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Metal tungsten vacuum sintering process

Tungsten is a widely used metal material with a variety of excellent properties. Its melting point can reach 3410 C, its density is high, and its solid theoretical density can reach 19.25g/em3. In addition, it has high elastic modulus, excellent thermal shock resistance, excellent thermal conductivity, low thermal expansion coefficient and high mechanical strength at high temperature. Based on these excellent properties, tungsten is often used as a protective material in extreme environments. The material must have the characteristics of high melting point, good thermal conductivity, high temperature strength, low sputtering yield, and no deposition with tritium. Metal tungsten just meets the above conditions, so tungsten is considered to be the most promising plasma-facing material (PFM). However, pure tungsten also has certain defects, such as: high ductile-brittle transition temperature (up to 400C), self-sputtering rate will increase sharply under high-energy irradiation, and low recrystallization temperature (1200C), especially in high temperature plasma Under bulk irradiation, atomic vacancies and dislocations in the tungsten material will increase, thereby forming a nanoporous layer, resulting in irradiation embrittlement, and the retention of protium, deuterium and tritium rapidly increases.

Although tungsten is a very promising PFM, it is difficult to sinter high-quality tungsten materials by traditional methods. This is because the melting point of tungsten is very high, and the sintering conditions required for sintering high-density bulk tungsten are relatively severe: one is higher than the high temperature of 2500C, and the other is to maintain a long sintering time at such a high temperature . However, prolonged high-temperature sintering will cause the tungsten grains to grow significantly. In recent years, many explorations have been made to improve tungsten sintering, such as changing the sintering conditions, the particle size or doping of the initial raw materials. At present, the main methods of sintering tungsten are: ball milling activated microwave sintering, spark plasma sintering, high pressure instantaneous sintering, hot isostatic pressing sintering, etc. However, the tungsten or tungsten alloy synthesized by these methods is not satisfactory.

It is difficult to obtain bulk tungsten with high density and no grain growth by using traditional sintering methods. The traditional sintering method not only has a long sintering time, but also it is difficult to sinter a bulk tungsten material with high density, small grain size and high hardness.

Vacuum sintering process

Raw material selection particle diameter is 1～5 μm, and the tungsten powder that purity is 99.98% and average particle diameter are 5 μm, and purity is 99.9%TiC.

Adopt pure tungsten and W-TiC alloy raw material through hydrogen reduction treatment, the performance test result of prepared bulk tungsten material and W-1.5%TiC alloy is listed in table 1 and table 2, wherein H is Vickers hardness .

The change of hardness of pure tungsten samples obtained by high temperature and high pressure vacuum sintering with sintering time and sintering temperature is shown in Table 1 and the following figure. It is obvious that under the sintering pressure of 5.0 GPa, the hardness of the pure tungsten sample increases with the increase of the vacuum sintering temperature. When the sintering temperature is 1 300 C and the sintering time is 15 min, the hardness of the pure tungsten sample is 6. 06 GPa, when the sintering temperature is increased to 1500C and the sintering time remains unchanged, the hardness of pure tungsten increases to 6.43GPa. At the same sintering temperature, when the sintering time is prolonged, the hardness of the pure tungsten sample will also increase. At the temperature of 1300C, when the sintering time is extended from 15min to 30min, the hardness of the pure tungsten sample increases from 6.06GPa to 6.19GPa. According to the analysis, the main factor affecting the hardness is the degree of densification of the sample. With the increase of sintering temperature, the porosity of the sample decreases, the density increases, and the hardness increases; and prolonging the sintering time will increase the creep of the material under high temperature and high pressure. As time changes, the porosity decreases, the density increases, and the sample hardness increases.

The hardness of W-TiC alloy obtained by high temperature and high pressure vacuum sintering varies with sintering time and sintering temperature. It can be seen that the hardness of W-TiC alloy increases with the increase of sintering temperature, which is consistent with the change trend of pure tungsten hardness. In addition, the hardness of W-TiC alloy is also closely related to the content of TiC. Under the experimental conditions, the hardness of W-TiC alloy increases with the increase of TiC content.

The effect of sintering temperature on the density of W-TiC alloy, it can be seen from the figure that the density of W-TiC alloy increases with the increase of sintering temperature. Under the conditions of sintering pressure of 5. 0 GPa, sintering temperature of 1 300 C and sintering time of 15 min, the density of W-1.5%TiC sintered body is 98. 5%; The time remains the same. The density increases to 99.0%.

Summarize

The pure tungsten bulk material and W-TiC alloy material with high density and high hardness were prepared by the high temperature and high pressure vacuum sintering method using the W powder and TiC powder treated by hydrogen reduction as raw materials. The results show that: under the conditions of pressure of 5.0 GPa, temperature of 1500 C and sintering time of 15 min, a pure tungsten bulk sample with good properties is obtained, the grain size is uniform, the grain does not grow up, and the density reaches 99.3 % , the hardness reaches 6. 43 GPa; under the same high temperature and high pressure vacuum sintering conditions, by adding TiC with a mass fraction of 1.5%, a high-density tungsten-based alloy W-1. 5% TiC is obtained by sintering. There is no growth, the grain size is uniform, the density reaches 99.0%, and the hardness is increased to 7.58 GPa. Compared with the W-1.5%TiC alloy prepared by the traditional sintering method, the hardness is increased by 30%. The analysis shows that in the process of high temperature and high pressure vacuum sintering, the growth of tungsten grains is suppressed, the activation energy between grains increases, and the tungsten grains soften and rheology, thereby improving the density of sintered samples. The research shows that the high temperature and high pressure vacuum sintering method has great potential in sintering refractory metals and high density materials.

Selection of vacuum sintering equipment: The RVS series vacuum sintering furnace produced by SIMUWU is a high-quality product for the vacuum sintering process of tooling and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum sintering process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.