A dilatometry study of the influence of green density on anisothermal and isothermal shrinkage of plain iron green parts

Abstract

The influence of green density between 6.5 and 7.3 g cm −3 on the anisothermal and isothermal shrinkage of atomized plain iron was investigated by dilatometry. The geometrical activity deriving from the extension of the interparticle contact areas and the structural activity provided by the defectiveness of the interparticle contacts promote anisotropic anisothermal shrinkage starting from 500°C up to the bcc to fcc iron transformation. It displays a minimum at 6.9 g cm −3 , resulting from the combined effects of the thermodynamic driving force and of geometrical and structural activity. Anisothermal shrinkage is caused by an anisotropic increase in the internal radius of the neck. Anisotropic isothermal shrinkage is smaller than the anisothermal one and is almost independent on green density. No anisothermal shrinkage was observed on the tapped powder, demonstrating that anisothermal shrinkage in green specimens is due to geometrical and to structural activity introduced by prior cold compaction.

Influence of the heating rate on sintering shrinkage of plain iron

“…In particular, it displays a minimum at the intermediate one and is higher at 7.3 g cm −3 than at 6.5 g cm −3 . This result is in agreement with the previous works [1,16].…”

“…In the previous works [1,2], the anisothermal shrinkage was determined as the difference between the extrapolated thermal expansion and the actual dimensional change, as shown in Figure 7(a). Thermal expansion was determined from the slope of the curve between 300 and 500°C interval.…”

“…Dislocation pipe diffusion is enhanced on approaching the Curie temperature by the disappearance of the magnetic moment of iron atoms in the pipes [13]. The hypothesis of the role of dislocations in the anisothermal shrinkage has been strengthened by the observation that it does not occur in a tapped powder [1]. T s does not depend on green density, while it increases with the heating rate Q , as shown in Figure 5.…”

“…The sintering shrinkage of prior cold compacted iron and steels is split into two contributions: a first shrinkage during the heating step in the alpha iron field, and a second shrinkage during the isothermal holding at the sintering temperature. Both the anisothermal and the isothermal shrinkage depend on the green density [1], as a result of the combination of the thermodynamic driving force and of the deformation of the powder particles promoted by cold compaction [2]. Deformation introduces a large density of structural defects in the interparticle regions, as it has been demonstrated by Torresani et al [3] and confirmed recently by Wendel et al [4].…”

Sintering shrinkage of cold compacted iron: effect of thermodynamic driving force, structural and geometrical activity

“…The former is affected by the particle size and morphology (and their distribution) and by green density (the higher the green density, the lower the total free surface of the material); the latter is influenced by the geometrical and the structural activity, both being affected by prior cold compaction, other than by temperature. The combination of these effects results in a minimum of shrinkage at an intermediate green density [14][15][16].…”