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A friction stir butt-welded Ti- and Nb-modified 12 % Cr ferritic stainless steel joint without any defects was successfully produced under low welding heat input. The welding heat cycles, temperature profiles, and degree of plastic deformation during welding were quantitatively evaluated based on the results of a rigid-viscoplastic thermo-mechanically coupled model. Microstructure development in the stir zone (SZ) and thermo-mechanically affected zone (TMAZ) was studied and comparatively analyzed. A fine duplex microstructure containing nearly 62.9 % martensite and 37.1 % ferrite was obtained in the SZ. Undissolved nanometer-sized precipitates during welding played a crucial role in grain refinement in the weld. Retardation of dynamic recrystallization (DRX) occurred in the SZ, and the recrystallized grains were no more than 8% of the materials. The dynamic recovery and continuous DRX experienced by the ferrite grains in the TMAZ of the advancing side were more intense than those on the retreating side, leading to a higher amount of high-angle boundaries and a 20.8 % reduction in dislocation density. The hardness and strength in the SZ apparently increased owing to refined grains and phase transformation. Both the heat-affected zone with a fully ferritic structure and the SZ exhibited superior low-temperature toughness.

The microhardness distribution in the different zones of a friction stir welded reduced activation ferritic/martensitic steel has been investigated and correlated to the hierarchical martensitic microstructure in the respective zones, characterized by electron backscatter diffraction orientation analysis. It is found that the variation of prior austenite grain size, packet size, and block width in different subzones is influenced by the peak temperature and effective strain rate during the friction stir welding process. The distribution of the microhardness correlates directly with the geometrically necessary dislocation density observed in the different zones.

Defect-free welded joints were obtained by placing 316 L stainless steel at the retreating side (case 1) and the advancing side (case 2) of friction stir welds when joining China low activation martensitic (CLAM) steel to 316 L steel. Quenched martensite formed in the weld on the CLAM side, and the precipitates coarsened and aggregated in the outer HAZ, however, uniform austenite microstructures formed on the 316 L side of the weld. The microhardness profiles were asymmetric, and the maximum hardness zones located at the nugget on the CLAM side ranged from 350 to 450 HV. Softening in the outer HAZ on the CLAM side was observed, which was more pronounced in case 2. The welded joints had excellent tensile properties and always reached the tensile strength of the 316 L base metal. Better tensile properties of the welded joint were obtained in case 1, and the bonding strength of interfaces between CLAM and 316 L steels were always excellent in both cases.

This research revisited the preparation of the pure titanium dioxide. Through the aerosol-assisted technology and our self-made setup, a kind of titanium dioxide with higher activity than commercial product P25 was successfully fabricated, while this aerosol-assisted process takes only about 10 s. By adjusting the operation temperature, the obtained titanium dioxide nanoparticles have different crystalline sizes and phase compositions, which are two major factors determining the catalytic activity of the materials. The results show that the optimum temperature was 600 °C, and the AST-600 particle prepared under this condition has the rate constant at 1.49 in the removal of methyl orange, which is 2.36 times that of Degussa P25. Although the AST-600 particle could only be activated by UV light similar to P25, the efficiency of AST-600/g-C3N4 is higher than that of P25/g-C3N4 under the visible condition. Meanwhile, the as-prepared titanium dioxide particles have been thoroughly characterized by TEM, SEM, XRD, PL, XPS, DRS, Raman and nitrogen adsorption in the study. Hence, we prepared a pure titanium dioxide with higher catalytic activity than P25, but the process is still very convenient, low cost and easy to scale up, holding great potential as an alternative to commercial product P25.

Friction plug welding (FPW) experiments were performed on 8-mm-thick 2219-T87 FSW welds to investigate the weld formation, microstructure, hardness distribution, mechanical property, and fracture behaviors of FPW joint. The main findings are as follows: geometric size of supporting plate hole and welding force exert great effect on weld formation and mechanical property. Recrystallized zone (RZ) with varied width is observed on the bonding interface between the plug and base metal due to huge friction heat and deformation. Softening is found near the bonding interface due to the disappearance of cold working and transformation of constituent particles. The ultimate tensile strength (UTS) and elongation of FPW joint can reach 336.3MPa and 8%, respectively. The initial fracture of tensile specimen is prone to locate at the lower part of RZ. The tensile fracture morphology of FPW joint is characterized by dimples.

Friction stir additive manufacturing (FSAM) was performed successfully using 2 mm thick sheets of 2195-T8 aluminum-lithium alloy. The influence of the tool pin shape and process parameters on the interfacial bonding features among the additive manufactured layers was discussed, and the effects of interfacial defects on the performances of the additive build were analyzed based on microstructures, hardness profiles, and mechanical property evaluations. It is shown that the shape of the tool pin is one of the key factors in influencing the bonding interface between two manufactured layers. The cylindrical pin and the conical pin with three flats are not suitable for the FSAM process since very poor material mixing features are produced along the bonding interface. Although the material mixing degree of bonding interface is obviously improved at the advancing side (AS) interface of the nugget zone (NZ) by using the convex featured pin or the pin with three concave arc grooves, the material mixing degree at the retreating side (RS) interface of the NZ is always insufficient. Meanwhile, the weak-bonding defects along the bonding interfaces could be formed, which are originated from the hooking defects on the RS. The weak-bonding defects are related to the oxides and impurities existing at the original bonding interfaces as well as the insufficient stirring action of the tool pin. The back and forth double passes welding is one of the effective methods to improve the material mixing in the whole NZ and eliminate the hooking defects extending into the NZ. The welding rotation speeds of 800, 900 and 1000 rpm for giving welding speed of 100 mm/min were used in the additive manufacturing processes of 2195-T8 aluminum-lithium alloy, in which the optimum microstructure is obtained with the rotation speed of 800 rpm. The soften degree for the multilayered build is obvious, and the hardness profiles across the different bonding interfaces are always uneven. Meanwhile, compared with the AS interface, the fluctuation of the hardness value at the RS interface is greater. The mechanical properties of the multilayered build are inhomogeneous, and the maximum tensile strength of the multilayered build is only reached the 56.6% of the base metal. The mechanical properties are closely associated with the soften tendency of the material and the degree of the amelioration of weak-bonding defect along the bonding interface.

The modified 9Cr–1Mo steel was selected to conduct friction stir welding experiment. Defect-free welded joints were successfully obtained with the welding parameters of 300 rpm–50 mm/min and 400 rpm–50 mm/min. The microstructures of the welded joint were observed by scanning electron microscopy, electron backscatter diffraction and transmission electron microscope. The main microstructural characteristics of the weld zone (WZ) and the high-temperature heat affected zone are the formation of fresh quenched martensite, dissolution of M23C6 carbides, grain refinement and increase in kernel average misorientation. The WZ is significantly hardened, and the maximum hardness of this region is about twice that of the base material (BM). The ultimate tensile strength of the welded joint is up to 98% that of BM. The absorbed impact energies of the SZ and HAZ reach 77.8% and 87.4% that of the BM, respectively. Besides, the influence of post weld heat treatment on the microstructure and mechanical properties of the welded joint was also investigated.

The effects of friction stir processing (FSP) on microstructures, tensile properties and work hardening behavior of reduced activation ferritic/martensitic steel (RAFM) under different rotation speeds were investigated. The microstructures of FSP samples were examined by scanning electron microscopy (SEM), Electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). Tensile test results show that the FSP samples not only have high strength, but also have good ductility simultaneously. The FSP samples of 150 rpm exhibit the best ductility with an elongation of about 1.5 times that of the base material (BM). The FSP samples of 200 rpm present a good match between strength and ductility, and the ultimate tensile strength (UTS) and total elongation are increased by 77.2% and 34.2%, respectively. FSP samples reveal lower hardening capacity and lower work hardening exponent than BM. As the rotation speed increases, the hardening capacity and the work hardening exponent of the FSP samples gradually decrease. Both BM and FSP samples can be divided into stage III and IV in the Kocks-Mecking type plot of work hardening rate. The FSP samples present the lower absolute slope value at stage III relative to BM. Besides, the FSP samples of 150 rpm and 250 rpm reveal the smallest and largest work hardening rate at stage IV, respectively.

Microstructure and mechanical properties of underwater wet friction taper plug-welded API X52 pipeline steel with 6500–7500-rpm rotational speed at 30–45-kN axial force have been investigated, and the defect-free friction taper plug-welded joints were obtained. It was found that the microstructure in welded joint was remarkably inhomogeneous and very different from the X52 base metal. The weld region could be divided into forged zone (FZ), final frictional plane (FFP), shear deformation zone (SDZ), bonding zone (BZ), and heat-affected zone (HAZ). The HAZ has the coarsest grain and mainly consists of martensite and bainite, and the FZ and SDZ are mainly characterized by a mixture of martensite, bainite, and various shapes of ferrites. The BZ is represented by the banding pattern of fine equi-axed grain ferrites because of local decarbonization around frictional interface in wet welding condition. The axial force has a greater impact on microstructure of welded joint as compared with rotational speed. The hardness profiles measured on cross section of welded joints are severely non-uniform, ranging from 200 to 400 HV1, due to the inhomogeneity of microstructures. The impact absorbed energy of bonding interface sites in welds was considerably lower than that of base metal (about 20% of parent metal) because of the local obviously coarse grain, Widmanstatten ferrites, and banding ferrite defects. The principle of “close mode” friction welding is illustrated, and the microstructural characteristics and mechanical properties of welds can be predicted by judging the type of friction welding.

The microstructural characteristics of underwater wet friction taper plug welded joints for API X52 pipeline steel were investigated and tensile strength and facture toughness of the welded joints were experimentally evaluated. The microstructure of welded joints is considerably inhomogeneous. The tensile strength of welded joints shows heterogeneity in thickness direction. All of the tensile specimens taken from upper side of welded joints fracture at base material, while those taken from the bottom are broken at bonding interface or base material. The fracture toughness of welded joints deteriorates seriously, as compared with base material. The welded joints with notches in weld center have higher fracture toughness than those in bonding zone and the axial force has few influences on fracture toughness of welded joints. The bonding zone between plug and base plate should play a role as fissure defect for the whole welded joint, considering that it is tens of microns and much softer than its vicinity. In addition, the coarse grain, quenched martensite and Widmanstatten ferrite in weld region should also be responsible for the reduction of fracture toughness.

Two phenanthroimidazole-acridine derivatives (DPM and TDPM) were designed and synthesized for deep-blue organic light-emitting diode (OLED). Twisted combined rigid structures with hybridized local and charge-transfer (HLCT) state properties enable them to achieve excellent OLED performance. Non-doped OLEDs based on DPM and TDPM show decent deep-blue narrow-spectrum emission with Commission International de L’Eclairage (CIE) coordinates of (0.157, 0.053) and (0.158, 0.045), as well as maximum external quantum efficiency (EQEmax) of 4.0% and 2.6%, respectively. More importantly, OLEDs based on TDPM exhibit a smaller efficiency roll-off (12%) than that of DPM (35%) at high brightness (820 cd m−2 for DPM and 893 cd m−2 for TDPM, respectively). Overall, our work provides a molecular design strategy of HLCT materials for efficient deep-blue OLED with high color purity using purely organic emitter.

Reduced activation ferritic/martensitic (RAFM) steels are among the most competitive candidates of structural materials for nuclear fusion reactors, due to their superior comprehensive properties. Friction stir welding (FSW) was investigated in joining RAFM steel, considering its potential advantages in obtaining an optimal microstructure and mechanical properties of welded joint. To evaluate the feasibility of FSW in joining RAFM steel, an in-depth understanding of the microstructure-property relationships for friction stir welded joints of RAFM steel is necessary. In this research, the quantitative relationships between microstructural evolution and tensile properties in the stir zone (SZ) of friction stir welded RAFM steel after post-weld tempering treatment (PWTT) were systematically studied. Three different post-weld tempering temperatures namely 720 °C, 760 °C, and 800 °C were adopted. Then the uniaxial tensile properties were tested at room temperature and 550 °C, respectively. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and the Thermo-Calc Calphad software were adopted to systematically investigate the microstructural evolution. Martensite lath width, precipitate number density, equilibrium solid solubility of alloying elements in the matrix, and geometrically necessary dislocation (GND) density were analyzed quantitatively. With the results obtained, we assessed the contribution of each strengthening mechanism to the 0.2% offset yield strength. According to the effective inter-barrier spacing theory, a microstructure-sensitive yield strength model was obtained to well predict the change in yield strength at different conditions. Finally, the results calculated by equivalent strengthening effect indicated that the crucial microstructure determining the yield strength of the SZ for RAFM steel after PWTT is the high density of dislocation substructures.

A zigzag line defect was detected in the stir zone (SZ) of friction stir butt-welded 12% Cr ferritic stainless steel joint. The characteristics and formation cause of the zigzag line defect was clarified and analyzed. The zigzag line defect presented the partly bonded feature but with relatively higher hardness, which did not affect the tensile properties at room temperature of the joint. The micro-void features of the zigzag line defect were considered to promote crack propagation under impact load, thereby significantly reducing the impact toughness of the SZ.

Underwater wet friction taper plug welding experiments have been performed on X52 pipeline steel with 6500–7500 rpm rotational speeds at 30–45 kN axial forces, and the microstructural characteristics and mechanical heterogeneity of defect-free friction taper plug welded joints are discussed thoroughly. It is found that the microstructure of welded joint is remarkably inhomogeneous and very different from the base metal. The welded joint has dramatically coarse grains and is dominantly characterized by a mixture of quenched martensite, upper bainite and various types of ferrites including Widmanstatten ferrite. Unlike the traditional solid-state friction welding processes, the relative homogeneous microstructure with fine and equiaxed grains cannot be obtained in the friction taper plug welding process. The axial force has a greater impact on microstructure of welded joint as compared to rotational speed. The hardness profiles measured in cross-section of welded joints are severely non-uniform, ranging from 200 to 400 HV1, due to the inhomogeneity of microstructure. The impact absorbed energy of welded joint with V-notch in the bonding zone was considerably lower than that of base material (about only 20% of parent metal) because of the local obviously coarse grain, Widmanstatten ferrite and banding ferrite defect. The microstructural inhomogeneity results in mechanical heterogeneity.

In this paper, the weakening mechanism and tensile fracture behavior of AA 2219-T87 friction plug weld were investigated. The as welded friction plug joint involves six regions of plug metal (PM), plug thermo-mechanically affected zone (PTMAZ), recrystallized zone (RZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM). Owing to the severe material flow and plastic deformation at high welding temperatures, dynamic recrystallization would occur near the bonding interface. In TMAZ and HAZ, dissolution of θ′ phase, θ′ to θ transformation and coarsening of θ particles are observed. Throughout the weld, the minimum hardness value 87Hv is found in TMAZ near RZ. During tensile process, the initial strain concentration generates in TMAZ, and then intensifies and extends until eventual fracture of the joint. The fracture morphology of the joint is characterized by large and shallow dimples with second phases.

Star-branched amphiphilic copolymer nanocarriers with high-density zwitterionic shell show great promise in drug delivery due to their controllable small size and excellent anti-biofouling properties. This gives the hydrophobic cargo with high stability and long blood circulation in vivo . In the present study, star-branched polylactic acid and poly(2-methacryloyloxyethyl phosphorylcholine) copolymers with (AB 3 ) 3 –type architecture (PLA- b -PMPC 3 ) 3 were conceived as drug vectors, and the copolymers were synthesized by an “arm-first” approach via the combination of ring opening polymerization (ROP), atom transfer radical polymerization (ATRP) and the click reaction. The self-assembled star-branched copolymer micelles (sCPM) had an average diameter of about 64.5 nm and exhibited an ultra-hydrophilic surface with an ultralow water contact angle of about 12.7°, which efficiently suppressed the adhesion of serum proteins. In vivo experiments showed that the sCPM loading strongly enhanced the blood circulation time of DiI and the plasma half-life of DiI in sCPM was 19.3 h. The relative accumulation concentration in tumor of DiI delivered by sCPM was 2.37-fold higher than that of PLA-PEG, at 4 h after intravenous injection. These results demonstrated that the star-branched copolymer (PLA- b -PMPC 3 ) 3 is a promising alternative carrier material for intravenous delivery versus classic PEG-modified strategies.

The present work reports the influence of thermal process on microstructure and mechanical behavior for friction taper plug welding (FTPW) in structural steel S355. The results indicate that there is a significant decrease in total energy input and cooling time in temperature interval from 800 to 500 °C (Δt8/5) with increasing the axial force. However, the varying axial force has little effect on the peak temperature. The microstructure of weld zone contains acicular ferrite, polygonal ferrite, grain boundary allotriomorphic ferrite, and lath bainite. With decreasing total energy input and Δt8/5, the formation of lath bainite is promoted as well as the precipitation and growth of grain boundary allotriomorphic ferrite is inhibited. The hardness tends to increase with decreasing total energy input and Δt8/5. Charpy impact energy at the bonding line is almost 35 J ± 7 J for all specimens. On the other hand, Charpy impact energy at the weld zone increases gradually with decreasing total energy input and Δt8/5.

The experiment on API X65 steel was carried out using friction taper plug welding (FTPW) under underwater wet conditions at room temperature. Sound weld formation can be achieved vastly at axial force ranging from 25 to 40 kN and a rotating speed of 7000 rpm. The microstructure of the weld zone consists of lath martensite, upper bainite, granular bainite, polygonal ferrite, and acicular ferrite. The microstructure of heat affected zone (HAZ) mainly consists of lath bainite as well as a bit of lath martensite and granular bainite. The tensile properties of defect-free welds are excellent. The maximum impact energy of the V-notch of bonding area can reach 110 J, which was conducted at 0°C with an error of ±12.5 J. The typical SEM impact fracture morphology presents areas of cleavage, quasi-cleavage, and dimples.

Underwater friction taper plug welding for X65 pipeline steel was performed using Q345 structural steel and 316L stainless steel plugs. Weld microstructures, defects formation and mechanical properties were investigated. Using Q345 plugs can produce defect free and overmatching welded joints whose impact toughness was also favourable. Lower bainite and lath martensite in the weld zone resulted in high hardness with maximum value up to 418 HV10. Using 316L plugs can significantly reduce weld zone hardness due to austenite dominated microstructure. Microcracks can easily emerge at and may propagate along γ–δ phase boundaries to form macrocracks. Intermittent cracks were found along the bonding interface of all the 316L plug welds, which should be caused by the relief of the residual stress.

In the present study, a number of joints were fabricated successfully on DH36 marine steel sheet in underwater wet based on friction taper plug welding (FTPW). The influences of the plug and hole geometry parameters combination on the quality of welded joint forming were first investigated. It was found that tapered holes and plugs were the preferred choice and a suitable cone angle range was obtained. Using the preferred hole and plug designed further expand the range of welding parameters to explore process parameters influence on the welding quality. Bonding mechanism and microstructural evolution to FTP welds were investigated with multiple observations. Mechanical properties of the obtained joints were also evaluated with tensile and Charpy impact tests by reference to AWS D3.6 Underwater Welding Code. The best result is found as the joint welded with 7500 rpm and 40 kN which has 535.6 MPa ultimate tensile strength, 22.5 % elongation, and 42.5 J impact energy at bonding line.

In this paper, friction plug welding (FPW) experiments of 2219-T87 aluminum alloy sheets with 10 mm thickness were performed. The material flow behavior, microstructures, second phases, mechanical properties and failure behaviors were also investigated. The results indicate that a sufficient volume of material from both plug and base metal flow upward and downward is critical obtaining defect free weld. The plug thermo-mechanically affected zone, plug recrystallized zone, thermo-mechanically affected zone (TMAZ), and heat affected zone could be weakened significantly owing to the thermal–mechanical process. The main reason is believed as the dissolution of precipitates and the redistribution of constituent particles. The maximum tensile strength and elongation of the joint would reach 329 MPa and 7%, respectively. The TMAZ closest to the bonding interface is the weakest location of the joints. The tensile crack initiates at TMAZ close to the bonding interface near the lower surface and then propagates along the soften area in TMAZ with forming a large amount of dimples with second phases existing at the bottom.

In this study, the local microstructure, strength and toughness of different zones in AA6061-T6 stationary shoulder friction stir welds were investigated. Results indicated that, the most significant weakening for both strength and toughness is found in HAZ that close to NZ. The microstructure of NZ includes recrystallized fine grains with high density of dislocations, homogeneous constituent particles distribution and re-precipitated of G.P. zones resulting in the recovery of local strength and the improvement for both plasticity and toughness. In HAZ, the grain structure and constituent particle distribution inherit the characteristics of base metal, but β″ precipitates are dissolved and the reprecipitated β′ particles are coarsened. This results in the decrease of yield strength, tensile strength, crack initial energy and propagation energy of HAZ, especially near NZ.

Friction taper plug welding experiment was conducted on X65 pipeline steel in underwater wet condition. Results indicate that the weld defects, microstructural evolution and mechanical properties are closely related to the material flow behavior during welding. The formation of weld defect characterized by incomplete filling or lack of bonding is mainly caused by the poor flowing property of the plug material especially in low axial welding force. In the experiment condition, rising the axial force would result in better material flow, higher weld power, more energy input and further improve the bond quality of the weld. Microstructure of both weld zone and heat-affected-zone mainly consists of lath bainite which results in an overmatching condition through the weld. High quality welds in this study also exhibit good tensile and impact properties that is equal to the base metal and acceptable bend property of 105° as the best. Keywords: Underwater welding, Friction taper plug welding, Material flow, Mechanical properties, Pipeline steel

A weakly-coupled hybridized locally-excited and charge-transfer hybridized locally-excited and charge-transfer (HLCT) dual-functional material TPA-2CNTAZ is designed and synthesized. The dispersed state of TPA-2CNTAZ exhibits deep blue electroluminescent (EL) with an exceeding exciton utilization of 53%, and the aggregated state of TPA-2CNTAZ displays efficient detection to the analogue of chemical warfare agents with an excellent limit of detection of 0.69 ppb.

A coupled thermo-mechanical model is developed for friction plug welding (FPW) process of AA2219 aluminum alloy, which is validated by the experimental results of the torque and temperature history. Both the deformation of plug and base metal are considered in this model. It is concluded that weak-bonding and unbonding is caused by the poor interface normal force and material flow based on the simulational and experimental results. Significant inhomogeneity in distribution of temperature, effective strain and effective stress is observed along the thickness direction of FPW joint, resulting in the inhomogeneity of microstructure and mechanical property. The peak temperature of bonding interface can reach 508 °C, which is 79% of the melting point of base metal. Higher welding force and rotational speed could promote material flow and improve interface normal force, which is beneficial to obtaining better connection.

Stationary shoulder friction stir welding (SSFSW) was successfully performed on 7075-T651 aluminum alloy. Defect-free joints with smooth weld surface were obtained. The redistribution of constituent particles and the dissolution of η′ precipitates resulted in the low crack initiation energy of the nugget zone. The low crack initiation energy in the heat affected zone (HAZ) can be attributed to both the presence of η precipitates and the redistribution of constituent particles. The presence of η precipitates in the HAZ was the major reason for the decreased tensile strength of SSFSW joint.

Additive and non-additive T joints were successfully fabricated by stationary shoulder friction stir welding using various welding parameters. No interior defects were observed in all joints by microstructural examination. The fatigue property of both additive and non-additive joints was revealed by four-point bending fatigue test. The microstructure and the stress concentration were investigated at the fatigue crack initiation sites of various T joints. Stress concentration determines the fracture location, and the surface microstructure influences the fatigue property. The stress concentration at the internal corner was evidently lower for additive joint than non-additive joint. The surface of stationary shoulder friction welded joint was characterised by ultrafine grains, which evidently enhanced the fatigue property.

The Q235 and X65 steel tapered plugs were used to perform friction taper plug welding (FTPW) on X65 pipeline steel in underwater wet condition. The work concentrates on defect formation, torque variation, energy input during FTPW, and also the weld microstructures and hardness. Energy input is proportional to torque-time integration under constant rotating speed and is critical to realize metallurgical bonding between the hole and plug. Increasing burn-off can prolong welding time, thus enhancing energy input, so that defect can be eliminated. Using X65 plug can produce defect-free welds since the energy input is adequate. However, when Q235 plug is used, lack of bonding is commonly found as a kind of root defect in friction taper plug (FTP) weld owing to the insufficient energy input. The weld zone microstructure mainly consists of upper bainite in X65 plug weld, but Widmanstatten structure in Q235 weld. In heat-affected zone, the microstructure is found as upper bainite in both X65 and Q235 plug welds.

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.

Refill friction stir spot welding (FSSW) is a solid state joining technology recently derived from conventional FSSW. In the present study, refill FSSW was performed in a 0·8 mm thick AA7075-T6 aluminium alloy with varying welding parameters (welding time and plunge depth). The influence of welding parameters on the microstructure and mechanical properties of the weld was investigated in terms of nugget thickness, hardness and overlap shear strength. The microstructural features and fracture mechanism were observed by optical microscopy and scanning electron microscopy. The results indicate that the nugget thickness increases with increasing welding time and plunge depth. Furthermore, melted films observed in the stir zone (SZ) were consistent with a maximum temperature of 470·9°C measured 2·6 mm away from the SZ. The overlap shear strength increases with the increase of weld time and plunge depth due to increasing nugget diameter.

A research investigation has been undertaken to identify the various stages and variation of welding parameters in friction taper plug welding (FTPW) process and to explore their effects on the performance and properties of the welds. According to the variation of axial force, the overall FTPW process is divided into feeding phase, pressing phase, welding phase, and forging phase. The rotating speed, welding force, and burn-off rate remain nearly constant in welding phase. However, the torque peaks in welding phase when after few seconds of welding force setting is reached. Rising the welding force would increase the peak torque, welding torque, and burn-off rate, but decrease the welding time. When improper welding parameter is used lack of bonding and incomplete filling defects would form within the weld. The microstructure of the weld metal is consist of retained austenite, pearlite, and various Widmanstatten ferrite. In heat affect zone, it is mainly of lathy upper bainite. Defect free welds exhibit favorable tensile properties of which 548.3 MPa tensile strength and 27.5% elongation that equal to the base metal could be found.

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.

Friction spot welding (FSpW) is a relatively new solid state joining technology developed by GKSS. In the present study, FSpW was applied to join the 6061-T4 aluminum alloy sheet with 2 mm thickness. The microstructure of the weld can be classified into four regions, which are stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base material (BM), respectively. Meanwhile, defects such as bonding ligament, hook and voids are found in the weld, which are associated to the material flow. The hardness profile of the weld exhibits a W-shaped appearance and the minimum hardness is measured at the boundary of TMAZ and SZ. Both the tensile/shear strength and cross-tension strength reach the maximum of 7117.0 N and 4555.4 N at the welding condition of the rotational speed of 1500 rpm and duration time of 4 s. Compared to cross-tension strength, the tensile/shear strength were stable with the variation of processing parameters. Three different fracture modes are observed under tensile/shear loading, which are plug type fracture, shear fracture and plug-shear fracture. There are also there different fracture modes under cross-tension loading, which are plug type fracture (on the upper sheet), nugget debonding and plug type fracture (on the lower sheet).

In this paper, AA6061-T4 T-joints with three different joint geometries of T-lap/T-butt-lap/T-butt were fabricated successfully by friction stir welding. The distributions and formation mechanisms of defects in friction stir welded (FSWed) T-joints were discussed through macro and micro-observations, respectively. Hardness profiles of the as-welded samples were also measured to evaluate the softening effect during the process. What’s more, influences of joint geometry and the traverse speed on the tensile properties of FSWed T-joints were investigated. All the experimental results indicate that tunnel defects and kissing bond are easily formed and vary significantly in T-joints of the three joint geometries. Defects are moderated to a large extent with decreasing the traverse speed, but the specific relationship to tensile properties is complicated. T-lap joints present the superior tensile properties along the skin direction among the three geometries, the same as T-butt joints along the stringer direction. All the as-welded samples almost fractured in the locations of softening zones and bonding surfaces.

In this study, AA6061-T4 T-joints characterized by combination modes of T-lap and T-butt were fabricated by friction stir welding with different processing parameters. Defects distributed in the obtained specimens were examined by using a stereo microscope and an optical microscope. Process parameter influence on the distribution and the size of original joint line with severe deformation (OJLwSD) defect was investigated. The microstructures and hardness profiles in the T-joints were studied. Influence factors on T-joints’ tensile properties were discussed. In addition, the fracture mode and the fracture surface of the failure samples were observed and discussed as well.

In the paper, the effects of processing parameters (rotational speed and duration time) on microstructure, mechanical properties of 6061-T4 aluminum alloy friction stir spot welds were investigated. Experimental results indicate that the microstructure and mechanical properties varied significantly depending on processing parameters, which significantly affect the hook geometry and material flow which in turn affected the size of stir zone. The hardness profile of the weld exhibited a W-shaped or an upside down V-shaped appearance and the minimum hardness was obtained in the periphery of the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The tensile/shear strength increased with increasing tool rotational speed and duration time, in which, the rotational speed played a crucial role in determining the strength. The cross-tension strength increased dramatically with increasing duration time at the highest rotational speed. There was a direct correlation between the effective weld width and the strength, the presence of larger effective weld width resulted in stronger weld. Two failure modes were observed: shear fracture of the nugget under tensile/shear loading, and nugget debonding under cross-tension loading, the fracture path was associated to the hook geometry.

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970 °C for 1 h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.

In this paper, the microstructure and mechanical properties of 7075-T6 aluminum alloy joints joined by refill friction stir spot welding (RFSSW) were investigated. The keyhole was refilled successfully, and the microstructure of the weld exhibited variations in the grain sizes in the width and the thickness directions. There existed defects (hook, voids, bonding ligament, etc.) associated to the material flow in the weld. Mechanical properties of the joint have been investigated in terms of hardness and tensile/shear and cross-tension test, and the fracture mechanisms were observed by SEM (scanning electron microscope). The hardness profile of the weld exhibited a W-shaped appearance in the macroscopic level, which reached the minimum at the boundary of the sleeve and the clamping ring. The variation laws between tensile/shear and cross-tension strength and processing parameters were rather complicated. The void in the weld played an important role in determining the strength of the joint. On the whole, the preferable strength can be obtained at lower rotational speed. Shear fracture mode was observed under tensile–shear loadings, and nugget debonding, plug type fracture (on the upper sheet) and plug type fracture (on the lower sheet) modes were observed under cross-tension loadings. It was also observed that the main feature affecting the mechanical properties of the joint is the alclad between the upper and lower sheets and the connecting qualities between the stir zone and thermo-mechanically affected zone.

The 300M steel was welded by electron beam welding (EBW) with optimized welding parameters in the annealed state. As-welded, for comparison, and as-quenched (oil quenching at 870 °C × 1 h and tempering at 315 °C × 2 h) welded joints were investigated in this paper. The microstructure and fracture morphology were analyzed using scanning electron microscopy (SEM) and optical microscope. X-ray energy spectrum analysis was used to determine chemical composition of phases formed at the joint. The microhardness and tensile strength were evaluated. Results indicate that the weld metal microstructures of the as-welded joint are lower bainite, retained austenite and pro-eutectoid ferrite; the heat affected zone microstructure is sorbite with undissolved particles. The microstructure of as-quenched joint is tempered martensite. The tensile strength of the joints after quenching reached 1900 MPa.

The sodium borosilicate glass doped with Cu 7.2 S 4 quantum dots was prepared by using both sol–gel and atmosphere control methods. The formation mechanism and the microstructure of the glass were examined using differential thermal analysis and thermal gravimeter (TG-DTA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectra (EDX), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The results revealed that Cu 7.2 S 4 quantum dots in orthorhombic crystal system had formed in the glass, and the size ranged from 9 nm to 21 nm. In addition, Z -scan technique was used to measure the third-order optical nonlinearities of the glass. The results indicated that the third-order optical nonlinear refractive index γ , the absorption coefficient β , and the susceptibility χ (3) of the glass were 1.11 × 10 − 15 m 2 /W, 8.91 × 10 − 9 m/W, and 6.91 × 10 − 10 esu, respectively.

In the present study, AA6061-T4 T-joints were successfully fabricated by friction stir welding (FSW) in three different combination ways of skins and stringers. Distributions and formation mechanisms of tunnel defects, kissing bond defects, original joint line with severe plastic deformation (OJLwSPD) defects, and zigzag line defects in T-joints were investigated by macro- and micro-observations. Influences of defects distributions and welding parameters on the tensile behaviors of T-joints were examined. To a better understanding of failure behaviors of T-joints, fracture locations and fracture surfaces of tensile samples were also investigated.

Microstructural features and fatigue properties of friction stir lap-welded joints for AA6061-T6 alloy are investigated and the influences of single pass welding (SPW) or double pass welding (DPW), hooking defects and fatigue stress ratio on the fatigue properties are analyzed. It is found that the fatigue strengths of FSW lap-welded joints are obviously lower than that of the fusion lap-welded joints of IIW FAT22 and only approximately correspond and close to the IIW FAT12 design curve. The higher fatigue stress ratio R will lead to the decrease of the fatigue strength of lap-welded joints. The existence of hooking defects is the key factor to reduce the fatigue strengths. Fatigue cracks always initiate at the tip of hooking defect in the RS of SPW and AS of DPW joints respectively. The severity of hooking defects and the quality of lap-welds could not be improved by the DPW process as compared with the SPW process. The fatigue strength Δσm and Δσk (R = 0.1) of DPW joints will be 21.26% and 17.88% lower than that of SPW joints respectively. The fracture of lap-welded joints exhibits multiple crack initiations from the tips of the hooking locations, and the crack propagation shows some brittle fracture and is mainly characterized by the fatigue striations. The locations of crack initiation and the amount of secondary cracks in DPW joints are more than that of in the SPW joints which leading to the lower fatigue properties. The ductile fracture mode is obviously shown in the final fracture zone with deep-hole type dimples in the SPW joints but shallow-hole type dimples in the DPW joints.

Friction stir spot welding (FSSW) is a newly-developed solid state joining technology. In this study, two types of FSSW, normal FSSW and walking FSSW, are applied to join the 5052-H112 aluminum alloy sheets with 1 mm thickness and then the effect of the rotational speed and dwell time on microstructure and mechanical properties is discussed. The lower sheet material underneath the hook didn’t flow into the upper sheet due to the concave surface in the shoulder and groove in the anvil. The hardness profile of the welds exhibited a W-shaped appearance and the minimum hardness was measured in the HAZ. The results of tensile/shear tests and cross-tension tests indicate that the joint strength decreases with increasing rotational speed, while it’s not affected significantly by dwell time. At the rotational speed of 1541 rpm, the tensile/shear strength and cross-tension strength reached the maximum of 2847.7 N and 902.1 N corresponding to the dwell time of 5 s and 15 s. Two different fracture modes were observed under both tensile/shear and cross-tension loadings: shear fracture and tensile/shear mixed fracture under tensile/shear loadings, and nugget debonding and pull-out under cross-tension loadings. The performance of the welds plays a predominant role in determining the type of fracture modes. In addition, the adoption of walking FSSW brings unremarkable improvements in weld strength.

The microstructure and fatigue properties of friction stir welded joints of 7075-T6 Al alloy were discussed. It was shown that the zigzag-curve defect at the root of welds is the key factor to reduce the fatigue performance of single-sided friction stir welded joints of 7075-T6 high strength aluminum alloy. On the other hand, the FSW joints of 7075-T6 Al alloy achieved higher fatigue strength compared to the traditional fusion design curves IIW FAT40 and Draft Eurocode 9 design category 55-6 for structural aluminum alloy components.

The comparative study on fatigue properties between AA2024-T4 friction stir welds and base materials have been performed and the influence of zigzag-curve defects across weld section on the fatigue properties of FSW joints were investigated. It was shown that the zigzag-curve defects were the inherent feature existed in the friction stir nugget and could make the characteristic fatigue strength decrease from 96.19 MPa for base material to 73.71 MPa for welded joints with a reduction of 23.4%. Although the lower fatigue strength than the corresponding base material, the FSW joints of AA2024-T4 Al alloy achieved higher fatigue strength compared to the traditional fusion design curves IIW FAT40 and Draft Eurocode 9 design category 55-6 for structural aluminum alloy components.

FS welds have a sound joint for the lack of voids, cracks and distortions. When compared to the base material, friction stir welding (FSW) joints exhibit a finer crystal grain in the weld nugget, and advancing side has a clearer boundary with the weld nugget than that of the retreating side. The fatigue life of FS welds is 6–14 times longer than that of MIG-pulse welds under the stress ratio R = 0.1 and the calculated fatigue characteristic values of each weld increase from 42.32 MPa for MIG to 68.47 MPa for FSW at 2 × 10 6 cycles. In the high-cycle regime, the fatigue strength of the FSW joint is almost equivalent to that of the base material. The fatigue fracture of FSW revealed regions of crack initiation, stable crack growth and overload.

The fatigue behaviour of single-sided friction stir (FS) butt welds containing oxide array was investigated. The oxide array makes cracks initiate easily under fatigue loading, and has a deleterious effect on the fatigue properties of FS welds. The fatigue characteristic values of FS welds with oxide array have been decreased by 35% for 5083 welds and 55% for 2024 welds at 2 × 10 6 cycles. The fatigue lives of FS welds with oxide array were 20–40 times shorter for Al 5083 welds and 30–80 times shorter for Al 2024 welds than those of sound welds under the stress ratio R = 0.1.

Fatigue properties of FS welds with a kissing bond (bonded welds) were studied by comparing the test results of bonded welds with those of sound welds. The fatigue life of bonded welds is 21∼43 times shorter than that of sound welds under the stress ratio R = 0.1, and the fatigue characteristic values of each welds have decreased from 100.24 MPa for sound welds to 65.57 MPa for bonded welds at 2 × 106 cycles. At the macroscopic level there is no evidence of failure by shear. The fatigue fracture revealed cracks initiated from the root tip of kissing bond.

The fatigue experiment of friction stir (FS) welds in 2024-T3 aluminum alloys were performed to investigate the influence of root flaws on the fatigue strength and life of FS welds. The test results of welds with flaws (flawed welds) were compared with the results suggested by the International Institute of Welding (IIW) recommendations and the welds without root flaws (flaw-free) in the published research reports. It was found that there was always existed flaws at the roots of FS welds because of unsuitable welding parameters and the vertical length of the flaws is about 0.31–0.33 mm for the FS butt-welded joint of 4 mm in thickness. The fatigue life of flawed welds is 33–80 times shorter than that of flaw-free welds, and the fatigue characteristic values have decreased from 120.6 MPa for flaw-free welds to 54.7 MPa for flawed welds at 2 × 106 cycles.

The alloy 5083 was welded by friction stir welding and inert-gas metal-arc welding (MIG-pulse). The fatigue life of friction stir welds is 9–12 times longer than that of MIG-pulse welds under R = 0.1 and the fatigue characteristic values of each welds have been increased from 39.8 MPa for MIG to 67.3 MPa for FSW.