The 19th Conference of Mechanical Engineering Network of Thailand 19-21 October 2005, Phuket, Thailand Modelling of Powders with Internal Pores in Cold Compaction Somboon Otarawanna 1 , Wiwat Tanwongwan 1 , Anchalee Manonukul 1 * and Julaluk Carmai 2 1 National Metal and Materials Technology Center, Klong Luang, Pathumthani 12120 2 Department of Production Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology North Bangkok, Bangsue, Bangkok 10800 *Corresponding author (E-mail: anchalm@mtec.or.th) Abstract Powders used in the powder compaction could be solid or sponge powders depending on the powder fabrication techniques. This paper is the first attempt to theoretically investigate sponge powder compactions with different levels of initial relative density. Cold compactions of regular-density, high-density sponge powders and solid powders have been studied using the ‘explicit particle modelling’ approach. The spongy morphology in each sponge powder is modelled by the porous constitutive model and ‘explicit modelling of internal pores’ approach for comparison purpose. The results suggest that the present of internal pore has an effect on the macroscopic stress response. The high- density sponge powder with a large number of very small isolated internal pores is apparently stiffer than the others. The regular-density sponge powder with high internal connected porosity requires lower macroscopic stress to deform than the others at the same relative density excluding internal pores. Where both internal and external pores are present at the initial stage of the compaction, the deformation is preferential towards the external pores. Keywords: Powder compaction; Constitutive material model; Sponge powder; Internal pore 1. Introduction Different powder fabrications can lead to powders with different characteristics. The fabrication of powders involves the application of energy to materials to create new surface area. Selection of a certain fabrication technique depends on understanding of a process, its economics, the resulting powder characteristics, and how those characteristics satisfy the desirable application. Two different popular fabrication techniques for metallic powders are oxide reduction and atomisation. Oxide reduction is a classical powder fabrication technique which can be achieved by thermochemical reactions of metallic oxide involving reducing gases such as hydrogen or carbon monoxide. Atomisation is an advanced powder fabrication technique which involves the formation of powder from molten metal using a spray of droplets. Typically, powders fabricated by oxide reduction are spongy, while powders fabricated by atomisation are solid as shown in figures 1(a) and (b) respectively. Sponge powders are commonly used for the production of low to medium density ferrous powder metallurgy parts, approximately 5.5 g/cm 3 to 7.0 g/cm 3 [1]. On the other hand, water atomised powders are commonly used for the production of high density alloyed parts. For powder compaction modelling, this can be carried out using the approach of ‘explicit particle modelling’ which models individual powder explicitly. From the literature [2-6], the explicit particle modelling has only been employed to model powders containing no internal pores regardless that the powders are solid or spongy. This work is the first attempt to truly represent spongy morphology. The compactions of solid powders and sponge powders with regular and high initial relative densities are modelled to investigate the deformation behaviour of sponge powders comparing to that of solid powders. (The relative density is defined as the ratio of the bulk density of porous media to its theoretical density.) Behaviour of each sponge powder is modelled using the constitutive material models for porous material. The modified Drucker-Prager Cap model and the porous metal plasticity (Gurson) model are employed to model sponge powders in the case of regular and high initial relative densities respectively. In addition, the compaction of high-density sponge powders is also performed using the approach of ‘explicit modelling of internal pores’ to compare the results with those of the constitutive modelling 2. Numerical Analysis Procedures The explicit particle modelling approach is used in this investigation to study the deformation of individual powder. The problem is solved numerically using a finite element analysis tool. In the past, only finite element modelling using two-dimensional plane strain elements has been reported in the literature [2-6]. The reasons for only using the two-dimensional element could be the limitation of computational power and the robustness of the contact algorithm in the finite element software. Comparison of finite element analysis using the two- dimensional plane strain, two-dimensional generalised plane strain and three-dimensional solid elements for modelling powder compaction was carried out and reported elsewhere [7]. It was found that the two- dimensional plane strain solid element is suitable for further investigation considering the accuracy and the effort required. Therefore, the two-dimensional plane AMM045