Polycrystalline diamond and carbide composite inserts (PCD/CC)
PCD/CC is the most widely used diamond for machining metal matrix composites. It is formed by sintering or pressing a layer of polycrystalline diamond PCD with a thickness of 0.5-1mm on the surface of a cemented carbide substrate with good strength and toughness. The bending strength of this composite blade is basically the same as that of cemented carbide, and the hardness of the working surface is close to that of the solid polycrystalline diamond PCD, and the weldability is good, the regrinding is easy, and the cost is low. A typical polycrystalline diamond-tungsten carbide insert is flat or sheet-like, and then laser-cut to the desired shape and size of the insert. PCD/CC are usually welded or machine-clamped, and are made into various cutting tools such as turning, drilling, milling, punching, reaming, boring and sawing. The performance of PCD/CC is related to the size of diamond grains. De Beers produces PCD inserts in 002, 010 and 025 varieties, with average grain sizes of 2µm, 10µm and 25µm, respectively. The larger the grain size, the higher the aggregation of diamond volume, the better the wear resistance and the longer the tool life, but the quality of the cutting edge is slightly worse, making it difficult to make a high-precision tool. On the contrary, fine-grained tools have a blunt radius of the cutting edge (generally 1-2 of PCD tools), and the machined surface quality is good. Therefore, the crystal grains of polycrystalline are continuously refined, and there are fine crystals with 1 acid or even less than 0.5 acid. To this end, the life of the tool and the quality of the machined surface should be considered comprehensively to select the appropriate grade of polycrystalline diamond and cemented carbide composite inserts.
CVD diamond
CVD diamond is a pure diamond material with high wear resistance, without binders, which is prepared at low pressure (<0.1MPa). CVD diamond comes in two forms: CVD thin film coating (CD) and CVD thick film (TFD).
CD is formed by depositing a layer of film-like diamond with a thickness of less than 50µm (usually 10~30µm) composed of polycrystals on a cemented carbide substrate (commonly known as K-type alloy) by CVD (chemical vapor deposition) process. Use has shown that CD diamond-coated tools are not suitable for machining metal matrix composites, because the hard particles in the composite can wear through the coating on the surface of the tool in a short time.
TFD is to deposit a substrateless diamond thick film with a thickness of up to 3-5mm, and then cut the thick film into small pieces of a certain shape according to the needs, and then weld it on the cemented carbide to form a composite blade or tool.
TFD has good comprehensive performance, it does not have the disadvantage of anisotropy of natural diamond, because there is no metal bond, the impurity content is low, and the purity is close to 100%, so the hardness and thermal conductivity are higher than PCD, the friction factor is smaller, chemical The stability is better, and the performance of metal matrix composites is quite good.
Shown are tool wear curves for 40% SiCP A356MMC with two diamonds, PCD and TFD. The cutting conditions used in the test in Figure 2 are: cutting speed of 400m/min, feed rate of 0.05mm/r, back cut amount (cutting depth) of 0.5mm, and cutting fluid. It can be seen from Figure 2 that the thick-film diamond TFD is the best for processing 40% SiCP metal matrix composites; PCD025 is the second, and PCD002 has the lowest tool life. Table 1 shows the cutting parameters recommended by De Beers for machining metal matrix composites with polycrystalline diamond grade SYNDITE 025 inserts. The research shows that with the increase of cutting speed and the decrease of feed rate, the value Ra of machined surface roughness will decrease, while the amount of back-cutting tool has no significant effect on Ra. Therefore, when finishing, the cutting speed should take the larger value in Table 1, and the feed rate should take the smaller value.
The machining of metal matrix composites with diamond can be dry cutting or wet cutting. However, due to the low thermal stability of diamond, it will be carbonized (ie graphitized) at 700-800 ° C, which will drastically reduce the tool life. If the cutting fluid is used for wet cutting, the cutting temperature can be reduced and the tool life can be increased. However, before the tool cuts into the workpiece, the cutting fluid should be poured until the cutting is completed. The cutting fluid must be supplied continuously, not intermittently, otherwise it will easily cause the tool to be damaged or chipped. In the same way, during the cutting process, it is necessary to avoid stopping or changing the cutting amount as much as possible.