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Ultrasonic vibration-assisted scratch characteristics of silicon carbide-reinforced aluminum matrix composites
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10817–10823 www.elsevier.com/locate/ceramint
Ultrasonic vibration-assisted scratch characteristics of silicon carbide-reinforced aluminum matrix composites Pingfa Fenga, Guiqiang Lianga,b,n, Jianfu Zhanga a
Department of Mechanical Engineering, Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipments and Control, Tsinghua University, Beijing 100084, PR China b College of Electrical Engineering, Binzhou Polytechnic, Binzhou 256603, PR China Received 27 February 2014; received in revised form 14 March 2014; accepted 16 March 2014 Available online 24 March 2014
Abstract The material removal of silicon carbide particle-reinforced aluminum (SiCp/Al) matrix composites during ultrasonic vibration-assisted scratch (UVAS) and traditional scratch tests was investigated by examining scratch load, coefficient of friction (COF) and scratch morphology. It indicated that the loads during both the UVAS and traditional tests fluctuated obviously. The average loads of the UVAS processes were lower than those of the traditional scratches. The COF caused by the traditional scratch process was more stable and larger than that of the ultrasonic one. Large particles were observed in the grooves formed by the traditional scratch method, whereas small chips of SiC were found in the scratches formed by UVAS, which indicated that the material removal ratio was higher for the scratches formed by UVAS than that of scratches formed by the traditional method. Ultrasonic vibration played an important role in reducing the grinding force and COF, as well as improving the morphology of the machined surfaces. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Ultrasonic vibration-assisted scratching; B. SiC/Al; C. Morphology
1. Introduction SiCp/Al is an aeronautical composite with high specific strength and stiffness, low coefficient of thermal expansion, high thermal conductivity and good wear resistance. SiCp/Al has been widely used in many fields including aerospace, automobile and optical precision instruments [1–3]. SiCp/Al has excellent mechanical properties, but it is typically difficult to process because of the SiC particles, which are known for their high hardness, strength and wear resistance [4]. This has limited the application of SiCp/Al. It is therefore important and meaningful to study the processing behavior of such materials. A great number of researches have been conducted on the processing of SiC/Al composites so far [5–8]. However, all of n Corresponding author at: College of Electrical Engineering, Binzhou Polytechnic, Binzhou 256603, PR China. Tel./fax.: þ 86 10 62781099. E-mail addresses: [email protected] (P. Feng), [email protected] (G. Liang).
http://dx.doi.org/10.1016/j.ceramint.2014.03.073 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
them only focused on optimizing the processing parameters and simulation analysis, which can only shed some light on the mechanism of macro material removal. The mechanism of microscopic material removal is yet to be studied. Ultrasonic vibration-assisted machining (UVAM) combines traditional processing and ultrasonic vibration. A large number of studies [9–11] have confirmed that UVAM is an effective method for the precision machining of ceramics, particle-reinforced composites and optical materials, and other difficult-to-process materials. Ultrasonic vibration can reduce the cutting load and improve the surface quality [12,13]. Azarhoushang et al. [14] drilled Inconel 1738-LC with high-frequency low-amplitude ultrasonic vibration. The results showed that the hole roundness, cylindricity and surface roughness were greatly improved using UVAM, with higher quality holes, smaller chipping sizes and less wear to the tools than traditional machining. Pei et al. [15] found that the surface quality of ceramics processed by rotary ultrasonic face milling was better compared with those formed by traditional processing. Experiments carried out by Zhao [16] on ceramic
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P. Feng et al. / Ceramics International 40 (2014) 10817–10823
Table 1 Physical characteristics of SiC and 2024Al-T6. Material
Elasticity modulus / GPa
Poisson's ratio
Yield stress / MPa
Density / kg/m3
Specific heat capacity / J/(kg 1C)
SiC 2024Al-T6
410 74.2
0.14 0.33
5700 345.4
3700 2780
670 875
Ultrasonic spindle
Diamond indenter
Dial gauge
Work piece Kistler 9257b
a single diamond indenter had not been investigated. Therefore, we compared the traditional scratch and UVAS tests in three ways: scratch loads, morphology and coefficient of friction. Various experiments were conducted in order to make comparative analysis on the mechanism of material removal between ultrasonic scratches and traditional scratches, we were able to comparatively analyze the material removal, COF and morphology of processed surfaces to determine the mechanism of material removal of particlereinforced aluminum matrix composites. 2. Experimental 2.1. Materials
Fig. 1. Schematic diagram of the experimental setup.
scratching with a single diamond with ultrasonic vibration assistance showed that surface damage was lessened by the presence of ultrasonic vibration. Besides, Chen designed and built an ultrasonic milling work table to perform tests on non-ablative carbon/carbon composite materials [17], which showed that the surface quality of ultrasonically machined work pieces was much higher with roughness (of Sa and Sq) decreasing by 16% to 36%, the force of the grinding tool reducing by approximately 44% in the feed direction and 46% in the pushing direction. Researchers have investigated the processing features and removal mechanism of particle-reinforced composites in scratch tests both theoretically and experimentally. Ghosh et al. [18] conducted scratch tests on ZrB2–SiC composites with a Berkovich nano indenter in 2008. Microstructure analysis revealed that the grooves contained many slip bands. The maximum shear stress occurred in front of the scratch tool and the largest tensile stress occurred behind it. Further static and dynamic indentation and ultra-high-speed scratch tests were performed by Ghosh et al. [19] to examine the inelastic deformation characteristics of ultra-high temperature ceramic composite, ZrB2–SiC, in 2009. They found that dynamic indentation could cause a large number of transgranular cracks and high-speed scratches could lead to severe brittleness removal. A custom-made diamond spherical indenter had been used in ultrasonic scratch tests on sapphire to reveal the material removal characteristics in some researches [20]. It was shown that ultrasonic assistance could effectively reduce scratching load and crack expansion. Extensive researches had revealed the material removal mechanism in scratch tests [21–23], but that of particle-reinforced metal matrix composites in ultrasonic vibration-assisted scratch tests with
The materials used for the tests were made of cast aluminum alloy and were first subjected to pressureless infiltration. As a result, the matrix materials were cast aluminum reinforced by SiC ceramic particles. The cast aluminum alloy had high plasticity and continuous plastic deformation characteristics. The SiC particles had an elasticity modulus of 420 GPa, Poisson's ratio of 0.15, particle volume fraction of 62% and average particle size of 40 μm, as shown in Table 1. The upper surface of SiC/Al was selected as the test surface and was cut into pieces with dimensions of 15 mm 35 mm 10 mm by wire cutters and an orientator. The samples were then polished, and a three-dimensional profilometer (Talysurf, CLI2000) was used to measure the surface roughness (Ra) of the substrates. After polishing, they had an Ra value of 100 nm. 2.2. Experiment setup and conditions The scratch equipment used in the tests was a rotary ultrasonic machine that mainly comprised an ultrasonic spindle system and a numerical control machining system. The maximum power of the rotary ultrasonic machine was 300 W. The ultrasonic generator could generate highfrequency axial vibration signals of 16.5–30 kHz, which were converted to mechanical vibrations through a transducer. The amplitude decreased with the increase of material hardness, but the impact of hardness was not very big. An amplitude transformer enlarged the amplitude to 1–20 μm and then passed it on to the indenter, which worked on the surface of the test samples at a very high frequency to achieve UVAS, as shown in Fig. 1. The resonance frequency between the indenter and ultrasonic equipment was checked before the tests. The ultrasonic frequency was set to 20 kHz and the amplitude was 4 μm. The diamond indenter was tetrahedral with a length of 200 μm. The two ends of the workpieces had a height
P. Feng et al. / Ceramics International 40 (2014) 10817–10823
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Fig. 2. Installation position of the diamond indenter and workpiece: (a) traditional scratch and (b) ultrasonic scratch.
difference of 20 μm in order to get the critical cutting depth that the ultrasonic assistance could be effective. The indenter moved from the lower end to the higher end. It was shown that the actual cutting depth of UVAS was determined by the ultrasonic amplitude and the axial feeding action [24]. Therefore, the assistance of ultrasonic amplitude should be compensated in traditional scratches so as to achieve the same depth as in UVAS. First, based on the assumption that the effects of ultrasonic amplitude were negligible, the actual cutting depths of UVAS and traditional scratch were measured. Then, adjustment was made based on the results to get the same depths. The installation of the diamond indenter and workpiece is shown in Fig. 2. The upper surface of the workpieces was scratched. The single-particle diamond indenter was mounted in the chuck and the aluminum SiC workpiece was fixed on the worktable. The diamond indenter scratched the workpiece from right to left at different depths at a certain amplitude and frequency. The experimental parameters were as follows: scratch depth was 0–20 μm, the feeding speed was 24,000 mm/ min and the total length was 15 mm. By comparing the cutting depths as is shown in the following figures, we find those made by UVAS are smaller. It indicated that the workpiece material could affect the ultrasonic amplitude, as is shown in Fig. 3. Thus, we made up for it by the average value of the depth differences between the UVAS and traditional scratches. Assuming the starting position of the indenter for traditional scratch tests was M and we recorded it as Zm, while that for UVAS tests was N and we could put Zn ¼ Zm þ A, in which A represented the ultrasonic amplitude. In order to have the same cutting depth, the position of the indenter in UVAS tests should be Zn ¼ Zm þ A δ, where δ was average value of cutting depth differences and turns out to be δ¼ 1.2 μm.
Fig. 3. Comparison of the groove size between the UVAS process and the traditional scratch process: (a) traditional scratch and (b) ultrasonic scratch.
2.3. Experimental measurement A high-precision micro dynamometer, Kistler dynamometer, was used to record the data from the scratch tests. The data collection frequency was 9 kHz. The load, vibration frequency and scratch process were recorded during the scratch tests. A scanning electron microscope (SEM, Quanta200 FEG, FEI, The Netherlands) was used to analyze the morphology of the workpiece after processing under different conditions. Because SiC is not conductive, a three-dimensional white-light interfering profilometer was used to observe the sample surfaces. To ensure the reliability of the test data, three tests were carried out using each set of process parameters to get the average values. Traditional scratch tests were performed by switching off the ultrasonic generator. 3. Results and discussion 3.1. Scratch loads The normal loads (perpendicular to the scratching direction) were very small compared with the axial (perpendicular to the workpiece surface) and tangential (parallel to the workpiece surface) loads in UVAS tests. Therefore, we only analyzed the axial and tangential loads in this paper. Fig. 4 shows the loads in UVAS and traditional scratch tests at the same speed. Because data were not reliable at the beginning and end of the
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Traditional Axial load
Scratch loads (N)
20
Ultrasonic Axial load
15
10
5
0
Traditional Tangential load Ultrasonic Tangential load 3 6 Scratch length (mm)
9
Fig. 4. Comparison of the scratch loads between the UVAS process and the traditional scratch process.
8 Axial loads FZ(N)
Traditional 6
Ultrasonic
4
2 0
50 100 Scratch length (µm)
150
Fig. 5. Comparison of the loads between the UVAS process and the traditional scratch process.
scratching process, only the middle part of each scratch was selected for analysis. The loads in both UVAS and traditional scratch tests fluctuated, which was mainly a result of the diamond indenter scratching the SiC. Compared with those during the traditional scratch tests, loads were decreased in UVAS tests. The average axial load decreased by 28.4% and the average tangential load was reduced by 46.9%. The axial loads were greater than the tangential ones in both the UVAS and traditional tests. To study the mechanism of axial load reduction, we analyzed the axial loads on scratches with a length of 150 μm to study the microscale properties of each workpiece. As shown in Fig. 5, the two curves separately reflected the axial loads on SiC (with a length of 40 μm) in both the UVAS and traditional scratch tests. The load abruptly increased and decreased in certain areas during the traditional tests. In contrast, there was only small fluctuation of axial load in the UVAS tests. To further study the mechanism of scratch load reduction in UVAS testing, we observed the morphology of the workpieces with SEM. Fig. 6(a) and (b) shows the morphology of samples scratched with the traditional and UVAS methods, respectively. The workpiece from the traditional tests was damaged and crushed to a greater degree than that subjected to UVAS. Large particles on the workpiece from the UVAS tests were broken
Fig. 6. Comparison of the surface morphology between the UVAS process and the traditional scratch process: (a) surface morphology for traditional scratch and (b) surface morphology for ultrasonic scratch.
down into chips under the high-frequency ultrasonic vibration. During the scratching process in the UVAS test, both the chips and aluminum substrate were removed, so the load was more stable and smaller than that in the traditional tests. Large SiC particles were broken down into small chips under the high-frequency ultrasonic vibration. These chips adhered to the workpiece surface. The cyclical changes of pressure in dynamic mode made the chips circulate and suspend on the surfaces to form a transfer film, which reduced the friction between the head and the workpiece considerably [25]. This might explain why the ultrasound scratches need smaller tangential loads than traditional ones. Fig. 7 depicts the microscale morphology of the scratch formed using the traditional process. The indenter and the workpiece kept constant contact during the scratching process, which caused extrusion and abrasions in the side of the indenter and the workpiece, thereby increasing the tangential load. In the UVAS process, the high-frequency vibration of the diamond indenter at a certain amplitude ensured that the contact between the indenter and the workpiece was intermittent, which reduced friction. As a result, the tangential load was also lowered in the UVAS process. UVAS is characterized by its intermittency [26], and the scratching speed is at periodical changes. Under the influence of these two features, the impacts of the diamond indenter on the workpiece yielded a large number of microcracks, which facilitated material removal and the expansion of cracks, thereby reducing the average load of the scratches.
P. Feng et al. / Ceramics International 40 (2014) 10817–10823
Fig. 7. Microstructure for traditional scratch.
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Fig. 8. Comparison of the coefficient of friction between the UVAS process and the traditional scratch process.
3.2. Coefficient of friction The COF for each sample was obtained from the formula k ¼ Ft/Fn, in which Ft represented tangential load and Fn was the axial load. Studies had shown that the COF of scratches could reveal the properties of material removal [27]. When the COF was small and the diamond indenter was sharp, the quality of the scratch was high. In contrast, if the COF was large and the diamond indenter was passivated, the scratch load increased dramatically and the head would be worn quickly. The COF of scratches formed by UVAS rapidly changed from 0.1 to 0.5, while that of scratches formed traditionally were consistently around 0.5. The enlarged view in Fig. 8 shows that the COF of scratch formed traditionally was relatively more stable and larger than that of scratch formed by UVAS in a small time window. The COF of the scratches formed by the UVAS process fluctuated within a certain range. Because the indenter kept intermittent contact with the workpiece during the UVAS process, the tangential load Ft and axial load Fn also changed periodically. As a result, the COF of the scratch formed by UVAS fluctuated more violently than that of the scratch fabricated by a traditional method. Based on the equation k¼ Ft/Fn, the COF was determined by the ratio of Ft to Fn. In the UVAS process, both the tangential and axial loads were smaller than those in the traditional scratch process. The tangential load was decreased to a greater extent than the axial load. Therefore, the COF of the UVAS process was smaller than that of the traditional scratch process.
3.3. Scratch morphology Fig. 9 compares the morphologies of the workpieces in the UVAS and traditional tests. For the workpieces scratched by the traditional method, the SiC particles were deformed and detached from the Al surface, so they could move and rotate easily. The workpiece surface exhibited many damaged areas and the groove was uneven. Under the same parameters, the workpiece surfaces subjected to the UVAS tests showed plastic material removal with lots of small chips. For particle-reinforced
Fig. 9. Comparison of the surface topography between the UVAS process and the traditional scratch process: (a) surface topography for traditional scratch and (b) surface topography for ultrasonic scratch.
composites, the synergy between the matrix and reinforcement affect cutting markedly [28]. During the traditional scratch process, the diamond indenter scratched the workpiece surface at a high speed and generated large stress, which caused the SiC particles to detach, move and rotate because of the different coefficients of thermal expansion of SiC particles and the Al matrix. The grinding force resulted in brittle damage to the surface. Conversely, during the UVAS process, the diamond indenter scratched the workpiece surface at a frequency of 20,000 times per second under ultrasonic longitudinal vibration. This greatly improved the surface quality and minimized the chips. Therefore, plastic material removal was observed on the workpiece surface after the UVAS process, resulting in good groove morphology.
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because the diamond indenter was a tetrahedral with a certain angle on both sides. However, because of the synergy between the Al matrix and SiC particles, the actual scratch depth was less than that expected from the amplitude and depth of ultrasonic vibration. 4. Conclusions
Fig. 10. The three-dimensional morphology of the scratch surface.
Traditional Ultrasonic
depth(µm)
12
8
4 0
100
200 300 400 distance(µm)
(1) Scratch loads showed obvious fluctuations in both the UVAS and traditional scratch tests. The average values during the UVAS tests were less than those of the traditional scratch tests. Axial loads were greater than tangential loads in both the UVAS and traditional scratch tests. (2) The COF of scratches fluctuated from 0.1 to 0.5 during the UVAS tests, whereas it remained around 0.5 for the traditional scratch tests. (3) Judging from the surface morphology, the SiC particles were more likely to be dislocated and deformed by loosening up, shifting and switching. There were a multiple of broken areas and the grooves were uneven. However, with the same parameters, the particles removed by the UVAS resulted in chips. Therefore, small chips were found on the workpiece. The ultrasonic vibration contributed to the depth and width of the scratches, both larger than those of the traditional ones.
500
Fig. 11. Comparison of the scratch depth between the UVAS process and the traditional scratch process.
The three-dimensional morphology of UVAS surface before making adjustment on amplitude is shown in Fig. 10. It can be clearly seen that the bottom of the groove is not flat and both sides bulge out. To further verify the influences of the workpiece material on ultrasonic amplitude, we measured the depth and width of the grooves, for which we chose the cross section perpendicular to the scratch direction. Ten sets of data were measured to ensure reliability and the results are shown in Fig. 11, from which we can see that the depth of UVAS was smaller than that of traditional scratch. It implied that the workpiece material affected the ultrasonic amplitude so that the actual depths of UVAS were smaller. Due to the fact that the indenter was tetrahedral and both sides had a certain angle, it was inevitable that the width became smaller as the depth decreased, which was caused by the rigid SiC particles in the composite and the deformation compatibility of aluminum matrix and SiC particles. The scratch width and depth by UVAS were larger than those of the scratch formed by the traditional method, indicating that the amount of material removed by the UVAS process was larger than that removed by the traditional one. It could be interpreted that the scratch depth of UVAS was larger than that formed by the traditional scratch process. As the indenter went deeper, the width of the scratch also increased
Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Grant no. 50975153), and the State Key Laboratory of Tribology Foundation (Grant no. SKLT08B11). References [1] B.G. Park, A.G. Crosky, A.K. Hellier, Fracture toughness of microsphere Al2O3–Al particulate metal matrix composites, Compos. Part B Eng. 39 (7–8) (2008) 1270–1279. [2] F.F. Xia, H.B. Xu, C. Liu, J.W. Wang, J.J. Ding, C.H. Ma, Microstructures of Ni–AlN composite coatings prepared by pulse electrodeposition technology, Appl. Surf. Sci. 271 (2013) 7–11. [3] A.B. Pandey, B.S. Majumdar, D.B. Miracle, Deformation and fracture of a particle-reinforced aluminum alloy composite: part I. Experiments, Metall. Mater. Trans. A 31 (3) (2000) 921–936. [4] R. Teti, Machining of composite materials, CIRP Ann. Manuf. Technol. 51 (2) (2002) 611–634. [5] Y. Wang, X.D. Liu, Present status and development trend of SiCp/Al composites, Res. Stud. Foundry Equip. 3 (2003) 18–22. [6] J.H. Xu, D.M. Zuo, M.D. Yang, Machining of metal matrix composites, Trans. Nanjing Univ. Aeronaut. Astronaut. 2 (1995) 161–167. [7] L. You, Study on Machining Properties of Ceramics Reinforced Aluminium Matrix Composites, Jimei University, Xiamen, 2012 pp. 97–99 (in Chinese). [8] K. Su, Study on the Ultrasonic Assisted Grinding Processing Technology of SiCp/Al Composite, Dalian University of Technology, Dalian, 2012 pp. 76–78 (in Chinese). [9] C.L. Zhang, P.F. Feng, Z.J. Wu, D.W. Yu, Experimental investigation on surface roughness of KDP crystal processed with rotary ultrasonic face milling, Key Eng. Mater. 499 (2012) 223–228.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10817–10823 www.elsevier.com/locate/ceramint
Ultrasonic vibration-assisted scratch characteristics of silicon carbide-reinforced aluminum matrix composites Pingfa Fenga, Guiqiang Lianga,b,n, Jianfu Zhanga a
Department of Mechanical Engineering, Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipments and Control, Tsinghua University, Beijing 100084, PR China b College of Electrical Engineering, Binzhou Polytechnic, Binzhou 256603, PR China Received 27 February 2014; received in revised form 14 March 2014; accepted 16 March 2014 Available online 24 March 2014
Abstract The material removal of silicon carbide particle-reinforced aluminum (SiCp/Al) matrix composites during ultrasonic vibration-assisted scratch (UVAS) and traditional scratch tests was investigated by examining scratch load, coefficient of friction (COF) and scratch morphology. It indicated that the loads during both the UVAS and traditional tests fluctuated obviously. The average loads of the UVAS processes were lower than those of the traditional scratches. The COF caused by the traditional scratch process was more stable and larger than that of the ultrasonic one. Large particles were observed in the grooves formed by the traditional scratch method, whereas small chips of SiC were found in the scratches formed by UVAS, which indicated that the material removal ratio was higher for the scratches formed by UVAS than that of scratches formed by the traditional method. Ultrasonic vibration played an important role in reducing the grinding force and COF, as well as improving the morphology of the machined surfaces. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Ultrasonic vibration-assisted scratching; B. SiC/Al; C. Morphology
1. Introduction SiCp/Al is an aeronautical composite with high specific strength and stiffness, low coefficient of thermal expansion, high thermal conductivity and good wear resistance. SiCp/Al has been widely used in many fields including aerospace, automobile and optical precision instruments [1–3]. SiCp/Al has excellent mechanical properties, but it is typically difficult to process because of the SiC particles, which are known for their high hardness, strength and wear resistance [4]. This has limited the application of SiCp/Al. It is therefore important and meaningful to study the processing behavior of such materials. A great number of researches have been conducted on the processing of SiC/Al composites so far [5–8]. However, all of n Corresponding author at: College of Electrical Engineering, Binzhou Polytechnic, Binzhou 256603, PR China. Tel./fax.: þ 86 10 62781099. E-mail addresses: [email protected] (P. Feng), [email protected] (G. Liang).
http://dx.doi.org/10.1016/j.ceramint.2014.03.073 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
them only focused on optimizing the processing parameters and simulation analysis, which can only shed some light on the mechanism of macro material removal. The mechanism of microscopic material removal is yet to be studied. Ultrasonic vibration-assisted machining (UVAM) combines traditional processing and ultrasonic vibration. A large number of studies [9–11] have confirmed that UVAM is an effective method for the precision machining of ceramics, particle-reinforced composites and optical materials, and other difficult-to-process materials. Ultrasonic vibration can reduce the cutting load and improve the surface quality [12,13]. Azarhoushang et al. [14] drilled Inconel 1738-LC with high-frequency low-amplitude ultrasonic vibration. The results showed that the hole roundness, cylindricity and surface roughness were greatly improved using UVAM, with higher quality holes, smaller chipping sizes and less wear to the tools than traditional machining. Pei et al. [15] found that the surface quality of ceramics processed by rotary ultrasonic face milling was better compared with those formed by traditional processing. Experiments carried out by Zhao [16] on ceramic
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P. Feng et al. / Ceramics International 40 (2014) 10817–10823
Table 1 Physical characteristics of SiC and 2024Al-T6. Material
Elasticity modulus / GPa
Poisson's ratio
Yield stress / MPa
Density / kg/m3
Specific heat capacity / J/(kg 1C)
SiC 2024Al-T6
410 74.2
0.14 0.33
5700 345.4
3700 2780
670 875
Ultrasonic spindle
Diamond indenter
Dial gauge
Work piece Kistler 9257b
a single diamond indenter had not been investigated. Therefore, we compared the traditional scratch and UVAS tests in three ways: scratch loads, morphology and coefficient of friction. Various experiments were conducted in order to make comparative analysis on the mechanism of material removal between ultrasonic scratches and traditional scratches, we were able to comparatively analyze the material removal, COF and morphology of processed surfaces to determine the mechanism of material removal of particlereinforced aluminum matrix composites. 2. Experimental 2.1. Materials
Fig. 1. Schematic diagram of the experimental setup.
scratching with a single diamond with ultrasonic vibration assistance showed that surface damage was lessened by the presence of ultrasonic vibration. Besides, Chen designed and built an ultrasonic milling work table to perform tests on non-ablative carbon/carbon composite materials [17], which showed that the surface quality of ultrasonically machined work pieces was much higher with roughness (of Sa and Sq) decreasing by 16% to 36%, the force of the grinding tool reducing by approximately 44% in the feed direction and 46% in the pushing direction. Researchers have investigated the processing features and removal mechanism of particle-reinforced composites in scratch tests both theoretically and experimentally. Ghosh et al. [18] conducted scratch tests on ZrB2–SiC composites with a Berkovich nano indenter in 2008. Microstructure analysis revealed that the grooves contained many slip bands. The maximum shear stress occurred in front of the scratch tool and the largest tensile stress occurred behind it. Further static and dynamic indentation and ultra-high-speed scratch tests were performed by Ghosh et al. [19] to examine the inelastic deformation characteristics of ultra-high temperature ceramic composite, ZrB2–SiC, in 2009. They found that dynamic indentation could cause a large number of transgranular cracks and high-speed scratches could lead to severe brittleness removal. A custom-made diamond spherical indenter had been used in ultrasonic scratch tests on sapphire to reveal the material removal characteristics in some researches [20]. It was shown that ultrasonic assistance could effectively reduce scratching load and crack expansion. Extensive researches had revealed the material removal mechanism in scratch tests [21–23], but that of particle-reinforced metal matrix composites in ultrasonic vibration-assisted scratch tests with
The materials used for the tests were made of cast aluminum alloy and were first subjected to pressureless infiltration. As a result, the matrix materials were cast aluminum reinforced by SiC ceramic particles. The cast aluminum alloy had high plasticity and continuous plastic deformation characteristics. The SiC particles had an elasticity modulus of 420 GPa, Poisson's ratio of 0.15, particle volume fraction of 62% and average particle size of 40 μm, as shown in Table 1. The upper surface of SiC/Al was selected as the test surface and was cut into pieces with dimensions of 15 mm 35 mm 10 mm by wire cutters and an orientator. The samples were then polished, and a three-dimensional profilometer (Talysurf, CLI2000) was used to measure the surface roughness (Ra) of the substrates. After polishing, they had an Ra value of 100 nm. 2.2. Experiment setup and conditions The scratch equipment used in the tests was a rotary ultrasonic machine that mainly comprised an ultrasonic spindle system and a numerical control machining system. The maximum power of the rotary ultrasonic machine was 300 W. The ultrasonic generator could generate highfrequency axial vibration signals of 16.5–30 kHz, which were converted to mechanical vibrations through a transducer. The amplitude decreased with the increase of material hardness, but the impact of hardness was not very big. An amplitude transformer enlarged the amplitude to 1–20 μm and then passed it on to the indenter, which worked on the surface of the test samples at a very high frequency to achieve UVAS, as shown in Fig. 1. The resonance frequency between the indenter and ultrasonic equipment was checked before the tests. The ultrasonic frequency was set to 20 kHz and the amplitude was 4 μm. The diamond indenter was tetrahedral with a length of 200 μm. The two ends of the workpieces had a height
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Fig. 2. Installation position of the diamond indenter and workpiece: (a) traditional scratch and (b) ultrasonic scratch.
difference of 20 μm in order to get the critical cutting depth that the ultrasonic assistance could be effective. The indenter moved from the lower end to the higher end. It was shown that the actual cutting depth of UVAS was determined by the ultrasonic amplitude and the axial feeding action [24]. Therefore, the assistance of ultrasonic amplitude should be compensated in traditional scratches so as to achieve the same depth as in UVAS. First, based on the assumption that the effects of ultrasonic amplitude were negligible, the actual cutting depths of UVAS and traditional scratch were measured. Then, adjustment was made based on the results to get the same depths. The installation of the diamond indenter and workpiece is shown in Fig. 2. The upper surface of the workpieces was scratched. The single-particle diamond indenter was mounted in the chuck and the aluminum SiC workpiece was fixed on the worktable. The diamond indenter scratched the workpiece from right to left at different depths at a certain amplitude and frequency. The experimental parameters were as follows: scratch depth was 0–20 μm, the feeding speed was 24,000 mm/ min and the total length was 15 mm. By comparing the cutting depths as is shown in the following figures, we find those made by UVAS are smaller. It indicated that the workpiece material could affect the ultrasonic amplitude, as is shown in Fig. 3. Thus, we made up for it by the average value of the depth differences between the UVAS and traditional scratches. Assuming the starting position of the indenter for traditional scratch tests was M and we recorded it as Zm, while that for UVAS tests was N and we could put Zn ¼ Zm þ A, in which A represented the ultrasonic amplitude. In order to have the same cutting depth, the position of the indenter in UVAS tests should be Zn ¼ Zm þ A δ, where δ was average value of cutting depth differences and turns out to be δ¼ 1.2 μm.
Fig. 3. Comparison of the groove size between the UVAS process and the traditional scratch process: (a) traditional scratch and (b) ultrasonic scratch.
2.3. Experimental measurement A high-precision micro dynamometer, Kistler dynamometer, was used to record the data from the scratch tests. The data collection frequency was 9 kHz. The load, vibration frequency and scratch process were recorded during the scratch tests. A scanning electron microscope (SEM, Quanta200 FEG, FEI, The Netherlands) was used to analyze the morphology of the workpiece after processing under different conditions. Because SiC is not conductive, a three-dimensional white-light interfering profilometer was used to observe the sample surfaces. To ensure the reliability of the test data, three tests were carried out using each set of process parameters to get the average values. Traditional scratch tests were performed by switching off the ultrasonic generator. 3. Results and discussion 3.1. Scratch loads The normal loads (perpendicular to the scratching direction) were very small compared with the axial (perpendicular to the workpiece surface) and tangential (parallel to the workpiece surface) loads in UVAS tests. Therefore, we only analyzed the axial and tangential loads in this paper. Fig. 4 shows the loads in UVAS and traditional scratch tests at the same speed. Because data were not reliable at the beginning and end of the
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Traditional Axial load
Scratch loads (N)
20
Ultrasonic Axial load
15
10
5
0
Traditional Tangential load Ultrasonic Tangential load 3 6 Scratch length (mm)
9
Fig. 4. Comparison of the scratch loads between the UVAS process and the traditional scratch process.
8 Axial loads FZ(N)
Traditional 6
Ultrasonic
4
2 0
50 100 Scratch length (µm)
150
Fig. 5. Comparison of the loads between the UVAS process and the traditional scratch process.
scratching process, only the middle part of each scratch was selected for analysis. The loads in both UVAS and traditional scratch tests fluctuated, which was mainly a result of the diamond indenter scratching the SiC. Compared with those during the traditional scratch tests, loads were decreased in UVAS tests. The average axial load decreased by 28.4% and the average tangential load was reduced by 46.9%. The axial loads were greater than the tangential ones in both the UVAS and traditional tests. To study the mechanism of axial load reduction, we analyzed the axial loads on scratches with a length of 150 μm to study the microscale properties of each workpiece. As shown in Fig. 5, the two curves separately reflected the axial loads on SiC (with a length of 40 μm) in both the UVAS and traditional scratch tests. The load abruptly increased and decreased in certain areas during the traditional tests. In contrast, there was only small fluctuation of axial load in the UVAS tests. To further study the mechanism of scratch load reduction in UVAS testing, we observed the morphology of the workpieces with SEM. Fig. 6(a) and (b) shows the morphology of samples scratched with the traditional and UVAS methods, respectively. The workpiece from the traditional tests was damaged and crushed to a greater degree than that subjected to UVAS. Large particles on the workpiece from the UVAS tests were broken
Fig. 6. Comparison of the surface morphology between the UVAS process and the traditional scratch process: (a) surface morphology for traditional scratch and (b) surface morphology for ultrasonic scratch.
down into chips under the high-frequency ultrasonic vibration. During the scratching process in the UVAS test, both the chips and aluminum substrate were removed, so the load was more stable and smaller than that in the traditional tests. Large SiC particles were broken down into small chips under the high-frequency ultrasonic vibration. These chips adhered to the workpiece surface. The cyclical changes of pressure in dynamic mode made the chips circulate and suspend on the surfaces to form a transfer film, which reduced the friction between the head and the workpiece considerably [25]. This might explain why the ultrasound scratches need smaller tangential loads than traditional ones. Fig. 7 depicts the microscale morphology of the scratch formed using the traditional process. The indenter and the workpiece kept constant contact during the scratching process, which caused extrusion and abrasions in the side of the indenter and the workpiece, thereby increasing the tangential load. In the UVAS process, the high-frequency vibration of the diamond indenter at a certain amplitude ensured that the contact between the indenter and the workpiece was intermittent, which reduced friction. As a result, the tangential load was also lowered in the UVAS process. UVAS is characterized by its intermittency [26], and the scratching speed is at periodical changes. Under the influence of these two features, the impacts of the diamond indenter on the workpiece yielded a large number of microcracks, which facilitated material removal and the expansion of cracks, thereby reducing the average load of the scratches.
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Fig. 7. Microstructure for traditional scratch.
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Fig. 8. Comparison of the coefficient of friction between the UVAS process and the traditional scratch process.
3.2. Coefficient of friction The COF for each sample was obtained from the formula k ¼ Ft/Fn, in which Ft represented tangential load and Fn was the axial load. Studies had shown that the COF of scratches could reveal the properties of material removal [27]. When the COF was small and the diamond indenter was sharp, the quality of the scratch was high. In contrast, if the COF was large and the diamond indenter was passivated, the scratch load increased dramatically and the head would be worn quickly. The COF of scratches formed by UVAS rapidly changed from 0.1 to 0.5, while that of scratches formed traditionally were consistently around 0.5. The enlarged view in Fig. 8 shows that the COF of scratch formed traditionally was relatively more stable and larger than that of scratch formed by UVAS in a small time window. The COF of the scratches formed by the UVAS process fluctuated within a certain range. Because the indenter kept intermittent contact with the workpiece during the UVAS process, the tangential load Ft and axial load Fn also changed periodically. As a result, the COF of the scratch formed by UVAS fluctuated more violently than that of the scratch fabricated by a traditional method. Based on the equation k¼ Ft/Fn, the COF was determined by the ratio of Ft to Fn. In the UVAS process, both the tangential and axial loads were smaller than those in the traditional scratch process. The tangential load was decreased to a greater extent than the axial load. Therefore, the COF of the UVAS process was smaller than that of the traditional scratch process.
3.3. Scratch morphology Fig. 9 compares the morphologies of the workpieces in the UVAS and traditional tests. For the workpieces scratched by the traditional method, the SiC particles were deformed and detached from the Al surface, so they could move and rotate easily. The workpiece surface exhibited many damaged areas and the groove was uneven. Under the same parameters, the workpiece surfaces subjected to the UVAS tests showed plastic material removal with lots of small chips. For particle-reinforced
Fig. 9. Comparison of the surface topography between the UVAS process and the traditional scratch process: (a) surface topography for traditional scratch and (b) surface topography for ultrasonic scratch.
composites, the synergy between the matrix and reinforcement affect cutting markedly [28]. During the traditional scratch process, the diamond indenter scratched the workpiece surface at a high speed and generated large stress, which caused the SiC particles to detach, move and rotate because of the different coefficients of thermal expansion of SiC particles and the Al matrix. The grinding force resulted in brittle damage to the surface. Conversely, during the UVAS process, the diamond indenter scratched the workpiece surface at a frequency of 20,000 times per second under ultrasonic longitudinal vibration. This greatly improved the surface quality and minimized the chips. Therefore, plastic material removal was observed on the workpiece surface after the UVAS process, resulting in good groove morphology.
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because the diamond indenter was a tetrahedral with a certain angle on both sides. However, because of the synergy between the Al matrix and SiC particles, the actual scratch depth was less than that expected from the amplitude and depth of ultrasonic vibration. 4. Conclusions
Fig. 10. The three-dimensional morphology of the scratch surface.
Traditional Ultrasonic
depth(µm)
12
8
4 0
100
200 300 400 distance(µm)
(1) Scratch loads showed obvious fluctuations in both the UVAS and traditional scratch tests. The average values during the UVAS tests were less than those of the traditional scratch tests. Axial loads were greater than tangential loads in both the UVAS and traditional scratch tests. (2) The COF of scratches fluctuated from 0.1 to 0.5 during the UVAS tests, whereas it remained around 0.5 for the traditional scratch tests. (3) Judging from the surface morphology, the SiC particles were more likely to be dislocated and deformed by loosening up, shifting and switching. There were a multiple of broken areas and the grooves were uneven. However, with the same parameters, the particles removed by the UVAS resulted in chips. Therefore, small chips were found on the workpiece. The ultrasonic vibration contributed to the depth and width of the scratches, both larger than those of the traditional ones.
500
Fig. 11. Comparison of the scratch depth between the UVAS process and the traditional scratch process.
The three-dimensional morphology of UVAS surface before making adjustment on amplitude is shown in Fig. 10. It can be clearly seen that the bottom of the groove is not flat and both sides bulge out. To further verify the influences of the workpiece material on ultrasonic amplitude, we measured the depth and width of the grooves, for which we chose the cross section perpendicular to the scratch direction. Ten sets of data were measured to ensure reliability and the results are shown in Fig. 11, from which we can see that the depth of UVAS was smaller than that of traditional scratch. It implied that the workpiece material affected the ultrasonic amplitude so that the actual depths of UVAS were smaller. Due to the fact that the indenter was tetrahedral and both sides had a certain angle, it was inevitable that the width became smaller as the depth decreased, which was caused by the rigid SiC particles in the composite and the deformation compatibility of aluminum matrix and SiC particles. The scratch width and depth by UVAS were larger than those of the scratch formed by the traditional method, indicating that the amount of material removed by the UVAS process was larger than that removed by the traditional one. It could be interpreted that the scratch depth of UVAS was larger than that formed by the traditional scratch process. As the indenter went deeper, the width of the scratch also increased
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