Welding of single crystal diamond cutting tools
At present, there are three main welding methods commonly used, namely bonding method, inlay method and brazing method.
The bonding method is to use organic binders such as epoxy resin to bond the diamond bit to the groove on the toolbar that matches the size of the diamond bit.
The temperature resistance of the organic binder generally does not exceed 200 °C. High temperatures may cause the binder to fail, and it is only suitable for ultra-precision machining with a small amount of cutting. And it is not possible to grind the diamond bit directly on the tool holder.
When the tool needs to be ground, it is necessary to use a solvent such as acetone to gel the bond first, grind the diamond bit, and then re-bond it to the toolbar. In addition, the positional accuracy of the diamond tip during the bonding process is difficult to guarantee.
The inlay method is to first sinter the diamond bit and the metal powder together, and then mechanically fix the diamond bit on the tool holder through the pressing plate.
Its technological process is as follows:
(1) Roughly grind the diamond into the shape of the upper and lower sides, and determine the position of the blade and the flank.
(2) Place the diamond face down in the cast iron mold as shown in the figure, fill an appropriate amount of copper-based metal powder (such as 663 copper powder) around it, and then insert the upper plunger to compact the metal powder with a little pressure.
(3) While the mold is slowly heated by a high-frequency induction device, the pressure between the upper and lower plungers is continuously increased until the temperature and pressure reach 750°C and 5.5kg/mm2 respectively.
After cooling, take out the sintered block and the diamond cutter head in it, and obtain the socket holes consistent with the shape and size of the diamond cutter head on the sintered block.
(4) According to the relative position of the designed blade and the shank, remove the excess part on the sintered block. Then the processed sintered block is brazed on the correct position of the arbor with lower-temperature copper brazing material, and the clearance angle and other parts of the arbor are precisely ground.
(5) Put the finely ground diamond cutter head into the socket hole of the sintered block and cover it with a suitable pressure plate, that is, the installation of the diamond cutter is completed. The installed diamond cutter head (as shown in the figure) only exposes the flank face and the rake face of about 1mm.
A large number of studies have found that soldiers containing active elements such as titanium, zirconium, niobium, and tantalum can directly wet the diamond surface in a vacuum. These active elements can also react with carbon atoms on the diamond surface to form stable carbides and form chemical bonds.
Metal molybdenum or cobalt-based cemented carbide has a relatively similar thermal expansion coefficient to diamond, and it can be used as a welding matrix to not damage the diamond and ensure brazing strength. On the basis of this research, the diamond brazing technology under vacuum conditions is developed.
The brazing filler metal used in shielding gas brazing of diamond is silver-copper-titanium alloy, and the composition ratios of silver, copper, and titanium in the alloy are 68.8%, 26.7%, and 4.5% respectively. The protective gas is a mixture of argon (95%) and hydrogen (5%). Welding takes place in a semi-open cavity as shown.
The brazing process is as follows:
(1) Fully remove oxides on the surface of diamond and metal substrate;
(2) Heating the base and brazing filler metal in a protective atmosphere until the brazing filler metal melts and spreads evenly on the designated position of the base, and then cools down;
(3) Place the diamond to be welded on the correct part of the substrate, fill it with protective gas, reheat it to the melting temperature of the solder, and then slowly cool it to room temperature.
Using the brazing method to install the diamond bit has the following advantages: high welding strength, the shear strength of the welding surface can reach 340MPa, and the diamond bit with a weight of only 0.02 carat can be firmly welded to the tool holder.
After brazing, the diamond cutter head can be finely ground to ensure the machining accuracy of the tool geometry angle.
Vacuum brazing method
At present, the vacuum brazing method is the most widely used, and the advantages are as follows:
Vacuum low pressure is used to remove the oxide film on the surface of the weldment and protect the weldment from being oxidized while avoiding the phenomenon of carbonization, decarburization, and pollution deterioration, and improving the strength of the welded joint.
The low pressure can also discharge the volatile gas released by the metal at the brazing temperature, and obtain a bright and dense joint.
In addition, because no flux is used in the vacuum atmosphere, there will be no defects such as pores and inclusions, which can improve the corrosion resistance of the base metal and save the process of cleaning the residual flux required for general welding.
In a vacuum furnace, vacuum brazing can braze many adjacent brazing seams at one time, and braze multiple components in the same furnace, which can greatly improve the welding efficiency and can be completed simultaneously with the heat treatment process.
In addition to these, there are many basic metals that can be welded by vacuum brazing, including aluminum alloys, titanium alloys, refractory metals, as well as ceramic glass, graphite, diamond, composite materials, etc.
In general, the thermal stress change value of the brazing filler metal is nickel-based brazing filler metal > nickel-copper-based brazing filler metal > copper-based brazing filler metal > copper-silver-based brazing filler metal > silver-based brazing filler metal.
Vacuum brazing has its great advantages:
1. Mass production can be realized, and more than 200 tools can be produced at one time;
2. It is easy to operate, and what is more convenient is that a tool with multiple heads can also be welded at the same time;
3. Increase the strength to ensure the consistency of welding.
Recommended brazing temperature
High Temperature Solder Paste
Containing active solder titanium, it can weld single crystal diamond, CVD, PCD, PCBN. Currently, it is mostly used for welding single crystal diamond and PCBN. It requires high vacuum (10-2Pa) and high strength.
Medium Temperature Solder Paste
Contains active solder titanium, can weld single crystal diamond, CVD, PCD, PCBN, suitable for welding PCD with temperature resistance within 800 degrees, high vacuum requirements (10-2Pa)
Low Temperature Solder Paste
Contains active solder titanium, can weld single crystal diamond, CVD, PCD, PCBN, suitable for most PCD welding (especially PCD with temperature resistance within 760 degrees), high vacuum requirements (10-2Pa)