Abstract：The heat transfer and material flow behavior during friction stir lap welding (FSLW) of Al alloy 5052 and high-strength steel DP590 were numerically simulated based on the coupled Euler-Lagrangian finite element method (CEL-FEM). The predicted results of the temperature histories and the deformation profile of the weld matched well with the experimental measurements. The simulation results show that when the welding speed is kept constant at 300 mm/min, as the rotation speed increases from 500 r/min to 1 000 r/min, the peak temperature position of the stirring zone is transferred from the Al alloy surface behind the shoulder on the advancing side (AS) to the bottom of the stirring pin, that is, the interface of the steel stirring zone. Meanwhile, the peak temperature increases from 545 °C to 635 °C during the welding process. Regardless of rotation speed, the temperature on the AS is always higher than that of the retreating side (RS). The material migration was studied by the tracer particle method. The Al alloy materials on the AS are eventually transferred to the rear region of the AS, bypassing the RS via the inner shear region, which is close to the stirring pin and shoulder root. The Al alloy materials on the RS were mainly migrated to the ipsilateral rear, and the migration trajectory is more divergent. The stirring pin acts on the Al/steel overlapping surface, driving the steel materials on the AS to move to the RS rear of the stirring pin and simultaneously extruding the steel materials into the Al weld area in the vertical direction. The steel material that migrates from the AS to the RS as the pin rotates eventually causes the RS to form a larger-sized hook-like structure. Compared with the Al alloy side, the increase in rotating speed more significantly enhances the material flow on the steel surface.
Keywords：friction stir welding;dissimilar joining;finite element simulation;temperature;material flow
Abstract：In this paper, the interface friction and material flow behavior of 6061 aluminum alloy during friction stir welding are studied by computational fluid dynamics. By adopting the shear stress boundary condition at the interface between the tool and the workpiece, the analysis of interface friction and material flow is realized in a fully coupled manner. The results show that the interfacial friction behavior between the tool and the workpiece presents significant non-uniform characteristics: approximate sticking friction ispresent atthe annulargroove of the shoulder, and slidingfriction ispresent at the periphery of the shoulder. The interfacial sliding in different degrees is present at the pin side, and the sliding index of the interface increases with the distance from the shoulder. There is a low viscosity zone with a thickness of 0.66~4 mm near the friction interface. In the low viscosity region, the material flow velocity on the upper portion of the workpiece reaches 0.17 m/s, and the strain rate reaches 85.7 s-1. On the lower portion of the workpiece, the material flow velocity reaches 0.017 m/s and the strain rate reaches11.7 s-1. Regarding the material flow path through the low viscosity zone, the material flow presents two modes: multi-circle rotation and straight-through flow. The simulated temperature, geometry of low viscosity zone and deposition position of marker materials are validated by the experimental results.
Keywords：Computational Fluid Dynamics;interfacial friction behavior;material flow;low viscosity region
Abstract：Thermoplastic composites is a new type of lightweight material, which have been widely used in the aerospace field. The dissimilar material connection between metals and thermoplastic composites has received much attention due to its ability to fully exploit the characteristics of both materials. Friction stir welding, as a low heat input welding technology, has broad application prospects in the field of welding between metals and thermoplastic composites. A systematic review of domestic and foreign research results on the friction stir welding of thermoplastic composites and metals is conducted, including welding methods, welding tool design, welding process parameters, and connection mechanisms. In addition, the future development trend of friction stir welding to fabricate thermoplastic composites/metal hybrid structures is prospected.
Abstract：As an important component of the launch vehicle, the fuel tank is subjected to random loads such as vibrations, sudden winds and impacts while in service. Therefore, it is especially important to ensure a reliable connection of the tank structure. In production, defects at the welds not only destroy the continuity of the joint, reduce its effective bearing area, but also cause local stress concentration, thus directly reducing the reliability of the rocket. On this basis, repair for welding defects is necessary. This paper focuses on several commonly used joint defect repair techniques at home and abroad, starting from the characteristics of different repair techniques, reviewing their development and application process, and analyzing their application objects and performance enhancement effects after repair. The existing problems of different repair methods are systematically summarized, and an outlook on the future development direction of structural defect repair technology for rocket fuel tanks is presented.
Abstract：With high specific strength and stiffness, high damage tolerance, good corrosion resistance and excellent low-temperature fatigue performance, Al-Li alloy is one of the ideal structural materials for realizing lightweight components in the fields of aerospace and rail trains. Friction stir welding is one of the preferred methods for welding high strength Al-Li alloys. This paper firstly, introduces the characteristics and difficulties in fusion welding of Al-Li alloy and analyzes the research status and challenges of friction stir welding of Al-Li alloy. Additionally, the modification process of friction stir welding of Al-Li alloy at home and abroad are summarized. Finally, the future development direction of friction stir welding of Al-Li alloy is prospected.
Abstract：A water cooled friction stir welding process was performed to solve the hole defects caused by overheating in the affected area of the shaft shoulder for 108 mm thick 5A06 (H112) aluminum alloy. The microstructure characteristics of the joint throughout its thickness were tested and analyzed. The results show that the microstructure of the double-side welding was symmetrical from top to bottom, and was still heterogeneous. For single pass welding, the grain and metal compounds size were different in the weld nugget zone. The grain size of the near surface nugget zone (NZ-upper) and the nugget overlap zone (OZ) were the smallest, and the particle size of the metal compound was the smallest with a dispersion distribution. At a depth of 7 to 22 mm from the surface, there was a coarse grain region, in which Al3Mg2 particles had grown up, and linear segregation of β Phase Al3Mg2, oxide, and Mg2Si was found at the transition position between it and TMAZ of the RS. A horizontal plastic material stacking model was used to explain the formation mechanism of of metal compound linear segregation.
Abstract：To avoid tool wear and break in conventional friction stir welding of aluminum/steel, 5083 aluminum alloy and 304 stainless steel were lap-welded by vortex- friction stir welding (VFSW). The effect of tool-tilted angle, rotating speed, and welding speed on the VFSW lap joint quality was investigated through the analysis of the weld formation, joint mechanical properties, and cross-sectional interface microstructure. The results show that good weld formation can be obtained at lower rotating speeds and lower welding speeds. Metallurgical bonding occurred between 5083 aluminum alloy and 304 stainless steel. At the rotating speed of 200 r/min, with the increase in welding speed from 15 mm/min to 30 mm/min, the bearable tensile-shear load of the bonding interface decreases from 9.14 kN to 4.59 kN. At the welding speed of 30 mm/min, with the increase in rotation speed from 200 r/min to 400 r/min, the bearable tensile-shear load of the bonding interface increases slightly, then decreases, and finally remains stable. Compared with the tool-tilted angle of 0°, the 2.5° tool-tilted angle can provide a larger process parameter window to obtain defect-free welds and higher metallurgical bonding strengths. The interface microstructure shows that a diffusion layer of aluminum atoms diffusing into stainless steel with a thickness of 1~2 μm was formed on the interface. The diffusion layer is relatively thicker at a lower rotating speed and welding speed.
Keywords：friction stir welding;Al/Steel dissimilar joint;vortex material flow;mechanical properties;welding speed
Abstract：In this study, aiming at solving the problems of large deformation and time-consuming of conventional double-sided friction stir welding (DS-FSW), synergetic double-sided FSW (SDS-FSW) was proposed. The microstructure and mechanical properties of the SDS-FSW 6061 aluminum alloy joints were studied by the developed special equipment. The results show that when the rotational speed is 1 800 r/min and the traverse speed is 1 200 mm/min, compared with the DS-FSW joint, the defects of the SDS-FSW joints are improved under the same welding parameters. The deformation of the SDS-FSW joint (0.15 mm) is much smaller than that of DS-FSW joint (1 mm) at this welding parameter. And the tensile performance of the SDS-FSW joints is better. The maximum tensile forces of SDS-FSW and DS-FSW joints are 36.8 kN and 34.9 kN, respectively.
Abstract：Aiming to solve the problems such as low weld pass rate and complicated welding process when welding non-ferrous metal with large diameter (M≥12 mm) in traditional arc stud welding, the static gantry friction stir welding equipment with special stud welding fixture was used to realize the industrial pure titanium（TA2）stud welding with size M12. The macroscopic morphology, microstructure, torsional strength and microhardness of joints were analyzed under different forging displacement and forging speed. The results show that when the forging displacement is small, the hole defect appears in the center of the welding joint, and when the forging displacement is medium, the strip defect appears in the center of the welding joint, while the joint is well welded and no defect is found in the larger parameters. When the top forging displacement is 3 mm, the joint's torsional strength reaches 117 N·m. The microhardness of the area near the weld is obviously increased by forging deformation.The microhardness near the welding joint is obviously increased by forging deformation.
Keywords：friction stud welding;commercial pure titanium;macroscopic morphology;microstructure;forging displacement;torsional strength
Abstract：Pissimilar friction stir welding of aluminum/magnesium with 6 mm thickness was carried out in air and under water of 26.5 ℃ and 5 ℃. The microstructures and mechanical property of joints welded in different environments were studied and compared. The mechanism of tool adhesion was explored. The results showed that submerged friction stir welding effectively could reduce the temperature of the welding process, inhibit the growth of brittle Al-Mg intermetallic compounds, alleviate the phenomenon of tool adhension, and improve the weld formation and mechanical properties of the joint. The aluminum/magnesium dissimilar joint fabricated under 5 ℃water had the highest tensile properties and elongation of 129.3 MPa and 4.4%, 1.8 and 3.9 times that of air-cooled joints, respectively. It also revealed that the tool adhesive materials were mainly composed of Mg2Al3 and Al solid solution, indicating that the tool adhesive phenomenon was directly related to the generation of Al-Mg intermetallic compounds.
Abstract：Dissimilar alloys of 6061 aluminum and T2 copper with thickness of 2 mm were butt joined by utilizing friction stir welding (FSW) to investigate the effect of pin offset values on microstructure and mechanical property of the Al/Cu joints. It was found that the temperature at the tool-workpiece interface was kept relatively stable, but the transverse force was increased gradually, and the material interlocking of dissimilar Al/Cu was also enhanced with the pin offsetting from Al side to Cu side, which was because of the dissimilar material flow characteristics between Al and Cu. The Scanning Electron Microscopy (SEM) analysis were performed on the microstructure of dissimilar Al/Cu FSW joints, it was demonstrated that the Intermetallic Compounds (IMCs) at the Al/Cu joining interface were distinct two-layers structure, which was Al2Cu layer at Al side and the Al4Cu9 layer at Cu side, and the IMCs were also distributed with various structures with both granular and banded shapes near the Al/Cu interface. The optimal material interlocking and IMCs distribution of Al/Cu FSW joint were achieved with pin offsetting 0.5 mm to Cu side, and the maximum Al/Cu joint tensile strength was 200 MPa.
Abstract：Friction stir welding (FSW) of Reduced activation ferritic/martensitic steel (RAFM) with a thickness of 5 mm was carried out. The high-temperature creep properties and failure mechanisms of FSW joints of were studied through high-temperature creep experiments. The results show that the mechanical properties of RAFM FSW joints are good at high temperature. The high temperature creep life of RAFM steel joints with high alloying elements W and Ta content is longer because Ta and W form more precipitated phases in high temperature environments, including MX and Laves phases, which have a stronger barrier effect on dislocation slip. The reason why the creep life at 650 ℃ is much shorter than that at 600 ℃ may be that high temperature make dislocations very prone to slip. When the Laves phase has not yet grown and the blocking effect of MX on dislocation slip is not strong enough, creep holes have been formed in some parts, resulting in a sharp reduction in the effective cross-sectional area, severe stress concentration, and initiation of cracks leading to fracture.
Keywords：friction stir welding;high temperature creep performance;welded joints;Reduced activation ferritic/martensitic steel
Abstract：Welding residual stress has a very important influence on the fatigue strength, stress corrosion resistance, dimensional stability and service life of bobbin tool friction stir welds. In order to study the magnitude and distribution of the internal welding residual stress of bobbin tool friction stir welds, the short wave X-ray diffraction was used to conduct the nondestructive testing analysis of the internal residual stress of the weldments; the microstructure evolution of the base metal, heat affected zone, thermo mechanical affected zone and nugget zone at the forward and backward sides of the weld was analyzed by optical microscopy, microhardness and electron back-scattering patterns (EBSD). The metallographic observation results show that the joint structure of double shaft shoulder friction stir welding is approximately symmetrical in the thickness direction, showing a "waist drum" shape. The interface between the nugget zone and the thermo mechanical affected zone is approximately hyperbolic, and the demarcation line of the thermo mechanical affected zone at the forward side is more obvious. EBSD scanning results show that there are strong deformation structures in the heat affected zone and the thermo mechanical affected zone; dynamic recrystallization occurred in the weld nugget zone under the dual influence of shear deformation and welding cycle heat, which is mainly a weakly oriented structure with a large content of small angle grain boundaries. The results of short wave X-ray diffraction show that the longitudinal residual stresses in the central layer of the inner plate thickness of the double shoulder FSW welded plate are greater than those in the transverse direction; along the weld, the section with large tensile stress is 150~250 mm away from the starting end of the weld, and the maximum tensile stress is 244 MPa.
Abstract：The linear defect on the back of 2A14 aluminum alloy friction stir welding (FSW) was repaired, analyzed the effects of different repair methods on the elimination of weld defects, mechanical properties and microstructure of the joint, and studied the process characteristics of the repair of weld back defects. The results show that four methods can eliminate incomplete penetration defects. Directly using FSW for repair can be repaired four times, but multiple repairs can cause weld thinning and tensile strength reduction. Repeated friction stir welding has a small impact on weld microstructure; After polishing the defects on the back of the weld, use TIG welding filler on the front side of the weld, and then conduct FSW repair, the weld has no obvious thinning, and the tensile strength has decreased to a certain extent. The tensile strength of the two repairs has decreased significantly compared to that of the first one, and the number of times should not exceed 2. The residual fusion welding dendrites in the weld reduce the performance of the repaired joint; After polishing, TIG welding filler was used and smoothed, and FSW repair was performed, the strength of the joint repaired twice did not decrease, and the microstructure of the fusion welding filler could be fully converted to FSW microstructure; After polishing the back defects, TIG filler can also eliminate the lack of penetration defects, but there are obvious air holes and even cracks, which seriously reduce the strength and are not suitable for use. In addition, selecting a larger size stirring head helps to maintain weld strength.
Keywords：2A14 aluminum alloy;friction stir welding;back linear defect;repair
Abstract：The lap welding of 1.2 mm 2A12 and 4.0 mm 7A09 aluminum alloys were carried out throgh refill friction stir spot welding technology by robot friction stir spot welding system. Specimen were field X-ray fluorescence exploration，subjected to tensile testing and observed by metallographic and scanning electron microscopy. The results showed that tensile force of joint was low when the welding speed was 30 mm/min. When the compaction/refilling rate increases to 40 mm/min and 50 mm/min, the tensile strength of the sample is relatively large, and the trend of change is similar with the increase of the rotational speed. The highest mechanical properties of the sample occur at the rotational speed of 2 100 r/min and the speed of 40 mm/min; The bonding area at the outer edge of a solder joint is the weakest area for material bonding. If there are defects on the surface of this area, the mechanical properties of the solder joint are poor. If the surface of this area is flat, the mechanical properties of the solder joint are high; the fracture mode of the sample is "perforation" on one side of the thin plate, and the solder joint remains intact on the side of the thick plate. The fracture mechanism is a mixed fracture of ductile fracture, intergranular slip, brittle displacement, and brittle fracture.