Tungsten metal is the metal with the highest melting point in the world, with a melting point of 3380 ° C. In comparison, the melting point of iron is only 1538 ° C, and the melting point of nickel is only 1455 ° C. Therefore, the filaments made of metal tungsten used in incandescent light bulbs, because only this metal can withstand the huge heat generated when the current passes.
Tungsten metal is the highest melting metal
Speaking of high temperature resistance, there may be few places where you need to find high temperature resistant materials than aero engines. After all, aero engines need to generate a lot of energy through extremely intense combustion to drive the aircraft to fly, resulting in the temperature behind the combustion chamber. Requires up to 2000 ° C and even higher.
△ Combustion chamber will generate high temperature air flow (Do you notice that the direction of the turbine has been reversed?) △
But we did not see metal tungsten in the main structure of the aero engine, or even the compounds containing tungsten. Why?
Whether a material can be used depends not only on a certain parameter
High melting point is the advantage of metallic tungsten, but a material is far more than just the "melting point" parameter, but includes many parameters such as stiffness, strength, density, price, processability, etc. These parameters are only A bright eye is not enough, but a comprehensive consideration of various factors.
For example, the material in aero engines must focus on the stiffness, strength and density of the material.
Materials with high rigidity will not produce great deformation when the structure is subjected to large forces. You must know that the rotor inside an aero engine may rotate tens of thousands of revolutions per minute. At such a high speed, if the structure has insufficient stiffness, , Then under such a large load there will be great deformation, it is easy to cause the engine to malfunction.
△ The object will be deformed by force, so the material stiffness is very important △
High strength means that the material can withstand greater forces without breaking. For example, the picture below is the static strength test of the metal. The strength of the material is tested by continuously increasing the tensile force until the test piece is broken.
The density of the material is also very critical in aero engines. In addition to the need to reduce the weight of the aero engine itself, it is more important that the aero engine is to fly to the sky and is constantly in high-speed motion. The internal rotor will also rotate at high speed. In the state, the higher the density of the material, the greater the inertial force, so reducing the density of the material can more easily obtain better performance.
These factors all restrict the use of various materials in aero engines. For example, although carbon fiber has low density and high strength, its stiffness is not sufficient, so it is rarely used in aero engines; for example, titanium alloy stiffness, strength, The density is good, but it is not resistant to high temperature, so titanium alloy cannot be used in high temperature parts … etc.
Carbon fiber blades on the GE90 require titanium alloy edging
Titanium alloys are generally only used in lower temperature parts
Therefore, even if metal tungsten has a high melting point, the disadvantage is also obvious. The density of metal tungsten is 19g per cubic centimeter, which is twice that of steel and four times that of titanium alloy. So it is difficult to apply such a large density to aero engines.
Tungsten also has a lot of problems in processing
No matter how good a material is, it can only be processed into the parts we need before it can be used. Therefore, the processing performance of a material also largely limits the scope of use of this material.
Tungsten has a high melting point, which is an advantage, but it also shows that many of the material processing methods we have become accustomed to are not suitable for tungsten. For example, "casting" related technologies are difficult to apply to tungsten.
At the same time, tungsten has high hardness and brittleness, so it is often used to manufacture machined tool bits, which also represents that tungsten steel is a very difficult material to process. You think: the machined tool head is made of tungsten steel, so what is the tungsten steel used for processing? Therefore, the above processing difficulties also limit the use of tungsten in aero engines.
Most importantly, a high melting point does not mean that it can be used at high temperatures
The so-called melting point of a metal means that the metal will melt into a liquid at a certain temperature. A metal with a high melting point means that the metal can remain in a solid state at high temperatures. But staying solid does not mean that the metal can have the necessary mechanical properties.
For example, titanium alloy, although we say that this metal can not be used in high temperature environment (600 ° C can not be used), in fact, the melting point of titanium alloy is often as high as 1600 ° C. So how can a titanium alloy with such a high melting point be used only in a low temperature environment?
The answer is that although titanium alloys remain solid at high temperatures, their mechanical properties have fallen too much. The following figure is the specific strength of titanium alloy. It can be seen that after the temperature has passed 200 ° C, the performance of the titanium alloy begins to decline.
Mechanical properties (gray) of titanium alloys decrease rapidly with increasing temperature
Tungsten has the same problem. Although the melting point is as high as 3380 ° C, but the temperature has exceeded 1000 ° C, the performance of the material has dropped drastically, not to mention the amazing density. Therefore, the high melting point of tungsten can be said to be "insignificant."
The high temperature performance of tungsten is actually not so good
In comparison, plutonium may be a more suitable metal for use in aircraft engines.
This metal element also has a very high melting point (3180 ° C), which is slightly lower than tungsten, but compared to tungsten, this metal is not so "brittle" and has better resistance to high temperature creep and high temperature mechanics. It is not as powerful as tungsten.
So even though the melting point of rhenium is lower than that of tungsten, the density is higher than that of tungsten (the density of rhenium is as high as 21.04g / cm?), And the processability is not much better than tungsten, but it is because it still maintains an acceptable material at high temperatures. Nature, so it is still a super heavy metal that has been initially applied in the aerospace field.