Lubricant-free forming by affecting thermoelectric currents

Abstract

Cold metal forming belongs to the most important manufacturing processes for sheet metal mass products. To meet increasing requirements concerning part quality and economic efficiency, tool wear has to be minimized. Therefore, most of the forming processes are only feasible with lubricant application. Especially for processes with high tool load, such as blanking or embossing, adhesive wear is a key challenge due to early initiation and high wear rates. Despite the high significance and consequences, like a reduction of part quality and process reliability as well as an increasing risk of severe tool damage, wear-causing interactions are insufficiently understood. One known influencing factor is the temperature in the forming zone, which arises due to the dissipation of conducted forming work. Besides the direct impact on adhesive wear, this temperature rise leads to thermoelectric voltages and currents, whose influence on adhesive wear development has been neglected so far. For this reason, typical tool and sheet materials have been characterized for the first time with regard to their thermoelectric behavior, represented by the Seebeck coefficient. Together with experimental blanking and embossing investigations, basic correlations between process parameters, temperatures, sheet and tool materials, thermoelectric currents as well as adhesive wear could be derived. The findings obtained confirm a strong influence of the direction and strength of thermoelectric currents, which are determined by the difference between the Seebeck coefficients of tool and sheet material, on adhesive wear. Similar thermoelectric behavior of both materials in contact suppresses thermocurrents and resulted in an adhesive wear reduction of 74% during blanking. In addition, the external generation of a regulated current enables to influence thermoelectric currents in strength and direction. An external current adapted to the process reduced adhesive wear by 31%. In sum, two new methods of adhesive wear reduction, which are applicable in every metal manufacturing process, were investigated within this project. Furthermore, the knowledge gained improves the fundamental understanding of wear-causing interactions.