3.1. Maximum undeformed chip thickness
As an important parameter in the grinding process, the agmax directly influences the wear condition of the abrasive grains. The ΔH can quantitatively reflect the wear condition of AcBN grain. As shown in Fig. 3, the ΔH of AcBN grain varies with the specific material removal volume (V′) for different agmax under CG and UVAG, respectively. Both CG and UVAG show that the value of ΔH increases and then decreases with an increasing value of agmax. This is because there are differences in the wear condition of the grains as the grinding process transitions from rubbing, plowing, and ploughing to the cutting stage. It can be clearly seen that the AcBN grain reaches the target radial wear height volume and loses its grinding capability at a V′ value of 2.4 mm3/mm for agmax = 1.2 µm, which is 0.6 mm3/mm less compared to the V′ with agmax < 1.2 µm. Secondary, the ΔH is larger than that of the CG at UVAG, and its average increase is 10.8%. On the one hand this is due to the introduction of ultrasonic vibration, which brings about periodic vibration of the abrasive grain, causing an increase in the actual agmax, which induces a cyclic impact force and accelerates the micro-fractrue of the AcBN grain. On the other hand, γ-TiAl is prone to material adhesion phenomenon in CG, which to a certain extent will make the measurement of the ΔH value of abrasive grains small. The periodic vibration can also effectively make the AcBN grain and the workpiece produce intermittent separation effect, effectively reducing the heat accumulation phenomenon in the grinding process and avoiding the influence of material adhesion on the measurement of the ΔH value.
The grain wear is an important factor affecting the grinding force, so the change of the grinding force can reflect the wear of abrasive grains laterally. Figure 4 shows the curves of the effect of agmax on normal Fn under CG and UVAG, respectively. Overall, the Fn value increases with the increase of the V′ in a class exponential function image. There exists a stage of smooth growth when the V′ = 1.2–2.4 mm3/mm, during which AcBN grains are in the steady wear stage. At present, the AcBN grains are constantly micro-fracturing to form multiple micro-cutting edges, which possess excellent cutting ability, while the Fn value increases gradually. After entering the severe wear stage of 2.4-3.0 mm3/mm, the abrasive grains are macro-fracture and pulled out, and large areas of material adhere to the AcBN grain., which leads to a sharp increase in the normal force. When agmax < 0.8 µm, the Fn increases at a faster rate with an increase in agmax. This is because, at this stage, the grinding process involves rubbing and ploughing, and the workpiece material flows along the front blade surface and the side of the grain under the action of the sassafras force. When agmax > 0.8 µm, grinding enters the cutting stage, and the material is removed in the form of chips. And the Fn under UVAG is smaller than that under CG, with an average reduction of 10.5%. The addition of ultrasonic tangential vibration increases the percentage of the cutting stage when the abrasive grain just contacts the workpiece, and at the same time reduces the effective contact arc length, which relatively reduces the percentage of the stage of rubbing, plowing force, and increases the percentage of the cutting force increases, so that more energy is used in the material removal process, which helps to reduce the Fn.
The \({\overline {R} _{\text{v}}}\) is an important parameter to characterize the abrasive grain removal capability, which also reflects the wear condition of the abrasive grain. The critical value of the agmax in the material removal into the cutting stage is called the critical chip thickness, and from Fig. 4, agmax = 0.8 µm is closer to the critical chip thickness, no matter it is CG or UVAG. When agmax < 0.8 µm, that is, agmax is less than the critical chip thickness, with the increase of agmax, the \({\overline {R} _{\text{v}}}\) value is increasing. Meanwhile, grinding is in the stage of rubbing and ploughing, the workpiece material from a single elastic deformation to elastic deformation and plastic deformation, increasing agmax will lead to more material pile-up, the \({\overline {R} _{\text{v}}}\) value becomes larger. When agmax > 0.8 µm, that is, agmax is greater than the critical chip thickness, grinding has been in the cutting stage, but with the increase of agmax, some of the cBN grains with low height of emergence are also involved in the grinding process, making the whole AcBN grain to withstand the increasing force, and some of the cBN grains with low holding force are directly pulled out, and the holes after pulling out are filled by the chips one after another. The remaining cBN grains undergo rapid macro-fracture, resulting in a further reduction in material removal capacity and a decrease in abrasion resistance of the abrasive grains. In the mass, the \({\overline {R} _{\text{v}}}\) value increases with the V′ value, and increases parabolically from 0.6 mm3/mm to 1.2 mm3/mm, then decreases sharply after the V′ value is larger than 1.2 mm3/mm, and then slows down in the range of 2.4 mm3/mm to 3.0 mm3/mm. In addition, the \({\overline {R} _{\text{v}}}\) value is smaller than that of CG under UVAG, and its average decrease is 22.2%, indicating that ultrasonic vibration can effectively enhance the material removal ability of AcBN grain. The reasons are as follows: from Fig. 2, it shows that the chips are longer in CG, and the AcBN grain undergoes a longer process of rubbing and plowing, which is prone to cause material pile-up on both sides of the grooves. As shown in Fig. 6, the AcBN grain is always in contact with the grinding arc area, it causes the abrasive chip to stay on the front blade surface of the cutting edge of the AcBN grain all the time, which reduces the sharpness of the abrasive edge and increases the pile-up of the material. While in UVAG, the AcBN grain is in intermittent contact with the grinding arc area, a grinding arc area is divided into multiple regions for cutting removal, the chips are shorter, the AcBN grain undergoes a shorter process of rubbing and plowing, and the material is mostly removed by the chips. In addition, due to the presence of ultrasonic vibration, the AcBN grain and the workpiece are constantly impacted, which is conducive to the detachment of chips from the surface of the AcBN grain, maintaining the sharpness of the cutting edge of the AcBN grain, which can effectively improve the quality of surface grinding.
3.2. Grinding speed
The vs is also an important parameter affecting the grinding process, which directly influences the wear condition of the abrasive grains. As can be seen from the previous section, when agmax > 0.8 µm, grinding is mainly in the cutting stage, so the agmax of a single grain at each vs is controlled at 1 µm. As shown in Fig. 7, the variation curves of the ΔH of AcBN grain with the V′ at different vs. Whether CG or UVAG, when vs ≥ 100 m/s, the ΔH of AcBN grain increases parabolically with the V′, and the V′ of AcBN grain only reaches 2.4 mm3/mm; while when vs ≤ 80 m/s, the ΔH of AcBN grain is closer to linear growth under UVAG, and the ΔH of AcBN grains firstly wear rapidly under CG, and then experienced steady wear, and finally severe wear, with the V′ of the AcBN grains reaching 3.0 mm3/mm. This is because that an increasing vs leads to an increase in the grinding power for the same parameters, which makes the grinding temperature rise. This not only elevates the thermal softening effect on the material, but also the AcBN grains are subjected to severe thermal damage during the grinding process, resulting in rapid grain wear at vs ≥ 100 m/s, and they have lost their grinding ability by the time the V′ value reaches 2.4 mm3/mm. Meanwhile, the ΔH of the AcBN grains is greater than CG at UVAG, and its average increase is the value varies at different vs. the average increase of the ΔH of AcBN grains at vs ≤ 80 m/s is 3.5%, 10.5%, and 12.1%. Respectively, the average difference between the two chances to be close to 0% when vs ≥ 100 m/s. As mentioned above, the addition of ultrasonic tangential vibration induces a cyclic impact force that accelerates the micro-fracture and increases the ΔH of the AcBN grains, but at higher vs, the intermittent separation of the UVAG is weakened, resulting in the ΔH of the AcBN grains being closer to the CG.
Figure 8 shows the variation curves of Fn with V′ at different vs. As mentioned before, at agmax = 1 µm, the grinding force is mainly determined by the cutting force. At the same time, due to the increase of the vs, the workpiece material is subjected to approximately significant thermal softening, and the cutting force is even smaller, so that the Fn decreases with the increase of the vs, both under UVAG and under CG. Secondly, the Fn increases with V', following an exponential-like function. There is a steady wear stage, and UVAG can reduce the Fn for the same reasons described in section 3.1. It is noted that the maximum reduction in Fn varies for different vs and is 10.9%, 10%, 12.8%, 8.3% and 3.8% in descending order with vs The interrupted cutting behavior caused by ultrasonic vibration is one of the main reasons for the reduction in grinding force, but due to the increase in the vs, the separation factor of the interrupted cutting decreases, resulting in a reduction in the Fn reduction at high vs.
Figure 9 illustrates that the \({\overline {R} _{\text{v}}}\) values exhibit a parabolic increase and then decrease with the V′. Additionally, under UVAG, the \({\overline {R} _{\text{v}}}\) values are smaller than those of CG, indicating that UVAG can improve the material removal rate with an average decrease of 23.7%. Secondly, the \({\overline {R} _{\text{v}}}\) decreases and then increases with the increase of vs, and the \({\overline {R} _{\text{v}}}\) of the material is the smallest when vs = 80 m/s, i.e., the material removal ability is the strongest. At low vs, the rubbing and plowing effects are more pronounced, and the chips are more likely to adhere to the AcBN grains, which reduces the material removal ability of the AcBN grains. After increasing the vs, the effect of cutting force is more obvious, and the material removal ability is enhanced. However, continue to increase the vs, the larger grinding power causes the AcBN grains to withstand higher thermal damage, resulting in a reduction in the material removal ability. γ-TiAl has a strong viscosity in the grinding process. Therefore, whether it is UVAG or CG, low-speed grinding is more likely to cause plastic flow, resulting in uneven groove bottom morphology, grooves on both sides of the pile-up is larger, it is generally advisable to use high-speed grinding on the γ-TiAl material grinding process.
3.3. Ultrasonic amplitude
Figure 10 shows the curves of ΔH with V′ for different A at agmax =1 µm and vs = 80 m/s. Overall the ΔH of AcBN grains increases with the A, and its average increase ranges from 11.7–82.2%. The difference is that when A = 0 or 3 µm, the variation curve of ΔH conforms to the general wear curve, which is divided into initial wear, steady wear, and severe wear stages. The periodic impact force leads to accelerated wear of the AcBN grains, and with the increase of A, the distance of the AcBN grains vibrating in the same cycle time increases, and the speed of the abrasive grains' movement in the vibration direction grows, which leads to an increase in the periodic impact force and exacerbates the wear of the AcBN grains.
The relationship between the change of Fn with V′ at different A is shown in Fig. 11. It can be seen when agmax =1 µm and vs = 80 m/s, the wear curves of the Fn under different A show a slow growth followed by a sharp increase with the increase of V′. Taking A = 6 µm as an example, specifically, the normal force increases rapidly by 0.77 N in the range of 0.6–1.2 mm3/mm, slowly by 1.7 N in the range of 1.2–2.4 mm3/mm, and sharply by 1.99 N in the range of 2.4-3.0 mm3/mm, which results in a final increase of 53.3% in Fn compared to that at 0.6 mm3/mm. In addition, the Fn shows a tendency to decrease and then get engaged with the increase of A, and reaches a minimum at A = 6 µm. This is because the application of ultrasonic vibration causes micro-fracturing of the AcBN grains between the AcBN grains and the workpiece to be intensified by periodic transient impacts, thus maintaining the dynamic sharpness over a long period of time, while increasing the proportion of cutting force so that more capacity is used to remove material. So moderately increasing the A can effectively reduce the Fn. However, as shown in Fig. 16, once the ultrasonic amplitude is too large, it is easy to increase the periodic transient impact force, the abrasive grains are prone to cleavage fracture and the bond cracks lead to pull out to aggravate the wear process. At this time, the grinding performance is greatly reduced, and the Fn shows a tendency to increase.
Ultrasonic vibration can accelerate the micro-fracture of the AcBN grains to a certain extent, so that the AcBN grains maintain a stable dynamic sharpness. However, it will also aggravate the degree of grain wear. In short, the AcBN grains of the material removal ability to be determined by the joint decision of the two. Figure 12 shows the relationship between the \({\overline {R} _{\text{v}}}\) and the V′ at different A. The \({\overline {R} _{\text{v}}}\) is always lower than that of CG at UVAG, even though the ultrasonic vibration increases the grain wear of the AcBN grains, its effect on the material removal capacity is greater than that of the grain wear of the abrasive grains. Meanwhile, the \({\overline {R} _{\text{v}}}\) increases and then decreases with the increase of A, which is corroborated by the change of Fn with A.
3.4. Grain wear morphology
Observing the surface morphology of abrasive grains is the most direct and intuitive way to show the wear characteristics of abrasive grains. The SEN image shows the morphology evolution of AcBN grains under UVAG and CG. The material adhesion phenomenon accompanies the whole grinding process, and AcBN grains are formed by multiple cBN grains layer by layer, which are continuously exposed and subject to the friction as well as the rebound caused by the elastic deformation of the machined surface of the workpiece, to maintain the ability to remove the material by the micro-fracture of the cutting edge. Figure 13 and Fig. 14 show the wear morphology of AcBN grains under different agmax. Under CG, the AcBN grains are occupied by the material adhesion over a large area, and some micro-fracture phenomenon also occurs under the agmax ≤ 0.8 µm, but it will also be adhered to by the chips and blocking. With the increase of agmax, most of the AcBN grains are involved in the cutting stage resulting in the basic invisible, from the side of the abrasive grains can be seen with each cutting movement of the material layers accumulated on the surface of the AcBN grains, and can be seen the long chip adhering to the surface of the AcBN grains. Comparatively, under UVAG, when agmax ≤ 0.8 µm, the AcBN grains are no longer adhered to by the material over a large area, and micro-fracture occurs in many places, which is conducive to the removal of the material. And the AcBN grains at the lower part were not involved in the grinding, and the intact single cBN grains could still be seen. When agmax =1.0 µm, the AcBN grains were pulled out due to that the left side of AcBN grains are involved in the grinding firstly and there was not enough bond wrapped around, which leading to the cBN grains pulling out. At agmax = 1.2 µm, the effect of ultrasonic vibration no longer counteracts the effect of increasing agmax, and the surface of the AcBN grains is covered by the material again, with only a small amount of micro-fracture and pull-out.
The vs on the wear morphology of AcBN grains is mainly reflected in the increase of grinding temperature and the weakening of the vibration separation effect of ultrasonic vibration with the increase of vs. It can be seen from Fig. 15 that at vs = 30 m/s, some of the cBN grains are still exposed, and the surface is micro-fractured under the impact of the grinding force. As it enters the high-speed grinding stage, the workpiece material is subjected to the more significant thermal softening effect, part of which adheres to the surface of the AcBN grain, covering most of the cBN grains. The material is removed into chips, adhering to the surface of the AcBN grains, and the holes where the cBN grains are pulled out are also blocked by the chips. When vs = 120 m/s, the grinding temperature rises sharply and the material flows along the sides of the AcBN grains eventually accumulating on the AcBN grains at the end of grinding. In Fig. 16, the wear morphology of AcBN grains with different vs under UVAG can be observed. Overall, compared to CG, AcBN grains are not extensively covered. It is evident that a considerable number of cBN grains undergo micro-fracture, and the holes formed by the pulled-out grains will not be blocked by the short chips under UVAG. At vs ≤ 80 m/s, the cBN grains are continuously micro-fracture under the reconstruction of the alternating load, and there is no chip adhesion on the surface. At vs ≥ 100 m/s, although the material adhesion is much better compared to CG, under the combined effect of increased grinding temperature and reduced vibratory separation by ultrasonic vibration, the cBN grains are adhered to the material after micro-fracture, which weakens the cutting ability of the AcBN grains.
Figure 17 shows the wear morphology of single AcBN grains at different A. when A = 0 µm is the CG, revealing that the AcBN grain is adhered to the cutting edge by the chips, and the holes left by pulling out the cBN grains are blocked by the chips. The AcBN grains are kept free of the chips by the holes that are pulled out under the action of ultrasonic vibration at A = 3 µm. It is also important to notice that the cBN grains shows cleavage fracture and the bond is cracked. With A = 6 µm, the effect of ultrasonic vibration becomes more pronounced, the cBN grains on the front face have been pulled out, and the cBN grains in the middle part have been micro-fractured to produce multiple cutting edges to enhance the grinding capability. While A ≥ 8 µm, the impact force of ultrasonic vibration becomes stronger and stronger, and the cBN grains on the front face have already been dislodged, and the cracks of the bond start to spread under the influence of shear stress on the back face.
In summary, UVAG has the following effects on the form of AcBN grain wear (Fig. 18): Firstly, the intermittent separation of the abrasive grains from the workpiece effectively reduces the grinding temperature and prevents the adhesion of γ-TiAl material. Secondly, UVAG frequently crushes the abrasive grains, exposing their sharp grinding edges and maintaining their sharpness, resulting in an effective increase in the self-sharpness of the abrasive grains. Thirdly, UVAG also causes periodic impacts that accelerate the expansion of cracks on the bond and the pull-out of abrasive grains.