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On the surface and tribological characteristics of burnished cylindrical Al-6061
ARTICLE IN PRESS Tribology International 42 (2009) 320–326
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
On the surface and tribological characteristics of burnished cylindrical Al-6061 N.S.M. El-Tayeb , K.O. Low, P.V. Brevern Faculty of Engineering and Technology, Multimedia University, 75450 Melaka, Malaysia
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 June 2006 Received in revised form 22 June 2008 Accepted 3 July 2008 Available online 16 August 2008
Surface treatment is an important aspect of all manufacturing processes to impart specific physical, mechanical and tribological properties. Burnishing process is a post-machining operation in which the surface irregularities of the workpiece are compressed by the application of a ball or roller. In the present study, simple and inexpensive burnishing tools, with interchangeable adapter for ball and roller were designed and fabricated. Ball burnishing processes were carried out on aluminium 6061 under different parameters and different burnishing orientations to investigate the role of burnishing speed, burnishing force and burnishing tool dimension on the surface qualities and tribological properties. The results showed that burnishing speed of 330 rpm and burnishing force of 160 N produce optimum results. Meanwhile, a decrease in the burnishing ball diameter leads to a considerable improvement in the surface roughness up to 75%. On the other hand, parallel burnishing orientation exhibits lower friction coefficient compared to cross-burnishing orientation. Furthermore, ball burnishing process is capable of improving friction coefficient by 48% reduction and weight loss by 60–80% reduction of burnished surface of aluminium 6061. These findings are further supplemented by the surface features as seen in SEM photomicrographs. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Burnishing Friction Wear Surface roughness Hardness
1. Introduction Burnishing process is a post-machining operation based on plastic deformation. In this process, the surface of the workpiece is compressed by the application of a highly polished and hardened ball or roller, Fig. 1. Unlike many conventional finishing processes such as grinding, lapping which depend on chip removal, burnishing is essentially a chipless post-machining process which is used to eliminate scratches, tool marks, pits and porosity. Previously published works indicated that burnished surfaces have many advantages over ground surfaces [1–4]. Some researchers concentrated on burnishing parameters such as burnishing speed [1–5], burnishing depth [5], burnishing force [1,2,7], burnishing feed rate [1,2], surface hardness [8], number of tool passes [2,6,7] and burnishing tool dimension [2,8], in relation to only surface roughness and hardness. From the mechanical properties standpoint, burnished surfaces gain some improvement in tensile and yield strengths [7,9,10]. In addition, burnishing process also imparts compressive residual stresses in the
Corresponding author. Tel.: +606 252 3926; fax: +606 231 6552.
E-mail addresses: [email protected] (N.S.M. El-Tayeb), [email protected] (K.O. Low), [email protected] (P.V. Brevern). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.07.003
surface region that could mitigate fatigue cracks that usually initiates at the surface [10,11]. Machined surfaces by conventional method may have inherent irregularities that cause energy dissipation (friction) and surface damage (wear) that affect element performance and reliability. For such surfaces, burnishing process is capable of improving the resistance to wear, corrosion and oxidation. These improvements can be extended to minimize friction and reduce adhesion. Although many investigators have pointed out that the burnishing reduces friction coefficient [3] and improves the wear resistance [4,12], surprisingly, little work has been done in this direction. It was revealed that the burnishing process helps to reduce friction up to a critical depth, beyond which cracks are initiated and the friction is increased [3]. In another study [4], burnishing process was found to reduce wear rate by 38% for copper and 44% for St-37 steel whereas an excessive burnishing depth accelerated the response for wear rate and this resulted in surface damage. The above literature review indicates that earlier investigations [1,2,5–11] concentrated on ball burnishing process dealing mostly with surface finish and surface hardness with little focus on tribological characteristics. Moreover, to the authors’ best knowledge, no works considered the influence of burnishing orientations on tribological properties. Therefore, the present work intended to study the effect of ball burnishing parameters, i.e. speed, force, ball diameter and orientation on the surface
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Normal Force
Rotational Motion Supporting Fluid Lateral Motion Burnishing Ball
321
Table 1 Summary of burnishing parameters Burnishing parameters
Values
Burnishing speed (rpm) Burnishing force (N) Ball diameter (mm) Burnishing feed rate (mm/rev) Lubricant
110, 230, 330, 496 155, 184, 212, 240 12, 14, 16 0.11 Kerosene
Asperities
Workpiece Valleys Fig. 1. Schematic illustration of burnishing mechanism.
present study. Fig. 2b shows a schematic representation of the burnishing tool in which a shank (1) is to be firmly clamped on the lathe machine. A helical compression spring (2) is used to exert the burnishing force during burnishing operations. A ball adapter (3), which is interchangeable with the roller adapter, can accommodate three burnishing carbon steel balls (4) with different diameters (12,14,16 mm). The ball adapter (3) contained a lubrication channel to lubricate the burnishing ball continuously during burnishing operation. A dial gage (5) of 0.01 mm sensitivity is attached at the end of the shank (1) and directly in contact with the spring guide (6). When burnishing force is exerted, the axial sliding motion of the spring guide rod (6) is detected by the dial gage. A calibration process was conducted using the actual burnishing operation setting to obtain a relationship between the burnishing force and the corresponding axial displacement. 2.3. Procedure of burnishing process The ball burnishing processes were performed by clamping the burnishing tool on the tool post of a LA430 lathe. The lathe machine has variable spindle speed up to 3500 rpm with a maximum power of 20 kW. The burnishing parameters considered are given in Table 1. 2.4. Surface roughness and hardness measurements
Fig. 2. (a) Top view of the ball burnishing tool and (b) sectional view of ball burnishing tool assembly.
qualities and tribological properties of burnished surfaces of aluminium 6061 for different burnishing orientations. In view of this, surface roughness and hardness were measured. Friction and wear tests were conducted using disc-on-ring and crossedcylinder techniques and the results are further supplemented by the SEM surface features.
2. Experimental details 2.1. Material specification and specimen preparation In this study, aluminium 6061 with chemical composition 0.40.8% Si, 0.7% Fe, 0.150.4% Cu, 0.15% Mn, 0.040.35% Cr, 0.25% Zn, 0.15% Ti, 0.2% Mg, remaining percentage is Al was used as workpiece material. Aluminium 6061 was selected because of its wide range of applications in the industry such as aircraft fittings, truck wheels, brake disc and hydraulic pistons [5,7]. Workpieces were received in the form of cylindrical rod and was initially turned into a circular disc of 25 mm diameter and 10 mm thickness. 2.2. Burnishing tools A burnishing tool (Fig. 2a) with interchangeable adapter for ball and roller were designed and fabricated for the purpose of the
Measurements of surface roughness and hardness were performed before and after burnishing. Surface roughness was measured using Perthometer S2 profilometer and drive PGK unit. Rockwell hardness of the burnished and non-burnished surfaces was measured using a Terco durometer with a 1/1600 Rockwell ball indenter. 2.5. Tribo-test apparatus Tribo-tests were performed using disc-on-ring and crosscylinder techniques to investigate the effect of burnishing parameters on friction and wear resistance under dry and lubricated contact conditions. The tribo-test machine (Fig. 3a) used for this study is shown schematically in Fig. 3b [13]. The burnished specimen (1) is clamped into a specimen holder (2) in away that burnishing orientation is in parallel or crossed direction of sliding (PB or CB) as shown in Fig. 3c. The specimen is loaded normally with dead weight (3) through a loading lever (4) which is supported on a frame (5). The loading arm (4) is balanced by a balancing weight (6). A 60 mm diameter, stainless steel counterface cup (7) was polished using abrasive paper (silicon carbide electro coated 2000 CW grade) to an average roughness of 2.19 mm. This surface was refreshed before each test to ensure its initial roughness is remained almost the same. The operating conditions were remained constant at 20 N normal load, 180 s test duration and 200 rpm rotational speed which is equivalent to sliding velocity of 0.6283 m/s and sliding distance 0.113 km. A light viscosity lubricant (SAE 20W/40) was used. Frictional forces at the sliding interface were measured using strain gauges
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Average Roughness vs Burnishing Speed
Average Roughness (μm)
0.8 0.7 0.6 0.5 0.4 0.3 0.2
Ball diameter 16mm Ball diameter 14mm Ball diameter 12mm Turned Surface
0.1 0 0
100
200
300
400
500
600
Burnishing Speed (rpm) Fig. 4. Effect of burnishing speed on average roughness for different ball diameters at F ¼ 212 N, f ¼ 0.11 mm/rev.
Fig. 3. (a) Tribo-test machine, (b) schematic diagram of the tribo-test machine and (c) tribo-test operations. PB: parallel-burnishing arrangment; CB: crossedburnishing arrangment.
mounted on the loading lever. A Setra El-4105 balance with 1 mg resolution was used to measure the weight loss of the specimens before and after each wear test.
3. Experimental results and discussions 3.1. Results of surface roughness and hardness Fig. 4 shows the effect of burnishing speed on surface roughness for different burnishing ball diameters. It is apparent that burnishing process improves the surface roughness over burnishing speed ranges from 160 to 440 rpm and above 440 rpm, surface roughness began to deteriorate. The results (Fig. 4) reveal that burnishing with ball diameters of 12 and 14 mm were capable of decreasing the surface roughness by about 61% and 75%, respectively. A 0.09 mm surface finish was achieved at optimal speed of 330 rpm. Clearly, smaller ball diameters (12 and 14 mm)
improve surface roughness by about 61% and 75%, respectively. On the other hand, burnishing with ball diameter of 16 mm did not have any improvement on the surface roughness but deterioration within the tested speed range. This is attributed to the increase in the real contact area at the interface followed by an increase in the friction coefficient and decrease in the contact pressure. This in turn unable the ball to indent the surface enough for any improvement; instead, it deteriorates the surface roughness (from 0.36 to 0.8 mm). A comparison between SEM photomicrographs of initial turned surface and burnished surface using different burnishing ball diameters are shown in Fig. 5. The initial turned surface of roughness 0.36 mm (Fig. 5a) shows turning marks, while burnishing has plastically deformed and removed these scars leading to reduced roughness (0.1 mm) and smoother surface texture (Fig. 5b). However, at higher burnishing speed, the surface roughness deteriorates. A comparison between these three surface morphologies (Fig. 5b–d) reveals that at 16 mm ball diameter (Fig. 5d), surface is deteriorated with excessive plastic deformation. The results of hardness (Fig. 6) indicate that burnishing at speed 110 rpm yields the highest improvement in hardness, as much as 39% increase. However, this improvement is reduced by increasing the burnishing speed but still, above the hardness of un-burnished surface. Basically, hardness depends on the amount of plastic deformation and the resultant internal compressive residual stresses. At higher speed, the duration of penetrating the tool into the burnished surface is less and this in turn induces less amount of plastic deformation followed by less amount of workhardening into the burnished surface. Another possible reason [14,15] is the instability (Chatter) of burnishing tool across the workpiece surface. This may be interpreted by inconsistent deforming action of the tool at high speed resulting in a decrease in the hardness. In addition, at high-speed lubricant may lose its effect due to insufficient time to penetrate between the tool and the workpiece surface. Fig. 7a shows that burnishing force up to 160 N can improve the surface roughness. Beyond this limit, the surface roughness is drastically increased from 0.38 to 1.3 mm (270%) at 210 N, after which it decreases but still above the initial roughness and could not be ascertain if further increase in burnishing force could improve the surface roughness. An excessive contact pressure due to large burnishing force may cause cold pressure welding [14,15] which in turn may damage the surface due to seizure failure. The SEM (Fig. 7b) shows a large amount of plastic deformation when burnishing force was increased. Fig. 8 shows that burnishing force improves the hardness by 35% at 240 N. However, at higher burnishing force, an excessive
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323
Hardness vs Burnishing Speed Rockwell Hardness B
120
Unburnish
100
0 rpm
80
Burnish
60
110 rpm
40
230 rpm
20
330 rpm
0
490 rpm Burnishing Speed (rpm)
Fig. 6. Effect of burnishing speed on hardness at F ¼ 212 N, f ¼ 0.11 mm/rev, d ¼ 12 mm.
Average Roughness, Ra (m)
Average Roughness vs Burnishing Force 1.7 1.5
Burnished Surface Turned Surface
1.3 1.1 0.9 0.7 0.5 0.3 0.1 140
160
180
200
220
240
260
Burnishing Force (N)
Fig. 7. (a) Effect of burnishing force on average roughness at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm and (b) SEM morphology of burnished surface at F ¼ 240 N.
Hardness vs Burnishing Force Rockwell Hardness B
100
Fig. 5. SEM morphology of (a) turned surface at n ¼ 330 rpm, f ¼ 0.11 mm/rev and (b)–(d) burnished surface at F ¼ 212 N, n ¼ 230 rpm, f ¼ 0.11 mm/rev, d ¼ 12, 14 and 16 mm, respectively.
Unburnish 0N
80
Burnish
60
155 N
40
184 N
20
212 N 240 N
0 Burnishing Force (N)
Fig. 8. Effect of burnishing force on hardness at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm.
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plastic deformation at the surface took place (Fig. 7b) and this is followed by pilling up of material in front of the burnishing ball, leading to an increase in surface roughness [2]. Further increase in burnishing force beyond 240 N will only produce flaking and eventually deteriorate the surface without any observable increase in hardness. The increase in the hardness, as pointed out by Hassan et al. [5], depends on the workpiece material and its capacity to accept plastic deformation.
3.2. Results of friction coefficient Fig. 9 shows a significant improvement in the friction coefficient of parallel- and cross-burnishing (PB and CB)-orientations surfaces under both dry and lubricated contact conditions. Moreover, PB-orientation consistently gives lower friction coefficient compared to CB-orientation (Fig. 9a). This may be attributed to several reasons, easy removal of wear debris due to free-path ahead of the wear debris during sliding, less mechanical interlocking of asperities, and no entrapped debris. When tests were conducted in PB-orientation lower friction was observed and this was because of removal of wear debris easily along the wear track (burnishing tracks) due to free-path ahead the wear debris. On the other hand, during CB-orientation tests, the counterface had to interact or slide transversely over different burnishing tracks (means more resistance) which necessitate relatively more energy to overcome that resistance. In general, dry friction coefficient tends to increase as burnishing speed increases for both parallel and crossed orientations.
Friction Coefficient vs Burnishing Speed
Parallel - Dry Crossed - Dry Parallel - Lubricated Crossed - Lubricated Turned Surface - Parallel Turned Surface - Crossed
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm)
Friction coefficient,
Friction Coefficient,
Friction Coefficient vs Burnishing Speed 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
On the other hand, interposing lubricant to the interface markedly decreased the friction coefficient to 0.181 (65%) at burnishing speed of 330 rpm (Fig. 9a). This decrease is associated with the decrease in the surface roughness as shown earlier in Fig. 4 besides the effect of hydrodynamic lubricant film. A closer SEM examination of the worn surface (Fig. 9b) reveals that interposing lubricant during tribo-test acts as a polishing agent, resulting in a smoother surface. Fig. 10 shows the effect of burnishing speed on friction coefficient under dry and lubricated contact for different burnishing ball diameters and PB- and CB-orientations, respectively. Interestingly, there is a clear define ranking order of the ball diameter, i.e. 12 mm ball diameter produces the lowest friction coefficient followed by 14 mm ball diameter and then 16 mm ball which produces the highest value of friction coefficient. Under lubricated contact condition (Fig. 10), an achievement of 40% improvement in friction coefficient when 14 and 16 mm ball diameters were used. At 12 mm ball diameter, the friction coefficient was improved by 65% (0.18) at burnishing speed 330 rpm. This trend supports the surface roughness results shown earlier where lower surface roughness (Fig. 4) led to lower friction coefficient (Fig. 9a). However, under crossed orientation, there is an almost definite relation between the friction coefficient and ball diameter, with the exception of the dissimilarity at lower burnishing speed when burnishing is carried out using ball diameter 12 and 14 mm. The effect of burnishing force on friction coefficient is illustrated in Fig. 11. Under lubricated condition, the friction coefficient tends to decrease with increasing burnishing force. This is obviously due to increasing the surface hardness beside the effect of introducing hydrodynamic film lubricant. In addition, parallel orientation generally exhibits lower friction coefficient compared to crossed orientation. It can be established that when
0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00
Lubricated - Ball Diameter 12 mm Lubricated - Ball Diameter 14 mm Lubricated - Ball Diameter 16 mm Dry - Ball Diameter 12 mm Dry - Ball Diameter 14 mm Dry - Ball Diameter 16 mm
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm) Friction Coefficient vs Burnishing Speed
Worn area Unworn (burnished Area)
Friction coefficient,
Unworn Unworn (burnished (burnished area) Area)
0.56 0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00
Lubricated - Ball Diameter 12 mm Lubricated - Ball Diameter 14 mm Lubricated - Ball Diameter 16 mm Dry - Ball Diameter 12 mm Dry - Ball Diameter 14 mm Dry - Ball Diameter 16 mm
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm) Fig. 9. (a) Effect of burnishing speed on friction coefficient for parallel and crossed burnishing orientations at F ¼ 212 N, f ¼ 0.11 mm/rev, d ¼ 12 mm (dry and lubricated contact condition) and (b) SEM morphology of worn surface for PB-O (lubricated contact condition).
Fig. 10. Effect of burnishing speed on friction coefficient for different burnishing ball diameter at F ¼ 212 N, f ¼ 0.11 mm/rev: (a) parallel burnishing orientation and (b) crossed burnishing orientation.
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Friction Coefficient,
Friction Coefficient vs Burnishing Force 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 140
Ploughing Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
Worn surface
Burnished surface
Parallel - Dry Crossed - Dry Parallel - Lubricated Crossed - Lubricated
160
180
200
220
240
260
Burnishing Force (N) Fig. 11. Effect of burnishing force on friction coefficient at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm. Fig. 13. SEM morphology of worn surface at F ¼ 212 N, N ¼ 490 rpm, f ¼ 0.11 mm/ rev, d ¼ 12 mm for crossed burnishing-orientation (dry contact condition).
Weight Loss vs Burnishing Speed
Weight Loss vs Burnishing Force
0.014
0.015 Ball Diameter 12 mm Ball Diameter 14 mm
0.010
Weight Loss, ΔW (g)
Weight Loss, Δ W (g)
0.012 Ball Diameter 16 mm Turned Surface
0.008
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
0.006 0.004 0.002
0.013 0.011
Parallel Crossed Turned
0.008 0.006 0.004 0.002
0.000 0
100
200
300
400
500
600
Burnishing Speed (rpm)
0.000 140
160
180 200 220 Burnishing Force (N)
240
260
Weight Loss vs Burnishing Speed Fig. 14. Effect of burnishing force on weight loss at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm (dry contact condition).
0.007 Weight Loss, Δ W (g)
0.006 0.005 0.004 0.003 Ball Diameter 12 mm
0.002
Ball Diameter 14 mm Ball Diameter 16 mm Turned Surface
0.001 0.000 0
100
200
300
400
500
600
Burnishing Speed (rpm) Fig. 12. Effect of burnishing speed and ball diameter on weight loss at F ¼ 212 N, f ¼ 0.11 mm/rev (dry contact condition): (a) PB-O and (b) CB-O.
the cup rotates in the direction parallel to the lay or directionality of burnished surfaces (parallel orientation), the resulting friction coefficient is lower than if the lay is at crossed orientation.
3.3. Wear results Fig. 12 shows the effect of burnishing speed on weight loss under dry contact condition for parallel and crossed orientations, respectively. The results indicate that weight loss, in general, increases with increasing burnishing speed. Burnishing with 16 mm ball diameter gave the highest weight loss in PB-O, Fig. 12a. The effect of burnishing force on weight loss under dry sliding condition is illustrated in Figs. 13 and 14. In general, the wear resistance of burnished surface is significantly better than those
for un-burnished surface. However, the trend of the weight loss of burnished surfaces somewhat increases as burnishing force increases. This implies that increasing burnishing force has a negative impact on the wear resistance of burnished aluminium 6061 surfaces. According to Archard [16], the amount of wear is inversely proportional to the hardness of the surface being worn away, which means the higher the hardness the lower the wear. In the current results, the hardness of the burnished surface increased with increasing burnishing force (Fig. 8). Consequently one should expect a decrease in the weight loss with increasing the burnishing force. Unexpectedly, instead of decreasing, the weight loss increases with the burnishing force as shown in Fig. 14. This could be due to the fact that all non-ferrous metal has a definite capacity for cold working. When this capacity is exceeded by applying further load during the sliding tests, considerable cracks and flakes are developed within the surface and subsurface and this accelerate more material removal. It seems that when the work-hardening effect of burnishing reaches its limit, any further plastic deformation will only generate flaking of the surface. This eventually leads to reduced wear resistance. However, there is no apparent trend to demonstrate the influence of burnishing ball sizes on wear behaviour of aluminium 6061 due to scattering of data. The SEM micrograph shown in Fig 13 depicts the worn surface (middle side) as rough and deep scars (ploughing) compared to the burnished surface (left side). Under lubricated contact, the weight loss was too small to be detected within the 1 mg accuracy of the available balance.
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4. Conclusions The conclusions of this work may be summarized as follows:
Within the range of experimental conditions tested, the
optimum ranges of burnishing speed and force are identified to be 200–380 rpm and 150–160 N, respectively, for 12 and 14 mm ball diameters. Other burnishing parameters were held constant, i.e. F ¼ 212 N, f ¼ 0.11 mm/rev, and n ¼ 330 rpm. Burnishing force below 160 N is capable of decreasing the surface roughness by 35%. Beyond this limit, the surface roughness starts to deteriorate plastically. Burnishing with smaller ball diameter of 12 and 14 mm is capable of improving the surface roughness up to 75%. Meanwhile, surface morphologies reveal that using 16 mm ball diameter, the surface deteriorates with excessive plastic deformation. Smaller diameters of ball burnishing are more effective in increasing the hardness in all burnishing speed and force considered. Burnishing speed 110 rpm yields the highest improvement in hardness, as much as 39% increase. However, the improvement reduces as higher burnishing speeds are applied. The friction coefficient of burnished surfaces is dependent on the surface roughness. Low friction coefficient corresponds to low surface roughness, which may be attributed to less mechanical interlocking of asperities and entrapped debris. SEM examination of the worn surface reveals that interposing lubricant during tribo-test acts as a cooler and polishing agent, resulting in smoother surface compared to the burnished surface. Under dry contact condition, burnished surface using smaller ball diameter produces the lowest friction coefficient. When the cup rotates in the direction parallel to the lay of burnished surfaces (parallel orientation), the resulting friction coefficient is consistently lower than if the lay is at crossed orientation. Increasing burnishing force has a negative impact on the wear resistance of burnished aluminium 6061 surfaces.
References [1] Hassan AM. An investigation into the surface characteristics of burnished cast Al–Cu alloys. Int J Mach Tools Manuf 1997;37(6):813–21. [2] Hassan AM, Al-Bsharat AS. Influence of burnishing process on surface roughness, hardness and microstructure of some non-ferrous metals. Wear 1996;199(1):1–8. [3] El-Tayeb NSM. Frictional behaviour of burnished copper surfaces under dry contact conditions. Eng Res Bull, HU Cairo 1994(1):171–84. [4] El-Tayeb NSM, Ghobrial MI. The mechanical wear behaviour of burnishing surfaces. In: Proceedings of the fourth international conference on production engineering design and development, Cairo, 1993. p. 198–209. [5] Hassan AM, Momani AMS. Further improvements in some properties of shot peened components using the burnishing process. Int J Mach Tools Manuf 2000;40(12):1775–86. [6] Hassan AM, Al-Jalil HF, Ebied AA. Burnishing force and number of ball passes for the optimum surface finish of brass components. J Mater Process Technol 1998;83:176–9. [7] Hassan AM. The effects of ball and roller burnishing on the surface roughness and hardness of some non-ferrous metals. J Mater Process Technol 1997;72(3):385–91. [8] Hassan AM, Maqableh AM. The effects of initial burnishing parameters on non-ferrous components. J Mater Process Technol 2000;102:115–21. [9] Preve´y PS, Cammett JT. The influence of surface enhancement by low plasticity burnishing on the corrosion fatigue performance of AA7075-T6. Int J Fatigue 2004. [10] Hassan AM, Al-Bsharat AS. Improvements in some properties of non-ferrous metals by the application of the ball-burnishing process. J Mater Process Technol 1996;59(3):250–6. [11] Zhuang WZ, Halford GR. Investigation of residual stress relaxation under cyclic load. Int J Fatigue 2001;23(1):31–7. [12] Hassan AM, Al-Dhifi SZS. Improvement in the wear resistance of brass components by the ball burnishing process. J Mater Process Technol 1999;96:73–80. [13] El-Tayeb NSM. Design and manufacturing tribotest apparatus. Tribology Laboratory, Faculty of Engineering and Technology, Multimedia University, 2003, /http://fet/fet/research/ccadkm/cad.htmlS. [14] El-Khabeery MM, El-Axir MH. Experimental techniques for studying the effects of milling roller-burnishing parameters on surface integrity. Int J Mach Tools Manuf 2001;41(12):1705–19. [15] Anantha BL, Ram A, Krishnamurthy R. Surface integrity studies in burnishing. In: Proceedings of the eighth AIMTDR Conference, IIT Bombay, 1978. p. 487–90. [16] Bhushan B. Introduction to tribology. New York: Wiley; 2002.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
On the surface and tribological characteristics of burnished cylindrical Al-6061 N.S.M. El-Tayeb , K.O. Low, P.V. Brevern Faculty of Engineering and Technology, Multimedia University, 75450 Melaka, Malaysia
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 June 2006 Received in revised form 22 June 2008 Accepted 3 July 2008 Available online 16 August 2008
Surface treatment is an important aspect of all manufacturing processes to impart specific physical, mechanical and tribological properties. Burnishing process is a post-machining operation in which the surface irregularities of the workpiece are compressed by the application of a ball or roller. In the present study, simple and inexpensive burnishing tools, with interchangeable adapter for ball and roller were designed and fabricated. Ball burnishing processes were carried out on aluminium 6061 under different parameters and different burnishing orientations to investigate the role of burnishing speed, burnishing force and burnishing tool dimension on the surface qualities and tribological properties. The results showed that burnishing speed of 330 rpm and burnishing force of 160 N produce optimum results. Meanwhile, a decrease in the burnishing ball diameter leads to a considerable improvement in the surface roughness up to 75%. On the other hand, parallel burnishing orientation exhibits lower friction coefficient compared to cross-burnishing orientation. Furthermore, ball burnishing process is capable of improving friction coefficient by 48% reduction and weight loss by 60–80% reduction of burnished surface of aluminium 6061. These findings are further supplemented by the surface features as seen in SEM photomicrographs. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Burnishing Friction Wear Surface roughness Hardness
1. Introduction Burnishing process is a post-machining operation based on plastic deformation. In this process, the surface of the workpiece is compressed by the application of a highly polished and hardened ball or roller, Fig. 1. Unlike many conventional finishing processes such as grinding, lapping which depend on chip removal, burnishing is essentially a chipless post-machining process which is used to eliminate scratches, tool marks, pits and porosity. Previously published works indicated that burnished surfaces have many advantages over ground surfaces [1–4]. Some researchers concentrated on burnishing parameters such as burnishing speed [1–5], burnishing depth [5], burnishing force [1,2,7], burnishing feed rate [1,2], surface hardness [8], number of tool passes [2,6,7] and burnishing tool dimension [2,8], in relation to only surface roughness and hardness. From the mechanical properties standpoint, burnished surfaces gain some improvement in tensile and yield strengths [7,9,10]. In addition, burnishing process also imparts compressive residual stresses in the
Corresponding author. Tel.: +606 252 3926; fax: +606 231 6552.
E-mail addresses: [email protected] (N.S.M. El-Tayeb), [email protected] (K.O. Low), [email protected] (P.V. Brevern). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.07.003
surface region that could mitigate fatigue cracks that usually initiates at the surface [10,11]. Machined surfaces by conventional method may have inherent irregularities that cause energy dissipation (friction) and surface damage (wear) that affect element performance and reliability. For such surfaces, burnishing process is capable of improving the resistance to wear, corrosion and oxidation. These improvements can be extended to minimize friction and reduce adhesion. Although many investigators have pointed out that the burnishing reduces friction coefficient [3] and improves the wear resistance [4,12], surprisingly, little work has been done in this direction. It was revealed that the burnishing process helps to reduce friction up to a critical depth, beyond which cracks are initiated and the friction is increased [3]. In another study [4], burnishing process was found to reduce wear rate by 38% for copper and 44% for St-37 steel whereas an excessive burnishing depth accelerated the response for wear rate and this resulted in surface damage. The above literature review indicates that earlier investigations [1,2,5–11] concentrated on ball burnishing process dealing mostly with surface finish and surface hardness with little focus on tribological characteristics. Moreover, to the authors’ best knowledge, no works considered the influence of burnishing orientations on tribological properties. Therefore, the present work intended to study the effect of ball burnishing parameters, i.e. speed, force, ball diameter and orientation on the surface
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Normal Force
Rotational Motion Supporting Fluid Lateral Motion Burnishing Ball
321
Table 1 Summary of burnishing parameters Burnishing parameters
Values
Burnishing speed (rpm) Burnishing force (N) Ball diameter (mm) Burnishing feed rate (mm/rev) Lubricant
110, 230, 330, 496 155, 184, 212, 240 12, 14, 16 0.11 Kerosene
Asperities
Workpiece Valleys Fig. 1. Schematic illustration of burnishing mechanism.
present study. Fig. 2b shows a schematic representation of the burnishing tool in which a shank (1) is to be firmly clamped on the lathe machine. A helical compression spring (2) is used to exert the burnishing force during burnishing operations. A ball adapter (3), which is interchangeable with the roller adapter, can accommodate three burnishing carbon steel balls (4) with different diameters (12,14,16 mm). The ball adapter (3) contained a lubrication channel to lubricate the burnishing ball continuously during burnishing operation. A dial gage (5) of 0.01 mm sensitivity is attached at the end of the shank (1) and directly in contact with the spring guide (6). When burnishing force is exerted, the axial sliding motion of the spring guide rod (6) is detected by the dial gage. A calibration process was conducted using the actual burnishing operation setting to obtain a relationship between the burnishing force and the corresponding axial displacement. 2.3. Procedure of burnishing process The ball burnishing processes were performed by clamping the burnishing tool on the tool post of a LA430 lathe. The lathe machine has variable spindle speed up to 3500 rpm with a maximum power of 20 kW. The burnishing parameters considered are given in Table 1. 2.4. Surface roughness and hardness measurements
Fig. 2. (a) Top view of the ball burnishing tool and (b) sectional view of ball burnishing tool assembly.
qualities and tribological properties of burnished surfaces of aluminium 6061 for different burnishing orientations. In view of this, surface roughness and hardness were measured. Friction and wear tests were conducted using disc-on-ring and crossedcylinder techniques and the results are further supplemented by the SEM surface features.
2. Experimental details 2.1. Material specification and specimen preparation In this study, aluminium 6061 with chemical composition 0.40.8% Si, 0.7% Fe, 0.150.4% Cu, 0.15% Mn, 0.040.35% Cr, 0.25% Zn, 0.15% Ti, 0.2% Mg, remaining percentage is Al was used as workpiece material. Aluminium 6061 was selected because of its wide range of applications in the industry such as aircraft fittings, truck wheels, brake disc and hydraulic pistons [5,7]. Workpieces were received in the form of cylindrical rod and was initially turned into a circular disc of 25 mm diameter and 10 mm thickness. 2.2. Burnishing tools A burnishing tool (Fig. 2a) with interchangeable adapter for ball and roller were designed and fabricated for the purpose of the
Measurements of surface roughness and hardness were performed before and after burnishing. Surface roughness was measured using Perthometer S2 profilometer and drive PGK unit. Rockwell hardness of the burnished and non-burnished surfaces was measured using a Terco durometer with a 1/1600 Rockwell ball indenter. 2.5. Tribo-test apparatus Tribo-tests were performed using disc-on-ring and crosscylinder techniques to investigate the effect of burnishing parameters on friction and wear resistance under dry and lubricated contact conditions. The tribo-test machine (Fig. 3a) used for this study is shown schematically in Fig. 3b [13]. The burnished specimen (1) is clamped into a specimen holder (2) in away that burnishing orientation is in parallel or crossed direction of sliding (PB or CB) as shown in Fig. 3c. The specimen is loaded normally with dead weight (3) through a loading lever (4) which is supported on a frame (5). The loading arm (4) is balanced by a balancing weight (6). A 60 mm diameter, stainless steel counterface cup (7) was polished using abrasive paper (silicon carbide electro coated 2000 CW grade) to an average roughness of 2.19 mm. This surface was refreshed before each test to ensure its initial roughness is remained almost the same. The operating conditions were remained constant at 20 N normal load, 180 s test duration and 200 rpm rotational speed which is equivalent to sliding velocity of 0.6283 m/s and sliding distance 0.113 km. A light viscosity lubricant (SAE 20W/40) was used. Frictional forces at the sliding interface were measured using strain gauges
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Average Roughness vs Burnishing Speed
Average Roughness (μm)
0.8 0.7 0.6 0.5 0.4 0.3 0.2
Ball diameter 16mm Ball diameter 14mm Ball diameter 12mm Turned Surface
0.1 0 0
100
200
300
400
500
600
Burnishing Speed (rpm) Fig. 4. Effect of burnishing speed on average roughness for different ball diameters at F ¼ 212 N, f ¼ 0.11 mm/rev.
Fig. 3. (a) Tribo-test machine, (b) schematic diagram of the tribo-test machine and (c) tribo-test operations. PB: parallel-burnishing arrangment; CB: crossedburnishing arrangment.
mounted on the loading lever. A Setra El-4105 balance with 1 mg resolution was used to measure the weight loss of the specimens before and after each wear test.
3. Experimental results and discussions 3.1. Results of surface roughness and hardness Fig. 4 shows the effect of burnishing speed on surface roughness for different burnishing ball diameters. It is apparent that burnishing process improves the surface roughness over burnishing speed ranges from 160 to 440 rpm and above 440 rpm, surface roughness began to deteriorate. The results (Fig. 4) reveal that burnishing with ball diameters of 12 and 14 mm were capable of decreasing the surface roughness by about 61% and 75%, respectively. A 0.09 mm surface finish was achieved at optimal speed of 330 rpm. Clearly, smaller ball diameters (12 and 14 mm)
improve surface roughness by about 61% and 75%, respectively. On the other hand, burnishing with ball diameter of 16 mm did not have any improvement on the surface roughness but deterioration within the tested speed range. This is attributed to the increase in the real contact area at the interface followed by an increase in the friction coefficient and decrease in the contact pressure. This in turn unable the ball to indent the surface enough for any improvement; instead, it deteriorates the surface roughness (from 0.36 to 0.8 mm). A comparison between SEM photomicrographs of initial turned surface and burnished surface using different burnishing ball diameters are shown in Fig. 5. The initial turned surface of roughness 0.36 mm (Fig. 5a) shows turning marks, while burnishing has plastically deformed and removed these scars leading to reduced roughness (0.1 mm) and smoother surface texture (Fig. 5b). However, at higher burnishing speed, the surface roughness deteriorates. A comparison between these three surface morphologies (Fig. 5b–d) reveals that at 16 mm ball diameter (Fig. 5d), surface is deteriorated with excessive plastic deformation. The results of hardness (Fig. 6) indicate that burnishing at speed 110 rpm yields the highest improvement in hardness, as much as 39% increase. However, this improvement is reduced by increasing the burnishing speed but still, above the hardness of un-burnished surface. Basically, hardness depends on the amount of plastic deformation and the resultant internal compressive residual stresses. At higher speed, the duration of penetrating the tool into the burnished surface is less and this in turn induces less amount of plastic deformation followed by less amount of workhardening into the burnished surface. Another possible reason [14,15] is the instability (Chatter) of burnishing tool across the workpiece surface. This may be interpreted by inconsistent deforming action of the tool at high speed resulting in a decrease in the hardness. In addition, at high-speed lubricant may lose its effect due to insufficient time to penetrate between the tool and the workpiece surface. Fig. 7a shows that burnishing force up to 160 N can improve the surface roughness. Beyond this limit, the surface roughness is drastically increased from 0.38 to 1.3 mm (270%) at 210 N, after which it decreases but still above the initial roughness and could not be ascertain if further increase in burnishing force could improve the surface roughness. An excessive contact pressure due to large burnishing force may cause cold pressure welding [14,15] which in turn may damage the surface due to seizure failure. The SEM (Fig. 7b) shows a large amount of plastic deformation when burnishing force was increased. Fig. 8 shows that burnishing force improves the hardness by 35% at 240 N. However, at higher burnishing force, an excessive
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Hardness vs Burnishing Speed Rockwell Hardness B
120
Unburnish
100
0 rpm
80
Burnish
60
110 rpm
40
230 rpm
20
330 rpm
0
490 rpm Burnishing Speed (rpm)
Fig. 6. Effect of burnishing speed on hardness at F ¼ 212 N, f ¼ 0.11 mm/rev, d ¼ 12 mm.
Average Roughness, Ra (m)
Average Roughness vs Burnishing Force 1.7 1.5
Burnished Surface Turned Surface
1.3 1.1 0.9 0.7 0.5 0.3 0.1 140
160
180
200
220
240
260
Burnishing Force (N)
Fig. 7. (a) Effect of burnishing force on average roughness at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm and (b) SEM morphology of burnished surface at F ¼ 240 N.
Hardness vs Burnishing Force Rockwell Hardness B
100
Fig. 5. SEM morphology of (a) turned surface at n ¼ 330 rpm, f ¼ 0.11 mm/rev and (b)–(d) burnished surface at F ¼ 212 N, n ¼ 230 rpm, f ¼ 0.11 mm/rev, d ¼ 12, 14 and 16 mm, respectively.
Unburnish 0N
80
Burnish
60
155 N
40
184 N
20
212 N 240 N
0 Burnishing Force (N)
Fig. 8. Effect of burnishing force on hardness at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm.
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plastic deformation at the surface took place (Fig. 7b) and this is followed by pilling up of material in front of the burnishing ball, leading to an increase in surface roughness [2]. Further increase in burnishing force beyond 240 N will only produce flaking and eventually deteriorate the surface without any observable increase in hardness. The increase in the hardness, as pointed out by Hassan et al. [5], depends on the workpiece material and its capacity to accept plastic deformation.
3.2. Results of friction coefficient Fig. 9 shows a significant improvement in the friction coefficient of parallel- and cross-burnishing (PB and CB)-orientations surfaces under both dry and lubricated contact conditions. Moreover, PB-orientation consistently gives lower friction coefficient compared to CB-orientation (Fig. 9a). This may be attributed to several reasons, easy removal of wear debris due to free-path ahead of the wear debris during sliding, less mechanical interlocking of asperities, and no entrapped debris. When tests were conducted in PB-orientation lower friction was observed and this was because of removal of wear debris easily along the wear track (burnishing tracks) due to free-path ahead the wear debris. On the other hand, during CB-orientation tests, the counterface had to interact or slide transversely over different burnishing tracks (means more resistance) which necessitate relatively more energy to overcome that resistance. In general, dry friction coefficient tends to increase as burnishing speed increases for both parallel and crossed orientations.
Friction Coefficient vs Burnishing Speed
Parallel - Dry Crossed - Dry Parallel - Lubricated Crossed - Lubricated Turned Surface - Parallel Turned Surface - Crossed
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm)
Friction coefficient,
Friction Coefficient,
Friction Coefficient vs Burnishing Speed 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
On the other hand, interposing lubricant to the interface markedly decreased the friction coefficient to 0.181 (65%) at burnishing speed of 330 rpm (Fig. 9a). This decrease is associated with the decrease in the surface roughness as shown earlier in Fig. 4 besides the effect of hydrodynamic lubricant film. A closer SEM examination of the worn surface (Fig. 9b) reveals that interposing lubricant during tribo-test acts as a polishing agent, resulting in a smoother surface. Fig. 10 shows the effect of burnishing speed on friction coefficient under dry and lubricated contact for different burnishing ball diameters and PB- and CB-orientations, respectively. Interestingly, there is a clear define ranking order of the ball diameter, i.e. 12 mm ball diameter produces the lowest friction coefficient followed by 14 mm ball diameter and then 16 mm ball which produces the highest value of friction coefficient. Under lubricated contact condition (Fig. 10), an achievement of 40% improvement in friction coefficient when 14 and 16 mm ball diameters were used. At 12 mm ball diameter, the friction coefficient was improved by 65% (0.18) at burnishing speed 330 rpm. This trend supports the surface roughness results shown earlier where lower surface roughness (Fig. 4) led to lower friction coefficient (Fig. 9a). However, under crossed orientation, there is an almost definite relation between the friction coefficient and ball diameter, with the exception of the dissimilarity at lower burnishing speed when burnishing is carried out using ball diameter 12 and 14 mm. The effect of burnishing force on friction coefficient is illustrated in Fig. 11. Under lubricated condition, the friction coefficient tends to decrease with increasing burnishing force. This is obviously due to increasing the surface hardness beside the effect of introducing hydrodynamic film lubricant. In addition, parallel orientation generally exhibits lower friction coefficient compared to crossed orientation. It can be established that when
0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00
Lubricated - Ball Diameter 12 mm Lubricated - Ball Diameter 14 mm Lubricated - Ball Diameter 16 mm Dry - Ball Diameter 12 mm Dry - Ball Diameter 14 mm Dry - Ball Diameter 16 mm
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm) Friction Coefficient vs Burnishing Speed
Worn area Unworn (burnished Area)
Friction coefficient,
Unworn Unworn (burnished (burnished area) Area)
0.56 0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00
Lubricated - Ball Diameter 12 mm Lubricated - Ball Diameter 14 mm Lubricated - Ball Diameter 16 mm Dry - Ball Diameter 12 mm Dry - Ball Diameter 14 mm Dry - Ball Diameter 16 mm
0
100
200
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
300
400
500
600
Burnishing Speed (rpm) Fig. 9. (a) Effect of burnishing speed on friction coefficient for parallel and crossed burnishing orientations at F ¼ 212 N, f ¼ 0.11 mm/rev, d ¼ 12 mm (dry and lubricated contact condition) and (b) SEM morphology of worn surface for PB-O (lubricated contact condition).
Fig. 10. Effect of burnishing speed on friction coefficient for different burnishing ball diameter at F ¼ 212 N, f ¼ 0.11 mm/rev: (a) parallel burnishing orientation and (b) crossed burnishing orientation.
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Friction Coefficient,
Friction Coefficient vs Burnishing Force 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 140
Ploughing Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
Worn surface
Burnished surface
Parallel - Dry Crossed - Dry Parallel - Lubricated Crossed - Lubricated
160
180
200
220
240
260
Burnishing Force (N) Fig. 11. Effect of burnishing force on friction coefficient at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm. Fig. 13. SEM morphology of worn surface at F ¼ 212 N, N ¼ 490 rpm, f ¼ 0.11 mm/ rev, d ¼ 12 mm for crossed burnishing-orientation (dry contact condition).
Weight Loss vs Burnishing Speed
Weight Loss vs Burnishing Force
0.014
0.015 Ball Diameter 12 mm Ball Diameter 14 mm
0.010
Weight Loss, ΔW (g)
Weight Loss, Δ W (g)
0.012 Ball Diameter 16 mm Turned Surface
0.008
Normal load 20 N Test duration 180 s Sliding speed 0.63 m/s Siding distance 113 m
0.006 0.004 0.002
0.013 0.011
Parallel Crossed Turned
0.008 0.006 0.004 0.002
0.000 0
100
200
300
400
500
600
Burnishing Speed (rpm)
0.000 140
160
180 200 220 Burnishing Force (N)
240
260
Weight Loss vs Burnishing Speed Fig. 14. Effect of burnishing force on weight loss at n ¼ 330 rpm, f ¼ 0.11 mm/rev, d ¼ 12 mm (dry contact condition).
0.007 Weight Loss, Δ W (g)
0.006 0.005 0.004 0.003 Ball Diameter 12 mm
0.002
Ball Diameter 14 mm Ball Diameter 16 mm Turned Surface
0.001 0.000 0
100
200
300
400
500
600
Burnishing Speed (rpm) Fig. 12. Effect of burnishing speed and ball diameter on weight loss at F ¼ 212 N, f ¼ 0.11 mm/rev (dry contact condition): (a) PB-O and (b) CB-O.
the cup rotates in the direction parallel to the lay or directionality of burnished surfaces (parallel orientation), the resulting friction coefficient is lower than if the lay is at crossed orientation.
3.3. Wear results Fig. 12 shows the effect of burnishing speed on weight loss under dry contact condition for parallel and crossed orientations, respectively. The results indicate that weight loss, in general, increases with increasing burnishing speed. Burnishing with 16 mm ball diameter gave the highest weight loss in PB-O, Fig. 12a. The effect of burnishing force on weight loss under dry sliding condition is illustrated in Figs. 13 and 14. In general, the wear resistance of burnished surface is significantly better than those
for un-burnished surface. However, the trend of the weight loss of burnished surfaces somewhat increases as burnishing force increases. This implies that increasing burnishing force has a negative impact on the wear resistance of burnished aluminium 6061 surfaces. According to Archard [16], the amount of wear is inversely proportional to the hardness of the surface being worn away, which means the higher the hardness the lower the wear. In the current results, the hardness of the burnished surface increased with increasing burnishing force (Fig. 8). Consequently one should expect a decrease in the weight loss with increasing the burnishing force. Unexpectedly, instead of decreasing, the weight loss increases with the burnishing force as shown in Fig. 14. This could be due to the fact that all non-ferrous metal has a definite capacity for cold working. When this capacity is exceeded by applying further load during the sliding tests, considerable cracks and flakes are developed within the surface and subsurface and this accelerate more material removal. It seems that when the work-hardening effect of burnishing reaches its limit, any further plastic deformation will only generate flaking of the surface. This eventually leads to reduced wear resistance. However, there is no apparent trend to demonstrate the influence of burnishing ball sizes on wear behaviour of aluminium 6061 due to scattering of data. The SEM micrograph shown in Fig 13 depicts the worn surface (middle side) as rough and deep scars (ploughing) compared to the burnished surface (left side). Under lubricated contact, the weight loss was too small to be detected within the 1 mg accuracy of the available balance.
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4. Conclusions The conclusions of this work may be summarized as follows:
Within the range of experimental conditions tested, the
optimum ranges of burnishing speed and force are identified to be 200–380 rpm and 150–160 N, respectively, for 12 and 14 mm ball diameters. Other burnishing parameters were held constant, i.e. F ¼ 212 N, f ¼ 0.11 mm/rev, and n ¼ 330 rpm. Burnishing force below 160 N is capable of decreasing the surface roughness by 35%. Beyond this limit, the surface roughness starts to deteriorate plastically. Burnishing with smaller ball diameter of 12 and 14 mm is capable of improving the surface roughness up to 75%. Meanwhile, surface morphologies reveal that using 16 mm ball diameter, the surface deteriorates with excessive plastic deformation. Smaller diameters of ball burnishing are more effective in increasing the hardness in all burnishing speed and force considered. Burnishing speed 110 rpm yields the highest improvement in hardness, as much as 39% increase. However, the improvement reduces as higher burnishing speeds are applied. The friction coefficient of burnished surfaces is dependent on the surface roughness. Low friction coefficient corresponds to low surface roughness, which may be attributed to less mechanical interlocking of asperities and entrapped debris. SEM examination of the worn surface reveals that interposing lubricant during tribo-test acts as a cooler and polishing agent, resulting in smoother surface compared to the burnished surface. Under dry contact condition, burnished surface using smaller ball diameter produces the lowest friction coefficient. When the cup rotates in the direction parallel to the lay of burnished surfaces (parallel orientation), the resulting friction coefficient is consistently lower than if the lay is at crossed orientation. Increasing burnishing force has a negative impact on the wear resistance of burnished aluminium 6061 surfaces.
References [1] Hassan AM. An investigation into the surface characteristics of burnished cast Al–Cu alloys. Int J Mach Tools Manuf 1997;37(6):813–21. [2] Hassan AM, Al-Bsharat AS. Influence of burnishing process on surface roughness, hardness and microstructure of some non-ferrous metals. Wear 1996;199(1):1–8. [3] El-Tayeb NSM. Frictional behaviour of burnished copper surfaces under dry contact conditions. Eng Res Bull, HU Cairo 1994(1):171–84. [4] El-Tayeb NSM, Ghobrial MI. The mechanical wear behaviour of burnishing surfaces. In: Proceedings of the fourth international conference on production engineering design and development, Cairo, 1993. p. 198–209. [5] Hassan AM, Momani AMS. Further improvements in some properties of shot peened components using the burnishing process. Int J Mach Tools Manuf 2000;40(12):1775–86. [6] Hassan AM, Al-Jalil HF, Ebied AA. Burnishing force and number of ball passes for the optimum surface finish of brass components. J Mater Process Technol 1998;83:176–9. [7] Hassan AM. The effects of ball and roller burnishing on the surface roughness and hardness of some non-ferrous metals. J Mater Process Technol 1997;72(3):385–91. [8] Hassan AM, Maqableh AM. The effects of initial burnishing parameters on non-ferrous components. J Mater Process Technol 2000;102:115–21. [9] Preve´y PS, Cammett JT. The influence of surface enhancement by low plasticity burnishing on the corrosion fatigue performance of AA7075-T6. Int J Fatigue 2004. [10] Hassan AM, Al-Bsharat AS. Improvements in some properties of non-ferrous metals by the application of the ball-burnishing process. J Mater Process Technol 1996;59(3):250–6. [11] Zhuang WZ, Halford GR. Investigation of residual stress relaxation under cyclic load. Int J Fatigue 2001;23(1):31–7. [12] Hassan AM, Al-Dhifi SZS. Improvement in the wear resistance of brass components by the ball burnishing process. J Mater Process Technol 1999;96:73–80. [13] El-Tayeb NSM. Design and manufacturing tribotest apparatus. Tribology Laboratory, Faculty of Engineering and Technology, Multimedia University, 2003, /http://fet/fet/research/ccadkm/cad.htmlS. [14] El-Khabeery MM, El-Axir MH. Experimental techniques for studying the effects of milling roller-burnishing parameters on surface integrity. Int J Mach Tools Manuf 2001;41(12):1705–19. [15] Anantha BL, Ram A, Krishnamurthy R. Surface integrity studies in burnishing. In: Proceedings of the eighth AIMTDR Conference, IIT Bombay, 1978. p. 487–90. [16] Bhushan B. Introduction to tribology. New York: Wiley; 2002.