Modeling of Rough Contact Interfaces with Trapped Compressive Liquid Pockets
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Boundary and mixed lubrication are usually used to reduce friction in sheet metal forming. In the boundary lubrication regime load is mainly carried by contacting asperities. Therefore, the tribological problem can be solved in the dry case using a geometrical correction of the real contact area [1]. On the other hand, when the lubricant quantity is not small enough to be neglected nor large enough to be assumed as a continuous layer, a compressible lubricant can contribute to the pressure and deformation of the solid. In this study, a fully coupled model for trapped compressible lubricant has been developed to predict the effect of partial lubrication on the friction coefficient during the aluminum deep-drawing process. A boundary element method (BEM) code in the Fourier space is used to solve the rough contact problem [2]. Moreover, the plastic behavior of the deformable solid is considered in the model using plastic saturation method. Once the tool reaches the liquid, the change of liquid depth induces pressure on the solid/liquid contact. A nonlinear loop iterates to find the proper liquid pressure and liquid volume change, which satisfy force equilibrium on dry and fluid contact. Contact area, distribution of contact pressure, and apparent coefficient of friction have been investigated at various normal pressures, different lubricant densities, and lubricant distribution. The model can predict the friction coefficient in the presence of trapped liquid pockets. It has been seen that lubricant distribution and volume, even in small amounts, have a non-negligible impact on the friction coefficient.
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