back extrusion process of a porous blank into two consecutive stages: I) densification of porous material under the ram face to some and density and II) flow (back extrusion proper) of the densified material into an annular gap. During the deformation of blanks of starting densities equal to or greater than their threshold values the extrusion process, as in the case of incompressible (bulk) materials, takes place in a single stage — the flow stage. In cases where ρ0 < ρt the maximum densification pressure of a porous material and the extrusion pressure, which is equal to it, are virtually independent of the starting blank density; the parameter determining these pressures is the threshold density, which depends on the magnitude of transverse strain. Optimum power and force conditions prevail when blanks of starting density equal to the threshold density are used. In the forging of iron powder blanks of this density the extrusion pressure is 20% less than with bulk St. 10 steel. Analytical expressions have been derived enabling the power and force parameters — pressure and work — in each stage of the extrusion forging process of porous sintered materials to be determined.