INVESTIGATIONS ON SHAPED TUBE ELECTROLYTIC DRILLING PROCESS

Abstract

Shaped Tube Electrolytic Drilling (STED) is an advanced adaptation of the Electrochemical Machining process. It was specifically developed to drill holes with a high depth to diameter ratio in conductive materials that are difficult to machine by traditional machining processes. In the ECM-based processes, the material removal takes place by anodic dissolution phenomenon atom by atom due to charge transfer between the tool and work sample. The work sample is the anode, and the tool serves as the cathode while the electrolyte solution keeps flowing between the two acting as a bridge to complete the electrical circuit for charge flow. The STED process utilizes a tubular tool through which the electrolyte is supplied to the machining zone i.e., the inter-electrode gap. Therefore, the incoming fresh electrolyte solution from the tubular tool sweeps away the reaction byproducts and exposes the new layer of anodic material to the dissolution process. The accumulation of machining byproducts in the form of sludge in the inter-electrode gap hampers the machining rate. Hence, the use of acidic electrolyte solutions to dissolve the reaction byproducts and flush the machining gap was a practice in the past. However, the acidic electrolytes pose problems; the corrosive nature reduces the facility life, toxicity cause health concerns for the operators, handling and disposal is also a problem. In recent years, the research is focused on introducing neutral electrolyte solutions in the STED process to tackle the shortcomings of harmful acidic electrolytes. The neutral electrolyte solutions with proper selection of input parameters according to the work material ensure a good machining behavior during the STED process. The issues like stray cutting, poor dimensional accuracy, and surface integrity faced with acidic electrolyte solutions stand reduced with neutral electrolyte solutions. However, obtaining high material removal rates with neutral electrolyte solutions requires tweaking of the input process parameters. Further, for different electrolyte work material combinations, the dissolution behavior during the process varies, yielding a different output. In the present research work, the STED process performance was investigated during the fabrication of holes on Inconel 718 to understand the effects of input process parameters. The experimentation was spread into four phases to attain the research objectives. In phase-I, an investigation to know the effects of non-electrical parameters such as tool insulation conditions, electrolyte types, and tool feed rate on the STED process was conducted. The assessment of the effects of process conditions was done on the basis of material removal rates and average diametral overcut, along with the surface morphologies obtained as a result of the STED experiments. In phase II, the investigation was focused on exploring the effect of using different electrolyte entry temperatures to check its effect on the STED process performance along with other input process parameters based on the one-factor-at-a-time approach. The effects of input process parameters were analyzed in terms of average diametral overcut, material removal rate, surface roughness, and roundness error. The impact of process conditions on the hole wall surface and the emerging defects during the fabrication of high aspect-ratio holes were also studied. In phase III, the dissolution behavior of Inconel 718 with different types of electrolyte solutions during the fabrication of high aspect-ratio holes was investigated for the STED process. The electrolytes used in the STED process are limited due to the dissimilar dissolution behavior of the work materials and sludge making tendency that cause choking of electrolyte flow. Thus, testing the feasibility of different electrolytes while using a microtubular tool in the STED process becomes interesting. Further, the mixing of neutral electrolytes with alkaline electrolytes enhances the localization of anodic dissolution and reduces stray dissolution. In this experimental phase, the effects of different types of electrolytes along with the chosen input process parameters such as applied voltage, tool feed rate, supplementary electrolyte concentration, electrolyte entry temperature on the roundness error, and hole wall surface roughness of the holes were studied using the experimental design based on response surface methodology (RSM). In phase IV, the experiments aimed to explore the enhancement of the STED process performance using the insights from the phase-III experiments about different electrolyte combinations and incorporate the tool modification to increase the process productivity. The experimental design was based on the RSM. The issues originating during the usage of added alkaline electrolyte for the fabrication of high aspect-ratio holes and the limitations posed by excessive sludge production while working with micro tools in the STED process have also been noted. The developed STED facility was a low-cost solution for the machining of high aspect-ratio holes on the difficult to machine materials (like HSS, Inconel, Titanium, etc.) at high material removal rates. The developed facility is capable of employing acidic, neutral and alkaline electrolytes alike without much concern about corrosion reducing the facility life due to the carefully designed fixtures and 3D printed PLA machining tank. The facility was able to utilize standard tubular tools of different sizes as well as modified tools as per different requirements. The thorough experimentation conducted on the developed facility demonstrated the ability of the STED process to utilize different neutral to alkaline electrolyte solutions. The material removal rates were noticed to be higher with NaCl as compared to the NaNO3 but the aggressive pitting on the hole wall surfaces with NaCl makes NaNO3 a preferable choice for machining Inconel 718 through the STED process.