Electrochemical machining assisted using pulsed electric fields, ultrasonic motion, and magnetic fields

Bradley, Curtis
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Samuel, Johnson
Walczyk Daniel F.
Chung, Aram
Chakrapani, Vidhya
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Mechanical engineering
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The electrochemical machining study presented in this thesis uses the novel testbed to reveal that phase-controlled waveform interactions between the three assistances affect both the material removal rate (MRR) and surface roughness (Ra) performance metrics. Experiments used a 7075 aluminum anode in an NaNO3 electrolyte with a 316 stainless steel cathode. The triad-assisted ECM case involving phase-specific combinations of all three high-frequency (15.625 kHz) assistance waveforms is found to be capable of achieving a 52% increase in MRR while also simultaneously yielding a 78% improvement in the surface roughness value over the baseline pulsed-ECM case. This result is encouraging because assisted ECM processes reported in literature typically improve only one of these performance metrics at the expense of the other.
Electrochemical machining (ECM) is a non-traditional machining process that uses an anodic dissolution electrochemical cell to preferentially dissolve a positively charged metal workpiece using a negatively charged tool, thereby creating a precise shape with a good surface finish. ECM is currently used to manufacture superalloy turbine blisks and medical implants as well as other complex profiles requiring superb surface finishes.
Finally, the thesis presents an ECM cell design methodology that spans two hierarchical time scales to effectively model magnetically assisted PECM. Total MRR is simulated in a large time scale model, on the order of the machining time duration (seconds). The diffusion layer thickness, simulated by a model on the order of the pulse duration, is then mapped to Ra by using an empirical function derived from the experimental data sets. The numerical models use characterizing minimum-order polynomials to estimate average conductivity, efficiency and peak current as functions of magnetic flux density and PECM voltage frequency. The models are then cross-validated by using 67% of the PECM test data for training the polynomial estimates. The average of the MRR and Ra model predictions are within 8% of the experimental value. The thesis concludes by presenting a microchannel use-case to demonstrate that capability of the proposed ECM cell design methodology to effectively navigate the complex ECM parameter space.
The research in this thesis presents a machining cell that combines pulsed electric waveforms, ultrasonic motion, and magnetic fields to form a novel, triad-assisted ECM cell. The pulsed electric field has a wide variety of asymmetric, bipolar configurations possible using two independent controlled power supplies. The workpiece is ultrasonically actuated from below the cell. Either a constant or sinusoidal magnetic field can be generated using an electromagnet or pair of permanent magnets, respectively.
August 2018
School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering
Rensselaer Polytechnic Institute, Troy, NY
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