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The effect of pre-weld heat treatment on the microstructure and low-temperature impact toughness of the coarse-grained heat-affected zone (CGHAZ) after simulated welding was systematically investigated through the utilization of scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test validated the presence of an optimal pre-weld heat treatment condition, resulting in the highest impact toughness observed in the CGHAZ. Three temperatures for pre-weld heat treatment (690, 720 and 750 °C) were used to obtain three different matrices (Steel 1, Steel 2, Steel 3) for simulated welding. The optimal pre-weld heat treatment is 720 °C for 15 min followed by water quench. Microstructure characterization showed that there is an evident microstructure comprising bainite (B) in Steel 1 and Steel 2 after pre-weld heat treatment, while the addition of martensite (M) with the pre-weld heat treatment temperature exceeds Ac1 by almost 60 °C (Steel 3). These differences in microstructures obtained from pre-weld heat treatment influence the refinement of high-temperature austenite during subsequent simulated welding reheating processes, resulting in distinct microstructural characteristics in the CGHAZ. After the optimal pre-weld heat treatment, Steel 2 subjected to single-pass welding thermal simulation demonstrates a refined microstructure characterized by a high density of high-angle grain boundaries (HAGBs) within the CGHAZ, particularly evident in block boundaries. These boundaries effectively prevent the propagation of brittle cracks, thereby enhancing the impact toughness.

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

The application of laser-arc hybrid welding (so-called, hybrid welding) to the fabrication of steel bridge members has recently been investigated. One-pass full-penetration butt joints of steels for bridge high-performance structure (SBHS400 and SBHS500) with a thickness of 15 mm were performed by hybrid welding. The sound butt joints by hybrid welding were confirmed by a series of tests. The Charpy impact test was performed on the test pieces extracted from the hybrid welded butt joints with specified test temperatures. A phenomenon known as fracture path deviation (FPD) occurred in most test pieces, due to a large variation in material properties of the heat-affected zone (HAZ), resulting in the difficulty of estimating the toughness of HAZ in hybrid welded joints. Therefore, the Charpy impact test was conducted on the test pieces subjected to the welding thermal cycle tests of hybrid welding, which can exclude the heterogeneity of material properties and obtain the Charpy absorbed energy of the HAZ with high accuracy. The test results indicated that FPD was not observed in all thermal cycle simulated test pieces because the uniform metallographic structures in the vicinity of the notch were formed by the simulated thermal cycle tests, and all thermal cycle simulated test pieces satisfied 47 J at the specified test temperatures, a value that prevents brittle fracture for SBHS. Besides, for investigating the effect of the high Charpy absorbed energy guaranteed by SBHS on the toughness of hybrid welded joints, the Charpy absorbed energy of the thermal cycle simulated test pieces of SBHS and those of conventional steel (SM400B) were compared. The results showed that some of the thermal cycle simulated test pieces of SM400B failed to satisfy 27 J, suggesting that SBHS may ensure a Charpy absorbed energy of 47 J or more in the HAZ of hybrid welded joints.

Aiming at the problem of complex path planning in the processing of curved surface workpieces of body-in-white, a hybrid path planning method based on a memetic algorithm is proposed. The method is divided into two parts: welding sequence planning and welding path planning between weld points. By establishing the kinematic model of a spot welding robot based on the pipper criterion and z - y - z Euler angle solution method, the motion constraints of path optimization are analyzed. Under the framework of the memetic algorithm, the improved A-star algorithm with redundant node deletion and a post-smoothing process is used to obtain the smooth collision-free optimal path set between weld points and to construct the objective function of travelling all weld points with the shortest path length and highest smoothness. The multiobjective elitist-simulated annealing genetic algorithm (MESAGA) is used to achieve the welding sequence planning of all weld points. The variable neighborhood search method improves the mutation operator; the elitist strategy is introduced to improve the probability of elitist individual crossover and mutation operation, and a simulated annealing algorithm is used to jump out of local search to obtain the global optimal solution. According to the motion constraints, the joint space path is obtained by the optimal path in Cartesian space. Simulation analysis results demonstrate that the hybrid path planning method based on the memetic algorithm can effectively optimize the path of spot welding robots and lay the foundation for control and trajectory planning during welding processes.

Laser-welding is a promising technique for welding NiTi shape memory alloys with acceptable tensile strength and comparable corrosion performance for biomedical applications. The microstructural characteristics and localized corrosion behavior of NiTi alloys in a simulated body fluid (SBF) environment are evaluated. A microstructural examination indicated the presence of fine and equiaxed grains with a B2 austenite phase in the base metal (BM), while the weld metal (WM) had a coarse dendritic microstructure with intermetallic precipitates including Ti2Ni and Ni4Ti3. The hardness decreased from the BM to the WM, and the average hardness for the BM was 352 ± 5 HV, while it ranged between 275 and 307 HV and 265 and 287 HV for the HAZ and WM, respectively. Uni-axial tensile tests revealed a substantial decrease in the tensile strength of NiTi WM (481 ± 19 MPa), with a reduced joint efficiency of 34%. The localized corrosion performance of NiTi BM was superior to the WM, with electrochemical test responses indicating a pitting potential and low corrosion rate in SBF environments. The corrosion rate of the NiTi BM and WM was 0.048 ± 0.0018 mils per year (mpy) and 0.41 ± 0.019 mpy, respectively. During welding, NiTi’s strength and biocompatibility properties changed due to the alteration in microstructure and formation of intermetallic phases as a result of Ti enrichment. The performance and safety of welded medical devices may be impacted during welding, and it is essential to preserve the biocompatibility of NiTi components for biomedical applications.

Friction stir welding (FSW) is a process by which a joint can be made in a solid state. The complexity of the process due to metallurgical phenomena necessitates the use of models with the ability to accurately correlate the process parameters with the joint properties. In the present study, a multilayer perceptron (MLP) artificial neural network (ANN) was used to model and predict the ultimate tensile strength (UTS) of the joint between the AA2024 and AA7075 aluminum alloys. Three pin geometries, pyramidal, conical, and cylindrical, were used for welding. The rotation speed varied between 800 and 1200 rpm and the welding speed varied between 10 and 50 mm/min. The obtained ANN model was used in a simulated annealing algorithm (SA algorithm) to optimize the process to attain the maximum UTS. The SA algorithm yielded the cylindrical pin and rotational speed of 1110 rpm to achieve the maximum UTS (395 MPa), which agreed well with the experiment. Tensile testing and scanning electron microscopy (SEM) were used to assess the joint strength and the microstructure of the joints, respectively. Various defects were detected in the joints, such as a root kissing bond and unconsolidated banding structures, whose formations were dependent on the tool geometry and the rotation speed.

This paper critically assesses phase transformations occurring after welding and subsequent post weld heat treatments in simulated sub-heat affected zones (HAZ) of P91B steel. Samples for weld-HAZ simulation were produced corresponding to grain-coarsened HAZ, grain-refined HAZ and inter-critical HAZ. Analyses revealed diverse phase transformation mechanisms (for GCHAZ = pipe-diffusion and for GR/ICHAZ = GB-diffusion) owing to manipulation in grain size and boron-enriched nanosized particles as regards virgin steel after welding. However, after PWHT, same phase transformation mechanism (interface diffusion) in all simulated sub-HAZs is observed. Hardness evaluations and prior austenite grain boundaries dissolution confirm GB embrittlement after welding. Boron segregation, the presence of borides and boron-enriched particles heads to ~ 50% drop in hardness deviations enhancing GB hardening after PWHT. Particle refinement is observed after PWHT which is further validated by numerical modelling. In addition, particle evolution during cooling from peak temperature of weld thermal cycle and isothermal holding of PWHT is analysed. Apparent activation energy of nucleation/growth follows descending order, i.e. GC/GR/ICHAZ for nanosized particles during welding.

Aluminium alloy AA2219 offers excellent weldability due to its low sensitivity to hot cracking, although it has inadequate weld joint strength. The reduction in the alloy's strength is mainly due to the melting and solidification process. The process parameters that influence the thermomechanical and metallurgical events that occur during welding are primarily responsible for the weld quality. The main objective of this study is to improve tensile strength and corrosion resistance by adopting post weld heat treatment. The influence of post weld heat treatment on the characteristics of AA2219-T87 aluminium alloy welded by gas tungsten arc (GTA), electron beam (EB), and friction stir (FS) processes was discussed in this paper. Experimentation was planned by utilizing a Box-Behnken design of experiments. Square butt joints were fabricated at planned experimental conditions, and samples were prepared for analysis. Response surface methodology (RSM) was employed and used to formulate mathematical models that incorporate welding process parameters to anticipate properties. A global optimization algorithm, simulated annealing (SA), was used to perform multi-objective optimization. For optimized welded samples, post weld heat treatment was shown. The findings indicate that post weld heat treatment improves the strength and corrosion resistance of AA2219-T87 aluminium alloy.

Recently, the pulsed current tungsten arc welding process (PC-TAW) has cemented their potential in various sorts of industrial application such as automobile, aerospace, and structural joining. However, the involvement of multiple process parameters in PC-GTAW process usually makes the process cumbersome to understand; and thereby, it is difficult to develop the mathematical model. Here, in this scientific work, the major efforts have been made to optimize multiple parameters for selected output responses through the use of evolutionary computational approaches. For this purpose, the particle swarm optimization (PSO), simulated annealing (SA) algorithm, and hybrid PSO-SA (HPSOSA) techniques have been employed and compared in terms of the quality responses for input parameters. From the soft computing modeling results, it has been observed that the HPSOSA improved the process performance and has revealed the global optimal solution within minimum interval of time. The developed models were statistically significant at 95% confidence interval. The experimental and mathematical outcomes for the welded specimens are duly supported with microscopic analyses.

The microstructure and impact toughness of the simulated heat affected zone (HAZ) of a high strength low alloy steels by laser-arc hybrid welding (LAHW) were investigated in this paper. Homogeneous simulated HAZ specimens with different grain sizes were prepared using the welding thermal simulating technique. Instrumented impact test was conducted to investigate the toughness of the simulated HAZ. Multi-scale sub-structure characterization of the simulated HAZ specimens was implemented by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD). The instrumented impact test found that the peak temperature T M mainly affected the crack propagation process, the crack propagation energy E p decreased as the increase of T M , while the crack initiation energy E i barely changed, the decrease of crack stable energy E a led to the change of E p . The multi-scale sub-structure characterization showed that the prior austenite grain size (PAGS), packet width, block width both increased with the rise of T M . The block width was similar to the facet size in the crack unstable propagation zone of the simulated HAZ specimens. The block was the microstructure unit controlling the crack propagation process of the LAHW simulated HAZ specimens.