Macro- and microstructuring of deep drawing tools for dry forming processes

Abstract

In forming processes, lubrication is needed to reduce friction and wear occurring at the contact areas between the work-piece and the tool. This is provided by separating these surfaces as well as transporting abrasive wear out of the sliding interface. With high interest in waste avoidance and efficient use of resources, today’s industry aims for substituting these lubricants. Thereby, deposition of coatings or local surface structuring offer a different way to reduce friction in forming processes. For instance, this can be reached by using different hard coatings as well as by utilizing manufacturing methods capable to control and reduce the contact area in the tribological system. In this project, the flange area of a deep drawing tool was macro-structured by turning-process (rotational symmetric geometry) and by milling-process (non rotational-symmetric part) to decrease the contact area of the work-piece up to 94% during the forming process. Also, the drawing edge radius of the tools was coated with different types of hydrogen-free tetrahedral amorphous carbon layers, e.g. with a sp³-ratios between 60% and 70%. Together with the subsequently introduced micro-features on the ta-C layer, employing the innovative method of Direct Laser Interference Patterning (DLIP), a further reduction of the contact areas was possible as well as to produce local rehybridization of the ta-C material. Also, high-throughput surface texturing of semi-finished sheet metals by DLIP process was performed, for reducing friction by minimizing also the contact area with the deep drawing tool. Forming of test strips showed the influence of the macro-structured flange area by reducing the friction force down to 75%. In addition, different modelling tools were developed for calculating both the process window and to determine the generation of wrinkles during deep drawing with macro-structured tools. The macro-structured deep drawing tools could also enlarge the process limits by increasing the stability against wrinkling of the sheet metal during the drawing process. The ta-C coating on the die radius without application of lubrication was capable to reduce the friction coefficient up to 20% compared to uncoated lubricated conditions. Performed tensile-bend tests on micro-structured ta-C coated cylinders also demonstrated the potential to reduce friction, as well as an excellent capability to reduce wear compared to the standard conditions.