Magnesium rare-earth (Mg-RE) alloy castings with a large size and complex structure exhibit versatile prospects in critical aircraft, aerospace, and defense fields owing to their ultralow density, excellent specific strength, and high-temperature resistance. The grain refinement of cast Mg-RE alloys can significantly improve their strength, plasticity, toughness, and casting performance, which are critical for expanding their applications. In this work, the grain refinement mechanism of Mg alloys by introducing RE elements and heterogeneous particles is first discussed based on the classical theory of constitutional supercooling and heterogeneous nucleation. Various grain refinement technologies for Mg-RE alloy casting using chemical and physical methods are comprehensively summarized. Further, the influence of grain refinement on the casting performance, mechanical properties, and corrosion properties of Mg-RE cast alloys is thoroughly discussed. Finally, the deficiencies and development trends of the current grain refinement of Mg-RE alloys are discussed from the point of actual application requirements.

This article reviews recent progress on the thermal conductivity of magnesium and its alloys. First, lattice distortion induced by solute atoms negatively impacts the thermal conductivity of Mg alloys, which is correlated to three properties of solute atoms: atomic radius, chemical valency, and extra-nuclear electrons. Second, the formation of intermetallic compounds, which is accompanied by the creation of new phase interfaces that act as barriers to the movement of electrons, negatively impacts the thermal conductivity of Mg alloys. This negative effect is correlated to the morphology, size, distribution, and content of secondary phases. Thermal conductivity along the transverse or normal direction is superior to that along the extrusion or rolling direction for wrought Mg alloys with basal texture. Further, temperature significantly influences the thermal conductivity of Mg alloys; however, the underlying mechanisms vary depending on the temperature range and should be discussed separately. These aspects of research on Mg alloys with high thermal conductivity are still necessary in the future: quantifying the relationship between microstructure and thermal conductivity; thermal behavior of multicomponent Mg alloys and the establishment of its thermal conductivity model; compositional design and microstructural control of high thermal conductivity Mg alloys; physical nature of thermal behavior in Mg alloys.

Superalloy casting is an important hot component in major aerospace equipment. It is being developed for larger and more complex structures and thinner wall thickness than traditional superalloy casting. The requirements of its internal metallurgical quality and external dimensional accuracy are becoming increasingly stringent, gradually exceeding the manufacturing limit of traditional investment casting technology. Shrinkage porosity defect control, thin-walled complete filling, dimensional accuracy, and surface quality control have become key challenges in the manufacturing of large and complex thin-walled superalloy castings. This paper systematically summarizes the research status of the superalloy casting process design, mold shell preparation, whole process dimensional accuracy, and adjusted pressure casting technique at home and abroad. This paper also analyzes and predicts the development trend of intelligent casting based on big data.

In situ TiB₂/Al-based composites are high-performance structural materials with excellent overall mechanical properties and machining properties. One of the critical factors in many practical situations is the composites' corrosive resistance. Microgalvanic corrosion occurs between TiB₂ particles and the Al matrix in TiB₂/Al composites as well as a negative effect of TiB₂ particles on the continuous passive layer is observed, resulting in lower corrosive-resistant performance. As a result, developing surface treatment and corrosion protection technology for TiB₂/Al composites is especially important. Regarding this problem, this paper mainly reviews the surface modification methods of in situ TiB₂/Al composites, including the anodic oxidation and rare earth conversion coating technology, low-temperature molten-salt deposition technology, and microarc oxidation technology. Furthermore, reasonable suggestions for future development on the surface protection technology of TiB₂/Al composites are made. The adoption of novel high-efficiency surface treatment and corrosion protection technology should provide effective technical support for the increasing large-scale application of in situ TiB₂/Al composites in aviation, aerospace, navigation, national defense, railway transportation, and automotive industrial fields.

Advanced high-strength steels have undergone rapid development from the first to third generation, which has considerably contributed to the continuous improvement of lightweight materials and safety in the automotive industry. The third generation representative steels, including quenching-partitioning (QP) and quenching-partitioning-tempering (QPT) steels have rapidly developed in the past 10 years. This article summarizes the preparation process as well as the strengthening and toughening mechanisms of QP and QPT steels from the following perspectives: (1) process design development and principles from QP to QPT, (2) carbon distribution and microstructure evolution during the partitioning process, (3) stability of metastable austenite and its influence on transformation-induced plasticity, (4) microstructure and heat-treatment process design of nanoprecipitation-strengthened QPT steel, (5) the integrated process of hot-forming QPT steel, and (6) the strengthening and toughening mechanisms and the service performance of QP and QPT steels. Finally, future prospects for manufacturing and using QP and QPT steels are discussed.

Although Al-based amorphous alloys have excellent mechanical properties and good corrosion resistance, their poor glass-forming ability makes large-scale samples difficult to obtain, limiting their engineering applications. Owing to the close relationship between the glass-forming ability of alloys and their structure, the development of Al-based amorphous alloys is first briefly reviewed to gain a thorough understanding of their component composition. On this basis, the microstructure, composition design theory, relationship between glass-forming ability and composition, and selection of primary phase during crystallization in Al-based amorphous alloys are discussed. Finally, the future study directions for Al-based amorphous alloys are prospected.

This paper concentrates on the research progress of titanium alloys and their diffusion bonding fatigue characteristics, and summarizes the laws of fatigue crack initiation and growth of titanium alloys with/without welding. The chemical composition, classification, and common welding method of titanium alloys are stated, with emphasis on the features and advantages of diffusion bonding. The phenomena of slip band formation and dislocation movement under cyclic loading are described, and the mechanism of fatigue crack initiation is clarified. The selection of microstructures is a common method to optimize mechanical properties of titanium alloys. Previous studies suggested that the laminated structure is an important mode to realize the low fatigue crack growth rate of titanium alloys. Improper parameters of the welding process can cause joint defects, and further heat treatment can reduce joint defects while improving the fatigue life and strength. Finally, the multilayer and heterogeneous laminates of titanium alloys produced by diffusion bonding are briefly described to realize the possibility of high damage tolerance.

Due to their unique structure and physicochemical properties, metal micro-nanostructured array materials are commonly used in optics, magnetism, electricity, catalysis, and other fields. The preparation of metal micro-nanostructured array materials by electrochemical technology has the advantages of high controllability, simple preparation without a template, and large-scale production, giving it broad application prospects. This review systematically summarizes the recent progress in the field of preparing metal micro-nanostructured array materials using electrochemical technology, combining recent work by the author's team. Furthermore, this review also introduces and comments on the feasibility of electrochemical methods without a template, the research status and formation mechanism of metal micro-nanostructured array materials, the application status of metal micro-nanostructured array in various fields, and future development challenges. This review is expected to serve as a useful source of reference and educational tool for future research in this field, thereby promoting the application and development of this template-free electrodeposition method.

Thermal barrier coatings (TBCs) are essential material and technology for modern high-performance aeroengines. They can promote the service temperature of hot components (such as turbine blades) while protecting them from oxidation and corrosion. A key component of TBCs is the metallic bond coat, which directly governs the service performance and lifetime of TBCs. However, due to its low oxidation resistance, severe bond coat-substrate interdiffusion, and low high-temperature strength, the MCrAlY bond coat cannot meet the temperature requirement of next-generation ultrahigh temperature TBCs because its service temperature is lower than 1100^oC. This study proposes a high-entropy alloy bond coat design strategy to address critical issues, aiming to break the temperature limitation of conventional bond coats. Herein, the oxidation and thermal corrosion resistance of Y/Hf-NiCoCrAlFe high-entropy alloy as well as the oxidation resistance of high-entropy alloy powder and bond coat are introduced and elaborated. Finally, the development trend of high-performance high-entropy alloy bond coat is summarized.

Laser cladding is a powerful surface strengthening technology that combines with high precision forming and low substrate damage to endow components with high surface performance. Fe-based coatings have been widely used in the surface engineering of many mechanical components. As the demand for higher performance and longer service life for components increases, the design and fabrication of new laser cladded coatings are expected to improve. This paper reviews recent research results of our team on laser cladding of novel Fe-based coatings, such as nano-bainite Fe-based coating, ultra-fine eutectic Fe-based coating, particle reinforced martensite coating, and high-hardness amorphous Fe-based coating. The study results are presented from the perspectives of the design, microstructure, and mechanical properties of these novel coating materials.

Nickel-based alloys and their welding materials have been the key materials in establishing key nuclear equipment due to their excellent corrosion-resistance and high-temperature mechanical properties. Therefore, the welding quality of nickel-based alloys is greatly responsible for the safe service of nuclear plants. However, ductility-dip crack (DDC) was commonly observed in the heat-affected zone during multipass welding. DDC is hard to detect by common nondestructive testing due to the micro‐size of the crack (approximately 100 μm in length). Hence, the high‐temperature DDC problem is a potential threat to the safety of nuclear plants. In this paper, the development of nuclear-level nickel-based alloys and their welding materials is reviewed. To solve the stress corrosion cracking occurring in the welding joint of Inconel 600, Inconel 690 was developed. However, a new reliability problem, DDC, was introduced. Researchers in the world developed Inconel 52, Inconel 52M, and Inconel 52MSS gradually from the design of the chemical components, and the DDC sensibility decreased. Presently, the DDC problem has not been solved completely. The evaluation methods of DDC were introduced, the cracking mechanisms were summarized, and the factors affecting the DDC were analyzed on the view of chemical components and microstructure. In conclusion, the research on DDC has been prospected briefly.

The robotic arc welding equipment and automated welding system can not fulfill the requirements in self-sensing, intelligent decision-making,and robust process-controlling in the complex welding environment. Artificial intelligence techniques were used to simulate observation actions, knowledge,and behavior of experienced welders, to realize functions,such as multi-mode information perception, knowledge judgment, and intelligent control during the welding process. Essential techniques in intelligent robotic welding as well as intelligent modeling and its coordinate control strategies of flexible welding manufacturing system were also proposed. These formed the technique,structure,and theoretical framework of intelligent welding manufacturing technique and system. They also provided a scientific method and technical realization to solve problems in intelligent manufacturing techniques and systems of other complex materials forming processes.

Steels have been critical in the rapid development of the global automobile industry. Among all automotive steels, dual phase (DP) steels have been extensively used as the mechanical components and outer plates in automobiles, owing to their excellent mechanical properties, desirable weldability and paintability, and low manufacturing cost. DP steels are beneficial in reducing the weight and increasing the safety of automobiles. The optimization of alloy elements and microstructure are essential for the engineering performance of DP steels. Understanding the relationship between their mechanical properties and microstructural features as well as the factors affecting the microstructure is of utmost importance. This study reviews the recent advances in the research on the microstructure evolution and mechanical properties of high strength cold-rolled DP steels for automobile applications. First, the alloy design principles and microstructure tailoring mechanism are summarized. Then, the microstructure evolution during thermal-mechanical processing, which includes rolling, intercritical annealing, subsequent cooling, and over-aging process is discussed. Thereafter, the mechanical properties and failure mechanism of DP steels as well as their relationship with the microstructural features are analyzed. Furthermore, the related challenges and future research directions are discussed and proposed, respectively.

Pore defects at the liquid/solid interface contribute to the interfacial bonding quality. Understanding the formation and growth mechanism of pores can help control their size distribution and eliminate their formation. However, the traditional static-research methods limit the in-depth study of the dynamic evolution behavior of bubbles. The growth, motion behavior, and morphological evolution of hydrogen bubbles at the liquid/solid bimetal interface during heating were characterized in situ, using synchrotron radiation X-ray imaging technology, and the bubbles' nucleation, growth mechanisms, and motion characteristics were investigated. The results show that the nucleation mechanism of hydrogen bubbles contains heterogeneous nucleation and bifilms. Bubble growth is divided into two stages: the inhomogeneous composition and bifilm transformation into hydrogen bubbles. The relationship between the bubbles' mean diameter and heating time conform to the stochastic model. The growth mechanism of bubbles exhibits jump merging and annexation behaviors, accompanied by the morphological transformation from spherical to elliptic and irregular shapes. The motion of the hydrogen bubbles includes upward migration and astatic jumping. The bubbles' growth, hindered by intermetallic compounds (IMCs), has experienced several jumps, attributed to the transformation of bifilms into bubbles, increasing bubble size and deformation, IMC dissolution, and IMC disturbance.