Analytical Model of the Anisotropic Dimensional Change on Sintering of Ferrous PM Parts

Abstract This work proposes an analytical model developed from experimental data to describe the anisotropic dimensional change on sintering. Axial-symmetric iron parts differing for geometry and sintering conditions have been investigated, aiming at highlighting the influence of geometry. The specimens were measured in the green and sintered state by a coordinate measuring machine (CMM). The dimensional changes of height, external diameter and internal diameter were derived from measurement results. The anisotropy of the dimensional variations has been studied with reference to the isotropic dimensional change derived from the change in volume of the parts. The influence of geometry and sintering temperature was highlighted. To properly describe the dimensional variations in the compaction plane, the dimensional change of the external diameter versus the dimensional change of the internal one has been analysed. By means of the experimental data, a reliable analytical relationship has been found, dependent on the parts geometry. An anisotropy parameter has been identified, which allows relating the dimensional change in the compaction plane and in the axial direction to the isotropic dimensional change. This parameter depends both on geometry and on sintering conditions. By means of the anisotropy parameter an analytical model for the anisotropic behaviour has been developed

Development of a design procedure accounting for the anisotropy of the dimensional change in Powder Metallurgy parts

The dimensional control is a crucial aspect for any manufacturing process. In Powder Metallurgy, and in particular in net shape press and sinter process, dimensional control assumes a particular relevance, since sintering of green parts involves dimensional variations that can be from 0 to 2-3% in volume. The dimensional variation in sintering is either shrinkage or swelling. Both depend on the material and on several process parameters relevant to the compaction and the sintering operations. Experimental evidences proved dimensional variations to be affected by an anisotropic behavior. This important phenomenon affects the effectiveness of the dimensional control if not opportunely taken into consideration in the design process. Professor Ilaria Cristofolini and Professor Alberto Molinari have started a deep investigation on this phenomenon, about five years ago, involving an important experimental campaign. The main idea is to collect a large quantity of data, both on ad-hoc designed samples and on parts produced by qualified PM companies cooperating with the University of Trento. The purpose is to develop a realistic model, able to explain and describe the mechanisms involved in the anisotropy of dimensional changes, and the dependence on the geometry of the parts, building a robust knowledge to improve the design methodologies in the industrial production. The present work investigates the effect of the geometrical characteristics of the part on the dimensional variations in sintering, giving a particular importance on its anisotropic behavior. The influence of geometry has been investigated using rings and disks with varying heights, external diameters and internal diameters. The influence of the sintering temperature has been also evaluated. The dimensional variation has been measured by a tri-dimensional Coordinate Measuring Machine. The anisotropy has been defined through a specifically determined parameter, which has been used to develop a predictive model estimating the anisotropy of the dimensional variations. This model has been then validated on complex parts produced by a Powder Metallurgy company