Why Airplanes Are Inseparable From Aluminum-lithium Alloys

Aluminum alloy is not unfamiliar to everyone, as it is cheap, beautiful, and durable, and has been widely used in various industries. Among them, there is a type of aluminum alloy that has attracted much attention due to its widespread application in the aerospace field, which is aluminum lithium alloy.

For aerospace, saving quality means significantly reducing costs. In order to reduce the weight of airplanes and rockets themselves, researchers have been working hard on manufacturing materials, hoping to find a material that is both lightweight and sturdy. Aluminum lithium alloy, as an advanced lightweight structural material, has many excellent comprehensive properties such as low density, high specific strength and stiffness, high elastic modulus, low fatigue crack propagation rate, good low-temperature performance, good corrosion resistance, and excellent superplastic forming performance. Most of the structural materials used in China's large aircraft C919 are aluminum lithium alloys. Below, we will briefly introduce aluminum lithium alloy to you.

What is aluminum lithium alloy

The so-called aluminum lithium alloy is an alloy formed by adding a small amount of lithium metal to aluminum alloy materials. Because lithium metal is the least dense metal on Earth, with a lower density than water. Adding this metal to other metals can reduce the density of the alloy. For example, adding 1% lithium metal to aluminum alloy can reduce its density by 3%. By adding lithium metal, lighter alloy materials can be produced than traditional aluminum alloys, meeting the requirements of the aerospace industry for continuously reducing quality and operating costs. Of course, through continuous research, it has been found that aluminum lithium alloys can not only achieve lower density but also higher strength, thereby reducing material usage and further reducing aircraft quality while ensuring aircraft strength.

Of course, aluminum lithium alloy is not just about adding lithium metal to aluminum. If it were that simple, it would not have become a costly aerospace material. Lithium metal itself is a reactive metal that can easily oxidize when exposed to air, and may even be ignited to cause a fire. If this lively metal is added to aluminum alloy, it is easy to oxidize, not only on its own, but also easily drives the aluminum alloy to oxidize. So, although adding lithium to aluminum metal will reduce the alloy density, it will not be added in large quantities. Currently, the lithium metal content added to various formed aluminum lithium alloys does not exceed 3%, and the lithium element content in mainstream aluminum lithium alloys is concentrated between 1-2%, with little fluctuation.

In addition, adding lithium alone is not enough to improve the strength of aluminum alloys, but may actually reduce their strength. Therefore, the mainstream aluminum lithium alloy is not just an alloy of aluminum and lithium elements, but a complex alloy with several or even more than ten elements added at the same time, and some elements may even exceed the content of lithium elements.

Development history of aluminum lithium alloys

Aluminum lithium alloys have mainly gone through four generations of products. The earliest lithium containing aluminum alloy was born in 1924, with a lithium element addition of only 0.1%. The special properties of this material were not discovered at the beginning, so it did not receive attention, nor did it further deepen the research direction of aluminum lithium alloy as a key focus. Until the 1950s, the United States and the Soviet Union successively developed the true first generation of aluminum lithium alloys. The main feature of the first generation aluminum lithium alloy is its low density, which can reduce the quality of aircraft manufacturing materials, further helping aircraft save fuel and reduce operating costs. The United States and the Soviet Union have successively applied aluminum lithium alloys to the manufacturing of aircraft components and achieved good economic benefits. Aluminum lithium alloy has attracted attention from various countries and has become an important aerospace material that everyone focuses on researching and developing.

However, the first generation of aluminum lithium alloys had poor ductility, were not easy to process, were prone to fracture, and were prone to corrosion after fracture, which determined that aluminum lithium alloys could not meet the application needs of aerospace very well. Taking airplanes as an example, if the structural materials are easily damaged or corroded, it will bring great safety hazards to the aircraft.

In the 1960s, the international market saw a surge in oil prices, giving rise to the second generation of aluminum lithium alloys. As long as the aircraft quality can be reduced, fuel consumption can be saved, thereby saving high operating costs. So, the second generation of aluminum lithium alloys mainly focuses on reducing density, increasing the lithium element content in aluminum lithium alloys, often adding more than 2% lithium element. The second-generation aluminum lithium alloy can reduce its mass by 8% to 10% compared to traditional aluminum alloys. But it is precisely because of the high lithium content that aluminum lithium alloys are subject to certain instability factors. The thermal stability of the second generation aluminum lithium alloy is relatively poor, and the processing and fracture prone properties have not been well improved. At the same time, there is anisotropy, which means that the structural strength and other indicators in different directions are different, which brings many difficulties to the practical application of the material.

The true arrival of the aluminum lithium alloy era began in the 1990s. By this time, human understanding of aluminum lithium alloys had reached a new height, no longer pursuing overall performance improvement, but studying aluminum lithium alloys with different properties based on different uses. The third-generation aluminum lithium alloy has derived various grades of aluminum lithium alloy products based on different uses, meeting the needs of different fields.

The third-generation aluminum lithium alloy has to some extent solved the legacy problems of the first generation and second generation aluminum lithium alloys. By reducing the amount of lithium element added to ensure material stability and further improving the formula with some special performance elements, aluminum lithium alloy has truly achieved a series of excellent properties such as low density, high structural strength, easy processing, and corrosion resistance. Aluminum lithium alloy has also been officially recognized by the aerospace industry, and its application proportion in aircraft and rockets is increasing. The second generation of aluminum lithium alloy is mainly used in some transport aircraft and helicopters, and with the third generation of aluminum lithium alloy, civil aviation large aircraft have also begun to use it on a large scale. The skin, crossbeams and other important structural components of large aircraft are all made of aluminum lithium alloy with specific performance. The use of third-generation aluminum lithium alloys not only reduces material density, but also significantly enhances material strength, truly giving airplanes a lightweight and safe fuselage.

Not only have airplanes benefited from aluminum lithium alloy materials, but rockets are also gradually embracing aluminum lithium alloys. The liquid hydrogen and liquid oxygen storage tanks of the US space shuttle use aluminum lithium alloy materials, reducing material density by 5% and weight by about 3 tons. This weight reduction is very beneficial for the operation of the space shuttle. Afterwards, many rocket fuel tanks also began to use aluminum lithium alloy materials. For example, SpaceX, which has developed rapidly in recent years, uses aluminum lithium alloy as the fuel storage tank material for its Falcon 9 rocket. Relatively speaking, the application of aluminum lithium alloy materials in China's rocket field was relatively late. Currently, the mainstream rockets and fuel tanks do not use aluminum lithium alloy materials. But it is worth mentioning that China has already conquered the development of large aluminum lithium alloy storage tanks with a diameter of 3.5 meters. Perhaps soon we will see the launch of Chinese rockets using aluminum lithium alloy storage tanks.

The third-generation aluminum lithium alloy has to some extent overcome the defects of the previous two generations of aluminum lithium alloy materials, and the safety and reliability of use have been greatly improved. However, there is still some room for improvement, which has given rise to the fourth generation of aluminum lithium alloy materials. The fourth generation aluminum lithium alloy material requires better performance and lower cost. The third-generation aluminum lithium alloy material has improved the performance of the alloy material by adding a small amount of silver element. However, the cost of silver element is still relatively high. The fourth generation aluminum lithium alloy aims to further improve the material performance by using as little or even no silver element as possible. The fourth generation aluminum lithium alloy materials that have been prepared currently have high strength, good crack resistance and corrosion resistance, as well as better toughness and static strength, which can further meet the needs of the aerospace industry.

It should be added that aluminum lithium alloys not only have special compositions, but also have many things to pay attention to during production and processing. If various factors cannot be fully considered, it is impossible to produce qualified high-performance aluminum lithium alloys, which is also an important reason why this material cannot yet become a popular material.

Process characteristics and prospects

Lithium is a very active element with a low density, so there are many factors to consider in the smelting process of aluminum lithium alloys with added lithium. The most important point is to primarily isolate air during the smelting process, as air can easily oxidize lithium, which can seriously affect the performance of aluminum lithium alloys. So the smelting device of aluminum lithium alloy needs to extract air to form a vacuum or add inert gas again before the smelting can begin. In addition, the raw materials used for smelting must also be strictly controlled. If any impurities, such as some oxides, are accidentally mixed in, it is easy to oxidize the lithium element, which seriously affects the performance of the final product.

There are also many aspects to pay attention to in the processing and welding of aluminum lithium alloy materials, and it is not enough to simply copy the processing techniques of other aluminum alloy materials. It is necessary to fully consider the unique properties of aluminum lithium alloys and scientifically choose processing techniques.

The production and processing of aluminum lithium alloys in China started relatively late, and the degree of industrialization is not yet high. They are mainly used in the military industry, and the aerospace industry has not yet been officially launched. There are also not many civil aviation applications, and there is considerable room for improvement in the future. Although the proportion of aluminum lithium alloy used in China's C919 aircraft is relatively high, currently, the import proportion of various types of aluminum lithium alloy materials in China is relatively high, and the road to complete domestic substitution is still relatively long. This is also an important opportunity to promote the transformation and upgrading of China's aluminum lithium alloy industry.