3.1. Effect of pitch of micro grooves on grinding force
The grinding performance was studied under varying pitches of micro-grooves at a constant spindle speed. The influence of different grinding depths on normal (Fn) and tangential (Ft) grinding forces were shown in Fig. 5. The measurement of the grinding force revealed a significant distinction between normal grinding wheels with different microstructure pitches and leaf-vein bionic grinding wheels.
The undeformed chip thickness of workpiece was the most important factor in determining grinding force and surface quality. The following equations (1) can be used to characterize the undeformed chip thickness of a single diamond grain workpiece :
where hm is the undeformed chip thickness, C is the abrasive grain density, α is the semi-included grit angle, vw is the workpiece speed, vs is the wheel speed, a is the depth of cut and de is the equivalent diameter of grinding wheel. When the grinding depth increased, the undeformed chip thickness of the workpiece became thicker, and the grinding force would increase accordingly.
It can be seen from the figure that when the cutting depth was small, the grinding force of different groove spacing had a small difference in grinding force. This might be because it was ductile removal at this time, and the groove had little effect on the grinding force. While the cutting depth increased, the grinding force fluctuated, which might be due to the change in the volume of coolant inside the groove and the thickness of the undeformed chip . As the grinding depth increased, the grinding force had an increasing trend, and the change trend of the normal grinding wheel (P=0) and the leaf-vein bionic grinding wheel was similar. When the grinding depth increased, the maximum undeformed chip thickness was larger than the critical depth of cut for ductile-brittle transition, and the material was removed by brittle fracture . Concurrently, the grinding depth increased, and the grinding contact arc length also enlarged.
At the same grinding depth, compared with normal grinding wheels, the normal grinding force of the vein bionic grinding wheel was effectively reduced by 9.6-63%, and the tangential grinding force is reduced by 8.3-42%. The change of grinding force becomes more and more as the cutting depth increased, which indicates that the way of material removal has gradually changed from plastic removal to brittle fracture and material powder removal.
The grinding force decreased with reducing of the pitch of micro-grooves. The smallest grinding force occurred at the smallest pitch. The reduction in grinding force can not only be attributed to the presence of grooves, which can promote better coolant fluidity and wear debris removal. It is also due to the reduction in the contact area between the grinding wheel and the grinding surface of the workpiece during operation, which leads to a reduction in the number of scratches. And the lower grinding force indicates that the vein bionic grinding wheel with low micro-groove pitch could remove more material from a workpiece before it burned or the tool failed.
For the grinding process, the grinding ratio G was a considerable index to evaluate the wear of the grinding wheel.
The normal-to-tangential grinding ratios for alumina ceramic using normal grinding wheel and leaf-vein bionic grinding wheels with different pitches were plotted in Fig. 6. By contrast, grinding wheel with P=10mm yielded the minimum G value, followed by grinding wheel with P=8mm. A larger grinding ratio usually indicates less tool wear, excellent wear resistance and long service life. Although the grinding force in both directions showed a similar downward trend, the normal grinding force reduced quicker than the tangential grinding force, as seen in Fig. 6. The larger the grinding force ratio, the sharper the abrasive grains and the better the performance of the grinding wheel. At the same time, the normal grinding force was lowerer, indicating that the load of the workpiece and the grinding wheel was lesser, so the wear of the grinding wheel was fewer and the workpiece appearance was better. The tangential grinding force was greater, which was conducive to the removal of the material, and the grinding efficiency was higher. Therefore, the comprehensive grinding performance of the two vein bionic grinding wheels with groove spacing of P=10 and P=8 was better.
3.2. Effect of pitch of micro-grooves on grinding quality
3.2.1 The nature of surface roughness: Ra and Rt
This part used the results of Ra and Rt to quantify the surface roughness of the workpiece. Even if two workpieces had the same Ra value, a surface with a larger Rt value was generally considered to be worse. Therefore, it was necessary to combine two parameter values to evaluate the surface quality.
Figure 7, 8 shows the Rt, Ra and surface morphology of the workpiece surface after four different grinding wheels. The influence of the variation of the micro-grooves spacing on the surface roughness Ra was analyzed and studied. The consequences revealed that when the feed depth was constant, the surface roughness Ra was improved when the pitch increases. The surface roughness produced by the wheel with normal grinding wheel (P=0mm) was the highest, Ra value was 1.26µm, and the wheel with P=8mm was the smoothest surface (Ra=1.17µm). Simultaneously, the Rt values were smaller for grooved grinding than the normal grinding wheel irrespective of groove pitch . From the analysis of the data, it can be known that the surface quality of the workpiece does not completely improve with the increase of the groove pitch. These results showed that the existence of grooves not only reduced the number of abrasive particles on the the grinding wheel surface, but also changed the relative distribution of coolant and chips, which has an essential impact on grinding quality. Due to the edge of the vein fractal structure was a relatively flat vertical section structure, the edge structure can also be used as an additional cutting edge throughout the grinding procedure, thereby further improving the grinding performance of the grinding wheel. And as the grinding depth increased, the over-squeezed abrasive particles were effectively crushed and allowed more abrasive particles in the lower layer to participate in the grinding process.
In three kinds of structured grinding wheels with different micro-groove pitches, when the micro-groove pitch increased from 6mm to 8mm, the grinding quality was improved. This could be due to the reduction of effective abrasive particles when a smaller pitch of micro-grooves was formed on the diamond wheel. In other words, when the pitch increased, the chip thickness becomes relatively larger, as shown in Fig. 9. When the micro groove pitch increased to 10mm, the number of grooves reduced, and the cooling performance of the grinding wheel reduced. Therefore, some burns were inevitably produced, and the roughness increased. The roughness of the unstructured grinding wheel was slightly higher than that of the leaf-vein bionic grinding wheel with P=8mm. This was because the fractal structure of leaf veins had different scales of macroscopic flow channels and microscopic grooves, and the coolant could flow better in the grinding contact area. Thus, we concluded that the effect of the pitch of micro-grooves on the surface roughness was eventful.
3.2.2 Analysis of workpiece surface defects
When the depth of cut was 8µm, the surface morphology of the workpiece produced using different grinding wheels were shown in Fig. 10. Several kinds of defects were usually observed on the surface of the workpiece: cavities, cracks, scratches, furrows, fragments, and material debris. Many scratches and cracks were heeded on the machined surfaces, as shown in Fig. 10(a) - (d). After the grinding experiment, due to the stress and the brittle fracture tendency of the ceramic, cracks would appear in the ceramic and began to grow. Since the groove pitch affected the grinding force, it can be seen that the formation and propagation of cracks were closely related to the grinding force. When the cracks extended to the surface and inside of the material, fragments would be formed. When the fracture propagation within the workpiece ultimately stopped, some residual cracks would form on the surface of workpiece. Furthermore, the number of scratches tended to increase as the micro groove pitch increased. This could be attributed to the fact that during the grinding process, with the shedding and wear of abrasive grains, the increase and accumulation of temperature in the processing area caused graphitization of the surface of the diamond grinding wheel, which affected the surface quality of the workpiece . In addition, the cavities were shown by magnifying the workpiece surface, seeing in Fig. 10(a1) - (d1). This was mostly due to the internal crack propagation during the material removal step and the intergranular breakage of the grinding wheel bond .
3.3. Wear morphology of wheel
This part focused on the wear of the wheel by observing the wheel topography. Grinding wheel wear was caused by the counteraction force of the workpiece on the abrasive grains and bonding agent during the grinding, which mainly includes wear flat, grain fracture, and grain pullout. In different stages of the grinding process, the main wear style of the grinding wheel was different. The generated force and moment increased rapidly when the grinding wheel comed into contact with the workpiece. When the stress exceeded the strength of bond, the abrasive particles started to fall off. The greatest force appeared first at the top of the abrasive grains, causing cracks to form, which then extended forward and expanded downwards, eventually causing the abrasive grains to fracture.During the initial grinding phase, the primary wear style of the grinding wheel was the fracture of the bond. As the grinding wheel gradually stabilizes, it turns into abrasion wear .
The wear surface of the grinding wheel under different groove pitches when the grinding depth was 8µm was observed, as shown in Fig. 11. Several different grinding wheels had different wear characteristics under the same grinding conditions. Compared with the the normal grinding wheel surface, the wear mode of the leaf-vein bionic grinding wheel was mainly wear flat, and the phenomenon of abrasive grain fracture and grain pullout was less. It was mainly due to the discontinuous the grinding wheel surface, which facilitated the grinding fluid to enter the grinding zone more acceptable. Only a little amount of coolant could flow into the grinding area without grooves, and the lubricating effect was poor. The groove served as a path for coolant inflow and remain in the channel. The lubricating coating could provide lubrication between the abrasive and the workpiece when the abrasive particles came into touch with it again. So as to achieve adequate lubrication and cooling, lessened the blockage of the grinding wheel surface, and reduced the generation of stress. When each diamond particle was shattered, the vibration between the grinding wheel and the material not only boosted the material removal efficiency of alumina, but it also caused mechanical strain. Simultaneously, the periodic fracture of diamond abrasive grains could form sharp cutting edges to maintain their self-sharpening properties during the grinding process, thereby improving the grinding performance .
Nonetheless, in the normal grinding wheel, it was found that the grain pullout and the grain fracture were more serious. This may be due to the small moving space of the grinding fluid and grinding debris in the grinding area, the increased load and the accumulation of grinding heat. As a result, extra predefined breaking points and edges were introduced into the abrasive layer. The number of effective cutting edges decreased as the abrasive granules prolapse. The adhesion layer, which was made up of abrasive debris, diamond debris, and bronze bond debris, enhanced friction between the grinding wheel surface and the working surface, resulting in a high grinding force and heat generation [30, 31]. Because the pressures exerted on the grain were high enough to break the connection, the bond fractures. This resulted in the formation of bulky pits, minor wheel loading, and chip adherence on the wheel surface. Although both the leaf-vein bionic grinding wheel and the normal grinding wheel had minor grain pullouts, the number of grain pullouts in the normal grinding wheel was more significant than that in the leaf-vein bionic grinding wheel. When the downward feed rate per unit time was very low, the grinding heat was transferred to the workpiece at a faster rate than the downward feed rate, which increased the temperature of the grinding process. As a result, the abrasive particles fell off due to the softening of the binder at the front of the grinding wheel. When the downward feed per unit time increased, this softening effect reduced, and the phenomenon of abrasive particle shedding decreased. When the downward feed rate per unit time increased to a certain extent, the volume of the generated wear debris exceeded the chip holding capacity between the abrasive particles, and the force of the abrasive particles increased and fell off. In the grinding process, the effective cutting edges number and cooling effect of the leaf-vein bionic grinding wheel surface were better than that of normal grinding wheels, so the surface appearance of the grinding wheel was better. And the number of abrasive particles per unit area reduced as the abrasive particles fell off, increasing the thickness of undeformed chips and resulting in poor surface quality of the workpiece .
On the surface of these grinding wheels, the fracture of the abrasive grains was also observed. This was because the fragments generated during the grinding of alumina entered the gap between the abrasive grains, and as the grinding progressed, the diamond abrasive grains were repeatedly pressed to cause indentation fatigue, which eventually led to the grinding process. The grains were broken. However, as the groove pitch decreased, the fracture situation improved significantly. Therefore, the fluidity of the coolant caused by different groove pitches had an important influence on the wear of abrasive particles.