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Abstract

High-efficiency deep grinding experiments of Inconel 718 nickel-based superalloy was carried out with the porous metal-bonded cubic boron nitride superabrasive wheel, in which the uniform and large pores were formed by the broken alumina bubble particles in the working layer after wheel dressing. Grinding temperature, energy partitioning into workpiece, and wheel wear were investigated. Results obtained show that long maintenance of low grinding temperature, that is, 50 °C–170 °C, is obtained in high-efficiency deep grinding with the porous metal-bonded cubic boron nitride wheel. The energy partitioning into the ground workpiece is ranged from 2% to 6%, which is smaller than that with the conventional vitrified cubic boron nitride wheels and alumina abrasive wheels. Sufficient storage space for chips and coolants contributes to the excellent performance of the porous metal-bonded cubic boron nitride wheel in high-efficiency deep grinding. Abrasion wear and grain fracture are the dominant wear patterns of the porous cubic boron nitride wheel in the steady wear stage, while chips loading and grain pullout play a critical role in the final dramatic wear behavior of the porous wheel.

Keywords

Porous metal-bonded cubic boron nitride wheel grinding temperature energy partitioning wheel wear high-efficiency deep grinding

Introduction

The well-known cubic boron nitride (CBN) superabrasive wheels have broad prospects in grinding difficult-to-cut metallic materials, namely, nickel-based superalloy and titanium alloy, due to high hardness, high thermal conductivity, and high wear resistance of CBN superhard material.^1–9 However, because the widely utilized conventional vitrified CBN wheels merely provide a small and irregular space to store chips and coolants in grinding, a great potential of CBN wheels could not be exhibited completely at present. Moreover, limited by the low fracture strength of vitrified bonding materials, the tool edges and working layer (also called as abrasive layer) of the vitrified CBN wheels break easily, particularly in heavy-load grinding and high-speed grinding.^10–12 The application of CBN superabrasive wheels is, therefore, restricted to a certain extent.

In order to improve the mechanical strength of working layer of the CBN wheels, the porous metal-bonded CBN wheels have been developed in the recent years. Pore-forming agents, namely, NH₄HCO₃, CaCO₃, and dry ice (CO₂), have been utilized according to the previous literatures.^13–15 Although the high-strength working layer of CBN wheels was fabricated using the metallic bonding material, the pores generated with the above-mentioned agents always had irregular size and shape. The storage space of chips and coolants was still poorly abundant. In addition, it was also rather difficult to dress the metal-bonded CBN wheels when they became blunt.

In order to solve the above-mentioned problems, the authors put forward to develop the new-generation porous metal-bonded CBN wheels, in which Cu-Sn-Ti alloy works as the bonding material of the working layer, and alumina (Al₂O₃) bubble particles work as the pore-forming agents. The fabrication mechanism and dressing technique of the porous wheels have been investigated and reported.¹⁶ In particular, the effects of porosity on the bending strength of the working layer of the porous wheels (i.e. the porous CBN abrasive blocks) have been provided.¹⁷ Although the bending strength was decreased from 103 to 51 MPa when the porosity was increased from 8% to 45%, the strength of the porous metal-bonded CBN wheels was always much larger than that of the vitrified CBN wheel (e.g. nearly 30 MPa). Furthermore, preliminary evaluation of grinding performance of the porous metal-bonded CBN wheels was also carried out in shallow surface grinding. However, little investigation on the wheel performance in high-efficiency deep grinding (HEDG) of difficult-to-cut metallic materials was provided. As reported in some literatures,^18,19 the HEDG technology offers a possibility to obtain very high material removal rates, while maintaining good surface integrity of the ground workpiece. But on the other hand, characterized by high wheel speed, high workpiece speed, and large depth of cut, HEDG always results in very different machining behavior in contrast to the conventional grinding with shallow depth of cut.

In this work, the porous metal-bonded CBN superabrasive wheels are utilized in the HEDG experiments of Inconel 718 nickel-based superalloy. Grinding temperature, energy partitioning into workpiece, and wheel wear are investigated, which are expected to establish an experimental and theoretical bases for further application of this new-generation CBN superabrasive wheels in HEDG practice.

Experimental materials and procedure

Characteristics of porous metal-bonded CBN superabrasive wheels

Figure 1(a) and (b) displays a porous metal-bonded CBN wheel of 400 mm in diameter and 12 mm in width. According to the previous publications,^16,17 the fabrication procedure of the porous wheels could be summarized briefly as follows: (1) The Cu-Sn-Ti alloy powders, graphite powders, and alumina bubble particles were prepared according to the optimal porosity of 40%. The addition of graphite powders is to control the excess flow of molten Cu-Sn-Ti alloy and avoid the abrasive blocks collapse and constriction during sintering. The weight ratio of Cu-Sn-Ti alloy powder to graphite powders is 9:1. At this time, the bending strength of the porous wheels could reach about 60–65 MPa; (2) after the raw materials were mechanically mixed, the CBN grains were placed into the mixture layer by layer through the mold plate; afterward, the porous CBN abrasive blocks was pressed into a mold with a pressure of 440 MPa for 40 s; (3) the porous CBN abrasive blocks were sintered at 880 °C for 30 min in the vacuum furnace with the heating and cooling rates of 10 °C/min. The vacuum was kept at below 10⁻² Pa; (4) the wheel substrate and the sintered porous blocks containing CBN grains were bonded using epoxy resin adhesive and dried in an oven with a temperature of 180 °C for 45 min. Accordingly, the porous metal-bonded CBN wheels were fabricated.

Figure 1.

Porous metal-bonded CBN wheel: (a) whole, (b) regional, (c) schematic diagram of broken alumina bubble particles after wheel dressing, and (d) protrusive CBN grains after wheel dressing.

In particular, after wheel dressing with a vitrified zirconia corundum (ZC) abrasive wheel at the grinding speed of 20 m/s, infeed speed of 200 mm/min, and depth of cut of 0.005 mm, some walls of the alumina bubble particles on the wheel working surface broke. The uniform and large pores were, therefore, formed, as demonstrated schematically in Figure 1(c). Accordingly, the abundant storage space for chips and coolants was obtained. Meanwhile, high exposure and uniform interval of the CBN grains were also observed after wheel dressing (Figure 1(d)). All above were obtained due to the inherent characteristics of the porous metal-bonded CBN wheels. It is noted that in the porous metal-bonded CBN wheel, the inner diameter and the outer diameter of alumina bubble particles is about 550 and 600 µm, respectively. They are quite different from the reported pore diameter of the vitrified CBN wheels, for example, ranged from 160 to 500 µm²⁰ or from 15 to 60 µm.²¹ The row interval of CBN grains is approximately 2.0 mm. The present pore size and grain interval are preliminarily selected according to the HEDG requirements.^22,23 Further optimization could be made in the later HEDG work based on the current research fruits.

Grinding experiments

Grinding is performed on a high-speed surface grinder, BLOHM PROFIMAT MT-408. The maximum rotational speed is 8000 r/min (i.e. the maximum wheel speed of 170 m/s for the wheel of 400 mm in diameter), and the output power is 45 kW. Figure 2 schematically shows the experimental setup of HEDG with the porous metal-bonded CBN wheel. Up-grinding mode was applied. The ground material was widely used Inconel 718 nickel-based superalloy. In particular, the grinding temperature was detected by means of the semi-artificial thermocouple technique; therefore, the workpiece samples were cut equally to two blocks with the size of 15 mm (length) × 5 mm (width) × 25 mm (height). A constantan foil of 0.02 mm thickness was sandwiched between thermocouples of workpiece and was insulated by two mica sheets. A hot junction for a constantan–workpiece semi-artificial thermocouple was made during grinding. The thermal electromotive force was, therefore, formed and measured using a LabView data acquisition card connected to a computer, as displayed in Figure 2. Grinding force signal was measured with the piezoelectric dynamometer Kistler 9272. The CBN wheel topography was observed by Hirox KH-7700 optical microscopy and Quanta-200 scanning electron microscopy (SEM).

Figure 2.

Schematic illustration of the grinding experiment setup.

Table 1 lists the grinding conditions. The material removal rates Q′ of Inconel 718 alloy are ranged from 1 to 10 mm³/mmċs in the present HEDG experiments, which are much larger than that in the previously reported shallow grinding. For example, it is about 0.67 mm³/mmċs with Inconel 718 alloy at the wheel speed of 17.5 m/s, workpiece speed of 4000 mm/min, and depth of cut of 0.01 mm;²⁴ about 0.75 mm³/mmċs with Ti-6Al-4V alloy at the wheel speed of 19 m/s, workpiece speed of 9000 mm/min, and depth of cut of 0.005 mm;²⁵ about 0.91 mm³/mmċs with SiCp-Al composites at the wheel speed of 23.55 m/s, workpiece speed of 10,900 mm/min, and depth of cut of 0.005 mm;²⁶ and about 0.70 mm³/mmċs with AISI 1018 steel at the wheel speed of 22 m/s, workpiece speed of 600 mm/min, and depth of cut of 0.07 mm.²⁷

Table 1.

Grinding experiment conditions.

Types	Contents
Machine tool	High-speed surface grinder PROFIMAT MT-408
Wheel	Porous metal-bonded CBN superabrasive wheel
Grinding mode	Up-grinding
Wheel speed V_s	30–120 m/s
Workpiece speed V_w	600–3600 mm/min
Depth of cut a_p	0.1–1 mm
Cooling fluid	Water-based emulsion 4%; pressure at 1.3 MPa; coolants velocity at 40 m/s

Experimental results and discussion

Effects of grinding parameters on grinding forces

Grinding force is an important index to evaluate the grinding performance. Figure 3 demonstrates the effects of the grinding parameters on the grinding forces and forces ratio. Particularly, the measured data in the figures are an average value of five repeated tests. The corresponding standard deviation is demonstrated through the error bars. In general, the normal grinding forces are significantly increased with the decrease in the wheel speeds and increase in the workpiece speeds and depths of cut. When the workpiece speed is 600 mm/min and the depth of cut is 0.2 mm, the specific normal forces $F'_{n}$ are decreased from 32.0 to 23.0 N/mm (by 28.1%) with the increasing wheel speed ranged from 30 to 100 m/s (by 2.3 times); when the wheel speed is 80 m/s and the depth of cut is 0.2 mm, the specific normal forces are increased from 26.8 to 39.6 N/mm (by 47.8%) with the increasing workpiece speed ranged from 600 to 3600 mm/min (by five times); when the wheel speed is 80 m/s and the workpiece speed is 600 mm/min, the specific normal forces are increased from 13.3 to 70.2 N/mm (by 428%) with the increasing depth of cut ranged from 0.1 to 1.0 mm (by nine times).

Figure 3.

Effects of grinding parameters on grinding forces and forces ratio obtained with sharp porous CBN wheel: (a) wheel speed, (b) workpiece speed, and (c) depth of cut.

However, during the present HEDG process with the porous metal-bonded CBN wheel, it is observed from Figure 3 that although the absolute values of tangential forces variance with the increase in grinding parameters are not the same significant as that of the normal forces variance, the relative magnitude of tangential forces variance is still significant. For instance, the specific tangential forces $F'_{t}$ are changed from 4.0 to 3.2 N/mm (by 20%) when the wheel speed is increased from 30 to 100 m/s (Figure 3(a)); increased from 3.4 to 5.8 N/mm (by 70.6%) when the workpiece speed is increased from 600 to 3600 mm/min (Figure 3(b)), and increased from 1.6 to 7.4 N/mm (by 430%) when the depth of cut is increased from 0.1 to 1.0 mm (Figure 3(d)).

Accordingly, the force ratio $F'_{n}$ / $F'_{t}$ could be generally kept constant to a certain extent, which is between 6.8 and 8.0 according to Figure 3(a) and (b) and between 7.8 and 9.5 according to Figure 3(c), even though the grinding parameters are varied in the present investigation.

Effects of wheel wear advance on grinding temperature

During grinding nickel-based superalloy, a great amount of energy is converted to the grinding heat, most of which eventually is transferred into the workpiece and causes a high grinding temperature. Furthermore, with wheel wear advance in grinding, the heat flux is generally increased, which results in a rise of grinding temperature. In this work, in order to investigate the grinding temperature change, at the same time, to evaluate the tool endurance capacity and wear advance of the porous metal-bonded CBN wheel, a grinding procedure was conducted at the constant material removal rate of 4 mm³/mmċs with the fixed grinding parameters, that is, the wheel speed of 80 m/s, the workpiece speed of 1200 mm/min, and depth of cut of 0.2 mm. Figure 4(a) and (b) displays two typical grinding temperature curves measured using the thermocouples, which correspond to different wear statuses of the porous metal-bonded CBN wheel. Figure 4(a) is obtained at the sharp status, in which the grinding temperature is below 120 °C. This low temperature is for most of the grinding period in this work, for example, for the accumulated material removal volume Q of 6000 and 12,000 mm³, as displayed in Figure 4(c). However, in the final dramatic wear stage, for example, for the accumulated material removal volume of 16,000 mm³, rapid wheel bluntness gives rise to the abrupt increase in grinding temperature beyond 800 °C. The corresponding wear morphology of the porous metal-bonded CBN wheel displayed in Figure 4(c) will be analyzed in the subsequent section “Wear behavior of porous metal-bonded CBN wheel in HEDG.”

Figure 4.

Typical curves and variation of grinding temperature: (a) in the steady wear stage with the sharp CBN wheel, (b) in the final dramatic wear stage with the blunt wheel, (c) grinding temperature variation with the wheel wear advance from sharp to blunt, (d) simulated grinding temperature distribution with the sharp wheel, and (e) simulated grinding temperature distribution with the final blunt wheel.

A finite element simulation based on a moving heat source was also carried out to predict the grinding temperature distribution. The heat flux applied for the finite element simulation was computed from the average values of the tangential grinding forces, which was varied from 3.67 × 10⁶ to 3.95 × 10⁷ W/m². The 20-node hex elements were utilized to mesh the simulated components. The mesh method was applied to deal with the sharp temperature gradient in the workpiece top region, while keeping a reasonable element size to reduce the calculation time. Therefore, the fine elements were designed in the workpiece top region, and the coarse ones were in the bottom region of the ground workpiece. A triangular heat flux was assumed to the wheel–workpiece contact width. Details and boundary conditions of the finite element analysis were not discussed any more here.

The simulated temperature distribution within the ground workpiece is displayed in Figure 4(d) with the sharp CBN wheel in the case of the accumulated material removal volume of 500 mm³, and in Figure 4(e) with the blunt wheel in the case of the accumulated material removal volume of 16,000 mm³, respectively. The simulated temperature values are generally consistent with the ones measured in the grinding experiments. Moreover, high grinding temperature is always found within the wheel–workpiece contact zone when the thermal source moves along the ground workpiece surface. The heat conducted to the workpiece is convected away to the surroundings in the grinding process, which results in a temperature field tail. The temperature in the wheel–workpiece contact zone rises at a rate of 315 °C/s and falls at 76 °C/s in the case of the sharp CBN wheel and rises abruptly at 4106 °C/s and falls at 564 °C/s in the case of the blunt CBN wheel. It is noted that too high grinding temperature and abrupt temperature variation always have negative effects on the ground surface quality because they may result in grinding burn and large residual tensile stresses in the workpiece.^28–33 Meanwhile, the dramatic wear behavior of CBN wheels is also induced.

Effects of grinding parameters on grinding temperature

When the porous metal-bonded CBN wheel is kept at the sharp status, the measured and simulated grinding temperatures as functions of grinding speeds, workpiece speeds, and depth of cut are displayed in Figure 5. According to Figure 5(a), it is found that the grinding temperature remains nearly stable (e.g. 70 °C–90 °C) with the increase in the grinding speeds ranging from 30 to 120 m/s. However, higher temperature within the grinding zone is formed due to the greater workpiece speed, as shown in Figure 5(b). For instance, when the workpiece speed is 600 mm/min, the grinding temperature is only 70 °C, while it is increased to 170 °C when the workpiece speed is increased to 3600 mm/min. Similarly, the increase in depth of cut from 0.1 to 1.0 mm also leads to a higher temperature ranging from 50 °C to 130 °C, as demonstrated in Figure 5(c). It is noted that all the grinding temperatures measured and simulated ranged from 50 °C to 170 °C when the porous CBN wheels are at the sharp status. Such low grinding temperature in the current HEDG experiments favors the good surface quality because grinding burn and thermal damage of the ground workpiece could be avoided effectively.

Figure 5.

Effects of grinding parameters on grinding temperature obtained with sharp porous CBN wheel: (a) wheel speed, (b) workpiece speed, and (c) depth of cut.

Energy partitioning into the workpiece in HEDG

The total heat flux generated within the grinding zone is transferred into the workpiece, tool, chips, and coolants, respectively. Therefore, the temperature rise of the ground surface is generally influenced comprehensively by the total energy produced in grinding and the energy partitioning into workpiece. Therefore, it is necessary to take into accounts the energy partitioning. The total heat flux q_t can be calculated according to the following equation³⁴

q_{t} = \frac{F_{t} V_{s}}{b l_{c}}

(1)

where F _t is the tangential grinding force, V_s is the wheel velocity, b is the grinding width, and l_c is the geometrical contact length, as follows

l_{c} = \sqrt{a_{p} d_{s}}

(2)

where a_p is the depth of cut and d_s is the outer diameter of the porous metal-bonded CBN wheel.

The heat flux entering the workpiece, q_w , could be further expressed as³⁵

q_{w} = \frac{k_{w} V_{w}^{1 / 2} θ_{m a x}}{β α_{w}^{1 / 2} a_{p}^{1 / 4} d_{s}^{1 / 4}}

(3)

where k_w is the workpiece thermal conductivity, V_w is the workpiece speed, and β is a constant that depends on the heat source model. As for the triangular heat source model utilized in this investigation (as shown in Figure 6), β could be chosen as 1.06.^35,36 α_w is the workpiece thermal diffusivity and θ _max is the maximum grinding temperature.

Figure 6.

Triangular heat source distribution in grinding.

Combining equations (1) and (3), the energy partitioning into the workpiece R_w is calculated by

R_{w} = \frac{q_{w}}{q_{t}} = \frac{q_{w} \sqrt{a_{p} d_{s}}}{F_{t} V_{s}}

(4)

Figure 7 displays the calculated results of the energy partitioning into the ground workpiece as functions of the wheel speeds, workpiece speeds, and depth of cut. Compared with the energy partitioning into the workpiece, that is, 4%–8% with the conventional vitrified CBN wheel and 25%–65% with conventional alumina abrasive wheel,^34,37 a lower energy partitioning value ranged from 2% to 6% is generally available with the porous metal-bonded CBN wheel, which contributes to the low grinding temperature obtained in the current HEDG experiments. Furthermore, according to Figure 7(a), the energy partitioning into the ground workpiece has a generally decreasing tendency with the increase in wheel speeds. However, higher workpiece speeds and larger depth of cut always results in the larger values of energy partitioning into workpiece, as shown in Figure 7(b) and (c).

Figure 7.

Effects of grinding parameters on the energy partitioning into workpiece: (a) wheel speed, (b) workpiece speed, and (c) depth of cut.

Discussion on the formation of low grinding temperature in HEDG with porous metal-bonded CBN wheel

It is an interesting phenomenon and great advantage that only if the porous metal-bonded CBN wheel is kept at the sharp status, lower temperature is always obtained in deep grinding than that in shallow grinding, even though the material removal rates in deep grinding are much larger. However, once the abrasive wheel becomes blunt, the grinding temperature will get a rapid and dramatic rise. For example, Jin et al.³⁸ found that the grinding temperatures generated in deep grinding AISI 1095 steel were either low (approximately 200 °C) or high (above 800 °C) with the increase in accumulated removal volume. Xu et al.³⁹ carried out creep-feed deep grinding experiments of K417 nickel-based superalloy. They found that when the depth of cut was lower than 1.0 mm, the grinding temperature was below 100 °C. With the further increase in depth of cut, however, the grinding temperature was raised to more than 800 °C. Ding et al.⁴⁰ also carried out creep-feed deep grinding experiments of K424 nickel-based superalloy and discovered that the grinding temperature was below 100 °C when the depth of cut was lower than 0.2 mm. Meanwhile, Kim et al.⁴¹ reported that the grinding temperature measured was approximately 30 °C–60 °C in creep-feed grinding of AISI 1020 steel with depth of cut of 0.5 mm, which was 10–20 times that of the conventional low grinding depth, that is, 0.02–0.05 mm. Based on the above analysis, it is known that such low grinding temperatures such as 50 °C–170 °C are reasonable and reliable in the current HEDG experiments with the porous metal-bonded CBN wheel at the sharp status.

However, low grinding temperature obtained in deep grinding is not only associated with high sharpness degree of the applied abrasive wheel but also contributed to the critical point of coolant film boiling.¹⁸ When the grinding temperature exceeds the critical point of coolant film boiling, grinding burn is caused by a rapidly deteriorating cooling and lubrication environment. Therefore, the cooling and lubrication condition within the grinding zone is also very important for the long maintenance of low grinding temperature in deep grinding. In addition, the heat produced in grinding is localized merely within a rather small wheel–workpiece contact zone and is mainly dependent on the quantity of the coolants penetrated into the zone.^42,43 Besides the mechanical properties of the applied coolants, the characteristics of the pores on the working surface of the abrasive wheel, for example, size, shape, and distribution, are also the key factors determining the cooling and lubrication conditions. Davis et al.⁴⁴ have pointed out that because high porosity improved the role of cooling and lubrication, excellent grinding performance was obtained for the wheels with high porosity under the condition of large material removal rates. Klocke et al.⁴⁵ also found that the effective flow of coolants was proportional to the porosity.

Figure 8(a) and (b) comparatively demonstrates the difference of cooling and lubrication conditions in deep grinding with the conventional vitrified CBN wheel and the new porous metal-bonded CBN wheel, respectively. It is evident that when the vitrified wheel is applied, the coolants could not take good effects due to the rather small space in the wheel–workpiece contact zone. However, as for the porous metal-bonded CBN wheel, because the alumina bubble particles are broken after dressing, sufficient space for storing chips and coolants is provided, which ensures the excellent cooling and lubrication effects in deep grinding. SEM morphology of pores in the final blunt porous metal-bonded CBN wheel is provided in Figure 9. Obviously, large quantities of pores generated by the broken alumina bubble particles still have good shape even when the CBN wheel becomes blunt. Therefore, low grinding temperature could be always obtained even for a large material removal volume in HEDG, which is a great potential of the new-generation porous metal-bonded CBN wheel.

Figure 8.

Comparison of the cooling and lubrication effects within the grinding zone with (a) conventional vitrified CBN wheel and (b) porous metal-bonded CBN wheel.

Figure 9.

SEM morphology of pores on the working surface of a porous metal-bonded CBN wheel.

Wear behavior of porous metal-bonded CBN wheel in HEDG

In general, similar to the vitrified abrasive wheels,^46–48 the wear feature of a porous metal-bonded CBN wheel is mainly governed by the strength of the wheel constituents, bonding material and CBN grains under the given grinding loads. Figure 10 provides the schematic illustration and photography of different wear patterns of the porous metal-bonded CBN wheel, that is, abrasion wear or grain fracture, chips loading, and grain pullout. The reason for the abrasion wear and grain fracture (Figure 10(a)) and chips loading (Figure 10(b)) has been investigated deeply in some literatures concerning grinding of metal materials.^49–51 It is not discussed any more here.

Figure 10.

Schematics and experimental observation of typical wear behavior of porous metal-bonded CBN wheel in HEDG: (a) abrasion wear or grain fracture, (b) chips loading, and (c) grain pullout.

The grain pullout phenomenon mainly resulted from the bond fracture or grain–bond interface fracture. As for the bond fracture, different from the previously reported porous metallic bonding layer of the porous wheels,^52,53 the bonding layer of the present porous metal-bonded CBN wheel could be regarded as the particular particle-reinforced metallic matrix composites (PRMMCs) due to the enhancing effect of alumina bubble particles on the bonding layer strength.⁵⁴ As a consequence, it is scarce for the new-generation porous metal-bonded CBN wheels to form grain pullout through bond fracture. However, some grains may be pulled out from the bonding layer due to the occasional weak chemical joining within the interface between CBN grain and Cu-Sn-Ti bonding alloy or due to the joining interface fracture by the increasing grinding loads on the worn grains, as displayed in Figure 10(c). In general, the grain pullout behavior is uncommon in the steady wear stage of the porous metal-bonded CBN wheel, while comparatively evident in the final dramatic wear stage.

Ten regional zones chosen from the working surface of the porous metal-bonded CBN wheel are traced in the present HEDG experiments with the fixed grinding parameters. The identical results of wheel wear behavior are obtained in the 10 regional zones, among which a typical regional zone is displayed in Figure 4(c). There are altogether 11 protrusive grains after wheel dressing. For easier discussion, all the 11 grains are numbered. When the accumulated removal volume arrives at 6000 mm³, the total number of protrusive grains is still kept at 11, which corresponds to the steady wear period of the porous metal-bonded CBN wheel applied. Under such condition, the grinding temperature has a general stable change tendency. Particularly, it is observed that the active cutting edges of grain 9 endure attrition wear, while another two grains, that is, grain 5 and grain 10, experience micro-fracture in grinding. However, once the accumulated removal volume reaches 16,000 mm³, except grain 3, the other 10 grains exhibit serious wear behavior, that is, grain macro-fracture and grain pullout. Furthermore, the obvious scratches on the bond surface and chips loading are formed, which is because the wheel bonding layer also takes part in grinding when all the CBN grains are worn out to a very low protrusive height and even pulled out under the heavy grinding loads. Under such condition, the porous metal-bonded CBN wheel goes into the abrupt wear period with an extremely accelerating wear rate, which also results in a sharp rise of grinding temperature, as displayed in Figure 4(e). At this time, the next dressing procedure is required.

In particular, there only exists a long steady wear stage and a subsequent short abrupt wear stage in the current HEDG experiments with the porous metal-bonded CBN wheel. It is advantageous upon the traditional three wear stages with the vitrified CBN wheels or alumina abrasive wheels, that is, the early wear stage with a rapid wear rate, the middle wear stage with a more or less constant wear rate, and finally the dramatic wear stage with a rapid wear rate.^55,56

Conclusion

Long maintenance of low grinding temperature ranged from 50 °C to 170 °C is obtained under the condition of high material removal rates and large material removal volumes in HEDG of Inconel 718 nickel-based superalloys with a porous metal-bonded CBN wheel.

When HEDG is carried out with the porous metal-bonded CBN wheel, the energy partitioning into workpiece is between 2% and 6%, which is smaller than that with the conventional vitrified CBN wheels and alumina abrasive wheels.

Compared with the conventional vitrified CBN wheels, excellent grinding performance of the porous metal-bonded CBN wheel is mainly contributed to the large storage space for chips and coolants provided by the broken alumina bubble particles in the bonding layer after wheel dressing.

The dominant wear patterns of porous metal-bonded CBN wheel in HEDG are abrasion wear and grain fracture in the steady wear stage, while chips loading and grain pullout play an important role in the final dramatic wear stage of the porous wheel.

Footnotes

Acknowledgements

The authors appreciate Dr Zhen-Zhen Chen for the experimental assistance.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the National Natural Science Foundation of China (nos 51235004 and 51375235), the Fundamental Research Funds for the Central Universities (no. NE2014103), and the Science and Technology Supporting Program of Jiangsu Province (no. BE2013109).

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Grinding temperature and wheel wear of porous metal-bonded cubic boron nitride superabrasive wheels in high-efficiency deep grinding

Abstract

Keywords

Introduction

Experimental materials and procedure

Characteristics of porous metal-bonded CBN superabrasive wheels

Grinding experiments

Experimental results and discussion

Effects of grinding parameters on grinding forces

Effects of wheel wear advance on grinding temperature

Effects of grinding parameters on grinding temperature

Energy partitioning into the workpiece in HEDG

Discussion on the formation of low grinding temperature in HEDG with porous metal-bonded CBN wheel

Wear behavior of porous metal-bonded CBN wheel in HEDG

Conclusion

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References