Advanced Friction Modeling in Sheet Metal Forming J.Hol 1,a , M.V. Cid Alfaro 2 , T. Meinders 3 , J. Hu´ etink 3 1 Materials innovation institute (M2i), P.O. box 5008, 2600 GA Delft, The Netherlands 2 Tata Steel Research - Development & Technology, P.O. box 10000, 1970 CA IJmuiden, The Netherlands 3 University of Twente, P.O. box 217, 7500 AE Enschede, The Netherlands a j.hol@m2i.nl Keywords: friction modeling, friction mechanisms, asperity contact, flattening, real contact area, ploughing, adhesion Abstract. The Coulomb friction model is frequently used for sheet metal forming simulations. This model incorporates a constant coefficient of friction and does not take the influence of important parameters such as contact pressure or deformation of the sheet material into account. This article presents a more advanced friction model for large-scale forming simulations based on the surface changes on the micro-scale. When two surfaces are in contact, the surface texture of a material changes due to the combination of normal loading and stretching. Consequently, shear stresses be- tween contacting surfaces, caused by the adhesion and ploughing effect between contacting asperities, will change when the surface texture changes. A friction model has been developed which accounts for these microscopic dependencies and its influence on the friction behavior on the macro-scale. The friction model has been validated by means of finite element simulations on the micro-scale and has been implemented in a finite element code to run large scale sheet metal forming simulations. Results showed a realistic distribution of the coefficient of friction depending on the local process conditions. Introduction Finite Element simulations of sheet metal products are everyday practice in the automotive industry. An accurate forming analysis can however only be made if, among others, the material behavior and friction conditions are modeled accurately. For material models, significant improvements have been made in the last decades, but in the majority of simulations still a simple Coulomb friction model is used. Consequently, it is still cumbersome to predict the draw-in and springback of a blank during forming processes correctly. To better understand contact and friction conditions during lubricated sheet metal forming pro- cesses, experimental and theoretical studies have been performed. On microscopic level, friction is due to the adhesion and ploughing effect between contacting asperities and the appearance of hy- drodynamic friction stresses. The real area of contact, which depends on different flattening and roughening mechanisms, plays an important role in characterizing friction. However, micro models encompassing these mechanisms are generally regarded as too cumbersome to be used in large scale simulations. An advanced friction model is proposed which couples the most important friction mechanisms. Based on statistical parameters a fast and efficient translation from micro- to macro modeling is included. A general overview of the friction model is presented and the translation from micro to macro modeling is outlined. The development of real area of contact is described by the flattening models proposed by Westeneng [1] and the effect of ploughing and adhesion on the coefficient of friction is described by the friction model of Challen & Oxley [1, 2]. The flattening models are validated by means of FE simulations on micro-scale and the feasibility of the advanced macroscopic