Experimental analysis and modeling of friction in sheet metal forming considering the influence of drawbeads

In sheet metal forming of automotive body parts, high quality standards are achieved by designing the material flow in the blankholder area. To reduce time and cost of the tool development, virtual methods are commonly used. Therefore, an accurate friction modeling is necessary to predict the material flow in the blankholder area correctly. The blankholder of industrial forming tools usually consists of a drawbead and an area with surface pressure. In this study, the influence of the drawbead on the friction is analyzed experimentally and considered in the simulation. The friction coefficient is determined before and after the drawbead pass in dependence of the normal pressure. A flattening of the surface peaks due to surface pressure and a roughening due to plastic strain are found as the main influencing factors on the friction after drawbead pass. In the simulation the friction is modeled based on the experimental results employing an enhanced method considering the influence of the drawbead. These modeling approaches are compared with the friction model of the commercial software TriboForm and the experimental results. For this purpose, the predictive accuracy of the restraining force of a drawbead with different pressure distributions on the blankholder is evaluated using a strip tensile test. This evaluation is further carried out for a different strain state at a cup drawing test. By considering the influence of the drawbead on the friction coefficient a significant increase in the predictive accuracy of the simulation can be achieved.