Fracture and stress in powder compacts
Abstract: In the field of powder metallurgy (PM), the production methods are constantly developed and improved to produce components 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 volume enclosed 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 narrow dimension tolerances. In the development process of new components finite element (FE) modelling and simulations are useful tools, especially to predict density distributions. Today, it is desired to include prediction of fracture and residual stress of powder compacts. The aim of this work is to increase the knowledge of the pressing process and its effect on green body properties. This includes a better understanding of residual stress development in the green body during pressing and the tensile fracture processes of unsintered powder. Both experimental and numerical investigations have been performed to gain knowledge in these fields. An elasto-plastic Cap model has been developed for powder pressing. To improve modelling of strength in the green state a density dependent failure envelope has been used. The model is implemented as a user material subroutine in a nonlinear finite element program. An inverse method is used to adjust the model behaviour to a water atomised metal powder. The fracture process of powder material is studied experimentally with diametral compression test. The properties of conventional and high velocity compacted (HVC) powder is also studied. Methods to determine the tensile strength and fracture energy in metal powder are presented. The test is virtually reproduced with finite element simulations. The residual stress field of a powder compacted rectangular bar is predicted with 3D and 2D finite element models. The effects of kinematics, friction, compacting pressures and die tapers have also been investigated. Numerical results show that the thickness of the small compressive residual stress region close to the side surface varies between 50 ìm and 600 ìm 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. The diametral compression test is an established method for measurement of the tensile strength in a brittle material. During the test a load, P, is applied along a diameter inducing compressive stresses. Stresses are tensile perpendicular to the compressed diameter. These tensile stresses act until failure. During fracturing a large crack along the compressive loaded diameter in the centre of the disc is visible. The crack development is studied with both experimental and numerical investigations. The results show that both tensile strength and fracture energy is strongly density dependent. A cohesive material behaviour is observed in the experiments. The central crack is virtually introduced in a finite element model and controlled with a proposed energy based fracture model. A numerical investigation of the tensile fracture process in powder compacts is performed and results are in agreement with experimental results. This work has given a better understanding of residual stress development in powder compacts. Another outcome from this work is a refined experimental method to determine tensile strength and fracture energy of powder material. An energy based fracture model is proposed for numerical simulations of tensile fracture in powder material.
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