High-density tungsten alloy, also known as tungsten-based heavy alloy, is a type of alloy made of tungsten as the matrix element (85% ~ 99% mass fraction) with the addition of Ni, Cu, Fe and other alloy elements by liquid phase sintering. The density is up to 16.5 ~ 19.0g / cc. The most commonly used are tungsten nickel copper and tungsten nickel iron alloy with tungsten content of 90% -97%. Tungsten nickel copper alloy is a non-magnetic alloy, while tungsten nickel iron is a magnetic alloy. W-Ni-Fe series alloys are widely used because they have better mechanical properties than W-Ni-Cu series alloys.
High-density tungsten alloy is traditionally prepared by powder metallurgy, and its sintering process is as follows:
Powder —- Mixing — Pressing — Sintering — Heat treatment — Plastic deformation (Forging, rolling) — Annealing-Product
Some products can also be directly obtained by sintering.
The sintering of high-density tungsten alloys mostly adopts liquid phase sintering process: normally, the sintering temperature is higher than the liquid phase temperature by 20 to 60 ℃. The sintering temperature of W-Ni-Fe alloy is 1500～1525 ℃, the density can be close to the theoretical density, the holding time is generally 60～90min. If it exceeds 120min, the performance will drop. The use of higher sintering temperature and shorter holding time is beneficial to improve the tensile strength and elongation of the alloy. The choice of cooling rate is related to the m(Ni)/m(Fe) ratio of the alloy. When this ratio is not in the range of forming brittle compounds, such as the m(Ni)/m(Fe) ratio of 2 to 4, any cooling rate will not generate intermetallic compounds. To avoid brittleness and reduce porosity, it is beneficial to use an appropriate cooling rate.
Post-sintering heat treatment includes: quenching, rapid cooling, atmospheric dehydrogenation treatment and surface hardening treatment, the purpose is to reduce the segregation of P, S and other impurities at the interface, reduce hydrogen embrittlement, improve alloy performance or meet other alloy performance requirements. Quenching and rapid cooling treatment can obtain a fine and uniform grain structure, better play the role of solid solution strengthening, and increase the strength of the alloy by 1/3 to 1/2.
Heat treatment can improve the mechanical properties of the alloy obviously. Studies have shown that if the 95W alloy is placed in an intermediate frequency induction furnace with a vacuum of 0.133-0.0133Pa for vacuum heat treatment (temperature 850 ℃ -900 ℃, holding 40min), the strength and impact toughness of the alloy will be significantly improved.
Plastic deformation processing
In order to improve the mechanical properties of the alloy, various plastic deformation processes are carried out on the sintered high-density tungsten alloy. At present, large-diameter tungsten alloy core materials with a length/diameter ratio greater than 8 are generally prepared by deformation strengthening technology. Deformation strengthening techniques include forging, hydrostatic extrusion, hot extrusion, hot rolling, or hot extrusion and forging composite deformation processes.
After dehydrogenation of high-density tungsten alloy by vacuum heat treatment, hydrogen embrittlement is weakened, phase boundary strength is improved, and the strength and plasticity of the alloy are greatly improved, which provides favorable conditions for high plasticization of the plastic deformation of the alloy. In the deformation strengthening process, the amount of deformation has a very important effect on the performance of the tungsten alloy. As the amount of deformation increases, the strength and hardness of the alloy increase, while the ductility decreases. In order to greatly improve the mechanical properties of the alloy, in recent years, a large deformation strengthening technology for high-density tungsten alloys has been developed. The strength of large deformation is increased by more than 300MPa than that of ordinary forging. For high-density tungsten alloy armor-piercing core materials with an aspect ratio of about 16 and a tensile strength of about 1200 MPa, one-time forging deformation strengthening can meet the requirements. For the new high-density tungsten alloy armor-piercing projectile in research and development, it must undergo multiple cycles of forging and deformation to meet the requirements.
After plastic deformation, the appropriate machining method can be selected according to the drawings, such as: turning, milling, cutting, drilling, polishing, etc.