Some aspects of cold upset forming of sintered aluminium preforms using graphite lubricant

Abstract

Cold upset forming and densification features in sintered aluminium preforms were investigated. Cylindrical preforms with four initial theoretical densities of 81, 85, 88 and 90% possessing three different aspect ratios of 0.50, 0.75 and 1.0 were prepared using a die and punch assembly with a 1.0 MN capacity hydraulic press. The preforms were packed in covered dry fine silica sand before the sintering operation was carried out in an electric muffle furnace at 525 ± 10 °C for a period of 1 h, and subsequently they were furnace cooled. Cold deformation experiments were conducted in incremental deformation steps. The experimental data showed that the ratio of the density of the preform after deformation/theoretical density of the fully dense material ρf/ρth value demonstrated an increasing trend with increasing value of exp (ϵz - ϵθ), where ϵ z is the true strain in the axial direction ϵθ is that in the hoop direction. It was established that a lower aspect ratio preform exhibited enhanced densification compared with the higher aspect ratio preforms, for any given initial preform density. Moreover, the applied stress had the tendency to increase rapidly during the initial stage of densification, thereafter continuing to increase at a low rate, as the densification was enhanced. The increase in applied stress was found to be associated with the combined effects of densification on the geometric and general work hardening parameters.