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Study on design and performance of metal-bonded diamond grinding wheels fabricated by selective laser melting (SLM)
Materials and Design 156 (2018) 52–61
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Study on design and performance of metal-bonded diamond grinding wheels fabricated by selective laser melting (SLM) Chenchen Tian a,b, Xuekun Li a,b,c,⁎, Shubo Zhang a,b, Guoqiang Guo d, Liping Wang a,b,c, Yiming Rong e a
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipment and Control, Tsinghua University, Beijing 100084, China State Key Lab of Tribology, Tsinghua University, Beijing 100084, China d Shanghai Spaceflight Precision Machinery Institute, Shanghai 201600, China e Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A novel method is proposed to fabricate porous metal-bonded grinding wheels based on selective laser melting technology. • The novel wheels possess excellent selfsharpening ability, controllable pore structure and good grinding performance. • Porous structures of grinding wheels have a significant influence on their grinding performance. • Compared with electroplated wheel, better machined surface quality can be achieved by the novel wheels.
a r t i c l e
i n f o
Article history: Received 12 May 2018 Received in revised form 12 June 2018 Accepted 15 June 2018 Available online 18 June 2018 Keywords: Grinding wheel Selective laser melting 3D printing Porous structure Metal bond Grinding performance
a b s t r a c t Porous metal-bonded grinding wheels with high porosity and controllable pore shape, size, quantity and distribution are in great demand but hard to realize for traditional process. In this study, the novel method is proposed to fabricate porous metal-bonded grinding wheels based on selective laser melting technology. Three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated using this novel method. Additionally, morphological characterization is conducted in terms of porous structure, microstructure and bonding condition. Furthermore, grinding performance is evaluated in terms of grinding force, material removal rate, ground surface roughness and hardness. Experimental results indicate that the SLM-fabricated grinding wheels possess the excellent dressing and self-sharpening ability, good bonding strength, controllable pore structure, high porosity and good comprehensive grinding performance. Compared with electroplated wheel, better machined surface roughness and slighter work hardening can be achieved by the SLM-fabricated wheels based on the similar material removal capacity. Among the SLM-fabricated wheels, octahedron structure wheel possesses the smallest extrusion and friction force, the highest instantaneous cutting force and the slightest work hardening, and solid structure wheel possesses the largest material removal rate. © 2018 Elsevier Ltd. All rights reserved.
⁎ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected], (C. Tian), [email protected], (X. Li), [email protected], (S. Zhang), [email protected], (G. Guo), [email protected], (L. Wang), [email protected] (Y. Rong).
https://doi.org/10.1016/j.matdes.2018.06.029 0264-1275/© 2018 Elsevier Ltd. All rights reserved.
C. Tian et al. / Materials and Design 156 (2018) 52–61
1. Introduction Metal-bonded grinding wheels are widely used in high efficiency and precision grinding of difficult-to-cut material such as cemented carbide [1], optical glass [2], engineering ceramic [3], nickel-based superalloy [4] and metal matrix composites (MMCs) [5]. Compared to resinbonded and vitrified-bonded grinding wheels, metal-bonded grinding wheels have many advantages, such as high holding force of abrasives, high bonding strength, high wear resistance, good shape retention, high durability and good loading capacity [6, 7]. However, due to the lack of pores, traditional metal-bonded grinding wheels face the challenges such as severe blockage, bad self-sharpening, poor truing and dressing, and even burn of workpiece surface [8–11]. Hot pressing is widely used in the fabrication of sintered metalbonded grinding wheels. However, because of its essence of passive formation, the pore shape, size, quantity and distribution cannot be controlled very well [10]. In addition, porosity will be restricted by the natural packing density of grains [12]. Furthermore, core elements of grinding wheels, such as bond, abrasive grain and porosity are coupled together and restrict each other during hot pressing process. For example, pore structure and porosity are influenced and restricted by bond and abrasive grain during passive compression forming process so that they cannot be controlled very accurately. In the past decades, the porous wheels are mainly fabricated by inorganic pore former (such as NH4HCO3, graphite and TiH2) [6] or organic pore former (such as granulated sugar, phenol resin and naphthalene) [13] or alumina bubble particles [10, 14, 15] or selective extraction with dense carbon dioxide [12, 16], and corresponding pore former removal techniques include sintering-dissolution, thermally stimulated decomposition, thermally melted elimination and embedding cenosphere technique [17]. However, the porosity cannot be increased to a high level based on hot pressing process because the natural packing behavior restricts the porosity. Furthermore, the shape, size and distribution of pores are not uniform [6]. Therefore, novel methods are in great demand to fabricate metalbonded grinding wheels with controllable pore shape, size, quantity and distribution. In this paper, a novel method is proposed to fabricate porous metalbonded grinding wheel based on selective laser melting technology. Numerous advantages can be realized with this novel fabrication method for metal-bonded grinding wheels. Firstly, it can improve the design freedom of pore structure and formula of grinding wheel, which can be optimized according to the specific working condition. Secondly, it can improve the porosity to a very high level. Thirdly, it can control the pore shape, size, quantity and distribution actively and accurately. Finally, it can decouple the coupling relationship of abrasive grain, bond and porosity. In this paper, three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated by selective laser melting technology. Additionally, morphological characterization, including porous structure, microstructure and bonding condition characterization, are
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performed via optical microscope, scanning electron microscope and energy dispersive spectroscopy. Furthermore, grinding performances of the SLM-fabricated wheels and electroplated wheel are evaluated and compared with each other in terms of grinding force, material removal rate, ground surface roughness and hardness. The results reveal that the SLM-fabricated grinding wheels possess excellent dressing and self-sharpening ability, controllable pore structure and porosity, as well as good comprehensive grinding performance. 2. Grinding wheel design and fabrication 2.1. Grinding wheel design In this study, three kinds of structures for grinding wheels are designed, including one 3D cellular structure – octahedron structure, one 2D porous structure – honeycomb structure and one solid structure. The CAD models for three kinds of grinding wheels are depicted in Fig. 1 and design parameters of grinding wheels are clarified in Table 1, including design porosity, outer diameter, inner diameter and wheel height. The dimension of octahedron cellular unit is defined as 2.83 ∗ 2.83 ∗ 2.83 mm and strut diameter is defined as 0.8 mm, making the porosity to a high level. The octahedron structure wheel is obtained by duplicating the cellular unit and then cut into a cylinder. The honeycomb structure wheel is obtained by slotting on the entity with regular hexahedral slot. The side length value of regular hexagon is defined as 0.58 mm and slot depth is defined as 6 mm. 2.2. Material preparation Diamond abrasive grains with grain size ranging from 62 μm to 75 μm and AlSi10Mg alloy powder with particle size ranging from 15 μm to 53 μm are well mixed into a mixture, shown in Fig. 2. The mixed powder is used in the fabrication of three kinds of grinding wheels based on SLM process. The abrasive concentration is defined as 60% for grinding wheel, so the volume fractions of abrasive grain and Al-based bond are 15% and 85% respectively. 2.3. Grinding wheel fabrication Three kinds of grinding wheels are fabricated in the SLM machine (AFS-M120, Beijing Longyuan AFS Corporation) based on the mixed powder. The SLM fabrication principle is depicted in Fig. 3 and fabrication parameters are illustrated in Table 2. The SLM machine consists of powder delivery system and laser scanner system. The laser beam will melt the AlSi10Mg alloy powder selectively layer by layer according to the CAD data. Then the melted alloy powder will solidify surrounding the diamond abrasive grain and become the bond of grinding wheel. The fabrication is performed in an inert atmosphere and wheel head is built on the top of the aluminum substrate. After fabrication, the wheel head is removed from the substrate using wire electro discharge machining. Finally, the wheel head is assembled to the cemented
Fig. 1. CAD models of grinding wheels: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel.
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Table 1 Design parameters of grinding wheels. Grinding wheel
Design porosity [%]
Outer diameter [mm]
Inner diameter [mm]
Wheel height [mm]
Abrasive concentration [%]
Octahedron structure wheel Honeycomb structure wheel Solid structure wheel
35 16 0
12 12 12
6 6 6
12 12 12
60 60 60
carbide rod using glue and the grinding wheel is completed, shown in Fig. 4. Therefore, the SLM-fabricated grinding wheels possess controllable pore structure and porosity with the help of SLM technology.
microscope. The surface to be observed should be polished and etched by Keller's reagent. 3.3. Bonding condition characterization
3. Morphological characterization 3.1. Wheel structure characterization
To investigate the bonding condition of diamond abrasive grain and Al-based bond, grain morphology and interface property are studied based on SEM images and EDS results.
To observe the surface topography of grinding wheel before and after grinding process, SEM images are derived from Field Emission Scanning Electron Microscope (Merlin compact, ZEISS Corporation).
4. Grinding performance test 4.1. Grinding platform
3.2. Microstructure characterization To observe microstructure and phase composition of SLM-fabricated composite, metallographic photos are obtained by using optical
Grinding experiments are performed in a jig grinder (Moore G48), shown in Fig. 5a. Before grinding procedure, the grinding wheels are dressed in the tool grinder using silicon carbide dressing wheel, shown in Fig. 5b. Dressing of SLM-fabricated wheel is much easier than conventional metal-bonded wheel due to the soft Al-based bond and high porosity. As depicted in Fig. 5a, the workpiece is fixed on the Kistler force sensor by the fixture and the sensor is mounted on the worktable by the vice. The coordinate system indicates the force direction of the sensor. For the grinding wheel, x y and z axis forces represent the tangential force, radial force and axial force respectively. 4.2. Grinding procedure
Fig. 2. Morphology of mixed powder used in SLM process.
Time dependent performance and depth dependent performance are investigated in this paper. Grinding and dressing parameters are illustrated in Table 3. To investigate the time dependent performance of grinding wheel, process data, such as grinding force, material removal rate, surface roughness and hardness, is needed. Therefore, some step surfaces are retained for the measurement of these data. The time dependent
Fig. 3. Principle of SLM process.
C. Tian et al. / Materials and Design 156 (2018) 52–61 Table 2 SLM fabrication parameters. Symbol
Parameter
Value
Unit
λ P v O δ
Laser wavelength Laser power Laser scanning speed Overlap rate Powder layer thickness
1064 300 2.5 30 30
nm W m/s % μm
grinding procedure is depicted in Fig. 6. During grinding process, the motion of grinding wheel can be decomposed into three parts: wheel rotation around z axis, vertical reciprocating feed motion along z axis and horizontal feed motion along x axis. The concrete procedure is clarified as follows: firstly, the 10 mm long base surface, marked as 0, is reserved along x direction from the end of workpiece and tool setting is completed. Secondly, 20 μm grinding depth is applied along y direction and the first single pass grinding is finished. Thirdly, the grinding wheel moves back to the original position and another 20 μm grinding depth is applied, then the second single pass grinding is finished. Fourthly, single pass grinding is repeated for five times, resulting in the whole grinding depth of 100 μm, creating the first step surface marked as 1, shown in Fig. 6a and b. As for the second step surface, another 10 mm length is reserved along x direction and above procedure is repeated, creating the second step surface marked as 2, shown in Fig. 6c. Finally, the third
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step surface marked as 3 is finished via repeating the same procedure, shown in Fig. 6d. The final shape of workpiece is presented in Fig. 6e. The whole grinding procedure is repeated for octahedron structure wheel, honeycomb structure wheel, solid structure wheel and electroplated wheel, respectively. To investigate the influence of grinding depth on the grinding performance, another depth dependent grinding experiment is conducted. The basic principle and procedure are identical with time dependent grinding experiment. However, the differences are grinding depth and corresponding feed times. The first step surface adopts 10 μm per pass and repeats for two times, creating the whole grinding depth of 20 μm. The second, third and fourth step surfaces adopt 20 μm, 30 μm and 40 μm per pass respectively, and then repeat for two times as well. The depth dependent grinding procedure is also repeated for four kinds of grinding wheels respectively. 4.3. Measurement method To reveal the influence of grinding time, grinding depth and wheel structure on the grinding performance, grinding force, material removal rate, ground surface roughness and hardness are measured. Grinding force is measured by the three-axis force sensor (Kistler 9256C2), charge amplifier (Kistler 5080) and data acquisition equipment. Force sensor adopts piezoelectric crystal to transform the force signal into the charge signal, which is amplified and converted into voltage signal
Fig. 4. SLM-fabricated grinding wheel before dressing: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel.
Fig. 5. Grinding platform: a) jig grinder for grinding and b) tool grinder for dressing.
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Table 3 Grinding and dressing parameters. Grinding & dressing parameter
Value
Workpiece material Workpiece dimension Vertical reciprocating frequency Grinding wheel speed Horizontal feed speed Grinding depth
Cr4W2MoV cold die steel (quench hardened) 100 mm (L) × 40 mm (W) × 15 mm (H) 2.6 Hz 12 m/s 10 mm/min Time dependent: 20 μm Depth dependent: 10, 20, 30, 40 μm Silicon carbide dressing wheel 0.8 Air cooling
Dressing tool Speed rate Coolant
ranging from −5 V to 5 V, and then converted to the digital signal via analog to digital converter. Material removal depth is measured by the ultra-depth three-dimensional microscopy (Keyence, VHX – 6000) and material removal rate is calculated considering the grinding area and grinding time. Ground surface roughness is measured by the optical surface profiler (ZYGO NewView 7300), which utilizes scanning white light interferometry to image and measure the microstructure and topography of surfaces in three dimensions. Ground surface hardness is measured by the hardness tester (Qness Q60A+) via measuring the micro Vickers hardness. Test points are hit on the specimen surface by penetrator with a pressing force of 1 kg and duration time of 10 s.
pressing grinding wheel, during grinding process, chip flute will occur around diamond abrasive grain on the surface of SLM-fabricated grinding wheel. The self-formed chip flute, presented in Fig. 7f, results from the cutting effect of grinding chip. The cutting edges of diamond abrasive grain are engaged with the workpiece and chips are formed. Furthermore, the chips are harder than Al-based bond, so they take some material of bond around diamond abrasive grain, resulting in the formation of chip flute shown in Fig. 7f. The phenomenon is unique for SLMfabricated grinding wheel and it can benefit the grinding performance because of large chip space and protrusion height. Therefore, the SLMfabricated grinding wheels possess excellent dressing and selfsharpening ability, which are very difficult to realize for hot pressing metal-bonded grinding wheels. To characterize the unique property of SLM-fabricated composite material, microstructures of side plane (parallel to the building direction) and scanning plane (perpendicular to the building direction) are observed. From the optical images shown in Fig. 8, the microstructures of side plane and scanning plane of SLM-fabricated two-phase composite have different patterns in terms of structure composition and void defect distribution. Side plane is characterized by scaly structures representing melting pools while scanning plane consists of interlaced and short strip structures representing scanning paths. Additionally, more void defects are formed in scanning plane compared with side plane. Therefore, SLM-fabricated composite shows strong anisotropy. 5.2. Bonding property
5. Results and analysis 5.1. Morphological property Surface topography of grinding wheels is obtained via SEM images shown in Fig. 7. It indicates the surface topography of octahedron structure wheel, honeycomb structure wheel and solid structure wheel in Fig. 7a, b and c respectively. The structures of SLM-fabricated grinding wheels are identical with CAD models depicted in Fig. 1 considering the fabrication accuracy. Fig. 7d and e represents the surface topography of solid structure wheel before and after grinding process respectively. After grinding process, the surface of SLM-fabricated grinding wheel becomes smoother due to the bond wear. Different from traditional hot
To investigate the bonding condition between diamond abrasive grain and Al-based bond, the morphology of grain and energy spectrum curve of the interface are obtained via SEM and EDS, shown in Fig. 9. Most abrasives are firmly embedded into bond and some of them are wrapped with bond, depicted in Fig. 9a, which indicates good infiltration between diamond and melted AlSi10Mg alloy. As presented in Fig. 9b, the transition regions (about 8–10 μm width) both exist for aluminum and carbon elements at the interfaces, suggesting the formation of C\\Al compound, which enhances the bonding strength of diamond abrasives. The C\\Al compound may be ionic compound Al4C3 with high hardness, shear strength and melting point, which benefits the bonding strength [18].
Fig. 6. Time dependent grinding procedure.
C. Tian et al. / Materials and Design 156 (2018) 52–61
(a)
(b)
500μm
(c)
500μm
(d)
500μm
(e)
(f)
200μm
200μm
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80μm
Fig. 7. Surface topography of grinding wheels: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel; d) wheel surface after dressing; e) wheel surface after grinding; f) chip flute formed during grinding process.
5.3. Grinding chips To study the grinding mechanism of SLM-fabricated grinding wheel, the morphology of chips is observed via SEM images after grinding process. The formation process of grinding chips can be divided into three stages: sliding, plowing and cutting. Because most abrasive grains are engaged in the cutting process with negative rake angle, most chips are segmental chips formed in grinding process, whose bottom surface is smooth but back surface presents sawtooth shape as shown in Fig. 10. Grinding chips to be observed possess various shapes, including C shape segmental chip (Fig. 10b), ribbon segmental chip (Fig. 10c), spiral segmental chip (Fig. 10d) and fracture segmental chip (Fig. 10e). The morphology of grinding chips indicates good cutting ability of SLMfabricated grinding wheel.
5.4. Grinding force Force sensor and signal acquisition equipment are used to record the three-axis grinding force during grinding process with different grinding wheels and grinding parameters. Because of the unique grinding motion in the experiments, force signals are periodic. As presented in Fig. 5, x y and z axis forces represent tangential force, radial force and axial force respectively. Tangential force represents the horizontal
(a)
Side plane
cutting force of abrasive grain and horizontal frictional force of bond. Radial force represents the normal force between wheel surface and workpiece caused by extrusion deformation. Axial force represents the vertical cutting force of abrasive grain and vertical frictional force of bond. Therefore, tangential force (x axis force) is taken as an example for the signal analysis. When grinding process is stable, a segment of x axis force original signal is intercepted and presented in Fig. 11a. The corresponding frequency spectrum is obtained through fast Fourier transform (FFT), presented in Fig. 11b. Several key frequency components exist in the frequency spectrum, including 2.6 Hz, 617 Hz, 50 Hz and its high order harmonic components. First of all, 50 Hz represents powder frequency and can be removed by band stop filter. Additionally, 2.6 Hz represents the frequency of vertical reciprocating motion while 617 Hz represents the frequency of wheel rotation. Because of the pneumatic spindle, wheel rotating speed is not controlled very accurately. So this frequency fluctuates in the range from 600 Hz to 700 Hz, corresponding with the wheel rotating speed of 40,000 r/min. Furthermore, low frequency (0 to 20 Hz) signal and high frequency (550 to 650 Hz) signal are decomposed and derived from the original signal through the low pass filter and band pass filter, depicted in Fig. 11c and d respectively. Low frequency signal reveals the force induced by the frictional effect between bond and workpiece while high frequency signal reveals the force induced by the instantaneous cutting effect between grain and workpiece. The low frequency signal and high frequency signal
(b)
Scanning plane
Melng pool Building direcon
Scanning path
500μm Void defect
500μm Diamond abrasive
Fig. 8. Microstructure of SLM-fabricated composite material: a) microstructure of side plane; b) microstructure of scanning plane.
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Fig. 9. Bonding condition between diamond abrasive grain and Al-based bond: a) SEM images; b) Line scanning energy spectrum of the grain-bond interface.
presented in Fig. 11c and d are intercepted from the same signal segment, so they are corresponding to each other. When the grinding wheel locates in the highest position, the low frequency force and high frequency force are both the smallest. When the grinding wheel moves down to the middle position of workpiece, the low and high frequency forces increase to the middle values. When the grinding wheel continues to moves down to the lowest position, the low and high frequency forces are both up to the largest. When the grinding wheel moves up to the middle position of workpiece, the low and high frequency forces decrease to the middle values again. When the grinding wheel continues to moves up to the highest position, the low and high frequency forces decrease to the lowest values, same as the original condition. Therefore, low frequency force and high frequency force are corresponding to the wheel position in one period. This phenomenon can be explained as follows: the grinding wheel can be regarded as a cantilever beam and deflection can be generated due to the grinding force. Moreover, the action point of force varies based on the wheel position. The deflection caused by bending moment will decrease as the wheel moves down due to the reduction of force arm, shown as L1, L2 and L3 in Fig. 11d, leading to larger practical grinding depth and larger extrusion pressure as well as larger cutting force. As shown in Fig. 11c and d, the peak value of low frequency signal (LFP) and peak value of high frequency signal (HFP) can be used as two metrics for the comparison of different grinding wheels. They represent the extrusion or friction force between bond and workpiece and the instantaneous cutting force of abrasive grains respectively. The comparison of different grinding wheels based on the two metrics is clarified
in Fig. 12. As presented in Fig. 12, some useful findings can be obtained. Firstly, grinding forces of electroplated wheel, including LFP and HFP, are smaller than SLM-fabricated wheels for every axis force. It means the electroplated wheel has advantages on the grinding force. Secondly, all the forces increase as grinding depth increases, which is the same as the traditional wheel. Thirdly, among the SLM-fabricated wheels, octahedron structure wheel possesses the smallest LFP but the largest HFP while solid structure wheel possesses the largest LFP but the smallest HFP for every axis force. This phenomenon can be explained as follows: LFP represents the extrusion and friction force of bond while HFP represents the instantaneous cutting force of grains. Octahedron structure wheel possesses the largest porosity and smallest contact area when grinding, so the extrusion and friction force contributed by the bond is the smallest. However, the quantity of the abrasive grains engaged in the cutting process is the smallest for the octahedron structure wheel, so the material removal amount for single grit is the biggest, leading to the largest instantaneous cutting force. 5.5. Material removal rate To compare the cutting ability of different kinds of grinding wheels, material removal rate is measured at grinding time of 27, 51 and 72 min. As presented in Fig. 13a, depth difference between two adjacent step surfaces represents the depth of material removal, and measured by ultra-depth three-dimensional microscope. Then the material removal rate is calculated and summarized in Fig. 13b. As presented in Fig. 13b, as time goes by, the material removal rate decreases slowly for all
Fig. 10. Shape of chips formed in grinding process: a) a pile of chips, segmental chips; b) C shape segmental chip; c) ribbon segmental chip; d) spiral segmental chip; e) fracture segmental chip.
C. Tian et al. / Materials and Design 156 (2018) 52–61
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Fig. 11. Grinding force signal analysis (x force e.g.): a) a segment of original force signal; b) frequency spectrum of original signal; c) one period low frequency signal (0–20 Hz); d) one period high frequency signal (550–650 Hz).
kinds of grinding wheels because of the wheel wear. Additionally, very small difference exists for the SLM-fabricated wheels and electroplated wheel in terms of material removal capacity. Moreover, among the SLM-
fabricated grinding wheels, solid structure wheel possesses the largest material removal rate due to the largest contact area and the largest quantity of grains engaged in the cutting process.
Fig. 12. Grinding force comparison of different structure grinding wheel: a) high frequency x axis force; b) high frequency y axis force; c) high frequency z axis force; d) low frequency x axis force; e) low frequency y axis force; f) low frequency z axis force.
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(a)
Grinding transion region
(b)
Material removal depth
Fig. 13. Material removal rate: a) 3D morphology of ground step surface; b) material removal rate comparison of different grinding wheels.
5.6. Surface roughness Surface roughness is a significant index to the ground surface quality. Thus, the measurement of machined surface roughness is performed based on the optical surface profiler, whose results are demonstrated in Fig. 14. Three roughness values are tested for one ground surface and average is calculated to decrease the measurement error. As presented in Fig. 14, it is obvious that the surface roughness obtained by the SLM-fabricated grinding wheels is smaller than electroplated wheel by a large margin. The phenomenon can be explained as follows: Albased bond of SLM-fabricated wheel possesses relatively low elastic modulus compared to the Cu-based bond and Fe-based bond. Furthermore, porous structure reduces the elastic modulus of grinding wheel further. Therefore, the bonding model between Al-based bond and diamond abrasive grain can be simplified as spring damping model. So the grits will move in the radial direction due to the elastic deformation of bond when radial grinding force is applied. Moreover, radial moving distance is larger for the grits with larger protrusion height because of larger radial force, resulting in that all the grits assemble to the same cylindrical surface, which benefits the surface roughness further. Generally, machined surface roughness will increase as grinding time goes by or grinding depth increases, just like the electroplated wheel shown in Fig. 14a and b. However, this regularity is not applicable to the SLM-fabricated wheels. As presented in Fig. 14a, the common regularity for octahedron structure wheel, honeycomb structure wheel and solid structure wheel is that the ground surface roughness is not sensible to the grinding time. It means that surface roughness has a small
change as time goes by after the initial wheel wear, which indicates the excellent self-sharpening ability and anti-wear ability of SLMfabricated grinding wheel. Additionally, as presented in Fig. 14b, after the initial wheel wear, the surface roughness is relatively stable as grinding depth increases for all the SLM-fabricated wheels. It is unique because bigger grinding depth can be adopted regardless of surface roughness, which benefits the grinding efficiency.
5.7. Surface hardness Surface hardness is another index to the ground surface quality and it can reflect the degree of work hardening. The measurement of surface hardness is performed by the micro hardness tester and the test results are clarified in Fig. 15. First of all, the hardness of base surface is tested as the original hardness of workpiece (approximate 670 HV). As presented in Fig. 15, different degrees of work hardening happen for different grinding wheels. First of all, ground surface hardness increases as grinding time goes by due to the wear of grinding wheel. Moreover, the machined surface hardness of the SLM-fabricated grinding wheels is obviously less than that of electroplated wheel, indicating that slighter metal plastic deformation of workpiece exists when grinding with the SLM-fabricated wheels. Furthermore, the lowest ground surface hardness can be realized using octahedron structure wheel due to the smallest low frequency force and the smallest extrusion and friction effect. Also, the highest porosity and the biggest chip space help to reduce the temperature of contact area between wheel and workpiece.
Fig. 14. Surface roughness: a) as a function of grinding time; b) as a function of grinding depth.
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Fig. 15. Ground surface hardness test results for different kinds of grinding wheels.
6. Conclusion In this paper, three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated by selective laser melting technology. Additionally, morphological characterization, including porous structure, microstructure and bonding condition characterization, are performed via optical microscope, scanning electron microscope and energy dispersive spectroscopy. Furthermore, grinding performances of the SLM-fabricated wheels and electroplated wheel are evaluated and compared with each other in terms of grinding force, material removal rate, ground surface roughness and hardness. The main findings are summarized as follows: 1) The SLM-fabricated grinding wheels possess excellent dressing and self-sharpening ability, controllable pore structure and porosity as well as good comprehensive grinding performance. 2) Most abrasives are firmly embedded into bond and C\\Al compound is produced at the grain-bond interface, indicating good bonding condition. During grinding process, chip flutes will occur around grains of SLM-fabricated grinding wheel, benefiting the chip space and protrusion height. 3) Among SLM-fabricated wheels, octahedron structure wheel possesses the smallest LFP, the largest HFP and the slightest work hardening while solid structure wheel possesses the largest LFP, the smallest HFP, as well as the largest material removal rate. 4) Compared with electroplated wheel, better machined surface roughness and slighter work hardening can be achieved by the SLM-fabricated wheels based on the similar material removal capacity. Different from electroplated wheel, the surface roughness ground by SLM-fabricated wheels is not sensible to the grinding time and grinding depth. Acknowledgement The authors would like to thank the technical support from Beijing Longyuan AFS Co., Ltd., Kunshan Hiecise Heavy Machinery Co., Ltd. and Institute of Process Engineering, Chinese Academy of Science. The research is financially supported by National Science and Technology Major Project (No. 2017ZX04007001), Tsinghua University Initiative Scientific Research Program and Tsinghua-RWTH Aachen Collaborative Innovation Funding. References [1] Y. Zhan, X. Xu, An experimental investigation of temperatures and energy partition in grinding of cemented carbide with a brazed diamond wheel, Int. J. Adv. Manuf. Technol. 61 (2012) 117–125, https://doi.org/10.1007/s00170-011-3706-7.
[2] B. Guo, Q. Zhao, X. Fang, Precision grinding of optical glass with laser microstructured coarse-grained diamond wheels, J. Mater. Process. Technol. 214 (2014) 1045–1051, https://doi.org/10.1016/j.jmatprotec.2013.12.013. [3] B.K. Rhoney, A.J. Shih, R.O. Scattergood, J.L. Akemon, D.J. Gust, M.B. Grant, Wire electrical discharge machining of metal bond diamond wheels for ceramic grinding, Int. J. Mach. Tools Manuf. 42 (2002) 1355–1362, https://doi.org/10.1016/S0890-6955 (02)00056-1. [4] W. Ding, C. Dai, T. Yu, J. Xu, Y. Fu, Grinding performance of textured monolayer CBN wheels: undeformed chip thickness nonuniformity modeling and ground surface topography prediction, Int. J. Mach. Tools Manuf. 122 (2017) 66–80, https://doi.org/ 10.1016/j.ijmachtools.2017.05.006. [5] C. Liu, W. Ding, T. Yu, C. Yang, Materials removal mechanism in high-speed grinding of particulate reinforced titanium matrix composites, Precis. Eng. 51 (2018) 68–77, https://doi.org/10.1016/j.precisioneng.2017.07.012. [6] W. Ding, J. Xu, Z. Chen, C. Yang, C. Song, Y. Fu, Fabrication and performance of porous metal-bonded CBN grinding wheels using alumina bubble particles as pore-forming agents, Int. J. Adv. Manuf. Technol. 67 (2013) 1309–1315, https://doi.org/10.1007/ s00170-012-4567-4. [7] B. Denkena, T. Grove, I. Bremer, L. Behrens, Design of bronze-bonded grinding wheel properties, CIRP Ann. Manuf. Technol. 65 (2016) 333–336, https://doi.org/10.1016/j. cirp.2016.04.096. [8] H. Xu, C. Liao, Q. Weng, Experimental study on porous metal bonded diamond grinding wheels - the selection of porosity inducers and agglomeration's parameter, Adv. Mater. Res. 415-417 (2012) 594–597, https://doi.org/10.4028/www.scientific. net/AMR.415-417.594. [9] B. Denkena, T. Grove, V. Suntharakumaran, Porous metal bonds increase the resource efficiency for profile grinding, Procedia CIRP 69 (2018) 265–270, https:// doi.org/10.1016/j.procir.2017.10.004. [10] C. Song, W. Ding, J. Xu, Z. Chen, Grinding performance of metal-bonded CBN wheels with regular pores, Appl. Mech. Mater. 217–219 (2012) 1857–1862, https://doi.org/ 10.4028/www.scientific.net/AMM.217-219.1857. [11] X. Wang, R. Kang, W. Xu, L. Wang, D. Guo, Modeling of laser dressing for metal-bond diamond grinding wheel, Key Eng. Mater. 329 (2007) 145–150, https://doi.org/10. 4028/www.scientific.net/KEM.329.145. [12] T.D. Davis, J. DiCorleto, D. Sheldon, J. Vecchiarelli, C. Erkey, A route to highly porous grinding wheels by selective extraction of pore inducers with dense carbon dioxide, J. Supercrit. Fluids 30 (2004) 349–358, https://doi.org/10.1016/j.supflu.2003.09.011. [13] J. Mao, F. Zhang, G. Liao, Y. Zhou, H. Huang, C. Wang, S. Wu, Effect of granulated sugar as pore former on the microstructure and mechanical properties of the vitrified bond cubic boron nitride grinding wheels, Mater. Des. 60 (2014) 328–333, https://doi.org/10.1016/j.matdes.2014.04.006. [14] B. Zhao, T. Yu, W. Ding, X. Li, Effects of pore structure and distribution on strength of porous Cu-Sn-Ti alumina composites, Chin. J. Aeronaut. 30 (6) (2017) 2004–2015, https://doi.org/10.1016/j.cja.2017.08.008. [15] C. Ma, W. Ding, J. Xu, Y. Fu, Influence of alumina bubble particles on microstructure and mechanical strength in porous Cu–Sn–Ti metals, Mater. Des. 65 (2015) 50–56, https://doi.org/10.1016/j.matdes.2014.09.002. [16] T.D. Davis, D. Sheldon, C. Erkey, Highly porous vitrified bonded abrasives by the selective extraction of butyl carbamate from green grinding wheels with supercritical CO2, J. Am. Ceram. Soc. 88 (7) (2005) 1729–1733, https://doi.org/10.1111/j.15512916.2005.00276.x. [17] B. Zhao, A.K. Gain, W. Ding, L. Zhang, X. Li, Y. Fu, A review on metallic porous materials: pore formation, mechanical properties, and their applications, Int. J. Adv. Manuf. Technol. 95 (2018) 2641–2659, https://doi.org/10.1007/s00170-017-14156. [18] E.M. Sharifi, M.H. Enayati, F. Karimzadeh, Fabrication and characterization of AlAl4C3 nanocomposite by mechanical alloying, Int. J. Mod. Phys. Conf. Ser. 5 (2012) 480–487, https://doi.org/10.1142/S2010194512002371.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Study on design and performance of metal-bonded diamond grinding wheels fabricated by selective laser melting (SLM) Chenchen Tian a,b, Xuekun Li a,b,c,⁎, Shubo Zhang a,b, Guoqiang Guo d, Liping Wang a,b,c, Yiming Rong e a
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipment and Control, Tsinghua University, Beijing 100084, China State Key Lab of Tribology, Tsinghua University, Beijing 100084, China d Shanghai Spaceflight Precision Machinery Institute, Shanghai 201600, China e Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A novel method is proposed to fabricate porous metal-bonded grinding wheels based on selective laser melting technology. • The novel wheels possess excellent selfsharpening ability, controllable pore structure and good grinding performance. • Porous structures of grinding wheels have a significant influence on their grinding performance. • Compared with electroplated wheel, better machined surface quality can be achieved by the novel wheels.
a r t i c l e
i n f o
Article history: Received 12 May 2018 Received in revised form 12 June 2018 Accepted 15 June 2018 Available online 18 June 2018 Keywords: Grinding wheel Selective laser melting 3D printing Porous structure Metal bond Grinding performance
a b s t r a c t Porous metal-bonded grinding wheels with high porosity and controllable pore shape, size, quantity and distribution are in great demand but hard to realize for traditional process. In this study, the novel method is proposed to fabricate porous metal-bonded grinding wheels based on selective laser melting technology. Three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated using this novel method. Additionally, morphological characterization is conducted in terms of porous structure, microstructure and bonding condition. Furthermore, grinding performance is evaluated in terms of grinding force, material removal rate, ground surface roughness and hardness. Experimental results indicate that the SLM-fabricated grinding wheels possess the excellent dressing and self-sharpening ability, good bonding strength, controllable pore structure, high porosity and good comprehensive grinding performance. Compared with electroplated wheel, better machined surface roughness and slighter work hardening can be achieved by the SLM-fabricated wheels based on the similar material removal capacity. Among the SLM-fabricated wheels, octahedron structure wheel possesses the smallest extrusion and friction force, the highest instantaneous cutting force and the slightest work hardening, and solid structure wheel possesses the largest material removal rate. © 2018 Elsevier Ltd. All rights reserved.
⁎ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected], (C. Tian), [email protected], (X. Li), [email protected], (S. Zhang), [email protected], (G. Guo), [email protected], (L. Wang), [email protected] (Y. Rong).
https://doi.org/10.1016/j.matdes.2018.06.029 0264-1275/© 2018 Elsevier Ltd. All rights reserved.
C. Tian et al. / Materials and Design 156 (2018) 52–61
1. Introduction Metal-bonded grinding wheels are widely used in high efficiency and precision grinding of difficult-to-cut material such as cemented carbide [1], optical glass [2], engineering ceramic [3], nickel-based superalloy [4] and metal matrix composites (MMCs) [5]. Compared to resinbonded and vitrified-bonded grinding wheels, metal-bonded grinding wheels have many advantages, such as high holding force of abrasives, high bonding strength, high wear resistance, good shape retention, high durability and good loading capacity [6, 7]. However, due to the lack of pores, traditional metal-bonded grinding wheels face the challenges such as severe blockage, bad self-sharpening, poor truing and dressing, and even burn of workpiece surface [8–11]. Hot pressing is widely used in the fabrication of sintered metalbonded grinding wheels. However, because of its essence of passive formation, the pore shape, size, quantity and distribution cannot be controlled very well [10]. In addition, porosity will be restricted by the natural packing density of grains [12]. Furthermore, core elements of grinding wheels, such as bond, abrasive grain and porosity are coupled together and restrict each other during hot pressing process. For example, pore structure and porosity are influenced and restricted by bond and abrasive grain during passive compression forming process so that they cannot be controlled very accurately. In the past decades, the porous wheels are mainly fabricated by inorganic pore former (such as NH4HCO3, graphite and TiH2) [6] or organic pore former (such as granulated sugar, phenol resin and naphthalene) [13] or alumina bubble particles [10, 14, 15] or selective extraction with dense carbon dioxide [12, 16], and corresponding pore former removal techniques include sintering-dissolution, thermally stimulated decomposition, thermally melted elimination and embedding cenosphere technique [17]. However, the porosity cannot be increased to a high level based on hot pressing process because the natural packing behavior restricts the porosity. Furthermore, the shape, size and distribution of pores are not uniform [6]. Therefore, novel methods are in great demand to fabricate metalbonded grinding wheels with controllable pore shape, size, quantity and distribution. In this paper, a novel method is proposed to fabricate porous metalbonded grinding wheel based on selective laser melting technology. Numerous advantages can be realized with this novel fabrication method for metal-bonded grinding wheels. Firstly, it can improve the design freedom of pore structure and formula of grinding wheel, which can be optimized according to the specific working condition. Secondly, it can improve the porosity to a very high level. Thirdly, it can control the pore shape, size, quantity and distribution actively and accurately. Finally, it can decouple the coupling relationship of abrasive grain, bond and porosity. In this paper, three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated by selective laser melting technology. Additionally, morphological characterization, including porous structure, microstructure and bonding condition characterization, are
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performed via optical microscope, scanning electron microscope and energy dispersive spectroscopy. Furthermore, grinding performances of the SLM-fabricated wheels and electroplated wheel are evaluated and compared with each other in terms of grinding force, material removal rate, ground surface roughness and hardness. The results reveal that the SLM-fabricated grinding wheels possess excellent dressing and self-sharpening ability, controllable pore structure and porosity, as well as good comprehensive grinding performance. 2. Grinding wheel design and fabrication 2.1. Grinding wheel design In this study, three kinds of structures for grinding wheels are designed, including one 3D cellular structure – octahedron structure, one 2D porous structure – honeycomb structure and one solid structure. The CAD models for three kinds of grinding wheels are depicted in Fig. 1 and design parameters of grinding wheels are clarified in Table 1, including design porosity, outer diameter, inner diameter and wheel height. The dimension of octahedron cellular unit is defined as 2.83 ∗ 2.83 ∗ 2.83 mm and strut diameter is defined as 0.8 mm, making the porosity to a high level. The octahedron structure wheel is obtained by duplicating the cellular unit and then cut into a cylinder. The honeycomb structure wheel is obtained by slotting on the entity with regular hexahedral slot. The side length value of regular hexagon is defined as 0.58 mm and slot depth is defined as 6 mm. 2.2. Material preparation Diamond abrasive grains with grain size ranging from 62 μm to 75 μm and AlSi10Mg alloy powder with particle size ranging from 15 μm to 53 μm are well mixed into a mixture, shown in Fig. 2. The mixed powder is used in the fabrication of three kinds of grinding wheels based on SLM process. The abrasive concentration is defined as 60% for grinding wheel, so the volume fractions of abrasive grain and Al-based bond are 15% and 85% respectively. 2.3. Grinding wheel fabrication Three kinds of grinding wheels are fabricated in the SLM machine (AFS-M120, Beijing Longyuan AFS Corporation) based on the mixed powder. The SLM fabrication principle is depicted in Fig. 3 and fabrication parameters are illustrated in Table 2. The SLM machine consists of powder delivery system and laser scanner system. The laser beam will melt the AlSi10Mg alloy powder selectively layer by layer according to the CAD data. Then the melted alloy powder will solidify surrounding the diamond abrasive grain and become the bond of grinding wheel. The fabrication is performed in an inert atmosphere and wheel head is built on the top of the aluminum substrate. After fabrication, the wheel head is removed from the substrate using wire electro discharge machining. Finally, the wheel head is assembled to the cemented
Fig. 1. CAD models of grinding wheels: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel.
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Table 1 Design parameters of grinding wheels. Grinding wheel
Design porosity [%]
Outer diameter [mm]
Inner diameter [mm]
Wheel height [mm]
Abrasive concentration [%]
Octahedron structure wheel Honeycomb structure wheel Solid structure wheel
35 16 0
12 12 12
6 6 6
12 12 12
60 60 60
carbide rod using glue and the grinding wheel is completed, shown in Fig. 4. Therefore, the SLM-fabricated grinding wheels possess controllable pore structure and porosity with the help of SLM technology.
microscope. The surface to be observed should be polished and etched by Keller's reagent. 3.3. Bonding condition characterization
3. Morphological characterization 3.1. Wheel structure characterization
To investigate the bonding condition of diamond abrasive grain and Al-based bond, grain morphology and interface property are studied based on SEM images and EDS results.
To observe the surface topography of grinding wheel before and after grinding process, SEM images are derived from Field Emission Scanning Electron Microscope (Merlin compact, ZEISS Corporation).
4. Grinding performance test 4.1. Grinding platform
3.2. Microstructure characterization To observe microstructure and phase composition of SLM-fabricated composite, metallographic photos are obtained by using optical
Grinding experiments are performed in a jig grinder (Moore G48), shown in Fig. 5a. Before grinding procedure, the grinding wheels are dressed in the tool grinder using silicon carbide dressing wheel, shown in Fig. 5b. Dressing of SLM-fabricated wheel is much easier than conventional metal-bonded wheel due to the soft Al-based bond and high porosity. As depicted in Fig. 5a, the workpiece is fixed on the Kistler force sensor by the fixture and the sensor is mounted on the worktable by the vice. The coordinate system indicates the force direction of the sensor. For the grinding wheel, x y and z axis forces represent the tangential force, radial force and axial force respectively. 4.2. Grinding procedure
Fig. 2. Morphology of mixed powder used in SLM process.
Time dependent performance and depth dependent performance are investigated in this paper. Grinding and dressing parameters are illustrated in Table 3. To investigate the time dependent performance of grinding wheel, process data, such as grinding force, material removal rate, surface roughness and hardness, is needed. Therefore, some step surfaces are retained for the measurement of these data. The time dependent
Fig. 3. Principle of SLM process.
C. Tian et al. / Materials and Design 156 (2018) 52–61 Table 2 SLM fabrication parameters. Symbol
Parameter
Value
Unit
λ P v O δ
Laser wavelength Laser power Laser scanning speed Overlap rate Powder layer thickness
1064 300 2.5 30 30
nm W m/s % μm
grinding procedure is depicted in Fig. 6. During grinding process, the motion of grinding wheel can be decomposed into three parts: wheel rotation around z axis, vertical reciprocating feed motion along z axis and horizontal feed motion along x axis. The concrete procedure is clarified as follows: firstly, the 10 mm long base surface, marked as 0, is reserved along x direction from the end of workpiece and tool setting is completed. Secondly, 20 μm grinding depth is applied along y direction and the first single pass grinding is finished. Thirdly, the grinding wheel moves back to the original position and another 20 μm grinding depth is applied, then the second single pass grinding is finished. Fourthly, single pass grinding is repeated for five times, resulting in the whole grinding depth of 100 μm, creating the first step surface marked as 1, shown in Fig. 6a and b. As for the second step surface, another 10 mm length is reserved along x direction and above procedure is repeated, creating the second step surface marked as 2, shown in Fig. 6c. Finally, the third
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step surface marked as 3 is finished via repeating the same procedure, shown in Fig. 6d. The final shape of workpiece is presented in Fig. 6e. The whole grinding procedure is repeated for octahedron structure wheel, honeycomb structure wheel, solid structure wheel and electroplated wheel, respectively. To investigate the influence of grinding depth on the grinding performance, another depth dependent grinding experiment is conducted. The basic principle and procedure are identical with time dependent grinding experiment. However, the differences are grinding depth and corresponding feed times. The first step surface adopts 10 μm per pass and repeats for two times, creating the whole grinding depth of 20 μm. The second, third and fourth step surfaces adopt 20 μm, 30 μm and 40 μm per pass respectively, and then repeat for two times as well. The depth dependent grinding procedure is also repeated for four kinds of grinding wheels respectively. 4.3. Measurement method To reveal the influence of grinding time, grinding depth and wheel structure on the grinding performance, grinding force, material removal rate, ground surface roughness and hardness are measured. Grinding force is measured by the three-axis force sensor (Kistler 9256C2), charge amplifier (Kistler 5080) and data acquisition equipment. Force sensor adopts piezoelectric crystal to transform the force signal into the charge signal, which is amplified and converted into voltage signal
Fig. 4. SLM-fabricated grinding wheel before dressing: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel.
Fig. 5. Grinding platform: a) jig grinder for grinding and b) tool grinder for dressing.
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Table 3 Grinding and dressing parameters. Grinding & dressing parameter
Value
Workpiece material Workpiece dimension Vertical reciprocating frequency Grinding wheel speed Horizontal feed speed Grinding depth
Cr4W2MoV cold die steel (quench hardened) 100 mm (L) × 40 mm (W) × 15 mm (H) 2.6 Hz 12 m/s 10 mm/min Time dependent: 20 μm Depth dependent: 10, 20, 30, 40 μm Silicon carbide dressing wheel 0.8 Air cooling
Dressing tool Speed rate Coolant
ranging from −5 V to 5 V, and then converted to the digital signal via analog to digital converter. Material removal depth is measured by the ultra-depth three-dimensional microscopy (Keyence, VHX – 6000) and material removal rate is calculated considering the grinding area and grinding time. Ground surface roughness is measured by the optical surface profiler (ZYGO NewView 7300), which utilizes scanning white light interferometry to image and measure the microstructure and topography of surfaces in three dimensions. Ground surface hardness is measured by the hardness tester (Qness Q60A+) via measuring the micro Vickers hardness. Test points are hit on the specimen surface by penetrator with a pressing force of 1 kg and duration time of 10 s.
pressing grinding wheel, during grinding process, chip flute will occur around diamond abrasive grain on the surface of SLM-fabricated grinding wheel. The self-formed chip flute, presented in Fig. 7f, results from the cutting effect of grinding chip. The cutting edges of diamond abrasive grain are engaged with the workpiece and chips are formed. Furthermore, the chips are harder than Al-based bond, so they take some material of bond around diamond abrasive grain, resulting in the formation of chip flute shown in Fig. 7f. The phenomenon is unique for SLMfabricated grinding wheel and it can benefit the grinding performance because of large chip space and protrusion height. Therefore, the SLMfabricated grinding wheels possess excellent dressing and selfsharpening ability, which are very difficult to realize for hot pressing metal-bonded grinding wheels. To characterize the unique property of SLM-fabricated composite material, microstructures of side plane (parallel to the building direction) and scanning plane (perpendicular to the building direction) are observed. From the optical images shown in Fig. 8, the microstructures of side plane and scanning plane of SLM-fabricated two-phase composite have different patterns in terms of structure composition and void defect distribution. Side plane is characterized by scaly structures representing melting pools while scanning plane consists of interlaced and short strip structures representing scanning paths. Additionally, more void defects are formed in scanning plane compared with side plane. Therefore, SLM-fabricated composite shows strong anisotropy. 5.2. Bonding property
5. Results and analysis 5.1. Morphological property Surface topography of grinding wheels is obtained via SEM images shown in Fig. 7. It indicates the surface topography of octahedron structure wheel, honeycomb structure wheel and solid structure wheel in Fig. 7a, b and c respectively. The structures of SLM-fabricated grinding wheels are identical with CAD models depicted in Fig. 1 considering the fabrication accuracy. Fig. 7d and e represents the surface topography of solid structure wheel before and after grinding process respectively. After grinding process, the surface of SLM-fabricated grinding wheel becomes smoother due to the bond wear. Different from traditional hot
To investigate the bonding condition between diamond abrasive grain and Al-based bond, the morphology of grain and energy spectrum curve of the interface are obtained via SEM and EDS, shown in Fig. 9. Most abrasives are firmly embedded into bond and some of them are wrapped with bond, depicted in Fig. 9a, which indicates good infiltration between diamond and melted AlSi10Mg alloy. As presented in Fig. 9b, the transition regions (about 8–10 μm width) both exist for aluminum and carbon elements at the interfaces, suggesting the formation of C\\Al compound, which enhances the bonding strength of diamond abrasives. The C\\Al compound may be ionic compound Al4C3 with high hardness, shear strength and melting point, which benefits the bonding strength [18].
Fig. 6. Time dependent grinding procedure.
C. Tian et al. / Materials and Design 156 (2018) 52–61
(a)
(b)
500μm
(c)
500μm
(d)
500μm
(e)
(f)
200μm
200μm
57
80μm
Fig. 7. Surface topography of grinding wheels: a) octahedron structure wheel; b) honeycomb structure wheel; c) solid structure wheel; d) wheel surface after dressing; e) wheel surface after grinding; f) chip flute formed during grinding process.
5.3. Grinding chips To study the grinding mechanism of SLM-fabricated grinding wheel, the morphology of chips is observed via SEM images after grinding process. The formation process of grinding chips can be divided into three stages: sliding, plowing and cutting. Because most abrasive grains are engaged in the cutting process with negative rake angle, most chips are segmental chips formed in grinding process, whose bottom surface is smooth but back surface presents sawtooth shape as shown in Fig. 10. Grinding chips to be observed possess various shapes, including C shape segmental chip (Fig. 10b), ribbon segmental chip (Fig. 10c), spiral segmental chip (Fig. 10d) and fracture segmental chip (Fig. 10e). The morphology of grinding chips indicates good cutting ability of SLMfabricated grinding wheel.
5.4. Grinding force Force sensor and signal acquisition equipment are used to record the three-axis grinding force during grinding process with different grinding wheels and grinding parameters. Because of the unique grinding motion in the experiments, force signals are periodic. As presented in Fig. 5, x y and z axis forces represent tangential force, radial force and axial force respectively. Tangential force represents the horizontal
(a)
Side plane
cutting force of abrasive grain and horizontal frictional force of bond. Radial force represents the normal force between wheel surface and workpiece caused by extrusion deformation. Axial force represents the vertical cutting force of abrasive grain and vertical frictional force of bond. Therefore, tangential force (x axis force) is taken as an example for the signal analysis. When grinding process is stable, a segment of x axis force original signal is intercepted and presented in Fig. 11a. The corresponding frequency spectrum is obtained through fast Fourier transform (FFT), presented in Fig. 11b. Several key frequency components exist in the frequency spectrum, including 2.6 Hz, 617 Hz, 50 Hz and its high order harmonic components. First of all, 50 Hz represents powder frequency and can be removed by band stop filter. Additionally, 2.6 Hz represents the frequency of vertical reciprocating motion while 617 Hz represents the frequency of wheel rotation. Because of the pneumatic spindle, wheel rotating speed is not controlled very accurately. So this frequency fluctuates in the range from 600 Hz to 700 Hz, corresponding with the wheel rotating speed of 40,000 r/min. Furthermore, low frequency (0 to 20 Hz) signal and high frequency (550 to 650 Hz) signal are decomposed and derived from the original signal through the low pass filter and band pass filter, depicted in Fig. 11c and d respectively. Low frequency signal reveals the force induced by the frictional effect between bond and workpiece while high frequency signal reveals the force induced by the instantaneous cutting effect between grain and workpiece. The low frequency signal and high frequency signal
(b)
Scanning plane
Melng pool Building direcon
Scanning path
500μm Void defect
500μm Diamond abrasive
Fig. 8. Microstructure of SLM-fabricated composite material: a) microstructure of side plane; b) microstructure of scanning plane.
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Fig. 9. Bonding condition between diamond abrasive grain and Al-based bond: a) SEM images; b) Line scanning energy spectrum of the grain-bond interface.
presented in Fig. 11c and d are intercepted from the same signal segment, so they are corresponding to each other. When the grinding wheel locates in the highest position, the low frequency force and high frequency force are both the smallest. When the grinding wheel moves down to the middle position of workpiece, the low and high frequency forces increase to the middle values. When the grinding wheel continues to moves down to the lowest position, the low and high frequency forces are both up to the largest. When the grinding wheel moves up to the middle position of workpiece, the low and high frequency forces decrease to the middle values again. When the grinding wheel continues to moves up to the highest position, the low and high frequency forces decrease to the lowest values, same as the original condition. Therefore, low frequency force and high frequency force are corresponding to the wheel position in one period. This phenomenon can be explained as follows: the grinding wheel can be regarded as a cantilever beam and deflection can be generated due to the grinding force. Moreover, the action point of force varies based on the wheel position. The deflection caused by bending moment will decrease as the wheel moves down due to the reduction of force arm, shown as L1, L2 and L3 in Fig. 11d, leading to larger practical grinding depth and larger extrusion pressure as well as larger cutting force. As shown in Fig. 11c and d, the peak value of low frequency signal (LFP) and peak value of high frequency signal (HFP) can be used as two metrics for the comparison of different grinding wheels. They represent the extrusion or friction force between bond and workpiece and the instantaneous cutting force of abrasive grains respectively. The comparison of different grinding wheels based on the two metrics is clarified
in Fig. 12. As presented in Fig. 12, some useful findings can be obtained. Firstly, grinding forces of electroplated wheel, including LFP and HFP, are smaller than SLM-fabricated wheels for every axis force. It means the electroplated wheel has advantages on the grinding force. Secondly, all the forces increase as grinding depth increases, which is the same as the traditional wheel. Thirdly, among the SLM-fabricated wheels, octahedron structure wheel possesses the smallest LFP but the largest HFP while solid structure wheel possesses the largest LFP but the smallest HFP for every axis force. This phenomenon can be explained as follows: LFP represents the extrusion and friction force of bond while HFP represents the instantaneous cutting force of grains. Octahedron structure wheel possesses the largest porosity and smallest contact area when grinding, so the extrusion and friction force contributed by the bond is the smallest. However, the quantity of the abrasive grains engaged in the cutting process is the smallest for the octahedron structure wheel, so the material removal amount for single grit is the biggest, leading to the largest instantaneous cutting force. 5.5. Material removal rate To compare the cutting ability of different kinds of grinding wheels, material removal rate is measured at grinding time of 27, 51 and 72 min. As presented in Fig. 13a, depth difference between two adjacent step surfaces represents the depth of material removal, and measured by ultra-depth three-dimensional microscope. Then the material removal rate is calculated and summarized in Fig. 13b. As presented in Fig. 13b, as time goes by, the material removal rate decreases slowly for all
Fig. 10. Shape of chips formed in grinding process: a) a pile of chips, segmental chips; b) C shape segmental chip; c) ribbon segmental chip; d) spiral segmental chip; e) fracture segmental chip.
C. Tian et al. / Materials and Design 156 (2018) 52–61
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Fig. 11. Grinding force signal analysis (x force e.g.): a) a segment of original force signal; b) frequency spectrum of original signal; c) one period low frequency signal (0–20 Hz); d) one period high frequency signal (550–650 Hz).
kinds of grinding wheels because of the wheel wear. Additionally, very small difference exists for the SLM-fabricated wheels and electroplated wheel in terms of material removal capacity. Moreover, among the SLM-
fabricated grinding wheels, solid structure wheel possesses the largest material removal rate due to the largest contact area and the largest quantity of grains engaged in the cutting process.
Fig. 12. Grinding force comparison of different structure grinding wheel: a) high frequency x axis force; b) high frequency y axis force; c) high frequency z axis force; d) low frequency x axis force; e) low frequency y axis force; f) low frequency z axis force.
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(a)
Grinding transion region
(b)
Material removal depth
Fig. 13. Material removal rate: a) 3D morphology of ground step surface; b) material removal rate comparison of different grinding wheels.
5.6. Surface roughness Surface roughness is a significant index to the ground surface quality. Thus, the measurement of machined surface roughness is performed based on the optical surface profiler, whose results are demonstrated in Fig. 14. Three roughness values are tested for one ground surface and average is calculated to decrease the measurement error. As presented in Fig. 14, it is obvious that the surface roughness obtained by the SLM-fabricated grinding wheels is smaller than electroplated wheel by a large margin. The phenomenon can be explained as follows: Albased bond of SLM-fabricated wheel possesses relatively low elastic modulus compared to the Cu-based bond and Fe-based bond. Furthermore, porous structure reduces the elastic modulus of grinding wheel further. Therefore, the bonding model between Al-based bond and diamond abrasive grain can be simplified as spring damping model. So the grits will move in the radial direction due to the elastic deformation of bond when radial grinding force is applied. Moreover, radial moving distance is larger for the grits with larger protrusion height because of larger radial force, resulting in that all the grits assemble to the same cylindrical surface, which benefits the surface roughness further. Generally, machined surface roughness will increase as grinding time goes by or grinding depth increases, just like the electroplated wheel shown in Fig. 14a and b. However, this regularity is not applicable to the SLM-fabricated wheels. As presented in Fig. 14a, the common regularity for octahedron structure wheel, honeycomb structure wheel and solid structure wheel is that the ground surface roughness is not sensible to the grinding time. It means that surface roughness has a small
change as time goes by after the initial wheel wear, which indicates the excellent self-sharpening ability and anti-wear ability of SLMfabricated grinding wheel. Additionally, as presented in Fig. 14b, after the initial wheel wear, the surface roughness is relatively stable as grinding depth increases for all the SLM-fabricated wheels. It is unique because bigger grinding depth can be adopted regardless of surface roughness, which benefits the grinding efficiency.
5.7. Surface hardness Surface hardness is another index to the ground surface quality and it can reflect the degree of work hardening. The measurement of surface hardness is performed by the micro hardness tester and the test results are clarified in Fig. 15. First of all, the hardness of base surface is tested as the original hardness of workpiece (approximate 670 HV). As presented in Fig. 15, different degrees of work hardening happen for different grinding wheels. First of all, ground surface hardness increases as grinding time goes by due to the wear of grinding wheel. Moreover, the machined surface hardness of the SLM-fabricated grinding wheels is obviously less than that of electroplated wheel, indicating that slighter metal plastic deformation of workpiece exists when grinding with the SLM-fabricated wheels. Furthermore, the lowest ground surface hardness can be realized using octahedron structure wheel due to the smallest low frequency force and the smallest extrusion and friction effect. Also, the highest porosity and the biggest chip space help to reduce the temperature of contact area between wheel and workpiece.
Fig. 14. Surface roughness: a) as a function of grinding time; b) as a function of grinding depth.
C. Tian et al. / Materials and Design 156 (2018) 52–61
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Fig. 15. Ground surface hardness test results for different kinds of grinding wheels.
6. Conclusion In this paper, three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated by selective laser melting technology. Additionally, morphological characterization, including porous structure, microstructure and bonding condition characterization, are performed via optical microscope, scanning electron microscope and energy dispersive spectroscopy. Furthermore, grinding performances of the SLM-fabricated wheels and electroplated wheel are evaluated and compared with each other in terms of grinding force, material removal rate, ground surface roughness and hardness. The main findings are summarized as follows: 1) The SLM-fabricated grinding wheels possess excellent dressing and self-sharpening ability, controllable pore structure and porosity as well as good comprehensive grinding performance. 2) Most abrasives are firmly embedded into bond and C\\Al compound is produced at the grain-bond interface, indicating good bonding condition. During grinding process, chip flutes will occur around grains of SLM-fabricated grinding wheel, benefiting the chip space and protrusion height. 3) Among SLM-fabricated wheels, octahedron structure wheel possesses the smallest LFP, the largest HFP and the slightest work hardening while solid structure wheel possesses the largest LFP, the smallest HFP, as well as the largest material removal rate. 4) Compared with electroplated wheel, better machined surface roughness and slighter work hardening can be achieved by the SLM-fabricated wheels based on the similar material removal capacity. Different from electroplated wheel, the surface roughness ground by SLM-fabricated wheels is not sensible to the grinding time and grinding depth. Acknowledgement The authors would like to thank the technical support from Beijing Longyuan AFS Co., Ltd., Kunshan Hiecise Heavy Machinery Co., Ltd. and Institute of Process Engineering, Chinese Academy of Science. The research is financially supported by National Science and Technology Major Project (No. 2017ZX04007001), Tsinghua University Initiative Scientific Research Program and Tsinghua-RWTH Aachen Collaborative Innovation Funding. References [1] Y. Zhan, X. Xu, An experimental investigation of temperatures and energy partition in grinding of cemented carbide with a brazed diamond wheel, Int. J. Adv. Manuf. Technol. 61 (2012) 117–125, https://doi.org/10.1007/s00170-011-3706-7.
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