Study On Pulsed Laser Dressing Of Coarse-Grained Bronze-Bonded Diamond Grinding Wheels And Its Grinding Performance

Exploring precise, efficient and green dressing methods with extensive applicability and low cost for superabrasive grinding wheels, especially for coarse-grained bronze-bonded diamond grinding wheels, is still one of the key research directions in the grinding field today. As a non-contact processing technology, laser dressing can avoid mechanical force, dressing tool wear, and tool wear-related or mechanical force-related errors. Moreover, laser dressing is suitable for the precise dressing of superabrasive grinding wheels of various bonds, grain sizes, shapes and dimensions. In spite of providing incomparable advantages over the conventional methods such as mechanical dressing and electrical dressing, laser dressing encounters problems of poor quality, low precision, low efficiency and poor surface topography, to some extent, which hinder the practical applications of laser dressing. Given this, with the goal of improving dressing quality, improving dressing precision, improving dressing efficiency and optimizing surface topography, an experimental platform used for pulsed fiber laser dressing was designed and established in this thesis. The pulsed laser dressing of coarse-grained bronze-bonded diamond wheels has been systematically studied using theoretical analysis, numerical computation, and experimental method. The detailed description is listed as follows:(1) The methods of improving profiling quality, improving profiling efficiency, and improving profiling precision were studied. The key processing parameters that determine the effects of laser profiling were summarized. The pulsed fiber laser tangential profiling experiments were conducted on coarse-grained bronze-bonded diamond wheels. The contour precision of the grinding wheels profiled by the laser beam and the diamond roller was compared. The results demonstrate that profiling quality is related to pulse width and protection measures. The shorter the pulse width is, the higher the profiling quality is. The graphitization of the diamond grains after profiling was inevitable but can be decreased by blowing argon from the side to improve profiling quality under the current experimental conditions. Profiling efficiency is related to the focal power density and the laser cutting depth. The higher the focal power density is, the higher the profiling efficiency is. The maximum profiling efficiency can be achieved if and only if the laser cutting depth matches the initial surface contour error of the grinding wheel, i.e., single-layer deep-cutting tangential profiling method with intermittent feeding being employed,. Profiling precision is related to the focal power density and the track-overlap ratio of laser cutting. The higher the focal power density is, the higher the profiling precision is. The selection of relatively high track-overlap ratios of laser cutting is beneficial to the increase of the profiling precision. When optimized process parameters were employed for laser profiling, the diamond grains were slightly graphitized and the circular run-out error and the parallelism error decreased from 83.1 μm and 324.6 μm to 6.8 μm and 3.8 μm, respectively. The surface contour precision of the laser-profiled grinding wheels was slightly lower than that of the grinding wheels profiled by the diamond roller.(2) The single-pulsed laser ablation of bronze bond was experimentally performed, and the micro-topography and geometrical shape of ablation craters were observed. The optical signal of the plasma generated in the ablation process was measured, and the physical parameters of the plasma were calculated. A solid-liquid-gas three-phase transient model based on phase-field method was developed to numerically simulate the thermodynamic processes of single-pulsed laser ablation of bronze bond, and the calculated results were compared with the experimental data. The results show that the electron temperature, the electron density, the inverse bremsstrahlung absorption coefficient had the same tendency of firstly increasing and then decreasing in the spatial distribution, and their maximum values were 3490 K,1.58×1016 cm-3,and 7.01×10-5 cm-1, respectively. The loss of laser energy caused by the inverse bremsstrahlung absorption was almost negligible. The bronze bond material was removed mainly through melting and vaporization. The bowl-shaped central section of ablation crater was wide at the top and narrow at the bottom. The molten sputter was produced on the edge of the crater. The heat-affected zone around the crater continued to expand with the increase of the average laser power. The crater diameter increased at an accelerating rate in the range between 33.2 and 60.2 μm and the crater depth increased linearly in the ranges between 0.9 and 3.4 μm as the average laser power increased. The required time for the ablation process to enter the quasi-steady state vaporization phase decreased as the average laser power increased. The maximum temperature(in the range from 2771 to 2952 K) and the maximum velocity(in the range from 80 to 186 m?s-1) of the metal vapor in the ablation zone continued to increase at an accelerating rate as the average laser power increased. The maximum error of the crater geometrical shape between the test results and numerical calculation results was only about 5%. Therefore, the model can be applied to simulate the dynamic evolution process of the crater’s solid/liquid and liquid/vapor interfaces during single-pulsed laser ablation.(3) The methods of improving smoothness of the bond surface, improving sharpening quality and sharpening efficiency, and controlling the grain protrusion height were studied. The key processing parameters that determine the effects of laser sharpening were summarized. The pulsed fiber laser ablation experiments and pulsed fiber laser radial sharpening experiments were performed on bronze-bonded wheels and coarse-grained bronze-bonded diamond grinding wheels, respectively. The surface topography of the grinding wheels sharpened by the laser beam and the silicon carbide wheel was compared. The results show that the smoothness of the bond surface is related to the energy distribution of the laser beam, the laser spot overlap ratio and the laser scanning track line overlap ratio. Compared with that of grinding wheels sharpened by the Gaussian laser beam, the smoothness of the bond surface of the grinding wheel sharpened by the square-shaped laser beam was higher. The increasing in the laser spot overlap ratio and the laser scanning track line overlap ratio was beneficial to the increase in the smoothness of the bond surface. The sharpening quality is related to the pulse width and the laser power density, and the sharpening efficiency is related to the laser power density. The shorter the pulse width is, the higher the sharpening quality is. The increasing in the laser power density was beneficial to the increase in the sharpening efficiency, while the sharpening quality increased firstly and then decreased with the increase of the laser power density. The grain protrusion height is related to the number of laser scanning cycles. The grain protrusion height continued to increase with an increase in the number of laser scanning cycles within a certain range. Compared with that of grinding wheels sharpened by the silicon carbide wheel, the lesser the removal of the grains is, the more consistent the grain protrusion height, the more appropriate the grain protrusion height, and the more sufficient the surface chip space and the better surface topography of grinding wheels sharpened by the laser beam can be achieved.(4) A comparative experiment was conducted using laser-dressed and mechanically dressed bronze-bonded diamond grinding wheels to grind an YG8 hard alloy workpiece. The surface quality of workpiece after grinding was analyzed, and the wear forms, the wear loss and the grinding ratio of laser-dressed grinding wheels in different grinding stages were studied. The results show that the surface roughness of workpiece basically decreased with the increase of the linear speed of the grinding wheel and increased with the grinding depth. The minimum value of the surface roughness was 0.425 μm. The workpiece after grinding by the laser-dressed grinding wheel had better micro-topography and lower surface roughness, compared with that of the wheel dressed by the silicon carbide wheel. The main wear forms of grinding wheel were grain wear and grain breakage in the initial wear stage. The grinding ratio of grinding wheel was about 205.4, and the surface roughness of workpiece after grinding was 0.314 μm. The main wear forms of grinding wheel were grain wear, accompanied with grain breakage and bond wear in the stable wear stage. The grinding ratio of grinding wheel was about 405.1, and the surface roughness of workpiece after grinding was 0.337 μm. The main wear forms of grinding wheel were grain breakage, grain wear and bond wear in the sharp wear stage. The grinding ratio of grinding wheel was about 96, and the surface roughness of workpiece after grinding was 0.454 μm.