Welding in the World最新文献

This research studies the influence of high-peak loads on local relaxation of residual stress and fatigue damage in high-strength steel welded joints treated by high-frequency mechanical impact (HFMI) treatment. The joint behavior is simulated with elastic–plastic finite element analyses that account for the combined effect of geometry, residual stress, and material properties. This simulation uses two treated geometry models: with or without surface roughness on HFMI groove, and two material properties: S690QL and AH36 structural steels. The results show that surface roughness and load history, including high-peak loads, significantly influence fatigue response. It is revealed that the model neglecting the surface roughness cannot represent the amount of residual stress change and fatigue damage at less than 100 µm depth from the surface. In addition, the local yield strength in the HFMI-treated zone affects the plasticity behavior near the surface imperfection under the high-peak loads, which provides comparatively different fatigue damage between S690QL and AH36 in some cases. As a result, this study provides the further understanding needed to develop a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

A strong effort has continued to optimize friction stir lap welding (FSLW) process and to achieve high strength of dissimilar metal welds. For more than a decade, various studies have shown that Al-to-Ti FSL welds can be strong under quasi-static loading. Recently, Al-to-Ti FSL welds have also been shown to be strong under cyclic loading, due to the unique and thin diffusion bonding layer formed at the interface. In this study, how readily high fatigue strength of AA2024-to-Ti6Al4V FSL welds can be achieved is assessed. First, the effect of increasing the pin size on the fatigue strength of the welds has been evaluated. Second, fatigue testing has been conducted on the welds of which diffusion bonding is more ascertained by the use of an interlayer. It has been found that the fatigue strength of the welds is insensitive to the pin size. This will be shown to be the result of the width of the diffusional bond to be insensitive to the use of a larger pin or whether the pin has penetrated to the bottom plate. Simulation has suggested that stress concentration in locations of lapping ends is not significantly affected by the increase of the metallurgically welded width, explaining that pin size and penetration-dependent weld width do not affect the fatigue strength. It has also been shown that interlayer assisted diffusion bonding has affected insignificantly the fatigue strength of the welds. Thus, the high fatigue strength is insensitive to process variation.

In the present study, laser welding of additively manufactured AlSi10Mg was undertaken with AlSi10Mg (similar) and Al6061 (dissimilar) alloy. The aim was to understand the laser weldability of selective laser melting (SLM)-printed AlSi10Mg alloy without filler material. The similar and dissimilar type of butt joints were  prepared, and it is found that dissimilar weldments had better mechanical properties than similar weldments. The heat treatment on these welded plates also improved their mechanical properties. The precipitation of Mg₂Si particles was evident from the XRD and TEM analysis. The as-built cellular structure was broken due to heat treatment and also near the weld zone in the as-welded plate. It was observed that microhardness increased with increase in Mg₂Si content after the heat treatment process. The strength of welded samples was less than that of the base metals. The heat treatment results in ~ 20% increase in the tensile strength of the welded samples with significant increase in elongation.

The serviceability of cladding layers manufactured by laser depends on the microstructure formed by the metallurgical solidification. The microstructure in the clad layer is influenced by several factors. Among them, the elemental distribution state of the molten powder in the molten pool plays a dominant role. The diffusion distribution of elements is closely related to the non-equilibrium metallurgical behavior in the additive manufacturing process. Therefore, it is important to conduct an in-depth study on the multi-field coupling behavior and the heat and mass transfer mechanism in laser additive manufacturing process. In this study, a coupled thermal-fluid–solid multi-physical field numerical model for the laser cladding of 316L stainless steel powder on 45 steels was developed. The transient change patterns of the temperature, flow and stress fields for the cladding process were quantitatively revealed. The diffusion process of the powder elements within the molten pool was considered to reveal the element distribution law in the clad layer. The effects of the surface tension, buoyancy for molten pool, and Marangoni convection on the flow field also were considered, and the validity of the numerical model was verified. This study provides a theoretical basis for optimizing the laser cladding process.

The influences of different laser beam offsets (LBOs) on the microstructures and mechanical performance of the welded joints were studied. In joints with LBO = +0.3, LBO = 0, and LBO = −0.3, the widths of fusion zones (FZs) are 0.98, 1.03, and 1.21 mm, respectively. Additionally, the equivalent grain diameters in the FZs are measured to be 45.3, 44.1, and 19.6 μm, respectively. The FZ of the joint with LBO = 0 contains β-Ti and Nb solid solution, as well as a small amount of α' phases. The FZ of the joint with LBO = +0.3 mainly consists of Nb solid solution and β-Ti phase. In contrast, the FZ of the joint with LBO = −0.3 is mainly composed of Nb solid solution as well as ω, α′, and α″ phases. The FZ of the joint with LBO = 0 shows the lowest Vickers microhardness (149.2 ± 10.4 HV on average), which decreases by 18.9% and 40.0% when composed with those of joints with LBO = +0.3 and LBO = −0.3. The joint with LBO = 0 is found to have the smoothest transition of the microhardness from the FZ to the base metal, which favors the performance of the joints. The average tensile strength values of joints with LBO = +0.3, LBO = 0, and LBO = −0.3 are separately 318.8 ± 20.6, 315.4 ± 19.7, and 317.9 ± 23.9 MPa. All tensile specimens are fractured on the side of Ti, which suggests the superior tensile performance of the welded joints.

A five-element medium-entropy filler alloy with composition of Ti-(18 ~ 24)Mn-(12 ~ 18)Fe-(3 ~ 8)Ni-(3 ~ 8)Zr (wt.%) was proposed for vacuum brazing of TiAl-based alloy. The filler alloy was mainly composed of Ti-based solid solution and Ti-(Fe, Mn) compound dissolved with elements of Ni and Zr. The filler alloy ingot was ground into powder and then the filler powder was preset into the V-shaped groove butt joint with a gap of 50 μm. The Ti-Mn-Fe-Ni-Zr brazing alloy showed the liquidus temperature of 1060.1 °C, and also presented excellent wettability on TiAl substrate at 1110 °C for 10 min. The brazed joint mainly consisted of γ-TiAl, α₂-Ti₃Al, and residual brazing filler reaction phase. The brazing condition of 1210 °C/45 min exhibited the maximum joint thickness of 308 μm and the maximum area percentage of γ-TiAl phase of 33.77%, with almost elimination of residual brazing filler reaction phase within the joint, and meanwhile offered the maximum room-temperature tensile strength of 418 MPa, 70.85% of the base alloy. The joint fracture showed a mixed mode of intergranular and transgranular fracture.

This paper presents an experimental and numerical investigation of the fatigue crack growth behavior in different regions of metal inert gas (MIG) butt welded joints of 6005-T6 aluminum alloy. A series of experiments were carried out, including the compact tensile specimen crack growth rate, microstructure, and failure fracture analysis, which reveals the crack growth mechanism in different weld regions. A fracture mechanics numerical model considering geometric features and material inhomogeneity was developed, and the crack growth life results were obtained in agreement with the test. The results show that the microstructure and inhomogeneous material properties significantly affect their crack growth behavior and life distribution. The life distribution of different weld regions was quantified based on the Weibull distribution parameters to guide crack growth life assessment.

There is a growing demand in the industrial sector for the use of high-strength structural steels (HSSSs), which can achieve a significant weight reduction in structures. These structural steels are usually produced by quenching and tempering (Q + T) or thermomechanical treatment (TM), and their applications in welded structures pose several challenges for the users. In industrial practice, gas metal arc welding (GMAW) is basically the most commonly used fusion welding process, which has a relatively high heat input. However, at HSSSs, there is a need for low heat input but, at the same time, productive welding processes. High-energy density welding processes, e.g., electron beam welding (EBW), offer a unique opportunity to weld these steels. The widespread use of HSSSs is also hampered by the fact that the benefits of high strength can be exploited primarily under static loading. At the same time, different welded structures made of HSSSs are often subjected to cyclic loading, and possible weld defects and material discontinuities are major risks in this case. During our experiments, GMAW and autogenous EBW processes were applied to make welded joints from S960 Q + T and TM structural steels. The fatigue resistance of the welded joints was characterized by fatigue crack growth (FCG) tests, considering the increased crack sensitivity of HSSSs. A statistical approach was followed both in the design of the experiments and in the evaluation of their results. Based on the test results fatigue crack propagation design curves were determined for the investigated GMAW and EBW welded joints. The design curves were compared with each other, with design curves of lower strength material (S690QL) and with the recommended fatigue crack growth laws of BS 7910.

Controlling the growth of brittle intermetallic compounds (IMCs) and interfacial structure is crucial to obtaining desirable mechanical properties of the high-temperature brazing joints of titanium-zirconium-molybdenum (TZM) alloy. The effect of B addition on the mechanical properties of the joints and the microstructures in the brazing seam was investigated. The results show that the addition of 1%B enhances significantly the wettability of Mo-45Ni filler metal. The strength of the brazed joint with Mo-45Ni-1B filler metal is 30% higher than that of the joint with Mo-45Ni filler metal. The microstructure of the brazed joint with Mo-45Ni filler metal consisted of TZM/NiMo/eutectic (Mo + NiMo) + NiMo /NiMo/TZM. B addition led to the formation of Mo₂B intermetallic compounds at the interface between NiMo layer and TZM alloy during heating and the precipitation of faceted Mo₂B IMCs in the brazing seam during solidification, promoting the brittle-to-ductile fracture transition. This study provides a deep insight into the HT brazing of TZM alloy.