Surface quality improvement by low plasticityburnishing (LPB) of mild steel

Ahmed Es. Nassef Port Said University, Egypt, Faculty of Engineering, Production Engineering and

Mechanical Design Department, Professor, nassef12 @yahoo.com Noha Fouad Naeim

Port Said University, Egypt, Faculty of Engineering, Production Engineering and MechanicalDesign

Department, Teaching Assistant, noha.naeim@yahoo.comAli El-Baghdady

Port Said University, Egypt, Faculty of Engineering, Production Engineering and MechanicalDesign

Department, Assistant professor, alielbagdady@yahoo.com

Abstract Low plasticity burnishing (LPB) process is chipless finishing methods which easilyproduce a smooth and hardened surface by plastic deformation of surface asperities.Burnishing is a process in which the peaks of metallic surfaces are displaced to fill the valleysby plastic deformation. This can simply achieved by pressing a hard and highly polished ballor roller against the surface of metallic work pieces. Burnishing is used increasingly as afinishing operation which gives additional advantages such as increased hardness, fatiguestrength and Wear resistance. The present study focuses on the effect of burnishingparameters (burnishing feed, burnishing depth and burnishing speed) upon surfaceroughness and surface hardness. It was found that by using LPB process that surfacehardness, surface roughness has been improved by 78.5%, 65% respectively. It was alsofound that the burnishing force and the initial roughness are the most pronouncedparameters which have a significant effect on the final surface roughness and the finalhardness of the surface of the work piece.

Keywords: Low plasticity burnishing (LPB), Ball burnishing, Surface roughness, Surface hardness.

Introduction

The quality of a machined surface is becoming more and more important insatisfying the increasing demands of performance and reliability ofengineering component. In the quality assurance of machine of surface the socalled "final finishing processes" have important role. During the lastdecades greater, and greater the significance of the different cold-plasticforming methods. Such kind of effective and applied plastic forming methodis the sliding burnishing using turning tool with super hard inserts. Thismethod can be used in piece production as well as mass production using NC,CNC. The principle of the burnishing process, shown in Fig.1 [9], is basedon the rolling movement of a tool (a ball or a roller) against the work

1

piece’s surface, a normal force being applied at the tool. As soon as theyield point of the work piece’s material is exceeded, plastic flow of theoriginal asperities takes place [10]. lathes or manufacturing centers [1].The burnishing of metals is a cold-working process that leads to an accuratechange on the surface profile of the work piece by a minor amount of plasticdeformation. In burnishing process, surface irregularities are redistributedwithout material loss [2, 3]. The burnishing process gives many advantagesin comparison with chip-removal processes. Burnishing increases the surfacehardness of the work piece, which in turn improves wear resistance, improvestensile strength, maintains dimensional stability and improves the fatiguestrength by inducing residual compressive stresses in the surface of thework piece [4–7]. The surface of the material is compressed by theapplication of a hard and highly polished tool (ball). The process ofburnishing can be applied to soft and ductile as well as very hard metals.Compressive action by the burnishing tool causes a slight plastic flow ofthe surface metal to a depth of a few micrometers. Due to the localizedcold plastic deformation by burnishing a residual compressive stress will beleft at the surface of the metallic component. This process improvessurface finish, fatigue resistance, wear and corrosion resistance ofsurfaces[8]. This paper presents an experimental based ball-burnishingprocess undertaken in order to study and determine the optimum values of arange of burnishing parameters for work piece material, namely mild steel.The burnishing parameters considered are the number of tool passes,burnishing feed, burnishing depth, and burnishing speed. These are regardedas the main influential parameters of surface finish and hardness.

Fig.1: Principle of burnishing [9].

Experimental Details

The work piece material used in this study is mild steel. Thechemical composition of which is presented in Table 1. The workpieces are received as cylindrical bright bars of 39 mm diameter.Then the experimental specimens are prepared as shown in Figure 2.Then, using the regular conditions for turning, moderate surfaceroughness is achieved, similar to that obtained in commonmanufacturing practices.

2

Table 1: Chemical composition of commercial mild steel (wt. %).

C Si Mn Cu Nb Fe0.118 0.113 0.394 0.185 0.259 98.72

Fig. 2: Work piece geometry.

A ball-burnishing tool as shown in Fig.3 was designed, fabricated andused for the work. The tool is made of steel 60 and was hardened by aheat treatment process according to steel heat treatment standard.

Fig.3: Ball-burnishing tool set up.(1. Tool head. 2. Diaghram. 3. Cup. 4. Cup cover. 5. Shank. 6. Strain gauge.)

Experimental ProcedureSurface roughness measurement

After the burnishing process, SRT-6200/6210 measuring equipment wasused to measure surface roughness of the test specimen. All of thetests achieved were repeated three times in order to guarantee itsprecision. The surface hardness values of the specimens were measuredusing XHB-3000 digital Brinell hardness tester. The work piece to beburnished is clamped by the three-jaw chuck of the lathe and guidedfrom other side by the lathe tailstock. The burnishing process wasapplied after turning without release the work piece from the lathechuck to keep the same turning alignment [11].

Initial dry turning conditions were unified for all work pieces asfollows:Cutting speed= 122.5 m/min., depth of cut =0.25 mm, feed rate= 0.028mm/rev. The surface hardness of specimens found to be 86.72 MPa underBrinell hardness tester using force = 187.5 N and the diameter ofball =2.5 mm. The surface roughness of machined specimens wasmeasured of 3.276μm

3

In the present study, a dynamometer is fabricated with boundedelectrical strain gauge (120 ohms resistance and 2.11 gauge factor of3x7 mm size).the strain gauge are bounded on the strain ring(Diaghram) and connected in the form of Wheatstone bridge. The strainring and the bridge circuit diagram for measuring the redialcomponent of burnishing force are shown in Fig.4 (a-b).

(a) Four arm bridge circuit.(b) Strain ring with strain gauge.

Fig.4: Bridge circuit set up.

Figure 5. shows the calibration set up of lathe tool dynamometer tocalibrate the dynamometer a vertical load in steps of 9, 16, 20, 23Kgf is loaded and the corresponding output voltage (mV)from thebridge is measured by means of P3 strain indicator and recorder are9, 16, 20, 23mV. The calibration chart is used to convert themeasured potential difference ΔV into burnishing force. Thecalibration chart shown in Fig.6 is most likely to the chart given inRef [12].

4

0 5 10 15 20 250

200

400

600

800

Load (Kgf)

volt

age

out

put

(mV)

Fig.5: calibration set up of dynamometer. Fig.6: Calibration curve ofdynamometer.Burnishing experiments were carried out for different burnishingparameter such as feed rate, compression force, revolution, andburnishing depth. The effects of burnishing process of surfaceroughness and surface hardness of test specimen have beeninvestigated. Burnishing tests were carried out according to workingconditions given in Table 2.

Table 2: Burnishing parameters for the test.

Depth of Burnishing(h ) mm

0.05 ,0.1 ,0.15 ,0.2 ,0.25

Number of passes 1

Feed (f) mm/rev 0.032 ,0.036 ,0.039 ,0.043 ,0.05

Spindle speed (N)rev/min

500 ,630 ,800 ,1000 ,1250

Burnishing condition Dry

Theoretical back ground

The ball-burnishing process is a local plastic deformation processacting on the work piece surface; the minimal needed burnishing forceshould be estimated, to perform the ball-burnishing work. To derivethe needed burnishing force, some assumptions were made [9] tosimplify the mathematical model Fig. 7, as the following:(a) The ball will not deform during the burnishing process.(b) The friction between the ball and the tested specimen is ignored.

5

(c) The derived burnishing force is derived for the first burnishingpath.

Fig.7. Force acting on the ball-burnishing tool

The needed burnishing force can be estimated by Eq. (1).

F = σ . A (1) Where, σ is the yielding stress of the material used (σ =225 MPa), A isthe contact area during the burnishing process. The contact area Acan be calculated through the surface integral.

A = L . W (2)

L = (r2 –(r-h)2 )0.5 (3)

Where r is the ball radius (r=5mm) and h is the depth of Burnishing, Lis the contact length between the ball burnishing and the burnishedsurface and W is the width of ball (W=5mm). The burnishing force iscalculated using mat lab program at different burnishing depth asshown in Table 3.

Table 3. Calculated burnishing force.

h (mm) 0.05 0.10 0.15 0.20 0.25

A (mm2)3.5267

4.9749

6.0776 7 7.806

2

F (N) 793.5

1119.4

1367.5 1575 1756.

4By comparison these results with the measurement burnishing force andcalculate the error % as shown in Table 4 by the equation:

Error % = Calculated force – Measurement force * 100%Calculated force

Table 4. Error % of burnishing force

h(mm) measurement Calculated Error

6

force(N) force(N) %0.05 293.7 793.5 62.980.1 453.9 1119.4 59.50.15 614.1 1367.5 55.10.2 827.7 1575 47.40.25 1068 1756.4 39.2

Results and Discussions

Effect of burnishing speed on surface roughness and surface hardness

The effects of burnishing parameters on surface roughness and surfacehardness have been evaluated. The variations in roughness andhardness values of the burnished surface with parameters such asrevolution, feed and burnishing depth, were experimentally found. Theeffects of burnishing revolutions on surface roughness and surfacehardness are shown in Fig.8 and 9 respectively. From these figures itcan be observed that increasing the burnishing revolutions decreasesthe surface roughness and improves the surface hardness of thespecimen. First the surface roughness decreased as the number ofrevolution increased until 1000 RPM, then when number of revolutionincreased above 1000 rpm the surface roughness was slightly increasedas shown in Fig.8 .The improvement of surface hardness whenincreasing the number of revolution is expected as the increase ofthe force increase the depth of penetration resulting in compressingmore asperities. Also, increasing the number of revolution increasesthe metal flow that feeds to filling of more valleys that wereexisted of subsurface due to the previous turning process [2].

400 600 800 1000 1200 14000.6000000000000010.800000000000001

11.21.41.61.8

Revolution(r.p.m)

Roughness(µm)

400 600 800 1000 1200 1400140

145

150

155

160

165

Revolution (r.p.m)

Hard

ness

(BHN

)

Fig.8: Effect of revolution on the surface roughness. Fig.9. Effect of revolution on the surfacehardness.

Effect of burnishing feed on surface roughness and surface hardness

7

The effects of burnishing feed on the surface roughness and surfacehardness for work piece material are shown in Fig .10 and 11respectively. Figure 8 shows that the surface roughness increaseswith the increase of the burnishing feed and the surface hardnessdecreases with the increase of feed until 0.039mm/rev, then when feedrate increased above o.039mm/rev the surface hardness then increaseas shown in Fig.11. Thus, to produce optimum surface roughness andhardness via burnishing, a burnishing feed of 0.032 mm/rev isconsidered to be the optimum value for Mild Steel. At high feeds, theball creates feed marks with a centre-line distance between twoconsecutive indentations that is large compared to the contact areabetween the tool and work piece, and hence less improvement in thesurface is available. To prevent this, the value of feed must be lessthan the length of the contact area between the tool and the workpiece.

0.03 0.04 0.050.6000000000000010.800000000000001

11.21.41.61.8

Feed (mm / rev)

Roughness (µm)

Fig.10: Effect of burnishing feed on the surface Fig.11: Effect of burnishing feed on the surface roughness. hardness.

Effect of burnishing depth on Surface roughness and surface hardness

The final surface hardness depends on the initial surface hardness ofthe materials to be burnished. The surface hardness is directlyproportional to applied force; i.e an increase in force increases thesurface hardness. This is due to the increase of depth of

8

penetration, increase in metal flow that leads to an increase in theamount of deformation. At 800 R..P.M spindle speed, surface hardnessvalue 153.52 BHN and surface roughness of 1.461 μm having feed rateof 0.036mm/rev with depth of penetration of 0.25 mm .The results ofFig.12. Indicate that increasing the burnishing depth decreases thesurface roughness. Fig.13. shows that the depth of penetration wasincreased from 0.05 to 0.25 mm, surface become more work hardened andincreased the surface hardness.

0 0.05 0.1 0.15 0.2 0.25 0.30.600000000000001

0.800000000000001

1

1.2

1.4

1.6

1.8

Burnishing depth (mm)

Roughness(µm)

0 0.05 0.1 0.15 0.2 0.25 0.3120

130

140

150

160

Burnishing depth (mm)

Hard

ness

(BH

N)

Fig.12. Effect of burnishing depth on the surface Fig.13. Effect of burnishing depth on thesurface

roughness. hardness.

Effect of burnishing force on Surface roughness and surface hardness

The surface hardness is based on the initial surface hardness to beburnished [13] the surface hardness is directly proportional toapplied force. The surface hardness increases with decrease inburnishing force also improvement in surface roughness with decreasein burnishing was noticed as shown in Fig. 14(a-b). The burnishingdepth has increased the working hardening effect and hence increasesin surface hardness as shown in Fig.15-a. As the burnishing depth wasincreased, more plastic deformation takes place, i.e. the peaks eased

9

out which produced rough surface and higher value as shown in Fig.15-b.

500 630 800 1000 1250200

600

1000

1400

1800

2200

2600

145

150

155

160

165

ForceHardness

Revolution(rpm)

Burn

ishi

ng F

orce

(N)

Surf

ace

Hard

ness

(BHN

)

(a) The effect of burnishing force on surface hardness.

500 630 800 1000 1250200

600

1000

1400

1800

2200

2600

0.0

0.5

1.0

1.5

2.0

ForceRoughness

Revolution(rpm)

Burn

ishi

ng F

orce

(N)

Surf

ace

Roug

hnes

s(μm

)

(b) The effect of burnishing force on surface roughness.

Fig.14. Effect of burnishing force on surface hardness and surface roughness.

0.05 0.1 0.15 0.2 0.25200

600

1000

1400

1800

2200

2600

120

130

140

150

160Force

Burnishing Depth(mm)

Burn

ishi

ng F

orce

(N)

Surf

ace

Hard

ness

(BHN

)

(a) The effect of burnishing force on surface hardness.

10

0.05 0.1 0.15 0.2 0.25200

600

1000

1400

1800

2200

2600

0.0

0.5

1.0

1.5

2.0Force

Burnishing Depth(mm)

Burn

ishi

ng F

orce

(N)

Surf

ace

Roug

hnes

s(μm

)

(b) The effect of burnishing force on surface roughness.

Fig.15. Effect of burnishing force on surface hardness and surface roughness.

It can be seen from Fig.16-a that the surface hardness was decreasedwith increasing the burnishing feed rate up to specific point then itstarts to increase , the best results was obtained at 0.05 mm/revburnishing feed rate. It can be seen from Fig.16-b that the surfaceroughness is increased with increasing the feed rate.

0.032 0.036 0.039 0.043 0.05200

600

1000

1400

1800

2200

2600

150

152

154

156

158

160Force Hardness

Feed Rate(mm/rev)

Burn

ishi

ng F

orce

(N)

Surf

ace

Hard

ness

(BHN

)

(a) The effect of burnishing force on surface hardness.

11

0.032 0.036 0.039 0.043 0.05200

600

1000

1400

1800

2200

2600

0.0

0.5

1.0

1.5

2.0Force

Feed Rate(mm/rev)

Burn

ishi

ng F

orce

(N)

Surf

ace

Roug

hnes

s(μm

)

(b) The effect of burnishing force on surface roughness.Fig.16: Effect of burnishing force on surface hardness and surface roughness.

Conclusion

The following conclusions were drawn based on the results byburnishing experiments on mild steel alloys

1. The test results produced significant improvement on surfaceroughness and surface hardness.

2. A lower surface roughness value obtained at spindle revolutionof 800r.p.m having feed of 0.036 mm/rev.

3. The surface hardness also increased as the revolution, feed,and depth of penetrations was increased.

References

[1] Dr. Gyula Varga, Prof. Illés Dudás, Examinations of Sliding BurnishingUsing for Improving the Surface Quality of External Cylindrical Surfaces,Egyetemváros, Hungary.

[2] A. M. Hassan, Burnishing force and number of ball passes for the optimumsurface finish of brass components, Journal of Materials ProcessingTechnology 72 (1997), 385–391.

[3] H. Luo, L. Wang, C. Zhang, Study on the aluminum alloy burnishingprocessing and the existence of the outstripping phenomenon, J. ofMaterials Processing Technology, 116 (2001),88–90.

[4] B.C. Yeldose, B. Ramamoorty, investigation into the high performance oftin-coated rollers in burnishing process, Journal of Materials ProcessingTechnology, 207 (2008), 350–355.

[5] F. J. Shiou, C. H. Chen, Determination of optimal ball burnishingparameters for plastic injection moulding steel, Int. J. Adv.Manufacturing Technology 3 (2003), 177–185.

[6] L. Luca, I. Ventzel-Marinescu, Effects of working parameters on surfacefinish in ball-burnishing of hardened steels, Precision Engineering, 29(2005), 253–256.

12

[7] N. H. Loh, S. C. Tam, S.Miyazawa, A study of the effects of ballburnishing parameters on surface roughness using factorial design, Journalof Mechanical Working Technology, 18 (1989), 53–61.

[8]Dabeer, P.S. & Purohit, G. K, “EFFECT OF BALL BURNISHINGPRAMETRS ON SURFACE ROUGHNESS USING SURFACE ROUGHNESS METHDOLOGY”,scientific paper , Dr. M.G.R Educational and Research Institute, Chennai,India, 5(2010)2, 111-116.

[9] Fang-Jung Shiou, Chien-Hua Chen, “Freeform surface finish of plasticinjectionmold by using ball-burnishing process”, Journal of MaterialsProcessing Technology 140 (2003) 248–254.

[10] Liviu Luca, Sorin Neagu-Ventzel, Ioan Marinescu,“ BURNISHING OFHARDENED STEEL COMPONENTS – AN ALTERNATIVE METHOD OF FINISHING”, Toledo,OH 43606.

[11] A. A. Ibrahim, “An investigation into ball burnishing process ofcarbon Steel on

a lathe”, Mechanical Engineering Department, Shoubra faculty ofEngineering, Benha University, Egypt

[12] J. Naga Malleswara Rao, A. Chenna Kesava Reddy and P.V. Rama Rao,” Design and fabrication of new type of dynamometer to measure radial component of cutting force and experimental investigation of optimum burnishing force in roller burnishing process”, Indian Journal of Science and Technology, Vol. 3 No. 7, (July 2010).

[13] S. Thamizhmnaii ; B. Bin Omar ; S. Saparudin ; S. Hasan,” Surface roughness investigation and hardness by burnishing on titanium alloy”, Journal of Achievements in Materials and Manufacturing Engineering, Volume: 28, 2, june 2008, pp.139-142.

13