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Study on dual rotary fretting wear behavior of Ti6Al4V titanium alloy
Wear 376-377 (2017) 670–679
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Study on dual rotary fretting wear behavior of Ti6Al4V titanium alloy Yan Zhou a, Ming-xue Shen b, Zhen-bing Cai a, Jin-fang Peng a, Min-hao Zhu a,n a b
Tribology Research Institute, Key Lab of Advanced Technologies of Materials, Southwest Jiaotong University, Chengdu 610031, China Engineering Research Center of Process Equipment and Its Remanufacture, Ministry of Education, Zhejiang University of Technology, Hangzhou 310032, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 13 October 2016 Accepted 27 October 2016
To investigate the wear mechanism and microstructural evolution of Ti6Al4V titanium alloy under the condition of dual rotary fretting (DRF), which mainly exists in the interfaces of ball-and-socket joints with the motion combing torsional fretting with rotational fretting. Using the Ti6Al4V titanium alloy flat specimen against the 52100 steel ball to investigate the DRF and damage behavior were carried out under the normal load of 50 N with different tilt angles (10° 60°) and varying angular displacement amplitudes (0.25° 5°). The morphologies of wear surface and their cross-sections were analyzed by scanning electron microscopy, electron probe microscopy analyzer and a surface profilometer etc. The results are as followed: 1) The DRF wear behavior of Ti6Al4V is strongly dependent on the tilt angle, which decides the scale for combination of rotation component and torsion component. 2) Fx/Fn curves presents different varied trends in different fretting regimes. The fretting wear is slight in partial slip regime due to the oxidative wear, and the surface bulge is formed in mixed fretting regime probably due to the plastic-flow, and in sliding regime, plough scratches caused by the abrasive wear. 3) As the tilt angle increase, DRF wear mechanism of Ti6Al4V titanium alloy transfers from oxidative wear to the abrasive wear. 4) The DRF damage of Ti6Al4V is the result of the competition between the rotational fretting component and the torsional fretting component. When the DRF behavior is mainly controlled by the torsional fretting component, the wear mechanism is the oxidative wear with the oxygen element distributing around the micro-slip circle zone. When the DRF behavior is mainly controlled by the rotational fretting component, the severe plastic deformation can be observed. & 2016 Elsevier B.V. All rights reserved.
Keywords: fretting wear Ti6Al4V rotational torsional oxidative wear
1. Introduction Since the characteristic of titanium alloy, such as low density and high strength and anti-corrosion, it has been widely used in the aerospace, ship manufacture, the chemistry industry, the joints implants [1–3] and so on. Ball-and-socket joint on dental titanium alloy implants was suffered the fretting with corrosion in mouth cavity [4]. A typical ball and socket joint of the artificial joint is which have the function to complete different relative motion such as rotation, torsion [5] [6] and other movements [7]. Moreover, the contact interfaces of ball and socket joint is tight assembled, therefore, it is similar to the rotational fretting model [8] with a little relative torsion angle which can be called the DRF [9,10]. It is known that fretting wear is a surface degradation process induced by small amplitude oscillatory movements between contacting bodies [11,12]. Briscoe et al. has investigated the DRF behavior of PMMA polymer against steel [13], and shown effects of varied tilt angles on the wear damage. n
Corresponding author. E-mail address: [email protected] (M.-h. Zhu).
http://dx.doi.org/10.1016/j.wear.2016.10.027 0043-1648/& 2016 Elsevier B.V. All rights reserved.
In addition, wear process of ductile materials is often accompanied with extensive plastic deformation and the ductile materials titanium alloy presents the poor tribological characteristic. In order to improve the wear resistance of titanium alloy through the surface modification method, such as C- DLC coating [3], Micro-Arc Oxidation TiO2 coatings [14,15] and so on, many researchers have investigated the wear resistance of these coatings under the reciprocating-sliding mode [16,17], pin-disc mode [18] and torsion fretting mode [6,19]. In order to better understand DRF behavior with our previous literatures [6,7,10,12,19–24], DRF tests of Ti6AL4V titanium alloy was carried out based on the DRF device. The DRF behavior and wear mechanisms have been investigated in detail, especially the discussion was emphasized that the effect of tilt angle on the DRF behavior.
2. Experimental procedure The DRF wear tests with a ball-on-flat contact were carried out on the DRF device improved from a low-speed reciprocating rotary motor system, as shown in Fig. 1 [9]. The flat specimens
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Fig. 1. Schematic diagram of DRF test device[9].
Table 1 Chemical composition (wt %) and mechanical properties of specimens. Alloy
TC4 52100
Chemical composition (wt %) C
Si
Mn
Ni
Cr
r0.10 1.0
0.25
0.30
0.20
1.50
(size of 10 10 20 mm) made of Ti6Al4V titanium alloy and the ball specimens (AISI 52100 steel) of 40 mm diameter were the friction pair. The surfaces were polished to a roughness (Ra E 0.04 μm). Table 1 summarizes the main chemical composition and mechanical properties of the materials. DRF tests were performed under laboratory control conditions(T ¼20 7 2 °C, RH ¼ 60 7 10%) by changing tilt angles (α ¼ 10°, 20°, 40°, 60°). The electric motor (5) under a constant rotary angle speed of 0.2°/s carry the ball specimen with the angular displacement amplitude varied from 0.25° to 5°, and the normal load of 50 N was imposed through the force sensor (12) for all tests to keep two specimens contact and ensure the constant contact pressure during testing, the number of the cycles are ranged from 1 to 103. Before testing, all specimens were ultrasonically cleaned in acetone. Microscopic examinations on specimen surfaces were performed by optical microscopy and scanning electron microscope after the tests. The worn surfaces and the cross-section morphologies are obtained by an optical microscope (OM, BX60MF5, OLYMPUS) and a scanning electron microscope (SEM, JSM-6610, JOEL), and the chemical compositions of the worn surface by the electron probe microanalyzer (EPMA, JOEL-8230x, JOEL). (1) An upper holder; (2) A flat specimen; (3) A ball specimen; (4) A lower holder, (5) a low-speed reciprocating rotary motor system to realize the fretting modes; (6) the centerline of the ball specimen overlapped to the rotary axis of the motor system; (7) a locating screw locked at a certain tilt angle α (in the range of 0– 90°); (8) a sleeve of motor system; (9) a base platform around the rotary axis.; (10) a vertical positioning system; (11) a lateral positioning system; (12) a 6-D forces/torques sensor.
Al
V
Ti
Fe
6.1
4
Bal.
r 0.50 Bal.
Hardness /HV
sb /Mpa
320 870
896 2000
3. Results and discussions 3.1. DRF kinetic behavior 3.1.1. Ft θ curves Ft-θ curves can show the information of friction force obtained from the DRF tests and reflect the state of the contact interface. Therefore, it is used to describe the fretting kinetic behavior. Fig. 2 shows the typical Ft θ curves as a function of increasing cycles under different tilt angles (α ¼20°, 40°) varied by the angular displacement amplitude (θ ¼ 0.25 5.0°). When the angular displacement amplitude θ is less than 1.0° with the tilt angle α of 20°or the angular displacement amplitude θ is less than 0.5°with the tilt angle α of 40°, all Ft θ curves shapes were quasi-linear during the whole DRF process, may the contact interface coordinate by the elastic deformation and the relative displacement hardly occur. Therefore, the fretting conditions ran in partial slip regime (PSR). As the tilt angle or the angular displacement amplitude increased, such as the angular displacement amplitude θ of 1.0°with the tilt angle α of 20°, the Ft θ curves presented the shape of parallelogram in the first several dozen cycles, then transformed to linear-shaped in succeeding cycles which implied that the relative motion between the contact interfaces changed from gross slip to partial slip. In addition, the sticking zone of the wear scar always can be observed. When the sliding condition evolves from one to another sliding condition, this is the mixed fretting regime (MFR) [13]. Therefore, the fretting ran in mixed fretting regime. However, When the angular displacement amplitude θ is more than 2.0° with the tilt angle α of 20° or 40°, all the cycle loops were of parallelograms which meant the sticking zone disappeared and sliding occurred all over the contact area, that meant the fretting entered slip regime (SR).
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Fig. 2. Variation of Ft-θ curves under different cycles: Fn ¼50N, α¼ 20° and 40°, θ: 0.25° 5°.
In conclusion, as the angular displacement amplitude θ increasing, the DRF can transfer from PSR to MFR, and then to SR. The tilt angle with the small values is used to represent the DRF controlled by the torsional fretting while it is large which represent the DRF controlled by the rotational fretting [14,15]. When the tilt angle is increasing, the friction force has a slight rise when the fretting runs in the PSR regime, while the friction force can keep at the same value when the fretting runs into the SR regime. 3.1.2. Running condition fretting map The fretting map can be used to characterize running behavior and damage mechanism of fretting [16–18]. According to the analysis of variation of Ft-θ curves in Fig. 2, the DRF map of titanium Ti6AL4V alloy for each point can be plotted one by one and the fretting regime can be defined by varying tilt angles and angular displacement amplitudes. The running condition fretting map and the corresponding OM morphologies of worn scars were shown in Fig. 3 and Fig. 4, respectively. As shown by Fig. 3, the MFR zone gradually decreased and the SR zone expanded to the side with the large tilt angle and small angular displacement amplitude. In addition, with the angular displacement amplitude increased, the fretting regime transformed from the PSR regime to the MFR regime and then to the SR regime successively. In conclusion, the fretting regimes were strongly dependent on the tilt angle and the angular displacement amplitude. According to the theory of fretting maps [13,19], both the tilt angle
Fig. 3. The running condition fretting map (RCFM) of Ti6Al4V with DRF conditions: N¼ 1000 cycles: Fn ¼ 50 N.
and the angular displacement amplitude strongly affect the DRF behavior of titanium Ti6AL4V alloy. 3.1.3. Ft/Fn curves The Ft/Fn (the ratio of friction force to the normal force) curves which can also be characterized the DRF behavior of Ti6Al4V
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Fig. 4. Worn scars of Ti6Al4V under the DRF conditions: Fn ¼ 50 N, N ¼1000 cycles.
Fig. 5. Evolution of Ft/Fn curves under the different tilt angle: (a) α ¼10°, (b) α ¼40°.
titanium alloy. Fig. 5 shows the evolution of Ft/Fn curves in different fretting regimes with different tendencies. In PSR, there was a larger sticking zone in the center of the contact interface surrounded by a bit slight slipping rim. The relative motion of the contact interface mainly coordinated by the elastic deformation, so the Ft/Fn curves kept a steady low value.It matched up to the friction force of the Ft-θ curves in PSR that also remained a quite steady low value during the whole DRF test. In MFR, the Ft/Fn curves with a rapidly climbing trend of the first several dozen cycles before it achieved a stable value and kept it. Referring to the Ft θ curves in Fig. 2, the result showed that the
Ft/Fn curves were climbing when the Ft θ curves transformed from the parallelogram to linear shaped with friction force increasing, and the ratio of Ft/Fn curve were steady as the Ft θ curves kept in a linear shape. In addition, the formation of the contact interfaces local bulge occurred during the Ft/Fn curves climbing process probably induced by the contact interfaces plastic flow and the wear, which is discussed in details in the following section. In SR, Ft/Fn curves are not strongly dependent on the angular displacement, which is similar to the Pearson's results [20] that indicates the friction coefficient is in fact independent of slip
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Fig. 6. SEM morphologies (a) α¼ 10°, (b) α¼ 40°and surface profiles (c) in PSR: θ¼ 0.25° N ¼ 1000 cycles.
Fig. 7. Wear scars and profiles of Ti6Al4V alloy in MFR after different number of cycles: α ¼ 20°, θ¼ 0.5°.
amplitude. In detail, the Ft/Fn curves can be divided into 4 stages based on the variation tendency: Stage I: at the initial fretting stage, the Ft/Fn curves started at a quite low value due to the small tangential force since the surface and polluted films on the contact surface. After several cycles, the Ft/Fn ratio rapidly achieved a low peak value due to the failure of surface and polluted films, the metal to metal contact friction occurred at the contact interface company with the contact force increasing. Stage II: The gradually accumulated wear debris as the third body played an important role in the DRF behavior, the contact interfaces were balance so the low Ft/Fn ratio value kept level temporarily. Stage III: As the cycles increased to 100–500, the Ft/Fn ratio was climbing rapidly again possibly due to the adhesion, plastic deformation and ploughing between the contact interfaces. Stage IV: after 500 cycles of the DRF, most wear debris ejected from wear scar that made the contact interfaces reached a dynamic equilibrium statement, the Ft/Fn ratio remained at a steady value. In addition, as the tilt angle increasing, the friction coefficient has a holistic ascend. For example, at the angular displacement amplitude of 0.5, which is very small value at 0.08 under the tilt angle of 10°,while the tilt angle increased into 40°, the friction coefficient value is about 0.4, which is 5 times than that with the tilt angle of 10°. It indicated the tilt angle has a significance effect to change the running condition of the DRF. 3.2. DRF wear behavior 3.2.1. In PSR regime Fig. 6 shows the worn scars morphologies and their 2D profiles under different tilt angles. When the tilt angle increased, the worn zone expanded to a moon-shaped worn scar, which means the distribution of contact force on contact interfaces was not uniform. As shown by Fig. 6(a) and (b), the worn scar is like a circle, and the
center sticky contact zone seems no wear damage, while the outside of circle edge can be observed some slight scratches. As the tilt angle increasing, the diameter of the sticky zone circle decreased, and the centers of these two circles are eccentric, which are the sliding circle and the sticky circle, respectively. In PSR, the wear scar like a circle without obvious damage due to the elasticity deformation of the contact interfaces, the increased tilt angle transformed the controlling fretting parameter from torsion component to rotation component, and made the sticky center shift to one side so a asymmetric wear scar was formed. In a word, the damage in PSR was quite slight without wear debris formed (which can see from the Fig. 6(c)), and the sticky zone always existed in PSR regime even with the cycles increased. 3.2.2. In MFR regime In MFR, as the cycles increased, the central sticky contact zone decreasingly narrowed then may disappear finally and the slight bulge in the central contact zone can be observed from the morphologies of the wear scar (which can see from the Fig. 7). There was few wear debris formed in MFR but some plowing tracks and visible plastic deformation can be observed, which may result in the bulges. Comparing the wear scars under different tilt angles, it was found that under a small tilt angle α of 10° in Fig. 8, the MFR-scar was similar to be a circle shaped which was the main feature of the torsional fretting [4,21]. However, under a large tilt angle α of 40° in Fig. 9, the MFR-scar became to a ellipse shaped with the major axis of the ellipse extended along the sliding direction which mainly control by the rotation component. With the tilt angle increased, the controlling parameter of the DRF behavior varied from the torsion component to the rotation component. The torsion component was a symmetrical component so the wear scar was like a circle or the bulge profile presented a slight
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Fig. 8. SEM morphologies and profiles of Ti6Al4V alloy in MFR: α ¼ 10°, θ¼ 1.0°.
Fig. 9. SEM morphologies and profiles of Ti6Al4V alloy in MFR: α ¼40°, θ¼ 0.5°.
structure than the matrix structure. It is reported by many researchers in fretting tests. While under the dual- rotary fretting condition, TTS was formed at the site close to the center zone where no obvious damage can be found. The SEM observations of TTS show in the central scar,but not near the edges in study of Sauger et al. [22]. The formation of TTS resulted from the driving force of plastic deformation. Fig. 10. SEM morphology of cross-section for Ti6Al4V alloy worn scar in MFR: α ¼10°, θ ¼1.0°, N ¼105.
symmetric flat, however, when the rotation component play a main role in DRF that resulted in a asymmetric state, then the wear scar was changed to ellipse or the bulge profile was more larger and asymmetrical [3]. Therefore, the wear damage of the DRF that mainly controlled by the rotation component was more severe than it controlled by the torsion component. Some studies show that, the fatigue crack induced by the fretting has always been observed in the MFR zone. For example, the crack can be found at MFR regime under the DRF of 7075 aluminum alloy [14]. The cross-section of worn scar for making sure whether there is the fatigue crack or not. Fig. 10 is the cross-section morphology of the Ti6Al4V titanium alloy worn scars in MFR. The crack can be observed only at the one side of the asymmetric circle, which looks like straight along the surface, no angle formed between the crack and the base material, it is typical different from the crack behavior of 7075 aluminum alloy under the same fretting condition. There are many cracks to propagate along with a fixed angle into the 7075 aluminum alloy matrix, so the 7075 aluminum alloy can be easier to cause the fretting fatigue than that of Ti6Al4V titanium alloy [14]. In addition, the tribology transformed structure (TTS) [22,23] can be formed which is more compacted
3.2.3. In SR regime In SR, there was a large amount of the oxidation debris ejected from the contact interfaces and some plough scratches along the rotational fretting direction (can be seen from Fig. 12).The profiles of wear scars showed that the wear depth in SR was the largest than other fretting regimes. In addition, the delamination can always be observed on wear surfaces (Fig. 11 and Fig. 12). Therefore, the wear mechanisms of DRF in SR were oxidation wear, abrasive wear and delamination. It is also valuable to pay some attention to the damages under different tilt angles. Under the small tilt angle, a large amount of compacted wear debris was squeezed by the contact press which finally resulted in some surface cracks. Under the large tilt angle, however, the ejection of wear debris was easier and timely, so the wear scar surface was relative smooth. With the increasing of the tilt angle, the plowing shape varied from arc-shaped to straightshaped (Fig. 11(b) and Fig. 12(b)). It also demonstrated that with the increasing of the tilt angle, the DRF controlling component changed from the torsional component to the rotational component. As mentioned above, for the wear mechanisms of Ti6Al4V titanium alloy under DRF conditions, it can be concluded that the wear in PSR was characterized by slight attrition, while the wear in MFR and SR involved a combination of plastic-deformation, abrasive and oxidation wear and delamination.
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Fig. 11. SEM morphologies and profiles of Ti6Al4V alloy in SR: α¼ 10°, θ¼ 5.0°.
Fig. 12. SEM morphologies and profiles of Ti6Al4V titanium alloy in SR: α¼ 40°, θ ¼5.0°.
The DRF runs into the SR regime, the rate of wear loss was higher and the damage of material was accelerated. Fig. 13 is the cross-section morphology of the Ti6Al4V titanium alloy worn scars in SR. No crack can be seen in SR regime, the similar results can be found in the study of Mohd Tobi et al. [24] only the local plastic deformation of the material near the worn surface is embedded in Fig. 13. The typical shallow “U” scallop shape can be observed by the cross-section morphology of worn scar in SR. It is also different the wear mechanism of the 7075 aluminum alloy, because the initiation of the fatigue crack can be found during the DRF, the wear loss rate of material is faster than that of crack propagation in SR regime. However, the Ti6Al4V titanium alloy is difficult for crack initiation, so the abrasive wear and the structure deformation is the mainly wear mechanism. 3.3. Tilt angle effect on DRF behavior 3.3.1. Ratio of rotational fretting and torsional fretting Since under the large angular displacement amplitude, the fretting behavior is like reciprocating sliding. Therefore the fretting transfers from the torsional to rotational can be observed with the small angular displacement amplitude. In other word, the fretting runs in PSR or MFR regimes. When the fretting mainly was controlled by the torsional component, the worn scars are always the circle-shaped. As the angular displacement amplitude increased, the diameter of the circle may increase slightly. The tilt angle increased can result in the eccentric circle worn scar, the center of adhesive circle zone is different from that of the sliding circle zone. When the fretting mainly control by the rotational component, the worn scars is a ellipseshaped or rectangle-shaped. The long axis length of the worn scar would be longer as the increase of angular displacement amplitude. To evaluate the effect of tilt angle on the DRF behavior, a ratio need to use for quantifying the function of rotational fretting and torsional fretting.
Fig. 13. SEM morphology of cross-section for Ti6Al4V titanium alloy worn scar in SR: α ¼40°, θ¼ 5.0°, N ¼105.
Considering the worn scar is not always like a circle, so the width of the worn scar called as X was defined as the variable of the torsional component. The length Y of worn scar was defined as the variable of the rotational component. Therefore, during the DRF is complex by rotational fretting and torsional fretting, the ratio Y/X of worn scars was used to evaluate the damage of the DRF. Fig. 14(a) shows the detail of the relationship between variables of X and Y. When the ratio value is equal to 2, it means the fretting zone is two times larger than the contact zone, the fretting behavior is gross-sliding which is similar to the reciprocating sliding friction. Fig. 14 shows the ratio of Y/X for worn scars with the small angular displacement amplitudes θ of 0.5° and 1.0° under different tilt angles. If the angular displacement amplitude is fixed, the variable X was similarly the same since the torsional fretting component hardly effect on the diameter of worn scars. When the tilt angle increased, the variable Y effect by the rotational component will increase. Therefore, the ratio curve with the fixed angular displacement amplitude presented a climbing trend as the tilt angle increasing. Comparing with the ratio curves under different angular displacement amplitudes, both of these curves maintain the similar climbing tendency. Specifically, when the angular displacement amplitudes θ is 0.5°, the ratio stay at 1.0 before the tilt angle of 40° and then begin to rise up to 1.4 since the tilt angle
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Fig. 14. Ratio curves of Ti6Al4V titanium alloy worn scars variation under angular displacement amplitude θ of 1.0° and 0.5° with the increasing tilt angles.
greatly decrease the wear volume. Specifically, comparing Fig. 15 (a) to (b), the wear volumes value under the angular displacement amplitude θ of 1.0° are approximately 10 times more than that under the angular displacement amplitude θ of 0.5°. At the tilt angle of 10°,when the angular displacement amplitude θ increased from 0.5° to 1.0°, the wear volume changed from 0.89 105 to 1.17 105 mm3, and the latter is 1.3 times more than the former one. When the tilt angle is 40°, increasing the angular displacement amplitude θ from 0.5°to 1.0° as 2 times, while the wear volume of latter one is 14 times more than that of former one. According to the result above, it indicated that the tilt angle has more significance effect on the variation of wear volume than the angular displacement amplitude in PSR and MFR regimes. What's more, the difference between the wear volumes from the tilt angle of 40°and 60° which is the 14 times. It indicated that when the tilt angle achieved at a constant value, the influence of angular displacement amplitude θ can be neglectful in SR regimes.
Fig. 15. Wear volumes of Ti6Al4V titanium alloy with different tilt angles under different angular displacement amplitude: (a) θ¼ 0.5°, (b) θ¼ 1.0°.
increase to 60°. The entire curve of the large angular displacement amplitude is little higher than that of small angular displacement amplitude. When the angular displacement amplitudes θ is 1.0°, the ratio increased linearly from 1.0 gradually to 1.4 as the tilt angle increasing to 60°. Obviously, as the ratio increasing, the size of worn scars gets bigger which is corresponding to the OM morphologies in Fig. 4 and the wear damage become more severe during DRF. Moreover, it also indicates that the wear damage induced by the rotational fretting is heavier than that induced by torsional fretting on DRF. 3.3.2. Wear volume effected by the tilt angle Fig. 15 shows the variation of wear volume as the tilt angle increasing under the angular displacement amplitude θ of 0.5° and 1.0°. The variation of wear volume is greatly dependent on the tilt angle. As shown by Fig. 15, the wear volumes get bigger as the tilt angle increasing at a constant value of angular displacement amplitude. Moreover, to decrease the angular displacement amplitude can
3.3.3. Tilt angle effect on oxidative wear In term of the torsional fretting, rotational fretting, or the dual rotational fretting, the typical chemical change on the worn surface is the O element [3,15,25,26]. It is significant to investigate the O elements distribution on the worn scars as the varied tilt angles, which is helpful to understand the oxidative wear mechanism during DRF. Fig. 16 shows EPMA distribution mappings of O and Ti elements on the worn scars companied with a line scan in each maps, varied as the increasing tilt angle. The average content of O element increase as the tilt angle increasing. The average content of O elements on worn scars is close to zero in the Fig. 16 (a) and (b), while in the Fig. 16 (c) is show the average content of 1.97% O. In details, Fig. 16 (a) and (b) is represented the torsional component, the distribution of O is like a concentric ring, the width of the ring is about 100 um, the center of contact zone, it is dark, is no O elements here because the zone is no relative displacement, the Oxygen do not enter in the interface. As the tilt angle increasing, the relative motion between two contact surfaces become easier, the more regimes can be oxidative. From the line scan distribution, we can find out that the asymmetry peaks of O elements along the line. What's more, the distribution of Ti element is also a shallow concentric circle, which is corresponding to the distribution of O element. So when the fretting behavior is controlled by the torsional component, the damage of the Ti6Al4V is oxidative wear. As the torsional component decrease, one side of the circle occur the heavier oxidative peaks. To observe Fig. 16(c), the tilt angle is 60°. The rotational component can make bigger relative motion of contact interface which make more contribute to the oxidative
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Fig. 16. EPMA distribution mappings of O (a–c) and Ti (d–f) elements on worn scars under different tilt angles α¼ 10°, α ¼20°, α¼ 60°.
Fig. 17. Cross-section of Ti6Al4V titanium alloy worn scars under angular displacement amplitude θ¼ 1.0° with different tilt angles: (a) α ¼10°; (b) α¼ 60°.
wear. So the average concentration of O element is higher. The highest O elements concentration is distributed along a circle zone around the worn scar which is mainly caused by the torsional component fretting. The rotational component make the contact center zone difficult to distinguish, and the line scan of O is like the random waved peak as well as the line distribution of Ti element, the results show that the O distributed uniformed on the
center contact zone. While the main zone for Ti elements lost is at the outside circle where shows the highest O may be covered with the oxides. 3.3.4. Tilt angle effect on wear mechanism The DRF wear mechanism of Ti6Al4V titanium alloy changed as the tilt angle increased, as shown in Fig. 17. The fatigue behavior-
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the crack is difficult to be observed under the DRF of Ti6Al4V alloy, it indicated that the fretting wear resistance of Ti6Al4V titanium alloy is better than that of 7075 aluminum alloy and medium carbon steel [9,14,15]. The significance wear mechanism of Ti6Al4V titanium alloy is oxidative wear, which is the mainly reason caused the failure of the materials. From the morphology of cross-section in Fig. 17(a), the microstructure near the worn surface can be observed that almost has no deformation when the tilt angle is small, it indicated that the DRF wear mechanism Ti6Al4V titanium alloy is oxidative wear when the DRF is controlled by the torsional fretting. Comparing to the Fig. 17(b), the worn surface was covered with the thin deformed layer, and the plastic deformation is the significance driving force to make the microstructure deformed thin layer. Due to the great ductility of Ti6Al4V titanium alloy, it may induce the microstructure changed such as tribological transformed structure (TTS). As the tilt angle increasing, the surfaces of contact zone ploughed by the debris particles to form a rough worn surface, and the plastic deformation of microstructure near by the worn surface occurred. In conclusion, when the DRF behavior is mainly controlled by the torsional fretting component, the wear damage is slight and the wear mechanism is oxidative wear. As the DRF behavior is mainly controlled by the rotational fretting component, the severe wear damage can be observed which is induced by the plastic deformation, the wear mechanisms are abrasive wear and oxidative wear.
4. Conclusions To investigate the wear mechanism and microstructural evolution of Ti6Al4V titanium alloy under the condition of DRF, Using the Ti6Al4V titanium alloy flat specimen against the 52100 steel ball to investigate the DRF and damage behavior were carried out with different tilt angles and varying angular displacement amplitudes. The conclusions are as followed: (1) DRF Ft-θ curves of Ti6Al4V titanium alloy have three types: linear-shaped, elliptical-shaped and parallelogram-shaped which corresponds to different fretting regimes. The angle displacement amplitude and the tilt angle intensively affect the dual-rotational fretting behavior. With the increase of tilted angle, the regime of MRF zone gradually decreases, and the ranges of SR and PSR zones expand. For the angle amplitude increasing, the fretting regime runs from PSR to MFR and then to SR. The Ft/Fn curves present different evolution characteristics in different fretting regimes. (2) The wear damage mechanisms of Ti6Al4V titanium alloy during DRF process are different. In PSR, the wear damage is slight. In SR, wear mechanism is the abrasive and oxidation wear. When the DRF runs into the MFR, a little bit debris can be observed and the bugles formed may be induced by the plastic-flow accumulation. (3) As the tilt angle increase, DRF wear mechanism of Ti6Al4V titanium alloy transfers from oxidative wear to the abrasive wear, as well as fretting is controlled by the torsion component transit to the rotation component. (4) When DRF is mainly controlled by the rotational component, the friction coefficient value has a slight increase corresponding to the variation of wear volumes. However, rotational component can result in severe wear damage with the ploughing scratches induced by the subsurface microstructure plastic deformation.
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Acknowledgements The authors would like to thank the National Nature Science Foundation of China, this research was supported by National Natural Science Foundation of China (Contract numbers (No.51375407, 51575459, U1530136, 51627806).
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Wear journal homepage: www.elsevier.com/locate/wear
Study on dual rotary fretting wear behavior of Ti6Al4V titanium alloy Yan Zhou a, Ming-xue Shen b, Zhen-bing Cai a, Jin-fang Peng a, Min-hao Zhu a,n a b
Tribology Research Institute, Key Lab of Advanced Technologies of Materials, Southwest Jiaotong University, Chengdu 610031, China Engineering Research Center of Process Equipment and Its Remanufacture, Ministry of Education, Zhejiang University of Technology, Hangzhou 310032, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 13 October 2016 Accepted 27 October 2016
To investigate the wear mechanism and microstructural evolution of Ti6Al4V titanium alloy under the condition of dual rotary fretting (DRF), which mainly exists in the interfaces of ball-and-socket joints with the motion combing torsional fretting with rotational fretting. Using the Ti6Al4V titanium alloy flat specimen against the 52100 steel ball to investigate the DRF and damage behavior were carried out under the normal load of 50 N with different tilt angles (10° 60°) and varying angular displacement amplitudes (0.25° 5°). The morphologies of wear surface and their cross-sections were analyzed by scanning electron microscopy, electron probe microscopy analyzer and a surface profilometer etc. The results are as followed: 1) The DRF wear behavior of Ti6Al4V is strongly dependent on the tilt angle, which decides the scale for combination of rotation component and torsion component. 2) Fx/Fn curves presents different varied trends in different fretting regimes. The fretting wear is slight in partial slip regime due to the oxidative wear, and the surface bulge is formed in mixed fretting regime probably due to the plastic-flow, and in sliding regime, plough scratches caused by the abrasive wear. 3) As the tilt angle increase, DRF wear mechanism of Ti6Al4V titanium alloy transfers from oxidative wear to the abrasive wear. 4) The DRF damage of Ti6Al4V is the result of the competition between the rotational fretting component and the torsional fretting component. When the DRF behavior is mainly controlled by the torsional fretting component, the wear mechanism is the oxidative wear with the oxygen element distributing around the micro-slip circle zone. When the DRF behavior is mainly controlled by the rotational fretting component, the severe plastic deformation can be observed. & 2016 Elsevier B.V. All rights reserved.
Keywords: fretting wear Ti6Al4V rotational torsional oxidative wear
1. Introduction Since the characteristic of titanium alloy, such as low density and high strength and anti-corrosion, it has been widely used in the aerospace, ship manufacture, the chemistry industry, the joints implants [1–3] and so on. Ball-and-socket joint on dental titanium alloy implants was suffered the fretting with corrosion in mouth cavity [4]. A typical ball and socket joint of the artificial joint is which have the function to complete different relative motion such as rotation, torsion [5] [6] and other movements [7]. Moreover, the contact interfaces of ball and socket joint is tight assembled, therefore, it is similar to the rotational fretting model [8] with a little relative torsion angle which can be called the DRF [9,10]. It is known that fretting wear is a surface degradation process induced by small amplitude oscillatory movements between contacting bodies [11,12]. Briscoe et al. has investigated the DRF behavior of PMMA polymer against steel [13], and shown effects of varied tilt angles on the wear damage. n
Corresponding author. E-mail address: [email protected] (M.-h. Zhu).
http://dx.doi.org/10.1016/j.wear.2016.10.027 0043-1648/& 2016 Elsevier B.V. All rights reserved.
In addition, wear process of ductile materials is often accompanied with extensive plastic deformation and the ductile materials titanium alloy presents the poor tribological characteristic. In order to improve the wear resistance of titanium alloy through the surface modification method, such as C- DLC coating [3], Micro-Arc Oxidation TiO2 coatings [14,15] and so on, many researchers have investigated the wear resistance of these coatings under the reciprocating-sliding mode [16,17], pin-disc mode [18] and torsion fretting mode [6,19]. In order to better understand DRF behavior with our previous literatures [6,7,10,12,19–24], DRF tests of Ti6AL4V titanium alloy was carried out based on the DRF device. The DRF behavior and wear mechanisms have been investigated in detail, especially the discussion was emphasized that the effect of tilt angle on the DRF behavior.
2. Experimental procedure The DRF wear tests with a ball-on-flat contact were carried out on the DRF device improved from a low-speed reciprocating rotary motor system, as shown in Fig. 1 [9]. The flat specimens
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Fig. 1. Schematic diagram of DRF test device[9].
Table 1 Chemical composition (wt %) and mechanical properties of specimens. Alloy
TC4 52100
Chemical composition (wt %) C
Si
Mn
Ni
Cr
r0.10 1.0
0.25
0.30
0.20
1.50
(size of 10 10 20 mm) made of Ti6Al4V titanium alloy and the ball specimens (AISI 52100 steel) of 40 mm diameter were the friction pair. The surfaces were polished to a roughness (Ra E 0.04 μm). Table 1 summarizes the main chemical composition and mechanical properties of the materials. DRF tests were performed under laboratory control conditions(T ¼20 7 2 °C, RH ¼ 60 7 10%) by changing tilt angles (α ¼ 10°, 20°, 40°, 60°). The electric motor (5) under a constant rotary angle speed of 0.2°/s carry the ball specimen with the angular displacement amplitude varied from 0.25° to 5°, and the normal load of 50 N was imposed through the force sensor (12) for all tests to keep two specimens contact and ensure the constant contact pressure during testing, the number of the cycles are ranged from 1 to 103. Before testing, all specimens were ultrasonically cleaned in acetone. Microscopic examinations on specimen surfaces were performed by optical microscopy and scanning electron microscope after the tests. The worn surfaces and the cross-section morphologies are obtained by an optical microscope (OM, BX60MF5, OLYMPUS) and a scanning electron microscope (SEM, JSM-6610, JOEL), and the chemical compositions of the worn surface by the electron probe microanalyzer (EPMA, JOEL-8230x, JOEL). (1) An upper holder; (2) A flat specimen; (3) A ball specimen; (4) A lower holder, (5) a low-speed reciprocating rotary motor system to realize the fretting modes; (6) the centerline of the ball specimen overlapped to the rotary axis of the motor system; (7) a locating screw locked at a certain tilt angle α (in the range of 0– 90°); (8) a sleeve of motor system; (9) a base platform around the rotary axis.; (10) a vertical positioning system; (11) a lateral positioning system; (12) a 6-D forces/torques sensor.
Al
V
Ti
Fe
6.1
4
Bal.
r 0.50 Bal.
Hardness /HV
sb /Mpa
320 870
896 2000
3. Results and discussions 3.1. DRF kinetic behavior 3.1.1. Ft θ curves Ft-θ curves can show the information of friction force obtained from the DRF tests and reflect the state of the contact interface. Therefore, it is used to describe the fretting kinetic behavior. Fig. 2 shows the typical Ft θ curves as a function of increasing cycles under different tilt angles (α ¼20°, 40°) varied by the angular displacement amplitude (θ ¼ 0.25 5.0°). When the angular displacement amplitude θ is less than 1.0° with the tilt angle α of 20°or the angular displacement amplitude θ is less than 0.5°with the tilt angle α of 40°, all Ft θ curves shapes were quasi-linear during the whole DRF process, may the contact interface coordinate by the elastic deformation and the relative displacement hardly occur. Therefore, the fretting conditions ran in partial slip regime (PSR). As the tilt angle or the angular displacement amplitude increased, such as the angular displacement amplitude θ of 1.0°with the tilt angle α of 20°, the Ft θ curves presented the shape of parallelogram in the first several dozen cycles, then transformed to linear-shaped in succeeding cycles which implied that the relative motion between the contact interfaces changed from gross slip to partial slip. In addition, the sticking zone of the wear scar always can be observed. When the sliding condition evolves from one to another sliding condition, this is the mixed fretting regime (MFR) [13]. Therefore, the fretting ran in mixed fretting regime. However, When the angular displacement amplitude θ is more than 2.0° with the tilt angle α of 20° or 40°, all the cycle loops were of parallelograms which meant the sticking zone disappeared and sliding occurred all over the contact area, that meant the fretting entered slip regime (SR).
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Fig. 2. Variation of Ft-θ curves under different cycles: Fn ¼50N, α¼ 20° and 40°, θ: 0.25° 5°.
In conclusion, as the angular displacement amplitude θ increasing, the DRF can transfer from PSR to MFR, and then to SR. The tilt angle with the small values is used to represent the DRF controlled by the torsional fretting while it is large which represent the DRF controlled by the rotational fretting [14,15]. When the tilt angle is increasing, the friction force has a slight rise when the fretting runs in the PSR regime, while the friction force can keep at the same value when the fretting runs into the SR regime. 3.1.2. Running condition fretting map The fretting map can be used to characterize running behavior and damage mechanism of fretting [16–18]. According to the analysis of variation of Ft-θ curves in Fig. 2, the DRF map of titanium Ti6AL4V alloy for each point can be plotted one by one and the fretting regime can be defined by varying tilt angles and angular displacement amplitudes. The running condition fretting map and the corresponding OM morphologies of worn scars were shown in Fig. 3 and Fig. 4, respectively. As shown by Fig. 3, the MFR zone gradually decreased and the SR zone expanded to the side with the large tilt angle and small angular displacement amplitude. In addition, with the angular displacement amplitude increased, the fretting regime transformed from the PSR regime to the MFR regime and then to the SR regime successively. In conclusion, the fretting regimes were strongly dependent on the tilt angle and the angular displacement amplitude. According to the theory of fretting maps [13,19], both the tilt angle
Fig. 3. The running condition fretting map (RCFM) of Ti6Al4V with DRF conditions: N¼ 1000 cycles: Fn ¼ 50 N.
and the angular displacement amplitude strongly affect the DRF behavior of titanium Ti6AL4V alloy. 3.1.3. Ft/Fn curves The Ft/Fn (the ratio of friction force to the normal force) curves which can also be characterized the DRF behavior of Ti6Al4V
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Fig. 4. Worn scars of Ti6Al4V under the DRF conditions: Fn ¼ 50 N, N ¼1000 cycles.
Fig. 5. Evolution of Ft/Fn curves under the different tilt angle: (a) α ¼10°, (b) α ¼40°.
titanium alloy. Fig. 5 shows the evolution of Ft/Fn curves in different fretting regimes with different tendencies. In PSR, there was a larger sticking zone in the center of the contact interface surrounded by a bit slight slipping rim. The relative motion of the contact interface mainly coordinated by the elastic deformation, so the Ft/Fn curves kept a steady low value.It matched up to the friction force of the Ft-θ curves in PSR that also remained a quite steady low value during the whole DRF test. In MFR, the Ft/Fn curves with a rapidly climbing trend of the first several dozen cycles before it achieved a stable value and kept it. Referring to the Ft θ curves in Fig. 2, the result showed that the
Ft/Fn curves were climbing when the Ft θ curves transformed from the parallelogram to linear shaped with friction force increasing, and the ratio of Ft/Fn curve were steady as the Ft θ curves kept in a linear shape. In addition, the formation of the contact interfaces local bulge occurred during the Ft/Fn curves climbing process probably induced by the contact interfaces plastic flow and the wear, which is discussed in details in the following section. In SR, Ft/Fn curves are not strongly dependent on the angular displacement, which is similar to the Pearson's results [20] that indicates the friction coefficient is in fact independent of slip
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Fig. 6. SEM morphologies (a) α¼ 10°, (b) α¼ 40°and surface profiles (c) in PSR: θ¼ 0.25° N ¼ 1000 cycles.
Fig. 7. Wear scars and profiles of Ti6Al4V alloy in MFR after different number of cycles: α ¼ 20°, θ¼ 0.5°.
amplitude. In detail, the Ft/Fn curves can be divided into 4 stages based on the variation tendency: Stage I: at the initial fretting stage, the Ft/Fn curves started at a quite low value due to the small tangential force since the surface and polluted films on the contact surface. After several cycles, the Ft/Fn ratio rapidly achieved a low peak value due to the failure of surface and polluted films, the metal to metal contact friction occurred at the contact interface company with the contact force increasing. Stage II: The gradually accumulated wear debris as the third body played an important role in the DRF behavior, the contact interfaces were balance so the low Ft/Fn ratio value kept level temporarily. Stage III: As the cycles increased to 100–500, the Ft/Fn ratio was climbing rapidly again possibly due to the adhesion, plastic deformation and ploughing between the contact interfaces. Stage IV: after 500 cycles of the DRF, most wear debris ejected from wear scar that made the contact interfaces reached a dynamic equilibrium statement, the Ft/Fn ratio remained at a steady value. In addition, as the tilt angle increasing, the friction coefficient has a holistic ascend. For example, at the angular displacement amplitude of 0.5, which is very small value at 0.08 under the tilt angle of 10°,while the tilt angle increased into 40°, the friction coefficient value is about 0.4, which is 5 times than that with the tilt angle of 10°. It indicated the tilt angle has a significance effect to change the running condition of the DRF. 3.2. DRF wear behavior 3.2.1. In PSR regime Fig. 6 shows the worn scars morphologies and their 2D profiles under different tilt angles. When the tilt angle increased, the worn zone expanded to a moon-shaped worn scar, which means the distribution of contact force on contact interfaces was not uniform. As shown by Fig. 6(a) and (b), the worn scar is like a circle, and the
center sticky contact zone seems no wear damage, while the outside of circle edge can be observed some slight scratches. As the tilt angle increasing, the diameter of the sticky zone circle decreased, and the centers of these two circles are eccentric, which are the sliding circle and the sticky circle, respectively. In PSR, the wear scar like a circle without obvious damage due to the elasticity deformation of the contact interfaces, the increased tilt angle transformed the controlling fretting parameter from torsion component to rotation component, and made the sticky center shift to one side so a asymmetric wear scar was formed. In a word, the damage in PSR was quite slight without wear debris formed (which can see from the Fig. 6(c)), and the sticky zone always existed in PSR regime even with the cycles increased. 3.2.2. In MFR regime In MFR, as the cycles increased, the central sticky contact zone decreasingly narrowed then may disappear finally and the slight bulge in the central contact zone can be observed from the morphologies of the wear scar (which can see from the Fig. 7). There was few wear debris formed in MFR but some plowing tracks and visible plastic deformation can be observed, which may result in the bulges. Comparing the wear scars under different tilt angles, it was found that under a small tilt angle α of 10° in Fig. 8, the MFR-scar was similar to be a circle shaped which was the main feature of the torsional fretting [4,21]. However, under a large tilt angle α of 40° in Fig. 9, the MFR-scar became to a ellipse shaped with the major axis of the ellipse extended along the sliding direction which mainly control by the rotation component. With the tilt angle increased, the controlling parameter of the DRF behavior varied from the torsion component to the rotation component. The torsion component was a symmetrical component so the wear scar was like a circle or the bulge profile presented a slight
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Fig. 8. SEM morphologies and profiles of Ti6Al4V alloy in MFR: α ¼ 10°, θ¼ 1.0°.
Fig. 9. SEM morphologies and profiles of Ti6Al4V alloy in MFR: α ¼40°, θ¼ 0.5°.
structure than the matrix structure. It is reported by many researchers in fretting tests. While under the dual- rotary fretting condition, TTS was formed at the site close to the center zone where no obvious damage can be found. The SEM observations of TTS show in the central scar,but not near the edges in study of Sauger et al. [22]. The formation of TTS resulted from the driving force of plastic deformation. Fig. 10. SEM morphology of cross-section for Ti6Al4V alloy worn scar in MFR: α ¼10°, θ ¼1.0°, N ¼105.
symmetric flat, however, when the rotation component play a main role in DRF that resulted in a asymmetric state, then the wear scar was changed to ellipse or the bulge profile was more larger and asymmetrical [3]. Therefore, the wear damage of the DRF that mainly controlled by the rotation component was more severe than it controlled by the torsion component. Some studies show that, the fatigue crack induced by the fretting has always been observed in the MFR zone. For example, the crack can be found at MFR regime under the DRF of 7075 aluminum alloy [14]. The cross-section of worn scar for making sure whether there is the fatigue crack or not. Fig. 10 is the cross-section morphology of the Ti6Al4V titanium alloy worn scars in MFR. The crack can be observed only at the one side of the asymmetric circle, which looks like straight along the surface, no angle formed between the crack and the base material, it is typical different from the crack behavior of 7075 aluminum alloy under the same fretting condition. There are many cracks to propagate along with a fixed angle into the 7075 aluminum alloy matrix, so the 7075 aluminum alloy can be easier to cause the fretting fatigue than that of Ti6Al4V titanium alloy [14]. In addition, the tribology transformed structure (TTS) [22,23] can be formed which is more compacted
3.2.3. In SR regime In SR, there was a large amount of the oxidation debris ejected from the contact interfaces and some plough scratches along the rotational fretting direction (can be seen from Fig. 12).The profiles of wear scars showed that the wear depth in SR was the largest than other fretting regimes. In addition, the delamination can always be observed on wear surfaces (Fig. 11 and Fig. 12). Therefore, the wear mechanisms of DRF in SR were oxidation wear, abrasive wear and delamination. It is also valuable to pay some attention to the damages under different tilt angles. Under the small tilt angle, a large amount of compacted wear debris was squeezed by the contact press which finally resulted in some surface cracks. Under the large tilt angle, however, the ejection of wear debris was easier and timely, so the wear scar surface was relative smooth. With the increasing of the tilt angle, the plowing shape varied from arc-shaped to straightshaped (Fig. 11(b) and Fig. 12(b)). It also demonstrated that with the increasing of the tilt angle, the DRF controlling component changed from the torsional component to the rotational component. As mentioned above, for the wear mechanisms of Ti6Al4V titanium alloy under DRF conditions, it can be concluded that the wear in PSR was characterized by slight attrition, while the wear in MFR and SR involved a combination of plastic-deformation, abrasive and oxidation wear and delamination.
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Fig. 11. SEM morphologies and profiles of Ti6Al4V alloy in SR: α¼ 10°, θ¼ 5.0°.
Fig. 12. SEM morphologies and profiles of Ti6Al4V titanium alloy in SR: α¼ 40°, θ ¼5.0°.
The DRF runs into the SR regime, the rate of wear loss was higher and the damage of material was accelerated. Fig. 13 is the cross-section morphology of the Ti6Al4V titanium alloy worn scars in SR. No crack can be seen in SR regime, the similar results can be found in the study of Mohd Tobi et al. [24] only the local plastic deformation of the material near the worn surface is embedded in Fig. 13. The typical shallow “U” scallop shape can be observed by the cross-section morphology of worn scar in SR. It is also different the wear mechanism of the 7075 aluminum alloy, because the initiation of the fatigue crack can be found during the DRF, the wear loss rate of material is faster than that of crack propagation in SR regime. However, the Ti6Al4V titanium alloy is difficult for crack initiation, so the abrasive wear and the structure deformation is the mainly wear mechanism. 3.3. Tilt angle effect on DRF behavior 3.3.1. Ratio of rotational fretting and torsional fretting Since under the large angular displacement amplitude, the fretting behavior is like reciprocating sliding. Therefore the fretting transfers from the torsional to rotational can be observed with the small angular displacement amplitude. In other word, the fretting runs in PSR or MFR regimes. When the fretting mainly was controlled by the torsional component, the worn scars are always the circle-shaped. As the angular displacement amplitude increased, the diameter of the circle may increase slightly. The tilt angle increased can result in the eccentric circle worn scar, the center of adhesive circle zone is different from that of the sliding circle zone. When the fretting mainly control by the rotational component, the worn scars is a ellipseshaped or rectangle-shaped. The long axis length of the worn scar would be longer as the increase of angular displacement amplitude. To evaluate the effect of tilt angle on the DRF behavior, a ratio need to use for quantifying the function of rotational fretting and torsional fretting.
Fig. 13. SEM morphology of cross-section for Ti6Al4V titanium alloy worn scar in SR: α ¼40°, θ¼ 5.0°, N ¼105.
Considering the worn scar is not always like a circle, so the width of the worn scar called as X was defined as the variable of the torsional component. The length Y of worn scar was defined as the variable of the rotational component. Therefore, during the DRF is complex by rotational fretting and torsional fretting, the ratio Y/X of worn scars was used to evaluate the damage of the DRF. Fig. 14(a) shows the detail of the relationship between variables of X and Y. When the ratio value is equal to 2, it means the fretting zone is two times larger than the contact zone, the fretting behavior is gross-sliding which is similar to the reciprocating sliding friction. Fig. 14 shows the ratio of Y/X for worn scars with the small angular displacement amplitudes θ of 0.5° and 1.0° under different tilt angles. If the angular displacement amplitude is fixed, the variable X was similarly the same since the torsional fretting component hardly effect on the diameter of worn scars. When the tilt angle increased, the variable Y effect by the rotational component will increase. Therefore, the ratio curve with the fixed angular displacement amplitude presented a climbing trend as the tilt angle increasing. Comparing with the ratio curves under different angular displacement amplitudes, both of these curves maintain the similar climbing tendency. Specifically, when the angular displacement amplitudes θ is 0.5°, the ratio stay at 1.0 before the tilt angle of 40° and then begin to rise up to 1.4 since the tilt angle
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Fig. 14. Ratio curves of Ti6Al4V titanium alloy worn scars variation under angular displacement amplitude θ of 1.0° and 0.5° with the increasing tilt angles.
greatly decrease the wear volume. Specifically, comparing Fig. 15 (a) to (b), the wear volumes value under the angular displacement amplitude θ of 1.0° are approximately 10 times more than that under the angular displacement amplitude θ of 0.5°. At the tilt angle of 10°,when the angular displacement amplitude θ increased from 0.5° to 1.0°, the wear volume changed from 0.89 105 to 1.17 105 mm3, and the latter is 1.3 times more than the former one. When the tilt angle is 40°, increasing the angular displacement amplitude θ from 0.5°to 1.0° as 2 times, while the wear volume of latter one is 14 times more than that of former one. According to the result above, it indicated that the tilt angle has more significance effect on the variation of wear volume than the angular displacement amplitude in PSR and MFR regimes. What's more, the difference between the wear volumes from the tilt angle of 40°and 60° which is the 14 times. It indicated that when the tilt angle achieved at a constant value, the influence of angular displacement amplitude θ can be neglectful in SR regimes.
Fig. 15. Wear volumes of Ti6Al4V titanium alloy with different tilt angles under different angular displacement amplitude: (a) θ¼ 0.5°, (b) θ¼ 1.0°.
increase to 60°. The entire curve of the large angular displacement amplitude is little higher than that of small angular displacement amplitude. When the angular displacement amplitudes θ is 1.0°, the ratio increased linearly from 1.0 gradually to 1.4 as the tilt angle increasing to 60°. Obviously, as the ratio increasing, the size of worn scars gets bigger which is corresponding to the OM morphologies in Fig. 4 and the wear damage become more severe during DRF. Moreover, it also indicates that the wear damage induced by the rotational fretting is heavier than that induced by torsional fretting on DRF. 3.3.2. Wear volume effected by the tilt angle Fig. 15 shows the variation of wear volume as the tilt angle increasing under the angular displacement amplitude θ of 0.5° and 1.0°. The variation of wear volume is greatly dependent on the tilt angle. As shown by Fig. 15, the wear volumes get bigger as the tilt angle increasing at a constant value of angular displacement amplitude. Moreover, to decrease the angular displacement amplitude can
3.3.3. Tilt angle effect on oxidative wear In term of the torsional fretting, rotational fretting, or the dual rotational fretting, the typical chemical change on the worn surface is the O element [3,15,25,26]. It is significant to investigate the O elements distribution on the worn scars as the varied tilt angles, which is helpful to understand the oxidative wear mechanism during DRF. Fig. 16 shows EPMA distribution mappings of O and Ti elements on the worn scars companied with a line scan in each maps, varied as the increasing tilt angle. The average content of O element increase as the tilt angle increasing. The average content of O elements on worn scars is close to zero in the Fig. 16 (a) and (b), while in the Fig. 16 (c) is show the average content of 1.97% O. In details, Fig. 16 (a) and (b) is represented the torsional component, the distribution of O is like a concentric ring, the width of the ring is about 100 um, the center of contact zone, it is dark, is no O elements here because the zone is no relative displacement, the Oxygen do not enter in the interface. As the tilt angle increasing, the relative motion between two contact surfaces become easier, the more regimes can be oxidative. From the line scan distribution, we can find out that the asymmetry peaks of O elements along the line. What's more, the distribution of Ti element is also a shallow concentric circle, which is corresponding to the distribution of O element. So when the fretting behavior is controlled by the torsional component, the damage of the Ti6Al4V is oxidative wear. As the torsional component decrease, one side of the circle occur the heavier oxidative peaks. To observe Fig. 16(c), the tilt angle is 60°. The rotational component can make bigger relative motion of contact interface which make more contribute to the oxidative
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Fig. 16. EPMA distribution mappings of O (a–c) and Ti (d–f) elements on worn scars under different tilt angles α¼ 10°, α ¼20°, α¼ 60°.
Fig. 17. Cross-section of Ti6Al4V titanium alloy worn scars under angular displacement amplitude θ¼ 1.0° with different tilt angles: (a) α ¼10°; (b) α¼ 60°.
wear. So the average concentration of O element is higher. The highest O elements concentration is distributed along a circle zone around the worn scar which is mainly caused by the torsional component fretting. The rotational component make the contact center zone difficult to distinguish, and the line scan of O is like the random waved peak as well as the line distribution of Ti element, the results show that the O distributed uniformed on the
center contact zone. While the main zone for Ti elements lost is at the outside circle where shows the highest O may be covered with the oxides. 3.3.4. Tilt angle effect on wear mechanism The DRF wear mechanism of Ti6Al4V titanium alloy changed as the tilt angle increased, as shown in Fig. 17. The fatigue behavior-
Y. Zhou et al. / Wear 376-377 (2017) 670–679
the crack is difficult to be observed under the DRF of Ti6Al4V alloy, it indicated that the fretting wear resistance of Ti6Al4V titanium alloy is better than that of 7075 aluminum alloy and medium carbon steel [9,14,15]. The significance wear mechanism of Ti6Al4V titanium alloy is oxidative wear, which is the mainly reason caused the failure of the materials. From the morphology of cross-section in Fig. 17(a), the microstructure near the worn surface can be observed that almost has no deformation when the tilt angle is small, it indicated that the DRF wear mechanism Ti6Al4V titanium alloy is oxidative wear when the DRF is controlled by the torsional fretting. Comparing to the Fig. 17(b), the worn surface was covered with the thin deformed layer, and the plastic deformation is the significance driving force to make the microstructure deformed thin layer. Due to the great ductility of Ti6Al4V titanium alloy, it may induce the microstructure changed such as tribological transformed structure (TTS). As the tilt angle increasing, the surfaces of contact zone ploughed by the debris particles to form a rough worn surface, and the plastic deformation of microstructure near by the worn surface occurred. In conclusion, when the DRF behavior is mainly controlled by the torsional fretting component, the wear damage is slight and the wear mechanism is oxidative wear. As the DRF behavior is mainly controlled by the rotational fretting component, the severe wear damage can be observed which is induced by the plastic deformation, the wear mechanisms are abrasive wear and oxidative wear.
4. Conclusions To investigate the wear mechanism and microstructural evolution of Ti6Al4V titanium alloy under the condition of DRF, Using the Ti6Al4V titanium alloy flat specimen against the 52100 steel ball to investigate the DRF and damage behavior were carried out with different tilt angles and varying angular displacement amplitudes. The conclusions are as followed: (1) DRF Ft-θ curves of Ti6Al4V titanium alloy have three types: linear-shaped, elliptical-shaped and parallelogram-shaped which corresponds to different fretting regimes. The angle displacement amplitude and the tilt angle intensively affect the dual-rotational fretting behavior. With the increase of tilted angle, the regime of MRF zone gradually decreases, and the ranges of SR and PSR zones expand. For the angle amplitude increasing, the fretting regime runs from PSR to MFR and then to SR. The Ft/Fn curves present different evolution characteristics in different fretting regimes. (2) The wear damage mechanisms of Ti6Al4V titanium alloy during DRF process are different. In PSR, the wear damage is slight. In SR, wear mechanism is the abrasive and oxidation wear. When the DRF runs into the MFR, a little bit debris can be observed and the bugles formed may be induced by the plastic-flow accumulation. (3) As the tilt angle increase, DRF wear mechanism of Ti6Al4V titanium alloy transfers from oxidative wear to the abrasive wear, as well as fretting is controlled by the torsion component transit to the rotation component. (4) When DRF is mainly controlled by the rotational component, the friction coefficient value has a slight increase corresponding to the variation of wear volumes. However, rotational component can result in severe wear damage with the ploughing scratches induced by the subsurface microstructure plastic deformation.
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Acknowledgements The authors would like to thank the National Nature Science Foundation of China, this research was supported by National Natural Science Foundation of China (Contract numbers (No.51375407, 51575459, U1530136, 51627806).
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