Chinese Journal of Aeronautics, (2015), 28(4): 1263–1272

Chinese Society of Aeronautics and Astronautics& Beihang University

Chinese Journal of Aeronautics

cja@buaa.edu.cnwww.sciencedirect.com

Electrochemical machining of burn-resistant

Ti40 alloy

* Corresponding author. Tel.: +86 25 84896304.

E-mail address: xuzhy@nuaa.edu.cn (Z. Xu).

Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.cja.2015.05.0071000-9361 ª 2015 The Authors. Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Xu Zhengyang *, Liu Jia, Zhu Dong, Qu Ningsong, Wu Xiaolong, Chen Xuezhen

College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

Received 6 November 2014; revised 5 January 2015; accepted 12 March 2015

Available online 20 June 2015

KEYWORDS

Burn resistant;

Electrochemical machining;

Material removal;

Surface roughness;

Titanium alloys

Abstract This study investigates the feasibility of using electrochemical machining (ECM) to pro-

duce critical aeroengine components from a new burn-resistant titanium alloy (Ti40), thereby reduc-

ing costs and improving efficiency relative to conventional mechanical machining. Through this, it

is found that an aqueous mix of sodium chloride and potassium bromide provides the optimal elec-

trolyte and that the surface quality of the Ti40 workpiece is improved by using a pulsed current of

1 kHz rather than a direct current. Furthermore, the quality of cavities produced by ECM and the

overall material removal rate are determined to be dependent on a combination of operating

voltage, electrolyte inlet pressure, cathode feeding rate and electrolyte concentration. By optimizing

these parameters, a surface roughness of 0.371 lm has been achieved in conjunction with a specific

removal rate of more than 3.1 mm3/AÆmin.ª 2015 The Authors. Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA. This is an

open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The high strength-to-weight ratio and corrosion resistance oftitanium and its alloys have seen them widely applied acrossa variety of industrial sectors, though the high material and

machining cost has resulted in aerospace applications beingthe more prevalent.1,2 More recently, a new type of burn resis-tant titanium (BuRTi) alloy, Ti40 (Ti-25V-15Cr-0.2 wt%Si),was developed for use in aeroengine components by the

Northwest Nonferrous Metal Research Institute of China

and has been shown to have excellent mechanical properties.3

However, this alloy’s high yield strength (940 MPa), tensile

strength (980 MPa) and yield ratio (0.959) mean that a highcutting force is required for machining; with a thermal conduc-tivity of only 5.6 W Æ m�1 Æ K�1, much of the heat generated by

cutting is concentrated in the vicinity of the cutting edge ratherthan being carried away with the chips, thereby furtherincreasing the tool wear. To make matters worse, the chemical

properties of Ti40 alloy mean that the chips generated have astrong affinity with the cutting tool, resulting in abrasion of thetool rack face. Meanwhile, friction between the workpiece and

tool flank is also increased by the fact that the machined sur-faces of Ti40 can easily rebound due to modulus of elasticityof only 104 GPa. This ultimately results in a high tooling cost,as well as a difficulty in achieving a high degree of machining

precision.Past research into traditional mechanical machining meth-

ods has explored several different approaches to reducing tool

1264 Z. Xu et al.

wear and improving surface integrity through optimizing thecutting tool material, lubrication system or cutting parame-ters.4,5 Studies into the milling of Ti40 with coated carbide

tools have demonstrated that high cutting speeds(>60 m/min)are not suitable for full-width machining of Ti40,6 with thehigh-speed-ball nose-end milling of BuRTi alloys resulting in

a relatively short tool life, high workpiece surface roughness(Ra) and poor surface integrity due to the presence of carbideparticles.7 This has led to the development of non-traditional

machining processes such as electrochemical machining(ECM), in which material is removed by anodic dissolutionduring electrolysis. This eliminates tool wear and the depen-dence on the mechanical properties of the material, creates

no residual stress or deformation in the work piece, and offersa material removal rate directly proportional to the currentapplied. Many studies have focused on the ECM of difficult-

to-machine materials. The electrochemical machining charac-teristics of nickel-base single-crystalline material LEK94 havebeen studied.8 The grid cathode method has been shown to

simplify free face cathode design.9 Hybrid ECM methods,including electrochemical grinding, laser assistant and vibra-tion ECM,10–13 extend the scope of ECM applications.

Modeling and simulation of ECM have been used to predictmachining results.14,15 The machining accuracy and stabilityof ECM can be improved by optimizing the flow field and feeddirection, by pulsating the electrolyte flow, or by using an aux-

iliary anode.16–20

The high corrosion and chemical resistance of the oxidelayer generated by ECM has seen it already used with titanium

in biomedical applications.21 It has also been used with tita-nium alloys such as TiAl and Ni–Ti shape memory alloy,22,23

with effective material removal rates Veff,Ti as high as

�1.78 mm3/AÆmin having been achieved.24 The greater surfaceintegrity afforded by ECM has also been shown to increase thefatigue strength of titanium,25 as well as help prevent pitting

corrosion in saturated ammonium bromide.26 However, inspite of some progresses being made in the optimization ofECM parameters for titanium alloys,27,28 there has so far beenvery little attention given to the ECM of BuRTi alloys. This

paper therefore takes an experimental approach to determinethe optimal electrolyte composition and machining parametersfor the ECM of Ti40 alloy, with the aim of applying this

knowledge to electrochemically machine cavities in Ti40.

Fig. 1 Schematic of flat-plane ECM apparatu

2. Methodology

2.1. Experimental device design

Two electrochemical machining apparatus were designed. Thefirst of these (see Fig. 1) was designed to test the flat-plane elec-

trochemical machining of Ti40 and was fabricated from epoxyresin fixtures to ensure both sufficient strength and electricallyinsulating properties. Into these fixtures were milled shallow

grooves measuring 1 mm deep and 20 mm wide to create anelectrolyte channel; lateral flow was appropriate in this situa-tion as there is no change in the surface curvature.Cylindrical (B20 mm diameter) stainless steel cathode and

Ti40 work piece were then installed, with the cylindrical shapebeing chosen so as to ensure a more reliable electrolyte seal andgreater ease of processing. During operation the Ti40 work-

piece is connected to the anode rod, which in turn is fixed tofeeding axle and fed toward the cathode with certain velocity(V, as shown in Fig. 1).

The second apparatus was designed to allow theelectrochemical machining of a square cavity in a50 mm · 25 mm · 20 mm Ti40 workpiece. For this, detachablesquare cathodes were fabricated to a size of 20 mm ·20 mm · 20 mm, onto which different outlet slot patterns werethen machined so as to optimize the uniformity of the elec-trolyte flow from cathode outlet to the machining area. A sche-

matic and photograph of this device are provided in Fig. 2,while different outlet slot patterns are shown in Fig. 3.

2.2. Experimental conditions

Since the low conductivity of the passive layer that is naturallyformed on Ti40 directly affects the stability and continuity of

ECM processing, especially given the high V concentrationof this alloy, five different electrolytes were tested to determinewhich is the most effective in terms of machining quality.These were

(a) Sodium nitrate (NaNO3): a non-linear electrolyte withgood machining accuracy.

(b) Sodium chloride (NaCl): the most popular electrolytefor titanium alloys.

s used and photograph of Ti40 workpiece.

Fig. 2 Schematic and photograph of apparatus used for ECM of a square cavity in Ti40.

Fig. 3 Four cathode outlet slot patterns tested.

Table 2 Parameters for flow field experiments.

Condition Value

Voltage 30 V

Pulse power frequency 1 kHz

Pulse duty cycle 0.7

Electrolyte 4%NaCl + 4%KBr

Temperature of electrolyte (35 ± 0.5) �CInitial gap 0.3 mm

Inlet pressure 0.6 MPa

Feeding rate 1.0 mm/min

Table 3 Design scheme of experimental parameters and

levels.

Levels A: voltage

(V)

B: pressure

(MPa)

C: feeding

rate

(mm/min)

D: concentration

(%)

1 30 0.6 0.9 4

Electrochemical machining of burn-resistant Ti40 alloy 1265

(c) A mixed solution of NaCl and NaNO3: for a given con-centration, the conductivity of NaCl is greater than thatof NaNO3.

(d) Potassium bromide KBr: has great activating ability andis highly corrosive.

(e) Mixed solution of NaCl and KBr: compared with Cl, Brhas a greater activating ability.

All experiments were conducted using the aforementionedflat-plane ECM apparatus with feeding rates of 0.8, 1.0,

1.2 mm/min. The main parameters for each experiment aregiven in Table 1.

The effect of using a pulsed or direct current (DC) power

supply on the surface quality achieved with ECM of Ti40was also studied using a 4%NaCl + 4%KBr solution at atemperature of 35 �C and an inlet pressure of 0.6 MPa.

The parameters used for the ECM of square cavities in Ti40

using various cathode outlet slot patterns are given in Table 2.Since the emphasis here was on improving the efficiencywhile maintaining the surface quality, the voltage, electrolyte

Table 1 Parameters for experiments of different electrolytes.

Condition Value

Workpiece material Ti40

Cathode material 1Cr18Ni9Ti stainless steel

Voltage 30 V

Inlet pressure 0.6 MPa

Temperature of electrolyte (35 ± 0.5) �CInitial gap 0.3 mm

Feeding rate 0.8, 1.0, 1.2 mm/min

2 34 0.75 1.2 6

3 38 0.9 1.5 8

pressure, concentration and feed rate were all selected as exper-

imental parameters, and the impact of these four factors on thebalanced gap Db, roughness Ra of the cavity surface and mate-rial removal rate MRR was determined through orthogonal

experiments. The experimental parameters and levels usedare shown in Table 3, with other experimental conditions asfollows: temperature T = 40 �C, a NaCl + KBr solutionelectrolyte, a pulsed power duty cycle s = 0.7, a frequency

Fig. 4 Ti40 surfaces after electrochemical machining in different electrolytes at different feed rates.

Fig. 5 Surface topographies of Ti40 after ECM in different electrolytes and 1.2 mm/min feed rate.

1266 Z. Xu et al.

Electrochemical machining of burn-resistant Ti40 alloy 1267

f= 1 kHz and a feeding distance of 8 mm. As the trialinvolved four factors and three levels of experiments, an L9

(34) array was used, with each experiment carried out 3 times.

3. Results and discussion

3.1. Electrolyte composition

Fig. 4 shows the electrochemically machined surfaces pro-

duced with different electrolytes and different feed rates:Fig. 4(a)–(c) showing corresponding to 8%NaNO3, 4%NaCl + 4%NaNO3 and 8%NaCl solution, respectively.

From the localized generation of a passive film on all of thesesamples, it is evident that the activating ability of these elec-trolyte solutions is not sufficiently strong enough. Moreover,

electrochemical dissolution of the surface is not continuousand uniform, suggesting that there is also an uneven distribu-tion of electrolyte. As result, the distribution of the inter-electrode gap and electrolyte flow are also not uniform, which

leads to unstable machining and a poor surface roughness. Incontrast, the electrolyte solutions containing Br have goodactivation, which accelerates the removal of the passive surface

film and helps ensure continuous electrochemical dissolution.This ultimately results in a noticeably superior surface quality,as can be seen in Fig. 4(d) and (e). The surface topography

data given in Fig. 5 shows similar results and confirms thatthe 4%NaCl + 4%KBr and 8%KBr electrolyte solutions are

Table 4 Parameters and results for comparison of pulsed current a

Power source Feed rate (mm/min)

DC 30 V 0.8

Pulsed current U= 30 V, f = 1 kHz, s = 0.7 0.8

Fig. 6 Ti40 surfaces after DC and

Fig. 7 Surface finish of Ti40 after E

the most effective of those tested. However, on the basis thatthe NaCl + KBr solution results in less metal corrosion andstray-current dissolution, it is considered as the best electrolyte

for the ECM of Ti40 and was therefore used in subsequenttesting.

3.2. Pulse current

The parameters and results of comparing pulsed and directcurrent are shown in Table 4, from which it is clear that for

a given set of parameters, the balance current and balancegap are smaller with a pulsed current. However, the currentdensity is improved from 0.876 A/mm2 of the direct current

to 1.091 A/mm2 of the pulse power. The electrochemical prod-ucts could be removed adequately within the pulse intervaltime, and the electrolyte conductivity could be kept relativelyconstant. As a result, the processing is more stable and the bal-

ance gap between the cathode and the workpiece is smallerwhen the feeding rate and dissolution rate are at equilibrium.This results in a significantly better surface finish. The images

provided in Fig. 6 confirm this difference in surface finish, withthe smaller band gap indicating that pulsed-current ECMcreates more concentrated erosion.

In Fig. 7 it can be seen that increasing the frequency ofpulsed-current ECM results in an increase in the surfaceroughness, which is likely due to the decrease in the electrolyteupdating time of a single pulse cycle. The conductivity of the

nd DC ECM of Ti40.

Current density (A/mm2) Balance gap (mm) Roughness (lm)

0.876 0.29 0.786

1.091 0.20 0.375

pulsed current ECM machining.

CM at different pulse frequencies.

Fig. 8 Balance current and surface with varying pulsed-current

duty cycle.

1268 Z. Xu et al.

electrolyte will become non-uniform because the products ofelectrochemical machining such as hydroxide precipitate,hydrogen bubbles, etc., are not cleared out of the machining

gap while there is the decrease in the electrolyte updatingtime. Moreover, the dissolution of each area on the planeis also uneven, thus resulting in a non-uniform machining

surface.As shown in Fig. 8, increasing the duty cycle of ECM

results in an increase in the balance current; as a result, there

is an increase in surface roughness to 0.66 lm at a dutycycle of 0.9. Similarly, the process becomes unstable at a dutycycle of 0.5, and so a duty cycle range of 0.6–0.8 was selected.

3.3. Flow field

Based on numerical simulations of flow field, two differentoutlet slot patterns of cathodes that could ensure stable elec-

trolyte flow are selected to investigate the effect of varying flowfield. The cathodes and work pieces are shown in Fig. 9, andthe 8 mm deep square cavities produced by these cathodes

are shown in Fig. 10. The fact that the undersurfaces of thetwo work pieces are both bright and smooth indicates thatthe ECM process was quite stable, and that the electrolyte in

the bottom machine gap was sufficient, uniform and consis-tent. However, Fig. 10 shows that the side planes A and B ofthe cavity machined by the cathode with 1st pattern slot areflatter than those produced with the 2nd pattern slot. This is

considered to be a result of the 2nd pattern inducing a less uni-form flow along the edges of the cavity, with the flow velocityand electrochemical dissolution being slower in the middle of

each edge than at its ends, thus resulting in a concave shapeto the cavity sides.

Fig. 9 Cathodes with different outlet slots and a Ti40 workpiece.

Fig. 10 Ti40 square cavities machined with different cathode outlet patterns.

Electrochemical machining of burn-resistant Ti40 alloy 1269

3.4. Electrochemical machining of a cavity

The cavities machined under different parameters are shown inFig. 11, while Fig. 12 shows the influence of these parameters

on the machining gap. This demonstrates that while there is anincrease in the machining gap with increasing voltage andelectrolyte concentration, the gap decreases with increasing

feed rate. These results do not clarify the existence of anyrelationship between the machining gap and pressure.However, in general, when the pressure increased, the machin-

ing product could be removed completely and the machiningprocess became more stable, thus resulting in a smallermachining gap.

Fig. 13 shows the influence of various ECM parameters onthe material removal rate MRR. This reveals that even thoughpressure has minimal effect, there is a notable increase in theMRR with increasing voltage and electrolyte concentration.

However, the greatest influence by far is increasing the feeding

Fig. 11 Cavities machined in Ti4

rate, which dramatically increases the MRR to more than3 mm3/AÆmin.

The effect of various ECM parameters on the surface

roughness is shown in Fig. 14, though all experimental condi-tions tested result in surface finish sufficient to meet designrequirements. Nevertheless, it should be noted that both volt-

age and feed rate have a much greater impact on the surfaceroughness than pressure and concentration.

On the basis of the experimental results presented thus

far, the optimal processing parameters for the ECM ofTi40 are considered to be a voltage of 38 V, pressure of0.6 MPa, feed rate of 1.5 mm/min and an electrolyte concen-tration of 8% (see Table 5). The cavity produced by these

ECM conditions is shown in Fig. 15, in which the balancemachining gap, Db, was measured as 0.26 mm, thematerial removal rate was more than 3.1 mm3/AÆmin, and

the surface roughness at the bottom of the cavity was0.371 lm.

0 under different parameters.

Fig. 12 Influence of machining parameters on end gap.

Fig. 13 Influence of machining parameters on MRR.

Fig. 14 Influence of machining parameters on surface roughness.

1270 Z. Xu et al.

Table 5 Optimized parameters for the cavity ECM of Ti40.

Condition Value

Electrolyte 8%NaCl + 8%KBr

Voltage 38 V

Pulse power frequency 1 kHz

Pulse duty cycle 0.7

Temperature of electrolyte (40 ± 0.5) �CInitial gap 0.3 mm

Inlet pressure 0.6 MPa

Machining depth 8 mm

Feeding rate 1.5 mm/min

Fig. 15 Cavity electrochemically machined in Ti40 with

optimized parameters.

Electrochemical machining of burn-resistant Ti40 alloy 1271

4. Conclusion

This study has shown that a mixed aqueous electrolyte of NaCl

and KBr is the best for ensuring the stability and quality of theelectrochemical machining of burn-resistant Ti40 titaniumalloy. In addition, the surface quality can also be improved

by the use of a pulsed-current rather than direct current powersupply, with the best flat-plane machining achieved with a fre-quency of 1 kHz. For the ECM of cavities in Ti40, a surfaceroughness as low as 0.371 lm and a material removal rate

greater than 3.1 mm3/AÆmin has been proven possible by opti-mizing the voltage, inlet pressure, cathode feeding rate andelectrolyte concentration. Because it provides benefits such as

high efficiency, good quality and no tool wear during ECMof burn-resistant Ti40 titanium alloy, this method would havewide applications in the fabrication of aeroengine parts with

difficult-to-machine materials.

Acknowledgements

This study was sponsored by the National Natural ScienceFoundation of China the Program for New CenturyExcellent Talents in University (NCET-12-0627) of China

and the Fundamental Research Funds for the CentralUniversities of China.

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Xu Zhengyang is a professor in College of Mechanical and Electrical

Engineering, Nanjing University of Aeronautics and Astronautics,

Nanjing, China. His area of research includes electrochemical

machining and hybrid machining, etc. He got the 12th Youth Science

and Technology Prize of Chinese Society of Aeronautics and

Astronautics in 2013.