Laser-assisted ultra-precision grinding process of tungsten carbide elements

Abstract

The present application relates to the technical field of machining, and provides a laser-assisted ultra-precision grinding process for a tungsten carbide element, comprising the following steps: selecting a corresponding matched four-axis ultra-precision machine tool and a grinding mode based on the machining characteristics of a workpiece, selecting a grinding wheel based on the grinding mode and different grinding stages, and performing shaping treatment and sharpening treatment on each initial grinding wheel through a grinding rod; clamping the workpiece on the four-axis ultra-precision machine tool, and grinding the workpiece based on a predetermined grinding method and different working grinding wheels; selecting a laser repair device based on the material characteristics of the formed element itself, and repairing the subsurface damage by using the laser repair device. Through the shaping treatment and sharpening treatment on the initial grinding wheel, the working grinding wheel can be accurately centered, the complexity of the device is reduced, interference is avoided, the error of in-situ measurement is eliminated, and the subsurface damage on the ground workpiece is repaired by laser repair, thereby improving the grinding precision of the workpiece.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of machining, in particular to a laser-assisted ultra-precision grinding process of tungsten carbide element. BACKGROUND

[0002] In recent years, small caliber aspheric optical lens is more and more widely used in the field of smart phone lens, vehicle-mounted imaging system, infrared optical imaging system and other fields with the advantages of simplifying the structure of optical system, improving the imaging quality and system resolution, and excellent optical refraction effect. Precision glass mold forming technology is the main technology for mass production of small caliber aspheric optical lens. Because the precision glass mold forming technology is formed by copying the surface profile of the mold, the surface accuracy and surface roughness of the mold determine the accuracy of the aspheric lens to a great extent.

[0003] The material of the optical mold grinding tool is usually selected from non-metallic materials such as tungsten carbide, silicon carbide and silicon nitride. In particular, binderless tungsten carbide not only has high hardness and high strength, but also has better wear resistance and fracture toughness compared to traditional tungsten carbide. It can also avoid surface quality degradation caused by the difference in thermal diffusion coefficient between the binder and the hard alloy particles, and has become an ideal mold material for precision glass mold pressing.

[0004] These mold pressing materials usually have very high hardness and brittleness. At present, the ultra-precision cutting technology based on single-point diamond lathe includes slow tool servo ultra-precision turning, fast tool servo ultra-precision turning, flying tool turning, etc. Due to serious tool wear during machining, it is impossible to realize the machining and forming of aspheric surfaces and other complex surfaces. Ultra-precision grinding is considered to be the main technology for machining complex surfaces on hard and brittle materials. However, the requirements of more complex surface, higher machining precision and more stringent surface quality for ultra-precision grinding of complex optical surfaces pose new challenges. High-precision and efficient dressing of grinding tools, full plastic domain grinding during machining, inhibition or elimination of subsurface damage of ground workpieces, and reduction of grinding tool wear are difficult problems that researchers in this field need to solve. The solution of these problems has very important theoretical significance and application value for mass production of complex curved optical elements and promoting the development of related optical fields. SUMMARY

[0005] The present application provides a laser-assisted ultra-precision grinding process of tungsten carbide element to solve the defects of low machining precision and high machining cost in the prior art.

[0006] The application provides a laser-assisted ultra-precision grinding process of a tungsten carbide element, comprising the following steps: selecting a corresponding matched four-axis ultra-precision machine tool and a grinding mode based on the processing characteristics of a workpiece; selecting a grinding wheel based on the grinding mode and different grinding stages to obtain an initial grinding wheel corresponding to different grinding stages, and installing the initial grinding wheel on the four-axis ultra-precision machine tool; performing shaping treatment and sharpening treatment on each initial grinding wheel by a grinding rod, so that the center of the grinding region of the processed working grinding wheel is accurately centered with the rotary table of the four-axis ultra-precision machine tool; clamping the workpiece on the four-axis ultra-precision machine tool, and grinding the workpiece based on a predetermined grinding method and different working grinding wheels, measuring the surface accuracy of the workpiece after grinding, and performing error compensation processing by the working grinding wheel until the accuracy requirement is met to obtain a formed element; selecting a laser repair device based on the material characteristics of the formed element itself, and using the laser emitted by the laser repair device to act on the surface of the formed element to repair the subsurface damage of the formed element.

[0007] According to the laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application, the corresponding matched four-axis ultra-precision machine tool and the grinding mode are selected based on the processing characteristics of the workpiece, and specifically, the binderless tungsten carbide is selected as the mold material, and the inclined-axis single-point grinding method is selected for grinding processing on the four-axis ultra-precision machine tool based on the aspheric surface processing characteristics of the binderless tungsten carbide.

[0008] According to the laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application, the grinding wheel is selected based on the grinding mode and different grinding stages to obtain an initial grinding wheel corresponding to different grinding stages, comprising the following steps: based on the inclined-axis single-point grinding method, a metal-based diamond grinding wheel with a particle size of 400#-500# is selected as a coarse grinding wheel in the coarse grinding stage; a resin-based diamond grinding wheel with a particle size of 800#-1000# is selected as a semi-fine grinding wheel in the semi-fine grinding stage; and a ceramic-based diamond grinding wheel with a particle size of 10000#-15000# is selected as a fine grinding wheel in the fine grinding stage.

[0009] According to the laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application, the initial grinding wheel is installed on the four-axis ultra-precision machine tool, specifically comprising the following steps: the grinding spindle is installed on the rotary table through the spindle sleeve and the displacement adjustment table, the displacement adjustment table can realize precise adjustment of the grinding spindle in X, Y and Z directions, and the initial grinding wheel is clamped on the grinding spindle.

[0010] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application comprises the following steps: installing the workpiece on the workpiece spindle of the four-axis ultra-precision machine tool through a transition piece, detecting and adjusting the round runout of the workpiece, keeping the workpiece spindle in a low-speed rotating state after passing the round runout adjustment, bonding and fixing the workpiece, and re-inspecting the round runout of the workpiece.

[0011] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application comprises the following steps: installing the workpiece on the workpiece spindle of the four-axis ultra-precision machine tool through a transition piece, detecting and adjusting the round runout of the workpiece, keeping the workpiece spindle in a low-speed rotating state after passing the round runout adjustment, bonding and fixing the workpiece, and re-inspecting the round runout of the workpiece.

[0012] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application comprises the following steps: installing the workpiece on the workpiece spindle of the four-axis ultra-precision machine tool through a transition piece, detecting and adjusting the round runout of the workpiece, keeping the workpiece spindle in a low-speed rotating state after passing the round runout adjustment, bonding and fixing the workpiece, and re-inspecting the round runout of the workpiece.

[0013] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application drives the grinding spindle, the workpiece spindle and the rotary table on the four-axis ultra-precision machine tool to perform inclined-axis grinding wheel dressing and X, Z-axis interpolation dressing, and specifically includes the following steps: when performing inclined-axis grinding wheel dressing, the initial grinding wheel shaft initial installation position is located on the rotary table, the initial grinding wheel end small circular arc center is substantially aligned with the rotary table center through rough centering; then the on-line dressing of the grinding wheel is realized by using the rotary motion of the rotary table, in the dressing process, the grinding rod rotates at a certain rotational speed ω1, the initial grinding wheel rotates at a rotational speed ω2, and the predetermined feed speed f is fed along the workpiece spindle axis, and the rotary table drives the initial grinding wheel to rotate to form an angle cylindrical grinding wheel, and a circular arc grinding area is formed at the end of the angle cylindrical grinding wheel; when performing X, Z-axis interpolation dressing, color marking is performed on the entire area of the circular arc at the end of the angle cylindrical grinding wheel before dressing, and the circular arc at the end of the angle cylindrical grinding wheel to be dressed is always tangent to the outer circular arc surface of the grinding rod during the dressing process, the angle cylindrical grinding wheel and the grinding rod rotate at a certain rotational speed, respectively, to enable the small circular arc at the end of the angle cylindrical grinding wheel to be completely dressed.

[0014] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application drives the grinding spindle, the workpiece spindle and the rotary table on the four-axis ultra-precision machine tool to perform inclined-axis grinding wheel dressing and X, Z-axis interpolation dressing, and specifically includes the following steps: when performing inclined-axis grinding wheel dressing, the initial grinding wheel shaft initial installation position is located on the rotary table, the initial grinding wheel end small circular arc center is substantially aligned with the rotary table center through rough centering; then the on-line dressing of the grinding wheel is realized by using the rotary motion of the rotary table, in the dressing process, the grinding rod rotates at a certain rotational speed ω1, the initial grinding wheel rotates at a rotational speed ω2, and the predetermined feed speed f is fed along the workpiece spindle axis, and the rotary table drives the initial grinding wheel to rotate to form an angle cylindrical grinding wheel, and a circular arc grinding area is formed at the end of the angle cylindrical grinding wheel; when performing X, Z-axis interpolation dressing, color marking is performed on the entire area of the circular arc at the end of the angle cylindrical grinding wheel before dressing, and the circular arc at the end of the angle cylindrical grinding wheel to be dressed is always tangent to the outer circular arc surface of the grinding rod during the dressing process, the angle cylindrical grinding wheel and the grinding rod rotate at a certain rotational speed, respectively, to enable the small circular arc at the end of the angle cylindrical grinding wheel to be completely dressed.

[0015] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application, after the repaired shaped element is obtained, the repair effect of the shaped element is evaluated by an evaluation method, the evaluation method comprises the following steps: cutting vertically to the machining surfaces of the shaped element before and after repair, and obtaining two cutting workpieces respectively; placing the cutting end faces of the two cutting workpieces of the shaped element before repair side by side downward in a cold inlay mold to perform cold inlay, placing the cutting end faces of the two cutting workpieces of the shaped element after repair side by side downward in a cold inlay mold to perform cold inlay, taking one of the two cutting workpieces before repair as a companion polishing piece, and taking one of the two cutting workpieces after repair as a companion polishing piece, so as to obtain one cold inlay piece before laser repair and one cold inlay piece after laser repair; placing the two cold inlay pieces on a polishing machine to polish to a mirror surface effect, and removing large-size damage and micro mechanical damage such as edge collapse and residual protrusions at the junction; configuring an etching solution of the cutting workpiece, and etching the cutting workpiece for a certain time to fully expose the real subsurface damage; using a transmission electron microscope to detect the subsurface damage, and evaluating the laser repair effect according to the detection result; using a needle tip enhanced laser confocal Raman spectrum system to analyze the polished surface, the ground surface and the laser repair surface of the tungsten carbide in sequence; and using a nano indentation technology to evaluate the mechanical properties of the laser repair surface, so as to realize the evaluation of the repair effect of the shaped element.

[0016] The laser-assisted ultra-precision grinding process of the tungsten carbide element provided by the application, by the grinding rod for shaping and sharpening treatment of the initial grinding wheel, the self-centering of the working grinding wheel end arc center and the rotary table is realized, the manual tool setting error is eliminated, the ideal grinding wheel end arc radius can be obtained, the numerical control programming for subsequent complex curved surface ultra-precision grinding can be directly used, the in-situ measurement of the small arc at the end of the grinding wheel is not needed, the complexity of the device is reduced, the interference is avoided, and the error of the in-situ measurement is eliminated, that is, the tool data is completely obtained by the machine tool system control, and the precision completely depends on the machine tool precision. And the laser repair equipment is used for repairing the processed shaped element, the repair of the subsurface damage of the shaped element, especially the crack, is realized, the surface roughness, the surface shape precision and the mechanical properties are improved, and the use performance and the service life of the finished workpiece are greatly promoted. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0018] Figure 1 is one of the flowchart of the laser-assisted ultra-precision grinding process provided by the present application.

[0019] Figure 2 is the second flowchart of the laser-assisted ultra-precision grinding process provided by the present application.

[0020] Figure 3 is the flowchart of the evaluation method in the laser-assisted ultra-precision grinding process provided by the present application.

[0021] Figure 4 is the overall structure schematic diagram of the four-axis ultra-precision machine tool in the laser-assisted ultra-precision grinding process provided by the present application.

[0022] Figure 5 is the structure schematic diagram of the grinding rod in the laser-assisted ultra-precision grinding process provided by the present application.

[0023] Reference signs:

[0024] 1, workpiece spindle; 2, grinding spindle; 3, displacement adjustment table; 4, rotary table; 5, machine tool body; 6, round corner cylindrical grinding wheel; 61, arc-shaped grinding area. DETAILED DESCRIPTION

[0025] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below in combination with the drawings in the present application. Obviously, the described embodiments are some embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0026] In the description of the embodiments of the present application, it should be noted that the orientations or positional relationships indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the embodiments of the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0027] In the description of the embodiments of the present application, it should be noted that unless specifically defined and limited otherwise, the terms "connected", "connected to", "connection" should be interpreted broadly, for example, can be fixed connection, can also be detachable connection, or integral connection; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0028] In the embodiments of the present application, unless specifically defined and limited otherwise, the first feature is "on" or "under" the second feature can be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, the first feature "above", "over" and "on" the second feature can be that the first feature is directly above or obliquely above the second feature, or only means that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "under" and "under" the second feature can be that the first feature is directly below or obliquely below the second feature, or only means that the horizontal height of the first feature is less than that of the second feature.

[0029] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in the present application and the features of different embodiments or examples without contradiction.

[0030] In the related art, there are many methods for repairing tungsten carbide subsurface damage, including magnetic fluid polishing, ion beam processing, HF etching, etc., but these conventional subsurface damage suppression or repair methods will more or less introduce various forms of new damage, and cannot achieve the ideal suppression or repair effect of subsurface damage, therefore laser irradiation is considered as a more effective repair method due to its high efficiency, non-contact and no introduction of new damage under suitable laser process parameters.

[0031] At present, the methods for reducing the depth of subsurface damage mainly include chemical mechanical polishing (CMP), magnetorheological finishing, ion beam polishing, HF etching, etc. The chemical mechanical polishing inevitably introduces new mechanical damages such as micro-cracks, fractures and holes due to the high normal load, and the optimization of the processing technology cannot fundamentally eliminate these subsurface mechanical damages. In addition, the uncontrolled material removal depth also leads to the deterioration of shape accuracy. The environmental pollution caused by the polishing liquid and the increase of production cost caused by the long polishing time are also inevitable problems.

[0032] The magnetorheological finishing technology is essentially the same as the traditional polishing due to the existence of the polishing liquid and the polishing particles. The surface will have a certain degree of hydrolysis and the embedding of the polishing particles. In addition, the unique magnetic sensitive particles will also be embedded into the material surface, so that the surface after magnetorheological finishing has improved light transmission and reflection performance, but the laser damage threshold is also reduced, so there is a certain limitation.

[0033] The ion beam processing combined with the magnetorheological finishing technology removes the subsurface damage. The ion bombardment injection effect in the ion beam processing process will introduce new impurities into the surface of the test piece. The introduction of new impurities will increase the refractive index of the material surface. Therefore, it is necessary to appropriately reduce the energy of the ion beam to reduce the introduction of new impurities. However, the reduction of impurities will also reduce the processing efficiency, so these problems greatly limit the repair effect of the subsurface damage.

[0034] In view of the problems in the related art, the present application Figure 1 、 Figure 2 describes a laser-assisted ultra-precision grinding process of a tungsten carbide element, comprising the following steps:

[0035] Step S10, selecting a corresponding matched four-axis ultra-precision machine tool and grinding mode based on the processing characteristics of the workpiece. Specifically, for the workpiece, the corresponding matched machine tool and the corresponding grinding mode are required under different processing characteristics. In this embodiment, the processing characteristics are ultra-precision grinding processing of small-diameter aspheric surfaces.

[0036] In a specific embodiment, the ultra-precision grinding process is carried out in a constant-temperature and constant-humidity clean room. For example, Figure 4As shown, the four-axis ultra-precision machine tool includes a grinding spindle 2 and a workpiece spindle 1, the grinding spindle 2 is installed on a displacement adjustment table 3, the displacement adjustment table 3 is installed on a rotary table 4, the rotary table 4 is arranged on a machine tool body 5, the workpiece spindle 1 is installed on one side of the grinding spindle 2, and the workpiece spindle 1 is arranged on the machine tool body 5 and is used to install a workpiece or a grinding bar, wherein the grinding spindle 2 is used to install a grinding wheel, the grinding spindle 2 adopts an ultra-high-speed air-floating electric spindle with a maximum rotating speed of 80000 rpm-110000 rpm, and the displacement adjustment table 3 can be finely adjusted in X, Y and Z directions, so as to realize the adjustment of the grinding spindle 2 in X, Y and Z directions.

[0037] In this step, the workpiece needs to be specifically selected, that is, the mold material is selected. Specifically, binderless tungsten carbide is selected as the mold material. Based on the non-spherical surface processing characteristics of binderless tungsten carbide, oblique single-point grinding method is selected for grinding processing on the four-axis ultra-precision machine tool.

[0038] It can be understood that among various mold materials, binderless tungsten carbide has higher hardness, wear resistance, fracture toughness and chemical stability, so the processing performance is better, and at the same time, the surface quality of the workpiece caused by the difference in thermal diffusion coefficient of the binder such as Co or Ni and the hard alloy particles can be effectively avoided. Therefore, M78 binderless tungsten carbide hard alloy is selected as the mold material, and the size of the workpiece is Φ25mmX10mm.

[0039] The grinding method is oblique single-point grinding method, which can realize deterministic precision removal of non-spherical surface and is suitable for ultra-precision grinding processing of small-caliber non-spherical surface. Among them, oblique single-point grinding is point contact, and the grinding wheel is a right-angle cylindrical grinding wheel or a round-angle cylindrical grinding wheel, which is explained in this specification. In the processing process, the grinding wheel axis is coplanar with the workpiece axis and has a fixed included angle. By controlling the rotating angle of the grinding shaft, single-point contact between the grinding wheel and the non-spherical workpiece is realized, and at the same time, the grinding point of the grinding wheel is always perpendicular to the normal of the workpiece grinding point.

[0040] Step S20, based on the grinding method and different grinding stages, the grinding wheel is selected, the initial grinding wheel corresponding to different grinding stages is obtained, and the initial grinding wheel is installed on the four-axis ultra-precision machine tool. Different grinding methods and different grinding stages need to select different initial grinding wheels. By selecting the grinding wheel, the matching of the grinding method and the grinding stage can be realized, so that the ultra-precision grinding of the workpiece can be realized.

[0041] According to the grinding method (oblique single-point grinding method, and the grinding processing is carried out in a step-by-step grinding manner) and the tool clamping manner of the grinding spindle 2, a round-angle cylindrical grinding wheel 6 is designed, and the parameters of the initial grinding wheel are selected as follows:

[0042] Abrasive selection: abrasive is the main raw material of grinding wheel, because ordinary corundum, silicon nitride and other abrasives are not suitable for hard and brittle material ultra-precision grinding, so diamond abrasive grinding wheel is selected.

[0043] Binder selection: binder is one of the components of grinding wheel, which is used to consolidate abrasive, and plays a decisive role in the strength, impact resistance, heat resistance and other properties of grinding wheel. Because the metal binder grinding wheel has good wear resistance, it is selected for rough grinding; resin binder grinding wheel and ceramic binder grinding wheel are mainly used to ensure the surface roughness in the grinding process, so resin binder grinding wheel and ceramic binder grinding wheel are selected for semi-fine grinding and fine grinding.

[0044] Particle size selection: particle size refers to the size of abrasive particles. Metal-based diamond grinding wheel with particle size of 400#-500# is selected as rough grinding wheel. During rough grinding, the grinding allowance is large, so coarse abrasive particles are selected. Coarse abrasive particles and large pores can remove large amount of processing allowance, and the grinding wheel is not easy to block and heat, thereby improving the efficiency of grinding process. Semi-fine grinding selects resin-based diamond grinding wheel with particle size of 800#-1000# as semi-fine grinding wheel, which realizes the transition processing between rough grinding and fine grinding. It not only removes the visual surface damage caused by coarse grinding and large abrasive depth, but also considers the processing efficiency. Fine grinding selects ceramic-based diamond grinding wheel with particle size of 10000#-15000# as fine grinding wheel. During fine grinding, the processing allowance is small, and the roughness value is required to be low. Therefore, fine abrasive particles are selected to meet this requirement.

[0045] S30, each initial grinding wheel is shaped and sharpened by the grinding rod, so that the center of the grinding area of the processed working grinding wheel is accurately centered with the rotary table 4 of the four-axis ultra-precision machine tool.

[0046] Grinding tool is an important factor affecting the final workpiece surface forming quality and the stability of the whole machining process. If necessary, the initial grinding wheel needs to be precisely trimmed in place. Grinding wheel trimming includes two processes: shaping and sharpening. The former is to maintain the correct geometric shape of the grinding wheel, and the latter is to protrude the abrasive particles to improve the grinding efficiency. The trimming process is as follows.

[0047] Grinding spindle 2 installation: the grinding spindle 2 is installed on the rotary table 4 through the spindle clamp and the displacement adjustment table 3. The displacement adjustment table 3 can realize precise adjustment of the grinding spindle 2 in X, Y and Z directions. In a specific example, the grinding spindle 2 adopts an ultra-high-speed air-floating electric spindle.

[0048] The grinding wheel dressing process: according to the non-spherical machining characteristics of the typical machining sample, the rotating green silicon carbide (GC) grinding rod outer cylindrical surface in-situ precision dressing method is selected. The initial grinding wheel dressing is carried out before clamping the workpiece, wherein the green silicon carbide (GC) grinding rod is installed at the end of the workpiece spindle 1, and the clamping is carried out by using the ER chuck (elastic chuck of round handle), which can be connected with the spindle by screw connection or hot mounting; the initial grinding wheel is installed on the ultra-high-speed air floating motorized spindle. The ER chuck is a common clamping tool, and the specific clamping method and specific mechanism are not described again. The water-based grinding fluid or oil-based grinding fluid is kept open during the dressing process, and the grinding wheel dressing is carried out in the following two steps:

[0049] Oblique axis grinding wheel dressing: the grinding wheel shaft (i.e. grinding spindle 2) is installed on the rotary table 4, and the initial grinding wheel end small circular arc center is basically aligned with the rotary table 4 center through rough centering (rough centering); then the on-line dressing of the grinding wheel is realized by using the rotary motion of the rotary table 4: during the dressing process, the grinding rod rotates at a certain speed ω1, the grinding wheel rotates at a suitable speed ω2, and the rotary table 4 drives the initial grinding wheel to rotate, and the circular arc grinding area 61 (end small circular arc area) of the round corner cylindrical grinding wheel 6 (i.e. working grinding wheel) is formed. After one cycle of axial feeding, the machined grinding wheel is fed in the radial direction, and the dressing depth of the grinding wheel gradually increases. Since the circular arc is formed by the rotary table 4, the center of the end circular arc of the dressed grinding wheel is naturally aligned with the rotary center of the rotary table 4, so there is no need to perform alignment operation, and the tool setting error of this part is eliminated.

[0050] X, Z axis interpolation dressing: the X, Z axis linkage function is used to make the grinding rod perform interpolation dressing on the diamond grinding wheel to form a circular arc. Before dressing, color marking is performed on the end circular arc of the round corner cylindrical grinding wheel 6 (i.e. working grinding wheel), and during the dressing process, the end circular arc of the working grinding wheel to be dressed is always tangent to the outer circular arc surface of the grinding rod. The grinding wheel and the grinding rod perform interpolation motion along the circular arc, and rotate at a certain speed respectively, so as to ensure the complete dressing of the end small circular arc of the grinding wheel, thereby improving the circular arc dressing precision of the grinding wheel.

[0051] In a specific example, in the interpolation dressing process, one interpolation movement of the diamond grinding wheel is one dressing cycle, after one dressing cycle, the diamond grinding wheel feeds a certain depth ap along the interpolation radius direction, and the dressing wheel arc profile is obtained through the mutual wear between the green silicon carbide rod and the diamond grinding wheel until the color mark at the end arc of the grinding wheel disappears completely, and the interpolation dressing is completed. In the interpolation dressing process, the relationship between each radius involved in the dressing process of the diamond grinding wheel is: R = r0 + r (in the formula, R is the interpolation arc radius, r0 is the radius of the green silicon carbide rod used for dressing, and r is the end arc radius of the dressing work wheel), the interpolation arc radius needs to be corrected after each interpolation dressing cycle, and the corrected interpolation arc radius is R-ap, so as to obtain r, which is the end arc radius of the dressing work wheel, which can be directly used in the numerical control programming of subsequent complex curved surface ultra-precision grinding, without the need for in-situ measurement of the small arc at the end of the grinding wheel, which reduces the complexity of the device, avoids interference, and eliminates the error of in-situ measurement, that is, the tool data is completely obtained by the machine tool system, and the precision completely depends on the machine tool precision.

[0052] Through the combination of the above two grinding wheel dressing methods, the center of the grinding area arc of the work wheel and the precision centering of the rotary table 4 can be realized, the angle compensation of the rotary table 4 in the grinding process is saved, the precise dressing of the work wheel can be realized, the precise end arc radius of the work wheel is obtained, and the basis for subsequent ultra-precision grinding of complex curved surfaces is provided.

[0053] Step S40, clamp the workpiece on the four-axis ultra-precision machine tool, and grind the workpiece based on the predetermined grinding method and different work wheels, measure the surface accuracy of the workpiece after grinding, and perform error compensation processing through the work wheel until the accuracy requirement is met to obtain the formed element. Different work wheels need to be replaced for grinding in different grinding stages, which are as follows:

[0054] Workpiece clamping: Since the workpiece size is small, a clamping assembly (not shown in the figure) is designed, which includes a transition piece, a transition disc and a vacuum chuck. The transition piece is used to clamp the workpiece, the transition disc is installed at the front end of the vacuum chuck, and a small hole is opened in the center of the transition disc for ventilation. When the workpiece is clamped, the transition piece is installed on the transition disc at the front end of the vacuum chuck by screws and the round runout is adjusted. At this time, the vacuum pump is in an open state, the vacuum pump is communicated with the small hole to adsorb the workpiece on the transition piece, the round runout of the workpiece is detected and adjusted, and after the workpiece is qualified, the workpiece spindle 1 is kept in a low speed rotating state, the workpiece is bonded and fixed, and the round runout of the workpiece is rechecked. After the bonding is firm, the vacuum pump is disconnected, and the vacuum pump is in a closed state during the grinding process to suppress the vibration caused by the air source and ensure the surface processing quality.

[0055] Grinding process: the grinding process adopts step-by-step grinding mode, and the specific process is divided into three stages of rough grinding-semi-fine grinding-fine grinding. The grinding conditions and process parameters of the three stages are shown in the following table. Since the three stages need to replace the grinding wheels with different particle sizes and binders, the grinding wheel coarse centering and dressing work need to be carried out respectively. First, a metal-based diamond grinding wheel with a particle size of 400# is used for rough grinding to quickly remove the material allowance and obtain a pre-formed curved surface. In the semi-fine grinding and fine grinding stages, small particle size grinding wheels are used to ensure the required surface type accuracy and surface quality. In the fine grinding stage, the surface profile accuracy of the fine grinding surface is measured in situ by using the measuring unit (i.e. LVDT) integrated in the machine tool after one fine grinding is completed, so as to obtain the profile error. The machining path of the grinding wheel is compensated according to the profile error, and the ultra-precision grinding process-surface profile accuracy measurement-error compensation cycle is carried out until the surface profile accuracy of the machined surface reaches the requirement. After fine grinding, the roughness of the fine grinding surface is detected by a laser interferometer.

[0056] The specific grinding process parameters are shown in the following table:

[0057]

[0058] Step S50, a laser repair device is selected based on the material properties of the forming element itself, and the laser emitted by the laser repair device is used to act on the surface of the forming element, so as to repair the subsurface damage of the forming element. The forming element itself causes many subsurface damages during the grinding process, and the laser repair device is used for repair, as follows:

[0059] The rough grinding of tungsten carbide causes many surface layer damages, such as microcracks, lattice distortion, etc., with a depth of about 20-30 μm. The removal of these surface layer damages is realized by subsequent fine grinding. At the same time, fine grinding also causes a thin subsurface damage layer, mainly including microcracks, amorphous layer, dislocation, stacking fault, etc. The existence of these damages not only leads to the decline of surface mechanical properties, but also makes the surface profile accuracy lose reference value when the damage layer depth exceeds the surface profile accuracy. Moreover, the subsurface damage that is too deep also greatly increases the post-processing work and reduces the processing efficiency. Therefore, it is very important to reduce or inhibit the subsurface damage.

[0060] Accordingly, a method for repairing the subsurface damage of tungsten carbide by laser is proposed, which can realize efficient and high-quality repair of subsurface damage without material removal and environmental pollution. According to the different pulse widths, the commonly used lasers mainly include millisecond, nanosecond, sub-nanosecond, femtosecond, picosecond, etc. Since the nanosecond laser has many advantages such as wide material application range, high material removal rate, moderate equipment cost, etc. compared with other lasers, the nanosecond laser is used for damage repair, and a laser repair evaluation method for subsurface damage is established to evaluate the repair effect.

[0061] The absorption of the high energy of the laser acting on the surface of the tungsten carbide instantaneously increases the temperature of the surface of the laser-acting area of the tungsten carbide, and the surface temperature continues to rise with the accumulation of the pulses until the melting point of the tungsten carbide, at which time the material in the laser-acting area is changed from a solid state to a molten state. If the pulsed laser continues to act, the temperature will continue to rise due to the accumulation of energy, so as to reach the surface vaporization temperature. The temperature range in the laser repair process is just this temperature interval between the melting temperature and the vaporization temperature. In this temperature interval, the tungsten carbide melts to a certain depth, so that the surface and subsurface micro-damage in the depth range are melted together. With the end of the laser pulse action, the material gradually cools down, and the damage such as micro-cracks is repaired in the cooling and recrystallization process, and the crystal lattice structure also changes accordingly, and the stress at the tip of the crack is also reduced. The present application selects a grating line scanning mode for subsurface damage repair.

[0062] Laser repair equipment: the laser repair system mainly includes a precision displacement platform (a three-axis displacement platform), a nanosecond laser, a mirror, a shutter, a Glan prism, and optical elements such as a beam expander, and other optical components can also be added as needed.

[0063] The laser repair process route and evaluation method are as follows:

[0064] Determination of the laser processing damage threshold of tungsten carbide: the laser beam propagation meets the Gaussian beam propagation characteristics, that is, the laser energy in the spatial position meets the Gaussian distribution function, and the beam cross section radius in the laser propagation direction also meets the Gaussian function.

[0065] Specifically, based on the laser repair mechanism of tungsten carbide, obtaining the laser processing damage threshold is the basis for repair, and the damage threshold is obtained by the extrapolation method through laser drilling test. The tungsten carbide workpiece is placed at the focal plane with a zero defocus amount, a water jet device is set to assist processing, and the relative position between the nozzle and the laser focal plane is fixed to avoid introducing new influencing factors and causing deviation of the final result. Drilling processing under different powers and action times is carried out on the tungsten carbide to ensure that a stable small hole ablation can be realized in a short residence time under the selected power. The ablation hole diameter under different powers and residence times is measured by using a scanning electron microscope (SEM). Since the ablation hole diameter of the laser on the material and the logarithm of the laser power have a linear relationship, a functional relationship between the laser output power and the hole diameter is established, and the corresponding laser processing damage threshold is calculated by an empirical formula.

[0066] Laser repair of tungsten carbide: select appropriate laser output power, scanning speed and scanning interval to raster line scanning on the surface of tungsten carbide, wherein the selection principle of scanning interval is to be as small as possible under the premise of ensuring sufficient processing efficiency. After laser processing is completed, the tungsten carbide workpiece is placed in a constant temperature room at 20±0.5℃ for cooling for 25-30 minutes, and then it is placed in a sealed device containing acetone solution and cleaned by an ultrasonic cleaning machine for 25-30 minutes to remove surface residual impurities. Finally, it is placed in a sealed device containing anhydrous ethanol and cleaned by an ultrasonic cleaning machine for 25-30 minutes. After drying, the surface morphology of the processed surface is observed and the surface roughness is analyzed by using instruments such as transmission electron microscope (TEM) and white light interferometer, and then the damage repair effect is analyzed.

[0067] As shown in Figure 3 , the evaluation is carried out by the constructed evaluation method. Specifically, the evaluation of the laser repair effect of the tungsten carbide workpiece: in order to more intuitively reflect the laser repair effect of the tungsten carbide workpiece, the following evaluation method is established for the laser repaired workpiece:

[0068] Cold mounting: the repaired and repaired tungsten carbide workpieces are respectively vertically cut into two parts along the repair surface (i.e. the grinding surface of the workpiece), and two cutting workpieces (two cutting workpieces before repair and two cutting workpieces after repair) are obtained. The cutting end faces of the two cutting workpieces before repair are placed side by side in a cold mounting mold for cold mounting, and the cutting end faces of the two cutting workpieces after repair are placed side by side in a cold mounting mold for cold mounting. One of the two cutting workpieces before repair is used as a companion for polishing, and one of the two cutting workpieces after repair is used as a companion for polishing, wherein the companion for polishing is used to reduce edge effect, and one cold mounting piece before and after laser repair is obtained.

[0069] Polishing: the two cold mounting pieces are placed on a polishing machine and polished to a mirror effect, and large-size damage and micro-mechanical damage such as edge chipping and residual protrusions at the junction are removed.

[0070] Polished surface corrosion: configure tungsten carbide corrosion solution and carry out corrosion treatment for a certain time to fully expose the real subsurface damage, especially cracks, wherein the corrosion time is determined by microscopic observation images of the same crack after short to long time corrosion. After corrosion is completed, the sample is immediately cleaned with acetone and deionized water to prevent the surface from being continuously corroded by the residual corrosion solution, thereby prolonging the corrosion time.

[0071] Subsurface damage detection and evaluation: 1) The subsurface damage is detected by using transmission electron microscopy, and the laser repair effect is evaluated according to the detection results (the detection results of the polished workpiece before and after repair are compared and analyzed). The repair effect obtained in this embodiment is: under suitable laser output power and scanning speed, the subsurface crack depth of the tungsten carbide workpiece after laser repair is reduced, and the temperature of the crack tip under the action of pulsed laser is increased to its melting temperature, so that the surface and subsurface micro-damage of the tungsten carbide workpiece are melted, and the stress concentration of the crack tip is greatly reduced, which effectively inhibits the expansion of the crack. It can be said that the laser repair realizes the healing of micro-cracks to a certain extent, and further plays a role in stopping the crack.

[0072] 2) The polished surface, ground surface and laser repair surface of tungsten carbide are analyzed in turn by using a needle-enhanced laser confocal Raman spectroscopy system. The analysis results obtained in this example are that the Raman spectrum obtained from the polished surface of the cut workpiece is very similar to the spectrum of the laser repair surface, that is, the polishing process does not bring new amorphous layer defects.

[0073] 3) The mechanical properties of the laser repair surface are evaluated by using nanoindentation technology. The analysis results obtained in this example are that the surface after laser repair has a higher fracture limit, the plastic domain processing range is widened, and it is more conducive to the subsequent processing and application of tungsten carbide.

[0074] In summary, compared with the existing precision ultra-precision grinding process of hard and brittle materials, the process scheme is more comprehensive, and higher grinding precision can be ensured by effective development of each technical point; further, the two kinds of dressing methods are combined during dressing of the grinding wheel, the basic purpose of dressing of the grinding wheel, i.e. dressing and sharpening of the grinding wheel, is achieved, the self-centering of the end arc center and the rotary table 4 is achieved, the manual tool setting error is eliminated, the ideal grinding wheel end arc radius can be obtained, which can be directly used in numerical control programming of subsequent complex curved surface ultra-precision grinding, without the need for in-situ measurement of the small end arc of the grinding wheel, which reduces the complexity of the device, avoids interference, and eliminates the error of in-situ measurement, that is, the tool data is completely obtained by the machine tool system, and the precision completely depends on the machine tool precision. Further, for the repair process of the subsurface damage of the tungsten carbide workpiece after ultra-precision grinding, the laser repair method and repair effect evaluation system established by the present application can effectively repair the subsurface damage, especially the cracks, and improve the surface roughness, surface accuracy and mechanical properties, and greatly promote the use performance and service life of the related workpiece.

[0075] It should be pointed out finally that the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit the same; and although the present application has been described in detail with reference to the foregoing embodiments, it should be appreciated by those skilled in the art that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features thereof can be replaced equivalently; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A laser-assisted ultra-precision grinding process of a tungsten carbide element, characterized in that, The method comprises the following steps: Selecting a corresponding matched four-axis ultra-precision machine tool and a grinding method based on the machining characteristics of the workpiece; Selecting a corresponding matched four-axis ultra-precision machine tool and a grinding method based on the machining characteristics of the workpiece; Specifically, the grinding wheel is installed on the workpiece spindle end of the four-axis ultra-precision machine tool, and the initial grinding wheel to be processed is installed on the rotary table of the four-axis ultra-precision machine tool; the grinding spindle, the workpiece spindle and the rotary table on the four-axis ultra-precision machine tool are driven to move, so as to perform inclined-axis grinding wheel dressing and X and Z axis interpolation dressing, so that the obtained working grinding wheel grinding area arc center is accurately centered with the rotary table; Specifically, the grinding wheel is installed on the workpiece spindle end of the four-axis ultra-precision machine tool, and the initial grinding wheel to be processed is installed on the rotary table of the four-axis ultra-precision machine tool; the grinding spindle, the workpiece spindle and the rotary table on the four-axis ultra-precision machine tool are driven to move, so as to perform inclined-axis grinding wheel dressing and X and Z axis interpolation dressing, so that the obtained working grinding wheel grinding area arc center is accurately centered with the rotary table; The workpiece is clamped on the four-axis ultra-precision machine tool, and the workpiece is ground based on the predetermined grinding method and different working grinding wheels, the surface accuracy of the workpiece after grinding is measured, and error compensation processing is performed by the working grinding wheel until the accuracy requirement is met, to obtain a shaped element; Based on the material characteristics of the shaped element itself, a laser repair device is selected, and laser emitted by the laser repair device acts on the surface of the shaped element, so as to repair the subsurface damage of the shaped element; Specifically, laser drilling test is adopted, and the laser processing damage threshold of the shaped element is obtained by extrapolation method; based on the laser processing damage threshold, appropriate laser output power, scanning speed and scanning spacing are selected for raster line scanning on the surface of tungsten carbide; after laser processing, the shaped element is placed in a constant temperature room at 20±0.5℃ for 25-30 minutes, then placed in a sealed device containing acetone solution, cleaned by an ultrasonic cleaning machine for 25-30 minutes to remove surface impurities, and finally placed in a sealed device containing anhydrous ethanol, cleaned by an ultrasonic cleaning machine for 25-30 minutes, and dried to obtain a repaired shaped element.

2. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 1, characterized in that, Selecting a corresponding matched four-axis ultra-precision machine tool and a grinding method based on the machining characteristics of the workpiece, specifically including: Selecting tungsten carbide without binder as the mold material, and selecting inclined-axis single-point grinding method on the four-axis ultra-precision machine tool based on the aspheric surface machining characteristics of the tungsten carbide without binder.

3. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 2, characterized in that, Selecting a corresponding matched four-axis ultra-precision machine tool and a grinding method based on the machining characteristics of the workpiece, specifically including: Based on the oblique single-point grinding method, a metal-based diamond grinding wheel with a particle size of 400#-500# is selected as a rough grinding wheel in the rough grinding stage; a resin-based diamond grinding wheel with a particle size of 800#-1000# is selected as a semi-precision grinding wheel in the semi-precision grinding stage; and a ceramic-based diamond grinding wheel with a particle size of 10000#-15000# is selected as a precision grinding wheel in the precision grinding stage.

4. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 3, characterized in that, The initial grinding wheel is installed on the four-axis ultra-precision machine tool, specifically including the following steps: The grinding spindle is installed on the rotary table through a spindle sleeve and a displacement adjustment table, and the displacement adjustment table can realize precise adjustment of the grinding spindle in X, Y and Z directions; and the initial grinding wheel is clamped on the grinding spindle.

5. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 2, characterized in that, The workpiece is clamped on the four-axis ultra-precision machine tool, including the following steps: The workpiece is installed on the workpiece spindle of the four-axis ultra-precision machine tool through a transition piece, and the workpiece round run-out is detected and adjusted; after passing the inspection, the workpiece spindle is kept in a low-speed rotating state, the workpiece is fixed by bonding, and the workpiece round run-out is rechecked.

6. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 1, characterized in that, Based on the predetermined grinding method and different working grinding wheels, the workpiece is ground, the surface shape precision of the workpiece after grinding is measured, and error compensation processing is performed through the working grinding wheel until the precision requirement is met to obtain a shaped element, including the following steps: The grinding is performed in a step-by-step manner, specifically, during grinding, the metal-based diamond grinding wheel is used for rough grinding to quickly remove the material allowance and obtain a pre-shaped curved surface; then the resin-based diamond grinding wheel is used for semi-precision grinding of the curved surface; and finally the ceramic-based diamond grinding wheel is used for precision grinding to obtain the shaped element. In the precision grinding stage, the profile error is obtained by in-situ measuring the precision of the precision ground surface using the measurement unit integrated in the four-axis ultra-precision machine tool after one precision grinding, and the processing path is compensated based on the profile error until the surface shape precision of the processed surface reaches the predetermined precision.

7. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 1, characterized in that, The grinding spindle, the workpiece spindle and the rotary table on the four-axis ultra-precision machine tool are driven to move to perform oblique grinding wheel dressing and X, Z axis interpolation dressing, specifically including the following steps: During the oblique grinding wheel dressing, the initial grinding wheel shaft initial installation position is located on the rotary table, and the initial grinding wheel end small arc center is basically aligned with the rotary table center after rough centering; then the on-line dressing of the grinding wheel is realized by the rotary motion of the rotary table, in the dressing process, the grinding rod rotates at a certain speed ω1, the initial grinding wheel rotates at a speed ω2, and the initial grinding wheel rotates at a predetermined feed speed f along the workpiece spindle axial direction, while the rotary table drives the initial grinding wheel to rotate to form a round corner cylindrical grinding wheel, and a circular arc grinding area is formed at the end of the round corner cylindrical grinding wheel. During the X, Z axis interpolation dressing, color marking is performed on the entire area of the circular arc at the end of the round corner cylindrical grinding wheel before dressing, and the end arc of the round corner cylindrical grinding wheel to be dressed is always tangent to the outer circular arc surface of the grinding rod during the dressing process; the round corner cylindrical grinding wheel and the grinding rod rotate at a certain speed on their own to make the small arc at the end of the round corner cylindrical grinding wheel realize complete dressing.

8. The laser-assisted ultra-precision grinding process of tungsten carbide elements according to claim 1, characterized in that, After the repaired molding element is obtained, the repairing effect of the molding element is evaluated by an evaluation method, the evaluation method comprising the following steps: Cutting is performed perpendicularly to the machining surfaces of the molding element before and after the repairing, and two cutting workpieces are obtained respectively; The cutting end faces of the two cutting workpieces of the molding element before the repairing are placed side by side downward in a cold inlay mold for cold inlay, the cutting end faces of the two cutting workpieces of the molding element after the repairing are placed side by side downward in a cold inlay mold for cold inlay, one of the two cutting workpieces before the repairing is taken as a polishing auxiliary piece, one of the two cutting workpieces after the repairing is taken as a polishing auxiliary piece, so that one cold inlay piece before the laser repairing and one cold inlay piece after the laser repairing are obtained respectively; The two cold inlay pieces are placed on a polishing machine tool to polish to a mirror effect, and large-size damage and micro mechanical damage such as edge collapse, residual protrusions at the junctions and the like are removed; An etching solution of the cutting workpiece is configured, and the cutting workpiece is subjected to etching treatment for a certain time, so that the real subsurface damage is fully exposed; Subsurface damage is detected by using a transmission electron microscope, and the laser repairing effect is evaluated according to the detection result, a needle tip enhanced laser confocal Raman spectrum system is used to analyze the polished surface, the grinding surface and the laser repairing surface of tungsten carbide in sequence, nanoindentation technology is used to evaluate the mechanical properties of the laser repairing surface, so that the evaluation of the repairing effect of the molding element is realized.

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