Abstract
Temperature rise in cold stamping processes due to frictional heating and plastic deformation of sheet metal alters the tool-sheet metal tribosystem. This is more prominent in forming advanced high strength steels and multi-stage forming operations where the temperature on the tool surface can rise significantly. The rise in temperature directly affects the friction due to break down of lubricant, change in physical properties of tribolayers and material behavior. This can result in formability issues such as workpiece-splitting, etc. Therefore, it is important to account for temperature effects on friction in sheet metal forming analyses. In this study, the temperature effect was included in a micromechanics-based friction model which allows calculation of local friction coefficients as a function of contact pressure, bulk strain and relative sliding velocity. The temperature influence on friction was introduced through material behavior of sheet metal, viscosity of lubricant and shear strength of boundary layer in the micromechanics-based model. The model validation has been done by comparing the calculated fractional real contact area with the experimental results. The model can be used in formability analyses and to predict optimum stamping press parameters such as the blank holder force and the press speed.
Highlights
Cold stamping processes, such as blanking, bending, drawing, flanging, etc., offer a variety of possibilities to form the sheet metal into a desired net shape in a cost-effective way
Results of fractional real contact area To further check the reliability of the model, the fractional real area of contact predicted from finite element (FE) analyses are compared with the real area of contact values measured in experiments
The temperature effect has been introduced through material behavior of sheet metal, viscosity of lubricant and shear strength of boundary layer in the micromechanics-based model
Summary
Cold stamping processes, such as blanking, bending, drawing, flanging, etc., offer a variety of possibilities to form the sheet metal into a desired net shape in a cost-effective way. To enable more accurate metal forming simulations, a micromechanics-based friction model has been developed for isothermal conditions [5, 6]. Defects in cold stamped products are commonly found during the start-up of the production line. This has been mainly attributed to an increase of temperature and a change of friction for an increased number of stamped parts. The effects of temperature on frictional behavior of sheet metal-tool contact system are investigated. These effects are implemented in an existing micromechanics-based friction model. The enhanced model accounts for the temperature effects on the sheet metal material behavior, lubricant viscosity and boundary layer shear strength
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