metals Article
Study on the Tribological Properties of Porous Titanium Sliding against Tungsten Carbide YG6 Zhiqiang Liu *, Feifei Ji, Mingqiang Wang and Tianyu Zhu School of Mechanical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China;
[email protected] (F.J.);
[email protected] (M.W.);
[email protected] (T.Z.) * Correspondence:
[email protected]; Tel.: +86-511-84401198 Academic Editor: Hugo F. Lopez Received: 6 September 2016; Accepted: 11 January 2017; Published: 20 January 2017
Abstract: In the metal cutting process, the friction and wear behavior between the cutting tool and machined surface is the most important factor that affects the surface quality and the service life of the cutter. The irregular pore structure of porous titanium alloy has changed its mechanical properties and the processing technology. The friction and wear mechanism of the cutting tool and the machined surface is greatly different from the traditional dense metal processing because of the crumbling at the edges, tearing phenomenon and the pore agglomeration effect of chips. In this paper, the tribological characteristics and the wear mechanism of friction pair which was formed by porous titanium alloy material and hard alloy cutter were studied from cutting force, cutting speed and temperature in micro-cutting condition, and the influence of porosity on the wear rate and friction coefficient was analyzed. Results show that the main factor which influences the friction coefficient and wear rate is the porosity. The wear mechanisms of porous titanium materials were abrasive and oxidation wear while the wear mechanism of tungsten carbide YG6 was abrasive wear. The friction coefficient and wear rate of the relatively stable state are beneficial to improve the surface quality and tool life. As a result, in the micro-cutting process of porous titanium alloys, the best choice of machining parameters for different porosity materials are as follows: the load is about 8 N, the sliding speed is about 400 r/min and the temperature is about 300 ◦ C. Keywords: porous titanium; tungsten carbides; friction and wear; wear morphology and mechanism
1. Introduction Porous titanium is an important structural metal that is widely used in aerospace, automobiles, shipbuilding, power generation equipment and other fields because of its high strength, low density, seismic resistance, good adsorption and other characteristics [1–3]. However, the porous titanium is hard to machine due to its low thermal conductivity, poor wear resistance, high chemical affinity, low elastic modulus and high-temperature oxidation resistance [4]. Especially, the friction and wear behavior between the tool and the machined surface have the most important impact on the surface quality and the machining precision. In the friction pair formed by the cutting tool and the machined surface, the tool material is an important factor of the performance of the machined surface. Due to the high strength of porous titanium alloy and the characteristics of its pore structure, the phenomenon that cutting tool impacts pore edges is serious [5]. Therefore, the tungsten carbide cutter with a higher hardness is the best choice and the commonly used cutters are YG6, YG8, K20, P30 and so on [6–8]. The sharp friction between the tool and the workpiece causes the temperature to rise, while the poor thermal conductivity of the porous titanium makes the temperature unable to diverge well [9]. On the one hand, oxidation will appear if the temperature becomes too high, and oxide will stick to the tool, resulting in adhesive wear if the temperature reaches approximately 300 ◦ C [10,11]. On the other hand, as the temperature Metals 2017, 7, 28; doi:10.3390/met7010028
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increases, the nitrogen in the titanium undergoes chemical reactions with the oxygen in air, which would deposit a hard layer on the cutting surface that hardens the work surface [12–14]. All these lead to more difficult cutting. Similarly, cutting fluid is another important factor that affects the performance of machining quality and reduces the temperature in the friction pair. In most cases, the cutting fluid plays a role in lubricating and cooling. However, the cutting fluid can also create the vibration of the cutter which affects the machining accuracy in the micro-machining of porous titanium [15]. Besides that, the cutting fluid contains some chemicals that could have reactions with some components of porous titanium. Furthermore, due to the pore structure of porous titanium, some of the lubricant remains in the pores, and is not easy to remove. Additionally, the lubrication creates some harmful gas at high temperatures that is not friendly to the environment [16]. Moreover, when the porous titanium is applied in the medical field, the residual component of the cutting fluid may cause harm to the human body [17,18]. Taking into account the above reasons, either dry or near dry cutting is generally used instead of lubrication design [19,20]. However, this method can also bring another problem: much heat will be produced, so it is necessary to investigate the influence of the temperature parameter. Micro-scale cutting is often used in modern industries and research on micro-cutting is mainly from three aspects: cutting speed, cutting depth and feed rate [21]. However, for micro-machining of porous materials, different loads, sliding speeds and other parameters have relationships with the surface quality as well [22]. Changing these parameters will also bring a rising surface temperature and a faster wear rate. In order to accomplish higher cutting accuracy, the load of the micro-machining of porous titanium should not fluctuate greatly. During the cutting process of porous titanium material, the friction and wear behavior is the main cause that leads to a short service life of the cutter. It is necessary to study the tribological properties, wear morphology and mechanism of porous titanium sliding against tungsten carbide tools [23]. The investigation is beneficial to further improve the friction and wear properties of porous titanium. Then, the friction and wear behavior was simulated using a tungsten carbide YG6 tool and two kinds of porous titanium with different porosity to trough a ball-on-flat sliding friction apparatus UMT-2. The first step is to prepare the titanium samples with porosities of ~24%–26% and ~38%–42% by sintering [24]. Next, we take advantage of a variable-controlled approach to carry out a single factor test from four aspects, porosity, load, sliding speed and temperature, and analyze the relationship between these factors and the friction coefficient and wear rate. In addition, we use a VHX-900 Super Depth of Field Digital Microscope and a JSM-6480 Scanning Electron Microscopy (SEM) to observe the surface of the wear trace and the micro-morphology of the carbide YG6 ball. Finally, the wear mechanism and the friction and wear properties of the porous titanium are obtained using an INCA Energy Spectrometer (EDS) to analyze the micro region and the spots of the wear surface. Consequently, the different studies conducted are beneficial to improve the processing technology, enhance the surface quality and reduce the friction and wear. 2. Experimental Procedures 2.1. Materials A pure titanium sample in the friction and wear experiments was obtained using the following procedure. First, two types of porous titanium materials with different porosities are created by sintering the sample. Second, the samples are processed into standard workpieces by Wire Cut Electrical Discharge Machining (WEDM, Suzhou Zhonghang Changfeng Technology Equipment Co., Ltd., Suzhou, China). After the cutting is finished, the friction surface is polished to eliminate the residual stress which may exist. Third, a scanning electron microscope (SEM, Shanghai Juyue Electronics Co., Ltd., Shanghai, China) is used to characterize the porosity and pore size of the sample surface. Finally, the SEM images are analyzed to choose test samples with uniform porosity distribution.
Two titanium powders, made by Shijiazhuang Yida Co., Ltd., Shijiazhuang, China, 500# and 200#, with diameters less than 27 μm and 74 μm, respectively, are used. The chemical composition of the two titanium powders is shown in Table 1 and the powder particles can be seen in Figure 1. Table 1. Chemical composition of titanium powder (wt %). Metals 2017, 7, 28
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Type Ti Fe Cu C O N 500# ≥99.7% ≤0.02% ≤0.005% ≤0.02% ≤0.06% ≤0.02% Two titanium ≥99.7% powders, made by Shijiazhuang Yida Co.,≤0.02% Ltd., Shijiazhuang, 500# and 200#, 200# ≤0.03% ≤0.006% ≤0.02%China, ≤0.03% with diameters less than 27 µm and 74 µm, respectively, are used. The chemical composition of the The sintering additive is an aqueous solution containing polyvinyl alcohol (2 wt two titanium powders is shown in Table 1 and the powder particles can be seen in Figure 1. %). The mixture was fed into the titanium body, with a diameter of 25 mm, by compression molding at a forming pressure of 180Table MPa. to titanium’s high chemical reactivity and its tendency to react 1. Due Chemical composition of titanium powder (wt %). with carbon, nitrogen, oxygen, hydrogen and other elements, the sintering was performed under vacuum atType 10−4 Pa. TheTisample wasFekept at 1200 Cu°C for 2 h and C then cooled O naturally N to obtain a titanium powder with a particle size of 500#. Conversely, a temperature of 1100 °C was used for the 500# ≥99.7% ≤0.02% ≤0.005% ≤0.02% ≤0.06% ≤0.02% process to200# acquire the≥sample particle size of 200#. As shown in Figure are two types 99.7% with≤a 0.03% ≤0.006% ≤0.02% ≤0.02%1, there ≤0.03% of specimens with different porosities.
(a)
(b)
Figure 1. Preparation of porous titanium test samples: (a) specimen with small porosity; and (b) Figure 1. Preparation of porous titanium test samples: (a) specimen with small porosity; and specimen with large porosity. (b) specimen with large porosity.
Next, in order to facilitate the clamping in the UMT-2 high temperature friction and wear The sintering additive is an aqueous solution containing polyvinyl alcohol (2 wt %). The mixture testing machine, the samples were cut into cubes by WEDM, with each cube measuring 15 mm was fed into the titanium body, with a diameter of 25 mm, by compression molding at a forming (length) × 15 mm (width) × 4 mm (height). In addition, to obtain the minimum error, the parallelism pressure of 180 MPa. Due to titanium’s high chemical reactivity and its tendency to react with of the sample was controlled to within 0.1 mm. The terminal faces of the samples were then polished carbon, nitrogen, oxygen, hydrogen and other elements, the sintering was performed under vacuum in a metallographic pre-grinding machine (M-2, Shandong Laizhou Huayin Testing Instrument Co., at 10−4 Pa. The sample was kept at 1200 ◦ C for 2 h and then cooled naturally to obtain a titanium Ltd., Shandong, China) after the samples had been washed in water. The sanding process was ◦ C was used for the process to powder with a particle size of 500#. Conversely, a temperature of 1100 divided into three stages: coarse grinding, fine grinding and burnishing. Coarse grinding was first acquire the by sample a particle size of 200#. Asthe shown 1, there two types of specimens performed usingwith metallographic paper, with typesinofFigure 360# and 600#,are during the rough milling with different porosities. process. Likewise, at the fine grinding stage, the papers were replaced with metallographic paper in order facilitate clamping in the UMT-2 highwas temperature and wear with Next, the types 800#, to 1000#, 1200#,the 1500# and 2000#, and grinding performed friction sequentially with testing machine, the samples were cut into cubes by WEDM, with each cube measuring mm each paper. Subsequently, specimens were polished on the M-2 using a diamond-abrading15agent (length) × 15 mm (width) × 4 mm (height). In addition, to obtain the minimum error, the parallelism with W1.0 to ensure that the surface roughness reached a value of 0.1 mm. The samples were then of the sample was alcohol controlled within The terminal faces of the samples were then polished cleaned with 90% andtodried in 0.1 the mm. shade. in a metallographic pre-grinding machine (M-2, Laizhou Testing Instrument Co., Porosity of materials can be nowadays wellShandong defined with manyHuayin methods such as the scanning Ltd., Shandong, China) after the samples had been washed in water. The sanding process was divided and subsequent computational analysis, porosimetry and the simple mass and volume into three stages:and coarse burnishing. wasand firstsubsequent performed measurements so grinding, on [24]. fine Thegrinding porosityand was defined Coarse by thegrinding scanning by using metallographic paper, with the types of 360# and 600#, during the rough milling computational analysis in this experiment, and was checked by the simple mass and process. volume Likewise, at the fine grinding stage, the papers were replaced with metallographic paper with measurements. Figure 2 shows the surface morphology of the standard workpieces. The total areathe of types 800#, 1000#, 1200#, 1500# and 2000#, and grinding was performed sequentially with each paper. Subsequently, specimens were polished on the M-2 using a diamond-abrading agent with W1.0 to ensure that the surface roughness reached a value of 0.1 mm. The samples were then cleaned with 90% alcohol and dried in the shade. Porosity of materials can be nowadays well defined with many methods such as the scanning and subsequent computational analysis, porosimetry and the simple mass and volume measurements and so on [24]. The porosity was defined by the scanning and subsequent computational analysis in
porosity of the material is given as follows:
ε
Ap A
100%
(1)
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where ε is the percentage of voids in the total area. Due to the complexity of the pore structure, it is difficult to define the exact aperture. Under this circumstance, theand concept of “equivalent size” is used by the Porous Materials Figure Corporation in this experiment, was checked by thepore simple mass and volume measurements. 2 shows America. core idea of is the thatstandard the perimeter of the The poretotal is actually equivalent to the diameter D. the surfaceThe morphology workpieces. area of observation is described by the 2 ). The Hence, theAequation for the equivalent (D) as is given below as porosity follows: of the material is given parameter (mm2 ). The areaaperture of the gap is expressed Ap (mm as follows: A p 4 Ac ε = D × 100% (1) (2) A P c where ε is the percentage of voids in the total area. where Ac is the true area of hole and Pc is the girth of concrete pore.
(a)
(b)
Figure 2. Highly processed test samples observed by VHX-900 super depth of field digital Figure 2. Highly processed test samples observed by VHX-900 super depth of field digital microscope microscope with a magnification of 20: (a) specimen with small porosity; and (b) specimen with large with a magnification of 20: (a) specimen with small porosity; and (b) specimen with large porosity. porosity.
Due surface to the complexity of the pore structure, it is difficult to define the exact aperture. Under this The morphology by SEM were analysed as shown in Figure 3. Here, the black shaded circumstance, the concept “equivalent pore size” is used by the Porous Corporation in region is the aperture gapofwhile the white part is the material base. The Materials porosity and equivalent America. The core idea is that the perimeter of the pore is actually equivalent to the diameter D. Hence, aperture are calculated from Equations (1) and (2). The average is taken and is used as the diameter thethe equation of piece. for the aperture equivalent (D) is given below as follows: D=
4Ac Pc
(2)
where Ac is the true area of hole and Pc is the girth of concrete pore. The surface morphology by SEM were analysed as shown in Figure 3. Here, the black shaded region is the aperture gap while the white part is the material base. The porosity and equivalent aperture are calculated from Equations (1) and (2). The average is taken and is used as the diameter of the piece. Because the pore structures of the porous titanium are not uniformly distributed, the measured values of different samples are not unique. To improve the reproducibility of the test data, which are influenced by the dispersion in sample porosity, test pieces with sizes that were either too large or too small were removed. Finally, two groups of samples meeting the requirements are selected. Table 2 shows the two groups of experimental data, along with the equivalent porosity and aperture.
Figure 2. Highly processed test samples observed by VHX-900 super depth of field digital microscope with a magnification of 20: (a) specimen with small porosity; and (b) specimen with large porosity.
The surface morphology by SEM were analysed as shown in Figure 3. Here, the black shaded Metalsregion 2017, 7, is 28 the aperture gap while the white part is the material base. The porosity and equivalent5 of 20
aperture are calculated from Equations (1) and (2). The average is taken and is used as the diameter of the piece.
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Figure 3. Image analysis of surface topography (QSTU threshold segmentation which was proposed by the Japanese scholar named Otsu (Nobuyuki Otsu) in 1979).
Because the pore structures of the porous titanium are not uniformly distributed, the measured values of different samples are not unique. To improve the reproducibility of the test data, which are influenced by the dispersion in sample porosity, test pieces with sizes that were either too large or too small were removed. Finally, two groups of samples meeting the requirements are selected. Table 2 shows the two groups of experimental data, along with the equivalent porosity and aperture. Table 2. Characteristics of porous titanium samples.
Type Porosity (%) Equivalent Aperture (mm) Figure 3. Image analysis of surface topography (QSTU threshold segmentation which was proposed 200# ~24–26 0.015 by the Japanese scholar named Otsu (Nobuyuki Otsu) in 1979). 500# ~38–42 0.25 2.2. Friction and Wear Tests Table 2. Characteristics of porous titanium samples. In order to simulate the friction and wear behavior of the cutting tool and the machined surface Type Equivalent Aperture during the micro-machining process,Porosity the tests(%) were carried out with the UMT-2 high (mm) temperature friction and wear testing machine (Nanjing Ranrui Technology Co., Ltd., Nanjing, China), which is a 200# ~24–26 0.015 ball-on-flat machine specifically for the friction Figure 4a, the porous 500# ~38–42 and wear test. As observed in0.25 titanium sample was glued to the plate. The grinding ball on the upper part of the machine consists of Carbide YG6 material; its detailed parameters are shown in Table 3.
2.2. Friction and Wear Tests
Table 3. and Detailed parameters of the grinding ball.tool and the machined surface In order to simulate the friction wear behavior of the cutting during the micro-machining process, the tests were carried out with the UMT-2 high temperature Material Diameter (mm) Hardness (HAR) Roughness (μm) friction and wear testing machine (Nanjing Ranrui Technology Co., Ltd., Nanjing, China), which is a Carbide YG6 9.53 ~89–92 0.01 ball-on-flat machine specifically for the friction and wear test. As observed in Figure 4a, the porous During the theplate. ball was fixed while the rotated onof thethe work table. In this titanium sample wasexperiment, glued to the The grinding ballworkpiece on the upper part machine consists of state, the contact area formed a friction ring, as shown in Figure 4b. Meanwhile, data for the load ( Carbide YG6 material; its detailed parameters are shown in Table 3.
Fz ) and friction ( Fx ) were collected by a computer.
Grinding ball
Porous titanium sample
(a) Figure 4. Cont.
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Fz Porous titanium sample Carbide YG6
Fx
Friction ring
Work table
(b) Figure 4. The Thediagram diagramof of experimental equipment: (a)testing the testing instrument; andprinciple (b) the Figure 4. thethe experimental equipment: (a) the instrument; and (b) the principle friction of frictionofand wear.and wear.
The experiment was designed a single-factor test, i.e.,grinding changesball. were only made to one factor, Table 3. as Detailed parameters of the while the other factors were kept unchanged. In micro-cutting, the cutting force is(mm) suitable from 0 N to 18 N and the spindle speed Material Diameter Hardness (HAR) Roughness (µm)is about 735 r/minCarbide [21]. However, considering the impact of pore structure and the heat dissipation of porous YG6 9.53 ~89–92 0.01 materials that seriously affect the wear rate of the micro-machining tool and the quality of the machined surface, the speed and load should be at a lower level. In order to obtain a clear difference During experiment, the ball was fixed whilethe theimpact workpiece rotated on the workby table. this between the the experimental phenomena and exclude of temperature caused largeInload state, the contact area formed a friction ring, as shown in Figure 4b. Meanwhile, data for the load z) or high speed, the range of test loads are from 2 N to 15 N and the speed should be kept between(F 40 and friction (Fxr/min. ) wereMore collected by aparameters computer. are listed in Table 4. r/min and 735 specific The experiment was designed as a single-factor test, i.e., changes were only made to one factor, while the other factors were kept unchanged. Table 4. Range of experimental conditions. In micro-cutting, the cutting force is suitable from 0 N to 18 N and the spindle speed is about Condition Speedthe ofimpact Revolution (r/min) Load (N) Temperature (K) 735Experimental r/min [21]. However, considering of pore structure and the heat dissipation of porous Range materials that seriously affect the wear rate of50–500 the micro-machining tool 4–14 and the quality 293–773 of the machined surface, the speed and load should be at a lower level. In order to obtain a clear difference between In addition, the tests also meet the following conditions: a dry friction in atmospheric the experimental phenomena and exclude the impact of temperature caused by large load or high environment, an external temperature of 20 °C, a relative air humidity range of ~20%–30%, a friction speed, the range ofmm, test loads are time fromof 2N 15 N and the speed should kept betweenof 40friction r/min ring diameter of 10 a friction 20 to min, a frequency of 0.65 Hz forbe the acquisition and 735 r/min. More specific parameters are listed in Table 4. data and a force sensing accuracy of 0.1 mN. To obtain a clear comparison of the effect of friction under different conditions, different testing Table 4. Range of experimental conditions. schemes were used, as shown in Table 5. In order to obtain convincing results, each group of samples were tested several times, and finally the most stable and the most representative data were Experimental Condition Speed of Revolution (r/min) Load (N) Temperature (K) selected. Range 4–14 293–773 As observed in Table 5, five groups50–500 of experiments were conducted for each scheme. The friction coefficient is the ratio of the load and the friction, i.e., In addition, the tests also meet the following conditions: a dry friction in atmospheric environment, F an external temperature of 20 ◦ C, a relative airμ humidity X range of ~20%–30%, a friction ring diameter (3) of 10 mm, a friction time of 20 min, a frequency of F 0.65 Hz for the acquisition of friction data and a Z force sensing accuracy of 0.1 mN. To obtain a clear comparison of the effect of friction under different conditions, different testing schemes were used, as shown in Table 5. In order to obtain convincing results, each group of samples were tested several times, and finally the most stable and the most representative data were selected.
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Table 5. Schemes of tests. Test Scheme Metals 2017, 7, 28
1
Test Scheme
Speed of Revolution (r/ min)
Load (N)
300 6 300 6 300 Table 5. Schemes of tests. 6 300 6 Load6 (N) 300 Speed of Revolution ( r/min ) 300 300 66 50 (1.57300 m/min) 100 (3.14300 m/min) 200 (6.28 m/min) 300 300 (9.43 m/min) 400 (12.56300 m/min) 500 (15.71300 m/min)
Temperature (◦ C) 20 100 200 300 Temperature400 (°C) 500 20
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66 100 20 66 200 20 1 6 20 6 300 2 6 20 66 400 20 66 500 20 50 (1.57 64 20 20 300m/min) 300 m/min) 100 (3.14 66 20 20 300 8 3 200 (6.28 m/min) 6 20 20 2 300 10 20 300 (9.43 m/min) 6 20 300 14 20 400 (12.56 m/min) 6 20 500 (15.71 m/min) 6 20 As observed in Table 5, five groups300 of experiments were conducted for each scheme. The friction 4 20 coefficient is the ratio of the load and the 300friction, i.e., 6 20 3 300 8 20 F µ= X (3) 300 10 20 FZ 300 14 20 Because the oil and powder particles can easily infiltrate in the process of wear and tear, the Because the measurements, oil and powderreferred particles easily infiltrate in thewould process of wear and tear, the traditional wear to can as the weighing method, result in obvious errors. traditional wear measurements, referred to as the weighing method, would result in obvious errors. Accordingly, the contour graph method was used to measure the wear volume. As shown in Figure 5a, Accordingly, the contourmarks graph of method was track used profile to measure wear volume. As depth shown extension in Figure perpendicular grinding the wear werethe acquired using the 5a, perpendicular grinding marks of the wear track profile were acquired using the depth extension function of the OLYMPUS DSX500 optical digital microscope (Nanjing Dongtu Technology Co., Ltd. function the OLYMPUS DSX500 optical digital microscope (Nanjing Dongtu Technology Co., Ltd. Nanjing, of China). Nanjing, China).
(a) Figure 5. Cont.
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Datum line
Contour line
Friction area Matrix (b) Figure 5. Image Image of the wear wear trace trace contour: contour: (a) Figure 5. of the (a) interception interception of of grinding grinding marks marks by by the the contour contour graph; graph; and (b) calculation of the wear scar cross-section area by dividing the meshed region. and (b) calculation of the wear scar cross-section area by dividing the meshed region.
In Figure 5b, a line is first selected to obtain the wear rate. Next, the area between the datum line In Figure 5b, a line is first selected to obtain the wear rate. Next, the area between the datum line and the contour line is counted using the grid method. Then, the same method is used repeatedly to and the contour line is counted using the grid method. Then, the same method is used repeatedly to measure the area of the wear trace contour and an average value is obtained. Lastly, the wear rate measure the area of the wear trace contour and an average value is obtained. Lastly, the wear rate can can be expressed as follows: be expressed as follows: Ap ∆V (4) Wr = V = Ap (4) W S nT r
S
nT
where Wr is the wear rate, ∆V is the wear volume, S is the sliding distance, A p is the average of the wear the rotational speed of the plate (which observed in Figure Ap4b), V is the wherescar is thearea, wearn is rate, wear volume, thebe sliding distance, is and the S is can Wrcontour T is the friction time. average the wear super scar contour is the rotational speed of the plate can be observed in TheofVHX-900 depth area, of field digital microscope (Keyence Co., (which Ltd., Osaka, Japan) and Figure 4b), andscanning frictionmicroscope time. T is theelectron the JSM-6480 (SEM, Shanghai Zhujin Analysis Instrument Co., Ltd., The VHX-900 super depth digital Co., Ltd., Osaka, Japan) and the Shanghai, China) are both usedoftofield observe themicroscope wear trace(Keyence of the surface and the micro-topography JSM-6480 scanning electron microscope (SEM, Shanghai Zhujin Analysis Instrument Co., Ltd., wear region of the carbide ball to analyze the friction characteristics of the friction pairs formed by Shanghai, China) are both used to observe the wear trace of the surface and the micro-topography the cutting tool and the machined surface. In addition, the INCA energy disperse spectroscopy (EDS, wear region of the carbide ball Co., to analyze the friction characteristics of the friction pairs formed by Oxford instrument (Shanghai) Ltd., Shanghai, China) is used to analyse the micro area and the the cutting tool and the machined surface. In addition, the INCA energy disperse spectroscopy (EDS, spots of the wear surface. The purpose is to obtain the wear mechanism of the cutting tool and the Oxford instrument machined surface. (Shanghai) Co., Ltd., Shanghai, China) is used to analyse the micro area and the spots of the wear surface. The purpose is to obtain the wear mechanism of the cutting tool and the 3. Results surface. and Discussion machined
n
3.1. Frictionand Coefficient and Wear Rate Curve 3. Results Discussion 3.1.1. Effects of Load on Friction Coefficient and Wear Rate 3.1. Friction Coefficient and Wear Rate Curve Two kinds of samples, small-porosity and large-porosity, were tested with loads of 4 N, 6 N, 3.1.1. Effects of Load Friction and Wear Rate 8 N, 10 N, and 14 N on in order to Coefficient study the effect of load on the friction coefficient and wear rate of porous materials in micro-cutting. Samples are all at the same sliding speed r/min) and the Two kinds of samples, small-porosity and large-porosity, were tested with(300 loads of 4 N, 6 N, 8 ◦ C). As shown in Figure 6, the changes in the friction coefficient were acquired by temperature (20 N, 10 N, and 14 N in order to study the effect of load on the friction coefficient and wear rate of the UMT-2. In Figure 6a, the frictionSamples coefficients porosity sample speed with the load less than N porous materials in micro-cutting. areofallsmall at the same sliding (300 r/min) and 8the increase firstly after declining. Finally, it tends to stabilize because the small-porosity sample has an temperature (20 °C). As shown in Figure 6, the changes in the friction coefficient were acquired by aperture with larger6a, contact area at coefficients the start of rubbing. As the friction the UMT-2. In aFigure the friction of small porosity sampledevelops, with the the loadphenomena less than 8 of N pore edge peeling and tearing becomes more serious, which leads to decrease in the contact area increase firstly after declining. Finally, it tends to stabilize because the small-porosity sample hasand an friction coefficient. When the load is 8the N,start the effect of a larger contact area of the small-porosity and aperture with a larger contact area at of rubbing. As the friction develops, the phenomena of the influence of theand pore edge becomes peeling and tearing disappears, which leads toin the small contact pore edge peeling tearing more serious, which leads to decrease the contact area area and
friction coefficient. When the load is 8 N, the effect of a larger contact area of the small-porosity and
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the influence of the pore edge peeling and tearing disappears, which leads to the small contact area canceling each smooth and almost a straight line.line. WithWith the increase of load, canceling each other otherout. out.Thus, Thus,the thecurve curveis is smooth and almost a straight the increase of the temperature increases dramatically because the friction decreases the shear yield strength of the load, the temperature increases dramatically because the friction decreases the shear yield strength material and reduces the friction coefficient at first. Figure shows6athat the load a great influence of the material and reduces the friction coefficient at first.6a Figure shows that has the load has a great on the friction coefficient of porous titanium alloys under the micro-machining. However, Figure 6b influence on the friction coefficient of porous titanium alloys under the micro-machining. However, shows that the friction coefficient of the large-porosity sample under different load is basically the Figure 6b shows that the friction coefficient of the large-porosity sample under different load is same. Thethe curves the curves large porosity change smoothly due smoothly to that thedue porosity has impact basically same.ofThe of the large porosity change to that thegreater porosity has on the friction coefficient than the factor of the load. As a result, the contact area changes little the greater impact on the friction coefficient than the factor of the load. As a result, the contactinarea process of friction. changes little in the process of friction. COF, 1.0
0.9 0.8 0.7
4N 6N 8N 10N 14N
0.6 0.5 0.4 0.3 0.2 0.1 0
200
400
600 Time,sec Time/s
800
1000
1200
(a)
Time/s (b) Figure 6. Friction coefficient curves of the two groups of samples with different porosities under Figure 6. Friction coefficient curves of the two groups of samples with different porosities under different loads:(a) (a)friction friction coefficient curves the porosity ε = ~24%–26%; and (b)coefficient Friction different loads: coefficient curves withwith the porosity ε = ~24%–26%; and (b) Friction coefficient curves with the porosity ε = ~38%–42%. curves with the porosity ε = ~38%–42%.
In order further investigate the influence of load and porosity on the friction coefficient and In order further investigate the influence of load and porosity on the friction coefficient and wear wear rate, the experimental data were analyzed. As shown in Figure 7a, the wear rate of each group rate, the experimental data were analyzed. As shown in Figure 7a, the wear rate of each group is is consistent with the change of loads. However, comparison of the two groups of different porosity consistent with the change of loads. However, comparison of the two groups of different porosity samples finds that the wear rate decreases as the porosity increases, which results in a reduction of samples finds that the wear rate decreases as the porosity increases, which results in a reduction the wear resistance of the material. The effect of porosity on the wear resistance of materials is of the wear resistance of the material. The effect of porosity on the wear resistance of materials is mainly to reduce the mechanical properties of materials, such as hardness, elastic modulus and so mainly to reduce the mechanical properties of materials, such as hardness, elastic modulus and so on. on. Moreover, the increase in the porosity of the material leads to an increase in the fracture Moreover, the increase in the porosity of the material leads to an increase in the fracture phenomenon phenomenon of the material edge and the wear debris, which results in the aggravation of the wear of the material edge and the wear debris, which results in the aggravation of the wear and tear. From and tear. From Figure 7a, when the load is 8 N–10 N, another important phenomenon is shown as the wear rate of the two groups of samples changes slowly. Similarly, the friction coefficients of the
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two samples in Figure 7b reach the same value when the load is 8 N–9 N. Thus, it can be concluded that the wear resistance of different porosity materials is relatively stable in the micro-cutting of Metals 2017, 7, 28 10 of 20 porous titanium alloys. In addition, the effect of porosity on the friction coefficient is minimum when the load is about 8 N. Figure 7a, when the load 8 N–10 N, important phenomenon shown the wearthat ratethe of Figure 7b shows theisvariation of another the friction coefficient with the is load and as indicates the twocoefficient groups of changes samples smoothly changes slowly. Similarly, the friction coefficients the two samples in friction as the load increases. This is because the of contact surface of the Figure 7b reach the same value whencontact the load is 8 micro-cutting N–9 N. Thus, process. it can beThe concluded that theinwear friction pairs is dominated by plastic in the slight decrease the resistance of different porosity materials samples is relatively stable in the micro-cutting of porous titanium friction coefficient of the small-porosity is due to the nonlinear relationship between the alloys. In the addition, the effect the friction minimum whencoefficient the load is of about load and contact area. It of canporosity also beon observed fromcoefficient the imageisthat the friction the 8 N. material would increase with increasing porosity. 35 Wear rate/10-6mm3/mm
30 25
Test specimen 1 =24%~26% Test specimen 2 =38%~42%
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(b) Figure 7. The variation curves of the wear rate and friction coefficient with the load: (a) relationship Figure 7. The variation curves of the wear rate and friction coefficient with the load: (a) relationship between the wear rate and the load of materials with different porosities; and (b) relationship between the wear rate and the load of materials with different porosities; and (b) relationship between between the friction coefficient and the load of materials with different porosities. the friction coefficient and the load of materials with different porosities.
3.1.2. Effects of Sliding Speed on Friction Coefficient and Wear Rate Figure 7b shows the variation of the friction coefficient with the load and indicates that the friction To study the effect of different speeds on theisfriction and wear of rate, load coefficient changes smoothly as the sliding load increases. This becausecoefficient the contact surface thethe friction applied to the test remained unchanged at 6 N and the temperature was maintained at room pairs is dominated by plastic contact in the micro-cutting process. The slight decrease in the friction temperature. Thesmall-porosity friction and wear tests were out atrelationship sliding speeds of 1.57 coefficient of the samples is due to carried the nonlinear between the m/min, load and3.14 the m/min, 6.28 m/min, 9.43 m/min, 12.56 m/min, and 15.71 m/min. As shown in Figure 8a, it can be contact area. It can also be observed from the image that the friction coefficient of the material would observed that increasing the coefficient of friction for small-porosity samples varies dramatically with speed. increase with porosity. The cause of this phenomenon is that debris is collected within the pores at low sliding speeds, and 3.1.2. Effectsgets of Sliding Speed on Friction Coefficient WearitRate the surface rougher under this condition. As aand result, leads the low value of the friction coefficient. If the is verysliding high, the contact temperature will and rise sharply, which To study thesliding effect speed of different speeds onsurface the friction coefficient wear rate, the may soften the contact surface and lead to a decrease in the coefficient of friction. It also indicates load applied to the test remained unchanged at 6 N and the temperature was maintained at room that the friction coefficient of the large-porosity sample seems to decrease at a low sliding velocity temperature. The friction and wear tests were carried out at sliding speeds of 1.57 m/min, 3.14 m/min, 6.28 m/min, 9.43 m/min, 12.56 m/min, and 15.71 m/min. As shown in Figure 8a, it can be observed
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that the coefficient of friction for small-porosity samples varies dramatically with speed. The cause of this phenomenon is that debris is collected within the pores at low sliding speeds, and the surface gets rougher under this condition. As a result, it leads the low value of the friction coefficient. If the sliding speed is very high, the contact surface temperature will rise sharply, which may soften the contact surface and lead to a decrease in the coefficient of friction. It also indicates that the friction coefficient of Metals 2017, 7, 28 11 of 19 the large-porosity sample seems to decrease at a low sliding velocity when the sliding speed increases. Comparing Figure 8a,b, can be found that the friction the found two porosities most when the sliding speedit increases. Comparing Figure coefficients 8a,b, it canof be that theare friction stable state when speed about 400 r/min. coefficients of the the twosliding porosities areismost stable state when the sliding speed is about 400 r/min. COF, 0.9
0.8 0.7
3.14 m/min 6.28 m/min 9.43 m/min 12.56 m/min 15.71 m/min
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0.5 0.4 0.3 0.2 0.1 0
200
400
600 Time,sec Time/s (b)
Figure 8. Friction coefficient curves of the two groups of samples with different porosities under Figure 8. Friction coefficient curves of the two groups of samples with different porosities under different sliding speeds: (a) friction coefficient curves with the porosity ε = ~24%–26%; and (b) different sliding speeds: (a) friction coefficient curves with the porosity ε = ~24%–26%; and (b) friction friction coefficient curves with the porosity ε = ~38%–42%. coefficient curves with the porosity ε = ~38%–42%.
Furthermore, the influence of the sliding speed on the wear rate and friction coefficient of the Furthermore, the in influence ofIn theFigure sliding speed wear rate andatfriction coefficient of the two two groups is shown Figure 9. 9a, it canonbethe observed that, the same sliding speed, the groups is shown in Figure 9. In Figure 9a, it can be observed that, at the same sliding speed, the wear wear resistance of the materials decreases as the porosity increases. Besides that, the wear rate resistance as of the decreases as theInporosity increases. Besides the wear rate decreases as decreases the materials sliding speed increases. particular, because of thethat, uneven distribution of space, the sliding speed increases. In particular, because of the uneven distribution of space, the wear rate of the wear rate of the sample with large porosity decreases abruptly at the speed of 9.43 m/min. the sample with large porosity decreases abruptly at the speed of 9.43 m/min. In Figure 9b, the friction coefficient increases as the sliding speed increases generally. Twists in the small-porosity and large-porosity samples are observed at sliding speed in the vicinity of 6.28 m/min and 3.14 m/min, respectively. Because the influence of the sliding speed on the friction coefficient is mainly through the change in temperature, high temperature can soften the material so as to slow down the change trend of friction coefficient. Larger pore lead to poorer heat dissipation.
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(b) Figure thethe wear rate and friction coefficient with the the sliding speed: (a) Figure 9. 9. The Thevariation variationcurves curvesofof wear rate and friction coefficient with sliding speed: relationship between the wear rate and sliding speed of materials with different porosities; and (b) (a) relationship between the wear rate and sliding speed of materials with different porosities; and relationship between thethe friction coefficient and sliding speed of of materials with different (b) relationship between friction coefficient and sliding speed materials with differentporosities. porosities.
3.1.3. Effects of Temperature on Friction Coefficient and Wear Rate In Figure 9b, the friction coefficient increases as the sliding speed increases generally. Twists in the To distinguish the effects of samples differentare temperatures on the speed friction wear rate, small-porosity and large-porosity observed at sliding in coefficient the vicinityand of 6.28 m/min large porosity samples were chosen. In addition, the test is carried out at the same sliding speed (300 and 3.14 m/min, respectively. Because the influence of the sliding speed on the friction coefficient is r/min) load (6 N).change The testing temperatures were 20 °C, 100 °C, °C, the 300 material °C, 400 °Csoand 500slow °C. mainlyand through the in temperature, high temperature can200 soften as to As observed in Figure the fluctuation of the pore friction is more intense than at room down the change trend of10, friction coefficient. Larger leadcurve to poorer heat dissipation. temperature. The increase in temperature softens the material and leads to a decrease in the 3.1.3. Effectsproperties of Temperature on Friction Coefficient and Wear mechanical and spalling of the sharp edges at theRate voids. Therefore, the contact surface area changes, and this results in an increase in the fluctuation of the friction curve. To distinguish the effects of different temperatures on the friction coefficient and wear rate, large COF,chosen. In addition, the test is carried out at the same sliding speed (300 r/min) porosity samples were 0.8 and load (6 N). The testing temperatures were 20 ◦ C, 100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C and 500 ◦ C. As observed in Figure0.7 10, the fluctuation of the friction curve is more intense than at room temperature. The increase in temperature softens the material and leads to a decrease in the mechanical properties 0.6 and spalling of the sharp edges at the voids. Therefore, the contact surface area changes, and this results in an increase 0.5in the fluctuation of the friction curve. In Figure 11a, the effects of the temperature on the wear rate and friction coefficient are shown. 20 0.4 With an increase in temperature, the wear rate first decreases and then increases. An inflection point 100 ◦ 200 appears near 300 0.3 C. The point in the material wear mechanism is explained as a progression from 300 minor to severe wear and tear. The main factors influencing this phenomenon are the production of 0.2 400 an oxide film on the friction surface and the temperature that results in the softening of the material. 500 0.1point, the wear rate decreases as a result of the production of the oxide film on Before the inflection ℃
℃
℃
℃
℃
℃
0
200
400
600
Time,sec Time/s
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Figure 10. Friction coefficient curves of sample with ε = ~38%–42%, under different temperatures.
3.1.3. Effects of Temperature on Friction Coefficient and Wear Rate To distinguish the effects of different temperatures on the friction coefficient and wear rate, large porosity samples were chosen. In addition, the test is carried out at the same sliding speed (300 Metals 2017, 28 (6 N). The testing temperatures were 20 °C, 100 °C, 200 °C, 300 °C, 400 °C and 500 13 of 20 r/min) and7,load °C. As observed in Figure 10, the fluctuation of the friction curve is more intense than at room temperature. The increase in temperature softens the material and leads to a decrease in the the friction surface. However, after the inflection point, the phenomenon of temperature softening mechanical properties and spalling of the sharp edges at the voids. Therefore, the contact surface controls the wear rate, and the wear rate increases as a result. area changes, and this results in an increase in the fluctuation of the friction curve. COF, 0.8 Metals 2017, 7, 28
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0.7 In Figure 11a, the effects of the temperature on the wear rate and friction coefficient are shown. 0.6 With an increase in temperature, the wear rate first decreases and then increases. An inflection point appears near 300 °C. The point in the material wear mechanism is explained as a progression from 0.5 minor to severe wear and tear. The main factors influencing this phenomenon are the production of 20 an oxide film on0.4 the friction surface and the temperature that results in the softening of the material. 100 Before the inflection point, the wear rate decreases as a result of the production of the oxide film on 0.3 However, after the inflection200 the friction surface. point, the phenomenon of temperature softening 300 controls the wear rate, and the wear rate increases as a result. 0.2 400 Figure 11b shows that the friction coefficient500 of the material decreases as the temperature rises. The relationship 0.1between the friction coefficient and the temperature can be simulated as a linear 0 200 400 600 800 1000 1200 equation, which was obtained by fitting the data and is given as follows: Time,sec ℃
℃
℃
℃
℃
℃
Time/s
μ T 0.5727 0.1304 T 20 / 1000 (5) Figure Figure 10. 10. Friction Friction coefficient coefficient curves curves of of sample sample with with εε == ~38%–42%, ~38%–42%,under underdifferent different temperatures. temperatures. where T (°C) is the temperature of the contact surface.
(a)
(b) Figure 11. Influence of different temperatures on the wear rate and friction coefficient under the
Figure 11. Influence of different temperatures on the wear rate and friction coefficient under the same load and sliding speed: (a) relationship between the wear rate and the temperature; and (b) same load and sliding speed: (a) relationship between the wear rate and the temperature; and experimental and theoretical curves of the friction coefficient at different temperatures. (b) experimental and theoretical curves of the friction coefficient at different temperatures.
3.2. Wear Morphology and Mechanism of Porous Titanium Materials The wear morphology of the two groups of samples with different porosities was recorded by SEM under different loads in Figure 12. Among them, Figure 12a–c shows that there is a slight
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Figure 11b shows that the friction coefficient of the material decreases as the temperature rises. The relationship between the friction coefficient and the temperature can be simulated as a linear equation, which was obtained by fitting the data and is given as follows: Metals 2017, 7, 28 14 of 19 µ( T )phenomenon = 0.5727 − 0.1304 T −load 20)/1000 (5) furrow and delamination peeling with (the and roughness increase. The surface roughness of the sample is small and the main wear mechanism is abrasive wear. The images in where T (◦ C) is the temperature of the contact surface. Figure 12e–f more evidently indicate that, as the load and roughness increases, the furrows become deeper a dense and black oxide film created. The wear mechanism is replaced by abrasive wear, 3.2. Wearand Morphology Mechanism of is Porous Titanium Materials which is accompanied by slight oxidation wear. The wear morphology of the of samples with different porosities was recorded by Furthermore, compared withtwo thegroups small-porosity sample, the sample with large porosity has SEM under different loads in Figure 12. Among them, Figure 12a–c shows that there is a slight furrow more serious wear on the surface and deeper furrows, which is mainly because the high porosity and delamination peeling with the load and roughness increase. The surface roughness causes the aperture edge phenomenon to tear leading to more obvious spalling. The gap becomes larger and the of the sample is small and the main wear mechanism is abrasive wear. The images in Figure 12e–f more debris that peels off participates in wearing the surface. These factors form a three-body abrasion evidently indicate that, as the load and roughness increases, the furrows become deeper and a dense process. Moreover, with the ongoing friction, a membrane body appears because of the repeated black oxidebetween film is created. The and wearthe mechanism is replaced bythis abrasive wear, which is accompanied extrusion the sample sphere (Carbide YG6); membrane can reduce the surface by slight oxidation wear. roughness and is conducive for improving the frictional performance.
(a)
(b)
(c)
(d)
(e)
(f)
Figure12. 12.Two Twogroups groupsof ofSEM SEMimages imagesshowing showingthe thesurface surfacemorphology morphologyof ofthe thesamples sampleswith withdifferent different Figure loads: (a) the porosity of the sample is ~24%–26% and the load is 4 N; (b) the porosity of the sample loads: (a) the porosity of the sample is ~24%–26% and the load is 4 N; (b) the porosity of the sample isis ~24%–26%and andthe theload loadis is porosity of the sample is ~24%–26% the load N;the (d) ~24%–26% 1010 N;N; (c)(c) thethe porosity of the sample is ~24%–26% andand the load is 14isN;14(d) the porosity of the sample is ~38%–42% and the load is 4 N; (e) the porosity of the sample is ~38%– porosity of the sample is ~38%–42% and the load is 4 N; (e) the porosity of the sample is ~38%–42% 42%the and theisload is 10 N;(f)and the porosity the sample is ~38%–42% and theisload is 14 N. and load 10 N; and the(f) porosity of theof sample is ~38%–42% and the load 14 N.
The wear morphology of the porous titanium materials with different porosity under different Furthermore, compared with the small-porosity sample, the sample with large porosity has more sliding speeds is shown in Figure 13. As the sliding speed increases, the wear surface deforms serious wear on the surface and deeper furrows, which is mainly because the high porosity causes plastically and the growing furrows roughen the surface. The increasing sliding speed also leads to the aperture edge to tear leading to more obvious spalling. The gap becomes larger and the debris an increase of temperature which produces the oxide film between the contact surfaces. that peels off participates in wearing the surface. These factors form a three-body abrasion process. This film can reduce the wear rate. The sliding speed used in this experiment is relatively small, Moreover, with the ongoing friction, a membrane body appears because of the repeated extrusion and the heat produced by friction behavior is not enough to cause the material to soften. between the sample and the sphere (Carbide YG6); this membrane can reduce the surface roughness Therefore, the phenomenon that sliding rate increases while the wear rate reduction is due to and is conducive for improving the frictional performance. the oxide film generated on the machined surface. The wear morphology of the porous titanium materials with different porosity under different Furthermore, in Figure 14, the wear surface morphology of the porous titanium samples with sliding speeds is shown in Figure 13. As the sliding speed increases, the wear surface deforms the porosity of ~38%–42% is shown at different temperatures. When the temperature is 300 °C, the surface is covered with a dense black oxide film and the pore edge fragmentation slows down, resulting in a low wear rate. On the other hand, when the temperature increases to 500 °C, the film is damaged and the fracture of the pore edge worsens, resulting in a more severe wear.
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plastically and the growing furrows roughen the surface. The increasing sliding speed also leads to an increase of temperature which produces the oxide film between the contact surfaces. Metals 2017, 7, 28 15 of 19
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Figure 13. Two groups of SEM images showing the surface morphology of the samples with different Figure 13. Two groups of SEM images showing the surface morphology of the samples with different sliding speeds: (a) the porosity of the sample is ~24%–26% and the speed is 100 r/min; (b) the porosity sliding speeds: (a) the porosity of the sample is ~24%–26% and the speed is 100 r/min; (b) the porosity ofthe thesample sample is is ~24%–26% ~24%–26% and (c)(c) thethe porosity of the sample is ~24%–26% and of and the thespeed speedisis200 200r/min; r/min; porosity of the sample is ~24%–26% the the speed is 400 r/min; (d) the of theofsample is ~24%–26% and the is 500isr/min; (e) the and speed is 400 r/min; (d)porosity the porosity the sample is ~24%–26% andspeed the speed 500 r/min; porosity of the sample is ~38%–42% and the speed is 100 r/min; (f) the porosity of the sample (e) the porosity of the sample is ~38%–42% and the speed is 100 r/min; (f) the porosity of the sample isis ~38%–42%and andthe thespeed speedisis200 200r/min; r/min; (g) (g)the theporosity porosityof ofthe thesample sampleisis~38%–42% ~38%–42%and andthe thespeed speedisis ~38%–42% 400 r/min; and (h) the porosity of the sample is ~38%–42% and the speed is 500 r/min. 400 r/min; and (h) the porosity of the sample is ~38%–42% and the speed is 500 r/min.
This film can reduce the wear rate. The sliding speed used in this experiment is relatively small, and the heat produced by friction behavior is not enough to cause the material to soften. Therefore, the phenomenon that sliding rate increases while the wear rate reduction is due to the oxide film generated on the machined surface. Furthermore, in Figure 14, the wear surface morphology of the porous titanium samples with the porosity of ~38%–42% is shown at different temperatures. When the temperature is 300 ◦ C, the surface is covered with a dense black oxide film and the pore edge fragmentation slows down, resulting in a low wear rate. On the other hand, when the temperature increases to 500 ◦ C, the film is damaged and the fracture of the pore edge worsens, resulting in a more severe wear.
(a)
(b)
Figure 14. SEM images showing the surface morphology of porous titanium materials at different temperatures: (a) the temperature of the sample is 300 °C; and (b) the temperature of the sample is 500 °C.
sliding speeds: (a) the porosity of the sample is ~24%–26% and the speed is 100 r/min; (b) the porosity of the sample is ~24%–26% and the speed is 200 r/min; (c) the porosity of the sample is ~24%–26% and the speed is 400 r/min; (d) the porosity of the sample is ~24%–26% and the speed is 500 r/min; (e) the porosity of the sample is ~38%–42% and the speed is 100 r/min; (f) the porosity of the sample is Metals~38%–42% 2017, 7, 28 and the speed is 200 r/min; (g) the porosity of the sample is ~38%–42% and the speed is 16 of 20 400 r/min; and (h) the porosity of the sample is ~38%–42% and the speed is 500 r/min.
(a)
(b)
Figure Figure 14. 14. SEM SEM images images showing showing the the surface surface morphology morphology of of porous porous titanium titanium materials materials at at different different temperatures: (a) the temperature of the sample is 300 °C; and (b) the temperature of the sample is 500 ◦ temperatures: (a) the temperature of the sample is 300 C; and (b) the temperature of the sample is ◦ °C. 500 C.
The micro region and the spots on samples were analysed by INCA Energy Disperse Spectrometer (EDS), and the results are shown in Figure 15 and Table 6. Figure 15a,b shows images of samples with different porosities at a sliding speed of 300 r/min. However, Figure 15b–d shows images of the large porosity samples with increasing temperatures. In addition, Table 6 shows the variation of the proportions of the elements in titanium samples under different conditions. A comparison of Figure 15c,d shows that with the increasing temperature, the Oxygen element content in the titanium sample increases. Although titanium is very stable at room temperature, the increased temperature is the main factor for oxidation. The EDS analysis shows that the wear process of porous titanium is accompanied by oxidation wear. Moreover, when the load is over 10 N, the oxidation wear is obvious, and the oxidation phenomenon is violent at a high temperature. Finally, the VHX-900 super depth of field digital microscope and the OLYMPUS DSX500 digital microscope are used to observe the wear morphology of the YG6 carbide grinding ball under the load of 6 N. The images are shown in Figure 16. The difference between Figure 16a,b is the sliding speed. As observed in Figure 16b, there are clearer scratches in the wear area due to furrows effects along the rubbing direction and abrasive grooves on the surface. Moreover, the furrows are unevenly distributed on the surface that along the sliding direction of the YG6 carbide grinding ball. In particular, the black region is the accumulation area of the debris. An apparent crack can be observed in Figure 16c, which is a result of the fatigue damage caused by abrasive wear. The peeling phenomenon can be observed in Figure 16d; the peeling occurs because the surface of the metal becomes brittle after being deformed and then strengthened. Then, micro-cracks are produced and the metal surface is peeled off under the load. As a result, the main form of wear in the contact area of the YG6 carbide grinding ball is abrasive wear.
variation of the proportions of the elements in titanium samples under different conditions. A comparison of Figure 15c,d shows that with the increasing temperature, the Oxygen element content in the titanium sample increases. Although titanium is very stable at room temperature, the increased temperature is the main factor for oxidation. The EDS analysis shows that the wear Metals 2017,of7,porous 28 17 of 10 20 process titanium is accompanied by oxidation wear. Moreover, when the load is over N, the oxidation wear is obvious, and the oxidation phenomenon is violent at a high temperature.
(a)
(b)
(c)
(d)
Figure15. 15. Wear Wearsurface surfaceanalysis analysisofofporous poroustitanium titaniummaterials materials EDS: porosity sample Figure byby EDS: (a)(a) thethe porosity of of thethe sample is is ~24%–26%, the load is 10 N and the temperature is 20 °C; (b) the porosity of the sample is ~38%– ◦ ~24%–26%, the load is 10 N and the temperature is 20 C; (b) the porosity of the sample is ~38%–42%, 42%, theisload 10 N the temperature is (c) 20 the °C; porosity (c) the porosity of the sample is ~38%–42%, the load 10 Nisand theand temperature is 20 ◦ C; of the sample is ~38%–42%, the loadthe is ◦ load is 6 N and the temperature is 200 °C; and (d) the porosity of the sample is ~38%–42%, the load 6 N and the temperature is 200 C; and (d) the porosity of the sample is ~38%–42%, the load is 6 N andis 6 Ntemperature and the temperature the is 300 ◦ C.is 300 °C.
Number
Porosity (%)
Load (N)
Temperature (°C)
Element Weight (%) Atom (%) O 2.26 6.43 a ~24–26 10 20 Al 1.03 1.74 Ti 96.71 91.83 O 6.40 17.00 ~38–42 10 20 Metals 2017, b 7, 28 18 of 20 Ti 93.60 83.00 O 10.77 26.55 c ~38–42 6 200 Ti 89.23 73.45 O under 11.89 conditions. 28.77 Table 6. Element composition in porous titanium samples different d ~38–42 6 300 Ti 88.11 71.23
Finally, the VHX‐900 super depth of field digital microscope and the OLYMPUS DSX500 digital Number Porosity (%) Load (N) Temperature (◦ C) Element Weight (%) Atom (%) microscope are used to observe the wear morphology of the YG6 carbide grinding ball under the O 2.26 6.43 load of 6 N. The images are shown in Figure 16. The difference between Figure 16a,b is the sliding a Al 1.03 1.74 ~24–26 10 20 speed. As observed in Figure 16b, there are clearer scratches in the wear area due to furrows effects Ti 96.71 91.83 along the rubbing direction and abrasive grooves on the surface. Moreover, the furrows are O 6.40 17.00 ~38–42 10 20 b unevenly distributed on the surface that along the sliding direction of the YG6 carbide grinding ball. Ti 93.60 83.00 In particular, the black region is the accumulation area of the debris. An apparent crack can be O 10.77 26.55 c observed in Figure 16c, which is a result of the fatigue damage caused by abrasive wear. The peeling ~38–42 6 200 Ti 89.23 73.45 phenomenon can be observed in Figure 16d; the peeling occurs because the surface of the metal O 11.89 28.77 becomes brittle after being deformed and then strengthened. Then, micro‐cracks are produced and ~38–42 6 300 d Ti 88.11 71.23 the metal surface is peeled off under the load. As a result, the main form of wear in the contact area of the YG6 carbide grinding ball is abrasive wear.
(a)
(b)
(c)
(d)
Figure 16. Surface morphology of the YG6 carbide grinding ball: (a) the sliding speed is 9.43 m/min
Figure 16. Surface morphology of the YG6 carbide grinding ball: (a) the sliding speed is 9.43 m/min and the magnification is 139×; (b) the sliding speed is 12.56 m/min and the magnification is 139×; and the magnification is 139×; (b) the sliding speed is 12.56 m/min and the magnification is 139×; (c) the sliding speed is 12.56 m/min and the magnification is 800×; and (d) the sliding speed is 12.56 (c) the sliding speed is 12.56 m/min and the magnification is 800×; and (d) the sliding speed is m/min and the magnification is 600×. 12.56 m/min and the magnification is 600×.
4. Conclusions
4. Conclusions A study on the friction and wear of porous titanium has been carried out under different porosity, load, sliding speed and temperature conditions. The following conclusions can be drawn from this study: (1)
In the micro-cutting process of porous titanium alloys, the best choice of machining parameters for different porosity materials are as follows: the load is about 8 N, the sliding speed is about 400 r/min and the temperature is about 300 ◦ C. This choice can satisfy the requirements that the material’s friction coefficient and wear rate should be kept at a relatively stable value to the maximum extent possible, to improve the quality of the machined surface and the service life of the cutter.
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The wear mechanism of porous titanium material is abrasive wear and slight oxidation wear at room temperature. However, the mechanism will change to severe oxidation wear and abrasive wear if the temperature is high. In addition, the main wear mechanism in the contact area of the YG6 carbide grinding ball is abrasive wear. Among the parameters of porosity, load, sliding velocity and temperature, the main factor that influences the friction coefficient and wear rate is the porosity. Porosity reduces the wear resistance of the material. In addition, due to the existence of the porosity, the abrasive wear mainly occurs between the friction pair, while the oxidation wear occurs only under large load. Under the micro-cutting condition, the contact surface of the friction pair is mainly plastic contact. The black oxide film formed on the surface of the processed surface at high speed or high temperature is mainly compounds of titanium and oxygen.
Acknowledgments: The work is supported by the National Natural Science Foundation of China (No. 51305174). Author Contributions: Zhiqiang Liu and Feifei Ji conceived and designed the experiments; Tianyu Zhu performed the experiments; Mingqiang Wang analyzed the data; Zhiqiang liu contributed reagents/materials/analysis tools; and Feifei Ji wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
References 1. 2. 3. 4. 5.
6. 7. 8. 9. 10.
11. 12. 13. 14.
Ezugwu, E.O.; Wang, Z.M. Titanium alloys and their machinability—A review. J. Mater. Process. Technol. 1997, 68, 262–274. [CrossRef] Liu, Y.; Jiang, G.F.; He, G. Enhancement of entangled porous titanium by BisGMA for load-bearing biomedical applications. Mater. Sci. Eng. C 2016, 61, 37–41. [CrossRef] [PubMed] Yetim, A.F. Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions. Surf. Coat. Technol. 2010, 205, 1757–1763. [CrossRef] Sui, X.D.; Li, G.J.; Qin, X.S. Relationship of microstructure, mechanical properties and titanium cutting performance of TiAlN/TiAlSiN conposite coated tool. Ceram. Int. 2016, 42, 7524–7532. [CrossRef] Ayed, Y.; Germain, G.; Ammar, A. Tool wear analysis and improvement of cutting conditions using the high-pressure water-jet assistance when machining the Ti17 titanium alloy. Mater. Lett. 2015, 42, 294–301. [CrossRef] Zhao, X.X.; Ke, W.; Zhang, S.S. Potential failure cause analysis of tungsten carbide end mills for titanium alloy machining. Eng. Fail. Anal. 2016, 66, 321–327. [CrossRef] Dearnley, P.A.; Grearson, A.N.; Aucote, J. Wear mechanisms of cemented carbides and ceramics used for machining titanium alloys. High Technol. Ceram. 1987, 38, 2699–2712. Corduan, N.; Hirnbert, T.; Poulachon, G. Wear mechanisms of new tool materials for Ti-6Al-4V high performance machining. CIRP Ann. 2003, 52, 73–76. [CrossRef] Igor, V.; Florian, A.; Stefan, K. The influence of temperature on friction and wear of unlubricated steel/steel contacts in different gaseous atmospheres. Tribol. Int. 2016, 98, 155–171. Dai, J.J.; Zhu, J.Y.; Chen, C.Z. High temperature oxidation behavior and research status of modification on improving high temperature oxidation resistance of titanium alloy and titanium aluminides: A review. J. Alloy. Compd. 2016, 685, 784–798. [CrossRef] Guo, C.; Zhou, J.; Chen, J.M. Improvement of the oxidation and wear resistance of pure Ti by laser cladding at elevated temperature. Surf. Coat. Technol. 2010, 205, 2142–2151. [CrossRef] Long, M.; Rack, H.J. Friction and surface behavior of selected titanium alloys during reciprocating-sliding motion. Wear 2001, 249, 157–167. [CrossRef] Matikas, T.E.; Nicolaou, P.D. Prediction of contact temperature distribution during fretting fatigue in titanium alloys. Tribol. Trans. 2009, 52, 346–353. [CrossRef] Yang, Y.; Zhang, C.H.; Wang, Y. Friction and wear performance of titanium alloy against tungsten carbide lubricated with phosphate ester. Tribol. Int. 2016, 95, 27–34. [CrossRef]
Metals 2017, 7, 28
15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
20 of 20
Wang, X.H.; Li, J.S.; Hu, R. Mechianical properties and pore structure deformation buhaviour of biomedical porous titanium. Trans. Nonferr. Met. Soc. China 2015, 25, 1543–1550. [CrossRef] Wang, G.; Rahman, M.; Wong, Y.S. Study on orthogonal turning of titanium alloys with different coolant supply strategies. Int. J. Adv. Manuf. Technol. 2009, 42, 621–632. [CrossRef] Quan, H.X.; Gao, S.S.; Zhu, M.H.; Yu, H.Y. Comparison of the torsional fretting behavior of three porous titanium coatings for biomedical applications. Tribol. Int. 2015, 192, 29–37. [CrossRef] Molinari, A.; Straffelini, G.; Tesi, B.; Baccai, T. Dry sliding wear mechanism of the Ti6Al4V alloy. Wear 1997, 208, 105–112. [CrossRef] Yang, D.; Liu, Z.Q. Surface topography analysis and cutting parameters optimization for peripheral milling titanium alloy Ti-6Al-4V. Int. J. Refract. Met. Hard Mater. 2015, 51, 192–200. [CrossRef] Tutunea Fatan, O.R.; Fakhri, M.A.; Bordatchev, E.V. Porosity and cutting forces: From macroscale to microscale machining correlations. Proc. Inst. Mech. Eng. 2011, 255, 619–630. [CrossRef] Lauro, C.H.; Filho, S.L.M.R.; Brandão, L.C.; Davim, J.P. Analysis of behaviour biocompatible titanium alloy (Ti-6Al-7Nb) in the micro-cutting. Measurement 2016, 93, 529–540. [CrossRef] Attanasio, A.; Gelfi, M.; Giardini, C.; Remino, C. Minimum quantity lubrication in turning. Wear 2006, 160, 333–338. [CrossRef] Farokhzadeh, K.; Edrisy, A. Transition between mild and severe wear in titanium alloys. Tribol. Int. 2016, 94, 98–111. [CrossRef] Fakhri, M.A.; Bordatchev, E.V.; Tutunea Fatan, O.R. An image-based methodology to establish correlations between porosity and cutting force in micromilling of porous titanium foams. Int. J. Adv. Manuf. Technol. 2012, 60, 841–851. [CrossRef] © 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
