Mechanism of anisotropic shrinkage during sintering of metalli powders

The anisotropic dimensional change on sintering of a prior cold compacted iron was investigated by dilatometry. Shrinkage is larger along the compaction direction than in the compaction plane. This phenomenon is very pronounced during the heating ramp in alpha phase and in particular below the Curie temperature, while in austenitic field is quite poor. The results of dilatometry tests were elaborated according to the shrinkage kinetics model of classical sintering theory, to calculate an effective diffusion coefficient along the two directions, which resulted higher for direction parallel to the compaction direction than perpendicular to it. In both directions, the effective diffusion coefficient is larger than that reported in the literature for pure iron, corresponding to an equilibrium density of structural defects. It also varies during the isothermal holding time. This discrepancy is attributed to the defectiveness introduced by cold compaction, that increases diffusivity through the activation of dislocation pipe mechanism, which is particularly intense below the Curie. This interpretation may also justify anisotropy of shrinkage due to the inhomogeneous deformation of interparticle contact regions that was measured with ISE method and EBSD analysis. The anisotropic shrinkage was also described through a modified micromechanical model proposed by the continuum mechanics approach, where the porous body is composed by aligned, elongated particles and elliptic pores, whose geometrical parameters were obtained through image analysis of SEM microhgraphs. The dislocation density calculated for different sintering temperatures was comparable to that measured experimentally. The effect of green density on anisotropy of shrinkage was investigated, too. Anisotropy tends to increase with green density, because of the larger plastic deformation introduced in the interparticle regions by the compaction pressure.