The efficiency of material removal using electrochemical machining (ECM) is highly dependent on the initial rim zone modifications of the material to be processed. The influence of the rim zone modifications, such as topography and microstructure, on ECM, is investigated on ferritic–pearlitic 42CrMo4 steel by experiment and simulation. 42CrMo4 steel in two different premachining states—ground and electric discharge machined (EDM)—is subjected to a subsequent surface finishing by ECM in sodium nitrate solution. Before and after ECM, the topography and microstructure are examined using scanning electron microscopy, X‐ray diffractometer, and topography analysis methods. The electrochemical properties of the material are determined by potentiodynamic polarization. The efficiency of surface finishing by ECM is quantified by mass spectroscopic analysis (inductively coupled plasma mass spectrometry) of the process electrolyte and related to the rim zone modifications by simulation. The results reveal that the efficiency of material removal during ECM is higher for EDM than for ground 42CrMo4. This is attributed to an increased roughness of EDM 42CrMo4 and to the unfavorable electrochemical properties of the cementite phase in ground 42CrMo4.
The oxidation mechanism of metals depends, among other factors, on the surface integrity. The surface and rim zone properties are often determined by the manufacturing process that was used to machine the material. Laser chemical machining (LCM) is a manufacturing process that uses laser radiation as a localized and selective heat source to activate a chemical reaction between an electrolyte and a metallic surface. The objective of this work is first to investigate how different LCM processes affect the rim zone properties of 42CrMo4. For this purpose, the surface chemistry is analyzed by EDS and XPS, phases and residual stresses are determined by XRD, and the morphology is investigated by SEM. Second, the influence of these modified rim zones on the oxidation properties of the steel at 500 °C in air is to be demonstrated in oxidation tests by in situ XRD and subsequent SEM/EDS investigations. A decisive influence of the oxides formed on the surface of 42CrMo4 during LCM in different electrolytes (NaNO3 solution and H3PO4) at two different laser powers on the high-temperature oxidation properties was demonstrated. These oxides were supposed to act as nucleation sites for oxide layer formation at 500 °C and led to an overall increase in oxide layer thickness after high-temperature oxidation compared to non-LCM-processed surfaces.
The present work will introduce a novel two-step etching procedure for grain analysis of intermetallic iron aluminides. A grain boundary etching technique from the literature will be complemented by subsequent grain-contrast etching to visualize different grains in polarized light. The use of an XY-table for the acquisition of MosaiX images also enables grain analysis over the entire cross-section of the sample.
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