Wear effects on coated tools during the alternating forming of case-hardening steel and aluminium wrought alloy

A major factor for a high economic profitability in industrial bulk metal forming is the material-specific wear of the forming tools, which has a significant influence on process reproducibility. Particularly in Tailored Forming, in which hybrid material combinations are processed, narrow and sensitive process windows are characteristic. During impact extrusion of joined hybrid semi-finished workpieces made of steel and aluminium, the extrusion shoulder is subjected to combined wear phenomena. Adhesion and abrasion are superimposed, whose interactions lead to process fluctuations and premature failures in the absence of sufficient wear resistance. To investigate the yet unknown combination of the interactions, upsetting tests are carried out in cycles of 100, in which steel (AISI 5120H) and aluminium (AA-6082) are hot-formed alternately. In addition to tempered and nitrided tools, duplex coatings such as chromium nitride and titanium aluminium nitride are used to evaluate the potential of reducing the occurring wear phenomena through surface engineering methods. After the first cycles, the tools exhibit needle-shaped adhesion marks consisting of aluminium, which were subsequently transferred primarily to the steel workpieces during upsetting. Since adhesion is more present at the tool edge, an increased adhesive tendency with longer flow paths is observed. Due to the improved tribological conditions, duplex coatings minimise the wear volume on the tool and potentially on the formed workpiece with short contact times.