Electrochemical micromachining behavior on 17-4 PH stainless steel using different electrolytes

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47th SME North Research Conference, NAMRC Pennsylvania, 47thSME SME North North American American Manufacturing Research Conference, NAMRC 47,State Pennsylvania, USA 47th AmericanManufacturing Manufacturing Research Conference, Penn47, Behrend USA Erie, 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA Pennsylvania, 2019

Electrochemical micromachining behavior on 17-4 PH stainless steel using Electrochemical behavior on 17-4 PH stainless steel using Electrochemical micromachining micromachining behavior on 17-4 PH stainless steel using different electrolytes different electrolytes Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June different electrolytes 2017, Vigo (Pontevedra), Spain * Aruna Aruna Thakur, Thakur, Mukesh Mukesh Tak, Tak, Rakesh Rakesh G. G. Mote Mote** Aruna Thakur, Mukesh Tak, Rakesh G. Mote Indian Institutte of Technology Bombay, Powai, Mumbai 400076, India

Institutte of Technology Bombay, Powai, Mumbai India Costing models forIndian capacity optimization in400076, Industry 4.0: Trade-off Indian Institutte of Technology Bombay, Powai, Mumbai 400076, India * Corresponding author. Tel.: +91-22 2576 7529; fax: +0-000-000-0000. address: [email protected] efficiency between used capacityE-mail and operational * Corresponding author. Tel.: +91-22 2576 7529; fax: +0-000-000-0000. E-mail address: [email protected] * Corresponding author. Tel.: +91-22 2576 7529; fax: +0-000-000-0000. E-mail address: [email protected]

A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb

Abstract Abstract a Abstract University of Minho, 4800-058 Guimarães, Portugal The microtextured surfaces are used to improvebUnochapecó, the efficiency of engineering components like aerospace, biomedical, tribology, defense, etc. Chapecó, SC, Brazil The microtextured surfaces are used to improve the efficiency89809-000 of engineering components like aerospace, biomedical, tribology, defense, etc. Electrochemical micromachining (ECMM) is a more for fabrication of microtextured surfaces because of its advantages The microtextured surfaces are used to improve the promising efficiency technique of engineering components like aerospace, biomedical, tribology, defense, like etc. Electrochemical micromachining (ECMM) is a more promising technique for fabrication of microtextured surfaces because of its advantages like high material removal rate, less energy require, less space and tool-material wastage over other methods. 17-4 PH SS alloy isadvantages a precipitation Electrochemical micromachining (ECMM) is a more promising technique for fabrication of microtextured surfaces because of its like high material removal rate, less energy require, less space and tool-material wastage over other methods. 17-4 PH SS alloy is a precipitation hardened (PH)removal martensitic stainless steel less (SS) used in thetool-material field of aerospace for over making turbine blades, power marine chemical high material rate,grade less of energy require, and wastage other methods. 17-4 PH plant, SS alloy is a and precipitation hardened (PH) martensitic grade of stainless steel (SS)space used in the field of aerospace for making turbine blades, power plant, marine and chemical industry. Properties such as highoftensile andsteel fatigue strength, good toughness, highforhardness and formability makeplant, its machinability inferior. hardened (PH) martensitic grade stainless (SS) used in the field of aerospace making turbine blades, power marine and Abstract industry. Properties such as high tensile and fatigue strength, good toughness, high hardness and formability make its machinabilitychemical inferior. ECMM behavior of 17-4 PH SS alloy is completely unfamiliar. Since, there is no information available on the suitability ofmachinability electrolyte dissolution industry. Properties such as high tensile and fatigue strength, good toughness, high hardness and formability make its inferior. ECMM behavior of 17-4 PH SS alloy is completely unfamiliar. Since, there is no information available on the suitability of electrolyte dissolution with 17-4 PH SS while using ECMM, the present study aims to provide a suitable electrolyte for microfabrication on 17-4 PH SS alloy using ECMM behavior of 17-4 PH SS alloy is completely unfamiliar. Since, there is no information available on the suitability of electrolyte dissolution with 17-4the PH concept SS while using ECMM, the 4.0", presentproduction study aims to processes provide a suitable electrolyte for microfabrication on 17-4 interconnected, PH SS alloy using Under of "Industry willused beforpushed to aqueous be increasingly ECMM with tungsten carbide for achieving better study surfaceaims integrity. Electrolytes ECMM are solution of NaBr andalloy combined with 17-4 PH SS while using ECMM, the present to provide a suitable electrolyte for microfabrication on 17-4 PH SS using ECMM with tungsten carbide for achieving better surface integrity. Electrolytes used for ECMM are aqueous solution of NaBr and combined information based on a real time basis and, necessarily, much more efficient. Inare this context, capacity optimization The performance characteristics such as radial overcut, material removal rate solution (MRR), dimple depth and some aqueous solution of NaCl+NaNO 3.achieving ECMM with tungsten carbide for better surface integrity. Electrolytes used for ECMM aqueous of NaBr and combined aqueous solution of NaCl+NaNO3. The performance characteristics such as radial overcut, material removal rate (MRR), dimple depth and some to non-passivating behaviour of material NaBr it actively participate with 17-4 PH aspects ofsolution surface have been goes beyond the traditional of capacityOwing maximization, also for electrolyte, organization’s profitability andand value. . Theinvestigated. performance characteristics suchcontributing as radial overcut, removal rate (MRR), dimple depth some aqueous ofintegrity NaCl+NaNO 3aim Owing to non-passivating behaviour of NaBr electrolyte, it actively participate with 17-4 PH aspects of surface integrity have been investigated. SS alloyofand results in overcuts and pits on the Owing surface. The better dimensional accuracy and surface integrity have been achieved with to non-passivating behaviour of NaBr electrolyte, it actively participate with 17-4 PH aspects surface integrity have been investigated. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of SS alloy and results in overcuts and pits on the surface. The better dimensional accuracy and surface integrity have been achieved with NaCl+NaNO SS alloy and33 electrolyte. results overcuts pits on optimization the surface. Theand better dimensional accuracy surface integrity beenthat achieved with electrolyte. NaCl+NaNO maximization. Thein study of and capacity costing models is an and important researchhave topic deserves NaCl+NaNO3 electrolyte. contributions fromPublished both thebypractical and theoretical perspectives. This paper presents and discusses a mathematical © Elsevier B.V. © 2019 2019The TheAuthors. Authors. Published by Elsevier © 2019 The Authors. Published by Elsevier B.V. B.V. model for capacity management based on different costing models (ABC and TDABC). A generic model has been This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) © 2019 The Authors. Published by Elsevier B.V.BY-NC-ND This anopen open access article under the BY-NC-ND CC (http://creativecommons.org/licenses/by-nc-nd/3.0/) This isisan access article under the CC licenselicense (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Scientific Committee of NAMRI/SME. developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-reviewunder under responsibility of Scientific the Scientific Committee of NAMRI/SME. Peer-review responsibility of the Committee of NAMRI/SME. Peer-review responsibility of the Scientific Committee of NAMRI/SME. value. Theunder trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Keywords: 17-4 PH SS; microstructure; radial overcut; MRR; dimple depth; surface integrity; X-ray diffraction.

Keywords: 17-4 PH SS; microstructure; radial overcut; MRR; dimple depth; surface integrity; X-ray diffraction. optimization might hide operational inefficiency. Keywords: 17-4 PH SS; microstructure; radial overcut; MRR; dimple depth; surface integrity; X-ray diffraction.

© 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 1. Introduction thermal conductivity [3]. 17-4 PH SS alloy is also used in 2017. 1. Introduction thermal conductivity [3]. 17-4 PH SS alloy is also used in turbine blades where extra is provided micro holes 1. Introduction thermal conductivity [3]. cooling 17-4 PH SS alloy with is also used in turbine blades where extra cooling is provided with micro holes 17-4 PH is a precipitation (PH) martensitic [4]. Operational Therefore, microtexture fabrication on 17-4 PHmicro SS alloy is Keywords: CostSS Models; ABC; TDABC;hardened Capacity Management; Idle Capacity; Efficiency turbine blades where extra cooling is provided with holes 17-4 PH SS is a precipitation hardened (PH) martensitic [4]. Therefore, microtexture fabrication on 17-4 PH SS alloy is grade of PH stainless steel (SS). In addition to iron-carbon, it has an attractive topic of researchfabrication interest. on 17-4 PH SS alloy is 17-4 SS is a precipitation hardened (PH) martensitic [4]. Therefore, microtexture grade of stainless steel (SS). In addition to iron-carbon, it has an attractive topic of research interest. chromium (16%), steel copper (12%) and nickel (3%) as its major Microtextured patterns like grade of stainless (SS). In addition to iron-carbon, it has an attractive topic ofsurfaces researchcovering interest. various chromium (16%), copper (12%) and nickel (3%) as its major Microtextured surfaces covering various patterns like alloying elements. Precipitation of Cu-rich phase in its 17-4 PH circular, rectangular, square etc. represent a newpatterns concept for chromium (16%), copper (12%) and nickel (3%) as major Microtextured surfaces covering various 1. Introduction alloying elements. Precipitation of Cu-rich phase in 17-4 PH circular, rectangular, square etc. represent a new concept like for SS is mainly responsible for itsofhigh strength. Owing to PH its enhancing the performance of industrial components. alloying elements. Precipitation Cu-rich phase in 17-4 circular, rectangular, square etc. represent a new concept for SS is mainly responsible for its high strength. Owing to its enhancing the performance of industrial components. outstanding suchfor as high tensile and fatigue strength, However, micro pattern fabrication difficult-to-cut alloy is SS is mainly itsahigh strength. Owing to its for companies enhancing the their performance of on industrial The costproperties ofresponsible idle capacity is fundamental information and management of extreme components. importance outstanding properties such as high tensile and fatigue strength, However, micro pattern fabrication on difficult-to-cut alloy is good toughness, high hardness, betterandweldability and aHowever, challenging task [5].fabrication In order ontodifficult-to-cut understand different outstanding properties such as high tensile fatigue strength, micro pattern alloy is good toughness, highsystems. hardness, better weldability a challenging [5]. In potential order toand understand different in modern production In general, it is definedand as unused capacity ortask production can be measured formability along with excellent resistance to weldability corrosion up to a characteristics of micromachining, electrical discharge good toughness, high hardness, better and a challenging task [5]. In order to understand different formability along with resistanceavailable to corrosion up toofa manufacturing, characteristics etc. of The micromachining, in several ways: tonsexcellent of PH production, hours management ofelectrical the idle discharge capacity temperature of 315°C, SS has immense application machining (EDM), processing micromilling, chemical formability along with 17-4 excellent resistance to corrosion up toin characteristics of laser micromachining, electrical discharge temperature of 315°C, 17-4 PH SS hasfax: immense application ina machining (EDM), laser processing micromilling, chemical * Paulo Afonso. Tel.: +351 253 510 761; +351 253 604 741 power plant, marine,17-4 aerospace and chemical industry etching, electrochemical machining (ECM), electrochemical temperature of 315°C, PH SS has immense application in machining (EDM), laser processing micromilling, chemical power plant, marine, aerospace and chemical industry etching, electrochemical machining (ECM), electrochemical E-mail address: [email protected] components [1,marine, 2]. However, these and properties also industry make it micromachining (ECMM) and (ECM), micromachining using power plant, aerospace chemical etching, electrochemical machining electrochemical components [1, 2]. However, these properties also make it micromachining (ECMM) and micromachining using difficult-to-cut especially, due tothese its high formability and low ultrashort voltage pulses were and employed and micro features components [1, 2]. However, properties also make it micromachining (ECMM) micromachining using difficult-to-cut duePublished to its high formability ultrashort voltage pulses were employed and micro features 2351-9789 © 2017especially, The Authors. by Elsevier B.V. and low difficult-to-cut especially, due to its high formability and low ultrashort voltage pulses were employed and micro features Peer-review under of the scientificbycommittee the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019responsibility The Authors. Published Elsevier of B.V. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. This is an©open access article underbythe CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) 2019 The Authors. Published Elsevier B.V. 2351-9789 This is an open access under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Thearticle Authors. Published by Elsevier B.V. 2351-9789 © 2019 Peer-review under responsibility of the Scientific Committee of NAMRI/SME. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. This is an open access article under the Scientific CC BY-NC-ND license 10.1016/j.promfg.2019.06.177 Peer-review under responsibility of the Scientific Committee of NAMRI/SME. Peer-review under responsibility of the Scientific Committee of NAMRI/SME.

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were produced [6-9] on various grades of stainless steel alloys. Among these, ECMM is a popular method to machine any hard to cut conducting material. It can be attributed to the fact that the material removal in ECMM is independent of material strength and melting point, thus producing structures, without residual stresses and heat-affected zone, along with practically nil tool wear [10-12]. Moreover, it solves the surface integrity problem, which is more dominant while using EDM and other conventional machining processes. Furthermore, ECMM poses a high material removal rate when compared with EDM [13, 14]. Therefore, many researchers [15-17] are interested in investigating ECMM on stainless steel in the field of microfabrication. During the process of ECMM of metallic alloys, insoluble electrolytic products restrict the electrolyte diffusion, resulting in better dimensional accuracy. Hence, selection of a suitable electrolyte plays a vital role in attaining the desired machining features. Some preferred electrolytes like NaCl, NaNO 3 and NaClO3 are commonly used for ECM on different grades of stainless steels [18, 19]. However, these electrolytes are identified to produce a large amount of sludge [20]. Therefore, ECM of the stainless steel was carried out using acidic NaNO3 electrolyte and improved MRR and overcut were found using acidic NaNO3 than NaNO3 electrolytes [21]. Electrochemical dissolution behavior of SS 304 was done with NaNO3 solution. It was found that machined surface was well protected by the formation of oxide layers on the surface [22]. Aggressive compounds like NaCl, NaBr and NaI promote dissolution of the passive oxides [23]. It was noted that dimensional and geometrical accuracy were deteriorated when NaBrO3 electrolyte is used during ECM of 1020 steel [24]. It can be attributed to the formation of a highly porous passive layer on the machined surface. From the past work, it has been understood that there is no information available regarding the role of electrolytes on machined surface characteristics through ECMM of 17-4 PH SS alloy. Therefore, the current work aims to fabricate microdimples analyze the dissolution behaviour of 17-4 PH SS using different electrolytes. Sodium bromide (NaBr) has the strongest ability to penetrate into the workpiece than other halogen salts [25, 26]. Hence, NaBr solution is used as an electrolyte for the first attempt. However, for improving dimensional accuracy and surface integrity, the electrolyte has been modified via combination of weak penetration halide salt sodium chloride in place of NaBr and a passivating salt sodium nitrate (NaCl+NaNO3). Such combination is expected to protect the machined surface dissolution by the formation of oxide layers on the surface. Machining characteristics like radial overcut, MRR, dimple depth and some aspects of surface integrity of machined surface obtained by ECMM of 17-4 PH SS at different voltages are analysed. 2. Material and methods A rectangular sample of 17-4 PH stainless steel with dimensions 30 mm × 15 mm × 5 mm was used for machining experiments. The examination of the microstructure of the asreceived 17-4 PH SS has been carried out using optical microscopy (Zeta Instrument, Zeta-20, USA). For revealing the

microstructure, sample was polished with the increasing grades of grid paper from 600 to 2500 and then finally polished with alumina suspension solution for 10 minutes. Kalling’s no 2 etchant was used to etch the polished sample for about 10 seconds. Immediately after the etching process, the sample was cleaned with deionized water and then dried to obtain the microstructure. Microstructure of 17-4 PH SS is shown in Fig. 1(a). The energy dispersive spectroscopy (EDS) (Zeiss, GeminiSEM 300, U.K.) was used to reveal the chemical composition of as received 17-4 PH SS alloy. The EDS spectrum is shown in Fig 1(b), indicating the chemical composition of 17-4 PH SS as 15.8 % Cr, 3.1 % Ni, 12.2 % Cu, 2% Si, 1.8 % Zn, 0.8 % Mn, 0.4 % S and rest Fe. Properties of 17-4 PH SS alloy are given in Table 1. Table 1: Typical properties of 17-4PH SS alloy. Properties of 17-4 PH SS Ultimate Tensile Strength Ksi (MPa): 0.2% Tensile Yield Strength Ksi (MPa): Hardness Rockwell C: Thermal conductivity (W/m•K) @300̊ C Electrical Resistivity (microhm-cm) Melting Range: °C Density: g/cm³

Values 160 (1103) 145 (1000) 35 17.9 50 1404-1440 7.8

All the machining experiements were carried out using an in-house electrochemical machine setup [27] which is shown in Fig. 2(a). The setup consists mainly four parts: microcontroller unit, machining chamber, DC power supply and electrolyte circulation system. The machining was employed in the chamber while various process parameters were controlled by microcontroller unit. Electrolyte was pumped through the reservoir in the machining area and return to the settling tank via filter. The electrode made up of tungsten carbide rod of 0.3 mm diameter was selected as a cathode and workpiece (17-4 PH SS) was as an anode. (a)



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(b)

(b)

Fig.1 (a) Optical microscopy image of microstructure and (b) EDS spectrum of as-received 17-4 PH SS.

Micro-dimples were electrochemically fabricated on the workpiece with the stationary electrode. Schematic diagram of a micro-dimple on the workpiece is indicated in Fig. 2 (b). The electrolytes used for the fabrication of dimples were aqueous solution of 1 M NaBr and the combination of 0.5 M NaCl and 0.5 M NaNO3 at room temperature. The applied voltages for machining were 5, 6 and 7V. Stationary tool was used with an initial interelectrode gap (IEG) of 0.1 mm and machining was carried out for 100 seconds for all the experiments. Voltage pulse frequency of 60 KHz and a duty cycle of 20% were used. All experiments were repeated thrice for quantifying the experimental errors. To observe the surface morphology of the machined microdimples, a scanning electron microscopy (SEM) (Zeiss, GeminiSEM 300, U.K was used. Radial overcut, dimple depth and material removal rate (MRR) were measured with an optical microscope (Zeta Instrument, Zeta-20, USA). To describe the radial overcut, a simple analytic formula was used and given by the equation 1. Whereas, dimple volume and dimple depth were directly calculated from optical image. After obtaining the dimple volume, MRR was calculated as the ratio of dimple volume and total machining time (100s). For identification of various precipitates of as-received 17-4 PH SS alloy and the surface obtained by ECMM were measured using X-ray diffraction (XRD) (PANalytical, EMPYREAN). A scan range 2𝜃𝜃 of 20°- 110° along with step size of 0.02° and count time of 2 s/step were used. 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 = (𝐷𝐷𝑑𝑑 − 𝐷𝐷𝑡𝑡 )⁄2

3

(1)

where, 𝐷𝐷𝑑𝑑 𝑎𝑎𝑎𝑎𝑎𝑎 𝐷𝐷𝑡𝑡 are the diameter of dimple and electrode respectively.

Fig. 2 (a) Experimental setup of ECMM and (b) schematic diagram of dimple also indicates Dt and Dd as a tool and dimple diameter respectively and dimple depth.

3. Results and discussion From ECMM of 17-4 PH SS the radial overcut, MRR, dimple depth and surface integrity have been analysed at applied voltage and electrolytes. The results are discussed as follows. 3.1. Effect of electrolyte on radial overcut Control of radial overcut is one of the major challenges in ECMM. Radial overcut is mainly affected by the types of electrolyte used, electrolyte concentration, electrolyte flow, its throwing pressure and formation of sludge [27-29]. Variation of radial overcut with voltage and electrolytes is shown in Fig. 3. It reveals that as voltage increases, radial overcut also increases [14, 24]. This is due to increase in average current with voltage as given in Table 2. Fig. 3 also shows that the radial overcut is more with NaBr electrolyte than compared to NaCl+NaNO3 electrolyte. It can be attributed to the solubility of halide salts increases with increasing ionic radius of the halide constituents. Therefore, solubility of 17-4 PH SS alloy is more with NaBr than NaCl. At the same time, due to actively participation of bromide ions, it form non-passivating layer on the surface while using NaBr [25]. On the contrary, the presence of NaNO3 in NaCl promotes the formation of passive layer results in low dissolution [24, 27]. Severe surface pitting was observed using NaBr electrolyte due to aggressive behavior of bromide ions. Similar observation was also reported by other researchers [23, 24, 27]. Table 2: Average current values Voltage, V Average current (mA) using NaBr 5.0 6.0 7.0

40.6 50.3 80.7

Average current (mA) using NaCl+NaNO3 20.4 30.8 40.1

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On the other hand, voltage increases depth also increases for combined aqueous solution of NaCl+NaNO 3 electrolyte.

0.3

0.5

Electrolyte NaBr Electrolyte NaCl+ NaNO3

0.2

0.1

0.0

5.0

5.5

6.0

6.5

7.0

Voltage, V Fig. 3 Variation of radial overcut with voltage and electrolyte of ECMM dimple.

0.3 0.2 0.1 0.0

3.2 Effect of electrolyte and voltage on MRR

MRR, mm /s

1E-3

Electrolyte NaBr Electrolyte NaCl+ NaNO3

1E-4 5.0

5.5

6.0

6.5

5.0

5.5

6.0

6.5

7.0

Voltage, V

Fig.4 shows the variation of material removal rate with different voltage and electrolytes. Figure indicates that the MRR is more with sodium bromide electrolyte than combined sodium chloride and sodium nitrate electrolyte. It might be due to higher solubility behavior of Br- ions resulted in deep penetration in comparison to NO3- ions. This result is consistent for all applied voltage.

3

Electrolyte NaBr Electrolyte NaCl+ NaNO3

0.4

Dimple depth, mm

Radial overcut, mm

0.4

7.0

Voltage, V Fig.4 Variation of MRR with voltage and electrolyte of ECMM dimple.

Fig. 5 Variation of dimple depth with voltage and electrolyte of ECMM dimple.

3.4 Effect of electrolyte and voltage on surface integrity 3.4.1 Surface depth and 3D surface profile Fig. 6 shows variation of 2D cross sectional profile of micro dimple with voltage using aqueous NaBr and combined solution of aqueous NaCl+NaNO3 electrolytes. Rougher surface and oversized dimple reveals with aqueous NaBr electrolyte. On the other hand, smoother as well as dimensional controlled surface obtained with aqueous NaCl+NaNO3 electrolyte. Representative 3D optical convex profile of micro dimples as shown in Fig. 7 are also depict the same. 3D optical convex profile of micro dimple with aqueous NaCl+NaNO3 electrolyte presents better circularity and less pitting surface than obtained from the aqueous NaBr electrolyte. It can be attributed to the active dissolution of 17-4 PH SS alloy with NaBr electrolyte and formation of carbide rich-phase on the surface whereas, the addition of aqueous NaNO3 to NaCl solution encourages the growth of a passive layer on the machined surface and reduces the dissolution rate [27, 30].

3.3 Effect of electrolyte and voltage on dimple depth

6V,NaBr, 6V, NaCl+NaNO3,

7V, NaBr 7V, NaCl+NaNO3

0 -100

Z, m

Depth of micro dimple is another important aspect as it directly depends on the tool, flow of electrolyte and formation of sludge. Variation of depth with voltage and electrolytes are shown in Fig. 5. Depth obtained from aqueous solution of NaBr is higher than using NaCl+NaNO3 electrolytes. This is because of strong ability to penetrate on the surface using NaBr than NaCl. It is also seen that the dimple depth for NaBr electrolyte first increases upto 6V and then decrease with increase in voltage. It can be attributed to that for better surface morphology, it is also essential that smooth dissolution of ions and simultaneously removal of the electrolytic sludge from the machining zone take place [27, 30]. Hence, proper removal of electrolytic sludge become difficult using NaBr electrolyte specially at high voltage. It becomes more severe while fabricating micro dimples. Therefore, depth of dimple is first increased upto 6V and then decreased using NaBr electrolyte.

5V, NaBr, 5V, NaCl+NaNO3,

-200 -300 -400 -600

-400

-200

0

X, m

200

400

600

Fig. 6 Variation of 2D cross sectional profile of ECMM dimple with voltage and different electrolytes after machining time of 100 s.



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5

Machined surface obtained with NaBr electrolyte 1000

(b)

400

Fe3C

200 0 20

40

60

80

Diffraction angle (2), degrees

Fe3C

600

Fe3C/CrC

Intensity, a.u.

Fe3C/Fe3O4

800

(a)

100

400 200

Fig. 7 Representative 3D optical convex profile of micro dimples obtained at 6V using (a) aqueous NaBr (b) combined aqueous NaCl+NaNO3 electrolytes.

3.4.2 X-Ray Diffraction analysis

12000

8000

(a)

As received 17-4 PH SS

2000 0 20

40

60

80

-Fe (1 1 0)

4000

-Fe (1 1 0)

6000 -Fe (2 0 0)

Intensity, a.u.

10000

-Fe (1 1 0)

To analyze the various phases present in as received 17-4 PH SS alloy and machined surface obtained from electrolytes XRD was carried out and shown in Fig 8. Diffraction peaks of as received 17-4 PH SS (Fig. 8 (a)) show that it has 4 peaks and mainly consists of alpha and gamma (α-Fe and ϒ-Fe) iron phases [31, 32]. Fig. 8(b) indicates the XRD spectrum of the machined surface using NaBr electrolyte, reveals metal carbide residues (M3C) mainly composed of Fe3C. Whereas, Fig. 8(c) shows XRD spectrum of the machined surface using NaCl+NaNO3 electrolyte. It reveals the presence of oxides mainly of iron and nickel.

100

Diffraction angle (2), degrees

0 20

40

60

80

Diffraction angle (2), degrees

Fe3C/Fe2O3/Cu5Zn8

600

Fe2O3/NiO2/FeCl3

Intensity, a.u.

800

(c)

Fe3C/Fe2O3/NaNiO2

(b)

Fe3O4/NiO2

Machined surface obtained with NaNO3+NaCl electrolyte 1000

100

Fig. 8 XRD spectra of (a) as received 17-4 PH SS, surface obtained using electrolytes (b) NaBr and (c) NaCl+NaNO3 solution.

3.4.3 Micro surface morphology Fig. 9 represents SEM images of electrochemical machined surface obtained from different electrolytes with varying voltage. Strong penetration of workpiece at different sites in the form of black and white solid surface film can be noticed from Fig. 9 (a,b,c) with aqueous NaBr electrolyte. It can be clearly noticed from EDS spectra as shown in Fig. 10 that the white solid film obtained from NaBr electrolyte is carbon-rich phase. This carbon-rich phase might be the undissolved globular carbides M3C where, M might be the Fe, Cr also confirmed from the XRD spectra as shown in Fig 8. It is a passive residual product. Whereas the solid black film indicates the active dissolution of Fe2+ ions with Br- ions present in NaBr electrolyte makes oxy bromide film (Fe(OBr)2) as written in Equation 7. This oxy-bromide film is strong enough to bind and keep undissolved carbide film on the surface of 17-4 PH SS alloy. In contrary, from Fig. 9 (d,e,f) with combined aqueous solution of NaCl+NaNO3 electrolyte shows oxidation of Fe2+ ions from the workpiece into Fe3+ ions. Further this Fe3+ ions react with electrolyte and form Fe(NO3)3 as written in Equation 8. Due to the formation of Fe(NO3)3 with aqueous NaCl+NaNO3 electrolyte, the metal carbide film dissolves. Similar phenomenon has also been reported [31, 33, 34]. Fig. 9 (a,b,c) also demonstrates that with increase in voltage, formation of residual passive product (white solid film)

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increases using NaBr electrolyte. On the other hand, from Fig.9 (d,e,f) surface cracks have been noticed with increase in voltage using NaCl+NaNO3 electrolyte. Some important chemical reactions supporting to the above mechanism occur at anode and cathode are shown below. At cathode 2𝐻𝐻 + + 2𝑒𝑒 − → 𝐻𝐻2 ↑ At anode and electrolyte interface 2𝐻𝐻2 𝑂𝑂 → 4𝐻𝐻 + + 𝑂𝑂2 + 4𝑒𝑒 − 𝐹𝐹𝐹𝐹 0 → 𝐹𝐹𝐹𝐹 2+ + 2𝑒𝑒 − 4𝐹𝐹𝐹𝐹 2+ + 3𝑂𝑂2 → 2𝐹𝐹𝐹𝐹2 𝑂𝑂3 (Surface oxide formation) 2𝐹𝐹𝐹𝐹2 𝑂𝑂3 → 4𝐹𝐹𝐹𝐹 3+ + 3𝑂𝑂2 2𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 + 𝑂𝑂2 + 𝐹𝐹𝐹𝐹 2+ → 𝐹𝐹𝐹𝐹(𝑂𝑂𝑂𝑂𝑂𝑂)2 + 2𝑁𝑁𝑁𝑁 + Oxy-bromide film 3𝑁𝑁𝑁𝑁𝑁𝑁𝑂𝑂3 + 𝐹𝐹𝐹𝐹 3+ → 𝐹𝐹𝐹𝐹(𝑁𝑁𝑁𝑁3 )3 + 3𝑁𝑁𝑁𝑁+

Electrolyte NaBr (a) Voltage = 5V

(2) (3) (4) (5) (6) (7) (8)

Electrolyte NaCl+NaNO3 (d) Voltage = 5V

Residual passive surface in the form of carbide segregation

(b) Voltage = 6V

(e) Voltage = 6V

White film Black film

(c) Voltage = 7V

(f) Voltage = 7V Crack

Fig. 9 SEM images of electrochemical machined surface of ECMM dimple with voltage and electrolytes.

(a)

(b)

Fig. 10 EDS spectra of white and black solid film surface of ECMM dimple obtained with NaBr electrolyte at 6 V (a) white solid residue (spectrum 3) and (b) black etchant surface (spectrum 4).

4. Conclusions The paper deals with the effect of applied voltage and electrolytes on electrochemical micromachining processes on 17-4 PH SS alloy. From the experimental investigations some of the important conclusions have been drawn here.  Owing to non-passivating behaviour of NaBr electrolyte, it actively participate with 17-4 PH SS alloy and results in formation of oversized dimples and pits on the surface. Therefore, it can be recommended that NaBr electrolyte is not a good option for ECMM of 17-4 PH SS alloy.  For better dimensional accuracy, use of combined aqueous solution of NaCl+NaNO3 electrolyte is a better choice. It can be attributed to addition of aqueous NaNO3 with NaCl solution encourages growth of a passive layer on the machined surface and reduces the dissolution rate.  The choice of selection of the electrolyte strongly affects the electrochemical reaction. Active dissolution of Fe2+ ion using NaBr electrolyte also evident from XRD in the form of carbide phases, whereas transpassive dissolution of Fe3+ ion with remarkable evolution of oxygen in the form of Fe(NO3)3 using combined solution of NaCl+NaNO3 electrolyte are found. References [1] Mohanty A., Gangopadhyay S., and Thakur A. On applicability of multilayer coated tool in dry machining of aerospace grade stainless steel. Mater. Manuf. Process 2016; 31(7): 1532–2475. [2] Sun Y., Hebert R. J., and Aindow M. Effect of heat treatments on microstructural evolution of additively manufactured and wrought 174PH stainless steel. Mater. Des 2018; 156: 429–440. [3] Hsiao C. N., Chiou C. S., and Yang J. R. Aging reactions in a 17-4 PH stainless steel. Mater. Chem. Phys 2002; 74: 134–142. [4] Yao J., Wang L., Zhang Q., and Kong F. Surface laser alloying of 17-4PH stainless steel steam turbine blades. Opt. Laser Technol 2008; 40: 838–843. [5] Masuzawa T. State of the Art of Micromachining. CIRP Ann. 2000; 49(2): 473–488. [6] Hyun S., Hyoung S., Ki D., and Nam C. Electro-chemical micro drilling using ultra short pulses. Precis. Eng. 2004; 28: 129–134. [7] Rajurkar K. P., Sundaram M. M., and Malshe A. P. Review of electrochemical and electrodischarge machining. Procedia CIRP 2013; 6; 13–26.



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