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Selective laser sintering and grinding performance of resin bond diamond grinding wheels with arrayed internal cooling holes
Ceramics International 45 (2019) 20873–20881
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Selective laser sintering and grinding performance of resin bond diamond grinding wheels with arrayed internal cooling holes
T
Zhi-jun Dua, Feng-lin Zhanga,*, Qiong-sheng Xua, Yao-jie Huanga, Ming-cong Lia, Hui-ping Huanga, Cheng-yong Wanga, Yu-mei Zhoub, Hong-qun Tangc a
School of Mechanical and Electronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China School of Mechanical and Electronic Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou, 510225, China c Guangxi Key Laboratory of Processing for Nonferrous Metallic and Featured Materials, Guangxi University, Nanning, 530004, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Selective laser sintering Resin bond diamond grinding wheel Internal cooling holes Grinding force
In this study, selective laser sintering was used to 3D print resin bond diamond grinding wheels with internal cooling holes. The mechanical properties and microstructures of the resin bond diamond grinding wheel were investigated. The grinding performance of 3D printed resin bond diamond grinding wheels with different internal cooling holes on the glass and cemented tungsten carbide was examined. The results show that 3D printed resin bond diamond wheels can be used to grind the glass and cemented tungsten carbide. Wheels containing internal cooling holes have lower grinding force. With increasing number and diameter of internal cooling holes, grinding forces on the glass decreased. Surface roughness up to 2–4 μm on the glass and cemented carbide was obtained by present 3D printed resin bond diamond wheel.
1. Introduction Resin bond diamond grinding wheels are widely used in the precision grinding of hard and brittle materials such as glass, ceramic, cemented carbide, sapphire, and semiconductor wafer. In resin bond diamond grinding wheels, diamond grits are bonded by thermosetting resin (e.g., epoxy or phenolic resins) and different filler materials, providing reinforced bond properties, inducing porosity and improving aesthetics. Both hard filler (e.g., aluminum oxide, silicon carbide, zirconia, and ceramic alpha alumina) and soft filler (e.g., petroleum coke, pyrophyllite, lime, and graphite) have been used to reinforce and control the breakdown rate of the grinding wheels [1]. The filler materials such as oxides (e.g., CaO and MgO), pyrite (FeS2), cryolite (Na3AlF6), zinc sulfide (ZnS), lithopone (ZnSBaSO4), potassium fluoroborate (KBF4) and potassium aluminum fluoride (KAlF4 and K3AlF6), potassium sulfate (K2SO4), and mixtures of these materials are often introduced to improve the grinding performance [2]. Moreover, metal powders such as Cu and Ni have been used as filler to increase the thermal conductivity and strength [3]. Resin bond diamond wheel is normally manufactured through mold pressing, hot pressing, and curing. Givot et al. [4] pressed (22.4 MPa) the mixture in a mold at room temperature and then cured the mixture by heating at 185 °C to prepare a resin bond diamond wheel. In order to
*
reduce the curing time, Goyal et al. [5] used microwave to reach temperature from 180 °C to 220 °C for curing of the resin bond grinding wheel. Mold pressing and subsequent hot pressing and curing were considered as the efficient routes for the mass production of diamond grinding wheel. However, preparing complex structures inside the wheel is difficult. In comparison with conventional grinding wheel, the textured grinding wheels with different dimensions and sizes have been proposed to improve the grinding performance. The texture on the wheel is commonly produced by mechanical machining [6–9] and laser beam ablation [10,11]. However, these conventional manufacturing processes have several limitations such as tools wear, thermal damage of the textured surface, limited texture accuracy, and geometrical complexity. 3D printing is known to be based on a layer-by-layer mode and can be applied to manufacture a component with complex structures. This technique has drawn significant attentions for applications in a variety of fields, including aviation, medical, biotechnology, electronic, and oceanography. Recently, it has also been proposed for manufacturing diamond tools. For example, Yang et al. [12] applied the additive manufacturing technology to design a metal bond diamond grinding wheel with regular grain distribution. Moreover, Tian et al. [13] used the selective laser melting (SLM) method to fabricate porous metal-
Corresponding author. E-mail address: [email protected] (F.-l. Zhang).
https://doi.org/10.1016/j.ceramint.2019.07.076 Received 19 April 2019; Received in revised form 6 July 2019; Accepted 7 July 2019 Available online 08 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 20873–20881
Z.-j. Du, et al.
Table 1 Details of raw materials. Name
Type
Resource
Nylon Glass bubble White corundum Diamond
PA2200 K46 500# W40
Commercial 3 M company Henan Tongfa Abrasive Co., Ltd. Henan Huifeng Diamond Technology Co., Ltd.
bonded grinding wheel. Tanaka et al. and Huang et al. [14,15] investigated resin bond diamond grinding wheel by stereolithography (STL) process. However, 3D printing to prepare a diamond wheel with internal connected cooling holes to improve grinding performance has rarely been studied. Herein, resin bond diamond grinding wheels with arrayed internal cooling holes were prepared by the SLS technique. The composition of the resin bond of the diamond wheel was optimized. Moreover, the performance of the 3D printed diamond wheel with different internal cooling holes was investigated. 2. Experimental The details of the resin, pore former (Glass bubble), filler (white corundum), and diamond grits for the 3D printing are presented in Table 1. Furthermore, Table 2 lists the specification of the printed wheel with characteristics of internal cooling holes. Fig. 1 shows the Scanning Electron Microscopy (SEM) images of nylon, diamond grit, glass bubble, and white corundum powders. Fig. 2 shows the detail of dimension and structure of the printed rim of the grinding wheel. Selective laser sintering (SLS) is based on layerby-layer additive manufacturing. In present study, the 3-D printed holes are based on the nylon powder which has not been scanned and sintered. After SLS, the unsintered nylon powder is loose and flow out to leave the shape of holes. And the laser scanned and sintered part owns enough strength, which can also keep the holes in good shape and strength. However, we found that it is difficult to build the internal connected holes with the diameter less than 1.5 mm. The printed holes will be clogged, and the internal connected channels cannot be formed when printing smaller holes. The experimental set-up for the 3D-printed wheel grinding is shown in Fig. 3. The grinding wheel rims were printed using an SLS machine (EOS P110). The printing power, scanning speed, and single layer thickness were set as 25 W, 2000 mm/s, and 0.1 mm, respectively. After trial sample printing, wheels with different fillers and cooling holes were printed at the sintering temperature varying from 171.5 to 173.5 °C. The printed rims were assembled with the aluminum alloy shank with a core diameter of 50.8 mm. Fig. 4(a) and (b) indicate that a commercial epoxy adhesive was used in this regard. Finally, the assembled diamond grinding wheels are dynamically balanced and corrected (< 0.1 g). In this study, the printed samples with a dimension of 5 × 5 × 25 mm3 were used. Moreover, the three-point bending strength was measured according to ASTM standard D7264/
D7264M − 15 [16]. The hardness tests were measured using a QT-1166 universal material testing machine and an LX-D Shore D hardness tester, respectively. The grinding performance of the 3D-printed diamond wheels was carried out on a face ultraprecision grinding lathe (WAZA520X-NC). The grinding parameters and conditions are listed in Table 3. Glass (BJ32) and cemented tungsten (YG15) were used as the workpiece materials, as listed in Table 4. The grinding forces were measured during the grinding experiments using a piezoelectric force dynamometer (Kistler 9257BA). The ground surface morphology of the workpiece and the printed diamond wheels was examined by SEM (FEI, Quanta 200). The surface topography of internal cooling holes was characterized using a stereomicroscope (MDA2000). The surface roughness of the ground workpiece was measured using a profilometer (XT20 surface profiler). 3. Results and discussion 3.1. Microstructure and mechanical properties of the 3D printed diamond wheel Fig. 5(a) and (b) show the optical microscopic images of the internal cooling holes with the diameters of 1.5 mm (G2) and 2.5 mm (G3), respectively. Fig. 6 illustrates the surface morphology of printed samples, indicating that the diamond grits, white corundum powder, and glass bubbles are evenly dispersed in the printed matrix. The hardness and bending strength of the grinding wheel of the 3D printing are shown in Fig. 7. With the addition of the white corundum powder in the bond, the hardness increased. This can be explained by the model of particle reinforced polymer composite as represented by Equations (1) and (2) proposed by Liang et al. [17]:
Ec = Em 1 +
s=
f
(m
1 + (1
1)
f )(m
1) s
(1)
7 5vm 15(1 vm)
(2)
where m = Ef /Em, Ef, Em, and Ec are the Young's modulus of the filler, matrix, and composite, respectively. Moreover, λ, ϕf, and vm are the parameters related to the particle diameter distribution and shape as well as interfacial adhesion, the filler volume fraction, and the matrix Poisson's ratio, respectively. Zorzi and Perottoni [18] derived the correlation between the
Table 2 Specification of 3D printed diamond wheels. Wheel No.
G1 G2 G3 G4 G5 G6
Composition
Cooling holes
Nylon(wt.%)
Glass bubble(wt.%)
White corundum (wt.%)
Diamond concentration (vol%)
Diameter d(mm)
67.5
20
0
12.5
0 1.5 2.5 0 1.5 1.5
65.5
2
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Number n (around the wheel) 90 90 90 120
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Fig. 1. SEM images of raw materials:(a) nylon; (b) diamond grit; (c) glass bubble; (d) white corundum.
Vickers hardness, Young's modulus, and Poisson's ratio by the instrumented indentation test, shown by Equation (3):
Hv = 2
9E (1 2v )2 8 (1 + v )3
0.585
3
(3)
Equations (1) and (2) show that the Young's modulus of grinding wheel increases with increasing amount of added white corundum. Therefore, the hardness increased in accordance with Equation (3). However, the bending strength slightly changed. The white corundum is popular reinforcement filler in the resin bond diamond wheel. Due to the low content of the white corundum (2 wt%), the reinforcement effect is limited in this study. Fig. 8 illustrates the fractured surface of 3D printed wheel samples. The diamond grits, white corundum particles, and glass bubbles are buried in the nylon matrix. The concentration of diamond grits is much higher than that of the white corundum powder and glass bubble, thus white corundum particles and glass bubbles are not easy to be identified such as diamond grits. 3.2. Effect of white corundum on grinding forces of the diamond wheel The grinding force of 3D printed wheels without (G1) and with (G4) addition of 2 wt% of the white corundum on the glass and YG15 cemented carbide for different depths of cut are shown in Figs. 9 and 10. The grinding forces of the G4 were found to be slightly higher than that of the G1 in the grinding of the glass. However, in grinding of the YG15 cemented carbide, the grinding force of G1 is similar to that of G4. The rise of the force for G4 in grinding the glass may be caused by the increase in the hardness of wheel's shore. Nylon with low elastic modulus 0.38 GPa [19] is a soft and resilient bond for the diamond wheel and may induce higher deformation of the wheel in the grinding of hard and brittle materials. With the addition of white corundum powder, the increase in the hardness accounts for the small rise in the grinding force. On other hand, solid loading of 3D-printed grinding wheel improved, and therefore, the grinding performance may be
affected by the addition of white corundum powder [20–23]. Table 4 shows that the YG15 cemented carbide has higher hardness and toughness in comparison to the glass and may account for the slight difference in the grinding force of G1 and G4. 3.3. Effect of printed internal cooling holes on grinding forces Internal cooling holes with the diameters of 1.5 mm and 2.5 mm in 3 × 3 array are printed in the diamond wheels G2 and G3 without the addition of the white corundum. Figs. 11 and 12 illustrate the effect of the hole diameter on the grinding force, indicating that the grinding force on the glass decreases with increasing cooling hole diameter, probably because of the improvement in the cooling and lubrication effect in the case of larger cooling hole. The increasing supply of the coolant at the grinding arc zone can reduce the friction between abrasives and the workpiece. In grinding with coolant supply, coolant can also be absorbed in the holes at the side face of the wheel and flow through the designed internal cooling as shown in Fig. 13. In contrast, increasing the diameter of the cooling hole also implies decreasing rigidity of the grinding wheel. It illustrates that the elastic deformation increases during the grinding process, and this may decrease the grinding force. However, this effect on the YG15 cemented carbide is not obvious and may be attributed to the fact that the printed wheel without the addition of the white corundum is soft for grinding the cemented carbide. In grinding the YG15 with larger holes, due to the decreasing rigidity of wheel, the deformation of the nylon may block the hole and reduce the cooling and lubrication efficiency. Samples with different numbers of holes containing the white corundum are 3D printed in this study. Figs. 14 and 15 present the effect of the hole number on the grinding force, indicating that the grinding force on the glass and YG15 cemented carbide slightly decreases as the cooling hole number varies from 0 to 120. It should be indicated that the same analysis mentioned for cooling and lubrication effect is valid for reducing the grinding force, in the wheel with more holes.
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Fig. 2. Structure of the printed rim of the grinding wheel: (a) and (b) 3D drawing of a solid wheel; (c) 3D and 2D drawings of the wheel with internal cooling holes.
Fig. 3. Schematic diagram of the 3D printed wheel grinding operation.
3.4. Surface morphology of the ground workpiece Fig. 16(a) and (b) show the surface morphology of the glass and cemented carbide, respectively, grounded by the G4 wheel. The ground surface of glass has a lot of fractured facets and cracks, indicating that the glass is ground mainly by a brittle material removal mode. Moreover, there are more plowing scratch on the cemented carbide, indicating that the material removal is dominated by the ductile mode.
Glass has much lower fracture toughness than the cemented carbide (YG15). The fracture toughness for glass and cemented carbide are KIC (glass) = 0.77MPa√m [24] and KIC (YG15) = 16.04MPa√m [25], respectively. This result indicates that the huge difference in the fracture toughness results in different material removal patterns. The maximum undeformed chip thickness hmax is expressed by the following Equation as reported by Maklin et al. [26]:
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Fig. 4. Picture of 3D printing grinding wheel: (a) without holes; (b) with internal cooling holes. Table 3 Grinding parameters. Grinding wheel speed(m/s)
7.1, 14.2
Grinding depth (μm) Workpiece feed speed(m/min) Grinding method Cooling condition
3, 5, 7, 9, 11 20 Up-grinding Wet (Deionized water)
Fig. 6. The surface morphology of the printed sample.
Table 4 Properties of the workpiece material. Properties
Workpiece material
Density (g/cm3) Compressive strength (MPa) Microhardness (HV) Bending strength (MPa)
BJ32 2.5-3.0 35 555.8 78.8
h max = (3/ C tan ) 0.5 (vw / vs )0.5 (a/ ds ) 0.25
YG15 13.5 86 1336.8 3100
(4)
where C, θ, vw, vs, a, are ds are the number of active grains per unit area of wheel surface, semi-included angle for the non-deformed chip crosssection, workpiece speed, wheel speed, wheel depth of cut, and wheel diameter, respectively. Huang et al. [27] suggested θ = 60°, and Xu et al. [28] showed that the value of parameter C can be determined by a simple geometric correlation presented by the following equation:
C=
4f /{dg2 (4
/3v )
2/3
}
(5)
Fig. 7. The bending strength and hardness of 3D-printed wheel samples with and without the addition of the white corundum.
Fig. 5. The surface topography of internal cooling holes with different diameters: (a) 1.5 mm (G2); (b) 2.5 mm (G3).
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Fig. 8. SEM images of the sample fractured surface: (a) (b) without the addition of white corundum; (c) (d) with the addition of white corundum.
Fig. 9. Grinding force of the 3D-printed diamond wheel with (G4) and without (G1) the white corundum for different depths of cut on the optical glass.
where dg is the equivalent spherical diameter of the diamond particle, v is the diamond volume fraction in the grinding wheel, and f is the fraction of diamond particles that actively cut in the grinding. Volume fraction v is 12.5 vol% in this study, and the value of parameter f was set to 0.5 in accordance with literature [29]. Moreover, the equivalent spherical diameter of the diamond grit (dg) is given by Malkin et al. [26] as shown by Equation (6):
dg = 15.2M
1
(6)
where M is the mesh size used in the grading sieve. Based on Equations (4)–(6), the value of the maximum non-deformed chip thickness (hmax) is 1.75 μm. Bifano et al. [30] revised Lawn's formula to reach the critical cut
Fig. 10. Grinding force of the 3D-printed diamond wheel with (G4) and without (G1) the white corundum for different depths of cut on the YG15 cemented carbide.
depth dc:
dc = 0.15(E /H )(KIC / H ) 2
(7)
where E is the Young's module, KIC is the fracture toughness, and H denotes the material hardness. According to Equation (7), the dc values for the glass and cemented carbide (YG15) are 0.03 μm and 7.54 μm, respectively. The maximum non-deformed chip thickness hmax value is higher than that of the critical cut depth dc (glass), but much less than dc (YG15), leading to different material removal modes in the grinding process. The surface roughness of the glass and cemented carbide was also examined as shown in Fig. 17. After grinding with G4 wheel, the surface roughness of the glass and cemented carbide varies from 2.0 to 4.0 μm
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Fig. 14. Effect of the hole number on the grinding force in grinding the glass.
Fig. 11. Effect of the hole diameter on the grinding force on grinding the glass.
Fig. 12. Effect of the hole diameter on the grinding force on grinding the cemented carbide.
Fig. 15. Effect of the hole number on the grinding force in grinding the cemented carbide.
Fig. 13. The illustration of the coolant supply in grinding: (a) the position of arrayed holes and coolant supply; (b) the illustration of the coolant flow in the arrayed internal cooling holes. 20879
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Fig. 16. The surface morphology of the ground workpiece: (a) Glass; (b) YG15.
Fig. 18. The diamond grits are still bonded within the matrix, as shown in Fig. 18(a) and (b). However, investigations show that grits are not highly protruded, which may also be caused by the lower hardness and rigidity of the binder. In contrast, Fig. 18(c) shows a smear coating on some regions of the wheel, which may come from the plastic flowing nature of the nylon, due to the grinding heat. The grinding temperature may be higher than the softening temperature of nylon. The melting temperature of nylon is 178 °C [31], which induces the adhering of nylon to the grinding wheel surface and may lead to form a nylon layer on the wheel surface. 4. Conclusions
Fig. 17. Workpiece surface roughness before and after grinding.
due to the difference in their original surface roughness, indicating that the 3D printed wheel can be used to grind the glass and cemented carbide in a half-precision machining stage. 3.5. The wear characteristics of the 3D printed diamond wheel After grinding the glass and cemented tungsten, the morphology of the grinding wheel (G4) was examined, and the results are presented in
SLS technique was applied to prepare a resin bond diamond grinding wheel with internal cooling holes. This study shows that the diamond grit can be well bonded and buried in the bond. Moreover, The addition of white corundum increased the hardness and bending strength of the grinding wheel and also slightly increased the grinding force in grinding of glass. The grinding force on the glass reduces with increasing diameter of the internal cooling holes. Furthermore, the grinding force of the printed wheel on the glass and YG15 cemented carbide reduces with increasing number of cooling hole number. In conclusion, the printed diamond wheel can be used to grind the glass and YG15 with a surface roughness varying from 2 to 4 μm. In the grinding, the diamond grits are not highly protruded, and some smear coatings are formed on the surface of wheel due to the plastic flowing characteristics of nylon.
Fig. 18. SEM images of the 3D printed diamond wheel after grinding.
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Acknowledgment The authors thank the financial support from the National Natural Science Foundation of China with Grant Nos. 51775118 and 51735003 and Guangxi Natural Science Foundation with grant Nos. 2018GXNSFAA281258.
[14] [15] [16] [17]
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[1] J. Hajduk, J.L. McArdle, Resin Bonded Grinding Wheel, U.S. Patent, 2014 No. 14/ 0057534 A1. [2] B. Linke, Manufacturing and sustainability of bonding systems for grinding tools, Prod. Eng. 10 (3) (2016) 265–276. [3] J.R. Thompson, Resin Bonded Diamond Wheels with Copper and Silicon Carbide Fillers, U.S. Patent, 1975 No. 3899307. [4] M. Givot, M.G. Schwabel, Bonded Abrasive Wheel, U.S. Patent, 2013 No. 13/ 0203328 A1. [5] P. Goyal, S. Borkar, R. Jaiswal, Rapid Curing of Resin Bonded Grinding Wheel, U.S. Patent, 2015 No. 15/0165594 A1. [6] A.M.O. Mohamed, R. Bauer, A. Warkentin, A novel method for grooving and regrooving aluminum oxide grinding wheels, Int. J. Adv. Manuf. Technol. 73 (5–8) (2014) 715–725. [7] T. Tawakoli, A. Daneshi, New kinematic in dressing of grinding wheels, Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition, 2013, pp. 1–5. [8] B. Kirsch, J.C. Aurich, Influence of the macro-topography of grinding wheels on the cooling efficiency and the surface integrity, Procedia CIRP 13 (2014) 8–12. [9] J.C. Aurich, B. Kirsch, Improved coolant supply through slotted grinding wheel, CIRP Ann. - Manuf. Technol. 62 (1) (2013) 363–366. [10] C. Walter, T. Komischke, F. Kuster, K. Wegener, Laser-structured grinding tools – generation of prototype patterns and performance evaluation, J. Mater. Process. Technol. 214 (4) (2014) 951–961. [11] B. Guo, Q. Zhao, X. Fang, Precision grinding of optical glass with laser microstructured coarse-grained diamond wheels, J. Mater. Process. Technol. 214 (5) (2014) 1045–1051. [12] Z. Yang, M. Zhang, Z. Zhang, A. Liu, R. Yang, S. Liu, A study on diamond grinding wheels with regular grain distribution using additive manufacturing (AM) technology, Mater. Des. 104 (2016) 292–297. [13] C. Tian, X. Li, S. Zhang, G. Guo, L. Wang, Y. Rong, Study on design and performance
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Selective laser sintering and grinding performance of resin bond diamond grinding wheels with arrayed internal cooling holes
T
Zhi-jun Dua, Feng-lin Zhanga,*, Qiong-sheng Xua, Yao-jie Huanga, Ming-cong Lia, Hui-ping Huanga, Cheng-yong Wanga, Yu-mei Zhoub, Hong-qun Tangc a
School of Mechanical and Electronic Engineering, Guangdong University of Technology, Guangzhou, 510006, China School of Mechanical and Electronic Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou, 510225, China c Guangxi Key Laboratory of Processing for Nonferrous Metallic and Featured Materials, Guangxi University, Nanning, 530004, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Selective laser sintering Resin bond diamond grinding wheel Internal cooling holes Grinding force
In this study, selective laser sintering was used to 3D print resin bond diamond grinding wheels with internal cooling holes. The mechanical properties and microstructures of the resin bond diamond grinding wheel were investigated. The grinding performance of 3D printed resin bond diamond grinding wheels with different internal cooling holes on the glass and cemented tungsten carbide was examined. The results show that 3D printed resin bond diamond wheels can be used to grind the glass and cemented tungsten carbide. Wheels containing internal cooling holes have lower grinding force. With increasing number and diameter of internal cooling holes, grinding forces on the glass decreased. Surface roughness up to 2–4 μm on the glass and cemented carbide was obtained by present 3D printed resin bond diamond wheel.
1. Introduction Resin bond diamond grinding wheels are widely used in the precision grinding of hard and brittle materials such as glass, ceramic, cemented carbide, sapphire, and semiconductor wafer. In resin bond diamond grinding wheels, diamond grits are bonded by thermosetting resin (e.g., epoxy or phenolic resins) and different filler materials, providing reinforced bond properties, inducing porosity and improving aesthetics. Both hard filler (e.g., aluminum oxide, silicon carbide, zirconia, and ceramic alpha alumina) and soft filler (e.g., petroleum coke, pyrophyllite, lime, and graphite) have been used to reinforce and control the breakdown rate of the grinding wheels [1]. The filler materials such as oxides (e.g., CaO and MgO), pyrite (FeS2), cryolite (Na3AlF6), zinc sulfide (ZnS), lithopone (ZnSBaSO4), potassium fluoroborate (KBF4) and potassium aluminum fluoride (KAlF4 and K3AlF6), potassium sulfate (K2SO4), and mixtures of these materials are often introduced to improve the grinding performance [2]. Moreover, metal powders such as Cu and Ni have been used as filler to increase the thermal conductivity and strength [3]. Resin bond diamond wheel is normally manufactured through mold pressing, hot pressing, and curing. Givot et al. [4] pressed (22.4 MPa) the mixture in a mold at room temperature and then cured the mixture by heating at 185 °C to prepare a resin bond diamond wheel. In order to
*
reduce the curing time, Goyal et al. [5] used microwave to reach temperature from 180 °C to 220 °C for curing of the resin bond grinding wheel. Mold pressing and subsequent hot pressing and curing were considered as the efficient routes for the mass production of diamond grinding wheel. However, preparing complex structures inside the wheel is difficult. In comparison with conventional grinding wheel, the textured grinding wheels with different dimensions and sizes have been proposed to improve the grinding performance. The texture on the wheel is commonly produced by mechanical machining [6–9] and laser beam ablation [10,11]. However, these conventional manufacturing processes have several limitations such as tools wear, thermal damage of the textured surface, limited texture accuracy, and geometrical complexity. 3D printing is known to be based on a layer-by-layer mode and can be applied to manufacture a component with complex structures. This technique has drawn significant attentions for applications in a variety of fields, including aviation, medical, biotechnology, electronic, and oceanography. Recently, it has also been proposed for manufacturing diamond tools. For example, Yang et al. [12] applied the additive manufacturing technology to design a metal bond diamond grinding wheel with regular grain distribution. Moreover, Tian et al. [13] used the selective laser melting (SLM) method to fabricate porous metal-
Corresponding author. E-mail address: [email protected] (F.-l. Zhang).
https://doi.org/10.1016/j.ceramint.2019.07.076 Received 19 April 2019; Received in revised form 6 July 2019; Accepted 7 July 2019 Available online 08 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 20873–20881
Z.-j. Du, et al.
Table 1 Details of raw materials. Name
Type
Resource
Nylon Glass bubble White corundum Diamond
PA2200 K46 500# W40
Commercial 3 M company Henan Tongfa Abrasive Co., Ltd. Henan Huifeng Diamond Technology Co., Ltd.
bonded grinding wheel. Tanaka et al. and Huang et al. [14,15] investigated resin bond diamond grinding wheel by stereolithography (STL) process. However, 3D printing to prepare a diamond wheel with internal connected cooling holes to improve grinding performance has rarely been studied. Herein, resin bond diamond grinding wheels with arrayed internal cooling holes were prepared by the SLS technique. The composition of the resin bond of the diamond wheel was optimized. Moreover, the performance of the 3D printed diamond wheel with different internal cooling holes was investigated. 2. Experimental The details of the resin, pore former (Glass bubble), filler (white corundum), and diamond grits for the 3D printing are presented in Table 1. Furthermore, Table 2 lists the specification of the printed wheel with characteristics of internal cooling holes. Fig. 1 shows the Scanning Electron Microscopy (SEM) images of nylon, diamond grit, glass bubble, and white corundum powders. Fig. 2 shows the detail of dimension and structure of the printed rim of the grinding wheel. Selective laser sintering (SLS) is based on layerby-layer additive manufacturing. In present study, the 3-D printed holes are based on the nylon powder which has not been scanned and sintered. After SLS, the unsintered nylon powder is loose and flow out to leave the shape of holes. And the laser scanned and sintered part owns enough strength, which can also keep the holes in good shape and strength. However, we found that it is difficult to build the internal connected holes with the diameter less than 1.5 mm. The printed holes will be clogged, and the internal connected channels cannot be formed when printing smaller holes. The experimental set-up for the 3D-printed wheel grinding is shown in Fig. 3. The grinding wheel rims were printed using an SLS machine (EOS P110). The printing power, scanning speed, and single layer thickness were set as 25 W, 2000 mm/s, and 0.1 mm, respectively. After trial sample printing, wheels with different fillers and cooling holes were printed at the sintering temperature varying from 171.5 to 173.5 °C. The printed rims were assembled with the aluminum alloy shank with a core diameter of 50.8 mm. Fig. 4(a) and (b) indicate that a commercial epoxy adhesive was used in this regard. Finally, the assembled diamond grinding wheels are dynamically balanced and corrected (< 0.1 g). In this study, the printed samples with a dimension of 5 × 5 × 25 mm3 were used. Moreover, the three-point bending strength was measured according to ASTM standard D7264/
D7264M − 15 [16]. The hardness tests were measured using a QT-1166 universal material testing machine and an LX-D Shore D hardness tester, respectively. The grinding performance of the 3D-printed diamond wheels was carried out on a face ultraprecision grinding lathe (WAZA520X-NC). The grinding parameters and conditions are listed in Table 3. Glass (BJ32) and cemented tungsten (YG15) were used as the workpiece materials, as listed in Table 4. The grinding forces were measured during the grinding experiments using a piezoelectric force dynamometer (Kistler 9257BA). The ground surface morphology of the workpiece and the printed diamond wheels was examined by SEM (FEI, Quanta 200). The surface topography of internal cooling holes was characterized using a stereomicroscope (MDA2000). The surface roughness of the ground workpiece was measured using a profilometer (XT20 surface profiler). 3. Results and discussion 3.1. Microstructure and mechanical properties of the 3D printed diamond wheel Fig. 5(a) and (b) show the optical microscopic images of the internal cooling holes with the diameters of 1.5 mm (G2) and 2.5 mm (G3), respectively. Fig. 6 illustrates the surface morphology of printed samples, indicating that the diamond grits, white corundum powder, and glass bubbles are evenly dispersed in the printed matrix. The hardness and bending strength of the grinding wheel of the 3D printing are shown in Fig. 7. With the addition of the white corundum powder in the bond, the hardness increased. This can be explained by the model of particle reinforced polymer composite as represented by Equations (1) and (2) proposed by Liang et al. [17]:
Ec = Em 1 +
s=
f
(m
1 + (1
1)
f )(m
1) s
(1)
7 5vm 15(1 vm)
(2)
where m = Ef /Em, Ef, Em, and Ec are the Young's modulus of the filler, matrix, and composite, respectively. Moreover, λ, ϕf, and vm are the parameters related to the particle diameter distribution and shape as well as interfacial adhesion, the filler volume fraction, and the matrix Poisson's ratio, respectively. Zorzi and Perottoni [18] derived the correlation between the
Table 2 Specification of 3D printed diamond wheels. Wheel No.
G1 G2 G3 G4 G5 G6
Composition
Cooling holes
Nylon(wt.%)
Glass bubble(wt.%)
White corundum (wt.%)
Diamond concentration (vol%)
Diameter d(mm)
67.5
20
0
12.5
0 1.5 2.5 0 1.5 1.5
65.5
2
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Number n (around the wheel) 90 90 90 120
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Fig. 1. SEM images of raw materials:(a) nylon; (b) diamond grit; (c) glass bubble; (d) white corundum.
Vickers hardness, Young's modulus, and Poisson's ratio by the instrumented indentation test, shown by Equation (3):
Hv = 2
9E (1 2v )2 8 (1 + v )3
0.585
3
(3)
Equations (1) and (2) show that the Young's modulus of grinding wheel increases with increasing amount of added white corundum. Therefore, the hardness increased in accordance with Equation (3). However, the bending strength slightly changed. The white corundum is popular reinforcement filler in the resin bond diamond wheel. Due to the low content of the white corundum (2 wt%), the reinforcement effect is limited in this study. Fig. 8 illustrates the fractured surface of 3D printed wheel samples. The diamond grits, white corundum particles, and glass bubbles are buried in the nylon matrix. The concentration of diamond grits is much higher than that of the white corundum powder and glass bubble, thus white corundum particles and glass bubbles are not easy to be identified such as diamond grits. 3.2. Effect of white corundum on grinding forces of the diamond wheel The grinding force of 3D printed wheels without (G1) and with (G4) addition of 2 wt% of the white corundum on the glass and YG15 cemented carbide for different depths of cut are shown in Figs. 9 and 10. The grinding forces of the G4 were found to be slightly higher than that of the G1 in the grinding of the glass. However, in grinding of the YG15 cemented carbide, the grinding force of G1 is similar to that of G4. The rise of the force for G4 in grinding the glass may be caused by the increase in the hardness of wheel's shore. Nylon with low elastic modulus 0.38 GPa [19] is a soft and resilient bond for the diamond wheel and may induce higher deformation of the wheel in the grinding of hard and brittle materials. With the addition of white corundum powder, the increase in the hardness accounts for the small rise in the grinding force. On other hand, solid loading of 3D-printed grinding wheel improved, and therefore, the grinding performance may be
affected by the addition of white corundum powder [20–23]. Table 4 shows that the YG15 cemented carbide has higher hardness and toughness in comparison to the glass and may account for the slight difference in the grinding force of G1 and G4. 3.3. Effect of printed internal cooling holes on grinding forces Internal cooling holes with the diameters of 1.5 mm and 2.5 mm in 3 × 3 array are printed in the diamond wheels G2 and G3 without the addition of the white corundum. Figs. 11 and 12 illustrate the effect of the hole diameter on the grinding force, indicating that the grinding force on the glass decreases with increasing cooling hole diameter, probably because of the improvement in the cooling and lubrication effect in the case of larger cooling hole. The increasing supply of the coolant at the grinding arc zone can reduce the friction between abrasives and the workpiece. In grinding with coolant supply, coolant can also be absorbed in the holes at the side face of the wheel and flow through the designed internal cooling as shown in Fig. 13. In contrast, increasing the diameter of the cooling hole also implies decreasing rigidity of the grinding wheel. It illustrates that the elastic deformation increases during the grinding process, and this may decrease the grinding force. However, this effect on the YG15 cemented carbide is not obvious and may be attributed to the fact that the printed wheel without the addition of the white corundum is soft for grinding the cemented carbide. In grinding the YG15 with larger holes, due to the decreasing rigidity of wheel, the deformation of the nylon may block the hole and reduce the cooling and lubrication efficiency. Samples with different numbers of holes containing the white corundum are 3D printed in this study. Figs. 14 and 15 present the effect of the hole number on the grinding force, indicating that the grinding force on the glass and YG15 cemented carbide slightly decreases as the cooling hole number varies from 0 to 120. It should be indicated that the same analysis mentioned for cooling and lubrication effect is valid for reducing the grinding force, in the wheel with more holes.
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Fig. 2. Structure of the printed rim of the grinding wheel: (a) and (b) 3D drawing of a solid wheel; (c) 3D and 2D drawings of the wheel with internal cooling holes.
Fig. 3. Schematic diagram of the 3D printed wheel grinding operation.
3.4. Surface morphology of the ground workpiece Fig. 16(a) and (b) show the surface morphology of the glass and cemented carbide, respectively, grounded by the G4 wheel. The ground surface of glass has a lot of fractured facets and cracks, indicating that the glass is ground mainly by a brittle material removal mode. Moreover, there are more plowing scratch on the cemented carbide, indicating that the material removal is dominated by the ductile mode.
Glass has much lower fracture toughness than the cemented carbide (YG15). The fracture toughness for glass and cemented carbide are KIC (glass) = 0.77MPa√m [24] and KIC (YG15) = 16.04MPa√m [25], respectively. This result indicates that the huge difference in the fracture toughness results in different material removal patterns. The maximum undeformed chip thickness hmax is expressed by the following Equation as reported by Maklin et al. [26]:
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Fig. 4. Picture of 3D printing grinding wheel: (a) without holes; (b) with internal cooling holes. Table 3 Grinding parameters. Grinding wheel speed(m/s)
7.1, 14.2
Grinding depth (μm) Workpiece feed speed(m/min) Grinding method Cooling condition
3, 5, 7, 9, 11 20 Up-grinding Wet (Deionized water)
Fig. 6. The surface morphology of the printed sample.
Table 4 Properties of the workpiece material. Properties
Workpiece material
Density (g/cm3) Compressive strength (MPa) Microhardness (HV) Bending strength (MPa)
BJ32 2.5-3.0 35 555.8 78.8
h max = (3/ C tan ) 0.5 (vw / vs )0.5 (a/ ds ) 0.25
YG15 13.5 86 1336.8 3100
(4)
where C, θ, vw, vs, a, are ds are the number of active grains per unit area of wheel surface, semi-included angle for the non-deformed chip crosssection, workpiece speed, wheel speed, wheel depth of cut, and wheel diameter, respectively. Huang et al. [27] suggested θ = 60°, and Xu et al. [28] showed that the value of parameter C can be determined by a simple geometric correlation presented by the following equation:
C=
4f /{dg2 (4
/3v )
2/3
}
(5)
Fig. 7. The bending strength and hardness of 3D-printed wheel samples with and without the addition of the white corundum.
Fig. 5. The surface topography of internal cooling holes with different diameters: (a) 1.5 mm (G2); (b) 2.5 mm (G3).
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Fig. 8. SEM images of the sample fractured surface: (a) (b) without the addition of white corundum; (c) (d) with the addition of white corundum.
Fig. 9. Grinding force of the 3D-printed diamond wheel with (G4) and without (G1) the white corundum for different depths of cut on the optical glass.
where dg is the equivalent spherical diameter of the diamond particle, v is the diamond volume fraction in the grinding wheel, and f is the fraction of diamond particles that actively cut in the grinding. Volume fraction v is 12.5 vol% in this study, and the value of parameter f was set to 0.5 in accordance with literature [29]. Moreover, the equivalent spherical diameter of the diamond grit (dg) is given by Malkin et al. [26] as shown by Equation (6):
dg = 15.2M
1
(6)
where M is the mesh size used in the grading sieve. Based on Equations (4)–(6), the value of the maximum non-deformed chip thickness (hmax) is 1.75 μm. Bifano et al. [30] revised Lawn's formula to reach the critical cut
Fig. 10. Grinding force of the 3D-printed diamond wheel with (G4) and without (G1) the white corundum for different depths of cut on the YG15 cemented carbide.
depth dc:
dc = 0.15(E /H )(KIC / H ) 2
(7)
where E is the Young's module, KIC is the fracture toughness, and H denotes the material hardness. According to Equation (7), the dc values for the glass and cemented carbide (YG15) are 0.03 μm and 7.54 μm, respectively. The maximum non-deformed chip thickness hmax value is higher than that of the critical cut depth dc (glass), but much less than dc (YG15), leading to different material removal modes in the grinding process. The surface roughness of the glass and cemented carbide was also examined as shown in Fig. 17. After grinding with G4 wheel, the surface roughness of the glass and cemented carbide varies from 2.0 to 4.0 μm
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Fig. 14. Effect of the hole number on the grinding force in grinding the glass.
Fig. 11. Effect of the hole diameter on the grinding force on grinding the glass.
Fig. 12. Effect of the hole diameter on the grinding force on grinding the cemented carbide.
Fig. 15. Effect of the hole number on the grinding force in grinding the cemented carbide.
Fig. 13. The illustration of the coolant supply in grinding: (a) the position of arrayed holes and coolant supply; (b) the illustration of the coolant flow in the arrayed internal cooling holes. 20879
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Fig. 16. The surface morphology of the ground workpiece: (a) Glass; (b) YG15.
Fig. 18. The diamond grits are still bonded within the matrix, as shown in Fig. 18(a) and (b). However, investigations show that grits are not highly protruded, which may also be caused by the lower hardness and rigidity of the binder. In contrast, Fig. 18(c) shows a smear coating on some regions of the wheel, which may come from the plastic flowing nature of the nylon, due to the grinding heat. The grinding temperature may be higher than the softening temperature of nylon. The melting temperature of nylon is 178 °C [31], which induces the adhering of nylon to the grinding wheel surface and may lead to form a nylon layer on the wheel surface. 4. Conclusions
Fig. 17. Workpiece surface roughness before and after grinding.
due to the difference in their original surface roughness, indicating that the 3D printed wheel can be used to grind the glass and cemented carbide in a half-precision machining stage. 3.5. The wear characteristics of the 3D printed diamond wheel After grinding the glass and cemented tungsten, the morphology of the grinding wheel (G4) was examined, and the results are presented in
SLS technique was applied to prepare a resin bond diamond grinding wheel with internal cooling holes. This study shows that the diamond grit can be well bonded and buried in the bond. Moreover, The addition of white corundum increased the hardness and bending strength of the grinding wheel and also slightly increased the grinding force in grinding of glass. The grinding force on the glass reduces with increasing diameter of the internal cooling holes. Furthermore, the grinding force of the printed wheel on the glass and YG15 cemented carbide reduces with increasing number of cooling hole number. In conclusion, the printed diamond wheel can be used to grind the glass and YG15 with a surface roughness varying from 2 to 4 μm. In the grinding, the diamond grits are not highly protruded, and some smear coatings are formed on the surface of wheel due to the plastic flowing characteristics of nylon.
Fig. 18. SEM images of the 3D printed diamond wheel after grinding.
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Acknowledgment The authors thank the financial support from the National Natural Science Foundation of China with Grant Nos. 51775118 and 51735003 and Guangxi Natural Science Foundation with grant Nos. 2018GXNSFAA281258.
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