Developing a novel porous aggregated cBN wheel and evaluating its grinding performance during machining Ti–6Al–4 V alloys

A novel porous aggregated cBN (AcBN) wheel was prepared under high-temperature sintering processes to ensure the desired machining efficiency and quality of Ti–6Al–4 V alloys. The designations of pore structures and AcBN grains within metallic grinding wheels operating at the high efficiency deep grinding processes were conducted. Variations of grinding forces and force ratio, grinding temperature, specific grinding energy along with grinding parameters, were carried out. In addition, characterization analyses of grain wear morphologies of wheel surface were then performed with vitrified monocrystalline cBN (McBN) wheels and porous AcBN wheels. Findings show that the employment of Ti-coated cBN particles contributes to the improvement of chemical bonding strength between grains and metallic matrix alloys inside AcBN grains. Compared with the severe grain wear of vitrified McBN wheels, porous AcBN wheels possess the excellent wear resistance ability and desired machining quality, owing to the abundant chip storage space and micro-fracture properties of cBN particles.


Introduction
A consistent increase in requirement of excellent grinding efficiency and quality for difficult-to-cut materials (e.g. titanium alloy, superalloy, etc.) of key components has been presented in modern aerospace industries [1][2][3][4]. The machining challenges are confronted by using traditional abrasive wheels, including the poor grinding efficiency and quality, as well as the heavy wheel wear [5][6][7][8]. Presently, the cubic boron nitride (cBN) superabrasive grinding wheels have attracted much attentions to solve the above problems, resulting from its superior properties (e.g. high strength, great impact-resistance performance etc.) [9][10][11][12][13]. However, the macro-fracture of grains caused by heavy loads appears easily, rooting from the anisotropic properties of monocrystalline cBN (McBN) superabrasives [14,15]. Subsequently, the machining efficiency and service life of grinding wheels will be drastically reduced due to the reduction of grains' macro-fracture along the cleavage plane and then decrease in dynamic sharpness of grinding wheels [16][17][18]. Therefore, it is particularly urgent to develop high grinding performance superabrasive grains, as well as the corresponding metalbonded superabrasive grinding wheels, aiming at improving grinding performance of wheels and being able to guarantee surface integrity.
Recently, numerous researches have focused on developing newly high-performance abrasive grains with good self-sharpening abilities and evaluating the corresponding grinding performance, wear characteristics, material removal mechanism at home and abroad [19][20][21][22][23][24][25][26][27]. Polycrystalline cBN (PcBN) grains fabricated using numerous tiny cBN superabrasive grains (mesh size of 2000#) and ceramic phases under the condition of high temperature and pressure were firstly proposed by Ichida et al. [19,20]. The higher grinding efficiency and lower grinding forces could be found during machining Inconel718 superalloys, compared with the commercial vitrified cBN grinding wheels. An enormous amount of native studies were also conducted, in terms of the PcBN grain wear mechanism, the development of high-performance PcBN grinding wheels, the formation mechanism of brazing interfaces, and associated high-efficiency grinding processes [21][22][23][24][25][26]. They revealed that the main reason of abrasion wear for PcBN grains was resolved into chip adhesion, and the initial crack and propagation appeared easily along the more brittle ceramic phase. In this case, the sharpness of PcBN grains could be maintained for a long time along with the exposure of micro cutting edges in the next layer. Noted that the tool's service life was severely affected as the macro-fracture of numerous grains occurred under continuous impacts of alternating heavy loads with PcBN grinding wheels. Similarly, McKie et al. [27] revealed that the micro-grain size and the concentration ceramic phases inside PcBN grains severely affected the grinding ability. In addition, compared to the brittle characteristics of ceramic phases, the employing of metal-bonded phases inside superabrasive grains contributed to improve its impact-resistance ability. However, PcBN grains have been always fabricated with ceramic phases by sintering processes in recent years and the size and content of micro-grains were relatively single, limiting the further application of corresponding superabrasive tools.
Furthermore, a new-type grinding wheel with 30% aggregated corundum abrasives was developed to guarantee the grinding efficiency and quality for titanium alloys, as revealed by Kacalak et al. [28] The better sharpness and higher material removal rate (appropriate 8.33 mm 3 /(mm·s)) could be obtained compared with conventional grinding wheels. However, the rapid grain wear and tiny chip storage space of aggregated corundum grinding wheels have limited the further improvement of the grinding performance and service life, resulting from the low wear-resistance of corundum abrasives. In the basis of the above-mentioned researches, a new-type aggregated cBN (AcBN) abrasive was developed and the corresponding grinding performance was then evaluated through the single-grain grinding trials, as reported in our previous studies [29]. The grinding performance of AcBN grains was improved significantly compared with the monocrystalline cBN superabrasives by altering the fracturing mode from brittle trans-granular fracture into the coexistence modes of inter-granular and trans-granular fractures, owing to the existence of ductile metallic phases and numerous micro cBN superabrasives. However, the sintering mechanism of AcBN grains and corresponding grinding performance of AcBN wheels should be further discussed.
In the current paper, the AcBN superabrasive grains were developed using Cu-Sn-Ti powders as metallic phases and cBN particles as material removal units under the hightemperature liquid sintering technology. The forming mechanism of reacted products and optimization of sintering parameters for AcBN grains were first studied. Subsequently, the grinding performance and wear characteristics were evaluated during grinding Ti-6Al-4 V alloys. Finally, the conclusions were summarized. Figure 1 shows the morphology of raw materials, including Cu-Sn-Ti alloy powders (i.e. Cu-18 wt.%Sn-10 wt.%Ti) and as-sintered AcBN grains (mesh size of 40/45#) detected by a scanning electron microscope (SEM, COXEM EM-30).

Fabrication of AcBN grains
Here, Cu-Sn-Ti alloys compose of near spherical Cu-Sn alloys (30 μm in diameter) and irregular titanium particles (10-40 μm in length). Besides the indispensable Cu-Sn-Ti alloys inside AcBN grains, cBN particles coated by Ti layers with the mesh size of 140/170# are the other main constituent. The volume fraction of cBN particles inside AcBN grains was optimized as 90 vol.% to guarantee the grain strength and provide sufficient number of cutting edges. Meanwhile, a few drops of volatilizable liquid paraffin were added to increase uniformity of cBN particles inside AcBN grains during materials blending processes. Subsequently, the mixtures (i.e. 10 vol.% Cu-Sn-Ti alloys and 90 vol.% Ti-coated cBN particles) and liquid paraffin were uniform blended, and then the above raw materials were placed in a cold uniaxial pressing module. After pressing 30 s at 20 MPa, the compacting process was completed and then sieved with a dimension of 40/45#. The sintering process was finally performed at 800 ℃ for 10 min to fabricate AcBN grains under a high-vacuum phenomenon (< 10 −2 Pa). Furthermore, a subsequent sieving procedure with a dimension of 40/45# was conducted to ensure the good uniformity of size and performance of fabricated AcBN grains, as detected in Fig. 1b and c.

Preparation of AcBN grinding wheels and experimental setups
In basis of the above-developed AcBN grains, the fabricating procedures of porous AcBN wheels could be divided as follows: (i) Cu-Sn-Ti alloys and molybdenum disulfide (i.e. MoS 2 ) powders were blended as main metallic matrix constituents uniformly, contributing to maintain the skeleton of abrasive layers during sintering processes; (ii) subsequently, the pattern arranging method was employed to put grains and near spherical carbamide particles in a metal pattern plate (total twelve layers). Furthermore, the mixed metallic matrix powders were divided into 13 parts and placed on each abrasive layer. The above arranging procedure was stopped once the total twelve layers were completed; (iii) these arranged powders (e.g. abrasives, carbamide particles, and metallic matrix powders) were placed into a cold uniaxial pressing module, and then the compact was formed. Prior to the sintering process at 880 ℃ for 30 min, porous structures were generated by removing carbamide particles inside compacts after dissolving 4 h in distilled water. Here, the heating and cooling at a rate of 10 °C/min were fixed at the pressure of 10-2 Pa in a VAF-20 vacuum furnace; (iv) the bonding process with 32 prepared segments and wheel substrates was conducted by using thermosetting adhesives at 150 ℃ for 90 min. Prior to experimental trials  Depth of cut a p (mm) Special grinding forces to machine Ti-6Al-4 V alloys, the truing and sharpening processes were employed for porous AcBN grinding wheels to ensure the machining stability. Subsequently, the running tests of dressed wheels should be performed in Safety Test Machine to guarantee the operation safety during grinding processes.
In this paper, the subsequent grinding performance trials with Ti-6Al-4 V alloys were performed in high-speed precision grinder using developed porous AcBN wheels. Here, this grinder (PROFIMAT MT-408) was equipped with desired coolant systems and precision electric spindle, satisfying the requirements of machining conditions. Meanwhile, the workpiece (dimension: 50 mm in length, 9 mm in width, and 60 mm in height) was first machined by using electrical wire-cutting techniques and then the grinding process on the top surface of workpiece was applied using alumina abrasive wheels until the machining roughness satisfied the requirements (R a < 0.4 μm). Various machining parameters were adopted as listed in Table 1. Prior to grinding processes, the acoustic emission instrument was assembled on the fixture and then the acoustic emission signal would be detected to distinguish the actual contacting state between the wheel and workpiece. In this case, the depth of cut could be ensured accurately. During grinding processes, the grinding forces were detected using force senor (Kistler 9253B) and the temperature was detected by the semithermocouple method. After each grinding parameter, a Mahr M2 instrument was employed to obtain the R a value of machined workpiece surface at entrance, middle, and export areas and the average R a value was used as evaluation indicators. In addition, the morphologies of machined workpiece and grinding wheel surface were achieved using the 3D optical microscope and SEM (Fig. 2).

Formation mechanism of AcBN grains bonding interface
The X-ray diffraction results of AcBN grains were measured to reveal chemical reaction products, as shown in Fig. 3.  (1)) as evaluation indicators were adopted to determine whether to conduct the chemical reaction spontaneously at a given temperature.
where ΔH (J), ΔS (J·K −1 ) and T (K) present an increment of reaction enthalpy and reaction entropy, as well as the current reaction temperature. The chemical reaction can proceed forward until the ΔG value is negative. When the reacting temperature ranges from 800 to 1300 K, the following chemical reactions will occur at the bonding interface between the grain and coating layer.
In this paper, the highest sintering temperature during preparing porous AcBN wheels reaches 880 ℃, the ΔG values in Eqs. (2), (3) and (4) are − 81.34 kJ/mol, − 156.85 kJ/ mol and − 292.73 kJ/mol, respectively, revealing that the above reactions can perform. The diffusion coefficient of N element into Ti element layer is 4.8 × 10 −7 exp(-5600/T) cm 2 /s, which is higher than that of B element. In this case, the Ti-N compound is first produced owing to the faster contact between N and Ti elements than the contact between B and Ti elements. Subsequently, the Ti-B compound will be generated on the surface of cBN grains. In addition, when the reacting temperature exceeds 620 ℃, the TiN layer is priority to generate on the internal surface of Ti layer and always exists as the reacting temperature raises. Once the reacting temperature exceeds 820 ℃, the TiB 2 is gradually produced on the surface of cBN particles. When the reacting temperature increases into 880 ℃, the thickness of TiB 2 layer tends to a steady increase and thus diffusion rate of B element slows down further, until the TiB 2 product reaches saturation point. The TiB compound will be generated along with the contact between the TiB 2 and Ti element. Therefore, the TiB 2 , TiB and TiN compounds are produced in turn from the surface of cBN particles to Ti-coating layer. Here, the growth process is mainly controlled by generated TiN compounds, whereas the introduction and diffusion of Ti element are the main factors of interfacial layer growth. Furthermore, the interface reaction between cBN particles and Ti-coating layer becomes weaker as the reaction continues. The Ti elements inside Ti-coating layers are prone to conduct chemical reaction with liquid Cu-Sn alloys and then CuTi 3 compound is generated. Along this line of consideration, the diffusion of active Ti elements during sintering processes attributes to generate high bonding interface strength. Figure 4 shows the influences of machining parameters on grinding forces and force ratio are studied. A rapid reduction and gradual smooth trends of special normal F ′ n and tangential F ′ t grinding forces can be observed as v s ranges from 30 to 120 m/s (Fig. 4a). Here, v w and a p are fixed at 3.6 m/min and 0.1 mm, respectively. When v s increases in the range of 30-50 m/s, F

Grinding temperature
Grinding temperature as a crucial indicator was employed to reveal the evaluation of machining performance as grinding parameters varies with porous AcBN wheels, as shown in Fig. 5. A linear increase of grinding temperature can be observed from 313.6 to 556.8 ℃, by 77.6% as v s increases in the range of 30-120 m/s (Fig. 5a). Here, v w and a p are fixed at 3.6 m/min and 0.1 mm, respectively. This phenomenon implies that a large amount of heat is generated and gathered at grinding arc area as the increase in v s and associated grinding power, leading to the rapid rise of grinding temperature on machined surface of workpiece. Furthermore, the effects of sliding and ploughing on grinding arc area are enhanced with a smaller a gmax . In this case, a large amount of heat caused by drastic friction is gathered at grinding arc area and temperature is then increased.
When v w increases from 0.6 to 6.0 m/min, a straight increase and then stable fluctuation of grinding temperature can be observed, which is affected by the comprehensive multifactor functioning by the heat source strength and its moving speed. As v w is less than 4.2 m/min, the temperature can be measured from 130.7 to 650.4 ℃, by approximately 4 times, with a straight slope equal to 144.4. Subsequently, a gradual increase of grinding temperature is seen from 650.4 to 693.7 ℃, by only 6.7% as v w reaches 6.0 m/min. This can be explained that the heat strength is gradually increased, owing to the increase in grinding load and heat source as the material removal rate raises. However, the contacting time of heat source on workpiece surface is then decreased, leading to a rapid increase and gradual reduction of temperature. In addition, when a p increases from 0.1 to 0.6 μm, a significant rise of temperature is found from 556.8 to 889.8℃, by 59.8% (Fig. 5c) and no burn on the workpiece surface is observed. As a p reaches 1.0 μm, a stable variation tendency of temperature can be observed. However, the similar color on workpiece surface reveals is detected with the material removal rate of 60 mm 3 /(mm·s) compared with other optical morphologies, indicating that no obvious burns appear.
Here, the largest material removal rate is significantly higher than that of the reported studies [31]. This phenomenon implies that the generated heat can be effectively conducted using the developed porous AcBN grinding wheels.

Specific grinding energy
It is widely known that the consumed energy to remove per unit materials (i.e. specific grinding energy: e s ) is usually used as an evaluation indicator to reflect the grinding performance of grinding wheels, which is affected by the sharpening ability of wheels. Seen from Eq. (5), the e s value varies in direct proportion to v s and F ′ t , as well as in inverse proportional to v w and a p . Here, the highest e s value reaches 620.7 J/mm 3 . With the increase in v w and Q ′ w , the effects of sliding and ploughing are weakened as the a gmax value increases. Here, the cutting process occupies the main proportion and thus the e s value is rapidly reduced. The corresponding a gmax value increases to 1.07 μm, the e s value is decrease to 40.0 J/mm 3 as the Q ′ w raises into 60 mm 3 /(mm·s).

Wear morphology
In this paper, comparative grinding trials were performed with vitrified McBN grinding wheels to reveal wearresistance abilities of fabricated wheels under the grinding parameters: v s of 120 m/s, v w of 3.6 m/min, and a p of 0.1 mm. Figures 7 and 8 show the surface wear morphologies for vitrified McBN and porous AcBN wheels, respectively. Numerous irregular pores with an equivalent diameter of approximately 100 μm can be seen and most of the pores is blocked with chips (Fig. 7). Meanwhile, the chip adhesion, as well as micro-and macro-fractures of cBN particles can be seen. In addition, porous AcBN wheels possess the adequate chip storage space with a high porosity of approximately 60 vol.%. Besides the generated intrinsic micropores, the open pores with a dimension of 0.6-0.8 mm can be detected. Here, the open pores are produced by removing near-spherical carbamide particles during the water soluble stage. However, the adequate chip storage space is maintained during wheel wear trials, which is prone to obtain the desired high machining quality and efficiency. Figure 9 depicts the reconstructed pore structures and the detected 3D optical topography, revealing the large open pores and micropores (within 100 μm). Here, X-ray scanning tests were performed on the X5000 (NSI) using spiral acquisition and reconstruction with aspatial resolution of 10 μm. Then, the porosity of a single segment of abrasive layers is calculated using the commercial software Avizo 9.4.

Conclusions
A new-type porous superabrasive wheels were fabricated using AcBN grains as material removal units and then the sintering mechanism on bonding interface inside AcBN grains was discussed. Subsequently, the grinding performance and wear characteristics were evaluated during grinding Ti-6Al-4 V alloys. Findings were summarized as follows:

Strong bonding interface between metallic phases and
Ti-coated cBN particles inside AcBN grains can be achieved through the diffusion of active Ti elements and thus new generated compounds (i.e. TiB 2 , TiB and TiN) on the surface of cBN particles during sintering processes. Data availability All data generated or analyzed during this study are included in the present article.

Declarations
Ethics approval and consent to participate The article follows the guidelines of the Committee on Publication Ethics (COPE) and involves no studies on human or animal subjects.

Competing interests
The authors declare no competing interests.