The physical property differences between tungsten and copper are significant (tungsten has high hardness and melting point, while copper has high electrical conductivity and good plasticity). The two elements are immiscible and have poor sintering properties. These differences provide a physical basis for balancing the hardness and toughness of copper-tungsten alloys. If the tungsten content is too high, the material is prone to embrittlement due to low bonding strength and excessive resistivity. Conversely, if the copper content is too low, it is difficult to form a continuous matrix to inhibit crack propagation. Below is a comparison of the specific physical properties of the two elements:
Physical Properties of Tungsten
Color and State: Tungsten appears steel-gray or gray-black and is a metallic element with the symbol W and atomic number 74.
Density: Tungsten has a density of 19.35 grams per cubic centimeter, significantly higher than that of copper.
Melting and Boiling Points: Tungsten has an extremely high melting point of 3410℃ and an even higher boiling point of 5927℃, making it the metal with the highest melting point.
Hardness: Tungsten has a high hardness, with a Mohs hardness of 7.5, and exhibits excellent wear resistance.
Thermal Expansion Coefficient: Tungsten has a low thermal expansion coefficient, meaning its dimensions change very little with temperature variations, which is beneficial for maintaining material stability.
Electrical Conductivity: Although tungsten’s electrical conductivity is not as high as that of copper, it still possesses good electrical conductivity and maintains stable electrical performance at high temperatures.
Other Properties: Tungsten also exhibits high-temperature resistance, corrosion resistance, high tensile strength, high thermal conductivity, and high creep resistance.
Physical Properties of Copper
Color and State: Copper has a purplish-red luster and is a metallic element with the symbol Cu and atomic number 29.
Density: Copper has a density of 8.96 grams per cubic centimeter (some sources state 8.92 grams per cubic centimeter, with differences possibly arising from measurement conditions or precision), which is relatively low.
Melting and Boiling Points: Copper has a melting point of 1083.4±0.2℃ and a boiling point of 2567℃, both far lower than those of tungsten.
Hardness: Copper has relatively low hardness and exhibits good ductility and malleability, making it easy to process into various shapes and sizes.
Thermal Expansion Coefficient: Copper has a relatively high thermal expansion coefficient, meaning its dimensions change significantly with temperature variations.
Electrical Conductivity: Copper has excellent electrical conductivity, second only to silver among all metals, and is an indispensable material in the electrical industry.
Thermal Conductivity: Copper also has good thermal conductivity and is commonly used in the manufacture of heaters and heat exchangers.
Other Properties: Copper also exhibits good corrosion resistance, diamagnetism, and antibacterial properties.
The optimal composition ratio must be determined through experimentation. For example, in high-voltage electrical contact applications, a W80Cu20 (80% tungsten, 20% copper) composition can balance hardness (>220HB) and flexural strength (980MPa) while utilizing copper’s ductility to alleviate stress concentration.
Improving Preparation Processes
Powder Metallurgy Method: Copper-tungsten alloys are prepared through steps such as powder production, blending, pressing, and sintering infiltration. Traditional processes suffer from closed pores and low density (<98%), which can be improved through the following methods:
- Adding a small amount of nickel to activate sintering, increasing density but reducing electrical and thermal conductivity.
- Using mechanical alloying to produce ultra-fine or nanoscale powders, enhancing sintering activity but potentially introducing impurities.
- Employing oxide co-reduction, which, although complex, can produce high-purity powders.
Injection Molding Method: Uniformly sized nickel, copper-tungsten, or iron powders are mixed with tungsten powder, combined with an organic binder, and injected into a mold. After steam cleaning, irradiation degumming, and hydrogen sintering, a high-density alloy is obtained. This method improves sintering properties, achieving a density of over 99%.
Infiltration Method: Tungsten or molybdenum powder is pressed and sintered to form a skeleton with a certain porosity, followed by copper infiltration. This method is suitable for low-copper-content products but requires control of infiltration temperature and time to avoid local embrittlement due to copper phase enrichment.
Controlling Heat Treatment Parameters
Annealing Treatment: Immediate annealing after forging can eliminate stress and prevent crack propagation. For example, brass annealed at 300-400℃ for 2 hours and copper annealed at 500-600℃ can enhance ductility. Aviation components require additional solution aging treatment to achieve a hardness of HB80-100.
Optimizing Rolling Temperature: Copper-tungsten alloys with 20%-30% copper content exhibit optimal comprehensive properties when rolled at 700-850℃. This temperature range effectively eliminates internal copper phase enrichment and pores while avoiding damage to the tungsten skeleton formed by infiltration-sintering. For example, W-20Cu alloys rolled at 650-750℃ and W-30Cu alloys rolled at 800-900℃, held for 30 minutes, achieve higher hardness and density.
Refining Grain Structure
Adding Trace Elements: Incorporating elements such as titanium (Ti), zirconium (Zr), and vanadium (V) into the copper matrix can refine grains and inhibit crack propagation. For example, adding 0.01% boron (B) to a CuZnAl alloy can refine grains to below 50μm, improving shape memory properties and fracture toughness.
Rapid Solidification Technology: Using melting, spraying, and roll water quenching to produce microcrystalline thin strips can reduce the grain size of copper-tungsten alloys to the micrometer level, enhancing plasticity. Although room-temperature aging may stabilize the material, it is necessary to weigh the loss of shape memory effect and superelasticity.
Avoiding Impurity Contamination
Strictly Cleaning Workpieces: Before welding or processing, alkali-acid cleaning or mixed acid cleaning methods should be used to remove surface oil, oxides, and impurities. For example, an alkali cleaning solution consisting of 93%-97% NaOH + 3%-7% KMnO₄ and an acid cleaning solution consisting of 54% H₂SO₄ + 45% HNO₃ + 18g/L Cr₂O₃ can effectively reduce non-metallic inclusions and enhance material purity.
Controlling Ambient Gases: Welding or heat treatment should be conducted in a vacuum or high-purity inert atmosphere (such as argon) to prevent embrittlement caused by the infiltration of oxygen, nitrogen, and other gases. For example, vacuum electron beam welding of copper-tungsten alloys requires a vacuum level of 133.3×10⁻⁶Pa, an accelerating voltage of 100-200kV, and a welding current of 10-40mA.