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Study on rotational fretting wear of 7075 aluminum alloy
ARTICLE IN PRESS Tribology International 43 (2010) 912–917
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Study on rotational fretting wear of 7075 aluminum alloy J.L. Mo, M.H. Zhu , J.F. Zheng, J. Luo, Z.R. Zhou Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e in f o
a b s t r a c t
Available online 21 December 2009
Rotational fretting wear tests in a ball-on-flat configuration have been successfully realized on a special rotational fretting rig developed from an ultra-low-speed reciprocating rotational driver. The rotational fretting behavior of 7075 aluminum alloy against 52 100 steel was studied under different angular displacement amplitudes and normal loads. The results showed that both Ft y and Ft/Fn curves can be used to characterize the rotational fretting running behavior, which exhibited different curve shapes and variation trends in different fretting running regimes. The rotational fretting behavior of 7075 aluminum alloy was strongly dependent on the angular displacement amplitude, normal load and number of cycles. The wear of 7075 aluminum alloy was characterized by slight attrition in the partial slip regime, while a combination of delamination, abrasive and oxidative wear was found in the slip and mixed fretting regimes. The formation of a central bulge probably due to plastic flow was observed under gross slip condition of the rotational fretting mode. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Fretting wear Rotational fretting 7075 aluminum alloy
1. Introduction Fretting is a small amplitude oscillatory movement which occurs between contacting surfaces, which are usually nominally at rest [1]. Four basic fretting modes, i.e. tangential, radial, rotational and torsional fretting, can be defined under a ball-onflat contact according to the directions of relative motions, as shown in Fig. 1. Many studies have been devoted to fretting wear and most of them have been focused on the tangential mode [2,3]. The rotational fretting can be defined as the relative motion which induced by reciprocating rotation under the oscillatory vibratory environment. Rotational fretting happens when two tight fitted parts suffered repeated fluctuating load, and therefore it is common in modern industrial or biomedical applications such as ball-and-socket joint, wheel-axle fit of train, yoke-axle fit, hip joint and knee joint of human bodies and so on [4,5]. However, few studies on rotational fretting have been reported until now. Briscoe et al. [6–9] reported the fretting wear behavior of PMMA polymer against steel under combined rotational and torsional contact conditions, and they found that the resulting contact zone kinematics has a great influence upon the accumulation, compaction and displacement of the debris particles from the contact. The entrapment and compaction of the debris into rolls resulted in a strong reduction in the total wear volumes when under rotational fretting condition [9]. Nowadays, the fretting behavior and damage mechanisms of metal-to-metal
Corresponding author. Tel.: + 86 28 87601304; fax: + 86 28 87603142.
E-mail address: [email protected] (M.H. Zhu). 0301-679X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2009.12.032
contact under rotational mode are still unclear. Therefore, in this paper, a special rotational fretting device was developed, and the rotational fretting behavior of 7075 aluminum alloy against steel was studied.
2. Experimental details 2.1. Experimental setup A special rotational fretting wear tester with a ball-on-flat configuration was developed by using an ultra-low-speed reciprocating rotational driver (rotational speed: 0.001–30 r/min, resolution of rotational angle: 0.011), as shown in Fig. 2. Ball specimen (1) was fixed on the lower holder (2) which was mounted on the rotational driver (3). The driver was mounted on the base platform (4). It should be ensured that the centerline of the ball specimen coincides strictly with the rotational axis of the rotational driver, and the rotational axis keeps a horizontal status. The ball specimen rotated following the motion of the rotational driver at a constant reciprocating rotational speed (in range of 0.01–5 1/s). Angular displacement of the contact pair was measured by a sensor, and was acted as the control signal feed back to the control unit of the tester. For this rotational driver the minimal rotational angle displacement amplitude of 0.051 can be reached. Flat specimen (5) was fixed on the upper holder (6) which mounted on a 6-D forces/torques sensor (7). This sensor was mounted on the X–Y moving stage, and consequently the flat specimen can be motorized by a lateral positioning system with
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Fig. 1. Schematic of the four simple fretting modes: tangential, radial, rotational and torsional fretting.
10 mm 10 mm 20 mm and a surface roughness (Ra) of 0.04 mm was chosen as the flat specimen. A chromium bearing steel ball (AISI 52100 steel) with a diameter of 40 mm and a surface roughness (Ra) of 0.04 mm was used as the counterpart. Rotational fretting tests were carried out under different normal loads of 10, 20 and 50 N; a constant rotational speed of 0.2 1/s; rotational angular displacement amplitudes varied from 0.1251 to 11; number of cycles varied from 1 to 1000. All the fretting tests were conducted in ambient atmospheric condition (2073 1C and 60710% RH) without lubrication. Fretting scars were evaluated by profilometer (AMBIOS XP-2), optical microscopy (OM), scanning electron microscopy (SEM, QUANTA200) and energy dispersive spectroscopy (EDX, EDAX-7760/68 ME).
3. Results and discussion 3.1. Rotational fretting kinetic behavior
Fig. 2. Schematic of the rotational fretting wear rig ((1) Ball specimen; (2) Lower holder for ball specimen; (3) Ultra-low-speed reciprocating rotational driver; (4) Mounting for rotational driver; (5) Flat specimen; (6) Upper holder for flat specimen; (7) 6-D forces/torques sensor; (8) Lateral positioning system; (9) Vertical positioning system).
position encoder (8) and a vertical positioning system with position encoder (9). During the test, an actual dynamic coefficient of friction is truly able to be recorded by the servocontrolled normal load. Besides, variations of the friction force (Ft) vs rotational angular displacement amplitude (y) can be recorded as a function of the number of cycles.
3.1.1. Ft y curves Variations of the friction force (Ft) vs rotational angular displacement amplitude (y) are the most important information obtained from the rotational fretting tests. Only three basic types of Ft y curves can be found in rotational fretting tests by varying test parameters, i.e. parallelogram cycles, elliptic cycles and linear cycles corresponding to gross slip and partial slip state, respectively. As shown in Fig. 3(a), all the Ft y curves under angular displacement amplitude of 0.1251 exhibited a linear shape except for the one in running-in stage corresponding to the number of cycles of 10, and it can be thus deduced that the rotational fretting ran in the partial slip regime (PSR) according to the theory of fretting maps [3,10]. For the intermediate angular displacement amplitude of 0.251, the shape of Ft y curves varied from parallelogram to elliptic with the increase of the number of cycles (Fig. 3(b)), suggesting that the fretting ran in the mixed fretting regime (MFR). When y =0.51, all the Ft y curves showed a parallelogram shape, indicating that the fretting ran in the slip regime (SR) (Fig. 3(c)). Therefore, the Ft y curves can be used to characterize the rotational fretting running behavior of 7075 aluminum alloy. The curves in different fretting running regimes exhibited different shapes.
2.2. Test procedures 3.1.2. Running condition fretting map (RCFM) A particularly severe fretting couple involving the contact of aluminum alloy and steel was used in this study. 7075 aluminum alloy (composition: 0.4% Si, 0.5% Fe, 1.2–2.0% Cu, 0.3% Mn, 2.1– 2.9% Mg, 0.1–0.2% Cr, 5.1–6.1% Zn, 0.2% Ti, wt%) with dimensions
As one of the fretting maps, the RCFM has been associated with three fretting regimes according to the previous analyses of fretting wear [11]. Additional tests under different normal loads
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Fig. 5. Ft/Fn curves of 7075 aluminum alloy under different angular displacements of 0.1251, 0.251 and 0.51: Fn = 10 N, N =1000 cycles.
Fig. 3. Ft y curves of 7075 aluminum alloy under different angular displacements amplitudes of 0.1251, 0.251 and 0.51and different number of cycles: Fn = 10 N.
Fig. 6. Trend of Ft/Fn and Ft Fn = 10 N.
Fig. 4. Running condition fretting map for 7075 aluminum alloy under rotational fretting condition, N= 1000 cycles.
and angular displacement amplitudes were performed in this study to build up a RCFM for 7075 aluminum alloy under the rotational fretting condition. As shown in Fig. 4, the fretting regimes were found to vary from PSR to MFR and then to SR with the increase of angular displacement amplitudes under constant imposed normal load. While under a constant angular displacement amplitude, the variation with the increase of normal load was reversed. Therefore, the fretting regimes of 7075 aluminum alloy were strongly dependent on the normal load and angular displacement amplitude. 3.1.3. Ft/Fn curves It was found that Ft/Fn (the ratio of friction force to normal force) curves can also be used to characterize the rotational fretting behavior of 7075 aluminum alloy. Ft/Fn curves in different fretting regimes showed significantly different tendency, as shown in Fig. 5. In PSR, i.e. y =0.1251, the Ft/Fn ratio reached a
y curves in MFR for 7075 aluminum alloy: y = 0.251,
steady value of 0.36 after a running-in stage. In the initial stage, Ft/Fn ratio increased rapidly because of the failure of adsorptive and polluted films on the surface. With the fretting wear proceeding, a stable partial slip condition was established at the contact interface, and consequently the Ft/Fn curve was leveled off and became linear. With the increase of the angular displacement amplitude, i.e. under angular displacement amplitudes of 0.251, a transition from the PSR to the MFR was observed. The Ft/Fn curve exhibited a significantly longer ascent stage of 520 cycles with strong fluctuation, and then reached a relatively high steady value of approximately 0.83. The rotational fretting process of 7075 aluminum alloy in the MFR can be divided into four stages according to the Ft y and Ft/Fn curves (Fig. 6): Stage I, for the first few cycles ( o30 cycles), Ft/Fn ratio increased rapidly because of the failure of adsorptive and polluted films on the surface; Stage II, with the increase of the number of cycles, the Ft/Fn curve showed a slower increase in slope, and correspondingly the Ft y curves changed to narrow parallelogram loops; Stage III, after 300 fretting cycles, the Ft y curves took an elliptic shape, indicating the transition from gross slip to partial slip conditions; Stage IV, as the number of cycles was increased to 500, the Ft y curves became linear and the Ft/Fn curve was leveled off accordingly, indicating the establishment of a stable partial slip condition at the contact interface. The fretting wear of the 7075 aluminum alloy in the MFR will be discussed in detail in the following section.
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In the SR, a quite different Ft/Fn curve evolution with strong fluctuation was observed, and a significantly higher Ft/Fn ratio of 0.98 was obtained at the end of 1000 cycles. The Ft/Fn ratio was found to increase with the transition from the PSR to the MFR and then to the SR under the rotational fretting condition. Therefore, the rotational fretting running behavior can be characterized by both the F y and Ft/Fn curves, which exhibited different shapes and variation in trends corresponding to different fretting running regimes.
was observed. As a result, the Ft y curves became elliptic, and the Ft/Fn ratio increased rapidly again. Stage IV: In this stage the contact interface was in state of partial slip. Wear debris was compacted tightly on the worn surface and hardly to be removed out of the wear scar from the edges. Reduction in the depth of profile due to the cover of debris was observed. Consequently, the wear state was transformed from the two-body contact to the three-body contact, and then the wear decreased.
3.2. Rotational fretting wear
3.2.3. In slip regime (SR) In the SR, plowing and detachment of particles was the main wear phenomena for 7075 aluminum alloy, as shown in Fig. 10. EDX analyses showed that the oxygen content on the worn surface was much higher than that on the original surface of specimen. Therefore, the main mechanisms of rotational fretting wear of 7075 aluminum alloy in the SR were abrasive wear, oxidative wear and delamination. Combining with the profile measurement, it was found that a central bulge was formed by plastic deformation accumulation when y =0.51, which was consistent with that observed for the rotational wear of PMMA polymer reported by Briscoe et al. [6–8]. This phenomenon was very different from that observed in the conventional reciprocating fretting tests (tangential fretting mode), and it is worthwhile to further investigate to understand the real mechanism. While for a larger angular displacement amplitude of 1.51, the debris generated was removed out of the wear scar which resulted in a significantly larger wear depth, and the bulge never appeared in this case. The difference was embodied in debris behavior, which probably resulted from their different relative movement between the contact surfaces, i.e. different
3.2.1. In partial slip regime (PSR) The wear of 7075 aluminum alloy in the PSR was very slight with few debris particles surrounding the wear scar, because the low imposed angular displacement was mainly accommodated by elastic deformation of the contact interface. The wear mechanism was a combination of slight abrasive wear and delamination. With the increase of normal load from 10 to 20 N, the wear damage on the surface was found to decrease greatly (Fig. 7), and correspondingly the Ft/Fn ratio showed a significant decrease (Fig. 8). 3.2.2. In mixed fretting regime (MFR) The rotational fretting wear morphologies in the MFR and their profile measurements are shown in Fig. 9. The four damage stages were analyzed as follows: Stage I: The fretting relative motion state was characterized as gross slip in this stage, and the relative slip was accompanied with visible wear. A bulge in the central zone can be observed from the profile of the wear scar. The formation of a bulge may have resulted from the plastic flow of material from the edges to the center of the wear scar under the condition of gross slip. SEM observation of the worn surface showed that visible plastic deformation and slight plowing were the main wear features in this stage. Stage II: With the increase of the number of cycles, relative slip reduced gradually, some particles were detached, oxidized and then accumulated between the contact surfaces. As a result, the tendency of the Ft/Fn curve in this stage was characterized by a slower increase in slope, and correspondingly the Ft y curves evolved to narrow parallelogram loops. The wear damage of 7075 aluminum alloy in this stage increased significantly accompanied with strong plastic deformation in the contact interface. The strong fluctuation of the Ft/Fn curve resulted from detachment of particles, where the bulge was disappeared, and a great amount of debris generated started to be removed out of the wear scar. Stage III: More particles were detached by delamination in this stage, and a transition from gross slip to partial slip conditions
Fig. 8. Ft/Fn curves of 7075 aluminum alloy under different normal loads of 10 and 20 N: y = 0.1251, N =1000 cycles.
Fig. 7. Wear scar morphologies of 7075 aluminum alloy under different normal loads of 10 N (a) and 20 N (b): y = 0.1251, N = 1000 cycles.
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Fig. 9. Wear morphologies (a) and profile measurement (b) of 7075 aluminum alloy in MFR after different number of cycles: y = 0.251, Fn =10 N.
Fig. 10. Wear morphologies and profile measurement of 7075 aluminum alloy in SR: y = 0.51 (a) and 1.51 (b), Fn = 20 N, N = 1000 cycles.
plastic deformation and relative slip. It also may be the result of the wear status transforming from fretting to sliding. As mentioned above, for the wear mechanisms of 7075 aluminum alloy under rotational fretting condition, it can be concluded that the wear in the PSR was characterized by slight attrition, while the wear in the MFR and SR involved a combination of delamination, abrasive and oxidative wear. The wear depth of 7075 aluminum alloy increased significantly with the fretting regime varying from PSR to MFR and then to SR (Fig. 11).
4. Conclusions The F y curves can be used to characterize the rotational fretting running behavior, which exhibited three basic types of
parallelogram, elliptical and linear cycles, respectively. The Ft/Fn curves, also used to characterize the rotational fretting behavior, showed significantly different variation trends in different fretting running regimes. The rotational fretting behavior of 7075 aluminum alloy against 52 100 steel was strongly dependent upon the angular displacement amplitude, normal load and number of cycles, and its running condition fretting map was built up in this study. A change in the angular displacement amplitude or normal load caused a transition from partial slip regime to mixed fretting regime or slip regime. The wear of 7075 aluminum alloy in the PSR was characterized by slight attrition. The main wear mechanisms of 7075 aluminum alloy in the MFR and SR were a combination of abrasive wear, oxidative wear and delamination. A special phenomenon was observed under gross slip condition of the rotational fretting mode that involved the formation of a central bulge due most likely to plastic flow.
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Development Program (2007CB714704).
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References
Fig. 11. Wear scar profile of 7075 aluminum alloy in PSR (y = 0.1251), MFR (y = 0.251) and SR (y = 0.51): Fn =10 N, N = 1000 cycles.
Acknowledgments This work was supported by National Natural Science Foundation of China (nos. 50775192, 50821063) and State Key
[1] Waterhouse RB. Fretting fatigue. London: Applied science; 1981. [2] Zhu MH, Zhou ZR. An experimental study on radial fretting behaviour. Tribology International 2001;34:321–6. [3] Zhou ZR, Fayeulle S, Vincent L. Cracking behavior of various aluminium alloys during fretting wear. Wear 1992;155:317–30. [4] Australian Transport Safety Bureau. Aviation Safety Investigation Report 200304074; 2003. [5] Bergmann G, Deuretzbacher G, Heller M, Graichen F, Rohlmann A, Strauss J, et al. Hip contact forces and gait patterns from routine activities. Journal of Biomechanics 2001;34:859–71. [6] Briscoe BJ, Chateauminois A, Lindley TC, Parsonage D. Contact damage of poly (methyl-methacrylate) during complex micro-displacements. Wear 2000;240:27–39. [7] Briscoe BJ, Chateauminois A, Lindley TC, Parsonage D. Fretting wear behaviour polymethyl-methacrylate under linear motions and torsional contact condition. Tribology International 2001;31:701–11. [8] Briscoe BJ, Chateauminois A. Nano-rheological properties of polymeric third bodies generated within fretting contacts. Surface and Coatings Technology 2003;163–164:435–43. [9] Chateauminois A, Briscoe BJ. Measurements of friction-induced surface strains in a steel/polymer contact. Tribology International 2002;35:245–54. [10] Zhou ZR, Vincent L. Mixed fretting regime. Wear 1995;181–183:531–6. [11] Zhou ZR, Fayeulle S, Vincent L. Cracking behaviour of various aluminium alloys during fretting wear. Wear 1992;155:317–30.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Study on rotational fretting wear of 7075 aluminum alloy J.L. Mo, M.H. Zhu , J.F. Zheng, J. Luo, Z.R. Zhou Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China
a r t i c l e in f o
a b s t r a c t
Available online 21 December 2009
Rotational fretting wear tests in a ball-on-flat configuration have been successfully realized on a special rotational fretting rig developed from an ultra-low-speed reciprocating rotational driver. The rotational fretting behavior of 7075 aluminum alloy against 52 100 steel was studied under different angular displacement amplitudes and normal loads. The results showed that both Ft y and Ft/Fn curves can be used to characterize the rotational fretting running behavior, which exhibited different curve shapes and variation trends in different fretting running regimes. The rotational fretting behavior of 7075 aluminum alloy was strongly dependent on the angular displacement amplitude, normal load and number of cycles. The wear of 7075 aluminum alloy was characterized by slight attrition in the partial slip regime, while a combination of delamination, abrasive and oxidative wear was found in the slip and mixed fretting regimes. The formation of a central bulge probably due to plastic flow was observed under gross slip condition of the rotational fretting mode. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Fretting wear Rotational fretting 7075 aluminum alloy
1. Introduction Fretting is a small amplitude oscillatory movement which occurs between contacting surfaces, which are usually nominally at rest [1]. Four basic fretting modes, i.e. tangential, radial, rotational and torsional fretting, can be defined under a ball-onflat contact according to the directions of relative motions, as shown in Fig. 1. Many studies have been devoted to fretting wear and most of them have been focused on the tangential mode [2,3]. The rotational fretting can be defined as the relative motion which induced by reciprocating rotation under the oscillatory vibratory environment. Rotational fretting happens when two tight fitted parts suffered repeated fluctuating load, and therefore it is common in modern industrial or biomedical applications such as ball-and-socket joint, wheel-axle fit of train, yoke-axle fit, hip joint and knee joint of human bodies and so on [4,5]. However, few studies on rotational fretting have been reported until now. Briscoe et al. [6–9] reported the fretting wear behavior of PMMA polymer against steel under combined rotational and torsional contact conditions, and they found that the resulting contact zone kinematics has a great influence upon the accumulation, compaction and displacement of the debris particles from the contact. The entrapment and compaction of the debris into rolls resulted in a strong reduction in the total wear volumes when under rotational fretting condition [9]. Nowadays, the fretting behavior and damage mechanisms of metal-to-metal
Corresponding author. Tel.: + 86 28 87601304; fax: + 86 28 87603142.
E-mail address: [email protected] (M.H. Zhu). 0301-679X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2009.12.032
contact under rotational mode are still unclear. Therefore, in this paper, a special rotational fretting device was developed, and the rotational fretting behavior of 7075 aluminum alloy against steel was studied.
2. Experimental details 2.1. Experimental setup A special rotational fretting wear tester with a ball-on-flat configuration was developed by using an ultra-low-speed reciprocating rotational driver (rotational speed: 0.001–30 r/min, resolution of rotational angle: 0.011), as shown in Fig. 2. Ball specimen (1) was fixed on the lower holder (2) which was mounted on the rotational driver (3). The driver was mounted on the base platform (4). It should be ensured that the centerline of the ball specimen coincides strictly with the rotational axis of the rotational driver, and the rotational axis keeps a horizontal status. The ball specimen rotated following the motion of the rotational driver at a constant reciprocating rotational speed (in range of 0.01–5 1/s). Angular displacement of the contact pair was measured by a sensor, and was acted as the control signal feed back to the control unit of the tester. For this rotational driver the minimal rotational angle displacement amplitude of 0.051 can be reached. Flat specimen (5) was fixed on the upper holder (6) which mounted on a 6-D forces/torques sensor (7). This sensor was mounted on the X–Y moving stage, and consequently the flat specimen can be motorized by a lateral positioning system with
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Fig. 1. Schematic of the four simple fretting modes: tangential, radial, rotational and torsional fretting.
10 mm 10 mm 20 mm and a surface roughness (Ra) of 0.04 mm was chosen as the flat specimen. A chromium bearing steel ball (AISI 52100 steel) with a diameter of 40 mm and a surface roughness (Ra) of 0.04 mm was used as the counterpart. Rotational fretting tests were carried out under different normal loads of 10, 20 and 50 N; a constant rotational speed of 0.2 1/s; rotational angular displacement amplitudes varied from 0.1251 to 11; number of cycles varied from 1 to 1000. All the fretting tests were conducted in ambient atmospheric condition (2073 1C and 60710% RH) without lubrication. Fretting scars were evaluated by profilometer (AMBIOS XP-2), optical microscopy (OM), scanning electron microscopy (SEM, QUANTA200) and energy dispersive spectroscopy (EDX, EDAX-7760/68 ME).
3. Results and discussion 3.1. Rotational fretting kinetic behavior
Fig. 2. Schematic of the rotational fretting wear rig ((1) Ball specimen; (2) Lower holder for ball specimen; (3) Ultra-low-speed reciprocating rotational driver; (4) Mounting for rotational driver; (5) Flat specimen; (6) Upper holder for flat specimen; (7) 6-D forces/torques sensor; (8) Lateral positioning system; (9) Vertical positioning system).
position encoder (8) and a vertical positioning system with position encoder (9). During the test, an actual dynamic coefficient of friction is truly able to be recorded by the servocontrolled normal load. Besides, variations of the friction force (Ft) vs rotational angular displacement amplitude (y) can be recorded as a function of the number of cycles.
3.1.1. Ft y curves Variations of the friction force (Ft) vs rotational angular displacement amplitude (y) are the most important information obtained from the rotational fretting tests. Only three basic types of Ft y curves can be found in rotational fretting tests by varying test parameters, i.e. parallelogram cycles, elliptic cycles and linear cycles corresponding to gross slip and partial slip state, respectively. As shown in Fig. 3(a), all the Ft y curves under angular displacement amplitude of 0.1251 exhibited a linear shape except for the one in running-in stage corresponding to the number of cycles of 10, and it can be thus deduced that the rotational fretting ran in the partial slip regime (PSR) according to the theory of fretting maps [3,10]. For the intermediate angular displacement amplitude of 0.251, the shape of Ft y curves varied from parallelogram to elliptic with the increase of the number of cycles (Fig. 3(b)), suggesting that the fretting ran in the mixed fretting regime (MFR). When y =0.51, all the Ft y curves showed a parallelogram shape, indicating that the fretting ran in the slip regime (SR) (Fig. 3(c)). Therefore, the Ft y curves can be used to characterize the rotational fretting running behavior of 7075 aluminum alloy. The curves in different fretting running regimes exhibited different shapes.
2.2. Test procedures 3.1.2. Running condition fretting map (RCFM) A particularly severe fretting couple involving the contact of aluminum alloy and steel was used in this study. 7075 aluminum alloy (composition: 0.4% Si, 0.5% Fe, 1.2–2.0% Cu, 0.3% Mn, 2.1– 2.9% Mg, 0.1–0.2% Cr, 5.1–6.1% Zn, 0.2% Ti, wt%) with dimensions
As one of the fretting maps, the RCFM has been associated with three fretting regimes according to the previous analyses of fretting wear [11]. Additional tests under different normal loads
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Fig. 5. Ft/Fn curves of 7075 aluminum alloy under different angular displacements of 0.1251, 0.251 and 0.51: Fn = 10 N, N =1000 cycles.
Fig. 3. Ft y curves of 7075 aluminum alloy under different angular displacements amplitudes of 0.1251, 0.251 and 0.51and different number of cycles: Fn = 10 N.
Fig. 6. Trend of Ft/Fn and Ft Fn = 10 N.
Fig. 4. Running condition fretting map for 7075 aluminum alloy under rotational fretting condition, N= 1000 cycles.
and angular displacement amplitudes were performed in this study to build up a RCFM for 7075 aluminum alloy under the rotational fretting condition. As shown in Fig. 4, the fretting regimes were found to vary from PSR to MFR and then to SR with the increase of angular displacement amplitudes under constant imposed normal load. While under a constant angular displacement amplitude, the variation with the increase of normal load was reversed. Therefore, the fretting regimes of 7075 aluminum alloy were strongly dependent on the normal load and angular displacement amplitude. 3.1.3. Ft/Fn curves It was found that Ft/Fn (the ratio of friction force to normal force) curves can also be used to characterize the rotational fretting behavior of 7075 aluminum alloy. Ft/Fn curves in different fretting regimes showed significantly different tendency, as shown in Fig. 5. In PSR, i.e. y =0.1251, the Ft/Fn ratio reached a
y curves in MFR for 7075 aluminum alloy: y = 0.251,
steady value of 0.36 after a running-in stage. In the initial stage, Ft/Fn ratio increased rapidly because of the failure of adsorptive and polluted films on the surface. With the fretting wear proceeding, a stable partial slip condition was established at the contact interface, and consequently the Ft/Fn curve was leveled off and became linear. With the increase of the angular displacement amplitude, i.e. under angular displacement amplitudes of 0.251, a transition from the PSR to the MFR was observed. The Ft/Fn curve exhibited a significantly longer ascent stage of 520 cycles with strong fluctuation, and then reached a relatively high steady value of approximately 0.83. The rotational fretting process of 7075 aluminum alloy in the MFR can be divided into four stages according to the Ft y and Ft/Fn curves (Fig. 6): Stage I, for the first few cycles ( o30 cycles), Ft/Fn ratio increased rapidly because of the failure of adsorptive and polluted films on the surface; Stage II, with the increase of the number of cycles, the Ft/Fn curve showed a slower increase in slope, and correspondingly the Ft y curves changed to narrow parallelogram loops; Stage III, after 300 fretting cycles, the Ft y curves took an elliptic shape, indicating the transition from gross slip to partial slip conditions; Stage IV, as the number of cycles was increased to 500, the Ft y curves became linear and the Ft/Fn curve was leveled off accordingly, indicating the establishment of a stable partial slip condition at the contact interface. The fretting wear of the 7075 aluminum alloy in the MFR will be discussed in detail in the following section.
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In the SR, a quite different Ft/Fn curve evolution with strong fluctuation was observed, and a significantly higher Ft/Fn ratio of 0.98 was obtained at the end of 1000 cycles. The Ft/Fn ratio was found to increase with the transition from the PSR to the MFR and then to the SR under the rotational fretting condition. Therefore, the rotational fretting running behavior can be characterized by both the F y and Ft/Fn curves, which exhibited different shapes and variation in trends corresponding to different fretting running regimes.
was observed. As a result, the Ft y curves became elliptic, and the Ft/Fn ratio increased rapidly again. Stage IV: In this stage the contact interface was in state of partial slip. Wear debris was compacted tightly on the worn surface and hardly to be removed out of the wear scar from the edges. Reduction in the depth of profile due to the cover of debris was observed. Consequently, the wear state was transformed from the two-body contact to the three-body contact, and then the wear decreased.
3.2. Rotational fretting wear
3.2.3. In slip regime (SR) In the SR, plowing and detachment of particles was the main wear phenomena for 7075 aluminum alloy, as shown in Fig. 10. EDX analyses showed that the oxygen content on the worn surface was much higher than that on the original surface of specimen. Therefore, the main mechanisms of rotational fretting wear of 7075 aluminum alloy in the SR were abrasive wear, oxidative wear and delamination. Combining with the profile measurement, it was found that a central bulge was formed by plastic deformation accumulation when y =0.51, which was consistent with that observed for the rotational wear of PMMA polymer reported by Briscoe et al. [6–8]. This phenomenon was very different from that observed in the conventional reciprocating fretting tests (tangential fretting mode), and it is worthwhile to further investigate to understand the real mechanism. While for a larger angular displacement amplitude of 1.51, the debris generated was removed out of the wear scar which resulted in a significantly larger wear depth, and the bulge never appeared in this case. The difference was embodied in debris behavior, which probably resulted from their different relative movement between the contact surfaces, i.e. different
3.2.1. In partial slip regime (PSR) The wear of 7075 aluminum alloy in the PSR was very slight with few debris particles surrounding the wear scar, because the low imposed angular displacement was mainly accommodated by elastic deformation of the contact interface. The wear mechanism was a combination of slight abrasive wear and delamination. With the increase of normal load from 10 to 20 N, the wear damage on the surface was found to decrease greatly (Fig. 7), and correspondingly the Ft/Fn ratio showed a significant decrease (Fig. 8). 3.2.2. In mixed fretting regime (MFR) The rotational fretting wear morphologies in the MFR and their profile measurements are shown in Fig. 9. The four damage stages were analyzed as follows: Stage I: The fretting relative motion state was characterized as gross slip in this stage, and the relative slip was accompanied with visible wear. A bulge in the central zone can be observed from the profile of the wear scar. The formation of a bulge may have resulted from the plastic flow of material from the edges to the center of the wear scar under the condition of gross slip. SEM observation of the worn surface showed that visible plastic deformation and slight plowing were the main wear features in this stage. Stage II: With the increase of the number of cycles, relative slip reduced gradually, some particles were detached, oxidized and then accumulated between the contact surfaces. As a result, the tendency of the Ft/Fn curve in this stage was characterized by a slower increase in slope, and correspondingly the Ft y curves evolved to narrow parallelogram loops. The wear damage of 7075 aluminum alloy in this stage increased significantly accompanied with strong plastic deformation in the contact interface. The strong fluctuation of the Ft/Fn curve resulted from detachment of particles, where the bulge was disappeared, and a great amount of debris generated started to be removed out of the wear scar. Stage III: More particles were detached by delamination in this stage, and a transition from gross slip to partial slip conditions
Fig. 8. Ft/Fn curves of 7075 aluminum alloy under different normal loads of 10 and 20 N: y = 0.1251, N =1000 cycles.
Fig. 7. Wear scar morphologies of 7075 aluminum alloy under different normal loads of 10 N (a) and 20 N (b): y = 0.1251, N = 1000 cycles.
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Fig. 9. Wear morphologies (a) and profile measurement (b) of 7075 aluminum alloy in MFR after different number of cycles: y = 0.251, Fn =10 N.
Fig. 10. Wear morphologies and profile measurement of 7075 aluminum alloy in SR: y = 0.51 (a) and 1.51 (b), Fn = 20 N, N = 1000 cycles.
plastic deformation and relative slip. It also may be the result of the wear status transforming from fretting to sliding. As mentioned above, for the wear mechanisms of 7075 aluminum alloy under rotational fretting condition, it can be concluded that the wear in the PSR was characterized by slight attrition, while the wear in the MFR and SR involved a combination of delamination, abrasive and oxidative wear. The wear depth of 7075 aluminum alloy increased significantly with the fretting regime varying from PSR to MFR and then to SR (Fig. 11).
4. Conclusions The F y curves can be used to characterize the rotational fretting running behavior, which exhibited three basic types of
parallelogram, elliptical and linear cycles, respectively. The Ft/Fn curves, also used to characterize the rotational fretting behavior, showed significantly different variation trends in different fretting running regimes. The rotational fretting behavior of 7075 aluminum alloy against 52 100 steel was strongly dependent upon the angular displacement amplitude, normal load and number of cycles, and its running condition fretting map was built up in this study. A change in the angular displacement amplitude or normal load caused a transition from partial slip regime to mixed fretting regime or slip regime. The wear of 7075 aluminum alloy in the PSR was characterized by slight attrition. The main wear mechanisms of 7075 aluminum alloy in the MFR and SR were a combination of abrasive wear, oxidative wear and delamination. A special phenomenon was observed under gross slip condition of the rotational fretting mode that involved the formation of a central bulge due most likely to plastic flow.
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References
Fig. 11. Wear scar profile of 7075 aluminum alloy in PSR (y = 0.1251), MFR (y = 0.251) and SR (y = 0.51): Fn =10 N, N = 1000 cycles.
Acknowledgments This work was supported by National Natural Science Foundation of China (nos. 50775192, 50821063) and State Key
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