Dry forming of low alloy steels by direct impact extrusion using a self-lubricating tool coating and structured workpieces

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Due to the ecological, economic, and legislative reasons, an exclusion of the lubricants during the full forward extrusion (VVFP) is demanding. This leads to an increased friction and thus, to an increased wear in the tribosystem. Therefore, the tribosystem has tobe adapted by a modification of the workpiece and the tool. This is possible by means of a structuring of the workpiece surface and the coating of tools. In the first project phase, the influence of structured workpiece surfaces and self-lubricating tool coatings in system (Cr,Al)N + XS2 (X = Mo, W) on the frictional shear stress between the workpiece and the coated tools was investigated. The results were transferred to a single-die dry VVFP process in the second project phase. However, it is not possible to transfer the findings of the VVFP to the multi-dies VVFP. This is due to the significantly increased and considerably different tribological and mechanical loads in multi-dies VVFP compared to the single-die process. Another challenge involved in the multi-dies dry VVFP is the varying tribological boundary conditions along the longitudinal axis of the tool. Furthermore, due to the longitudinal varying tribological conditions in the two-dies tool, it is still unclear, whether and to what extent the existing approach of coating a not segmented, mono-block tool is sufficient. The overall objective of the third project phase is thus, to develop an explanatory model of the dry multi-dies VVFP, which describes the wear behavior of the tribosystem, consisting of the workpiece with a structured surface and the tool with a self-lubricating coating as a function of the temporal and local tribological boundary conditions. For this purpose, the tribological boundary conditions as well as the tool wear in multidies dry VVFP will be examined under consideration of selected surface integrities by means of experimental and simulativeinvestigations at the Laboratory for Machine Tools and Production Engineering (WZL). At the Institute of Surface Engineering (IOT), an adjustment of the existing self-lubricating hard coating (Cr,Al)N+XS2 (X = Mo or W), deposited using the hybrid technology dcMS/HPPMS (direct current- / High Power Pulsed Magnetron Sputtering), with respect to the varying tribological boundary conditions along the longitudinal axis of the tool will be investigated. Here, two coating concepts will be investigated, the coating of the not segmented, mono-block tool and the coating of the axially segmented tool. The more economic concept will be subsequently identified using the field tests. Within a close cooperation between WZL and IOT, it will be possible for the first time to conduct an economic dry metal forming in a multi-dies full forward extrusion.