Compared green silicon carbide and resin bond CBN wheel for Titanium alloy
When the green silicon carbide grinding wheel grinding the titanium alloy, the grinding wheel adheres to the titanium alloy more seriously, and the grinding force is large, and the processing quality is poor. This paper studies the resin bond CBN grinding wheel for titanium alloy and explores new ways to improve the grinding for titanium alloy.
1.Cutting and grinding experiment
This experiment was carried out on the MGK7120×6 grinding machine. The workpiece was fixed on the Kistler9257BA dynamometer. The workpiece is TC4 titanium alloy, the size is 20mm×20mm×5mm, the density is 4.5g/cm3, the hardness is 30HRC, the yield strength is 860Mpa, the tensile strength is 950Mpa.
The grinding wheels used in the comparative experiment were conventional resin bond CBN grinding wheel and silicon carbide grinding wheel with grain size of 80, outer diameter of φ150 mm, inner diameter ofφ32 mm and thickness of 20mm. In order to exclude the interference of other factors in the experiment, 600# diamond was used for polishing before the experiment, and the surface roughness after polishing was Ra 0.2μm. The processing parameters are shown in Table 1, and the cutting and grinding experiment device is shown in Fig.1.
Table1 Processing Parameters in the Experiment
|Cutting Depth ap/(μm)||Grinding Wheel/Tools Linear Velocityvs/(m/s)||Workpiece speed vm/(mm/s)|
2 Experimental results and analysis
2.1 Surface morphology of green silicon carbide and resin bond CBN wheel
The feed rate of the SiC grinding wheel is 20mm/s in the experiment. The surface morphology obtained by grinding the workpiece at different grinding depths is shown in Fig.2. It can be seen from Fig.2(a) that when the grinding depth is 5μm, there are many plastic grooves and scratches formed by the abrasive grains on the surface of the workpiece.
As shown in Fig.2(b), after the grinding depth increasing, the material accumulation is significantly increased, and the workpiece material has irregular flow perpendicular to the machine direction.
Figure 2 Processing Morphology of SiC Grinding Wheel for Grinding TC4 Titanium Alloy
The surface morphology of CBN grinding wheel for titanium alloy is shown in Figure 3.Comparing Fig.2 and Fig.3, it can be found that the surface quality ground by CBN grinding wheel becomes worse with the increase of grinding depth, the surface quality ground by CBN grinding wheel is obviously better compared with that ground by SiC grinding wheel.
Compared with SiC grinding wheel, CBN grinding wheel has better thermal conductivity and high heat transfer efficiency. The heat in the grinding zone can be diffused out by itself in the grinding process, so the temperature in the grinding zone is reduced faster and reduces the fusion adhesion, making CBN grinding wheel sharper and reducing the pressing effect of the workpiece.
2.2 Roughness of green silicon carbide and resin bond CBN wheel
The grinding surface is composed of a large number of extremely fine grooves carved by many abrasive grains on the grinding wheel. Due to the grinding features, the roughness perpendicular to the grinding direction is much larger than that of along to the grinding direction, so the test measures the roughness perpendicular to the grinding direction.
The surface roughness of titanium alloy at different grinding depths is shown in Table 2. The surface roughness of titanium alloy increases with the increase of grinding depth, especially when machining with SiC grinding wheel, the increasing trend is more obvious.
Table 2 Surface Roughness of Titanium Alloy at Different Depths
|CBN Grinding Wheel||0.28||0.38||0.45||0.52|
|SiC Grinding Wheel||0.3||0.45||0.59||0.75|
If the grinding surface roughness is only considered from the geometrical factor, it can be concluded that the shallower the surface depth of the abrasive cutting into material, the lower the surface roughness is. Since the depth of the surface depth of the abrasive cutting into the materials and the furrow become deeper as the depth of the grinding increases, the surface roughness of the grinding should be large.
Compared with general cutting and grinding, it features high grinding speed, large grinding ratio and high temperature in the grinding zone, which cause hot-melt deformation of the metal surface and even grinding burns. Therefore, compared with the plastic deformation of the workpiece during turning or milling, the plastic deformation of the workpiece during the grinding process is much larger. Under the joint effect of grinding force and grinding heat, the metal grains on the surface of the titanium alloy are stretched in the lateral direction, and even there are fine cracks and local metal accumulation.
In addition, the factors affecting the plastic deformation of the surface metal are often the decisive factors affecting the surface roughness. When the grinding depth is 5μm, it is found that the grinding wheels in the grinding process have less heat. At this time, the plastic deformation of the workpiece is small. Comparing the surface roughness of 2 grinding wheels after grinding, it is found that the roughness is similar.
When the grinding depth is increased to 20μm, the heat transfer of SiC wheel is poor, so that the heat in the grinding zone cannot be transferred through the grinding wheel in time, and the grinding temperature is increased, so the thermoplastic deformation of the workpiece is large, and the surface roughness is increased to 0.75 μm.
2.3 Grinding force of green silicon carbide and resin bond CBN wheel
The grinding force curves of the CBN grinding wheel and SiC grinding wheel are shown in Fig.4 and Fig.5. The deeper the grinding depth, the bigger the grinding force and the better linear relationship is.
Comparing Fig.4 and Fig.5, it can be found that the grinding force of the CBN grinding wheel is obviously smaller than that of the SiC grinding wheel, and the ratio of normal force to tangential force is also smaller. This is mainly because during the machining process, as the temperature of the grinding zone increases, the cutting edge adheres seriously, which makes SiC grinding wheel passivate and weakens the grinding ability. CBN grinding wheel has self-sharpening and the abrasive gains can keep sharp for a long time, it can remove the material better and the corresponding grinding force is lower.
The surface quality ground by resin bond CBN wheel is better than that of the silicon carbide grinding wheel, and the surface integrity is better, macroscopic crack and surface damage are relatively less. CBN grinding wheel has better toughness than the silicon carbide grinding wheel when grinding depth is increased, the contrast is more obvious. Compared with the conventional silicon carbide wheel, the CBN wheel has the less tangential force and normal force. When the feed speed is 20mm/s and the cutting depth is 20μm, the tangential force and normal force of tool reduced by a maximum of 25.3% and 48.9%.