Friction modelling of coated sheets for forming applications
Due to the COVID-19 crisis measures the PhD defence of Meghshyam Shisode will take place (partly) online in the presence of an invited audience.
The PhD defence can be followed by a live stream.
Meghshyam Prabhakar Shisode is a PhD student in the research group Nonlinear Solid Mechanics (NSM). His supervisors are prof.dr.ir. A.H. van den Boogaard and Dr. J. Hazrati from the Faculty of Engineering Technology (ET).
The main challenges in the sheet metal forming industry are zero defect production, reducing the cost and lead time of new products. Finite element (FE) analysis is a standard tool for studying the formability of sheet metals in forming processes. Friction is a determining parameter which influences the simulation accuracy and reliability of these analyses. In the past, it has been verified that the coefficient of friction is not constant for given contacting surfaces during forming but varies with local contact conditions. Therefore, in forming processes the coefficient of friction is a transient phenomenon and changes from location to location. Nowadays, textured zinc coated steel sheets (GI) are widely used in forming applications to improve formability, corrosion resistance, durability and paint appearance. Therefore, an accurate description of friction for coated sheets is required in FE formability analyses.
Friction is a local phenomenon which depends on the local contact conditions at the tool--sheet metal interface. The contact condition is defined mainly by the real area of contact. Therefore, understanding asperity flattening mechanism is vital for a reliable modelling of friction. In a typical deep drawing process, the sheet metal is subjected to normal load, sliding and stretching. In this thesis, asperity flattening models are presented for normal loading, sliding and combined normal loading and stretching. The models account for the coating and substrate material behaviour and measured surface topography of the rough surface. In this study, an experimental setup is developed to investigate the effect of combined normal load and bulk strain on real area of contact. The experimental results are used to validate the normal load flattening model and to calibrate model parameters of combined normal load and bulk strain flattening model. The experimental investigation revealed that the surface deformation depends not only on the material behaviour and coating thickness but also on surface topography.
In deep drawing processes, the tribological system is defined by lubricant, surface topographies of tool and sheet metal and their materials. A boundary lubrication or mixed lubrication regime may occur at the tool--sheet metal contact depending on the lubricant amount, tool and sheet surface topographies and other process parameters. If the lubricant amount is not enough to fill the valleys of contacting surfaces, a boundary lubrication condition exists in which the total contact load at the interface is carried by asperity contacts. On the other hand, if the lubricant amount is enough, a hydrodynamic pressure may develop in the lubricant which shares some part of the contact load. Therefore, the coefficient of friction in the mixed lubrication regime is lower than in the boundary lubrication regime. In this thesis, the multi-scale friction model in the boundary and mixed lubrication regimes are presented for coated sheets. Measured surface topographies of sheet and tools are used as the input. The local contact locations and their sizes at the tool-sheet interface are determined for a given loading condition using the flattening models. The friction force at each contact patch is determined by adapting a single asperity micro-scale ploughing model from which the overall coefficient of friction is determined. A recently developed 3D elliptical paraboloid shaped single asperity ploughing model which accounts for 3D asperity shape, size, and orientation is used. The boundary friction model is implemented in an in-house FE code DiekA for forming simulations. The boundary friction model is coupled with the average Reynolds equation to describe friction in the mixed lubrication regime. The hydrodynamic pressure developed in the lubricant is determined by solving the average Reynolds equation in the FE domain. To account for the influence of surface topographies, flow factors are determined for measured surface topographies of tool and sheet which are used in the average Reynolds equation. Deep drawing experiments are performed to demonstrate the validity of the boundary and mixed lubrication friction model. The comparison of experiments and simulations shows good agreement.
Sheet surface texturing is critical in the steel industry since it influences the tribological performance, formability, and paint appearance of the sheet surface. In forming processes, variations in tribological systems due to tool wear and applied lubricant amount can lead to undesirable variation in friction behaviour. Designing a surface texture can be an option to control and improve the robustness in friction behaviour. Surfaces textured by electro-discharge textured (EDT) rolls are widely used in industry, resulting in semi-deterministic surface texture. Similarly, laser textured (LT) surfaces are gaining increasing attention due to its better control on resulting surface texture. In this thesis, the influence of surface texture parameters on friction behaviour is studied. For this purpose, surface generation algorithms are implemented to generate EDT and LT surfaces. The effect of change in tool roughness and lubricant amount on the coefficient of friction is studied to determine the surface texture parameters which result in the least variation in friction coefficient. Furthermore, a component level case study is presented to demonstrate the relation between surface texture and robustness in friction coefficient. The results show that the developed friction model can be used to design surface textures for forming processes under given process parameters for robust friction behaviour.