Views:7 Author:Site Editor Publish Time: 2019-08-09 Origin:Site
Sintering of tungsten-based heavy alloys is an important part. The sintering process has a great influence on the density, grain size, segregation and microstructure of the alloy. During the sintering process, sintering temperature, time, sintering atmosphere and cooling rate are the main process parameters, which have a very important influence on the density, microstructure and mechanical properties of the alloy.
The main methods for sintering tungsten-based heavy alloys include liquid phase sintering, solid phase sintering, two-step sintering, and repeated sintering.
Sintering temperature and holding time have a great influence on the uniformity of the alloy. When the sintering temperature is low or the holding time is short, the binder phase in the liquid phase sintered tungsten-based heavy alloy partially forms a liquid phase, and the tungsten particles cannot be effectively wrapped, and the distribution of tungsten and the binder phase is extremely uneven.
When the content of tungsten in the alloy is high and the content of binder phase is small, the sintering temperature should be increased; for alloys with low tungsten content and high binder phase content, the sintering temperature should be appropriately reduced. At the appropriate sintering temperature, the length of the sintering time should be selected to take into account the size of the sintered part and the amount of tungsten. The optimum sintering time is 60-90 min. Liquid phase sintering is usually carried out in a pure hydrogen atmosphere; when sintered in a vacuum, the density is higher than that in a hydrogen atmosphere; when nitrogen or hydrogen is used as a protective atmosphere, the alloy has a large porosity and a low density. The hardness of the binder phase in the alloy decreases with the decrease of the cooling rate, while the hardness of the tungsten phase does not change much. The main factor affecting the toughness of the cooling rate is impurity segregation.
A reasonable sintering process should achieve the following objectives: increase the density of the alloy after sintering, reduce or eliminate porosity, avoid hydrogen embrittlement and formation of intermetallic compounds, and prevent grain growth and coarsening.
When a tungsten-based heavy alloy is sintered in a liquid phase, the sintering temperature is high and the crystal grains are easily grown. Moreover, during the sintering process, the solid/liquid density difference is large, and the viscous flow is generated under the action of gravity, and the tungsten crystal grains are "segregated", and the sample is prone to severe collapse deformation. The application of liquid phase sintering is greatly limited for parts with high performance, uniformity of structure, strict dimensional accuracy or complex structure. Solid phase sintering can reduce or eliminate the deformation of the sample due to the avoidance of liquid phase flow and sinking of tungsten particles during liquid phase sintering, and the process has practical value. However, due to the relatively low relative density of solid phase sintering, the strength is much lower than that of liquid phase sintering, and some other means must be used.