The influence of the heterogeneity of the pore size distribution on the “zonal isolation” during sintering


I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Krzhizhanovsky str., 3, Kyiv, 03142, Ukraine
Mathematical Models and Computing Experiment in Material Science - Kiev: Frantsevich Institute for Problems of Materials Science NASU, 2019, #21


One of the problems of powder metallurgy technologies is the heterogeneity of shrinkage of powder billets during sintering. Such heterogeneity leads to distortion of the desired shape of the products, reducing their strength, and in some cases, to complete destruction during sintering. This heterogeneity is explained primarily by the nonuniform distribution of the initial density in the volume of the powder billet. Another common cause of nonuniform sintering may be nonuniform heating of the workpiece or its interaction with the furnace wall or tool. Continuous sintering theories, in which the structure of the pore space at the point of the effective porous material is described only by the volume fraction of pores, make it possible to simulate the kinetics of shrinkage heterogeneity during sintering due to the above factors. However, there are phenomena of local irregularity of shrinkage, which are not caused by the aforementioned macroscopic factors, but primarily due to the complex kinetics of the structure of the porous material in the sintering process, which is not limited to a change in density. Such phenomena were named, in particular, as "zonal embodiment" during sintering. Continuous sintering theory of biporous materials developed by the authors describes the structure of the pore space for two parameters, not just the average density. Therefore, it becomes possible to describe the change in the structure of the porous material related not only to the shrinkage. The kinetics of the porous structure with homogeneous initial density but with nonuniform pore size distribution for sintering cases with kinematic constraints are considered. For this purpose, in accordance with the methods of mechanics of heterogeneous materials, a unit cell of such material was considered, consisting of regions of two different types, which are randomly distributed in volume. The consideration is based on the continuous theory of sintering of biporous materials. It is shown that the inhomogeneous distribution of pores in size leads to the emergence of local micouniformities of the density field, or in other words to the "zonal isolation" during sintering.