A new constitutive model for the finite element analysis of metal powder compaction

Bibliography: leaves 60-61.During the commercial compaction of metal powder in a punch-die setup and the subsequent sintering of the compacted preform, the large confining pressures, friction and heat that arise determine the properties of the final component. An important application of the modelling of metal powder compaction is the determination of the properties of the compacted preform as these affect the properties of the final sintered component. An uneven density distribution in the compacted preform, for example, can cause cracking in the component during the sintering stage. The mechanical behaviour of the metal powder during compaction largely determines the properties of the compacted preform. To date the consolidation of metal powders has been conveniently represented using constitutive theories based on elastic-plastic material models. Most of the work has concentrated on the use of quadratic yield surfaces but experimentation has shown that this type of yield surface does not always correctly represent metal powder behaviour (Brown, 1994 ). This thesis details the development of a new constitutive model for the finite element analysis of metal powder compaction based on extensive experimental testing done by Watson(1993) on aluminium powder. Watson found that a cap yield surface, often used in soil plasticity, best fitted the experimental data obtained from the aluminium powder. A model similar to that proposed by Watson was implemented in this thesis in an attempt to show that a cap yield surface is more accurate for modelling the compaction of aluminium powder and other powders than a quadratic yield surface. The new constitutive model combines a Drucker-Prager or shear yield surface and a density evolving cap or consolidation yield surface to model powder compaction and differs from other cap models as the shear yield surface also evolves with density. A combination of an associative flow rule on the consolidation yield surface and a von Mises flow rule on the shear yield surface made for easier numerical implementation of the model. The model was implemented in ABAQUS as a FORTRAN 77 User-Material Subroutine using an Euler Backward integration scheme