Investigation of hole roundness-error using different electrolytes in STED process

Shaped-Tube Electrolytic Drilling (STED) is an electrochemical dissolution-based process able to fabricate high aspect-ratio (AR) holes rapidly on the superalloys. The electrolyte flowing in the side gap between the tool's lateral surface and hole wall in the presence of an electric field during the entire process duration leads to non-uniform dissolution and stray-cutting. The localization of anodic dissolution mitigates the stray cutting, which contributes to the non-uniformity of micro-holes. The enhanced localized anodic dissolution is attainable by using neutral electrolytes mixed with the alkaline electrolytes that promote passivation on the surface of Inconel 718. The hole roundness error is crucial for the dimensional accuracy of micro-holes due to the overcutting constraints. Therefore, the present research investigates the feasibility of using electrolyte solutions of dissimilar anionic activity mixed with alkaline electrolytes to mitigate hole roundness error in high AR micro-holes. The effects of process parameters, such as applied voltage, tool feed rate, supplementary electrolyte concentration, and electrolyte entry temperature on the hole roundness error were studied to understand the dissolution behavior with different electrolyte solutions. The investigation results revealed that adding alkaline electrolytes in NaCl and NaNO3 based electrolyte solutions mitigate hole roundness error, whereas the KBr-based electrolyte solution performs unsatisfactorily.