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Vector beams with spiral phase and spatially varying polarization profiles have many applications from optical micromanipulation to materials processing. Here, we propose and demonstrate an atomic spatial mode extracting scheme for the vector beam based on polarization-dependent absorption in the atom vapor. By employing the linear polarization pump beam which induces polarization sensitive absorption in the atomic ensemble, a counter-propagated weak probe vector beam is extracted by spatial absorption, and extracted part still maintains the original polarization and the vortex phase. The topological charges of the extracted mode are verified by interfering with the Gaussian beam, and it can be found that the orbital angular momentum is conserved in the extracting process. Our work will have potential applications in non-destructive spatial mode identification, and is also useful for studying higher-dimensional quantum information based on atomic ensembles.

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.

In this study, Friction plug welding (FPW) for 8 mm thickness AA2219-T87 sheets were carried out, and defect-free joints were obtained. The geometric size of plug and plate hole, rotational speed and welding force exhibit significant effects on the weld formation. Meanwhile, it is concluded that significant inhomogeneity of microstructure and mechanical properties exists in FPW joints. The recrystallization zone has the highest mechanical properties owing to the fine equiaxed grains and uniformly distributed θ precipitates. The entire plug, thermo-mechanically affected zone and nugget thermo-mechanically affected zone closed to the bonding interface are significantly softened due to the deformation of the grains and θ’ precipitate dissolution. The ultimate tensile strength (UTS) and elongation of the FPW joints can reach 359 MPa and 7.3% at 77 K and 305 MPa and 5% at 298 K, respectively.

The air supply terminal located at the floor level attached to side-wall is widely used in large space buildings, leading to potential energy saving as well as significant vertical thermal stratification. The cooling load calculation of such system is challenging, especially the calculation of the load gained from unoccupied zone. This paper adopts experiment and computational fluid dynamics (CFD) methods to study the heat transfer upward and downward across the stratified surface in large space building with floor-level side wall air-supply system. Five experimental cases with different heat source power and exhaust airflow ratios are performed to study their effects on the indoor thermal environment. We investigate the same cases in CFD and verify the result of vertical temperature distribution and cooling load components. As a critical parameter in evaluating the thermal stratification environment of large space building, the inter-zonal heat transfer coefficient Cb is emphatically discussed. By comparing the Cb value obtained through the two methods, the accuracy of the microscopic method is verified by the heat balance method. The results show that the Cb value is mainly affected by the zonal division and air distribution, but less prominently by exhaust airflow ratio and heat source in the occupied zone.

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.

Stratum ventilation with air supply terminal located on the floor level near side-wall is typical for large space building, consequentially resulting in vertical thermal stratification. Heat accumulation in the upper part of unoccupied zone will have significant effect on the cooling load of occupied zone. In order to accurately predict the indoor thermal environment and load characteristics of large space building, and to calculate the heat migration occurring in the thermally stratified environment, the determination of inter-zonal heat transfer coefficient Cb is critical. This paper discusses the method to calculate inter-zonal heat transfer coefficient to represent a large space building. In a scaled laboratory, five experiment cases with different exhaust air flow ratios, heat source powers are studied. CFD simulations of the same cases are performed. The vertical temperature distribution and detailed cooling load results of the entire system and occupied zone are verified based on the experiment results. By dividing the whole space vertically into two and five zones, inter-zonal heat transfer coefficient Cb under different heat source heights is calculated from the CFD numerical simulation results. The results show that the area division method, airflow pattern, airflow state of occupied zone and the heat source affect Cb value.

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.

Compared with nanoscale zero-valent iron particles (nZVI), sulfidated zero-valent iron nanoparticles (S-nZVI) exhibit higher reactivity and selectivity towards the remediation of halogenated organic pollutants such as trichloroethene (TCE). However, traditional methods of preparing S-nZVI are not safe and environmentally friendly to some degree. In this study, we developed a biological precipitation method to prepare S-nZVI, in which sulfate-reducing bacteria (SRB) played an important role for the in situ formation of ferrous disulfide on the surface of nZVI. The results show that the obtained sulfidated nZVI particles (BP-S-nZVI) exhibit excellent performance in TCE degradation, and the first-order rate coefficient (Kobs) and the efficiency of electron utilization were up to (1.66 ± 0.1)× 10-1 h-1 and 85%, respectively. Moreover, zero-valent iron utilization efficiency of BP-S-nZVI reached 95%. Meanwhile, the particles were well characterized and the mechanism for enhanced reactivity was explored. The prominent advantage of this novel method is that the harmful chemical sulfidating agents such as dithionite (S2O42−) and sodium sulfide (Na2S) are not required, making the preparation process more environmental-friendly, green and sustainable.

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

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.