Enhancing Surface Integrity of Micro-Holes Machined on Inconel 718 using Shaped Tube Electrochemical Machining via Hydroxide Mixing

Abstract

This research explores enhancing surface integrity of high aspect ratio (AR>5) micro-holes machined on Inconel 718 using Shaped Tube Electrochemical Machining (STEM) by mixing limited amount of metal hydroxides in the different electrolyte solutions. Typically, STEM employs hazardous acid solutions. These solutions cause machine corrosion, environmental risks, and harm to operators. This research aims to explore eco-friendly and safe alternatives. The efficacy of adding NaOH or KOH to eco-friendly neutral aqueous solutions of NaCl, KBr, and NaNO3 on machined surfaces is expounded. We assess electrolyte mixing parameters, including concentration and composition, and explore the effects of varying electrolyte temperature, voltage, and tool feed rate. Surface assessment uses SEM, EDX, XRD, and profilometry. Results reveal that adding hydroxides to neutral salt solutions improves surface roughness and reduces pitting. Hydroxide ions aid in dissolving oxide films during STEM of Inconel 718. Under specific conditions (9 g l−1, 18°C, 8 V, 0.8 mm min−1), NO3│OH yielded the lowest Ra (1.64 μm), while Br│OH resulted in the highest Ra (2.14 μm) with pitting, cracks, and deposits. NO3│OH exhibited a 25% decrease in Ra when a hydroxide concentration rise from 3 g l−1 to 15 g l−1. This research supports eco-friendly STEM for improved surface quality without process performance compromise.

On Performance Enhancement of the STED Process Using Modified Tool Electrode

“…Using a neutral to alkaline electrolyte solution was motivated by the benefits encountered in the form of hole wall surface morphology while machining Inconel 718. 21 The hole wall surface obtained from the STED process after machining with a modified tool shows no major defects, as seen from the FESEM micrograph in Fig. 13.…”

“…5,19 It is difficult to achieve tool feeding rates above 1 mm min −1 while using micro-tools due to the low material dissolution rates associated with micro tools. [19][20][21] The difficulty also arises from the frequent shortcircuiting between the tool and the work sample when setting the TFR above 1 mm min −1 , which deteriorates process productivity. Thus, sustaining the STED process with minimal short-circuiting and tool feed rates above 1 mm min −1 to obtain acceptable quality high aspect-ratio holes poses problems that require process tweaking.…”

“…[22][23][24] The electrolyte type combination and concentration were selected considering the merits of mixing different electrolytes on the electrochemical dissolution behaviour of Inconel 718. 21,25,26 The range of process parameters, such as applied voltage and electrolyte concentration (EC), were selected based on literature and pilot experiments. 13,20 The parametric ranges and the values for the input variables corresponding to the responses are shown in Table I.…”

Editors' Choice—Improving Quality of EDMed Micro-Holes on Titanium via In Situ Electrochemical Post-processing: A Transient Simulation and Experimental Study

“…These side reactions may involve gas evolution, changes in local pH, or other factors affecting machining precision. 7 Regions experiencing sustained high current density may undergo more aggressive dissolution, resulting in irregularities in the hole's shape. As a consequence, the achieved dimensional accuracy differed from the expected outcome.…”

“…Non-contact micromachining techniques, commonly employed for fabricating miniaturized holes, encompass Electrochemical Machining (ECM) 6 and Electric Discharge Machining (EDM). 7,8 ECM operates on the principle of anodic dissolution, resulting in smooth surfaces without thermal damage, residual stresses, burrs, or tool wear. However, the challenge arises when machining materials like titanium and its alloys due to the formation of impervious and stable oxide films.…”