Selective thermally oxidised tool surfaces for dry deep drawing

Abstract

Within the scope of German Research Foundation (DFG) Priority Programme (SPP) 1676, this subproject deals with the research of a technology to substitute lubricants in the deep drawing process by selective thermally oxidised tool surfaces. In this context, different atmospheres and methods for selective oxidation of the tool surfaces were investigated. The oxide systems generated were characterised and analysed using highresolution analysis for examining and classifying their tribological properties [1]. With regard to the layer adhesion and the resulting measurements of coefficients of friction (COF), an α-Fe2O3 layer system was chosen, which proved promising for application in deep drawing processes. A special specimen geometry was developed that allows investigation of the tribological wear behaviour of the layer in the deep drawing process. This geometry was intended to mimic the conditions in a deep-drawing process with high cycle numbers of up to 5000 on a laboratory scale and to be suitable for the subsequent analysis methods [2, 3]. The wear specimens were examined on a developed wear test bench and, according to loading cases at dry deep drawing, tested with the sheet metal DP600 + Z under 90° deflection. [4, 5] Based on the results from the wear tests, a numerical FE model was developed capable of simulating the material wear as a function of the process parameters. This served as a numerical tool for predicting layer failure as well as for analysing wear critical areas on the deep drawing tools. In the further course, the heat treatment equipment was upgraded so that samples could be heated inductively in a batch process. This led to increased surface activation and more economical oxidation processes [6]. Since near-surface chromium carbides strongly influence and even prevent selective oxidation, a powder-metallurgically manufactured steel (PM) was also examined. Those surfaces featured chromium carbides in small quantities and to a lesser extent. The selective oxidation of these PM steel surfaces showed excellent results in terms of oxidisability compared to conventionally manufactured steel, as well as constant lower COFs [7]. Following the tribological wear investigations, FE analyses were used to create a modular tool concept enabling the installation of oxidised inserts in defined sections of the tool. After manufacturing, this modular tool was used to validate the applicability of the developed layer system as well as the numerical wear model [8]. In addition to the deep drawing steel DP600 + Z, the electrolytically galvanised deep drawing steel DC04 + ZE was used in these experiments [9]. Tests with both deep-drawing steels demonstrate the good deep-drawability in the dry state with oxidised mould inserts.00102