Inclusion of initial powder distribution in FEM modelling of near net shape PM hot isostatic pressed components C. Van Nguyen*, A. Bezold and C. Broeckmann The final shape of powder metallurgy hot isostatic pressing (HIP) components is determined by many factors, including the shape and size of the capsule, the initial density distribution of the powder before HIP, and HIP process parameters. Current HIP simulation, which usually assumes that the powder inside the capsule is homogeneous, cannot predict non-uniform shrinkage. Therefore, the purpose of this work is to develop a methodology to quantitatively determine the initial powder density distribution inside HIP capsules, which were precompacted by different techniques. The relative density distribution obtained from experimental work is used as the initial condition for numerical simulations. The final shapes of the capsules produced by HIP are compared to simulation results. The paper shows the sensitivity of the final shape on the initial powder density distribution and illustrates the need to implement the initial powder distribution into the finite element model to improve the quality of the near net shape HIP simulation. Keywords: Hot isostatic pressing, HIP, Relative density, Density distribution, Density measurement, Near net shape, Numerical simulation, Non-uniform shrinkage List of symbols A Dorn’s constant c Porosity coefficient f Porosity coefficient N Creep exponent RD Relative density of the powder RD o Initial relative density of the powder q von Mises stress S ij Deviatoric stress tensor T Temperature : e ij Creep plastic strain rate d ij Kronecker Delta s eq Equivalent stress for the porous material s ij Cauchy stress tensor s kk Hydrostatic stress Introduction Near net shape hot isostatic pressing (HIP) simulation has been used for a long time in order to reduce development costs and design time. Many different constitutive densification models and simulation tools have been reported. Most of the publications show that good agreement between numerical simulation and experiments can be obtained, but so far, no summarising comparison of the results of the different finite element model (FEM) simulations was given. 1 In theory, the hot isostatically pressed components should have isotropic shrinkage under isostatic pressure, but in practice, this is often not the case. Non-uniform shrinkage of compo- nents and even high distortion (Fig. 1) have been observed, which is not predictable with the current FEM simulation method because the models assume that the powder distribution inside a capsule before HIP is homogeneous. Anisotropic shrinkage has been observed and verified in the sintering process for cases in which the powder body was not homogeneous. An inhomogeneous body has been created artificially from two powder layers. 2 The bilayer powder compact was bent after sintering toward the lower relative density (RD) side. In the HIP process, pressure and temperature are combined to achieve a full density material at a lower temperature than would be required for sintering alone. Moreover, not only the initial powder distribu- tion, but also capsule shape, size and thickness, and the process parameters pressure and temperature will have influence on the final shape of hot isostatically pressed components. Therefore, the influence of an inhomoge- neous powder distribution on the final shape of hot isostatically pressed components can differ from its influence on pressureless sintered components. The results of previous studies 3 show that an initial inhomogeneous powder distribution after preconsolida- tion (tapping or vibration of the filled capsules before HIP) results in anisotropic shrinkage or even high distortion (bending) for HIPed tubes as shown in Fig. 2, which is similar to the case of sintering. The hot isostatically pressed capsule bent toward the side, which has lower RD. These simulation results were based on Institute for Materials Applications in Mechanical Engineering, RWTH Aachen University, Aachen, Germany *Corresponding author, email c.nguyenvan@iwm.rwth-aachen.de ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 7 November 2013; accepted 9 March 2014 DOI 10.1179/1743290114Y.0000000087 Powder Metallurgy 2014 VOL 57 NO 4 295