To meet the demand for high-efficiency and high-reliability spot joining in lightweight structures, a rapid welding mode involving only plunging and retracting stages was implemented based on the previously developed synergistic double-sided probeless friction stir spot welding (SDP-FSSW) process. Using 6061-T6 aluminum alloy as the base material, the effects of rotation speed on joint microstructure, mechanical performance, and fracture behavior were systematically investigated. The results show that, even without a dwelling stage, the rapid SDP-FSSW process achieves stable high-strength joints, with tensile/shear strengths exceeding 5 kN and reaching a maximum of 10,064.3 N at 600 rpm. During welding, the cooperative rotation of the two shoulders induces pronounced wavy hook structures on both sides of the interface, which effectively suppress crack propagation along the interface through the combined effects of mechanical interlocking and metallurgical bonding, thereby enabling short-cycle, high-strength joining. Fracture analysis indicates that the joint failure mode transitions from plug-pull fracture to shear fracture with increasing rotation speed, and high-strength joints exhibit abundant tensile dimples and pronounced plastic deformation, characteristic of typical ductile fracture behavior.

This study used Linear Friction Welding (LFW) to join dissimilar titanium alloys joints of TC4/TC17, to investigate diffusion effects of interface elements and their relationship to phase transformations and nano-mechanical properties. Results showed that short-range diffusion of β-stabilizing elements like Mo and Cr occurred across the interface: with these elements migrating from the TC17 side toward the TC4 side. This movement induced the development of a narrow acicular band dominated by orthorhombic martensite (α′′). The transmission electron microscopy analyses confirmed that the α′′ phase exhibits a Burgers orientation relationship with both α and β phases on either side of the interface, demonstrating good crystallographic orientation inheritance following welding. Combined high-angle annular dark-field and energy dispersive spectroscopy analyses identified inside the recrystallized β grains on the TC17 side of depletion locally of β-stabilizing elements. In these grains nanoscale superlattice β precipitates were identified; which were consistent with β matrix and appeared as one-third order superlattice reflections in selected area electron diffraction patterns. However, in the TC4 side and further away from the interface, the lack of β stabilizers and cooling down below the β-transus temperature, produced solely equiaxed α instead of α′ grains. Nanoindentation measurements showed a graded range of mechanical properties across the interface, across α′′ > α > β, with α′′ showing the largest hardness value (4.96 GPa). These findings show the important role of β-stabilizer diffusion to interface phase transformations and its effects on mechanical properties locally, and provide essential insights into microstructural changes and allow to design for performance the TC4/TC17 linear friction welded joints.

The third-generation nickel-based powder metallurgy superalloy FGH99 was joined by inertia friction welding, and its microstructure evolution, tensile property and failure mechanism were investigated. Results show that gamma' dissolution occurs in thermo-mechanically affected zone (TMAZ) via diffusion of solute elements under high temperature during welding, resulting in large number of primary, secondary and tertiary gamma' phases dissolving into gamma grains. Complete gamma' dissolution takes place in weld zone (WZ) owing to its welding temperature exceeds gamma' solution temperature of FGH99, and few gamma' phases are re-precipitated after rapid cooling; sufficient dynamic recrystallization appears in WZ and leads to many fine recrystallized gamma grains with sizes smaller than 5 mu m. The tensile strength and elongation of joint are 1642.7 MPa and 17.6%, respectively, revealing a sound joint is obtained. The failure zone of joint after tensile test is located at TMAZ, where few gamma' phases are reserved after welding and without gamma dynamic recrystallization. During tensile loading, high density dislocations are generated inside gamma grains in TMAZ and formed pile-up at gamma/gamma' boundaries. Microvoids will form at gamma/gamma' boundaries and propagate to failure. Therefore, the failure mechanism of joint can be concluded as cracking of gamma/gamma' boundaries in TMAZ.

Interface residual stress concentration is one of the key factors limiting the mechanical reliability of ceramic/metal brazed joints. Etching subtractive micro-scale structures on ceramic surface can be used to modulate residual stress in brazed joints; however, micro-scale structures designed via the ”subtractive” approach share the same composition as the ceramic matrix, resulting in a significant thermal expansion mismatch between the joint components. This study investigates brazed joints between ZrO2 and GH536. By introducing additive micro-scale structures with designable material properties, multiple regulatory effects are achieved: these micro-scale structures not only interrupt continuous stress transmission on the ceramic side and promote the redistribution and transfer of residual stress but also alleviate thermal mismatch in the joint through a low coefficient of thermal expansion (CTE) phase, thereby significantly enhancing the mechanical performance of the joint. This work provides a new strategy for interface design in ceramic/metal brazed joints and offers a generalizable approach for residual stress management and performance optimization in dissimilar brazed components.

In this study, microstructure evolution and the interface bond of linear friction welded dissimilar Ti2AlNb/Ti60 joint were studied. The relationship of microstructure to tensile and impact strength of the joint were analyzed. The results show that continuous dynamic recrystallization occurred on two weld zones during welding, forming equiaxed fine grains which were smaller than those of the two base metals. On the Ti60 side thermo-mechanically affected zone, α grains were elongated under the high temperature and shear force effects; while on the Ti2AlNb side, the large B2 grains were elongated where a large number of O phases were dissolved. In the Ti2AlNb side heat-affected zone, the B2 grains were not deformed, and the O phases inside B2 grains were decomposed, as the welding temperature was lower than that in the TMAZ. The interface metallurgical bond was achieved through mutual diffusion of Ti, Al, and Nb, producing an element transition layer of 1.54 µm composed of TiAl3 and NbAl3 intermetallic compounds. The tensile strength of the joint was 920 MPa, which is 97.9

This study develops an integrated framework combining coupled Eulerian-Lagrangian (CEL) simulation with experimental validation to systematically investigate synergistic double-sided probeless friction stir spot welding (SDP-FSSW) of 2198 aluminum-lithium alloy sheets under counter-rotational conditions. The simulation results reveal that the pronounced thermo-mechanical coupling and localized strain concentration induced by counterrotation are the dominant driving factors governing the microstructural evolution. The tracer-particle tracking results reveal that the opposing shear actions from the upper and lower shoulders drive the material to flow toward the lower and upper sides of the interface, respectively, thereby forming a Hook morphology with opposite upward and downward deflections. This feature is fundamentally different from the single-side deflected Hook commonly observed in conventional single-sided spot welding. Mechanical testing shows that the optimal joint performance is achieved at a welding force of 5 kN and a dwell time of 3 s, yielding a peak tensile/shear strength of 14.62 kN, approximately 62 % higher than that of conventional single-sided spot welding. Fracture analysis further indicates that, under counter-rotational conditions, the deflected Hook morphology enhances the effective load-bearing area and joint strength, while an excessively deflected Hook can promote crack propagation and drive the transition from shear fracture to plug-type fracture. These findings not only elucidate the previously unclear mechanisms of material flow and fracture behavior in SDP-FSSW, but also provide theoretical insight and practical guidance for the development of high-reliability friction stir spot welded joints in Al-Li alloys.

The rapid development of modern railways imposes higher requirements on the welding quality of seamless rails, and linear friction welding (LFW) is expected to become a preferred welding method. In this study, LFW experiments and finite element simulations were conducted on scaled-down U75V rail specimens to systematically reveal the microstructural and mechanical evolution and the underlying thermo-mechanical coupling mechanisms of the rail-profile joint. Experimental results indicate that the joint exhibits significant heterogeneity: the welding process leads to complete austenitization of the interface, and rapid cooling post-weld causes the weld zone (WZ) to transform into a mixed microstructure of martensite and pearlite. The thermo-mechanically affected zone (TMAZ) undergoes partial dynamic recrystallization, predominantly consisting of fine pearlite. Furthermore, the hardness and tensile strength distributions across the rail head, web, and base differ significantly, and the intricate plastic flow induced by the complex cross-section makes the joint susceptible to local unbonded defects. Finite element simulations confirm that the differences in thermal cycles, caused by the severely uneven distribution of the interfacial temperature field, are the fundamental cause of the microstructural heterogeneity. Meanwhile, the significant stress concentration near the interface at the rail head and base explains the local deterioration of mechanical properties. Based on the aforementioned mechanisms, this study clarifies the regulation direction for process parameters, providing a theoretical and empirical basis for exploring the engineering application of LFW in seamless rails.

This study investigates linear friction welding of GH4169 alloy for aero-engine integrally blisks, with particular focus on elucidating the mechanisms by which combined pre- and post-weld heat treatments influence microstructural evolution and corrosion behavior of welded joints. Microstructural characterization reveals that composite heat treatment promotes the formation of large-scale spherical u03B3u2032 and disc-shaped u03B3u2032u2032 phases in the Base Material (BM), while in the Thermo-Mechanically Affected Zone (TMAZ), original precipitates coarsen and fine u03B3u2032 and u03B3u2032u2032 phases reprecipitate. Additionally, needle-like u03B4 phases precipitate along grain boundaries. The synergistic effect of grain refinement and precipitation strengthening results in superior joint mechanical properties, including a microhardness of 540 HV0.5, a tensile strength of 1400 MPa, and a fracture elongation of 18%, with the joint strength comparable to that of the BM. Electrochemical analysis shows that the joint exhibits significantly lower corrosion resistance than the BM, due to enhanced micro-galvanic coupling between the u03B3-matrix and precipitated u03B3u2032 or u03B3u2032u2032 phases. This is evidenced by an increase in corrosion current density from 1.62u00D710-6 A/cm2 of BM to 3u00D710-6 A/cm2 of joint. High-temperature molten salt corrosion tests indicate that corrosion mainly occurs through the combined action of oxides and soluble salts. The joint shows accelerated corrosion, reaching a peak value in the average corrosion rate of 269.9 g/m2/h, characterized by a fine-grained microstructure and loosely oxide films. The electrochemical impedance of these is measured at 1.83 ku03A9 u00B7cm2, attributed to thermo-mechanical effects. In contrast, the coarse-grained BM forms dense and protective oxide layers, with a higher impedance of 14.20 ku03A9 u00B7cm2 and a lower peak corrosion rate of 134.9 g/m2, reflecting more stable corrosion behavior. This work deciphers how heat treatment controls corrosion resistance through the regulation of precipitate distribution in welded joints, providing valuable guidelines for the optimization of integrated welding and heat treatment process in blisk production.

In this study, a new linear friction welding (LFW) process, embedded LFW process, was put forward, which was mainly applied to combination manufacturing of long or overlong load-carrying titanium alloy structural components in aircraft. The interfacial plastic flow behavior and bonding mechanism of this process were investigated by a developed coupling Eulerian-Lagrangian numerical model using software ABAQUS and a novel thermo-physical simulation method with designed embedded hot compression specimen. In addition, the formation mechanism and control method of welding defects caused by uneven plastic flow were discussed. The results reveal that the plastic flow along oscillating direction of this process is even and sufficient. In the direction perpendicular to oscillation, thermo-plastic metals mainly flow downward along welding interface under coupling of shear stress and interfacial pressure, resulting in the interfacial plastic zone shown as an inverted “V” shape. The upward plastic flow in this direction is relatively weak, and only a small amount of flash is extruded from top of joint. Moreover, the wedge block and welding components at top of joint are always in un-steady friction stage, leading to non-uniform temperature field distribution and un-welded defects. According to the results of numerical simulation, high oscillating frequency combined with low pressure and small amplitude is considered as appropriate parameter selection scheme to improve the upward interfacial plastic flow at top of joint and suppress the un-welded defects. The results of thermo-physical simulation illustrate that continuous dynamic recrystallization (CDRX) induces the bonding of interface, accompanying by intense dislocation movement and creation of many low-angle grain boundaries. In the interfacial bonding area, grain orientation is random with relatively low texture density (5.0 mud) owing to CDRX.

The phase transformation behaviors of the joint are studied using scanning electron microscope and electron backscattered diffraction， and the comprehensive mechanical properties of joint are enhanced by annealing heat treatment. The results show that complete dynamic recrystallization and β→α΄ phase transformation occur in the weld zone（WZ） of the as welded joint. Under the thermo-mechanical coupling effect， the rod-shaped α phase exhibits significant elongation deformation along the friction direction in the thermo-mechanically affected zone， accompanied by obvious decomposition and breaking spheroidization. In the heat-affected zone， the rod-shaped α phase within the grains is similar as the base metal， whereas the secondary α phase in the original β matrix is completely dissolved under high temperature during welding. After welding， fine secondary lamellar α phase precipitates at the boundary between α and β phase. The tensile property tests reveal that WZ is the weak area of the joint. The tensile strength， elongation and section shrinkage of the joints are 1048.6 MPa， 6.1% and 5.7%， respectively. After annealing heat treatment of 570 ℃， large number of fine acicular secondary α phases whose are interweaved are precipitated in WZ， resulting in the damage tolerance microstructural characteristics （interweaved secon-dary α） of TC21 titanium alloy are restored in WZ. Therefore， the tensile strength and elongation are obviously increased to 1131.4 MPa and 9.4%， respectively.

In this study, the synergistic double-sided probeless friction stir spot welding (SDP-FSSW) was innovatively employed to weld 6061-T6 aluminum alloy with a thickness of 2 mm. The correlation between the welding process and joint forming was clarified as well as mechanical properties. The joint formation mechanism and failure behavior were revealed. The results show that effective double-sided synchronous controllable forming is achieved during the SDP-FSSW process, resulting in sound joints. Four onion rings are present in the stirring zone (SZ) of the joint, and the hook exhibits a wavy-shaped morphology at the interface. Dynamic recrystallization, disruption of oxide film, and atomic diffusion play crucial roles in promoting grain boundary migration across interfaces, grain growth, and achieving high-strength metallurgical bonding. Two types of micron-sized second phases are observed in the joint, namely AlFeSi phase and beta-Mg2Si phase. The hardness distribution of the joint cross-section is symmetrical overall, initially increasing and then decreasing with the increase of rotation speed, reaching the highest hardness value of about 95.7 HV on the upper plate of the SZ. The joint strength is attributed to both interface metallurgical bonding and mechanical interlocking formed by hook defects. The maximum value of the joint tensile-shear strength can reach 11,493 N at a rotation speed of 600 rpm, dwell time of 2 s, and plunge depth of 0.3 mm. The joint fracture modes gradually transition from mixed fracture mode to plug fracture mode with the increase of rotation speed.

This study introduces Synergistic Double-Sided Probeless Friction Stir Spot Welding (SDP-FSSW) as a novel solidstate technique for joining dissimilar aluminum alloys, specifically AA7075-T8 and AA6061-T4, which differ markedly in their mechanical properties. The primary objective was to systematically investigate how varying the rotational speeds of the upper and lower tool shoulders affects weld formation, microstructural evolution, and joint mechanical performance. By independently controlling the rotation speeds of both tool shoulders, the process enabled precise regulation of interfacial morphology, heat input, and plastic flow, resulting in flat, defect-free welds with controlled hook formation and significant material intermixing. Tensile-shear tests showed that all joints exhibited strengths above 5 kN. Under optimal conditions-600/200 rpm with a dwelling time of just 3 s-the joint achieved a maximum tensile-shear strength of 9585.9 N. These results demonstrate that SDP-FSSW provides superior control over interface morphology and joint properties, offering significant potential for fabricating high-strength, lightweight, multi-material structures in advanced manufacturing.

The pre-weld heat treatment was carried out to obtain different initial microstructures of the GH4169 superalloy, and then Linear Friction Welding (LFW) was performed. The effect of the pre-weld heat treatment on the microstructure evolution and mechanical properties of the joint was analyzed, and the joint electrochemical corrosion behavior as well as the hot corrosion behavior was studied. The results show that the joint hardness of Base Metal (BM) increases after pre-weld heat treatment, and the strengthening phases gamma ' and gamma '' further precipitate. However, the precipitation phases dissolve significantly in the Weld Zone (WZ) due to the thermal process of LFW. The corrosion resistance in BM is reduced after the pre-weld heat treatment, while it is similar in WZ with a slight decrease. The surface morphology of the BM and WZ can be generally divided into a loose and porous matrix and a scattered oxide particle layer after hot corrosion. The joint cross section exhibits a Cr-depleted zone with the diffusion of Cr to form an oxide film. The corrosion product mainly consists of Fe2O3/Fe3O4 as the outer layer and Cr2O3 as the inner layer. (c) 2024 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics andAstronautics This is an open access article under the CC BY-NC-ND license

The probeless friction stir spot welding (P-FSSW) process of 2198-T8 aluminum-lithium alloy was investigated to elucidate material flow mechanisms and their influence on joint interface bonding. A coupled numerical simulation and texture analysis approach revealed the complex flow behavior. Tracer particle simulations demonstrated a laminar, radially inward spiral flow pattern in the stirring zone, with weaker flow intensity along the thickness direction due to thermomechanical gradients. Electron backscatter diffraction (EBSD) analysis indicated that dynamic recrystallization was the primary mechanism for grain refinement, creating fine equiaxed grains and forming shear-dominated textures ({111} (110)) that reflect localized material flow trajectories. The bonding mechanism at the joint interface was driven by the fracture of the surface oxide layers, atomic diffusion, and dynamic recrystallization, which facilitated void closure and grain coalescence. Grain boundary migration across the interface was observed, resulting in a metallurgical bond. These results provide an in-depth understanding of the coupling between material flow and the evolution of interface bonding. The research emphasizes the critical role of plastic deformation and recrystallization gradients, determined by the extent of downward material flow, in forming defect-free, metallurgically bonded interfaces.

In this research, linear friction welding of TC4-DT titanium alloy with Widmannstatten microstructure characteristic was conducted. The microstructure evolution, mechanical properties and fracture mechanism of this joint were carried out in detail. Results reveal that the joint could be divided into four zones according to their various microstructure: base metal (BM), heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and weld zone (WZ). The HAZ showed coarsening of the (3 phase, while the TMAZ showed alpha colony deformation and precipitation of the fine secondary alpha phase. In WZ, complete transformation to alpha' martensite structure occurred. An "M" shaped hardness profile developed across the weld, with maximum hardness in the TMAZ due to work hardening and precipitation strengthening effects. The WZ also showed higher hardness than the base metal because of the alpha' martensite strengthening. The average ultimate tensile strength, average yield strength and average elongation measured were 855.5 MPa, 745.1 MPa and 7.6 % as well, all satisfying the TC4-DT standard. Fracture testing showed that failure occurred in the base metal rather than the weld line, as the weld joint had higher strength. The crack propagation was not in a straight line, following the lamellar alpha/(3 boundaries, which is associated with the damage tolerant characteristics of TC4-DT with a Widmannstatten microstructure.

This study established a thermal-mechanical coupling numerical simulation model for linear friction welding of GH4169 superalloy blisks using ABAQUS/Explicit and FRANC3D. A systematic analysis was conducted of the evolution laws of the temperature and residual stress fields under different process parameters during the welding process. Additionally, the effect of crack parameters on crack propagation and fatigue life under multiaxial loading was simulated. The results show that the peak temperature at the center of the friction interface is approximately 1300 degrees C. The stress distribution within the joint demonstrates periodic fluctuations. Axially, the friction interface experiences the maximum compressive stress. The crack parameters had a substantial effect on crack propagation characteristics. As the initial crack size increases from 0.6 mm to 1.0 mm, there is a concomitant 40.4 % reduction in fatigue life, from 75,089 cycles to 44,767 cycles. Moreover, crack propagation is highly sensitive to the crack orientation. As the crack width increases from 0.375 mm to 1 mm, the fatigue life decreased by approximately 28.2 %.

In this study, the TA15 titanium alloy specimen was manufactured by selective laser melting (SLM) technology. The forming mechanism and microstructure of SLM-produced TA15 components were analyzed by numerical simulations and experimental methods. Abaqus software was employed to clarify the temperature field characteristics during SLM process, and a thermal conduction model was developed. The results showed that the peak temperature within melt pool exceeds the beta-transus during SLM, which promotes dynamic recrystallization and Widmanstatten alpha structures. The microstructure of the SLM-produced TA15 components was almost entirely composed of the alpha phase, with the beta phase present in only 0.6 % and 0.5 % in scanning and building directions, respectively. The microstructure exhibited a high degree of uniformity, with grains displaying various orientations, and grains had no obvious local plastic deformation with few substructures. In the SLM process of TA15 titanium alloy, rapid temperature fluctuations are observed, accompanied by uniform heat distribution and symmetrically arranged temperature field centered around the heat source. The temperature gradient ahead of the heat source is less steep than that behind it. Additionally, the tensile test of TA15 titanium alloy produced by SLM was carried out at 500 degrees C, microscopic holes are generated at grain boundaries. The specimen had obvious plastic deformation before tensile fracture, and fracture mode was mainly ductile fracture. It was worth noting that the tensile strength of the specimen exceeds 800 MPa, and hardness of the samples is similar in two directions, with an average hardness of 420.2 +/- 12.6 HV and 400.6 +/- 22.5 HV, showing that TA15 titanium alloy processed by SLM has excellent mechanical properties.

To clarify the creep properties of linear friction welding superalloy joint, providing reference for engineering application, reliable GH4169 superalloy joints were achieved by linear friction welding (LFW), the creep properties were tested, creep mechanism and microstructure evolution were analyzed. This study investigated the creep failure mode and microstructure evolution of LFW GH4169 joint at 650 degrees C/700 MPa and 700 degrees C/660 MPa. Optical microscopy, Electron backscatter diffraction, Vickers microhardness and transmission electron microscopy were used to characterize microstructure evolution of the as-weld joint and crept features. The experimental results show that LFW joint exhibits excellent creep resistance at 650 degrees C, and creep life rapidly decreases with increase of creep temperature. The fracture took place within base metal, which is different from conventional creep rule, and main failure mode is grain boundary debonding or sliding. The plastic deformation capacity of large-sized grains in the base metal is low, resulting in a high degree of stress concentration. The high stress local concentration at interface leads to initiation, coalescence and propagation of microcracks, resulting in failure of joint.

In order to promote linear friction welding (LFW) on combination manufacture of load-carrying structural components produced in aircraft, the microstructure evolution, failure behavior and strengthening mechanism of LFWed TC21 damage tolerance titanium alloy joint were investigated in detail. Results show that joint can be divided into four zones according to their various microstructure: base metal (BM), heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and weld zone (WZ). alpha -> metastable (3 transformation takes place in HAZ, leading to secondary alpha phase dissolution; in TMAZ, large degree of alpha -> metastable (3 transformation occurs due to its higher welding temperature, accompanyed by lamellar alpha and metastable (3 deformed severely along friction direction; alpha ->(3 ->alpha" transformation happens in WZ in conjunction with (3 dynamic recrystallization, leading to a large number of alpha" martensite precipitating in recrystallized (3 grains. The tensile strength, elongation, and reduction of area of as-welded joint are 1048.6 MPa, 6.1 % and 5.7 %, respectively. And the failure of the as-welded joint is located at TMAZ, which is caused by the microcracks formed due to high-density dislocation pile-up at deformed lamellar alpha/ metastable (3 boundary under tensile loading. After annealing at 570 degrees C for 4 h, dense and fine secondary alpha phases are re-precipitated in the whole weld. The tensile strength, elongation and reduction of area after heat treatment increased to 1131.4 MPa (with Orowan increment of 229 MPa), 9.4 % and 23.9 %, respectively. The weak TMAZ of the as-welded joint has been strengthened, which is because the dislocation pile-up at lamellar alpha/(3 boundary under tensile loading is relieved owing to the re-precipitated fine secondary alpha increasing dislocation movement resistance.

Ni-based superalloys are one of the most important materials employed in high-temperature applications within the aerospace and nuclear energy industries and in gas turbines due to their excellent corrosion, radiation, fatigue resistance, and high-temperature strength. Linear friction welding (LFW) is a new joining technology with near-net-forming characteristics that can be used for the manufacture and repair of a wide range of aerospace components. This paper reviews published works on LFW of Ni-based superalloys with the aim of understanding the characteristics of frictional heat generation and extrusion deformation, microstructures, mechanical properties, flash morphology, residual stresses, creep, and fatigue of Ni-based superalloy weldments produced with LFW to enable future optimum utilization of the LFW process.