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.