Accelerated Electrochemical Machining Tool Design Via Multiphysics Modeling
Authors: Skinn Brian , Hall Timothy D , Snyder Stephen , Rajurkar K. P. , Taylor E. J. 
Published: February 2020

Electrochemical machining (ECM) is a manufacturing technology that allows metal to be precisely removed by electrochemical oxidation and dissolution into an electrolyte solution. ECM is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. In ECM, the workpiece is the anode and the tool is the cathode in an electrochemical cell; by relative movement of the shaped tool into the workpiece, the mirror image of the tool is “copied” or machined into the workpiece. Figure 1 shows a schematic and photograph of a shaped ECM tool (cathode) and the resulting part (anode) surface after machining. Compared to mechanical or thermal machining processes where metal is removed by cutting or electric discharge/laser machining, respectively, ECM does not suffer from tool wear or result in a thermally damaged surface layer on the workpiece. Consequently, ECM has strong utility as a manufacturing technology for fabrication of a wide variety of metallic parts and components, and includes machining, deburring, boring, radiusing and polishing processes. ECM provides particular value in that application is straightforward to high strength/tough and/or work-hardening materials such as high strength steel, chrome-copper alloy (C18200), nickel alloy (IN718), cobalt-chrome alloy (Stellite 25) and tantalum-tungsten alloy (Ta10W), since the material removal process involves no mechanical interaction between the tool and the part. A variety of commercial and military production applications are envisioned as well suited for ECM techniques.

One notable difficulty with ECM, common to a variety of manufacturing operations, is an inability to predict a priori the tool and process parameters required in order to satisfy the final specifications of the fabricated part. In this talk, Faraday will present preliminary results from a Phase I SBIR program aiming to demonstrate the potential for a phenomena-based design platform to predict optimal ECM tool shape using commercially available multiphysics simulation software. This capability is anticipated to dramatically shorten the process/tooling development cycle, eliminating much or all of the iterative prototyping necessary in the absence of a predictive tool. The validation argument will be presented via comparison of simulation results with data from parallel ECM experiments. The initial validation work encompasses a small subset of experimental parameters: electrolyte salt (“active” NaCl vs “passive” NaNO3) and electrolyte flow/tool geometry (“cross flow” past a solid tool, and “through-flow” in a tubular tool). The initial feasibility demonstration of the tool design platform will include only ECM based on potentiostatic direct currents, constant tool advancement rates, and simulations including only primary current distributions. Reasonable agreement between the simulated and experimentally derived profiles is observed. Eventually, however, the goal is to extend the model to other modes of ECM processing, including: 1) pulse-current ECM (PECM), where the tool is withdrawn from the workpiece during pulse current off-times to flush the gap; and also 2) pulse/pulse-reverse ECM (P/PR ECM) in metal-solubilizing electrolytes, where the tool gap is maintained relatively constant. An approach to modeling the optimal inter electrode gap dimension during PECM has been reported by Rajurkar and collaborators [[1]]; to our knowledge no modeling has been performed on constant-gap pulse/pulse-reverse ECM in solubilizing electrolytes. Faraday has developed and patented novel approaches to ECM based on pulse reverse currents [[2],[3],[4]] that do not require complicated electrolytes and results in improved surface finishes and better process control; application of the predictive model to these approaches is anticipated to provide substantial gains in both manufacturing logistics and economics.

The authors acknowledge the financial support of U.S. Army Contract No. W15QKN-16-C-0070.

References

[[1]] B. Wei, K.P. Rajurkar, S. Talpallikar. “Identification of Interelectrode Gap Sizes in Pulse Electrochemical Machining.” J. Electrochemical Society
144(11): 3913-19, 1997.

[[2]] C. Zhou, E.J. Taylor, J. Sun, L. Gebhart, R. Renz. “Electrochemical Machining using Modulated Reverse Electric Fields.” U.S. Patent No. 6,402,931, issued 11 June 2002.

[[3]] E.J. Taylor. “Sequential Electromachining and Electropolishing of Metals and the like using Modulated Electric Fields.” U.S. Patent No. 6,558,231, issued 6 May 2003.

[[4]] E.J. Taylor, M. Inman. “Method and Apparatus for Pulsed Electrochemical Grinding.” U.S. Patent No. 9,403,228, issued 2 August 2016.

Figure 1