External energy field assisted friction stir welding (FSW) is an improved solid-phase welding method that is widely used in aerospace, automotive manufacturing, and energy industries. By introducing external energy, the microstructure and properties of the joint can be improved, thereby enhancing the welding quality and efficiency. This article classifies external energy field-assisted FSW techniques, summarizes the selection and control of energy fields, optimization of process parameters, and summarizes the effects of external energy fields on welding temperature, axial load, and joint microstructure and properties. Finally, this article summarizes the future development trends of external energy field-assisted FSW technology.

As the thickness of the friction stir welding plate increases, the bearing capacity of the equipment cannot meet the requirement of high forging force of unilateral friction stir welding, so it is difficult to form a good quality weld. To reduce the forging force on the equipment, a dual-robot mirror friction stir welding process is proposed in this paper. The variation and influencing factors of the forging force are studied, and the feasibility and superiority of the process are verified. Firstly, a thermodynamic coupling finite element simulation model is established to obtain the variation law of the forging force with plate thickness, and the conclusion that the dual-robot mirror friction stir welding process can reduce the forging force is obtained. After that, the forging force estimation model is established, and the experimental correction and verification of the model based on data-driven is carried out to realize sensorless forging force monitoring. Finally, the mirror welding experiment, three-group process parameter multi-factor experiment, and unilateral friction stir welding comparison experiment are carried out on the prototype platform. The variation of the forging force with time and process parameters is obtained, and it is proved that the dual-robot mirror friction stir welding process can reduce the forging force on the stirring head and the bearing capacity requirement of the equipment.

For the inertial friction welding (IFW) of GH4169 superalloy, a finite element (FE) simulation model was established to investigate the transition of interfacial friction regime and transient evolution law of thermal processes variables such as heat generation and temperature. The accuracy of the FE model is verified by the experimental data. The results show that the Coulomb friction zone occupies the whole interface at the beginning of the welding process, and gradually shrinks from 0.54 s and disappears completely at 9.60 s, while the shear friction zone builds up at 0.31R0 (R0 is the radius of the workpiece) and expands to the entire interface. Based on the simulation results, an analytical equation describing the evolution of the Coulomb friction zone is proposed. The coupling relationship and variations of heat flux, frictional stress and temperature at the interface caused by the transition of friction regime are analyzed. The results show that the transition of interfacial friction regime from the Coulomb friction regime to the shear friction regime is beneficial for temperature homogenization at the interface.

In order to improve the operation life of welding tool in friction stir welding (FSW) of titanium alloy, the coating was added to the surface of the welding tool, and the effects of surface coating on the operation life of welding tool was studied. FSW of TC4 titanium alloy was conducted by using uncoated and AlCrN coated welding tool. The wear of the welding tool was characterized by 3D imaging instruments, and the microstructure and composition of the joint are analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of AlCrN coating on the service life of the welding tool was studied. The results are as follow: Compared to the uncoated welding tool, the effective welding distance of AlCrN coated welding tool has increased from 600 mm to 800 mm, and after welding in the length of 800 mm, a well formed welding joint with a tensile strength of 95.32% of the base material can still be obtained. In addition, the wear rates of the pin and shaft shoulder of AlCrN coated welding tool are reduced by 50.41% and 22.41%, respectively. AlCrN coating can effectively improve the service life of the welding tool. Comparative analysis of the wear of the welding tool reveals that the wear of the uncoated welding tool includes abrasive wear, bonding wear, and oxidation wear. The wear process mainly includes three stages: initial wear stage, severe wear stage, and stable wear stage. Comparatively, the wear process of the coated welding tool consists of two stages:“ breaking” and“ tearing”.

The composite structure of aluminum alloy and steel has the characteristics of light weight, high strength and low cost. It plays an important role in aerospace, shipbuilding, automobile manufacturing and other fields, obtaining reliable and stable aluminum/steel composite structure is the requirement of the times. Friction stir welding (FSW), as a new type of solid phase bonding technology, has the advantages of high efficiency, energy saving and environmental protection. In the regular friction stir welding process of aluminum/steel dissimilar metals, it needs enough welding axial force and stirring head torque to generate heat for welding. In this process, it will lead to the wear of the stirring head and seriously limit the welding speed. Based on the optimization of aluminum/steel dissimilar metal friction stir welding, this paper summarizes the external auxiliary methods that have made progress in aluminum/steel friction stir welding at present, and divides them into mechanical structural external auxiliary FSW, intermediate transition layer auxiliary FSW, temperature controll external auxiliary FSW and ultrasonic vibration auxiliary FSW. On this basis, the development direction of external auxiliary aluminum/steel dissimilar metal friction stir welding is further prospected.

The heterogeneous structure between carbon fiber reinforced polymer and metal is one of effective ways to realize the structural light weighting of aerospace equipment. Friction stir welding is a low heat input solid-phase welding technology, can achieve high-quality joining between carbon fiber reinforced polymer and metal dissimilar materials. This paper summarizes the domestic and foreign research results in the direction of friction stir welding between carbon fiber reinforced polymer and metal in recent years. A systematic review was conducted from three aspects: process exploration, interface joining mechanism and joint performance control based on friction stir welding, and it also looks forward to the future main research directions in terms of the joint use of multiple joining techniques between carbon fiber reinforced polymer and metal heterostructures, improving the fatigue performance of the joint and exploring the corrosion mechanism of the joint.

Al alloy (Al)/steel dissimilar metals structures have a critical application in the aerospace industry, especially in the delivery pipeline of high thrust liquid launch vehicles. Compared with welding methods such as fusion welding and brazing, rotary friction welding can achieve a reliable joint between Al and steel with high quality, high strength and high corrosion resistance. The research progress of rotary friction welding of Al/steel dissimilar metals was reviewed from the optimization of welding process parameters, analyses of interface microstructures, control of interface inhomogeneity and metallurgical control by adding inter-layer metals. Finally, the development trends and scientific problems to be solved in rotary friction welding of Al/steel dissimilar metals were summarized.

In order to improve the service reliability of friction stir welded joints of high-strength aluminum alloy components for aviation and aerospace vehicles in corrosive environments, the stress corrosion behavior of 6 mm thick 6082–T6/7075–T6 dissimilar aluminum alloy friction stir welded joints treated with T6 was studied using metallographic analysis, electron microscopy analysis, and four-point bending stress corrosion method. The results showed that under the conditions of rotating speed 1200 r/min and weeding speed 80 mm/min, the 6082–T6/7075–T6 base material obtained a dense and well formed friction stir welded joint structure with an onion ring structure. The four-point bending stress corrosion of 6082–T6 and 7075–T6 base metal friction stir welded joints mainly occurs on the 7075–T6 aluminum alloy side on the retreating side, while the 6082–T6 aluminum alloy side on the advancing side shows good corrosion resistance. This is mainly due to the difference in electrode potential between dissimilar aluminum alloy base materials, which leads to galvanic corrosion of welded joints under stress. In addition, The formation of micro electric couples between the α–Al matrix and the second phase, as well as the presence of large angle grain boundaries in the TMAZ, HAZ, and BM regions of the welded joint, are also important reasons for stress corrosion.

The age-hardened AA7075 in the friction stir welding (FSW) joints exhibits lower plasticity, posing challenges for further deformation processing. In this study, this paper employed a 3 mm thick AA7075–O plate for stir friction butt welding, investigating the impact of welding speed on the joint microstructure and strength-ductility, aiming to provide solutions for achieving highly ductile FSWed joints. The results reveal that the weld nugget zone (NZ) in the AA7075–O joint consists of fine equiaxed grains. With an increase in welding speed, the average grain size (AGS) in the weld NZ initially decreases and then gradually increases. At a welding speed of 40 mm/min, the smallest AGS in the weld NZ is approximately 1.96 μm. The microhardness distribution across the transverse section of the joint shows a convex shape at different welding speeds, indicating significant hardening in the joint region. The weld NZ exhibits the highest hardness, reaching approximately 145HV. The process window for FSW of AA7075–O is broad, with joint strength exceeding the base material (BM) strength at welding speeds within the range of 20–80 mm/min. The elongation of the welded specimen reaches 82% of the BM. The use of AA7075–O not only overcomes the softening issue in the agehardened AA7075 FSWed joints but also yields a higher elongation. Bending test results demonstrate that the FSWed joints of AA7075–O can withstand larger bending angles, with a maximum bending angle reaching 105°, indicating superior plastic deformation capabilities.

The brazing of DD26 single crystal superalloy was investigated with NiNbCoWCrAlSiMo filler metal under the condition of 1225 ℃/30 min. Microstructure and mechanical properties of joint corresponded to brazing gaps of 0.05 mm and 0.20 mm were analyzed. It was indicated that the joint microstructure was composed of γ+γ′ particles and W-rich and Nb-rich phases distributed among the particles. When the brazing gap was 0.05 mm, the joint tensile strength at room temperature was 614 MPa, about 75.9% of DD26 alloy; the joint tensile strength at 1000 ℃ was 466 MPa, about 78.8% of DD26 alloy; and the creep rupture life under the condition of 1000 ℃/75 MPa was 44.6 h. When the brazing gap was 0.20 mm, the joint tensile strength at room temperature was 686 MPa, about 84.8% of DD26 alloy; the joint tensile strength at 1000 ℃ was 479 MPa, about 81.0% of DD26 alloy; the creep rupture life under the condition of 1000 ℃/75 MPa was 17.5 h. The influence of heat treatment on the creep rupture life of joint with brazing gap of 0.05 mm was insignificant, while that of joint with brazing gap of 0.20 mm increased. The fracture analysis showed that the joint fracture presented the characteristics of crack propagation along the edge of granular γ+γ′ dual phase structure.

Cold expansion technology with forcemate bushings is a coldworking method to install the forcemate bushing into the fastening hole by drawing the interference mandrel through the hole. A new cold expansion method with forcemate bushing is proposed to avoid the presence of non-uniform distribution of residual stress. The new method adds an auxiliary bushing with a slope on the inner side of the forcemamte bushing, and the slope of the bushing is the same as that of the mandrel, so that a uniform interference is applied along the axial direction of the hole. Through finite element simulations, the residual stress distribution obtained from the new method is compared with that of the traditional method on the expansion percentage of 1.5%, 1.8%, 2.0%, 2.3% and 2.5%. The results show that the new method avoids the generation of residual tensile stress around the hole, the residual stress is more uniform and the residual compressive stress area is larger than that of the traditional method. Compared with the traditional cold expansion technology, the fatigue life is increased by 33% when the expansion amount of cold expansion is 2.5%, and this advantage increases with the increase of the interference.

Introducing the second phase particles into metal materials is one of the important means to improve the mechanical properties of metal materials. In this paper, a new arc additive manufacturing method of TiC/Al6061 composite material coated with TiC reinforced particles in aluminum foil to form a special filler welding wire is presented. The effects of 1%, 2% and 3% TiC reinforced particles on the microstructure and properties of aluminum matrix composites were investigated. The results show that: Compared with the matrix material, the average grain size of the composite material with TiC mass fraction of 3% decreased from 45.5 μm to 25.3 μm, and the tensile strength and yield strength increased from 148.5 MPa and 118.0 MPa to 178.1 MPa and 157.3 MPa, which increased by 19.9% and 33.3%, respectively. The average microhardness increased from 50.5HV to 65.2HV, which increased by 29.1%. The results show that the load transfer strengthening and grain refinement of TiC and Orowan strengthening mechanism are the main reasons for the improvement of mechanical properties of the materials.

The damage evolution and final damage mode of glass fiber/epoxy resin reinforced magnesium alloy laminates with different pore sizes were studied by experiments and numerical simulation, and the microscopic damage mode of fracture was analyzed by scanning electron microscopy. At the same time, the fabrication process of fiberreinforced magnesium alloy laminates was improved. The results show that the surface treatment of AZ31 magnesium alloy can effectively reduce the delamination effect, and the specimen tensile strength is 347.62 MPa, compared with the untreated specimens which is improved by 11.33%, the interlayer microstructure morphology is more compact and uniform. With the increase of the pore size, the residual tensile strength decreases gradually. Fiber reinforced magnesium alloy laminates gradually expand from“ X shape” to“ funnel shape” in the progressive damage failure process. The failure modes of laminates are complicated mixed failure modes, which are mainly ductile brittle fracture of metal layer, tensile fracture of fiber layer and interlayer delamination. The experimental results are nearly consistent with the finite element numerical simulation results, which verifies the validity of the numerical simulation model and provides a reference for practical engineering applications.