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Vacuum sintering of tungsten alloy in controlled micro-atmosphere

High-density tungsten alloy is an alloy system composed of tungsten as the matrix (the content of tungsten is 85% ~ 98%), and a small amount of Ni, Cu, Fe, Co, Mo and other elements are added. Generally, it is divided into W-Ni-Cu and W -Ni-Fe two series.

This kind of alloy has a series of excellent physical and mechanical properties, such as high strength and hardness, good ductility and toughness, small thermal expansion coefficient, good corrosion and oxidation resistance, good machining and weldability, etc. It has been widely used in both industrial and civil industries.

Element content in tungsten powder, element content in nickel powder, element content in copper powder, analysis of impurity elements in the above three main raw materials, it can be seen that the content of C, S and P in the raw materials is very small. However, the content of C, S and P in the binder is relatively large. If the C content of the system is about 0.045wt%, the slow cooling after sintering will generate (Ni, Fe)6W6C cubic carbides at the interface of the W matrix. When the Ni/Fe ratio is 1/1, an orthorhombic (Ni, Fe)W intermetallic compound is easily formed at the interface of the W matrix phase. When the S content reaches a certain value, the impact toughness of the alloy will decrease significantly. In addition, S will also segregate in the interphase of W/binder phase and these interphase precipitation at the interface will lead to a significant decrease in mechanical properties. The alloy is embrittled due to the tendency of P to segregate to the tungsten/binder phase interface. Another impurity associated with P is S. On the interface, it can also form compounds with K and O, and gather on the inner surface of the pores.

Due to the small atomic radii of these elements, they have strong diffusivity in tungsten alloys, so it is easier to segregate at high-energy positions such as grain boundaries and phase boundaries, and even generate brittle phases, thereby reducing the performance of the alloy. Therefore, it is necessary to reduce their concentration in the solid solution, so as to inhibit their segregation at the interface of tungsten/binder phase, and achieve the purpose of improving the strength and plasticity of the material.

The shape of the tungsten powder is a hexagonal structure, the particles are relatively regular, and the size is about 2 μm; the nickel powder is in the shape of a polyp, and the size is about 20 μm; and the copper powder is a long strip structure without agglomeration, and the size is about 20 μm.

Technical solutions

(1) Controlled micro-atmosphere vacuum sintering activation and cleaning technology: Atmospheric conditions are very important for the sintering of tungsten alloy products. If the vacuum degree is too high, it is easy to ionize the air, causing the molybdenum boat and the tungsten alloy compact to spark and burn out the tungsten alloy device. Or molybdenum boat, especially tungsten alloy devices containing volatile valence of Cu, Ni and other metal elements, with the change of oxygen partial pressure and temperature during sintering, the change of electricity price and even phase change, excessive oxidation and reduction, precipitation of NiO, Ni2O3, CuO, Cu2O and other impurity phases, which lead to a significant decrease in the activation and cleaning properties of key components of tungsten alloys, and ultimately affect the overall performance of tungsten alloy products. This project improves the high-temperature sintering activity of the active components of tungsten alloy by strictly controlling the vacuum degree in the sintering furnace before sintering, and the automatic air pressure adjustment system continuously injects inert gas + trace controllable reduction activation atmosphere, and at the same time trace controllable reduction activation The atmosphere can also balance and adjust the cleaning and purification of impurity components such as C, S, P, etc., which can significantly reduce the internal defects of the alloy, ensure the density, strength, hardness and other performance indicators of the product, and improve the product yield.

(2) High-activity composite component technology: The tungsten alloy structure is composed of a tungsten phase and a binder phase (or matrix phase). The tungsten skeleton is filled in liquid phase during sintering to ensure the strength and plasticity of the alloy. Experiments have confirmed that adding some highly active substances to tungsten alloys, such as trace amounts of cobalt, rhenium, vanadium, tantalum, rare earth, etc., these highly active substances can be uniformly distributed around and on the surface of the tungsten phase and the binder phase at high temperatures. The alloy layer is formed at the interface of adjacent tungsten grains, thereby reducing the volume migration activation energy of the matrix phase. Through diffusion and viscous flow similar to liquid, the contact between the two phases is greatly enhanced, so that the hard phase and the binder phase are strengthened and uniformly dispersed. , thereby greatly increasing the strength and density of the alloy; in addition, the high active material can also reduce the liquid phase sintering temperature, effectively inhibit the growth of tungsten grains and the formation of brittle intermetallic compounds, and improve the tensile strength of the alloy.

(3) Rapid and thorough compound dewaxing technology: In order to prevent the wax vapor from overflowing through the thermal insulation material, the pore spreading technology is adopted, and a gas shunt pipe is installed in the vacuum sintering furnace to spread the argon gas evenly to every corner and from every corner. The wax vapor is evenly sucked away from each corner, combined with the action of the argon gas carrier, the flow of the wax vapor is accelerated, which ensures the rapid and complete dewaxing, thereby ensuring that the tungsten alloy products have excellent organizational structure.

(4) High temperature liquid phase sintering active ingredient volatilization suppression technology: Before sintering, the tungsten alloy compact is embedded in a composite covering agent mainly composed of Al2O3. When the temperature rises to 1200°C, the composite covering agent begins to volatilize to form a trace amount of composite gas , fully inhibits the volatilization of active components such as Cu and Ni during high temperature liquid phase sintering, and ensures that the density and linear expansion coefficient of the sintered tungsten alloy products meet the performance requirements of aerospace and military products.

(5) Carbon-free heating and thermal insulation technology: the heating element is a wide-band molybdenum-lanthanum alloy wire, and the thermal insulation material is a wide-band molybdenum-lanthanum alloy sheet, which replaces the traditional carbon felt heating and thermal insulation materials, and solves the carburization of tungsten alloy devices during sintering. (If carburization occurs during sintering, the tensile strength and elongation of tungsten alloy devices will be significantly reduced), thus ensuring good mechanical properties of high specific gravity tungsten alloys.

The surface layer of high-density tungsten alloy sintered with carbon heating element has carbon infiltration, which seriously reduces the mechanical properties of high-density tungsten alloy (tensile strength 450MPa, elongation 1.5%); After the density tungsten alloy is broken, its bonding phase is torn into cotton wool, which shows good plasticity. At this time, both the strength and plasticity reach the best (tensile strength> 650MPa, elongation> 3%), and the fracture shape is the bonding phase. Tear toughness fracture is dominant.

The high-density tungsten alloy products prepared by the controlled micro-atmosphere vacuum sintering process have the characteristics of high tensile strength, good plasticity, moderate surface hardness, standard density and qualified linear expansion coefficient, and are mature and reliable. Its performance indicators are as follows:

(1) Density: ≥17.2g/cm3;

(2) Tensile strength: ≥650MPa;

(3) Elongation: ≥3%;

(4) Linear expansion coefficient: ≥4.5×10-6/℃;

(5) Surface hardness: 24～33HRC;

(6) Yield: ≥98%.

Conclusion

The controlled micro-atmosphere vacuum sintering process for high-density tungsten alloys has been carried out in more than two years of research: controllable micro-atmosphere vacuum sintering activation cleaning technology, high-activity composite component technology, rapid and thorough composite dewaxing process, high temperature liquid Research on the volatilization inhibition process of phase sintering active ingredients and carbon-free heating and heat preservation technology. Through extensive research, some progress has been made in the vacuum sintering process for preparing high-density tungsten alloys. The main summary is as follows:

(1) The change from hydrogen sintering to vacuum sintering greatly improves the safety of the high-density alloy sintering process.

(2) The prepared high-density alloy products have high tensile strength, good plasticity and appropriate hardness.

(3) Significantly increased the production capacity of high-density alloys.

Selection of vacuum sintering furnace equipment: RVS series vacuum sintering furnace produced by SIMUWU is a high-quality product for vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum sintering process.