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, postsintering 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 13 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. International literature addresses the issue of sintering shrinkage and its anisotropy according to two approaches: physical metallurgy and continuum mechanics. For some years, the Metallurgy Group of the University of Trento has started a research program aiming at studying the dimensional changes in sintering and their anisotropy according to the physical metallurgy approach and the present PhD thesis is part of this project. The objectives are: - Study of the effect of the green density on the sintering shrinkage and validation of a kinetic model for the isothermal shrinkage - Study of the effect of the powder morphology, and then of the driving force correlated to the excess of surface area, on the sintering shrinkage - Comparison of the results obtained with iron with those gained with austenitic stainless steel. AISI 316L is poorly affected by structural activity introduced by prior cold compaction since high sintering temperature is required to have shrinkage and, being paramagnetic, does not undergo the increase in volume diffusivity due to the dislocation pipe diffusion mechanism. The work is organized as follow. Chapter 2 is an overview of the state of the art in which the two theories (classical and continuum mechanics) that allow the study of the dimensional variations in sintering and their anisotropy are described and the important parameters identified. 14 For this study, two iron powders (atomized and sponge) and two austenitic stainless steel AISI 316L powders (coarse and fine) have been considered. First of all, they were characterized and the microstructure of the green parts, in terms of contact lengths between the particles, was studied. The dimensional changes during the whole sintering cycle were measured by means of dilatometry tests carried out both in the longitudinal and in the transversal direction, to investigate the anisotropy. The experimental procedures are comprehensively explained in Chapter 3.
A new kinetic equation for the isothermal shrinkage accounting for the extension of the interparticle contact areas is proposed in Chapter 4. Bulk diffusivity is the mass transport mechanism that determines the dimensional changes. The shrinkage is enhanced by the increase of the internal radius of the neck, that represents the geometrical activity in sintering of prior cold compacted powders. After that, the focus passes to the results that are divided depending on the material in Chapter 5. Through atomized iron, the effect of green density on the longitudinal and transversal shrinkage was investigated. In addition, the shrinkage kinetic model was validated. In particular, considering the geometrical activity of the green parts and the corresponding shrinkages on heating, the geometrical activity at the sintering temperature was estimated and, based on it, the theoretical isothermal shrinkages were calculated. Thanks to the effective diffusion coefficient as a function of the isothermal holding time it has been possible to evaluate the structural activity, that was found to be isotropic. From the comparison of the shrinkages of the samples produced with the atomized iron and sponge iron at green density of 6,9 g/cm3 , the influence of the morphology of the powder particles was assessed.
The results showed that, on heating, the effect of deformation prevails and the morphology of the powder does not influence the shrinkage. On the contrary, the isothermal 15 shrinkage depends on the specific surface area since it affects the driving force of sintering. AISI316L requires higher sintering temperature than iron to have shrinkage. Moreover, the contribute to the dimensional variations due to the dislocation pipe diffusion mechanism is expected to be modest. The dependence of the sintering shrinkage of such a material on the driving force related only to the particle size excluding others morphological aspects was evaluated considering two powders with different particle sizes. At the end of the chapter, the differences between the behavior of iron and AISI 316L are pointed out. Chapter 6 is dedicated to a preliminary study of the shrinkage that occurs on heating and it may represent a starting point for further development of the topics of this PhD thesis. The work ends with some concluding remarks.