Experimental investigation on titanium alloys for machining of stepped circular holes using ultrasonic-assisted hybrid ECM

In the recent development of high-performance gas turbine engine, there is a tendency to design the cooling holes in order to improve the heat transfer and cooling efficiency. Titanium alloy is the most preferred material for these blades. It is hard material, and hence, the traditional drilling is not appropriate for “Ti” alloy. This research presents the design and optimization required to manufacture contoured holes on a titanium-based superalloy using hybrid electrochemical machining (ECM) under different operating conditions. The circular holes are developed and are analyzed by ANOVA. The experimental results are further optimized using a Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method. The voltage ( V ), feed rate ( f 1 ), and feed rate ( f 2 ) were identified as the most influencing factors which are further used for stepped circular hole machining. A design of the experiment is also optimized using the TOPSIS method. The obtained experimental results are verified using a SEM analysis to confirm the uniformity of the machined surfaces and the inverse relationship of the overcut with the increasing values of the voltages and feed rates. Optimal machining conditions for the stepped hole machining were determined for voltage 18 V, feed rate “f 1 ” at 0.8 mm/min, and feed rate “f 2 ” at 1.35 mm/min.


Introduction
The input temperatures of gas turbines are observed with higher values over the period of time on account of a regular improvement in the efficiency and higher firing temperatures.In the 1960s, the properties of the materials used limited the temperature of gas turbine firing and the temperature of turbine blades to approximately 800 °C.To achieve maximum power output and thermal efficiency, modern gas turbine engines operate at temperatures ranging from 1200 to 1500 °C.The failure of thermal insulation of the blade is observed due to the rising temperature of the gas and a transfer of heat to it.The temperatures observed in the first stage of todays modern gas turbine engines and its surroundings are significantly high.High-pressure stage airfoils need to be cooled in order to keep their structural integrity.These cooling techniques typically consist of internal passages of varying shapes [1].Film cooling is a method that is well accepted and frequently used in modern gas turbine cooling system to maintain the surface temperatures of components within acceptable limits.To ensure the safe operation of the engine blade, both external and internal cooling methods have been selected.By injecting cool air from the inside of the blade to the outside surface, external cooling is achieved, and a film layer forms between the blade surface and hot gas-pass flow.The shaped hole film-cooling technology is now standard in highly cooled gas turbine aerofoils.It is common practice to distribute the ribs on the wall of an inlet cooling hole in order to enhance the heat transfer rate.It is also known as turbulators.A thermal barrier is created by these ducts between the blade and the hot gases that are flowing through the main flow path of the engines.The increased contact surface between the metal and the cooling air along with a heat exchange due to turbulence contributes to the enhancement of the effectiveness of these holes in the cooling process [2].
In general, a turbine blade could have several hundred film-cooling holes, each of which is between 0.2 and 0.8 mm in diameter [2,3].Titanium is widely used in turbine blades due to its excellent combination of properties such as high strength, low density, and good corrosion resistance.The use of titanium in turbine blades has been extensively studied and documented in the literature.The manufacturing of micro holes in titanium-based super alloys can be challenging due to the material's unique properties.Titanium-based super alloys are also very hard, which can make them difficult to machine.The high hardness of the material can cause excessive tool wear, which leads to a shorter tool life and increased manufacturing costs.It is also reactive with material like cutting tools and coolants.To address these issues, manufacturers may use specialized machining techniques, cutting tools, and coolants that are designed specifically for titanium-based super alloys [4,5].Micro electrical discharge machining (micro EDM), laser beam machining, and ultrasonic machining are the three methods that have been utilized to create micro holes.Micro EDM and laser beam machining are both thermal manufacturing processes, and hence, the machined workpiece shows heat-affected zones as well as micro cracks.Ultrasonic milling presents a few challenges, including increased tool wear and a heightened sensitivity to variations in material strength.Electrochemical machining techniques, on the other hand, do not leave any heat-affected layers and do not cause tool wear because ECM involves an anodic electrochemical dissolution process in an electrolyte.
ECM is frequently utilized in the machining of hard alloys because it is capable of removing electrically conductive materials irrespective of their hardness and toughness [5,6].The absence of metal cutting forces, micro cracks, and a good surface quality makes an ECM process as a mainstream manufacturing technology for a part with complex structures in aeroengines [7][8][9].The ECM is further advanced as hybrid ECM and ultrasonic-assisted ECM.
The ECM process involves the controlled anodic electrochemical dissolution of components in an electrolytic cell [10].The ECM process has certain features, such as a high rate of material removal and no wear on the tools.It provides a good surface quality without the occurrence of re-solidified layers, heat-affected zone, and strain hardening [11].The complexity of profile and coupling of multiple physical fields lead to difficulty in removing material from the interelectrode gap.Klocke et al. [12,13] developed an interdisciplinary model to simulate the ECM process for aero-engine blades.It is reported that the suitable cathode geometries can be calculated directly in a virtual design step by using inverse simulation.The external cooling methods are studied by Sargison et al. [14].They present flow visualization experiments for a new film-cooling hole called the converging slot hole or console, which has been shown to improve heat transfer and aerodynamic performance in turbine vane and rotor blade cooling systems.Results showed that the console film was similar to a slot film and remained thin and attached to the surface for certain coolant-to-mainstream momentum flux ratios.Gritsch et al. [15] presents measurements of local heat transfer coefficients in the vicinity of three film-cooling holes with different geometries.Results showed that expanded holes had lower heat transfer coefficients than the cylindrical hole, particularly at high blowing ratios, and the laidback fan-shaped hole provided better lateral spreading of the injected coolant.The superposition method was applied to evaluate the overall film-cooling performance of the hole geometries.Garg [16] presented external flow, and heat transfer characteristics over smooth and rough turbine blades for a range of parameter values are described, as well as the effect of film-cooling holes and internal cooling channels with ribs and bleed holes on smooth blades.Several studies on the blade tip region, susceptible to burnout and oxidation, are also described.Han and Dutta [17] in his research on the internal cooling methods has reported.This paper provides a review of recent developments in turbine blade internal cooling, with a focus on the use of rib-enhanced serpentine passages, jet impingement.The machining debris of zirconia is examined by He et al. [18] using Raman spectroscopy during spark-assisted chemical engraving.It is concluded that due to differences in phase composition, the material removal is influenced by physical and chemical phenomena in 75.8% and 24.2%, respectively.Gruner et al. [19] studied hole drilling on stainless steel using a high repetition frequency femtosecond laser (220 fs and 1030 nm).This research determines the optimal parameters for the highest available drilling quality and speed.
Burger et al. [20] found that the ECM reduces the cost of manufacturing a single blade by 30% in batch production due to reduced tooling costs.Zhang et al. [21] advocated for precision drilling with an ultrashort pulse laser (such as picosecond and femtosecond) to improve the quality of holes, and they demonstrated that picosecond laser helical drilling can manufacture high-quality holes with a significantly reduced recast layer thickness (less than 5 µm).Liu et al. [22] developed a modified tool to eliminate overcut in ECM of repaired turbine blade tips and obtained an optimal machining time with an appropriate voltage.They did this by obtaining an optimal machining voltage.After using a millisecond laser and a femtosecond laser in a two-step process, Wang et al. [23] finally succeeded in producing high-quality holes in thermal barrier-coated nickel-based alloys.Romoli et al. [24] designed a micro-drilling cycle that used ultrashort laser pulses and was based on three phases in sequence: drilling through, enlarging, and finishing.These authors showed that this drilling strategy was able to produce cylindrical holes with diameters of 180 ± 2 μm on a plate that was 350 μm thick with the complete absence of burrs and debris.Jain et al. [25] proposed analytical equation techniques in order to predict stepped holes and most significant parameters.These models were used to predict and analyze the behavior of the workpiece.Reddy et al. [26] developed a tool design model that is based on the use of a correction factor method to repeatedly modify the shape of the tool until an anode profile is obtained that is within the prescribed tolerance limits.Dutta and Sarma [27] examines how input process parameters like capacitance, gap voltage, and pulse on time (ton) affect response variables like material removal rate (MRR), tool wear rate (TWR), and diametral overcut (DOC) during μ-EDM of Hastelloy C276.RSM-DF optimizes capacitance, gap voltage, and ton for maximum MRR, minimum TWR, and DOC.MOGA shows that at 1001.610 pF, gap voltage of 137.6671V, and ton of 48.025 μs, MRR is 0.005153 mm 3 /min, TWR is 0.002423, and DOC is 0.028691 mm.Pawar et al. [28][29][30] presented research work carried out on super alloys by researchers in the area of ECM, EMM, STEM, hybrid ECM, etc., for a wide range of super alloys and metals to advanced alloys.
From the literature review, it is seen that the machining of titanium alloy using ECM plays an important role which overcomes the difficulties and complexity arising during the machining.The productivity of the ECM process can be further improved by using a hybrid approach like the use of ultrasonic pulses, abrasive, and laser.From the review of the literature, it is also clear that stepped hole machining of titanium alloy using hybrid ECM needs more attention because it needs to be done with the best machining parameters to get better hole profiles.The multi-criteria decision-making technique has various methods which are found to be useful in multi-objective optimization.Many researchers use the TOPSIS method to solve their multi-objective optimization problems, which shows how important the TOPSIS method is in the multi-objective optimization domain [31].TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) is a decision-making method that evaluates and ranks alternatives based on their proximity to the ideal solution and distance from the worst solution.It uses the concept of similarity to determine the best alternative among a set of options.TOPSIS involves creating a normalized decision matrix, determining the ideal and worst solutions, calculating the similarity and dissimilarity scores, and then ranking the alternatives accordingly.
The purpose of this research is to use ultrasonic-assisted ECM to identify the ideal machining conditions for plane circular holes and stepped circular holes.Under various operation conditions, two different steeped hole profiles, namely circular and stepped circular, have been produced.In this work, the impact of several factors, including voltage, feed rate, pulse on time, on material removal rate, overcut, and surface roughness, is investigated.Two phases were taken in the research process for this project.First, a circular hole was optimized multi-objectively by identifying the most significant components using ANOVA and TOPSIS.The factors that have been established are updated further for the stepped hole machining through screening trials.The remaining half of this paper describes how the TOPSIS method was used to optimize the stepped hole machining process after the indicated parameters were further studied.As a result, this study evaluates the TOPSIS method's applicability in various settings.The experimental approach of hybrid ECM, ANOVA, and TOPSIS analysis for both flat and stepped hole machining is explained in the remaining section of this study.

Experimental
The experiments are conducted on the ultrasonic-assisted pulse electrochemical machine (USAPECM).It is a nontraditional machining process that combines electrochemical machining (ECM) with ultrasonic vibration.It is used to machine complex shapes in conductive materials such as metals, alloys, and composites.The experimental setup consists of power supply, a tool electrode (anode), a workpiece (cathode), an electrolyte solution, and an ultrasonic transducer.The tool electrode is made of a conductive material and is shaped to the desired geometry of the part to be machined.The workpiece is clamped onto a fixture and submerged in the electrolyte solution.The ultrasonic transducer generates high-frequency mechanical vibrations that are transmitted to the electrolyte solution and the workpiece, as illustrated in the Fig. 1a.The photograph of different tools used and hybrid ECM is shown in Fig. 1b.These vibrations help to enhance the machining process by improving the electrolyte flow, increasing the rate of material removal, and reducing the formation of surface defects.The power supply generates the DC current to electrochemically machine the workpiece.The tool electrode and workpiece are submerged in an electrolyte solution that reacts with the workpiece material to remove material from the workpiece.The potential difference between the workpiece and the tool electrode was provided by a pulse generator.
The ECM tool electrode plays a critical role in achieving high precision and accuracy during the machining process.ECM tool electrode is a small electrically conductive tool.It is made of a conductive material copper.It is designed for a shape that can be used to machine complex and intricate hole profiles (circular, rectangular, and triangular).A tool electrode of the desired shape is fabricated using wire electrical discharge machining (Fig. 2).In EDM, sequential spark discharge occurs between the tool and the workpiece.The measurement of the electrode tool is 158-mm length and 2-mm inner diameter, and 3-mm outer diameter was used as the circular tool.Similarly, the measurement of the rectangular and triangular electrode tool is 158mm length and 2-mm inner diameter, and 3-mm outer diameter is used.The sodium chloride (NaCl) and 5% HCl having the conductivity of about 120 mS/cm is used for the machining purpose.
Ti-6-6-2 is selected as the material for the workpiece due to its high strength-toweight ratio as requirement for turbine blade stators of aeroengines.The chemical  1.The specifications of a titanium alloy workpiece for machining-shaped hole are shown in Fig. 3.The thickness of the workpiece is selected 10 mm, and diameter is 3 mm.
In the ECM process, the electrolyte performs three functions: it transmits current between the tool and the workpiece, eliminates reaction products from the IEG, and removes heat created by current passage [3,29].In order to maintain the proper MRR, the electrolyte must be pumped through the IEG at a very high pressure.It is also important to pump the electrolyte through the IEG at a very high pressure in order to maintain  a consistent MRR [11].The material removal rate (MRR) is determined by measuring the initial weight of the workpiece and subtracting the final weight after machining.This difference in weight is then divided by the time taken for the machining process.Using a weighing machine, this straightforward calculation provides a quantitative measure of the efficiency and speed at which material is being removed during the manufacturing or machining operation.Overcut, in electrochemical machining (ECM), refers to the variance between the size of the electrode and the cavity or hole being machined.This discrepancy is a critical parameter in ensuring precision and accuracy during the ECM process.The measurement of overcut is effectively carried out using the SIPCON vision measuring system.The stylus-type surface tester MITUTOYO make is employed to measure the surface roughness of holes machined using hybrid electrochemical machining (ECM).This specialized device utilizes a stylus or probe to traverse the contours of the machined surface, capturing detailed height variations.

Design of experiments and optimization using TOPSIS
Taguchi methods involve the use of orthogonal arrays to determine the most important factors affecting a process.These factors can be optimized.The parameters and their respective levels for machining of the circular micro holes are mentioned in Table 2.The DoE output response table for three different outputs for plain circular holes is given in Table 3.Total five parameters and five levels are considered for the analysis.The L25 orthogonal array is a specific type of design matrix that consists of 25 experimental runs, each of which represents a unique combination of the experimental factors being tested.A separate analysis is performed for the material removal rate (MRR), overcut, and surface roughness.
The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is a multi-criteria decision-making method that can be used to evaluate the performance of different process parameters in the fabrication of micro holes using electrochemical machining (ECM).The parametric design for the machining of titanium alloy involves the monitoring of four parameters such as voltage, micro-tool feed rate, and pulse on time.
By conducting trials with the workpiece and assuming that certain actual parameter values from earlier research efforts will be used, a general concept of the range of values to use in each level of output parameters such as MRR, overcut, and surface finish selected using the TOPSIS method rank for the circular hole as shown in Table 4.

Analysis for stepped circular hole machining
Based on a circular hole investigation followed by a screening experiment, it was seen that the voltage and two feed rates are the most responsible factors to generate the desired stepped hole profiles which were further used for investigation.The following mathematical relation was assumed to figure out two feed rates and their levels.For a stepped circular hole, the relationship between the outer diameter and the inner diameter is defined as the following: It is observed from the experimental study that to generate the outer diameter and inner diameter, two feed rates are responsible which are again defined by equation [25].Therefore, from the Taguchi analysis of the TOPSIS and from the experimental study, the determined input factor combinations are shown in Table 5.
In Table 5, the feed rate "f1" is responsible for the outer diameter, while the feed rate "f2" is responsible for the inner diameter.From Table 5, the new design of the experiment using the Taguchi method was used for the machining of the stepped circular hole which has nine experimental runs shown in Table 6.
For stepped circular hole machining, TOPSIS analysis was performed to identify the best experimental run which is shown in Table 7.
Each of the three output responses was given the same weight of 0.33 because they were all just as important.Using the TOPSIS method, the best combinations of the test  runs for the stepped circular hole were found to be 5-8-7-4-9-6-1-2-3.In Table 7, the rank is given to the experimental run 5.For this experiment, the factor combinations are 18 V, 0.8 mm/min for "f1" feed rate, and 1.35 mm/min for "f2" feed rate.These combinations were thought to be the best ones to start with, and the Taguchi analysis was used to find out more about them.

Results and discussion
A Taguchi design experiment was conducted to assess the influence of five input variables on three output responses: Material Removal Rate (MRR), overcut, and surface finish.The primary objective of the TOPSIS method is to identify the experimental run with the highest performance relative to the others that were considered as an initial input parameter.To comprehend the importance of these input variables, an analysis of variance (ANOVA) was conducted, as shown in Table 8.
In ANOVA analysis, the input factor having P-value less than 0.05 is considered a significant value.The ANOVA analysis reveals that the voltage is the significant factor with a percentage contribution of 90% for MRR, 4.31% for overcut, and 4.31% for surface roughness.Additionally, the feed rate is a significant input factor with a  percentage contribution of 10% for MRR, 93.20% for overcut, and 92.46% for surface roughness.The pulse on time is found to be the least significant factor having a percentage contribution of 0% for MRR, 0.79% for overcut, and 0.93% for surface roughness.For the TOPSIS method, the feed rate is found to be the most significant factor with a percentage contribution of 93.76% followed by pulse on time at 1.62% and voltage with the least significant of 1.12%.The Taguchi analysis of the TOPSIS method for the plain circular hole was performed.The main effect plot for means of the TOPSIS method is shown in Fig. 4. From Fig. 4., final optimal machining conditions are identified as the voltage at 12 V, feed rate at 0.6 mm/min, pulse on time at 750 µs, ultrasonic on time at 180 s, and the amplitude at 80 µ.For these conditions, confirmation experiment was performed, and the result of the confirmation experiment is shown in Table 7.The design of experiments (DoE) for stepped circular hole machining primarily focuses on the analysis of voltage and feed rates using ANOVA.The MRR is improved by 46.10%, overcut is reduced by 7.14%, and surface roughness is increased by 3.10% as per Table 7.Thus, overall, the performance of the Taguchi analysis is reduced by 42.06% (Table 9).Hence, the final optimal conditions for plain circular hole machining are confirmed as the same conditions determined by the TOPSIS method.For the stepped hole machining, voltage (v), feed rate (f 1 ), and feed rate (f 2 ) are chosen as input factors.The ANOVA analysis results clearly highlight the significance of these three factors.Similar to the Taguchi analysis of the TOPSIS method for the plane circular hole, the Taguchi analysis of the TOPSIS method for the stepped circular hole is also performed (Fig. 5).It is observed that the optimal machining conditions are identified as the voltage at 19 V, feed rate "f 1 " at 0.8 mm/ min, and feed rate "f 2 " at 1.35 mm/min.
The confirmation experiment for the optimal machining conditions determined by the Taguchi analysis of the TOPSIS method for the stepped circular hole was performed.The results of the confirmation experiment are shown in Table 10.
The MRR is improved by 6.71%, overcut is reduced by 40.26%, and surface roughness is increased by 53.30% as per the results shown in Table 10.Thus, overall  performance is reduced by 86.84%.Therefore, the optimal machining conditions for the stepped circular hole were confirmed to be the same as those determined by the TOPSIS method.
The SEM images presented in Fig. 6 illustrate the results from various experimental runs involving stepped circular holes.These images provide clear evidence that the profiles of the stepped holes were exclusively influenced by the voltage and feed rates (f1 and f2), all of which yielded distinct output response values.This observation serves to validate and affirm the accuracy of both the ANOVA analysis and the TOPSIS method employed for plain circular hole assessment, establishing their applicability in determining the optimal input parameters for stepped circular hole machining.

Morphology of square hole and surface analysis
Figure 7 shows a SEM image of the stepped circular hole.The voltage (V), feed rate (f 1 ), and feed rate (f 2 ) have a uniform effect on the stepped hole profile.The SEM analysis also shows that there is no generation of the built-up edge formation.This reveals that the chip and bur produced during the machining are removed efficiently by the electrolyte.Also, at the same time, the electrolyte helps to reduce the rubbing action of the bur over the surface.The SEM image reveals that the voltage (V), feed rate (f1), and feed rate (f2) have a uniform effect on the profile of the stepped hole.This suggests that the changes in these parameters have consistent and predictable impacts on the machining results.The efficient chip removal prevents accumulation and interference, contributing to a smoother and more precise machining process.As a result, the ECM achieves a superior surface finish with uniform material phase, justifying the advantage over the conventional machining process.The SEM analysis also shows that the inner and outer diameters of the stepped circular holes have a direct relationship with the feed rate (f 2 ) and feed rate (f 1 ).

Overcut analysis
The SEM images of the machined hole fabricated through the developed hybrid ECM setup are shown in Fig. 6.The holes were generated at different machining conditions mentioned in Table 4. From Fig. 7, the volcanic features with distinct craters were observed across the machined surface.The SEM analysis shows that at higher values of voltage and feed rates, the overcut decreases linearly.The reason for this is that higher voltage and feed rate values effectively regulate the material removal phenomenon.For the first experimental trial as per Table 4, the workpiece provides initial inertial resistance during machining, which results in the generation of higher overcut, as seen in the SEM image of overcut.Thus, an increase in voltage and feed rate values has an inverse relationship with overcut.

Fig. 1 a
Fig. 1 a Schematic of hybrid ECM.b Experimental setup of hybrid ECM

Fig. 2 2 Material
Fig. 2 Electrode a circular, b square, and c triangular for machining stepped holes

Fig. 3
Fig. 3 SEM images of stepped hole for a square profile and b triangular profile

Fig. 4 a
Fig. 4 a Main effect plot for stepped profiles.Main effect plots of TOPSIS method for the plane circular hole

Fig. 5
Fig. 5 Main effect plot of the TOPSIS method for a stepped circular hole

Fig. 6
Fig. 6 SEM images for square Square hole 1. b Square hole c hole 3. d Square hole 4

Fig. 7
Fig. 7 SEM image for square hole profile with dimensions

Table 2
Parameters and the levels of USAPECM

Table 3
Taguchi DoE with output responses for plain circular hole

Table 4
TOPSIS analysis of the plain circular hole

Table 5
Factors with their levels for stepped circular hole machining

Table 6
Taguchi DoE with output responses for the stepped circular hole machining

Table 7
TOPSIS analysis for a stepped circular hole

Table 8
ANOVA for the plain circular hole machining

Table 9
Confirmation experiment for the plain circular hole

Table 10
Confirmation experiment for the stepped circular hole