Preparation and Performance Analysis of Porous

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Preparation and Performance Analysis of Porous


    Preparation and Performance Analysis of Porous

    Composite-bonded CBN Grinding Wheels

    DING Wenfeng

    5 (College of Mechanical and Engineering School, Nanjing University of Aeronautics and

    Astronautics, Nanjing 210016, China)

    Abstract: Fabrication experiments of porous composite-bonded CBN wheels are conducted using alumina (Al2O3) bubbles, CBN grains, Cu-Sn-Ti alloy and graphite particles. Influence of sintering parameters and porosity on the bending strength of the CBN composite blocks is measured and

    10 analyzed. Dressing and grinding practice is carried out. The results show that the optimal sintering temperature of the CBN composite blocks is 880?. When the porosity of composite blocks is 8-45 %,

    the strength reaches 51-103 MPa. Regular shape of the pores is obtained after dressing. Both the grinding force and grinding temperature of the composite-bonded CBN wheel are lower than that of the vitrified one under the same grinding condition, which indicates the better grinding performance of the

    15 new-type porous composite-bonded CBN wheel.

    Key words: sintering; porous CBN wheel; alumina bubbles; bending strength; grinding performance

0 Introduction

    Grinding with CBN grains is the most commonly used machining process suitable for manufacturing aircraft structural components made of difficult-to-cut materials, especially

    [1-3]20 nickel-based superalloy and titanium alloy, bearing steel . Machining efficiency is a major

    factor that hinders the application of grinding with CBN grains. The electroplated CBN wheels are mostly utilized in grinding these difficult-to-cut materials. However, due to the seriously wear, the tool life is still a big disadvantage for single layer CBN wheels. For multilayer CBN wheels, the bond especially metallic bonds are very closed structure and unless outside agents like

    25 pore-forming agent were added, no controlled porosity could assist with chip clearance. As a consequence, high intension of adhesion of workpiece material to grinding wheel will easily increase the temperature in contact zone and damage the ground surface. At the same time, the frequent dressing of grinding wheel will cause higher machining cost and lower grinding efficiency. High porosity and self-sharpening character are necessary for metallic bonds CBN

    [4] grinding wheels 30.

    To overcome the inherent shortcomings of conventional metal-bonded grinding wheels, researchers had developed a series of investigations. Tanaka et al had tried to develop a new

    [5]porous grinding wheel using cast iron as bonding material . Truong and Tomino have developed

    and researched the porous grinding wheel by resistance sintering, hot isostatic pressing and

    [6, 7] 35 vacuum sintering . Su and Dai used a pore-forming agent with a low melting point to fabricate[8, 9]porous grinding wheels by hot pressing . There are always some shortcomings involved. Firstly,

    the bonding strength of metal binder material to the abrasive grains was inadequate so that the grains easily pulled out during grinding. Secondly, the formation of pores was mainly relying on the pore-forming agent in the working layer. Therefore, the shape and size of the pores were not

    40 uniform and could not be controlled. Whats more, the strength of the working layer was not so [10, 11]high that the wheel edge breaks easily during grinding .

    In order to solve the problems mentioned above, the alumina (AlO) bubbles are added into 23

    Foundations: Natural Science Foundation of China (No. 51005116); the Ph.D. Program Foundation of Ministry of Education of China (No. 20103218120026)

    Brief author introduction:DING Wenfeng(1978-), male, professor, the main research is focused on high-efficiency precision grinding technology and equipments. E-mail:

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    the working layer to fabricate the porous composite-bonded CBN abrasive wheels. The composite

    blocks with various porosities, which serve as the working layer, are fabricated using sintering 45 technology. Accordingly, the porous composite-bonded CBN grinding wheels were fabricated.

    Firstly, the sintering temperature was optimized. Then the effect of porosity on the bending

    strength of the CBN abrasive composite blocks was discussed. The dressing process was carried

    out and the pores formed on the working layer surface were observed. Finally, the grinding forces

    and temperatures of the composite-bonded CBN wheel were measured and compared with that of 50 vitrified CBN counterpart.

    1 Experimental materials

    Because Cu-Sn-Ti active alloy has some advantages, such as higher strength, lower melting

    temperature, better cost-efficient, it is chosen as the bonding material of composite-bonded CBN

    wheel in this investigation. At the same time, the graphite particles are added into Cu-Sn-Ti alloy 55 to improve effectively the toughness and brittleness of the bonding material due to the lamellar

    structure and softness of the graphite materials. Thus, the dressing property of the

    composite-bonded CBN wheels may be improved. In addition, as a solid lubricant of the bonding

    system, the graphite particles play an important role in decreasing the friction action in grinding

    contact zone.

    60 The CBN grains with the size of 151~181 µm (80/100) is chosen. The concentration of CBN

    grains in the working layer of the grinding wheel is controlled at 100%. According to the

    high-porosity requirements for the CBN wheels during high-efficiency grinding process, alumina

    (AlO) bubbles with the size from 250 µm to 300 µm are adopted, shown in Fig.1. The bubble 23

    hollow may be observed obviously from the cracked one (Fig.1(b)).

65 (a) Whole bubbles (b) Cracked bubble

    Fig. 1 The morphology of alumina bubbles

    2 Strength and porosity analysis of composite-bonded CBN wheels

    2.1 Fabrication of porous CBN composite blocks

    70 The manufacturing process of the CBN composite blocks is demonstrated in Fig.2. The

    specimens are manufactured using the mixture of Cu-Sn-Ti alloy powder, graphite particles, CBN

    grains, alumina (AlO) bubbles as pore-forming media. The mixture is firstly compressed through 23

    cold pressing in a mould. Subsequently, the specimens are sintered in the vacuum furnace. The

     sintering process is carried out at the heating temperature from 860 ? to 920 ? for 30 min?2 75 under a high vacuum of better than 1×10Pa. Afterwards, the specimens are cooled in the furnace.

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    The porous composite-bonded CBN wheel (shown in Fig.3) with a diameter of 400 mm are

    accordingly fabricated based on the porous CBN composite blocks.

     Fig. 2 Schematic of the manufacturing process of composite blocks

80 2.2 Influence of sintering temperature on composite block strength In general, sintering temperature and holding time are the two important parameters to determine the sintering process and quality. Based on the previous research results, the sintering

     temperature is set individually at 860 ?, 880 ?, 900 ? and 920 ?. The holding time is kept at 30 min. To achieve good bonding effect, sintering experiments of CBN composite blocks with

    different porosity are carried out at the different temperatures mentioned above. 85

     The bending strength of composite block specimens was measured by three-point bending [12] method, as described in the literature . The result is displayed in Fig.4. Though the strength of the composite block with varied porosity is different, the influence of the sintering temperature on the bending strength of the composite blocks is identical. The peak value of the bending strength

     always reaches at 880 ?, which is tightly correlated with the better combination is obtained90

     between the binding material, CBN grains and AlObubbles at 880 ?. Particularly, the details as 23 follows, the fracture morphology of the composite blocks, interfacial microstructure between the bonding material and AlObubbles, and the chemical compositions of the composite blocks as 23 [13] well, are not discussed because it has been reported in our previous work .

     95 Fig. 3 Porous composited-bonded CBN wheels Fig. 4 The bending strength of composite blocks

     2.3 Influence of porosity on composite block strength The whole volume of CBN composite block V is described as: V=V+V+V(1) GBP

    here Vis the CBN grain volume, Vis the bonding material volume, and Vp is the pore volume. 100 G B

    In particular, if the tiny pores formed through flowing of liquid bonding material at elevated

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     temperature are ignored, the whole volume of the pores is equal to the volume of all the alumina

     bubbles in the composite block. That is to say, the porosity η of the working layer of the composite-bonded CBN grinding wheel could be calculated:

    105 /V (2) η=Vp Table 2 displays the influence of the composition of CBN composite blocks on the porosity. When the porosity is above 45 %, its difficult to press in the mould. Therefore, the potosity is designed from 8 % to 45 % in this investigation.

    Tab. 2 Material composition of composite block specimens 110

     Specimens Material

    1 2 3 4 5 6 7

    Cu-Sn-Ti+graphite (wt.%) 80 80 75 70 65 60 55

    CBN grains (wt.%) 15 15 15 15 15 15 15

    Alumina bubbles (wt.%) 5 10 15 20 25 30 35

    Porsity (%) 8 15 21 28 34 38 45

     Fig.5 shows the bending strength of the composite blocks. Obviously, the strength is

     decreased with the increasing porosity. When the porosity is changed from 8 % to 45 %, the strength is varied from 51 MPa to 103 MPa. Compared with the bending strength of the working

    115 layer of the vitrified CBN wheels with the same porosity, the strength of the CBN composite block is increased greatly. For most vitrified CBN grinding wheels, the bending strength of the working layer is about 30 MPa. Fig. 5 The bending strength of composite blocks with different porosity

    120 3 Dressing and grinding performance of composite-bonded CBN wheels

     3.1 Experimental set-up of dressing and grinding The tool conditions especially the wheel topography after dressing plays a significant role on the grinding performance of CBN abrasive wheels. The dressing and grinding experiments of the

    CBN wheels are carried out on BLOHM Profimat MT408 surface machine. Fig.6 shows the 125

    configuration of dressing and grinding experimental setup, respectively.

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     (a) Dressing (b) Grinding Fig. 6 Experimental setup of dressing and grinding process

    The dressing and grinding conditions are summarized in Table 3. To generate a desired surface 130

     topography, the dressing method with zirconia corundum (ZC) grinding wheel has been utilized to profile and sharpen the porous composite-bonded CBN wheel in this investigation (Fig.6(a)).

     During grinding, the workpiece material is nickel-based superalloy GH4169. Meanwhile, a vitrified CBN grinding wheel with the same conditions as grain size, concentration and porosity

    has been adopted for comparing the grinding performance with the porous one. The grinding force 135

    and temperature are measured simultaneously, shown in Fig.6(b).

    Tab. 3 Dressing and grinding conditions Type Parameters Value

     Wheel velosity (m/s) 30

     Dressing wheel velosity (m/s) 24 Dressing

     Longitudinal table speed (mm/min) 60

    Depth of cut (mm) 0.1

     Workpiece material GH4169

     Wheel velosity (m/s) 50 Grinding

     Infeed speed (mm/min) 4000

     Depth of cut (µm) 5, 10, 15, 20

     3.2 Pore formed after dressing

    Fig.7 displays the pore formed on the surface of the working layer of the composite-bonded 140

     CBN wheels after dressing. The regular pores are formed through the removal of the wall of the alumina bubbles. Seen from Fig.7, the pore of the alumina particle is exposed completely. Thus, the storage space may be provided for chips and coolant in the grinding process. 3.3 Comparison of grinding force and temperature of two types of CBN wheels

    145 Fig.8 shows the grinding forces and temperatures of both the composite-bonded CBN wheels

     and the conventional vitrified counterpart under the constant wheel speed of 50 m/s and workpiece infeed speed of 4 m/min. The specific grinding force of the composite-bonded CBN wheel is obviously lower than that of vitrified one. At the same time, the grinding temperature of the composite-bonded wheel is nearly 10 % lower, which indicates the performance advantage of the

    high-porosity composite-bonded CBN wheel. The detailed grinding characteristics of porous 150

    composite-bonded CBN wheels will be discussed in the further reports.

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     Fig. 7 Morphology of pore after dressing

     155 (a) Grinding force (b) Grinding temperature

     Fig. 8 A comparison of grinding force and temperature of two types of CBN wheels

     4 Conclusion In this paper, a new method to fabricate porous composite-bonded CBN wheels is adopted. Alumina (AlO) ceramics bubbles are used as pore-forming material for the abrasive wheels. 23

     160 Optimal sintering process of the CBN composite blocks is determined at 880? for 30 min. When

     the porosity of composite blocks is changed from 8 % to 45 %, the strength of the composite blocks is changed from 51 MPa to 103 MPa. After dressing, the regular pores are formed in the working layer of the CBN wheels. Grinding practice has shown that the grinding force and

    temperature of composite-bonded CBN wheel are obviously lower than that of the vitrified one

    under the same grinding condition. 165

     References [1] Aspinwall D K, Soo S L, Curtis D T, et al. Profiled superabrasive grinding wheels for the machining of a nickel based superalloy[J].Annals of the CIRP, 2007, 56(1): 335-338. 170 [2] Pal B, Chattopadhyay A K, Chattopadhyay A B. Development and performance evaluation of monolayer brazed

     cBN grinding wheel on bearing steel[J]. International Journal of Advanced Manufacturing Technology, 2010, 48(9-12): 935-944. [3] Kompella S, Zhang K, Pakalapati R. A new CBN crystal for improved grinding performance in vitrified bonds[J]. Key Engineering Materials, 2009, 389-390: 211-216.

    175 [4] Neslusan M. Grinding of Ni-based alloys with grinding wheels of high porosity[J]. Advances in Production

     Engineering & Management. 2009, 4(1-2): 29-36. [5] Tanaka T. New development of metal bonded diamond wheel with pore by the growth of bonding bridge[J]. Japan Society of Precision Engingeering, 1992, 26(1):27-32. [6] Truong S H, Isono Y, Tanaka T. Scanning electron microscopic study and mechanical property examination of 180 abond bridge: development of a porous metal bonded diamond wheel[J]. Journal of Materials Processing

    Technology, 1999, 89(8): 385-391.

    [7] Tomino H. Influence of porosity on grinding performance of porous cast iron bonded diamond grinding wheel

    made by pulse current sintering method[J]. Journal of the Japan Society of Powder and powder Metallurgy, 1999,

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     46(3): 257-261.

    185 [8] Su H H, Fu Y C, Xu J H, et al. Porous metal bond matrix with high porosity and large pore size fabricated using pore-forming agent[J]. Key Engineering Materials, 2009, 410: 279-283. ls (II)- [9] Dai Q L, Luo C B, Liao C J. Experimental study on porous metal bonded diamond grinding whee grinding performance of porous wheels[J]. Key Engineering Materials, 2008, 359-360: 48-52. [10] Xu X P, Yu Y Q, XPS and SEM characterization of wheel-workpiece interface in grinding of superalloy[J].

    Surface and Interface Analysis, 2002,33(4): 343-350. 190

     [11] Morgan M N, Rowe W B, S.C.E. Black S, D.R. Allanson. Effective thermal properties of grinding wheels and grains[J]. Process Installation Mechanical Engineers, 1998, 212: 661-669. e CBN wheel segments based on graphite [12] Chen Z Z, Xu J H, Ding W F, et al.Fracture morphology of composit self-lubricating effects[J]. Solid State Phenomena. 2011, 175: 33-37. 195 [13] Pan Q, Ding W F, Xu J H. Fabrication of composite blocks containing alumina bubble particles for porous

    CBN abrasive wheels[J]. Key Engineering Materials, 2012, 499: 229-234.

     多孔复合结合剂 CBN 砂轮的制作与性能分析

     丁文锋200 ?南京航空航天大学机电学院!南京 210016 摘要,采用 Al2O3 空心球、CBN 磨粒、Cu-Sn-Ti 合金与石墨颗粒开展了多孔复合结合剂 CBN 砂轮的制备试验。分析了烧结工艺与气孔率对 CBN 复合块抗弯强度的影响!开展了砂轮修 整与磨削试验。结果显示!优化的烧结工艺是 880?。当气孔率为 8-45%时!复合块抗弯强 度达到 51-103 MPa。修整后砂轮表面出现了规则气孔。同等条件下!与陶瓷结合剂 CBN 205 轮相比!多孔复合结合剂 CBN 砂轮的磨削力和磨削温度更低!这显示出新型多孔砂轮具有


     关键词,烧结(多孔 CBN 砂轮(Al2O3 空心球(抗弯强度(磨削性能



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