Materials and Design 63 (2014) 531–537

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Materials and Design

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Wear properties of brass samples subjected to constrained groovepressing process

http://dx.doi.org/10.1016/j.matdes.2014.06.0430261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +98 914 401 7268; fax: +98 421 2240862.E-mail address: mebrahimi@iust.ac.ir (M. Ebrahimi).

M. Ebrahimi a,⇑, Sh. Attarilar b, F. Djavanroodi a,c, C. Gode d, H.S. Kim e

a Department of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iranb Department of Materials Science and Engineering, Sahand University of Technology, Tabriz, Iranc Department of Mechanical Engineering, Prince Mohammad Bin Fahd University, Al Khobar 31952, Saudi Arabiad School of Denizli Vocational Technology, Program of Machine, Pamukkale University, Denizli, Turkeye Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea

a r t i c l e i n f o

Article history:Received 31 March 2014Accepted 17 June 2014Available online 2 July 2014

Keywords:Constrained groove pressingBrassWearScanning electron microscopy

a b s t r a c t

Ultrafine grained materials have experienced a rapid development during the last two decades. Con-strained groove pressing (CGP) process is one of the severe plastic deformation methods to fabricateultrafine grain sheet materials. In this research, wear behavior of brass sheet subjected to CGP processwas investigated. Generally it is shown that CGP process enhances the wear resistance of the materialand this behavior is improved by increasing pass number. Also, the effect of initial pass and lower appliednormal load on the wear resistance is more profound than subsequent passes and higher applied normalload, respectively. In addition, the influence of normal load is more profound than pass number at theincrement of friction force. Although CGP process results in reduction at the specific wear rate, the influ-ence of the first pass is much higher than the subsequent ones. Furthermore, lower specific wear rate isoccurred at the higher applied normal load. The scanning electron microscopy analyses indicated that thewear mechanism is transferred from adhesion, delamination, abrasion and oxidation for the annealedcondition to abrasion and adhesion for the third pass CGP sample. Also, it is found that there is a reverserelationship between specific wear rate and hardness.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Various metals and alloys are desired to have enhancedphysical, mechanical, superplasticity, wear, corrosion and fatigueproperties for various applications [1–3]. Over the past twentyyears, microstructural refinement by imposing ultra large plasticdeformation for producing ultrafine grained (UFG) and nanostruc-tured (NS) materials have gained researchers’ attention [1,4,5].Fig. 1 shows the terminology that corresponds different ranges ofgrain size for the material microstructure. Two examples aboutthe application of these methods are aerospace and automobileindustries in which the importance of light weight parts with highperformance are obvious in order to reduce fuel consumption andenvironmental issues [6]. Recently, the application of the severeplastic deformation (SPD) materials in the biomedical implantsand also, the biocompatibility property of such advanced materialshave been investigated [3].

During the last two decades, several SPD methods have beenproposed and developed such as equal channel angular pressing(ECAP) [7], high pressure torsion (HPT) [8], accumulative roll bond-ing (ARB) [9], repetitive corrugation and straightening (RCS) [10],tubular channel angular pressing (TCAP) [11], constrained groovepressing process (CGP) [12], surface mechanical attrition treatment(SMAT) and severe shot peening (SSP) [13,14]. Although ARBmethod has been designed and manufactured for the sheet metalsamples similar to CGP one, its application has been limitedbecause of bonding defects, edge cracking and less feasible prob-lems leading to the material waste and mechanical properties dim-inution [9,12]. It can be said that the dominant rule of all SPDmethods is the increment of the polycrystals’ free energy and thegeneration of more defects and interfaces (grain boundaries) [13].

Depending on the limitation of a sheet sample elongation dur-ing the process, three types of GP process are defined as con-strained GP (CGP), semi constrained GP (SCGP) and GP. In theCGP type, the sheet material is confined by a die set-up (container)in which the sheet is not free to extend during the process in anydirection. In the SCGP, the sheet sample is constrained by a channel[15,16]. In the CGP process, a sheet material is repetitively

Fig. 1. The terminology and grain size ranges for describing materials microstructure.

532 M. Ebrahimi et al. / Materials and Design 63 (2014) 531–537

subjected to shear plastic deformation by application of asymmet-rically grooved and flat dies, respectively as can be seen in Fig. 2. Inthis method, the gap between the upper and lower dies is equiva-lent to the thickness of the sheet sample (Fig. 2b), hence theinclined regions of the sample are exposed to the pure shear defor-mation in the plain strain condition (Fig. 2c) [17]. Then during thesecond cycle, the grooved sample is straightened using the flat die;see Fig. 2d. During this process, a sheet material is placed in thedies and it cannot freely elongate in any directions, therefore thisprocess can be repeated up to the desired strain magnitude andhigher strength. The width (t) and depth (t) of corrugated die areequal hence, the inclined angle is 45�. The theoretical magnitudesof shear and effective strains (c and e) after one cycle CGP processhas been represented in Eqs. (1) and (2) [10]:

c ¼ t=t ¼ 1 ð1Þ

e ¼ 43

c=2ð Þ2� �1=2

¼ 0:58 ð2Þ

Every pass of the CGP process has four cycles. As mentionedabove; at first, the flat sheet sample is corrugated by the grooveddie. By this way, inclined regions which are shown with the verti-

Fig. 2. The schematic representation

cal lines are imparted to the effective plastic strain of 0.58 as can beseen in Fig. 2c. Subsequently, the grooved sample is straightenedby the flat die where in previously pressed inclined regions aresubjected to the plastic shear deformation in the reverse directionleading to a total effective strain of 1.16; see Fig. 2d. Then, the pro-duced flat sample in the previous two cycles is rotated 180� aroundthe axis perpendicular to the plane of the sheet (Fig. 2e). This rota-tion allows the undeformed regions to be deformed by furtherpressings due to the asymmetry characteristic of the grooved dieand so, the grooving and softening stages are carried out againaccording to Fig. 2(f and g). Finally, material with the homogenousand uniform effective plastic strain of about 1.16 is successfullyattained [12,16].

The CGP process was firstly invented by Shin et al. [18] for fab-rication of UFG sheet metal. Lee and Park investigated the plasticdeformation during the CG pressing and rolling in both numeri-cally and experimentally methods [19]. They revealed that thedeformed specimen is not uniform in the thickness direction andthe magnitude of effective strain at the center of the specimen isgreater than that at the surface. In the other study, Krishnaiahet al. studied the production of UFG aluminum sheet by applicationof CGP process at the ambient and cryogenic temperatures [20].They showed that pressing of commercial pure (CP) Al in the

of CGP process during one pass.

Fig. 3. (a) The CGP die and also, (b) brass sample before and after the process.

M. Ebrahimi et al. / Materials and Design 63 (2014) 531–537 533

cryogenic temperature is more effective to improve both yield andultimate tensile strengths in comparison with the room tempera-ture; but, there is not any significant difference at the microstruc-ture. It is also proved that both temperature conditions have abilityto produce UFG structure. In the case of the CGP process of purecopper, they reported that the grain size is reduced from 75 lmto a cell block structure approximately 0.5 lm in the both roomand cryogenic temperatures and also, the temperature does effectthe microstructure [21]. Peng et al. investigated the applicabilityof the CGP process for grain refinement and mechanical propertiesof Cu–Zn alloy [22]. They demonstrated that CGP process can besuccessfully utilized for two-phase alloys, but there is a possibilityof cracking, since two-phase alloys are less ductile than pure met-als. Also, they indicated that wider grooved die can reduce concen-trated stress at the most intensive deformed regions, hence thesample can bear more cycles of CGP process in order to reach finergrain size and higher mechanical properties. Finite element analy-sis of CGP by Yoon et al. indicated that the strain behavior achievedat the first pass is more homogenous than the second pass [23].Zrnik et al. studied the deformation behavior and structure devel-opment during the CGP process of aluminum sample at the ambi-ent temperature [24]. They proved that the sample structuresubjected to between one and four passes shows the formationof banded subgrained microstructure with dislocation cells. Inaddition, the amount of plastic strain does not make a significantinfluence on the microstructural characteristics. It is also foundthat there is a heterogeneous plastic strain distribution acrossthe sheet sample after the final pass. It is believed that the defor-mation mechanisms of deformed materials are often consideredto be governed not only by the activity of the lattice dislocations,but also by mechanisms such as grain rotation and grain boundarysliding. The influence of CGP orientation conditions on the micro-structure of Cu–38Zn alloy was investigated by Peng et al. [25]. Itis concluded that the cross-orientation pressing condition permitsmore pass number, hence higher effective plastic strain and finergrain size can be obtained without any crack formation in the spec-imen as compared to the single-orientation pressing one. Further-more, the cross-orientation pressing situation has higher effect onthe grain refinement rate than the single-orientation pressing atthe range of relatively small effective strain. The investigation byKazeminezhad et al. leaded to a presentation of a new die designentitled modified repetitive corrugation and straightening (MRCS)method [16]. They concluded that the material waste in the CGPmethod is close to zero, while only 84% of the pressed sample’slength is usable in the both RCS and MRCS methods. The wasteof material is occurred from the nonuniform thickness throughthe sample’s length. In addition, the thickness variation in theCGP method is the least. Shirdel et al. evaluated the SPD of purealuminum using semi CGP (SCGP) [15]. Their results showed thatthe real nature of the SCGP method is a combination of both planestress and plane strain situations. A study by Morattab et al. on thefractography of commercial pure Al fabricated by SCGP techniqueshowed that the fracture mode is ductile even after four passes[26]. Also, finer and more dislocation free cells are observed afterthe final pass. The investigation by Mou et al. on the multi-cycleCGP of H62 brass indicated that the surface region where higheffective strain is imposed has finer grain size and higher hardnessvalue than the interior region for a constant pass number [27]. Thisfinding is also independent of the pass number. Study on hardnessand effective plastic strain of H62 brass was conducted by Penget al. [28]. The numerical results indicated that the CGP processunder fully constrained condition leads to the increment of effec-tive strain by 40% compared with the CGP under partially con-strained condition. Hence, the value of the effective plastic strainand its distribution depend strongly on the constrained conditionof CGP process. In addition, simulated results were in good agree-

ment with the experimental test and revealed that the nonuniformstrain distribution leads to nonuniform profile of the both hardnessand grain size. Kumar and Raghu [29] studied the microstructuraland mechanical behavior of Al work-piece processed by CGP. Theydemonstrated that Vickers microhardness (HV) value and its homo-geneity improve continuously with increasing pass number. Also, aconsiderable enhancement in yield and ultimate tensile strengthsis obtained after the first pass whereas subsequent passes lead toimprovement of strengths with the lower rate.

It is well known that wear is the result of erosion, abrasion,impact, metal to metal contact, oxidation and corrosion or a com-bination of these circumstances. Although this property is rarelycatastrophic, it really decreases operating efficiency of machinery.There are number of methods such as pin on disc, and reciprocat-ing pin to characterize the wear behavior of materials [30,31].Brass is one of the important class of materials that refers to a widerange of Cu–Zn alloys and it is used in many applications such asbearings, gears and automobile synchronizing rings where a goodwear resistance is one of the most important considerations [32].There has been several studies to evaluate the effect of SPD tech-nique on the wear property of materials [33–35].

The present study focuses on the wear behavior of brass sheetsamples after it has been subjected to CGP process. Wear propertyare characterized for different pass number of process. Also, fieldemission scanning electron microscopy (FE-SEM) and energy dis-persive X-ray spectroscopy (EDX) were utilized to analyze the wearmechanism changes during the process.

2. Experimental procedure

The brass alloy in the yellow brass series with the chemicalcomposition of (in wt.%) 71.03% Cu, 28.62% Zn, 0.10% Pb, 0.08%Fe and 0.17% other elements were used for this research. The spec-imens with the dimension of 50 mm � 50 mm � 1 mm were pre-pared from the as-received material. All specimens wereannealed at 600 �C for 2 h and then cooled slowly at the furnaceto the room temperature prior to CGP process. The CGP operationwas carried out on the brass specimens up to three passes at roomtemperature, there by imposing a total effective plastic strain of3.48 according to the Eq. (2). It is needed to say that the fracturewas occurred at the first cycle of the fourth pass of brass specimen.Molybdenum disulfide (MoS2) was utilized during the CGP processto reduce frictional effect and also, total required force. The CGP dieand also, the brass sample before and after the process are repre-sented in Fig. 3.

Wear test of the brass sheet specimens was carried out usinglinearly reciprocating ball-on-flat dry sliding method according to

534 M. Ebrahimi et al. / Materials and Design 63 (2014) 531–537

the ASTM: G133 before and after the CGP process. Prior to weartest, the specimens’ surface were grinded and then polished upto 1 lm. The schematic representation of this method is displayedin Fig. 4. All tests were done at the ambient temperature, in thehumidity of about 45% and without lubricant condition. In addi-tion, the ball material, diameter and hardness were tungsten car-bide, 5 mm and 75 HRC, respectively. Furthermore in all tests,constant frequency of 10 Hz and constant stroke length of 6 mmwere used. Also, the magnitude of line speed was considered tobe 0.24 m/s. The end of each test corresponded to a wear time of1200 s, i.e. the total sliding distance was equal to the 288 m. Atleast, two samples were tested for each wear condition. All speci-mens were cleaned before and after the wear test in acetone usingan ultrasonic bath for about 6 min and subsequently air dried. Tomeasure wear mass loss, the difference in the weight of the samplebefore and after the wear test was recorded using a digital balancedevice with a sensitivity of 0.0001 g.

Four different normal loads of 2.5, 5, 10 and 20 N were utilizedfor the annealed and pressed samples and then, influences of thenormal load and pass number were investigated on the wear char-acteristics such as wear mass loss and specific wear rate. Followingrelationship is used to calculate wear quantities [36].

V=L ¼ KðF=HV Þ ¼ KF ð3Þ

where V, L, K, F, HV and k are wear volume (mm3), total slidingdistance (m), Archard’s constant (wear coefficient), applied normalload (N), Vickers hardness value and specific wear rate (mm3/N m),respectively. After wear test, field emission scanning electronmicroscopy (FE-SEM) was used to investigate the morphologiesof the worn surfaces. Also, energy dispersive X-ray spectroscopy(EDX) was performed using the same FE-SEM to analyze the wornsurfaces.

Vickers microhardness (HV) measurements were carried out onthe brass sheet specimens before and after each CGP processaccording to the ASTM: E384. The magnitude of load and dwelltime were 300 gf and 10 s, respectively. Six points at the randomlocation was measured and the average magnitude was reported.Furthermore, optical microscopy (OM) was used for obtaining thegrain size of the annealed sample and classical Williamson–Hall(W–H) method by X-ray diffraction (XRD) was utilized for determi-nation of the crystalline size of the final pass in order to verify thegrain refinement of brass sheet sample during the CGP process. Thedetails of W–H method via XRD technique can be found elsewhere[29].

Fig. 4. The schematic representation of linearly reciprocating ball-on-flat drysliding wear test.

3. Results and discussion

Table 1 lists the wear mass loss magnitudes of brass material forthe annealed (0) and pressed samples up to the three passes for thenormal loads of 2.5, 5, 10 and 20 N. As the first output, it can beseen that the wear mass loss is reduced by utilizing of CGP processirrespective of the applied normal load and the CGP pass number.Also, the second finding is that the wear resistance is enhanced byincreasing pass number irrespective of the applied normal loadmagnitude. For example, there are about 23%, 37% and 45% reduc-tions at the wear mass loss for the first, second and third passes ofCGP sample as compared to the annealed one, respectively whenthe magnitude of applied normal load is equal to the 5 N. The samebehavior has been previously reported on the ECAP process of alu-minum alloy [37] and low carbon steel [38] and also, HPT processof pure copper [39] and aluminum alloy [40]. It can be concludedthat enhancement of wear resistance behavior has been witnessedby various SPD processes.

On the other hand, the effect of the first pass of process is morenoticeable than subsequent passes in wear mass loss of sample. Forexample for the applied normal load of 2.5 N, approximately 33%reduction at the wear mass loss of brass sample is obtained afterthe first pass as compared to the annealed state, while only 22%and 20% reductions at the wear mass loss are attained after the sec-ond and third passes as compared to their previous passes (the firstand second passes). Similar pattern is observed for the otherapplied normal loads as shown in Fig. 5a. The other interestingfinding is that the magnitude of applied normal load has someeffects on the wear mass loss. As an example approximately(33%, 48% and 59%) and (13%, 20% and 25%) diminutions areobtained at the wear mass loss magnitude for the first, secondand third passes of brass sample as compared to the annealed sit-uation for the applied normal loads of 2.5 and 20 N, respectively. Inother words, this indicates that the effect of CGP process on thewear resistance at the lower applied normal loads is more consid-erable than the higher ones as seen in Fig. 5b.

Fig. 6 depicts the effect of applied normal load and CGP passnumber on the average friction force magnitude. The friction forceis calculated by the following relationship:

Ff ¼ l � N ð4Þ

where Ff, l and N are friction force, friction coefficient andapplied normal load, respectively. As can be seen, the effect of nor-mal load is more significant than pass number at the development

Table 1The magnitudes of wear mass loss for various CGP pass numbers and applied normalloads.

Pass number Applied normal load (N) Wear mass loss (mg)

0 2.5 1.145 1.4110 3.2420 5.83

1 2.5 0.765 1.0810 2.7120 5.05

2 2.5 0.595 0.8910 2.3720 4.65

3 2.5 0.475 0.7710 2.1820 4.39

Fig. 5. The wear mass loss reduction in percent after the CGP process up to three passes for the various applied normal loads as compared to the:(a) previous pass and (b)annealed condition.

Fig. 6. The effect of applied normal load and CGP pass number on the friction force.

Fig. 7. The effect of applied normal load and CGP pass number on the specific wearrate.

Fig. 8. The effect of CGP pass number on the both specific wear rate and hardnessmagnitude.

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of the friction force. One can see that about 478% change isobserved at the friction force value for the annealed condition forthe applied normal load of 20 N instead of 2.5 N, while only 3.8%change in friction force is seen at the friction force magnitude forthe normal load of 10 N when third pass CGP sample is testedinstead of annealed sate.

Fig. 7 represents the specific wear rate of brass sample beforeand after CGP process up to the three passes for the four normalloads according to Eq. (5). Interestingly, the magnitude of specificwear rate is reduced by increasing pass number and also, thisreduction is more notable after the first pass of the process. Forexample for the normal load of 2.5 N, about 33%, 22% and only

10% reductions are attained after the first, second and third passesof brass sample as compared to their previous passes, respectively.Furthermore, higher applied normal load leads to lower specificwear rate. There is approximately 27% reduction for the third passCGP sample by using the normal load of 20 N instead of 2.5 N. Theinfluence of CGP pass number on the average specific wear rate hasbeen represented in Fig. 8 irrespective of applied normal load. Thisfigure shows that lower specific wear rate is obtained on the CGPbrass sheet sample. The reduction on the specific wear rate valueis about 41% after the third pass CGP brass sample as comparedto the annealed one.

KF ¼ V=L! K ¼ m=FLq ð5Þ

The improvement of the wear behavior after the CGP processmay be attributed to the grain size refinement and also, hardnessenhancement [12,37]. Fig. 8 shows the Vickers hardness

Fig. 9. (a) The field emission scanning electron microscopy and (b) energy dispersive X-ray spectroscopy of brass samples for the initialed and final pass conditions.

Fig. 10. The optical microscopy of annealed brass sample before CGP process.

536 M. Ebrahimi et al. / Materials and Design 63 (2014) 531–537

magnitudes before and after the process. The results show that theaverage magnitude of hardness is reached 198.5 HV from 63 HV

after three passes of CGP process. In detail, about 170%, 194% and215% improvements at the hardness magnitude are obtained afterthe first, second and third passes of CGP process as compared tothe annealed condition. These findings indicate that there is areverse relationship between the hardness and specific wear rateof brass sheet sample processed by CGP method.

To investigate the worn surface morphology before and afterthe final stage of the CGP process, FE-SEM and EDX analyses havebeen utilized as shown in Fig. 9. It is found that the worn surface of

annealed condition is made up of adhesive, abrasive, delaminationand oxidation regions; see Fig. 9a. The presence of oxidation zonehas been confirmed by the EDX analysis on the annealed samplewith the oxygen amount of 2.66% in weight percent. The delamina-tion condition can be described as material separation from thesample surface and adhesion of it to the ball. With further sliding,the process is reversed, i.e. the brass material which was stuck onthe ball is adhered back to the sample surface. On the other hand,the wear mechanism of the final pass sample is a combination ofabrasion and adhesion. It is also noted that the main wear mecha-nism for the final pass brass sample is abrasive and the degree ofthe adhesive mechanism is severely reduced after the CGP processas comparison with the initial state; see Fig. 9b. Also, the EDX anal-ysis shows that there is no oxidation zone in the 3rd pass sample.

Fig. 10 shows the microstructure of the annealed brass beforethe CGP process which depicts the grain size of about 55 lm, whilethe W–H method by XRD technique indicates that the final pass ofCGPed sample has the crystalline size of approximate 425 nm.These findings reveal that about 99% reduction has been achievedfor the final pass as compared to the as-received condition. Thisintense reduction at the grain refinement can be attributed tothe production, multiplication and locking of dislocations, estab-lishment of low angle and high angle grain boundaries and finallyformation of UFG structure [1]. These phenomena lead to enhance-ment of hardness value and alteration of wear mechanism for thefinal pass CGPed sample as compared to the initial condition. Inother words, significant microstructural refinement together withenhanced mechanical and wear characteristics of brass samplecan be achieved through CGP process.

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4. Conclusions

For this research, CGP die has been designed and manufactured.Then, brass sheet sample has been processed up to three passes atroom temperature. Finally the wear characteristics of the brasssheet sample were investigated. The following conclusions can bedrawn as:

� The wear mass loss of brass material has been diminished byapplication of CGP process. Although the wear resistance hasbeen improved by adding pass number, the effect of first passis more impressive than the subsequent passes. In addition,the effect of the CGP process on the wear behavior at the lowerapplied normal load is more considerable than the higher one.� The influence of applied normal load is more profound than CGP

pass number at the increment of friction force value. In addi-tion, higher normal load causes increment at the friction forcevalue. Although CGP process leads to reduction at the specificwear rate of brass sample, the effect of first pass is more signif-icant than the subsequent ones. Also, lower specific wear ratehas been achieved at the higher applied normal loads.� The wear mechanism has been transferred from adhesion,

delamination, abrasion and oxidation for the annealed condi-tion to abrasion and adhesion for the third pass CGP.� CGP process of brass sheet sample not only leads to more resis-

tance to wear, but also results in enhancement of hardnessproperty. In addition, intense grain refinement has beenachieved after the CGP process in comparison with the annealedstate. It can be said that UFG structure of final CGPed samplegives better wear resistance because of alteration at the wearmechanism.

Acknowledgements

One of the authors, M. Ebrahimi, would like to thank the Scien-tific and Technological Research Council of Turkey (TÜBITAK) forsupporting the portion of this project under the 2216 Research Fel-lowship Program for Foreign Citizens.

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