Fracture in metal powder compaction

Abstract

This paper presents a preliminary assessment and qualitative analysis on fracture criterion and crack growth in metal powder compact during the cold compaction process. Based on the fracture criterion of granular materials in compression, a displacement based finite element model has been developed to analyse fracture initiation and crack growth in metal powder compact. Approximate estimation of fracture toughness variation with relative density is established in order to provide the fracture parameter as compaction proceed. A single crack initiated from the boundary of a multi-level component made of iron powder is considered in this work. The finite element simulation of the crack propagation indicates that shear crack grows during the compaction process and propagates in the direction of higher shear stress and higher relative density. This also implies that the crack grows in the direction where the compaction pressure is much higher, which is in line with the conclusion made by previous researchers on shear crack growth in materials under compression. In agreement with reported work by previous researchers, high stress concentration and high density gradient at the inner corner in multi-level component results in fracture of the component during preparation.

Shear cracking in an Al powder compact studied by X-ray microtomography

“…how it leads to a softening of the material response) needs to be taken into account to identify the location where a shear crack is initiated, but they were unable to follow the development of the crack. Tahir and Ariffin [12] studied the compaction of an axisymmetric flanged component using a Mohr-Coulomb constitutive law. They introduced a small crack at the location of the maximum shear stress developed in the component and allowed the crack to grow based on a mode II propagation criterion.…”

Numerical modeling of crack formation in powder forming processes

“…Density distributions remain practically unchanged during these process steps and, therefore, examination of density fields can hardly reveal these post-pressing defects. Attempts to specifically model cracking in P/M compaction have been made by Coube and Riedel (2000), Lewis and Khoei (2001), Tahir and Ariffin (2006), ). In Tahir's work, a stress-based fracture criterion in conjunction with an advanced remeshing technique are employed to track meticulously the propagation of a single crack during the pressing of a flanged component.…”

Finite element modelling of crack propagation in metal powder compaction using Mohr–Coulomb and Elliptical Cap yield criteria

“…In the recent development in modelling the powder compaction process, efforts have been made to accurately predict the crack formation during compaction [4][5][6], as well as taking into account the die filling and powder transfer process to accurately predict the density distribution within the green parts [7,8]. Coube and Reidel [4] focused on the causes of crack formation during metal powder compaction of multilevel parts.…”

“…Recent numerical and experimental works present evidences of the presence of a shear crack (Mode II) occurring during the densification stage of the powder compaction process [5,9]. Hence, it is the purpose of this paper to assess the suitability of two common yield criteria for powder compaction; the Elliptical Cap and Mohr-Coulomb yield criteria in simulating a shear crack (Mode II) during the densification stage of the powder compaction process.…”