Study of the anisotropic sintering shrinkage of green iron and stainless steel

Powder metallurgy (PM) is a near net-shape technology that guarantees excellent dimensional control and surface finishing in the production of parts with complex shapes. Press and sinter is the conventional PM process. The powder mix is compacted into rigid dies to obtain the so called green part that is subsequently sintered to promote the formation of metallic bonding between the powder particles. The differences in terms of geometrical and dimensional features between the green compact and the final part are related to the dimensional variations occurring during sintering, therefore, to preserve the cost effectiveness of the process, their in-depth knowledge is crucial. In this context, post- sintering machining should be limited to the realization of geometrical details that cannot be directly obtained through uniaxial cold compaction or to achieve strict tolerances required by specific applications. The dimensional changes in sintering that may be either shrinkages or swellings are affected by many parameters and, among them, we can consider: material, presence of lubricant and additives, green density, compaction strategy, sintering temperature and time, atmosphere, heating and cooling rate. Furthermore, the dimensional variations along the direction parallel to compaction (longitudinal, axial) are different from the dimensional variations in the compaction plane (transversal, radial) causing an anisotropy that depends, in addition to the mentioned parameters, on the geometry and the dimension of the part. At present, in absence of an adequate designing tool to account for the anisotropy, engineers rely on empirical methods often based on the trial-and-error approach.