The importance of microstructure in electrochemical jet processing

© 2018 Electrochemical jet processing (EJP) is an athermal technique facilitating precision micromachining and surface preparation, without recast layer generation. The role of the microstructure in determining machining characteristics has been largely overlooked. In this study, we show that in order to optimise EJP for a given material, fundamental material factors must be considered to ensure the desired near-surface response in terms of metallurgy, topography and dimensional accuracy. In this work, specimens have been prepared from the same feedstock material (brass, Cu39Zn2Pb), to appraise the role of microstructure in the determination of key removal characteristics, such as resultant topography, removal efficiency and form. Topography is shown to be highly dependent upon microstructure across large current density ranges, whereby the phase ratio is generally the dominant amplitude-defining material property, where preconditions with divergent ratios result in lower amplitudes. The microstructure, specifically the phase ratio, significantly changes the form, where predominantly single-phase conditions result in deeper and narrower features (up to 15% deeper compared with as-received condition). In addition, removal efficiency is greater (by 6%) at low current density for small grained dual-phase conditions, than for predominantly single-phase, due to erosion complementing anodic dissolution. Mechanisms are discussed for these removal phenomena and used to inform industrial practice

Direct Writing Unclonable Watermarks with an Electrochemical Jet

Counterfeit parts result in significant losses per annum and are often dangerous, therefore they represent a serious concern for manufacturers and end users alike. Easily written but unclonable watermarks undermine the proposition of the counterfeiter. Here, a rapid electrochemical jet engraving routine is presented to encode robust materials with self-organized dendritic structures at length scales that can be imaged with a smartphone. Surface defects act as stochastically distributed seeds from which discrete pitting events can be propagated by translating the electrochemical field. While the vascular pathways can be directly written at the macro scale, the formation and propagation of microscale dendritic arms is chaotic, caused by the implicit randomness of the defect seeds and the supply of ions to the surface. The latter is confounded by random perturbations in the flow condition. Each engraved dendrite is unique, stable at high temperature (>500°C) and can be subjected to rapid image recognition to allow individual mark identification at any point during part production and delivery, or through part lifetime

Ambient grain orientation imaging on complex surfaces

Crystal orientation imaging is generally confined to the laboratory, typically following destructive sectioning, with most current techniques reliant on electron-material interactions that require a vacuum. This information is gathered in a manner that requires careful planning, however a more desirable approach would allow the manufacturer to acquire this data non-destructively at the point of manufacture, with little or no time penalty. We show that coupling a numerically controlled etching method to topographical data processing can be used to spatially map grain orientations over planar and non-planar surfaces. Our method allows the construction of large area orientation maps (≈400 mm 2) in agreement with electron backscatter diffraction datasets. We have characterised spatial and angular resolution limits for the technique, which are correlated to length scales of microscale etch surfaces and our ability to measure their geometries. This approach has the potential to augment materials processing technologies, where resultant microstructures require strict control in order to guarantee through-life integrity

Direct-writing by active tooling in electrochemical jet processing

Recent innovations in electrochemical jet processing have caused step changes in process flexibility and precision. However, utilisation of these innovations requires the development of new machine tool technology. Presented here is a new methodology enabling the exploitation of highly customisable energy density profiles regardless of toolpath vector whilst minimising any error from the intent profile. A further approach is defined whereby active tooling allows the energy density profile to be modulated as a function of position within the toolpath, giving rise to dynamic feature creation. Adoption of this methodology allows a new design freedom within electrochemical jet processes

Unveiling surfaces for advanced materials characterisation with large-area electrochemical jet machining

Surface preparation for advanced materials inspection methods like electron backscatter diffraction (EBSD) generally involve laborious and destructive material sectioning and sequential polishing steps, as EBSD is sensitive to both sample topography and microstrain within the near-surface. While new methodologies, like focussed ion beam and femtosecond laser milling are capable of removing material in a layer-by-layer manner to enable the construction of tomographic datasets within the electron microscope, such techniques incur high initial capital cost for slow removal and reconstruction rates. In this study, ambient condition electrochemical slot jets are applied to rapidly etch (e.g. 31 s) large surface areas (e.g. 160 mm 2) at controlled depths (e.g. 20 µm) with no in-process monitoring. Unveiled surfaces are conducive to measurement by EBSD (raw index rates between 75-95%), despite topographic anisotropy arising both from the process and the material. The mechanisms of topography formation during dissolution under the slot jet are analysed and understood. It is proposed that this slot jet method can be applied to create measurement surfaces for analysis with optical-based microstructural measurement routines reliant on topography and directional reflectance, at a significantly lower cost and time intervention than electron beam-based analysis methods

Crystallographic texture can be rapidly determined by electrochemical surface analytics

Orientation affects application-defining properties of crystalline materials. Hence, information in this regard is highly-prized. We show that electrochemical jet processing (EJP), when coupled with accurate metrological appraisal, can characterise crystallographic texture. Implementation of this technique allows localised dissolution to be anisotropic and dependent on etch-rate selectivity, defined by the crystallography. EJP therefore, generates complex, but characteristic topographies. Through rapid surface processing and analysis, textural information can be elucidated. In this study, samples of polycrystallineAl and Ni have been subjected to EJP, and the resulting surfaces analysed to generate three-colour orientation contrast maps. Comparison of raw data acquired through our method with prior electron back-scatter diffraction data shows broad correlation and assignment (68% on a pixel-by-pixel basis), showcasing rapid large-area analysis at high efficiency

Advancing electrochemical jet machining through process monitoring and control

Electrochemical jet machining (EJM) is a non-conventional surface texturing process based on the localised anodic dissolution of a metallic work piece under the influence of an impinging electrolyte jet and an applied electrical potential. Although the surface modification of metallic components through EJM has been widely researched, there is a limited understanding of the electrical response of the system and unexploited opportunities to utilise this for process monitoring and integrated surface measurement. The research presented in this thesis addresses this knowledge gap and investigates the development of on-machine metrology and advanced process control through electrical measurements of the electrolyte jet. The application of a high-frequency electrical excitation allowed the measurement of work piece surface topography through mapping jet resistance as a function of spatial coordinates without affecting the surface. The performance of this novel measurement system was evaluated by comparison against established measurement techniques and an analytical model describing the response was proposed. This was further applied to investigate the resistance during machining, and it was demonstrated that the removal depth can be correlated to the measured resistance in-process. This provides a route for real-time monitoring of the live process and implementation of closed-loop control. A method for automated tracking of the geometry of the work piece during machining based on the resistance was developed. The resolution of electrochemical jet machining and the jet-based surface characterisation methods presented in this thesis is constrained by the diameter of the jet. To address this limitation, a novel end-effector was developed that can constrict the electrolyte jet through the hydrodynamic effect of flow focusing, and its machining and measurement performance was characterised. It was demonstrated that the jet diameter can be reduced 79% with corresponding 54% reduction of machine kerf width. The tool size can be continuously varied in-process through a simple variation of process parameters as part of the machine control programme

Advancing electrochemical jet machining through process monitoring and control

Electrochemical jet machining (EJM) is a non-conventional surface texturing process based on the localised anodic dissolution of a metallic work piece under the influence of an impinging electrolyte jet and an applied electrical potential. Although the surface modification of metallic components through EJM has been widely researched, there is a limited understanding of the electrical response of the system and unexploited opportunities to utilise this for process monitoring and integrated surface measurement. The research presented in this thesis addresses this knowledge gap and investigates the development of on-machine metrology and advanced process control through electrical measurements of the electrolyte jet. The application of a high-frequency electrical excitation allowed the measurement of work piece surface topography through mapping jet resistance as a function of spatial coordinates without affecting the surface. The performance of this novel measurement system was evaluated by comparison against established measurement techniques and an analytical model describing the response was proposed. This was further applied to investigate the resistance during machining, and it was demonstrated that the removal depth can be correlated to the measured resistance in-process. This provides a route for real-time monitoring of the live process and implementation of closed-loop control. A method for automated tracking of the geometry of the work piece during machining based on the resistance was developed. The resolution of electrochemical jet machining and the jet-based surface characterisation methods presented in this thesis is constrained by the diameter of the jet. To address this limitation, a novel end-effector was developed that can constrict the electrolyte jet through the hydrodynamic effect of flow focusing, and its machining and measurement performance was characterised. It was demonstrated that the jet diameter can be reduced 79% with corresponding 54% reduction of machine kerf width. The tool size can be continuously varied in-process through a simple variation of process parameters as part of the machine control programme

Precision enhanced electrochemical jet processing

Through electro-physical modification of the electrode gap in electrochemical jet techniques, precision has been shown to be greatly increased. Repeatable kerf widths approaching the diameter of the nozzle are demonstrated and the individual contributing effects are quantified across energy density and length scales. This allows the feature resolution achievable through electrochemical jet processing to be comparable to other surface structuring techniques, albeit with zero thermal loading of the surface. This is applied to demonstrate the machining of complex geometric features, not previously produced by electrolyte jet techniques