Lithium is the third element in the periodic table of elements, with a body centered cubic structure and a specific gravity of 0.534g/cm3. The solid solubility in aluminum is about 4% at 601 ℃ and less than 1% at room temperature. Therefore, as a precipitation strengthening element, it has long been valued by people. In 1957, Alcoa company of the United States developed an Al Li alloy with the brand of 2020. Its composition is Al-4.5% Cu 1.0% Li 0.6% mn-0.15%. Its strength is equal to that of 7075 alloy, but its density decreases by 3% and its elastic modulus increases by 7%. As a substitute for 7075 alloy, it has been used in the main wing and vertical tail of the U.S. Navy aircraft ra-5c, reducing the weight by 6%.
After the oil crisis in the 1970s, the aviation industry put forward higher requirements for the lightweight of aircraft materials in order to reduce fuel costs. Therefore, while actively developing titanium alloys and fiber-reinforced composites, people naturally think of
aluminum lithium alloys. With the continuous improvement of modern manufacturing technology and equipment, it is possible to improve the toughness and plasticity of alloys and produce excellent light-weight structural materials that can meet the requirements of aerospace industry. Therefore, since the entry, countries around the world have carried out the research and development of Al Li alloys again, which has promoted the development of Al Li alloys into a new period.
Aluminum lithium alloys has the following advantages:
1. Low density Li is the lightest metal element next to h and he, with a specific gravity of one fifth of al.
Every 1% Li added to Al can reduce the density by 3%, with the greatest reduction. Obviously, Al Li alloy is very attractive as an aviation material
2. High elastic modulus
The addition of Cr, Si, Mn, Ti, Li, Cu, Ag and other alloy elements to Al alloy can increase the elastic modulus, and the role of Li is the most obvious. When 1% Li is added, the elastic modulus is increased by 6%, which is 9% higher than the elastic modulus. The increase of specific elastic modulus is due to the low density of Li and the large solid solution in al.
3. High strength
The strengthening factors of Al Li alloy include precipitation strengthening, solid solution strengthening and fine grain strengthening.
4. Abnormal intensity temperature dependence.
5. Good superplasticity. A small amount of Zr is usually added to Al Li alloy to prevent recrystallization and refine grains. The grains can be reduced to several um by appropriate thermomechanical treatment Therefore, Al Li alloy has good superplasticity.
6. Good fatigue resistance.
A large number of studies since the mid-1970s have proved that compared with 2024 and 7075 industrial Al alloys,
Al Li alloys not only have low density, high strength and high stiffness, but also have good fatigue properties, superplasticity, corrosion resistance and so on. Therefore, it is entirely possible for
aluminum lithium alloys to be actually used in the aerospace industry at the end of this century and the beginning of the next century to replace the existing Al alloy. At present, Boeing has used Al Li alloy produced by Alcoa company to carry out experiments on Boeing aeroplanes. If the structural materials of Boeing 747 are all replaced by Al Li alloy, the body weight can be reduced by 45 tons.
