C919 front fuselage skin, nose skin, middle fuselage skin, middle and rear fuselage skin are made of 2198-T8 aluminum-lithium alloy thick plate and Al-Li-S4-T8 aluminum-lithium alloy thick plate The thick plate referred to here refers to the plate with a thickness of >6mm; the front fuselage long raft, the long head of the nose, the middle fuselage long sill and the floor beam, the pillar and the seat rail are made of 2196-T8511/2099-T83 aluminum - Lithium alloy extruded material, etc.; the middle and rear fuselage is made of 2196-T8511/2099-T83 aluminum-lithium alloy extruded material, and its floor beam and pillar material is 2099-T83, 2196 -T8511 aluminum-lithium alloy extruded material. It can be seen that there are three kinds of common aluminum-lithium alloys used in the C919: 2196, 2198, and 2099. They are all registered with the American Aluminum Association (AA), and the registration year is 2000, 2005, and 2003. It is an American alloy and also uses an aluminum-lithium alloy of the grade Al-Li-S4-T8, but its composition has not been reported.
As can be seen from the data in Table 2, 2099 alloy is a very pure alloy. Its impurity iron and silicon content are very low. It must be formulated with high-purity original aluminum ingot, and its lithium content is also high, so its material value is better than the other two. All kinds of alloys are high. The 2196 and 2198 alloys also contain a small amount of silver, which significantly accelerates the ageing strengthening of Al-Li alloys. In general, the overall performance of Ag-containing aluminum-lithium alloys is superior to silver-free aluminum-lithium alloys, silver. Not only can the grain of the alloy be refined, but also the aging precipitation process of the alloy can be accelerated, so that the plasticity of the alloy after aging is greatly reduced.
The 2099 alloy also contains 0.40% to 1.0% Zn. Zinc can form n' (MgZn2) phase with Mg and promote the precipitation of δ' and S' phases, thereby improving its mechanical properties. Pre-strain before aging can promote Al-LI- The tensile strength of the Cu-Mg-Zn-Zr alloy after aging at 190 ° C is increased by about 20 N/mm 2 .
2. Where does the aluminum-lithium alloy for the C919 come from?
C919 fuselage skin panels, extruded materials for long rafts, floor beams, pillars, etc. All aluminum-lithium alloy materials used for straight sections such as aluminum-lithium alloy materials such as seat rails and fuselage are American aluminum. The company (Alcoa) supplies all other aluminum materials imported from Alcoa. The sheet is produced at Davenport Works, and the extrusions and forgings are processed by Lafayette Works. of. In addition to aluminum-lithium alloy extruded materials, some of the extruded aluminum materials are produced by an aluminum extrusion company based in South Korea.
The C919 is assembled by connecting more than one million parts. The connection of aluminum parts to other parts, such as the connection between the engine and the wing, plays a very important role in aircraft manufacturing. All the connecting fasteners. It is produced at two plants of Alcoa in Suzhou, Jiangsu Province, China, and some may be imported. The old riveting method is still the main joining process for aircraft parts. The fuselage of the C919 is assembled from 1,670 pieces of sheet metal. Only rivets are used for more than 60,000, some are 2xxx alloys, and some It is a lithium-containing 2xxx alloy.
As early as mid-2009, China Commercial Aircraft Co., Ltd. contacted Alcoa to negotiate matters related to the supply of aluminum used in large aircraft projects. The negotiations were very smooth, and Alcoa promised to provide all the high-end needed. Aluminum, and the price is cheaper than China's own production, delivery time is also guaranteed, Alcoa also sent material experts to go to the relevant departments of China Commercial Aircraft Corporation to help aluminum materials and introduce aluminum properties, for both sides The cooperation is even better. The company's Asia Pacific headquarters has deliberately moved from Beijing to Shanghai.
3. Why do aluminum-lithium alloys win the favor of C919?
As we all know, the aerospace industry is a must for the quality of parts. In the present words, it is necessary to accurately measure the quality of parts. The density of lithium is 540 kg/m3, which is the lightest metal element, which is 20% of the density of aluminum. For every 1% of Li added to aluminum, the density of the alloy decreases by about 3%, and the positive elastic modulus increases by about 5%. Therefore, lithium-containing aluminum-lithium alloys have excellent properties such as low density, high specific strength, specific stiffness, specific modulus of elasticity, etc., and have become excellent aluminum materials for aerospace vehicles. It is reported that if a lithium-lithium alloy is used to manufacture a Boeing aircraft. The structure, its quality can be reduced by 14.6%, fuel savings by 5.4%, aircraft costs will be reduced by 2.1%, and the annual flight cost per aircraft will be reduced by 2.2%. This is why aircraft manufacturers have a soft spot for aluminum-lithium alloys.
On September 11, 2011, the manned aircraft developed by China, Tiangong No. 1, was successfully launched and successfully entered the scheduled orbit. Its resource compartment is made of aluminum-lithium alloy, which is about 10% lighter than the 2xxx series and 7xxxx conventional alloy. The cabin is equipped with solar cells, power system, propulsion system, and control torque gyro. The quality is about 1.4t.
4. Development history of aluminum-lithium alloy
The world's first lithium-containing aluminum-lithium alloy was born in Germany in the early 1920s under the trade name Scleron, but it has not been used in industrial applications. In 1942, the X2020 alloy (Al-4.5Cu-1.0Li-0.8Mn-0.6Cd) invented by Lebaron of American Aluminum Corporation was patented. In 1957, its plate was applied on the US Navy's A3J reconnaissance aircraft; 1960 At the beginning of the year, a small amount of application was obtained on the supersonic bomber B-58 and the fighter, but the fracture toughness of this alloy was low, especially the notch sensitivity was larger than that of the 7075-T6 alloy.
In 1955, British scientists Hardy and Silcock studied the aging process of AL-Li and AL-Cu-Li alloys and found an ordered δ'Al3Li phase. In 1960, the Soviet Union developed D1420 alloy, its composition (W%): 4 ~ 7Mg, 1.5 ~ 2.6Li, 0.2 ~ 1.0Mn or 0.05 ~ 0.3Zr, 0.015 ~ 0.15Ti, 0.05 ~ 0.3Cu, the rest is AL. In 1971, the Fylmer Institute in the UK developed a new AL-Mg-Li-Zr alloy, especially after the oil crisis of 1973, once again set off a wave of research on aluminum-lithium alloys, in order to solve its low toughness and A breakthrough in the brittleness of the grain boundary, the birth of the third generation of aluminum-lithium alloys, such as the AirlinkTM alloy of Kenli Aluminium and its production process, has been widely used on the A380 wide body large aircraft, now on the aircraft The applied aluminum alloys are all third generation (Table 3).
Now, no matter what deformed aluminum alloy, any aluminum alloy containing an alloying element is called an aluminum-lithium alloy. As of January 2015, there were 25 aluminum-lithium alloys registered by Alcoa, including 22 of the 2XXX series, and the maximum content of Li was 2.6% (2090 alloy); 3 of 8XXX series, including Li Up to 8091 alloy, containing 2.4% to 2.8%, the lithium content in the currently used (active) aluminum-lithium alloy does not exceed 2.8%.
In the global development of aluminum-lithium alloys, it can be clearly seen that the Soviet-Russian focus on the development of lithium-containing alloys based on Al-Mg alloys, while the United States and France have developed lithium-based alloys based on 2XXX alloys. Due to the ideology, the deformed aluminum alloy developed by the Soviet Union was not registered by the American Aluminum Association. Until the Soviet Union was dissolved in 1991, Russia registered a non-prototype alloy 2224A in 1997. The prototype alloy I refer to is the second digit of the “0” alloy.
The only countries with independent aluminum-lithium alloy industries are the United States, Russia, France and China, which have established a complete system from smelting-casting-processing, which is a key process for the production of aluminum-lithium alloy materials. Companies that produce aluminum-lithium alloys include Alcoa's Davenport Rolling Mill, Lafayette Extrusion-Forging, UK's Kitts Green Works, and Russia's Samar. Samara, Fusina rolling mill in Italy; Issoire rolling mill in Kental Aluminium, France; Kamensk Metallurgical Company of Russia United Aluminum Corporation; China Southwest Aluminum (Group) Co., Ltd. The global production capacity of aluminum-lithium alloy materials in 2015 is about 200 kt/a, of which Alcoa accounts for more than 50%.