Aluminum Alloys and Aluminum-Based Matrix Composites

1. Introduction and Scope

Due to air pollution and energy shortages in the contemporary world, weight lighting for transportation vehicles and energy conservation, as well as emission reductions, are necessary to achieve carbon neutrality and fuel conservation. As a structural material with high specific strength, good process performance, and abundant reserves, aluminum alloy is undoubtedly becoming a substitute for steel materials.

Since the invention of electrolytic aluminum technology, aluminum alloys have been widely used in the fields of aviation and automobiles. The aerospace industry mainly develops aluminum alloys with high strength, high toughness, and excellent stress corrosion resistance to meet the strict usage conditions. 2xxx series and 7xxx series aluminum alloys are typical structural materials for aviation [1]. The current research hotspot lies in the optimization of processing technology and improving the material composition. Powder metallurgy and spray deposition are typical innovative technologies that can avoid compositional segregation and obtain higher element contents. Research on aluminum matrix composites and superplastic aluminum alloy materials is also ongoing. In the industries of new energy vehicles and intelligent connected vehicles, 4xxx and 6xxx series aluminum alloys are widely used. The application of aluminum alloys in a vehicle body and chassis can reduce the weight of the entire vehicle by 20–40%, which effectively extends its range. For car wheels with a complex shape, high-pressure die-casting technology is now commonly used. Compared to steel wheels, the weight of Al-Si series wheels is greatly reduced, effectively reducing the vehicle’s fuel consumption and carbon dioxide emissions. Automotive power battery shells made of aluminum alloy can reduce the weight by nearly 20%. The front and rear anti-collision beams made of optimized 7xxx aluminum alloy profiles have energy absorption values of no less than those of steel, achieving weight reduction and improving safety. In addition, aluminum alloys can also be used to manufacture components such as cylinder blocks, cylinder heads, crankshafts, connecting rods, and pistons for automotive engines. Plate and profile goods are commonly used products of aluminum alloys. Different deformation technologies are required to obtain the various shapes. The hot deformation behavior or the thermal deformation constitutive equation is a useful tool to obtain the optimized process parameters. Based on these mathematical models, the deformation defects, and even the service performances, of certain aluminum alloys can be predicted. Computational materials science can greatly shorten the cycle of material preparation processes. This Special Issue’s scope embraces several types of aluminum alloys and the interdisciplinary work aimed at introducing the emerging area of technologies and theories.

2. Contributions

Ten articles have been published in the current Special Issue of Metals, encompassing the fields of hot deformation behavior, constitutive modeling, performance prediction modeling, structure designation, composition designation and quenching sensitivity. Current Special Issue papers can also be classified based on material composition, including AlZnMgCu, AlMg, AlSi, and AlLi alloys.

2.1. Hot Deformation Behavior

The stress–strain curve helps scholars understand the dynamic hardening and dynamic softening behaviors during hot deformation, such as dynamic recovery and dynamic recrystallization [2], which are the basis for optimizing hot-working process parameters. In addition, the constitutive models can also be established based on stress–strain curves under different temperatures and strain rates. The constitutive equation is a necessary model for finite element simulation of plastic deformation to obtain the deformation state, temperature distribution, and the stress concentration during processing. This Special Issue presents two typical constitutive models, which are often used to describe warm forging and hot compression, respectively.

2.2. Performance Prediction Modeling

The prediction of processing defects and performance is an important step during component production. An accurate damage model or state equation under a complex loading environment can precisely calculate the service life of components, laying a theoretical foundation for material selection and structural designation [3], effectively avoiding material failure and shortening the product development cycle. The Special Issue also presents two prediction models, which are applicated to the real-time monitor of surface defects and the performance life under a cyclic tension–compression condition, respectively. The first-principle investigation for the mechanical properties of an Al-Mg-Zr alloy under uniaxial tension is also presented.

2.3. Composition Designation

The Zn/Mg ratio and Mg/Si ratio determine the strength of 7xxx and 6xxx aluminum alloys [4], respectively. The increase in Cu content is conducive to improvements in the aging hardening rate and the corrosion resistance. The element Cu is also one of the constituent elements of the nano strengthening phase in AlCu and AlCuLi alloys. For high-strength aluminum alloys, Fe and Si are impurities that can cause a sharp decrease in plasticity. However, Si is the main alloying element in a 4xxx alloy. The increase in Si content improves the fluidity of a AlSi alloy. Adding Cu and Mg to AlSi can also form the θ, β or Q phases, similar to those in AlCu and AlMgSi alloys [5]. Of course, the plasticity will be deteriorated when Si exceeds the eutectic composition. At this point, it is necessary to combine this with rapid solidification technology to improve the morphology of primary Si. It should be pointed out that there is a suitable content for any alloying element. Too little addition does not have a strengthening effect, while too much addition may lead to precipitation of the coarse second phase at the interface, reducing the strength, plasticity, and even corrosion resistance.

2.4. Quenching Sensitivity

Quenching is one heat treatment technology that obtains excellent strength, toughness and corrosion resistance, and so on. For high-performance aluminum alloys with a high alloying element content, a lower quenching cooling rate will cause a large number of alloying elements to precipitate along grain boundaries during the cooling, forming coarse and incoherent compounds [6]. Quenching precipitation greatly reduces the mechanical properties of the alloy while also deteriorating the corrosion resistance. Where are these coarse compounds formed? What are the ingredients? How does it affect the material properties? At what quenching rate level can cooling precipitation be suppressed? The answers to the above questions form the foundation for obtaining a high-quality supersaturated solid solution, which results a high-aging strengthening effect. Quenching sensitivity is particularly prominent in AlZnMgCu alloys. The 7085 aluminum alloy is currently known as the most excellent hardenability aluminum alloy.

2.5. Structure Designation

Even with the same material, different structural designations can be used to achieve different performances, such as increasing stiffness and improving a material’s resistance to external loads [7]. In this research neighborhood, computational materials science is also an important application technology that can help researchers quickly judge the rationality of structures and reduce the number of physical experiments.

2.6. External Field-Assisted Manufacturing

External field-assisted manufacturing is a highly innovative technology that can be used to compensate for the shortcomings of traditional techniques, thereby obtaining a better processing experience or a superior performance. Lasers, magnetic fields, and ultrasound are commonly used external media [8]. The input of these external energy fields changes the processing rate and may also alter the law of the microstructure evolution, resulting in unexpected performances.

3. Conclusions and Outlook

With the vigorous development of the manned aerospace and intelligent driving vehicle industries, aluminum alloys have shown increasingly vigorous vitality. Whether it is the breakthrough in composition design concepts or the endless emergence of new technologies and processes, something has greatly expanded the application breadth and depth of aluminum alloys. This Special Issue gathers multiple related topics and provides an overview of the latest developments in aluminum alloys, with different compositions and their related technologies.

As Guest Editors of this Special Issue, we hope that these published papers can be helpful to scientists and engineers engaged in the research and development of high-performance aluminum alloys. We also hope that these contributors can establish connections, accomplish interdisciplinary and professional complementarity work, and then achieve greater successes. At the same time, we would like to warmly thank all the authors for their contributions, and all of the reviewers for their efforts in ensuring a high-quality publication. We offer our sincere thanks to the Editors of Metals for their continuous help and support during the preparation of this issue. In particular, my sincere thanks goes to Toliver Guo for his help and support.