ORIGINAL ARTICLE

Ductility improvement of aluminum 1050A rolled sheetby a newly designed ball burnishing tool device

Fathi Gharbi & Salem Sghaier & Hedi Hamdi &Tarek Benameur

Received: 12 January 2011 /Accepted: 16 August 2011 /Published online: 30 August 2011# Springer-Verlag London Limited 2011

Abstract Conventional ball burnishing processes using aroller or a ball pressed against round or small flat surfaceshave long been used to improve hardness, fatigue strength,and wear resistance of mechanical parts by plastic defor-mation. However, the treatment of large flat surfaces usingconventional techniques is rarely considered because of itstime consumption. In the present work, the optimalburnishing parameters of rolled sheets of aluminum1050A are determined by means of a newly developedburnishing tool device especially designed to treat large flatsurfaces with orders of magnitude reduction in burnishingtime. Experiments were designed and performed on amachining center based on response surface methodologywith central composite design. The burnished specimenswere then tested to find the burnishing condition underwhich ductility was improved. This study has resulted insignificant new insights into the effect of burnishing on thesurface quality and workpiece properties of aluminum1050A plates. A second-order mathematical model, vali-dated using data obtained from atomic force microscopy,was developed to predict the surface roughness as functionsof speed, force, and feed rate. The results indicate that

burnishing of aluminum 1050A plates improves its ductility,but not its micro-hardness. Following the various burnishingconditions, the micro-hardness measurements range from 40to 43 HV (50 g), indicating that there is little or no hardening.Although a moderate effect with varied degrees is found onthe surface roughness as functions of the investigatedparameters, the burnishing force has a significant effect onductility. The results also indicate that lower values ofroughness do not guarantee better ductility for aluminum1050A plates. Furthermore, the effect of the burnishingloads on the residual stresses was found to depend on thefeed direction.

Keywords AFM . Burnishing . Hardness . Surfaceroughness . Tensile strength

Abbreviationsn Spindle speed (rpm)Vc Cutting speed (mm/min)d Burnishing tool diameter (mm)A Burnishing speed (mm/min)f Burnishing feed (mm/rev)Z Number of ballsTcycle Cycle time (s)L Burnished workpiece length (mm)W Burnished workpiece width (mm)e Tool release (mm)Nc Cycle numberTt1 Required burnishing time for the simple burnishing

tool (s)Tt2 Required burnishing time for the new burnishing

tool device (s)C Course (mm)F Burnishing force (N)HV Vickers micro-hardness

F. Gharbi (*)Institut Supérieur des Sciences Appliquéeset de Technologie de Kairouan,Kairouan, Tunisiae-mail: fathi.gharbi@yahoo.fr

F. Gharbi : S. Sghaier : T. BenameurLaboratoire de Génie Mécanique LGM-MA05, ENIM,5019( Monastir, Tunisia

H. HamdiUniversité de Lyon, ENISE, LTDS CNRS UMR 5513,58 rue jean Parot,42023 Saint Etienne, France

Int J Adv Manuf Technol (2012) 60:87–99DOI 10.1007/s00170-011-3598-6

Ra Arithmetical surface roughness average (μm)Rq Root mean square (nm)Rmax Maximum roughness depth (nm)Zmin Minimum peak depth (nm)Zmax Maximum peak depth (nm)Npic Number of peaks foundZhmax Depth of histogram maximum (nm)Lp Peak-to-peak distance (nm)

1 Introduction

Regardless of the machining methods used, all surfaces ofmechanical parts consist of peaks and valleys with irregularheight and spacing. Burnishing is a widely used processthat can be applied to improve surface quality and producelow values of roughness for various materials such as steel,aluminum, brass, and so on. Burnishing is a cold treatmentthat consists in a deforming tool passing under a generatedpressure onto the workpiece surface. This can be done byrolling (roller burnishing) or by ball (ball burnishing).During burnishing, the generated pressure exerted by thetool exceeds the yield point of part surface at the point ofcontact, causing a small plastic deformation. This plasticdeformation created by roll or ball burnishing is adisplacement of the material from the peaks that flowsunder pressure into the valleys, resulting in a mirror-likefinish with a strain-hardened, wear-, and corrosion-resistantsurface [1]. In addition, this process transforms tensileresidual stress induced in the near surface by machininginto compressive residual stresses, giving several improve-ments to mechanical properties [2].

Several authors have investigated the effect of burnish-ing on mechanical properties improvement such as hard-ness [3–5], surface quality [6–12], state of residual stress[13–16], fatigue life, and corrosion of the workpiece [17,18]. In a recent study, Gharbi et al. [8] have identified theeffect of the burnishing parameters on the surface qualityand microstructure properties of AISI 1010 steel plates.They found that the burnishing force is the most influentialparameter. A force of more than 400 N was found to causeflaking of the burnished surface. In another study, Korzynskiand Panaca [19] introduced a centerless roller burnishing tostudy 41Cr4 steel and the influence of burnishing parameterson machining effects. They found that the average of surfaceroughness was three to six times smaller than that of theunburnished surface. Shiou and Ciou [20] have developed avibration-assisted mechanism on a machining center thatuses spherical polishing to determine the optimal planesurface of STAVAX plastic mold stainless steel. The authorsreported that their method significantly improves the surfaceroughness and is capable of increasing the volumetric wear

of the polishing ball up to 72% in comparison with the non-vibrated polishing process.

Furthermore, El-Taweel and El-Axir [21] used theTaguchi technique to analyze and optimize the ballburnishing process of brass. Their results show that theburnishing force had a dominant effect on both the surfaceroughness and the micro-hardness of brass. The improve-ment in surface roughness and micro-hardness was reportedrespectively as 39.87% and 42.85%. In addition, El-Tayebet al. [22] have found that the roller burnishing ofaluminum 6061 can decrease the surface roughness asmuch as 40% and increase the hardness from 20% to 30%in comparison with turning. In general, the improvement ofthe surface roughness due to burnishing ranged between40% and 90% [4–10].

In order to properly design a burnishing process, severalcontrolling parameters have to be considered, namely,burnishing speed, feed rate, force (or pressure) and ballsize, number of burnishing passes, workpiece material, ballmaterial, and lubricant [6]. However, among these param-eters, most of the literature generally indicated that theburnishing force (or pressure) and the feed rate are the twomain parameters that affect the surface finish of a particularmetal. To limit the number of experiments needed and tooptimize the input parameters that give the best responsesurface, two methods are used: response surface methodology(RSM) with factorial design [23] and Taguchi’s method withanalysis of variance [24].

Unlike round or small flat surface workpieces, thetreatment of large flat surfaces is hardly ever consideredin the literature. The reason for this is that burnishing oflarge flat surface using a conventional simple tool is a time-consuming process. The purpose of this study was todetermine the optimal burnishing parameters of rolledsheets of aluminum 1050A by means of a newly developedburnishing tool device especially designed to treat large flatsurfaces. This unique tool device is designed by the authorsand ensures orders of magnitude reduction in burnishingtime. The reduction in the burnishing time obtained usingthe new tool device is compared with that obtained using aconventional simple tool. Several experiments weredesigned and performed on a machining center based onRSM with central composite design [23]. In order toexplore the optimum combination of burnishing param-eters, the effects of burnishing speed, force, and feed rateon the surface roughness (Ra) and surface micro-hardness(HV, 50 g) were investigated. A second-order mathemat-ical model was developed to predict the surface roughnessas a function of these parameters. The model wasvalidated using data obtained from an atomic forcemicroscopy (AFM). The burnished specimens were thentested to find the burnishing condition under whichductility was improved.

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2 Comparison with the conventional method

Figure 1 compares a conventional simple burnishing tool withthe designed burnishing tool device used in this study. Asindicated, the new burnishing tool device can handle more thanone simple tool depending on the number of holes located inthe disc (Fig. 1). For example, Fig. 1b shows a burnishing tooldevice with four holes in which four conventional tools aremounted. It should be noted that before burnishing, all thetools are mounted so that they touch the surface of theworkpiece at the same time. This is done manually before thestart of the burnishing process. More detailed information onthis new burnishing tool device is reported in the reference byGharbi et al. 2010 [8]. Since large flat surfaces are consideredin this study, it would be interesting to compare theburnishing time obtained using a conventional burnishingtool mounted on a planing machine (Fig. 2) with that usingthe new burnishing tool device mounted on a milling or amachining center (Fig. 3). To derive an equation for thepercent reduction in time obtained with the new tool, supposea machine slide surface of dimensions (L × W) is consideredfor burnishing, as shown in Figs. 2 and 3. Furthermore, it isassumed for simplicity that one pass using the new tooldevice can cover the entire machine slide surface. That is, thediameter between two opposing holes is equal to the width ofthe machine slide. The required burnishing times using theconventional simple tool and the new rotary burnishing tooldevice are assumed to be Tt1 and Tt2, respectively.

In the case of Fig. 3, the cutting speed (millimeters perminute) can be expressed as:

Vc ¼ p d n ð1Þwhere n is the spindle speed (revolutions per minute) and dis the tool diameter (millimeters). The burnishing speed isgiven by:

A ¼ f � n � Z ¼ f � Z � Vc

p � d ð2Þ

where f is the feed (millimeters per revolution) and Z is thenumber of balls. According to Fig. 2 (conventionalmethod), the burnishing time (Tt1) can be expressed as:

Tt1 ¼ Nc � Tcycle ð3Þ

where Nc is the number of cycles and Tcycle is the time ofone cycle. Nc and Tcycle are also given as:

Nc ¼ W

fð4Þ

Tcycle ¼ 2 � Lþ 2eð ÞVc

ð5Þ

Therefore,

Tt1 ¼ Nc � 2 � Lþ 2eð ÞVc

¼ 2 �W � Lþ 2eð Þf � Vc

ð6Þ

Given the assumption that one pass during the burnish-ing process can cover the entire machine slide surface(Fig. 3), the burnishing time (Tt2) is given as:

Tt2 ¼ C

Að7Þ

where C is the burnishing tool course given by:

C ¼ Lþ d þ 2e ð8Þand A is the burnishing speed expressed as:

A ¼ f � n � Z ¼ f � Z � Vc

p � dThe burnishing time (Tt2) is therefore equal to:

Tt2 ¼ Lþ d þ 2eð Þ � p � df � Z � Vc

ð9ÞFig. 1 a One simple burnishing tool. b New burnishing tool device [8]

Fig. 2 Schematic diagram of the ball burnishing process (conven-tional method)

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Dividing Eq. 6 with Eq. 9 gives the ratio R for timereduction using the new burnishing tool device as follows:

R ¼ Tt1Tt2

¼2�W � Lþ2eð Þ

f �Vc

Lþdþ2eð Þp�df �Z�Vc

¼ 2 �W � Lþ 2eð Þ � ZLþ d þ 2eð Þp � d ð10Þ

If the following data are used: d=W=95 mm, L=1,000 mm, and e=2 mm, then the values of R can beexpressed as a function of the number of balls (Z) as follows:

R ¼ Tt1Tt2

¼ 2 94ð Þ 1; 000þ 4ð ÞZ1; 000þ 94þ 4ð Þp 94ð Þ ¼ 0:582 Z ð11Þ

Table 1 lists the values of R and the percent timereduction of burnishing time obtained using the new tooldevice as a function of the number of balls (Z). Asindicated, an increase in the number of balls leads to asignificant reduction of the burnishing time. For example, ifsix simple burnishing tools are mounted on our newburnishing device, this would reduce the time by morethan 249% as compared with using one simple burnishingtool (milling vs. planing). It should be noted from Eq. 11that when the number of balls is equal to 1, burnishing large

flat surfaces using milling would require more burnishingtime than using planing. However, if our burnishing tooldevice was used with a single tool (Z=1) to burnish a PDS5specimen using the same optimal burnishing parameters asreported by Shiou and Chen [25] (milling vs. milling), thereduction in burnishing time would reach 3,642%. Thisindicates a tremendous benefit when using our new tooldevice to burnish large flat surfaces.

3 Experimental work

The newly developed burnishing tool was used device todetermine the optimal burnishing parameters of rolledsheets of aluminum 1050A. The material was received ina form of cold-rolled plates with a thickness of 3 mm. Thismaterial was selected because of its importance in themanufacturing industries. The chemical compositions andthe mechanical properties of the selected material are listedin Table 2. As listed in Table 2, the yield and tensilestrengths are 109.6 and 121.24 MPa, respectively.

The surface micro-hardness of the unburnished surfaceswas measured using a hardness tester (Shimadzu). A

Fig. 3 Schematic diagram of the ball burnishing process (with new tool device)

Table 1 Time reduction usingthe new tool device Number of balls (Z)

2 3 4 5 6

Ratio (R) 1.164 1.746 2.328 2.91 3.492

Time reduction (%) 16.4 74.6 132.8 191 249.2

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roughness tester of type RT-10 was used to measure thesurface roughness before and after burnishing. For betteraccuracy, a total of nine measurements were made for boththe roughness and the surface micro-hardness at differentlocations of the workpiece surface. The mean values of theroughness and the micro-hardness are calculated byaveraging these measurements. A mean value of 41±1HV (50 g) was obtained for the micro-hardness. The meansurface roughness (Ra) of the unburnished material wasmeasured to be 1.5±0.5 μm. The tensile tests wereperformed using an Instron Servohydraulic UTS 8502machine with a maximum capacity of 250 KN. All thetests were conducted with a speed of 5 mm/min. ANanoScope IV Multimode AFM and a JSM-6300 SEM

were used to examine the surface topography of theunburnished and burnished samples.

Tensile specimens were laser-cut along the rolling directionof the aluminum sheets so that they can be burnished and thenused to investigate their mechanical behavior. The lasercutting guarantees the smoothest surfaces with the bestpossible dimensional precision. The assembly drawings ofthe ball burnishing tool device and the test specimen areshown in Fig. 4 [8]. The experimental work was carried outusing a C-tek machining center (CNT 830) with one simpleburnishing tool fixed at a radius of 47.5 mm from the centerof the disc. As a consequence, a 95-mm width of burnishedsurface is covered for every course C of the machine table. Inour case, when the machine table completes a course C, theburnishing tool touches the workpiece surface twice as it isrotating (i.e., front and back). Therefore, if a pass is definedwhen the ball of the burnishing tool touches the whole areaof the workpiece once, two passes are completed for everycourse C of the machine table.

4 Experimental design and analysis

A simple and adequate experimental design, RSM withcentral composite design, was used to optimize theburnishing process using three parameters, namely, bur-nishing speed, force, and feed rate. All experiments wereperformed using a ball diameter of 10 mm, a number ofburnishing passes of 2, and a penetration depth of 0.2 mm.ESSOLUBE HD 15W-40 is used as lubricant oil betweenthe tool and the workpiece. The oil has a kinematicviscosity of 113 and 15.4 mm2 s−1 at 40°C and 100°C,respectively.

Table 2 Chemical composition of the workpiece material (aluminum1050A)

Element Weight (%)

Al 99.574

Si 0.069

Fe 0.293

Cu 0.007

Ni 0.002

Zn 0.006

Sn 0.002

Ti 0.047

Mechanical properties of the workpiece material

Yield strength (MPa) 109.6

Tensile strength (MPa) 121.24±1

Hardness (HV, 50 g) 41±1

Tensile elastic modulus (MPa) 68,000

Fig. 4 a Burnishing tool device mounted on the machining center. b Machining setup [8]

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Table 3 summarizes the burnishing conditions andtheir coded levels. As indicated, five levels wereconsidered for each investigated parameter. Accordingto the central composite design with three independentparameters, 20 experiments were conducted with thecombination of values listed in Table 3. The experimen-tal plan was simplified using five coded levels of −2, −1,0, 1, and 2 for each parameter using the followingtransformations:

x1 ¼ n� 600

200ð12Þ

x2 ¼ F � 150

50ð13Þ

x3 ¼ f � 0:15

0:05ð14Þ

where x1, x2, and x3 represent the burnishing speed,force, and feed rate, respectively. Table 4 shows thearrangements and the results of the 20 experiments thatwere performed. The results, indicated in Table 4 in terms

Table 3 Parameters of burnish-ing process Factors Symbol Levels

−2 −1 0 1 2

Speed (rpm) x1 200 400 600 800 1000

Force (N) x2 50 100 150 200 250

Feed (mm/rev) x3 0.05 0.1 0.15 0.2 0.25

Ball diameter (mm) 10

No. of passes 2

Burnishing depth (mm) 0.2

Burnishing conditions Lubricated

Table 4 Experimental design matrix and result of surface quality

Exp. no Speed (rpm) Force (N) Feed (mm/rev) Response

Coded value Act. value Coded value Act. value Coded value Act. value Ra (μm)

1 −1 400 −1 100 −1 0.1 0.70

2 1 800 −1 100 −1 0.1 0.71

3 −1 400 −1 100 1 0.2 0.93

4 1 800 −1 100 1 0.2 1.33

5 −1 400 1 200 −1 0.1 1.20

6 1 800 1 200 −1 0.1 1.18

7 −1 400 1 200 1 0.2 1.11

8 1 800 1 200 1 0.2 1.15

9 −2 200 0 150 0 0.15 0.79

10 2 1000 0 150 0 0.15 2.13

11 0 600 −2 50 0 0.15 0.75

12 0 600 2 250 0 0.15 2.20

13 0 600 0 150 −2 0.05 0.75

14 0 600 0 150 2 0.25 1.85

15 0 600 0 150 0 0.15 0.72

16 0 600 0 150 0 0.15 0.70

17 0 600 0 150 0 0.15 0.68

18 0 600 0 150 0 0.15 0.70

19 0 600 0 150 0 0.15 0.73

20 0 600 0 150 0 0.15 0.71

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of surface roughness, represent the mean of nine measure-ments made at different locations of the workpiecesurface. Measurements of the micro-hardness indicate thatunder these burnishing conditions, their values range from40 to 44 HV, suggesting there is little or no hardening ofthe surface. Figure 5 compares the SEM morphology ofaluminum 1050A under various burnishing conditions. Asindicated, burnishing has a significant effect on theworkpiece surface as a result of plastic deformation,which is clearly visible in the images.

Using the results of the mean surface roughness, asecond-order response model of ball burnishing processon aluminum 1050A plates was developed in terms ofthe burnishing speed (x1), force (x2), and feed (x3) asfollows:

Ra ¼ 0:6709þ 0:1944x1 þ 0:2419x2 þ 0:1831x3þ0:1704x21 þ 0:1742x22 þ 0:1304x23

ð15Þ

In real values, this equation can be rewritten as follows:

Ra ¼ 3:0877� 0:00414 n� 0:01607F � 11:986 fþ426:10�8n2 þ 6968:10�8F2 þ 52:16f 2

ð16Þ

It should be noted that to obtain Eq. 15, some coefficientswere omitted. A backward elimination procedure with asignificance level of 5% was used. According to theStudent’s t test, which determined the significant and non-significant parameters, these coefficients were found to benon-significant. Table 5 lists the results of the t test.Furthermore, in order to test its adequacy, the final modelwas tested using the variance analysis (F test). Table 6 liststhe results of this test. The “lack of fit F value” of 419.26implies that it is significant with the p value being close tozero. With the significant lack of fit, this suggests that athird-order polynomial model may be needed to betterdescribe the performance behavior of the surface roughness.On the other hand, for the current model, R2≈77%, which is

Fig. 5 SEM morphology of 1050A aluminum specimens. a Unbur-nished. b Burnished (burnishing conditions: F=200 N, n=800 rpm, andf=0.05 mm/rev). c Burnished (burnishing conditions: F=200 N, n=

600 rpm, and f=0.1 mm/rev). d Burnished (burnishing conditions: F=100 N, n=600 rpm, and f=0.2 mm/rev)

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only slightly less than R2 ≈ 82% for the full quadratic modeland comparable with those values reported in similar studies[3, 5, 7]. Therefore, for these reasons and for simplicity, thecurrent model for the surface roughness is kept.

5 Results and discussions

5.1 Effects of burnishing parameters on the surfaceroughness

Once the model was developed, it can then be used topredict the variations of the surface roughness fordifferent combinations of burnishing speed, feed rate,and force. These variations are presented in terms oftwo-dimensional plots shown in Figs. 6, 7, and 8. Eachcurve in a given plot shows the effect of one burnishingparameter on the mean roughness with the other twoparameters constant. As indicated in Figs. 6, 7, and 8, the

relationships between the mean roughness and each ofthe burnishing parameters are parabolic. An increase ofthe burnishing speed, the feed, or the force causes adecrease in mean roughness down to a minimum value.Beyond this minimum value, the mean roughness startsto increase with the increase of each of the burnishingparameters. It should be noted that the optimumsolution for this convex function of x1, x2, and x3 couldbe easily determined. The calculated optimum values arex1=−0.5704 (speed=486 rpm), x2=−0.6943 (force=115 N), and x3=−0.7021 (feed=0.115 mm/rev). Thesevalues are indicated in the figures on the x-axis. However,we used graphical analysis to obtain optimum burnishingparameters with values that we have already used in ourexperiments, so further analyses can be done on thespecimen. The analyses include the effect of burnishingon the tensile properties and the distribution of compres-sive residual stresses, as will be indicated later inSections 5.2 and 5.3.

Table 5 Results of the Student’st test

Standard critical value of t test:t0.05,10=2.12aCoefficients that are significant

Coefficient Value of coefficient Computed t value Significant coefficientsRa Ra Ra

b0 0.67091 5.709 0.67091a

b1 0.19437 2.639 0.19437a

b2 0.24188 3.284 0.24188a

b3 0.18313 2.486 0.18313a

b11 0.17045 2.901 0.17045a

b22 0.17420 2.965 0.17420a

b33 0.13045 2.220 0.13045a

b12 −0.04875 −0.468 –

b13 0.05625 0.540 –

b23 −0.12125 −1.164 –

Table 6 Ra response using theF test Source df Seq SS Adj SS Adj MS F p

Regression 6 3.43905 3.43905 0.573175 7.23 0.001

Linear 3 2.07712 2.07712 0.692373 8.74 0.002

Speed 1 0.60451 0.60451 0.604506 7.63 0.016

Force 1 0.93606 0.93606 0.936056 11.81 0.004

Feed 1 0.53656 0.53656 0.536556 6.77 0.022

Square 3 1.36193 1.36193 0.453978 5.73 0.010

Speed × speed 1 0.37038 0.73052 0.730519 9.22 0.010

Force × force 1 0.56366 0.76302 0.763016 9.63 0.008

Feed × feed 1 0.42789 0.42789 0.427891 5.40 0.037

Residual error 13 1.03013 1.03013 0.079241

Lack of fit 8 1.02859 1.02859 0.128574 419.26 0.000

Pure error 5 0.00153 0.00153 0.000307

Total 19 4.46918

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5.1.1 Burnishing speed

Based on the developed mathematical model, Eq. 15, theeffects of burnishing speed on the mean roughness forvarious feed rates and forces are shown in Fig. 6a, b,respectively. Figure 6a shows the effect of burnishing speedon the mean roughness for different burnishing feeds and ata fixed force (F=150 N). As observed from Fig. 6a, thelowest value of the mean roughness was achieved at a feedrate of 0.1 mm/rev for F=150 N. Furthermore, Fig. 6bshows that for a burnishing feed of 0.15 mm/rev, the lowestvalue of the mean roughness was achieved at a burnishingforce of 100 N. Therefore, the optimal feed rate andburnishing force for aluminum 1050A plates are 0.1 mm/rev and 100 N, respectively. Examining all the values ofburnishing forces and feed rates, a minimum roughness isachieved at a burnishing speed ranging from 400 to600 rpm. A marginal increase of the mean roughnessoccurred below 400 rpm, whereas a significant increasetook place above 600 rpm. This is because beyond600 rpm, an increase in the burnishing speed causes a rise

in the temperature in the burnishing ball–workpieceinterface, in addition to possible chattering. The combina-tion of these factors may have increased the possibilities ofmicrostructural evolution, leading to higher values ofsurface roughness.

5.1.2 Burnishing feed

Alternative presentations of the above results are shown inFig. 7a, b. The figures demonstrate the variations of themean roughness with the feed rate for different values offorce and burnishing speed, respectively. Both figuresindicate that the lowest values of surface roughness wasachieved at a feed rate ranging from 0.1 to 0.15 mm/rev.Figure 7a shows the effect of feed rate on the surfaceroughness at different burnishing speeds at a fixed force ofF=150 N. As indicated, burnishing conditions with speedsof 400 and 600 rpm yielded the same results. In addition,Fig. 7b indicates that for a burnishing speed of 600 rpm, thelowest values of mean roughness are obtained at values ofburnishing force of 100 N. The results also indicate that a

Fig. 6 Effect of burnishing speed on mean roughness: at different feeds (millimeters per revolution) (a) and at different forces (Newton) (b)

Fig. 7 Effect of burnishing feed on mean roughness: at different speeds (revolution per minute) (a) and at different forces (Newton) (b)

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change in the feed rate above the optimum value of 0.1 mm/rev increases the surface roughness. This may be due to theincrease of the distances between the successive burnishingball traces. As the feed rate increases, the burnishing ball hasless and less chances to smooth out the previous traces. This inturns will cause an increase in surface roughness. As aconsequence of Figs. 6 and 7, a feed rate of 0.1 mm/rev and aburnishing force of 100 N with a speed ranging from 400 to600 rpm yielded the lowest values of surface roughness foraluminum 1050A plates.

5.1.3 Burnishing force

The effects of burnishing force on the mean roughness forvarious burnishing speeds and feed rates are shown in

Fig. 8a, b, respectively. The figures indicate that the meanroughness can be significantly affected by the burnishing force.Similarly to Fig. 7, the results show that burnishing speeds of400 and 600 rpm yielded comparable output at a feed of0.15 mm/rev for the full spectrum of burnishing forces.

It should be noted that a combination of very highburnishing force with high feed is harmful to the surfaceroughness, as indicated in Fig. 8b. This is because highforces cause a high amount of work hardening, which inturn may cause flaking on the metallic surface layer of theworkpiece. In this case, deterioration in the surface of thealuminum 1050Aworkpiece will occur, causing an increasein the surface roughness. It should also be noted that for agiven burnishing force, an increase in the feed rate or in theburnishing speed decreases and then increases the mean

Fig. 8 Effect of burnishing force on mean roughness: at different speeds (revolutions per minute) (a) and at different feeds (millimeters perrevolution) (b)

Fig. 9 Evolution of AFM image parameters for different burnishing conditions

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roughness. Examining Figs. 6, 7, and 8, the values of theburnishing parameters at which minimum roughness wasachieved are 0.1 mm/rev for the feed, 100 N for the force,and 400–600 rpm for the speed.

Figure 9 shows a radar plot indicating the evolutionof AFM image parameters for different burnishingconditions. As shown, the optimum burnishing param-eters using AFM data are 0.1 mm/rev, 100 N, and400 rpm for the feed, force, and speed, respectively.Even at a small-scale investigation, these values are inagreement with those obtained by our model. The useof AFM data confirmed that the optimum speed is400 rpm. Therefore, the developed mathematical modelcan be used with good confidence to predict the surfaceroughness as functions of feed, speed, and force foraluminum 1050A. Examples of the AFM imagesrepresenting different burnishing conditions are shownin Fig. 10.

5.2 Effect of burnishing on the tensile properties

The tensile test curves for cold-rolled aluminum 1050Afor different burnishing conditions are illustrated in

Fig. 11. As shown in Fig. 11, burnishing aluminum1050A plates can have a significant effect on its stress–strain behavior. The stress–strain behavior for aluminum1050A remains more or less constant in the plastic zone.Depending on the burnishing force, the yield strength andthe ultimate tensile strength may decrease or increase as

Ra = 50.6 nm, Rmax = 527 nm Ra = 29 nm, Rmax = 268 nm

Ra = 11.2 nm, Rmax = 146 nm

Fig. 10 Comparison of AFM images for different burnishing conditions within an area of 7×7 μm. a Unburnished. b Burnished (burnishingconditions: F=200 N, n=400 rpm, and f=0.1 mm/rev). c Burnished (burnishing conditions: F=100 N, n=400 rpm, and f=0.1 mm/rev)

Fig. 11 Stress–strain curves of tensile tests for burnished andunburnished specimens

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compared with the unburnished condition. Figure 11 alsoshows that the plasticity increases with no increase in thestress. Both the energy and percent elongation at fractureincrease with burnishing, as listed in Table 7. Thisincrease is may be due to the removal of the micro-cracks on the edges of the specimen during tool rotationor may be caused by the improvement in the surfacefinish that significantly reduces the surface flaws whencracks can initiate.

In comparison with the unburnished specimen, theimprovement in elongation is up to 48% when thespecimen is burnished with burnishing conditions of200 N, 400 rpm, and 0.1 mm/rev. In this optimum case,the yield strength, ultimate tensile strength, energy, andpercent elongation at fracture for aluminum 1050A were108.2 MPa, 125.16 MPa, 21.01 J, and 12.94%, respec-tively. In this case, the burnished specimen under theoptimal conditions resists tensile well and its plasticityincreases. In addition, following the various burnishingconditions of aluminum 1050A plates, the micro-hardness measurements show that there is little or nohardening of the specimen surface. The average of themicro-hardness measurements on the burnished andunburnished specimens remains in the interval of 40–44 HV. Therefore, based on these results, burnishing ofaluminum 1050A plates improves its ductility, but not itsmicro-hardness.

5.3 Distribution of compressive residual stress

Further investigation was carried out to study the effect ofburnishing loads on the residual stresses (σxx) in aluminum1050A rolled sheets. The residual stresses were determinedusing a Proto XRD unit. The latter is fitted with achromium Kα radiation tube (wavelength, l=2.291Å) at20 kV, 4 mA to acquire the (hkl)=(222) diffraction peak atBragg’s angle 2θ=156.31°, and using a collimator size of2 mm in diameter. Sin2y method measurements wereperformed at seven β angles from 25° to −25° using 3°oscillations at each β angle (with Proto XRD unit

y ¼ b � p�2qð Þ2 ).

The mechanical stresses exerted by ball burnishing onthe specimen surface led to sustained modification on theresidual stress state. Figure 12a, b illustrates the distributionof residual stress profiles measured beneath the surfaceaccording to the feed direction and the cross-feed direction(Fig. 3), respectively. In both directions, residual stresses inthe case of unburnished specimens are considered as null. Forthe two mentioned conditions of Fig. 12a, b, residual stressesare compressive with low values between 0 and −45 MPa. Itseems that in the feed direction, burnishing normal load hasan influence on the affected depth corresponding to theoptimum value of the residual stress σxx. When the normalload increases from 100 to 200 N for the same ball diameter

Table 7 Comparison of the mechanical characteristics between unburnished and burnished specimens

Ultimate stress(Mpa)

Energy atfracture (J)

Elongation atfracture (%)

Microhardness,HV (50 g)

Unburnished 121.24±1 14.6 8.70±0,03 41±1

Burnished: 50 N–800 rpm–0.05 mm/rev 115.11±1 15.23 9.60±0,03 40±2

Burnished: 100 N–400 rpm–0.1 mm/rev 115.19±1 16.8 12,04±0,03 41±3

Burnished: 200 N–400 rpm–0.1 mm/rev 125.16±1 21.01 12.94±0,03 43±1

Fig. 12 Residual stress profiles measured from surface: in the feed direction, σxx (a), and in the cross-feed direction, σyy (b)

98 Int J Adv Manuf Technol (2012) 60:87–99

(10 mm), the affected depth increases from 40 to 110 μm,respectively, for an optimum value of σxx −30 and −45 MPa(Fig. 12a). However, this was not the case in the cross-feeddirection. This is probably due to the material flow around theburnishing ball when rolling on the surface, where the lateralflow takes a greater place than the frontal one. For eachloading condition, the affected depth for σyy remains constantat 110 μm and the optimum value of the residual stress σyy isequal to −30 MPa (Fig. 12b).

6 Conclusion

The effects of burnishing on the surface quality andworkpiece properties of aluminum 1050A rolled sheetshave been determined. Experiments were designed andperformed on a machining center based on RSM withcentral composite design. Based on this study, severalconclusions may be drawn. First, a newly developedburnishing tool device especially designed to treat largeflat surfaces is successfully used in this study. Whencompared with conventional burnishing tools, the newone ensures orders of magnitude reduction in burnishingtime. Second, optimal burnishing parameters of rolledsheets of aluminum 1050A have been determined. Asecond-order mathematical model was then developed topredict the surface roughness as functions of speed,force, and feed rate. This model can be used with goodconfidence as it was validated using data obtained fromAFM. Furthermore, even though burnishing of alumi-num 1050A plates improves its ductility, the micro-hardness measurements show that there is little or nohardening of the specimen surface. While a moderateeffect was found on the surface roughness as functionsof the burnishing parameters, the burnishing force has asignificant effect on ductility. Lower values of roughnessdo not guarantee better ductility for aluminum 1050Aplates. Finally, burnishing loads have an influence onthe residual stresses σxx and the affected depth in the feeddirection, but not in the cross-feed direction.

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