Modelling and simulation of powder pressing with consideration of residual stresses

Abstract: In powder metallurgy (PM), production methods are constantly developed and improved to produce component with high precision and strength. Cold uniaxial pressing of powder into a green body is a common process in PM. During uniaxial die powder pressing, the enclosed volume between the die and punches is reduced and the powder consolidates until a final height is obtained or a prescribed compacting pressure is reached. Desired properties of the green body are high strength, uniform density, no defects and tight dimension tolerances. For many cases it is beneficial to perform the design changes on the computer. In the development process are finite element (FE) modelling and simulations useful tools. Many of the commercial nonlinear FE codes available today can be used for powder pressing simulations. Some critical features in simulation of the powder pressing process are; material behaviour, large displacement, contact boundary conditions and friction variations. The aim of this work is to numerically capture and understand the development of residual axial stresses in the green body. The used constitutive model is based on a soil mechanical elasto-plastic model by DiMaggio-Sandler. The model has been refined for powder material behaviour. To improve modelling of strength in the green state a new density dependent failure envelope has been used. The model is implemented as a user material subroutine in the non-linear finite element program LS-DYNA. An inverse method is used to adjust the model to a water atomised metal powder from Höganäs AB. The residual stress field of a powder compacted rectangular bar is predicted, three dimensional and two dimensional finite element models are used. The influences of different kinematics, friction, compacting pressures and die tapers have also been investigated. Numerical results show that the thickness of the small compressive stress region close to the side surface varies between 50 to 600 micrometer along the surface. Compacting pressure, upper punch hold down and die taper geometry have all a significant influence on the residual stress state while die wall friction has only a small influence. The numerical results are in agreement with results from X-ray and neutron diffraction measurements.