Development Direction and Process Treatment of Aerospace Aluminum Alloy Materials

Published by Yinsheung CNC machining August 8, 2023

Ultra high strength aluminum alloys generally refer to aluminum alloys with a yield strength of over 500MPa, commonly known as the 7 series of superhard aluminum. This series of aluminum alloys was initially developed in the context of aerospace applications and has now developed into the main structural material for military and civil aircraft around the world, accounting for 70-80% of the proportion in aircraft structural components. They have replaced expensive titanium alloys in many fields and become an indispensable and important lightweight structural material. With the continuous development of modern aerospace, nuclear industry, and transportation industry, higher requirements have been put forward for the comprehensive performance of structural components. The new generation of ultra-high strength aluminum alloy, which combines lightweight, high-strength, high toughness, high fracture toughness, and stress corrosion resistance, is undoubtedly the preferred solution.

Development Background and Status of Aluminum Alloys for Aerospace Applications

Aluminum alloy, as a relatively mature lightweight and high-strength alloy material, is widely used in aerospace. Aluminum alloy materials are generally used as structural materials, with higher specific strength and superior processing performance than steel.

The aerospace industry mainly develops aluminum alloy materials with high strength, high toughness, and strong corrosion resistance to meet the harsh usage conditions of aerospace. The 2000 series and 7000 series aluminum alloys are widely used, and process improvements and material formulations are made on the basis of high-strength aluminum alloys. Innovative production processes such as powder metallurgy and spray forming are used to develop lightweight aluminum alloy materials with better performance, Conduct research on aluminum matrix composites and superplastic aluminum alloy materials.

In the development and application process of lightweight and high-strength aluminum alloys, stress corrosion is a major issue that accompanies the entire application history of aluminum alloys. How to weaken or delay the stress corrosion problem of high-strength aluminum alloys during use has become a major challenge in the application process of aluminum alloys.

The main components of 2000 series aluminum alloys that are widely used in the aerospace field are aluminum (Al), copper (Cu), and magnesium (Mg). The main components of 7000 series aluminum alloys are Al, zinc (Zn), Mg, and Cu. There are also high-performance (high-strength, high toughness, and corrosion resistance) aluminum alloy materials obtained by adding some special elements. At present, the main method to obtain high-performance aluminum alloy materials is to change the melting and casting conditions.

The 2000 series aluminum alloy mainly consists of Cu as the main alloying element. Adding an appropriate amount of Cu to the aluminum alloy material can improve the strength, heat resistance, and processing performance of the alloy, but the corrosion resistance will decrease. The introduction of Cu will make the aluminum alloy more prone to intergranular corrosion, and the material composition elements directly affect the performance of the aluminum alloy. Therefore, for 2000 series aluminum alloys, pure aluminum or 6000 series aluminum alloy coating treatment is generally applied on the surface as an electrochemical protective film for the bulk aluminum alloy to improve its corrosion resistance performance. Later scholars conducted many studies on the methods of improving stress corrosion performance of different grades of aluminum alloys, which to some extent delayed the degree of stress corrosion of aluminum alloys.

Aluminum alloy material is an important component material for ensuring the safe flight of aircraft. The selection and performance prediction of aluminum alloy materials in different parts are directly related to the safety and reliability of aircraft. Predicting the failure of aviation aluminum alloy materials is directly related to life safety and requires widespread attention.

High strength aluminum alloys with excellent performance are mainly used in the aerospace and military fields, as aerospace and military products have extremely high requirements for weight reduction. Materials with high specific strength are the preferred materials for aerospace. Among aircraft aluminum materials, the 7000 series high strength and high toughness aluminum alloy and the 2000 series medium strength and high toughness aluminum alloy play an important role.

Development of Aluminum Alloy Materials for Aerospace Applications Abroad

The 2000 series aluminum alloy has excellent temperature resistance, mainly used in high-temperature components of aerospace. The excellent temperature resistance is mainly due to the complex chemical and phase composition inside the 2000 series aluminum alloy, which can maintain good strength stability and process performance under high temperature conditions. It is mainly used for heat-resistant parts and heat-resistant weldable structural components and forgings working within the temperature range of 150~250 ℃.

There are impurities of iron (Fe) and silicon (Si) in the 2000 series alloys, which can produce coarse impurity phases, seriously affecting fracture toughness and short transverse mechanical properties. Therefore, researchers aim to improve the strength and toughness of 2000 series aluminum alloys by adjusting the content of alloy elements and reducing Fe and Si impurities. They also aim to improve the heat resistance of aluminum alloys by adding nickel (Ni) elements, and improve the welding performance of aluminum alloy materials by adjusting the Cu content. For thin-walled aluminum alloy materials, the main focus is on improving their damage tolerance performance, while for thick walled aluminum alloy materials, the main focus is on improving their stress corrosion resistance and toughness. By adjusting the element content and heat treatment methods in different applications of aluminum alloy, the most excellent matching material can be found.

The 7000 series aluminum alloy is mainly added with Zn as the main element, and the toughness of the alloy can be strengthened through heat treatment. Adding Mg element to the alloy can improve its thermal deformation performance and expand the quenching range. Changing the heat treatment conditions can improve strength, weldability, and corrosion resistance. However, the introduction of Mg element makes the stress corrosion tendency of the aluminum alloy severe. Therefore, the 7000 series aluminum alloy is a high-strength, weldable, and highly sensitive alloy to stress corrosion. The Al/Zn/Mg/Cu alloy with the addition of Cu element has higher strength and belongs to the category of ultra-high strength aluminum alloy. The yield strength and tensile strength are very close, with high yield and specific strength, but poor plasticity and low strength at high temperatures. It is often used for load-bearing structural components with temperatures below 120 ℃.

The excellent processing performance, corrosion resistance, and high toughness of the 7000 series aluminum alloy make it the main structural material for aerospace. As early as the 1820s, it was recognized internationally that the addition of Mg and Zn to aluminum alloys through joint heat treatment has a strengthening effect. However, there is a serious problem of stress corrosion cracking, which can be improved by adding trace amounts of chromium (Cr), manganese (Mn), and molybdenum (Mo), and has been widely used in shipborne fighter jets. Especially in 1943, the 7075 alloy developed by the United States was first applied to the B-29 bomber, bringing revolutionary changes to the aircraft structure and performance. Subsequently, the 7000 series aluminum alloy was imitated and developed by various countries like mushrooms after rain, and widely used in high-end manufacturing products.

In the 1960s, the United States improved on the 7075 aluminum alloy and developed a stronger, more ductile, and stress corrosion resistant 7050 alloy, which was mainly used in F-18 compression resistant structural components. Subsequently, 7150 alloy was developed for manufacturing the upper wing structures of civil aircraft such as Boeing 757/767 and Airbus A301. In the 1980s, the United States and others successfully developed 7055 alloy based on the 7150, which has a strength about 10% higher than the 7150 and has high comprehensive performance. It is used for the upper wing skin and wing stringers of Boeing 777 aircraft.

In order to improve safety and reliability, the selection of aircraft design materials has gradually shifted from pursuing high static strength methods to high damage tolerance methods, requiring aluminum alloy materials to have higher fracture toughness values and stress corrosion cracking resistance values. In the design and heat treatment process research of aluminum alloy materials, it is necessary to focus on improving fracture toughness values and stress corrosion cracking resistance values.

The main problem in the application process of 7-series aluminum alloys is the contradiction between high strength and stress corrosion sensitivity. Scholars have made a lot of research and development to solve the problem of high stress corrosion sensitivity of high-strength aluminum alloys. By changing the element composition, heat treatment process, and deformation heat treatment methods, the stress corrosion sensitivity of 7-series aluminum alloys is continuously improved. Countries have made a lot of research and accumulated a large amount of data, We have also achieved certain results.

Overall, the development direction of aluminum alloys is from high strength, low toughness to high strength, high toughness to high strength, high toughness, and corrosion resistance. The development of heat treatment states follows T6 to T73 to T76 to T736 to T77, with higher alloying degree, lower impurity content such as Fe and Si, and more reasonable addition of trace transition group elements, ultimately improving the overall performance of the alloy.

02 Current status of domestic aluminum alloy research and development

The research on high-strength and high toughness aluminum alloys in China started relatively late, starting in the 1960s. Initially, it mainly imitated the United States, mainly copying the 2000 and 7000 series alloys from the United States, such as 2014, 2024, 2324, 2525, 7075, 7001, 7475, 7055 alloys, etc. In recent years, China has continuously increased its research and development efforts in high-strength aluminum alloys, focusing on advantageous research institutes and enterprises to form a joint force. It has organized Guangxi South South Aluminum Processing Co., Ltd., Northeast Light Alloy Co., Ltd., Central South University, Northeast University, Shanghai Jiao Tong University, Beijing Institute of Aeronautical Materials, and Nonferrous Metals Institute to complete a series of national major projects, achieving good results, Basically possessing the mass production capacity of high-strength aluminum alloy materials, meeting the application needs of national major projects for high-strength aluminum alloy materials.

03 Gap and Development Direction between China and International Advanced Level

In China, although aluminum alloy materials have undergone years of development, there is still a significant gap compared to the international advanced level. Specifically, firstly, the products are mostly imitations, and aluminum alloy materials with comprehensive independent intellectual property rights have not yet been developed, and a standard system for producing aluminum alloy materials has not been established yet; Secondly, basic research is weak, development time is short, and data accumulation is insufficient; Thirdly, the production and processing equipment is outdated, and the product quality control system is not sound. In view of this, the future development direction of aluminum alloy materials in China should be:

1) Change the content and ratio of constituent elements;

2) Develop aluminum alloy material formulas corresponding to different performance requirements, improve the performance of aluminum alloys by changing the solidification external field conditions, increasing the solid solution of alloy elements, and adding elements such as zirconium (Zr), scandium (Sc), and erbium (Er) to improve the performance of aluminum alloys through microalloying methods;

3) Further purify the alloy, reduce impurities such as Fe and Si, control the impurity content, and produce high toughness and strength aluminum alloy materials;

4) Develop new heat treatment process technologies, establish different heat treatment process cards for aluminum alloys with different performance requirements, establish corresponding technical standard specification systems, and ensure the process stability of different grades of aluminum alloys.

Application of high-strength aluminum alloys in the aerospace field

Aluminum alloy has become the main structural material for aerospace equipment due to its excellent performance, and the development process of aluminum alloy has mainly gone through five generations.

The first generation of aluminum alloy, obtained through age hardening treatment to obtain high static strength aluminum alloy, mainly used in Y-5 and H-5; The second generation aluminum alloy is a high-strength and corrosion-resistant aluminum alloy obtained through aging treatment process, mainly used in military aircraft such as Y-6, H-6, J-11, and Y-8; The third generation aluminum alloy, based on high purity, produces high-purity aluminum alloys with excellent properties of high strength, toughness, and corrosion resistance, mainly used in J-10, Xiaolong, and ARJ21; The 4th generation aluminum alloy, obtained through precise control of the multi-scale second phase based process, is a super strong, high toughness, corrosion resistance, and fatigue resistance aluminum alloy, mainly used in ARJ21 and large transport machinery; The 5th generation aluminum alloy is under research and development, developing high comprehensive performance aluminum alloys with high hardenability, mainly used in large aerospace vehicles.

With the continuous improvement of material technology, high-performance materials are constantly iterating in the development of aerospace processes, and the emergence of new materials is constantly pushing traditional material technology forward. Aluminum materials face great challenges in terms of performance and specifications, requiring aluminum alloy materials to be stronger, lighter, more reliable, and longer lived, with service performance in extreme environments, high strength, toughness, corrosion resistance, and fatigue resistance, Large specification high-performance materials with low residual stress after processing. By controlling the heat treatment process and accurately adjusting the microstructure of materials, the comprehensive properties of aluminum alloys can be improved to meet the stringent requirements of the aerospace industry for materials. This requires us to apply the latest international and domestic scientific research achievements to process high-precision aluminum alloy materials (such as thin plate continuous casting and rolling, electromagnetic casting, rapid solidification, rheological casting, spray deposition, deep cooling processing, etc.) to meet the demand for high-strength aluminum alloys in high-end application scenarios.

The main applications of 01 aluminum alloy in the aviation industry

In aviation equipment, different parts have precise selection requirements for aluminum alloy materials. The aluminum alloys used in different parts of the aircraft mainly include 2X24, 7X75, 7X5X, etc. With the continuous improvement of aluminum alloy material technology, the application grades in different parts of the aircraft are also constantly adjusted (see Table 1).

The main application of high-strength aluminum alloy materials on Boeing 747 aircraft is the use of 7075T6 and 2024T3 materials. The main aluminum alloy grades used on the Boeing 777 aircraft are 7055T77, 7150T77, and 2X24T3; The advanced aluminum alloys used on the Airbus A380 are 7055/7449/7085/2024/6013/5076 and other alloys. The lower and upper fuselage panels are made of aluminum alloy materials, while most of the others are made of carbon fiber composite materials for weight reduction. The main aluminum alloy material grades used in domestic large aircraft ARJ21 are 2X24, 7050, 7175, 7075, etc. Whether in civil or military aircraft, the proportion of material structure is still mainly composed of aluminum alloy materials. Aluminum alloy still has irreplaceable advantages in material cost, process stability, comprehensive mechanical properties, and other aspects.

The application grades and heat treatment status of aluminum alloys for aircraft in the aviation industry are summarized in Table 2

Application of 02 aluminum alloy in the aerospace field

Aluminum alloy is widely used in the aerospace field of launch vehicles, with main grades being 7075, 20X4, 2219, etc. The engine equipment, main components, rotary table, remote control parts, etc. are mainly made of aluminum alloy 7075. For example, the liquid oxygen tank is mainly 2219, and 20X4 is mainly used for satellite streamlined covers and temperature control valve components. In addition, the skeleton of the manned aircraft is made of 2024 and 7075 aluminum alloys, as these two aluminum alloys have high strength, light weight, and thin thickness. Aluminum alloy plates are also frequently used on these objects, such as protective plates, safety devices, thrusters, etc.

Process Technology of Aviation Aluminum Alloy Materials

The final end use scenario of aluminum alloy is directly related to the entire production process, and different application scenarios depend on the process control of the production process, namely the processing process.

01 Heat treatment process

The comprehensive performance improvement of aluminum alloy materials depends largely on the control of process technical parameters during the production process, and the appropriate heat treatment method can greatly affect the comprehensive performance of aluminum alloy materials. Therefore, for aluminum alloys with different performance requirements, appropriate heat treatment technology should be developed to improve the comprehensive performance of aluminum alloy materials.

The use of high-temperature homogenization annealing process to treat aluminum alloy can maximize the solid solution of the aging strengthening phase and residual non-equilibrium phase into the matrix, and make them uniformly distributed, increase the concentration of the solid solution after solid solution, and achieve the effect of improving aging strengthening. At the same time, the parameter design of the entire heat treatment process is carried out for the combined heat treatment process of large aluminum alloy forgings, which includes hot deformation, intermediate high-temperature homogenization, and high-temperature solid solution treatment, It can improve stress corrosion performance while increasing strength.

There are two types of solid solution treatment processes for general aluminum alloys: conventional solid solution treatment and composite solid solution treatment. Among them, composite solid solution treatment refers to the treatment of strengthening solid solution and high-temperature pre precipitation. In the early stage of ingot casting, the homogenization annealing process of room temperature treatment and low temperature treatment can control the precipitation of transition group elements, which have a significant inhibitory effect on recrystallization and can improve the substructure strengthening effect of the alloy to a certain extent, thereby improving the fracture toughness and stress corrosion resistance of the alloy, and effectively weakening the anisotropy of the material.

The aging treatment in the heat treatment of high-strength aluminum alloys also plays a crucial role in the performance of aluminum alloys. There are three main forms of aging treatment: peak aging, bipolar aging, and regression reaging. The goal of developing aging treatment methods is also to enhance the comprehensive properties of aluminum alloys, such as higher strength, toughness, corrosion resistance, and fatigue resistance. The development of heat treatment states follows the direction of T6 to T73 to T76 to T736 to T77. The aging treatment methods develop sequentially from peak aging to outdated effect, and then to regression and aging treatment.

The aging temperature and time have an impact on the effect of aging strengthening, and different aging treatment processes can directly affect the tensile strength, yield strength, elongation, and intergranular corrosion level of aluminum alloys. As early as 1989, Alcoa Company in the United States registered and applied for the first RRA treatment process specification using the T77 heat treatment state. This is also the first heat treatment process specification that can be used for industrial applications. This process specification can be used as a guide for the heat treatment process of 7150 aluminum alloy. The 7150 aluminum alloy thick plates and extruded parts produced by this process are widely used in C-17 military transport aircraft in China, The key technology of high-performance aluminum alloy using T77 heat treatment technology is still in the development process and has not yet been industrialized.

The heat treatment process also includes thermomechanical treatment, which is a process that combines thermoplastic deformation and heat treatment. The use of thermomechanical treatment can be used to improve the distribution of transitional precipitates and the fine structure inside the alloy. Reasonable thermomechanical treatment can enable aluminum alloys to achieve high strength, toughness, and corrosion resistance. The deformation heat treatment process was proposed as early as 1981 and is mainly applied to aerospace structural alloys. The deformation heat treatment has a significant effect on improving the mechanical properties of 7050 and 7475 alloys.

In China, there are only over 100 types of heat treatment processes for aluminum alloys, which is still a long way from over 370 types abroad. Efforts should be made to develop heat treatment processes and shorten the distance between basic heat treatment technologies for aluminum alloys and developed countries.

02 Production process of high-strength aluminum alloy extruded profiles

There are various forms of high-strength aluminum alloys in the application process, mainly including aluminum profiles, aluminum plates, 3D printing powders, and other forms. Among them, aluminum alloy profiles have excellent characteristics such as light weight, high strength, and mature welding processes. Aluminum profiles can be widely used as large structural load-bearing components in the fields of aerospace and rail transit. The production process of aluminum profiles mainly adopts continuous extrusion molding to improve production efficiency and achieve a certain degree of prestressing orientation, thereby improving the mechanical properties of the profiles. In the extrusion process of aluminum profiles, in the continuous extrusion method with multiple extrusion cycles, an interface is formed between two adjacent extruded billets, which increases the extension length of the interface in the profile. This is because the transverse weld seam greatly affects the service life of the aluminum profile, leading to a sharp decrease in fatigue life.

03 High strength aluminum alloy 3D printing process

The development of low-cost, high-efficiency, and automated high-strength aluminum alloy process technology has received attention from aerospace, and large-scale aluminum or titanium alloy 3D printing technology is currently the focus of attention in aerospace. 3D printing technology, as a forward-looking strategic technology in China, plays a crucial role in the development of engineering applications.

In the field of aerospace, although aluminum alloys have been widely used, there are still certain drawbacks in practical applications compared to titanium alloys and composite materials. For example, when aluminum alloys are exposed to applications above 160 ℃, their mechanical properties, corrosion resistance, and fatigue properties will decrease, and they will soften and age with prolonged use. Therefore, there is still a lot of work to be done to improve the comprehensive performance of aluminum alloys in extreme working conditions.

With the continuous maturity of 3D printing technology, the development of high-strength aluminum alloy powders is also ongoing, and new aluminum alloy materials are constantly emerging, continuously breaking new performance records. Amaero HOT Al, a new type of aluminum alloy jointly developed by Amaero and Monash University in Australia, can achieve long-term stability at 260 ℃ after 3D printing, followed by heat treatment and age hardening. Developing commercial high-strength aluminum alloy new materials to adapt to 3D printing technology to achieve intelligent manufacturing performance and high complex shapes of aluminum alloys has become the main trend of future development. The development prospects of 3D printing of aluminum alloys are promising, mainly applied in the fields of aerospace and military industry.

epilogue

With the continuous development and progress of high-strength and lightweight aluminum alloy material forming technology and processing technology, the performance of the body material is continuously improved, and the processing and forming process is continuously improved. It has played a typical advantage of lightweight metal in high-end manufacturing industry, but there are still a series of problems that need to be further carried out on the existing basis, as follows:

Firstly, continuously increase the research and development of scientific and basic technologies for high-strength aluminum alloy materials, and improve the performance of high-strength aluminum alloy materials from various aspects such as material composition, heat treatment process, and processing technology;

The second is to strengthen the development and development of high-strength aluminum alloy production equipment, so as to quickly transform the traditional process of aluminum alloy into intelligent manufacturing process;

The third is to strengthen international technological cooperation and learn from advanced foreign technology development experience and research and development concepts

#aluminiumprofile #Aerospace #aircraft #cncmachining #aluminumalloy