1 Introduction Polycrystalline Diamond (PCD) is a new type of superhard material made of specially treated single crystal diamond micropowder and a small amount of binder sintered under high temperature and high pressure. A PCD made of a K-type cemented carbide insert and pressed on a substrate is called a PCD composite sheet. The disorderly arranged diamond particles in the PCD have uniform high hardness and high wear resistance and are widely used in industries such as tools, tools and molds. The PCD composite sheet, due to the action of the substrate, balances the strength and toughness to a certain extent under the premise of ensuring hardness and wear resistance, thereby further expanding the application field and adding low cost, so it has more use value and application prospect.
However, ultra-hardness and super-abrasiveness have always been the biggest obstacles to the precision machining of PCD materials, and the traditional processing methods are almost impossible. With the development of processing technology, special processes are gradually used for the processing of PCD materials, but there are still many deficiencies, and the processing quality is more difficult to achieve. Therefore, in order to meet the growing demand for PCD materials in industrial development, the in-line electrolytic dressing (ELID) precision mirror grinding technology of metal bond superabrasive grinding wheel was introduced, aiming to explore the precision machining of PCD materials through the test and research of this technology. New way.
2 PCD material ELID precision mirror grinding experimental material experiment using PCD-1500 series polycrystalline diamond composite sheet produced by American GE company. Its physical and mechanical properties are shown in the table to the right.
Experimental conditions The self-developed ELID grinding electrolytic electrode device was installed on the MM7120A horizontal axis moment table precision surface grinder, together with the grinding wheel, grinding fluid and power source developed by ourselves to form the ELID grinding system.
The experimental grinding wheel is a cast iron fiber bond diamond grinding wheel CIFB, the specification is Ø240mm×90mm×10mm×5mm, and the particle size is W5. The trimming power supply used in the experiment is a self-developed special high-frequency pulse power supply for ELID grinding. The output voltage of the power supply is 0-140V, the current is 0~10A, and the pulse frequency is 0-500kHz. The grinding fluid uses a special grinding fluid developed by ourselves. Since the grinding fluid of ELID grinding also serves as an electrolyte, an alkaline water-soluble grinding fluid is used, and in addition to adding a rust inhibitor, a passivating agent, an extreme pressure additive, and a synthetic lubricant, a certain amount of inorganic salt is required. In order to make the grinding fluid have electrolysis capability.
In the experimental process, the ELD grinding device is used to grind the PCD composite sheet. Firstly, the grinding wheel is subjected to EDM precision shaping to eliminate the roundness and cylindricity error of the grinding wheel, so that the fine abrasive is distributed on the surface of the grinding wheel as high as possible. Then, the power is turned on, and the electrolysis pre-sharpening is performed to form a sufficient oxide film on the surface of the grinding wheel for about 30 to 45 minutes. Next, online electrolytic dynamic grinding is performed. Due to the high hardness of the PCD material, this stage is long, and the processing parameters must be strictly controlled to ensure sufficient cooling. After the grinding is completed, the power is cut off, and the speed of the table is increased. The workpiece is lightly ground for 30 to 45 minutes by the oxide film on the surface of the grinding wheel.
The surface roughness was measured using a SE-3H profiler manufactured by Kosaka Laboratory Ltd. of Japan, in which the microscopic size magnification was V=50000, the paper direction magnification was H=10, the sampling length was R0=0.25 mm, and the measurement length was measured. L = 2.5 mm. After measurement, the surface roughness of the PCD composite sheet after grinding reached Ra0.012 μm.
3 Removal Mechanism of ELID Grinding of PCD Material Due to the high hardness of PCD material, the grinding process is obviously different from hard and brittle materials such as hard alloy, engineering ceramics and optical glass. This is mainly manifested in the fact that the PCD material has high grinding pressure, low grinding efficiency, large grinding wheel consumption and small grinding ratio. In particular, the introduction of online electrolytic dressing technology makes the whole grinding process very complicated and has many factors. Therefore, the processing and removal mechanism of PCD materials has its particularity.
Although the diamond abrasive grains in the grinding wheel are similar in hardness to the diamond layer of the PCD composite sheet, the microstructures of the two are quite different. The former is to hold the diamond abrasive grains by a certain metal bond, and form an oxide film with good elasticity and plasticity on the surface of the grinding wheel, while the latter is to solidify the diamond particles together in the form of a diamond CC bond strong structure. Therefore, the diamond layer of the PCD composite sheet has an extremely high hardness, is almost non-plastic, and is extremely resistant to impact. During the grinding process, due to the high contact pressure, with the high-speed rotation of the grinding wheel, the abrasive grains on the surface of the grinding wheel have strong impact and friction on the surface of the PCD material, forming a severe mechanical alternating impact on the PCD composite sheet. It is also accompanied by intense friction, mutual extrusion and sliding, and causes bonding, scoring, tribochemical reaction and surface fracture. In this case, a part of the crystal grains on the surface of the PCD material is cleaved, peeled off or broken. At the same time, the grinding temperature rises sharply due to severe friction and sliding, and part of the diamond on the surface of the PCD is graphitized or oxidized due to poor thermal stability, thereby being removed.
Due to the online electrolytic dressing effect, the grinding wheel always maintains a sharp state to participate in the grinding, overcomes the obstacle of the grinding wheel being easy to block, and is beneficial to improve the processing quality and efficiency: the formed oxide film has a certain thickness, strength and compactness, covering the surface of the grinding wheel. Reduce the loss of the grinding wheel: the oxide film contains a large number of fine-grained abrasives. On the one hand, it increases the number of abrasive grains participating in the grinding, which is beneficial to improve the grinding efficiency. On the other hand, it is equivalent to grinding and polishing the workpiece, which is beneficial to the polishing. The formation of precision mirrors. Therefore, the removal of PCD material by ELID grinding method is the result of the combined action of mechanical, thermochemical and electrolytic reactions, especially the oxide film formed by electrolytic reaction, which plays a key role in the formation of the final precision mirror.
4 Conclusions Through the ELID precision mirror grinding experiment and analysis of PCD composites, the following conclusions are drawn:
1. Precision grinding of polycrystalline diamond with metal bond diamond grinding wheel and on-line electrolytic dressing (ELID) precision grinding technology can achieve good results. The grinding surface roughness value is very small, it is a kind of polycrystalline diamond. A new approach to precision machining has great promotional value and application prospects.
2. Due to the super-hardness of PCD materials, the grinding process is very different from the general hard and brittle materials. When using ELID grinding technology, material removal is the result of a combination of mechanical, thermochemical and electrolytic reactions.
3. Due to the online electrolytic dressing effect, an oxide film with a certain strength, thickness and compactness is formed on the surface of the grinding wheel. The oxide film plays an extremely important role in the whole grinding process and the final surface quality.
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