Nonuniform temperature field upsetting is prone to oxide inclusions, and the temperature field of rail flash butt welding (FBW) is primarily formed because of the Joule heat generated by the end-face current. The current distribution at the end face largely determines the heat distribution; thus, the current distribution and heat production at the end face of an alternating-current (AC) FBW must be investigated. This study combined finite element simulation and experimental validation to establish an AC rail FBW electric–magnetic–thermal coupling model to explore the influence of current parameters, end-face temperature, and feed mode on the distribution of the end-face current. The results show that a reduction in the welding current, current frequency, and time in low- and medium-temperature stages can improve the uniformity of the temperature field. The electrode clamping method determines the shape of the temperature field, whereas the proposed hybrid clamping method is the most conducive to uniform heat generation at the end face. Moreover, electrode clamping at 210 mm near the end face yielded uniform temperature fields. The experimental validation results were consistent with the calculated results, indicating that the proposed model is reasonable and reliable. In practical welding operations, it is advisable to optimize current and frequency to achieve an end face temperature > 1000 °C swiftly. This study provides a direction for enhancing the uniformity of the temperature field and improving the expulsion capability of impurities during the upsetting process, thereby optimizing the flash butt welding process for rails.
This study addresses the effects of alloying elements and radius curvature of the electrode on the degradation behavior during resistance spot welding (RSW) of A6451-T4. The importance of electrode characteristics is emphasised according to changes in hardness and electrical conductivity by electrode composition and radius curvature. The electrodes that were alloyed with Ag, Cr, and Be with varied radii were used in this study. The endurance limit of electrode was investigated by producing 100 welds with the optimised welding parameters. In addition to mechanical characterisation of the weld samples, comprehensive analyses of the electrode surfaces were carried out by carbon imprint, 3D digital microscope profiling, and electron microscopy. A computational analysis using the commercialised SORPAS software was also conducted to analyse heat generation according to the electrode characteristics. The results demonstrate that the electrode degradation proceeds by four discrete stages: aluminum pick-up and alloying, contact area increase, pitting, and cavitation. It was confirmed that load-bearing capacity and nugget diameter also change in proportion to the generated heat between the electrode and welded sheet. Among the physical properties of the electrode, the hardness and electrical conductivity most influence the electrode wearing behavior.
Proper parameter selection is crucial for obtaining the required shape of the beads and reducing defects like uneven welds, cracks, porosity, and irregularities while creating wire arc additive manufacturing (WAAM) samples. This study aims to investigate the impact of three input process parameters (current, welding speed, and gas flow rate) at three different levels on the properties (weld bead width, bead height, and dilution) of samples made from aluminum 4047 using the CMT-WAAM process. The study will analyze the data using response surface methodology (RSM). A central composite design (CCD) matrix was employed to develop a design of experiment incorporating three process factors. The appropriateness of the design was assessed by ANOVA analysis. The upper limits for the height and penetration of the weld bead were 2.83 mm and 3.12 mm, respectively. The lowest level of width measured was 9.44 mm. The forecasted ideal input parameters were a current of 150 A, a welding speed of 50 cm/min, and a shielding gas flow rate of 15 l/min. The findings demonstrated that the current exerted the most significant impact on determining the various responses, with welding speed and gas flow rate being the subsequent influential factors. The microstructures were analyzed using optical microscopy, revealing that the microstructure of the wall region comprised columnar and equiaxed grains. This study has considerable potential for manufacturing aluminum items utilizing a CMT-based arc welding technique.
The fatigue mechanism of a Mg-4%Al-1%Ca (hereinafter referred to as AX41) non-combustible magnesium alloy and its TIG (tungsten inert gas) and MIG (metal inert gas) weld joint was investigated through the plane bending fatigue and crack propagation tests. Results of plane bending fatigue tests showed that fatigue strengths of TIG and MIG weld joints were lower than that of the base metal (BM), while a similar fatigue strength at 107 cycles was found in both TIG and MIG weld joints. However, a TIG weld joint showed the highest crack propagation resistance among tested samples in crack propagation tests. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis were used to investigate the relationship between microstructural factors and the crack initiation mechanism of AX41 alloy and its weld joint. Cracks were found to initiate from grains in BM, while weld defects became a crack initiation site in a MIG weld joint. However, both grains and weld defects were observed as a crack initiation site in a TIG weld joint. Results of EBSD analysis indicated that fatigue crack initiation in TIG weld joint was favored in large grains with high Schmid factors. The critical value of initial crack length was discussed in the correlation between the maximum grain size and the maximum weld defects/inclusions size on the fatigue crack initiation mechanisms. As a result, a prediction method of the fatigue strength at 107 cycles was proposed based on the hardness and weld defects/inclusions size.
In this paper, the results of microstructural analyses, including optical microscopy, scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction analysis, of the Ni-based self-fluxing alloys NiCrBSi, NiCrBSi–WC, and NiBSi–WC deposited on a previously quenched and tempered (QT) steel substrate 42CrMo4 by flame spraying with simultaneous fusing and plasma transferred arc (PTA) process are presented. The aforementioned microstructural analysis was carried out to determine the microstructural characteristics of the investigated coatings, especially at the coating/substrate interface, and the influences of the spraying and welding technology on the steel substrate. The analysis revealed a change in the microstructure of the coating/substrate interface. Specifically, the diffusion characteristics of certain chemical elements (carbon and iron) from the coating to the substrate and from the substrate to the coating were observed. Additionally, the analysis established the existence of new phases within the coating that arose as a result of the aforementioned diffusion and reaction with chemical elements from the coating. The diffusion of chemical elements was most pronounced in the area of the coating/substrate interface, while it decreased away from this area.
Friction Stir Processing (FSP) is an innovative and promising technique for microstructure refinement, material property enhancement, and surface composite production. The current study describes the fabrication of AA8090/boron carbide surface composites (SCs) by FSP. Experimental studies were conducted by varying the FSP parameters, specifically the rotational speed (800–1400 rpm), traverse speed (25–75 mm/min), and groove width (1–1.8 mm). Ultimate Tensile Strength (UTS), Surface Roughness (SR), and Percentage Elongation (El) were used as response measures. Experiments were planned based on the central composite design (CCD) of Response Surface Methodology (RSM) and a mathematical relationship between the input parameters and UTS, SR and El, and were obtained by RSM. The model adequacy was tested using analysis of variance (ANOVA). The models enabled the examination of individual and interaction effects of input parameters on the UTS, SR, and El of the produced SCs. AA8090/boron carbide SC strength was optimal of 366 MPa at 800 rpm, 75 mm/min, and 1.8 mm and optimal 21.13% elongation at 1400 rpm, 25 mm/min, and 1 mm. A smoother surface with 0.82-μm roughness was optimal at 1400 rpm, 25 mm/min, and 1.2 mm. The present study uses the FSP method to synthesize near-net-shaped SCs without further machining by systematically selecting process parameters. The study shows that the increase in rotational speed during AA8090/boron carbide SC fabrication produces composites with a good surface finish, lower UTS, and good ductility. However, the increase in the other two parameters, namely, traverse speed and groove width, produces low ductile composites with rougher surfaces and higher strengths.
The fatigue failure rule of 7075-T651 aluminum alloy stir welded joint is studied under two-stage variable amplitude loading by fatigue experiments and finite element simulation. The results show that the friction stir welding (FSW) joints have different weak areas under different loading conditions. The fracture position of the FSW joint is related to the loading sequence of variable amplitude load and cycle number. The cycle number and crack length under the first-stage loading influence the fatigue life and fracture location of the FSW joints. When the cycle ratio of the first-stage loading is 70 ~ 80%, the fracture position of the corresponding specimen does not change. The simulation results show that the corresponding cycle ratios of critical damage under low–high load and high-low load are 77.8% and 74.4%, respectively, which are consistent with the experimental results. When the crack length is greater than 400 μm under low–high load, or when the crack length is greater than 500 μm under high-low load, the crack location does not change.
High-frequency mechanical impact (HFMI) is a user-friendly and efficient mechanical post-weld treatment method, and the achieved fatigue life improvement is statistically proved and is attributed to HFMI-induced compressive residual stresses amongst other effects. Several studies have shown in the past that the process parameters (treatment time and working speed) have an influence on the stress state introduced by the HFMI treatment. Thus far, however, only device-specific quantitative recommendation for the HFMI treatment exists based on the instructions of each HFMI device manufacturer. It is not clear if a certain treatment time for a given intensity leads to optimum results regarding the enhanced fatigue life and the treatment parameters of the several HFMI devices cannot be directly compared with each other. For these reasons, a universal and simple definition of the HFMI treatment’s intensity based on the kinetic energy of the HFMI pin was used to quantitatively correlate the HFMI-induced compressive residual stress with the HFMI-process parameters for two different HFMI devices: pneumatical impact treatment (PIT) and high-frequency impact treatment (HiFIT). To this purpose, data from former studies of HFMI-treated base material and welded specimens are revaluated. It is shown, that the compressive residual stresses show only slight changes after reaching a threshold value of the applied kinetic energy ((approx) 50 to 100 J/mm). This energy-based approach for the quantification of the treatment intensity was also used for a case study with PIT- and HiFIT-treated transverse stiffeners with different treatment intensities (2 J/mm and 7 J/mm). A high influence of the treatment intensity on the residual stress state was determined.
This paper examines the issue of wire deflection in wire and arc additive manufacturing (WAAM) and robotic welding, which leads to process instability and defects in printed geometry. The study focuses on the deflection of alloy 625, alloy 718, and 3Si1 welding wires during the deposition process. Measurements were taken to determine the relationship between wire deflection and the amount of wire used. Regression models were developed for each material to predict initial wire deflection and changes in deflection due to contact tip wear. The results showed that the deflection of alloy 625 and alloy 718 wires followed a nonlinear pattern for the first 500 m of wire, while the deflection of 3Si1 wire followed a nearly linear trend. The intensity of the contact tip wear is dependent on the normal contact load, which decreases as the wear increases. A generalized regression model of wire deflection was constructed based on the obtained regressions and the study of the wire’s deformed state. Based on these models, an algorithm was developed to correct the wire deflection by adjusting the tool center point coordinates. The effectiveness of the developed algorithm was verified in practice.
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