J. Hol

Friction and lubrication modelling in sheet metal forming simulations of the Volvo XC90 inner door

The quality of sheet metal formed parts is strongly dependent on the friction and lubrication conditions that are acting in the actual production process. Although friction is of key importance, it is currently not considered in detail in stamping simulations. This paper presents project results considering friction and lubrication modelling in stamping simulations of the Volvo XC90 inner door. For this purpose, the TriboForm software is used in combination with the AutoForm software. Validation of the simulation results is performed based on door-inner parts taken from the press line in a full-scale production run. The project results demonstrate the improved prediction accuracy of stamping simulations.

Advanced friction modeling in sheet metal forming

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 between 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.

Boundary and mixed lubrication friction modeling under forming process conditions

A multi-scale friction model for large-scale forming simulations is presented. A framework has been developed for the boundary and mixed lubrication regime, including the effect of surface changes due to normal loading, sliding and straining the underlying bulk material. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. To account for the lubricant effects special hydrodynamic contact elements have been developed. Pressure degrees of freedom are introduced to capture the pressure values which are computed by a finite element discretization of the 2D averaged Reynolds equations. The boundary friction model and the hydrodynamic friction model have been coupled to cover the boundary and mixed lubrication regime. To prove the numerical efficiency of the multi-scale friction model, finite element simulations have been carried out on a top hat section. The computed local friction coefficients show to be dependent on the punch stroke, punch speed and l...

Multi-scale friction modeling for manufacturing processes : the boundary layer regime

This paper presents a multi-scale friction model for largescale forming simulations. A friction framework has been developed including the effect of surface changes due to normal loading and straining the underlying bulk material. A fast and efficient translation from micro to macro modeling, based on stochastic methods, is incorporated to reduce the computational effort. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. A discrete model has been adopted which accounts for the formation of contact patches ploughing through the contacting material. To simulate metal forming processes a coupling has been made with an implicit Finite Element code. Simulations on a typical metal formed product shows a distribution of friction values. The modest increase in simulation time, compared to a standard Coulomb-based FE simulation, proves the numerical feasibility of the proposed method.

Friction in Sheet Metal Forming : Forming Simulations of Dies in Try-Out

The quality of sheet metal formed parts is strongly dependent on the tribology and friction conditions that are acting in the actual forming process. This paper focuses on the tribology conditions during early try-out of dies for new car models. The motivation for the study is that the majority of the forming simulations at Volvo Cars are performed to secure the die try-out, i.e. solve as many problems as possible in forming simulations before the final design of the die and milling of the casting. In the current study, three closure parts for the new Volvo V60 model have been analysed with both Coulomb and TriboForm friction models. The simulation results from the different friction models are compared using thickness measurements of real parts, and 3D geometry scanning data of the parts. Results show the improved prediction capability of forming simulations when using the TriboForm friction model, demonstrating the ability to accurately describe try-out conditions in sheet metal forming simulations.

A Multi-scale Friction Model for Sheet Metal Forming Simulations

This paper presents a multi-scale friction model for large-scale forming simulations based on the surface changes on the micro-scale. The surface texture of a material changes when it is subjected to normal loading and stretching. Consequently, the frictional behavior between 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 the change of the surface texture on the micro-scale. Statistical parameters have been introduced to make a fast and efficient translation from micro to macro modeling. The flattening models are validated by means of FE simulations on micro-scale and the implementation of the advanced macroscopic friction model in FE codes is discussed.

A multi-scale friction model framework for full scale sheet forming simulations

In this paper a numerical framework is proposed which accounts for the most important friction mechanisms. Static flattening and flattening due to bulk strain are accounted for by theoretical models on a microscale. 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 and the effect of ploughing and adhesion on the coefficient of friction is described by a micro-scale friction model. A brief theoretical background of these models is given. The flattening models are validated by means of FE simulations on microscale and the feasibility of the advanced macroscopic friction model is proven by a full scale sheet metal forming simulation.