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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.

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.

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 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.

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.

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 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.

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.

The analysis results of long-distance oil and gas pipeline failures are important for the industry and can be the basis of risk analysis, integrity assessment, and management improvement for pipeline operators. Through analysis and comparison of the statistical results of the United States, Europe, the UK, and PetroChina in pipeline failure frequencies, causes, consequences, similarities, and differences of pipeline management, focusing points and management effectiveness are given. Suggestions on long-distance pipeline safety technology and management in China are proposed.

Girth weld cracking of long-distance oil and gas pipelines yields substantial harm to pipeline safety and may cause serious accidents. As of today, non-destructive testing has been one of the most common methods for predicting potential faults and ensuring safe operation. Classical pipeline non-destructive testing methods include magnetic flux leakage testing and the use of ultrasonic testing by electromagnetic acoustic transducers. However, they are incapable of identifying the defects in complex surfaces like girth welds. Magnetic flux leakage testing exhibits poor anti-interference abilities and low space resolution. Ultrasonic testing by electromagnetic acoustic transducers suffer from low conversion efficiency and poor signal quality. In order to overcome the disadvantages of conventional pipeline non-destructive testing methods, we propose an embedded eddy current testing system by leveraging image processing and neural networks. Hough transform and the contour extraction technique are employed to extract the characteristic features from the two-dimensional (2D) eddy current impedance image. Experiment results show that the system can effectively identify the girth weld defects, featuring an accuracy of up to 92%. The low power consumption and compactness of the proposed system makes it a great candidate for pipeline inner inspection.

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.

In this article, a J-integral estimation scheme of circumferential through-wall cracked pipe welds under pure bending is introduced. Three-dimensional elasto-plastic finite element computations are carried out, as pipe materials usually have a yielding plateau. Compared with the solution of the finite element method, the results indicate that the LBB.ENG3 scheme is easy to use and can give a satisfactory and reasonable J-integral estimation solution. Finally, the effect of mismatching on the J-integral for pipe-welded joints is discussed in detail.