Welding in the World最新文献

This study investigates the implementation of mechanized high-frequency mechanical impact (HFMI) treatments to enhance the fatigue life of complex welded structures. Digital visual inspection and path planning are utilized through digital laser scanning system to automate the HFMI process for more complex weld shapes in S355 steel grade, such as circular weld seams, which are more challenging to treat consistently. The mechanized HFMI setup was calibrated to ensure precise alignment and consistent application of the HFMI tool along the weld toe line. Finite element analysis was used to simulate the HFMI process, focusing on the effects of multi-pass HFMI treatments and optimizing treatment parameters. The simulations identified fatigue initiation sites which were confirmed with the mechanized HFMI treated specimens. Fatigue testing of both as-welded and HFMI-treated samples was conducted, revealing significant improvements in fatigue strength for the HFMI-treated specimens, which is in line with IIW recommendations. The fatigue test results were also compared with previous testing of manually treated samples. It was concluded that the fatigue life scatter, expressed in log C (load capacity), resulted in a smaller standard deviation for the mechanically HFMI-treated samples than the manually treated samples. Geometric measurements from laser scanning post-HFMI treatment also indicated reduced scatter in weld toe radius and HFMI groove depth compared to manual treatments, demonstrating the robustness and consistency of the mechanized process.

Directed energy deposition-arc (DED-arc) of duplex stainless steels faces challenges in achieving a balanced ferrite/austenite ratio in the as-deposited condition. Consequently, solution annealing is often necessary to optimize the microstructure and attain good mechanical and corrosion properties. This study explores an approach to improve the phase balance using AM 2205 wire and Ar + 2% N₂ as shielding gas. Thin-walled structures comprising 10 layers were fabricated using a waveform-controlled gas metal arc welding (GMAW) process at two different travel speeds and arc power levels. Additionally, reference samples were produced with Ar + 2.5% CO₂ to quantify the influence of shielding gas composition on austenite formation. Microstructural investigations revealed that samples produced with nitrogen-containing shielding gas had an average austenite fraction ranging from 39 to 46 vol.%, depending on the travel speed and arc power combination. This represents an average increase of 3–9 vol.% compared to traditional shielding gas.

This study investigated the fatigue behavior of butt weld joints in a Ni–Fe-based superalloy GH1140 at 650 °C. The welded joints were prepared by the gas tungsten arc welding process on GH1140 plates with a thickness of 20 mm and a 45° K-shape groove. Vickers hardness tests on the cross-section of the welded joints showed slightly lower hardness in the weld center compared to the base metal. Fatigue test results demonstrated a reduction in fatigue lifetime for the welded joints compared to the base metal, attributable to the combined effects of reduced hardness in the weld metal and material heterogeneity. The reduction in lifetime was found to be stress-dependent, with a transition from fatigue fracture at lower stress levels to shear fracture at higher stress levels. Moreover, an empirical fatigue lifetime prediction model was used to predict the fatigue lifetime of the welded joints, showing good agreement with experimental data.

Welding aluminum castings is known for a high sensitivity for hydrogen-induced weld porosity. Research has shown that the porosity detected in mixed material resistance spot welds between die-cast and wrought aluminum is predominantly classified as solidification porosity. But, nugget penetration depths into the wrought aluminum alloy are low. In this work, the effect of a forging electrode force during welding current time t_i or after the welding current is shut off on weld spot characteristics is investigated, when resistance spot welding (RSW) mixed material joints between EN AC-43500-T6/T7 and EN AW-6082-T6. A current cut-off test based on a welding profile according to VDA 238–401 reveals a welding current time t_i between 60 ms ≤ t_i ≤ 80 ms as relevant for porosity detection in cross-section analysis. Based on this, different welding profiles are tested, and resulting weld spot characteristics are analyzed when applying chisel tests, cross-section analysis, quasi-static shear tensile tests, and scanning electron microscopy. The application of a forging electrode force reduces weld spot porosity in RSW significantly, while a current downslope leads to an increase. Further, the application of a forging electrode force during welding current time leads to a significant decrease in the standard deviation of s = 1.07 kN to s = 0.08 kN in shear tension force, although the mean shear tension force is increased marginally from 7.07 kN to 7.15 kN. Yet, an increase in weld penetration depth ≥ 20% into EN AW-6082-T6 is only achieved when reducing the initial electrode force to 5 kN and initiating forging electrode force during t_i.

This paper examined the characteristics and significance of fracture toughness for LFW (linear friction welding) joints of weathering steel SPA-H processed under different joining conditions to control maximum temperature during LFW process. Two types of LFW joint were prepared; one was jointed under higher compressive pressure so as a maximum temperature during LFW process to be lower than the A₁-transformation temperature (A1-LFW), and the other was jointed under lower compressive pressure to be higher than the A₃-transformation temperature (A3-LFW). The fracture toughness of both joints where a crack was located at the joint interface exhibited a higher value than that of the heat-affected zone of MAG (metal active gas) welds for the same steel. These results indicated that the LFW was more effective for the joining of weathering steel compared with conventional arc welding in terms of fracture toughness. However, A1-LFW exhibited lower fracture toughness (critical CTOD) than that of base metal or A3-LFW. Thus, the significance of the test results was discussed from mechanical and metallurgical viewpoints. The fracture toughness for A1-LFW found to be deteriorated due to work hardening associated with compressive plastic straining during LFW under higher compressive pressure, where the metal heated under A₁-temperature was not completely ejected by friction but remained around the joint interface. On the other hand, the deterioration of fracture toughness for A3-LFW was found to be caused by hardening due to bainitic transformation near the joint interface, whereas the narrowness of the hardened region provided a little bit higher toughness than the intrinsic toughness of the transformed phase due to plastic constraint loss.

Inner defects in welded joints constitute concurrent sites for fatigue failure in welds, especially when high fatigue resistance is enforced at weld toe and root. Existing standards limit the size of pores and other inner defects in proportion to the sheet or weld thickness, but for larger dimensions cutoff values are in place. This leads to overly high acceptance criteria when it comes to thicker sheets, as for example those used for wind turbine towers. To analyze the influence of pores on the fatigue life, specimens with targeted pore sizes larger than the usually accepted were tested under fatigue loading. In the present study, a numerical model for fatigue assessment was developed based on the experimental results, which allowed further variance of pore sizes and locations. It is shown, that for larger weldments the acceptable pore size can be increased by up to 87.5 % without the necessity to reduce the applicable design curve. But, the location of the pore with regard to the specimens surface has a large influence, with pores in a distance to the surface smaller than 13 % of the sheet thickness leading to a reduction in the applicable design curve by up to 4 FAT classes — an effect that is currently not considered in common weld quality documents.

The study investigates the effects of friction stir processing (FSP) and silicon carbide (SiC) nanoparticle dispersion on the mechanical properties and microstructure of 316L stainless steel produced by selective laser melting (SLM). To achieve defect-free samples, FSP was conducted at 710 rpm and 31.5 mm/min, providing sufficient heat input to prevent void formation. Significant grain refinement was observed, with SiC nanoparticle–reinforced samples achieving an average grain size of 205 nm, compared to 715 nm for FSP-only samples and 12 µm for the base metal. X-ray diffraction confirmed the material remained single-phase austenitic. Mechanical testing showed that hardness increased by 95% with SiC nanoparticles and 55% without, while tensile strength and yield stress improved by 30–40% for SiC-reinforced sample and 15–20% for only FSPed sample, without significant reduction in elongation and ductility. The study concludes that FSP, especially with SiC nanoparticles, effectively enhances the mechanical properties and refines the microstructure of 316L stainless steel, making it a promising post-processing technique for SLM-produced parts.

The present study deals with the development of a continuous cooling transformation diagram corresponding to the coarse-grained heat-affected zone of a high-strength all-weld metal with a minimum yield strength of 1100 MPa fabricated via gas metal arc welding. Dilatometry tests were conducted to determine the transition temperatures. High-resolution imaging methods, such as transmission electron microscopy and atom probe tomography, as well as nanoindentation, were employed to resolve the microstructural constituents. At fast cooling rates (t_8/5 from 1.4 to 25 s), the microstructure comprises a mixture of martensite and coalesced bainite, with a slight increase in the content of coalesced bainite with faster cooling. This demonstrates that coalesced bainite cannot be avoided in the coarse-grained heat-affected zone of the current alloy by increasing the cooling rate. With slower cooling (t_8/5 ≥ 50 s), the microstructure becomes increasingly bainitic, accompanied by a marginal drop in Vickers hardness. At t_8/5 times of 500 s and 1000 s, the all-weld metal consists of granular bainite with significant amounts of retained austenite and different shaped martensite-austenite constituents. The coarser massive-type constituents contain body-centered cubic grains, sized in the hundreds of nanometers, with a hardness approximately twice as high as that of the surrounding bainitic matrix.

Jacket foundations are lattice-like structures, whose assembly requires the welding of a large number of tubular joints. Such foundations type is suitable to support wind energy converters in deeper water with large turbine size. In order to increase the production speed and its quality, robot systems were developed to produce tubular joints. As fatigue is dominating the design of these structures, an assessment of the performance of tubular joints produced by four different robots was performed and compared with the performance of manually welded joints. In total, 18 large-scale tests were performed on joints with dimensions representative for offshore structures, which were produced in industrial environment. Almost all breakthrough cracks occurred through the chord, with cracks initiated at the weld toe, although in some cases cracks were also initiated and propagated between weld beads. Strain measurements have demonstrated that when multiple cracks are present in one specimen, they interact only in the last phase of the fatigue life, when they are so large that they affect the stiffness of the tubular components. The measured fatigue strengths of joints produced by robot were similar or higher than the T-curve of DNV-RP-C203. Two fabricators delivered components of which the fatigue strength was more than 20% higher than the standard curve. These results emphasize that mastering the welding process with robots is necessary to achieve superior levels of fatigue strength.

Analysis of mechanical properties of welded joints considering material and geometric inhomogeneity helps to improve the accuracy of joint performance prediction. The influence of material and geometric inhomogeneity on the mechanical properties of welded joints, encompassing stress–strain behavior, microstructural characteristics, and hardness profiles, was investigated. The present study focuses on the aluminum alloy oscillating laser welded (OLW) joint and employs response surface method and entropy weight method to establish an approximate model, thereby determining the optimal welding parameters. Subsequently, the stress–strain characteristics and microstructure of the weld (WM), base metal (BM), and heat-affected zone (HAZ) were obtained. To characterize the overall stress–strain characteristics of welded joints, three equivalent meso-damage model methods based on the Gurson-Tvergaard-Needleman (GTN) model were proposed, and the advantages and disadvantages of the methods were comprehensively evaluated by comparing the accuracy and efficiency of joint performance prediction. In addition, the stress–strain simulation analysis of the joint’s sub-region was performed to verify the effectiveness of the three-material equivalent model method in predicting the performance of the welded joint sub-region. Built on the study contents discussed above, the equivalent meso-damage model suggested in this paper completely accounts for the joint’s inhomogeneity and can accomplish high-precision prediction of the joint’s overall and local performance.