Surface Evolution and Lubricant Distribution in Deep Drawing

Abstract

Deep drawing is one of the most important processes applied in industrial production. Here the Finite-Element-Method (FEM) is an important tool in the development and optimization process. One aspect to optimize simulations is to consider real friction behavior. Thus the friction phenomenon has to be describable. In addition to contact normal pressure and velocity the surface topography and the lubricant amount have a great influence on friction. This paper illustrates the influence of surface evolution in real, inhomogeneous processes on the lubricant distribution. For this a rectangular cup with four different corner radii is used to evaluate local surface topographies and lubricant amounts in deep drawing. The lubricant amount is measured by fluorescence technique and the surface topography is evaluated by a confocal white-light microscope. Due to hydrodynamic effects the lubricant is squeezed out and displaced to adjacent regions. Further hydrostatic pressures built up in closed lubricant pockets force the lubricant to stay in the forming zone to bear a part of the load. In free forming zones without contact between the sheet and tool the surface roughens due to grain dislocations in the microstructure. This paper also presents the results of lubricant distribution and surface evolution by varying the initial lubricant amounts and drawing depth. It can be recognized that the different corner radii of the rectangle cup have a great influence on the surface evolution and lubricant distribution. Moreover it can be clearly seen that surface parameters correlate with the lubricant amount. By means of the described evaluation it is also possible to correlate these values with load histories consisting of contact pressures and strain evolution, evaluated in FEM. All the results contribute to a better understanding of the friction behavior in deep drawing and point out the inhomogeneous character of friction.