The electrochemistry of electrochemical machining

• Main Author: Howarth, Paul
• Published: University of Edinburgh 2003
• Subjects: 671.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669291

Electrochemical machining is a process that has the potential to machine complex shapes at high production rates. However, the expansion of ECM in industry has been impeded by the iterative trial and error approach that is often required to generate process parameters for any one machining set-up. This arises due to the incompleteness of models used to describe the complex physical, chemical and hydrodynamic parameter interdependencies. Such interdependence results in non-ideal effects that distort the transfer geometry between the tool shape and the required workpiece shape. The aim of this thesis is to address some of these problems, working towards developing a predictive stoichiometric model for the chemical interdependencies of ECM that can be applied to advanced alloys and therefore further the use of ECM in industry. This has been achieved by extending the planar tool system to a segmented tool arrangement capable of measuring the chemical parameters (dissolution valencies, n, and overpotentials, V0) along the flow path length. In addition, electrolyte sampling tubes have also been incorporated into this arrangement enabling the electrolyte to be sampled along the flow path length to determine the conductivity, K and the pH. This system has been applied to the stainless steel group of alloys. A systematic study of a variety of stainless steels [SS3 16, SS4 10, Jethete (J), Duplex (D) and Super Duplex (SD)] has been performed, measuring their electrochemical machining (ECM) characteristics in chloride and nitrate electrolytes. Theoretical current-time analysis using the segmented tool was used to determine the dissolution valency, n, and overpotential, V0, along the flow path length. Electrolyte samples from the interelectrode gap, taken at intervals along the flow path, and the bulk were analysed for conductivity, pH and the soluble products characterised by visible absorption spectroscopy. The insoluble products were analysed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicate that the ECM dissolution characteristics of stainless steels are controlled by the surface oxide structure, which is primarily determined by the chromium content of the steel. An interesting "runaway current" phenomena was observed when machining J with nitrate electrolyte where dissolution valencies of n = 9 were observed. This phenomenon was also observed when machining pure iron. The effect was found to be caused by a short circuit reaction occurring in the interelectrode gap resulting in inefficient ECM. This short circuit problem was stopped either by using a chloride/nitrate mixed electrolyte or by the addition of a complexing agent such as EDTA. Therefore, an understanding of the dissolution chemistry has proved vital to the successful application of ECM to industrially important modern alloys.