As a light alloy that is crucial for aviation transportation equipment, the macroscopic mechanical properties of aluminum alloy are closely related to its microstructure. By changing the main alloying elements in the aluminum alloy structure, the microstructure of the aluminum alloy can be altered, and the macroscopic mechanical properties and other properties (such as corrosion resistance and welding performance) of the material can be significantly improved. Up to now, microalloying has become the most promising technological development strategy for optimizing the microstructure of aluminum alloys and improving the comprehensive properties of aluminum alloy materials. Scandium (Sc) is the most effective microalloying element enhancer known for aluminum alloys. The solubility of scandium in aluminum matrix is less than 0.35 wt.%, Adding trace amounts of scandium element to aluminum alloys can effectively improve their microstructure, comprehensively enhance their strength, hardness, plasticity, thermal stability, and corrosion resistance. Scandium has multiple physical effects in aluminum alloys, including solid solution strengthening, particle strengthening, and inhibition of recrystallization. This article will introduce the historical development, latest progress, and potential applications of scandium containing aluminum alloys in the field of aviation equipment manufacturing.
Research and Development of Aluminum Scandium Alloy
The addition of scandium as an alloying element to aluminum alloys can be traced back to the 1960s. At that time, most of the work was carried out in binary Al Sc and ternary AlMg Sc alloy systems. In the 1970s, the Baykov Institute of Metallurgy and Materials Science of the Soviet Academy of Sciences and the All Russian Institute of Light Alloy Research conducted a systematic study on the form and mechanism of scandium in aluminum alloys. After nearly forty years of effort, 14 grades of aluminum scandium alloys have been developed in three major series (Al Mg Sc, Al Li Sc, Al Zn Mg Sc). The solubility of scandium atoms in aluminum is low, and by using appropriate heat treatment processes, high-density Al3Sc nano precipitates can be precipitated. This precipitation phase is nearly spherical, with small particles and dispersed distribution, and has a good coherent relationship with the aluminum matrix, which can greatly improve the room temperature strength of aluminum alloys. In addition, Al3Sc nano precipitates have good thermal stability and coarsening resistance at high temperatures (within 400 ℃), which is extremely beneficial for the strong heat resistance of the alloy. In Russian made aluminum scandium alloys, 1570 alloy has attracted much attention due to its highest strength and widest application. This alloy exhibits excellent performance in the working temperature range of -196 ℃ to 70 ℃ and has natural superplasticity, which can replace the Russian made LF6 aluminum alloy (an aluminum magnesium alloy mainly composed of aluminum, magnesium, copper, manganese, and silicon) for load-bearing welding structures in liquid oxygen medium, with significantly improved performance. In addition, Russia has also developed aluminum zinc magnesium scandium alloys, represented by 1970, with a material strength of over 500MPa.
The Industrialization Status of Aluminum Scandium Alloy
In 2015, the European Union released the “European Metallurgical Roadmap: Prospects for Manufacturers and End Users”, proposing to study the weldability of aluminum magnesium scandium alloys. In September 2020, the European Union released a list of 29 key mineral resources, including scandium. The 5024H116 aluminum magnesium scandium alloy developed by Ale Aluminum in Germany has medium to high strength and high damage tolerance, making it a very promising material for fuselage skin. It can be used to replace traditional 2xxx series aluminum alloys and has been included in Airbus’ AIMS03-01-055 material procurement book. 5028 is an improved grade of 5024, suitable for laser welding and friction stir welding. It can achieve the creep forming process of hyperbolic integral wall panels, which is corrosion-resistant and does not require aluminum coating. Compared with 2524 alloy, the overall wall panel structure of the fuselage can achieve a 5% structural weight reduction. The AA5024-H116 aluminum scandium alloy sheet produced by Aili Aluminum Company has been used to manufacture aircraft fuselage and spacecraft structural components. The typical thickness of the AA5024-H116 alloy sheet is 1.6mm to 8.0mm, and due to its low density, moderate mechanical properties, high corrosion resistance, and strict dimensional deviation, it can replace 2524 alloy as the fuselage skin material. Currently, the AA5024-H116 alloy sheet has been certified by Airbus AIMS03-04-055. In December 2018, the Ministry of Industry and Information Technology of China released the “Guiding Catalogue for the First Batch of Secondary Application Demonstrations of Key New Materials (2018 Edition)”, which included “high-purity scandium oxide” in the development catalogue of the new materials industry. In 2019, the Ministry of Industry and Information Technology of China released the “Guiding Catalogue for the First Batch of Demonstration Applications of Key New Materials (2019 Edition)”, which included “Sc containing aluminum alloy processing materials and Al Si Sc welding wires” in the development catalogue of the new materials industry. China Aluminum Group Northeast Light Alloy has developed an Al Mg Sc Zr series 5B70 alloy containing scandium and zirconium. Compared with the traditional Al Mg series 5083 alloy without scandium and zirconium, its yield and tensile strength have increased by more than 30%. Moreover, the Al Mg Sc Zr alloy can maintain comparable corrosion resistance to the 5083 alloy. At present, the main domestic enterprises with industrial grade aluminum scandium alloy production capacity are Northeast Light Alloy Company and Southwest Aluminum Industry. The large-sized 5B70 aluminum scandium alloy sheet developed by Northeast Light Alloy Co., Ltd. can supply large aluminum alloy thick plates with a maximum thickness of 70mm and a maximum width of 3500mm; Thin sheet products and profile products can be customized for production, with a thickness range of 2mm to 6mm and a maximum width of 1500mm. Southwest Aluminum has independently developed 5K40 material and made significant progress in the development of thin plates. Al Zn Mg alloy is a time hardening alloy with high strength, good processing performance, and excellent welding performance. It is an indispensable and important structural material in current transportation vehicles such as airplanes. On the basis of medium strength weldable AlZn Mg, adding scandium and zirconium alloy elements can form small and dispersed Al3 (Sc, Zr) nanoparticles in the microstructure, significantly improving the mechanical properties and stress corrosion resistance of the alloy. The Langley Research Center of NASA has developed a ternary aluminum scandium alloy with the grade C557, which is ready to be applied in model missions. The static strength, crack propagation, and fracture toughness of this alloy at low temperature (-200 ℃), room temperature, and high temperature (107 ℃) are all equal to or better than those of 2524 alloy. Northwestern University in the United States has developed the AlZn Mg Sc alloy 7000 series ultra-high strength aluminum alloy, with a tensile strength of up to 680MPa. A pattern of joint development between medium high strength aluminum scandium alloy and ultra-high strength Al Zn Mg Sc has been formed. Al Zn Mg Cu Sc alloy is a high-strength aluminum alloy with a tensile strength exceeding 800 MPa. At present, the nominal composition and basic performance parameters of the main grades of aluminum scandium alloy are summarized as follows, as shown in Tables 1 and 2.
Table 1 | Nominal Composition of Aluminum Scandium Alloy
Table 2 | Microstructure and Tensile Properties of Aluminum Scandium Alloy
Application prospects of aluminum scandium alloy
High strength Al Zn Mg Cu Sc and Al CuLi Sc alloys have been applied to load-bearing structural components, including the Russian MiG-21 and MiG-29 fighter jets. The dashboard of the Russian spacecraft “Mars-1″ is made of 1570 aluminum scandium alloy, with a total weight reduction of 20%. The load-bearing components of the instrument module of the Mars-96 spacecraft are made of 1970 aluminum alloy containing scandium, reducing the weight of the instrument module by 10%. In the “Clean Sky” program and the EU’s “2050 Flight Route” project, Airbus conducted integrated cargo hold door design, research and development, manufacturing, and installation test flights for A321 aircraft based on the successor grade AA5028-H116 aluminum scandium alloy of 5024 aluminum scandium alloy. Aluminum scandium alloys represented by AA5028 demonstrated excellent processing and welding performance. Utilizing advanced welding techniques such as friction stir welding and laser welding to achieve reliable connection of scandium containing aluminum alloy materials. The gradual implementation of “welding instead of riveting” in aircraft reinforced thin plate structures not only maintains the consistency of aircraft materials and structural integrity, achieving efficient and low-cost manufacturing, but also has weight reduction and sealing effects. The application research of aluminum scandium 5B70 alloy by China Aerospace Special Materials Research Institute has broken through the technologies of strong spinning of variable wall thickness components, control of corrosion resistance and strength matching, and control of welding residual stress. It has prepared aluminum scandium alloy adaptive welding wire, and the joint strength coefficient of friction stir welding for thick plates in the alloy can reach 0.92. China Academy of Space Technology, Central South University, and others have conducted extensive mechanical performance testing and process experiments on the 5B70 material, upgraded and iterated the structural material selection scheme for 5A06, and have begun to apply the 5B70 aluminum alloy to the main structure of the overall reinforced wall panels of the space station’s sealed cabin and return cabin. The overall wall panel of the plate structure pressurized cabin is designed with a combination of skin and reinforcement ribs, achieving higher structural integration and weight optimization. While improving overall rigidity and strength, it reduces the number and complexity of connecting components, thereby further reducing weight while maintaining high performance. With the promotion of the application of 5B70 material engineering, the usage of 5B70 material will gradually increase and exceed the minimum supply threshold, which will help ensure the continuous production and stable quality of raw materials, and significantly reduce raw material prices. As mentioned earlier, although many properties of aluminum alloys have been improved through scandium microalloying, the high price and scarcity of scandium limit the application range of aluminum scandium alloys. Compared with aluminum alloy materials such as Al Cu, Al Zn, Al ZnMg, scandium containing aluminum alloy materials have good comprehensive mechanical properties, corrosion resistance, and excellent processing characteristics, which make them have broad application prospects in the manufacturing of main structural components in industrial fields such as aerospace. With the continuous deepening of research on scandium microalloying technology and the improvement of supply chain and industrial chain matching, the price and cost factors that restrict the large-scale industrial application of scandium aluminum alloys will gradually improve. The good comprehensive mechanical properties, corrosion resistance, and excellent processing characteristics of aluminum scandium alloys make them have clear structural weight reduction advantages and wide application potential in the field of aviation equipment manufacturing.
Post time: Oct-29-2024