Salient features in the cold upset-forming of sintered aluminium-3.5% alumina powder composite preforms

The present investigation involves the evaluation of cold upset-forming and densification features in aluminium-3.5% alumina sintered powder preforms. Cylindrical preforms of three initial theoretical densities (75, 80 and 90%) and three initial aspect ratios (0.36, 0.54 and 0.72) were prepared using a suitable die, a punch and a die bottom insert on a 0.60 MN capacity hydraulic press. The preforms were coated with an indigenously developed ceramic coating and dried for a period of 9 h under room-temperature conditions. The ceramic-coated preforms were sintered in an electric muffle furnace for a period of 100 min at 550 degrees C and then furnace-cooled. Cold-deformation experiments were carried out in several steps, at the end of each step, dimensions such as height, contact and bulged diameters and densities being measured. In general, the deforming load was applied in steps of 0.02 MN until cracks began to appear on the free surfaces. Analysis of the experimental data showed an increasing trend in the attained fractional theoretical density w.r.t. the new strain parameter psi (= e(epsilon z-epsilon 0)). Preforms possessing lower initial aspect ratios have shown enhanced densification compared to preforms of higher initial aspect ratios, subject to the condition where the initial preform densities were maintained constant. However, the Poisson's ratio w.r.t. the fractional theoretical density attained exhibited three distinct stages, namely, a steep rise, a steady-state, followed by a rapid rise approaching to value of 0.5 in the close vicinity of the theoretical density. An attempt has also been made to derive suitable expressions for the deformed density, strains, stresses and the radius of curvature of the barrel during the upsetting of powder preform. (C) 1997 Elsevier Science S.A.