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[Display omitted] • A interlayer surface reconstruction system was developed through the electron beam luminescence technology. • Electron beam scanning offers a viable alternative to traditional structural light, effectively avoiding metal vapor contamination. • An appropriate image processing technique was effectively developed to extract the centerline of the electron beam strip.. • The functionality of the established system was validated through multiple typical surfaces. Electron beam freeform fabrication (EBF3) technology is primarily known for its vacuum environment, high deposition efficiency, and versatile electron beam capabilities. However, this technology suffers the challenges of poor stability, repeatability, and forming quality for its complex deposition process. Timely interlayer surface inspection can effectively reflect the actual deposition state, thereby providing valuable guidance for the operators. Therefore, this study proposes an interlayer surface reconstruction technique based on the electron beam scanning luminescence. Specifically, a reconstruction system was first developed based on the electron beam AM equipment. Subsequently, the system was calibrated, and an appropriate image processing algorithm was developed to extract the electron beam central line. Finally, several surfaces were reconstructed to verify its practicality. Results indicate that the representative samples can be successfully rebuilt with a maximum reconstruction error within 0.15 mm. This research is of great significance in enhancing the deposition success rate, stability, and part quality. [ABSTRACT FROM AUTHOR]

Dual-wire additive manufacturing (AM) couples traditional wire-based AM for part fabrication and the molten pool metallurgy for material-preparation with high deposition efficiency and material utilization. However, compared with traditional single-wire AM technology, it has a more complex and sensitive dual-droplet transition distance (TD), which not only affects the forming quality but also the metallurgical quality. Therefore, it is necessary and urgent to monitor and control its TD value online. In this study, we systematically investigated the sensing, controlling, and influential mechanism of the TD value in dual-wire AM technology, and Ti 2 AlNb was taken as the target alloy owing to its great application prospects in the aerospace field. Specifically, a deposition experiment with different initial TD value was conducted to study the effect on the morphology and composition distribution of the as-printed part. Based on the optimal distance, the related image extraction algorithms and closed-loop control methods are developed. The closed-loop controlled verification experiment on the slope and step substrate, as well as the multi-layer deposition test, were carried out and analyzed. The results indicate that the developed system can control the TD to the desired value with good robustness. In addition, the controlled deposited multi-layer part exhibited good morphology and composition homogenizing in the post-characterization experiment. This study is of great significance for the intelligent and industrial development of dual-wire AM technology. [Display omitted] • Ti 2 AlNb alloy was in-situ synthesized through twin-wire electron beam AM. • Droplet transition mode significantly affects the as-deposited sample quality. • Droplet transition distance can be identified via the proposed extraction algorithms. • Optimal droplet transition mode can be sustained via the developed control system. • Closed-loop printed part exhibits well morphology and composition uniformity. [ABSTRACT FROM AUTHOR]

Electrons, which carry abundant information, can be used for the in situ monitoring of electron beam additive manufacturing, such as in the emerging electron optics method for monitoring surface morphology. In electron beam directed energy deposition (EB-DED), droplet transfer, which affects the stability and quality of the deposition, is a unique and critical process. In this study, we developed a dual-channel absorbed current monitoring method based on absorbed electrons for the in situ monitoring of droplet transfer in the EB-DED process, which differs from the common vision-based monitoring technology. The ability to identify different droplet transfer modes and parameter changes was verified through a series of experiments. The results show that this method exhibited prominent signal characteristics corresponding to typical droplet transfer modes in EB-DED, accurately identified the droplet transfer mode during part fabrication, and provided a clear criterion for identifying the stability of the deposition process. It also distinguished changes in the droplet transfer distance and wire-feeding speed via a quantitative analysis of the signal variation caused by the corresponding droplet behavior. We also analyzed and explained the generation mechanisms of the signal characteristics. The dual-channel absorbed current monitoring method exhibited high reliability and efficiency during the in situ monitoring of the droplet transfer of EB-DED. The proposed method represents a new tool for information sensing in closed-loop control and facilitates the study of droplet transfer behavior in EB-DED. [Display omitted] • A dual-channel absorbed current monitoring method is developed for EB-DED. • Typical droplet transfer modes in EB-DED can be identified by absorbed currents. • The method provides a clear criterion for identifying the stable deposition state. • Parameters variation can be monitored by analyzing the change of droplet behavior. [ABSTRACT FROM AUTHOR]

• Images of vapor plume in high-speed laser welding are captured through high-speed imaging. • Influences of laser power and welding speed on the behavior of vapor plume are analyzed. • Humping occurrence are predicted with the random forest and a sliding time window. The increase of welding speed can improve the productivity of laser welding. The behavior of vapor plume is an important indicator reflecting the state of the laser welding process. However, the behavior of vapor plume in high-speed laser welding (whose welding speed is higher than 10 m/min) has not been studied adequately. In this research, images of the vapor plume in high-speed laser welding of SUS304 austenitic stainless steel are captured through high-speed imaging for the monitoring of the welding process. Characteristics of the vapor plume are extracted with image processing, and the influences of laser power and welding speed on these characteristics are analyzed. The relationships between the behavior of vapor plume and melt pool are discussed, and three ejection regimes of the vapor plume are proposed. In order to predict the occurrence of humping, which is a typical defect in high-speed laser welding, samples are built by applying a time sliding window and then a classification model is built with the random forest. In addition, the importance of features in the classification is analyzed. The test results show that the proposed method can predict the humping defect in high-speed laser welding accurately. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three twin-wire feeding methods were adopted to fabricate Ti-Al-Nb intermetallics in-situ through electron beam freeform fabrication. • The twin droplet transition mechanism of the respective wire feeding methods was revealed. • The effect of different droplet transition modes on the morphology, composition, and mechanical properties of the deposited was studied. • Finite element model of twin-wire heating was proposed to assist explaining droplet transition. As a challenging but promising technology, wire-based in-situ additive manufacturing shows great advantages in material preparation and part manufacturing integration. However, compared with single-wire AM technology, it faces a more complex droplet transition pattern, which is closely related to the feeding modes of the elemental wires. In the current work, the effect of wire feeding methods on the droplet transition mode, the morphology, composition, and mechanical properties is for the first time systematically studied. Three wire feeding methods, including double-side-feeding, single-side-feeding, and parallel-feeding, were adopted to fabricate the Ti-Al-Nb intermetallics in-situ. The results show that the TiNb wire and pure-Al wire enter the molten pool separately and with a totally inconsistent droplet transition mode in the double-side-feeding and single-side-feeding methods. This droplet transition results in poor morphology, macro-segregation, and inconsistent mechanical properties. While in the parallel-feeding method, the TiNb and Al droplet transfer to the molten pool after forming a common droplet, which provides a better guarantee for the morphology and performance of the deposited parts. As a research oriented to the deposition process, the current work is of great significance for enriching the fundamental theory and promoting the development of the wire-based in-situ AM technology. [ABSTRACT FROM AUTHOR]

Abstract: In search of high speed and miniaturization, the welding of NdFeB permanent magnet material is currently of increasing interest. Previous studies have shown that dissimilar material joining between magnet and steel sheets can be realized by laser irradiation, but it is still not clear how the welding parameters affect the weld quality. In this paper, the results are reported of experiments studying effects of laser pulse power, pulse duration, and defocusing distance on joint dimensions and mechanical behavior in laser spot welding of an NdFeB magnet (N48) with a low carbon steel (AISI 1006). Both conduction mode and keyhole mode welding were performed in the present study. The welding modes can be altered by changing peak power or defocusing distance, but not by changing pulse duration. Three fracture modes were found in shear tests, i.e., ‘nugget pullout’, ‘through nugget’ and ‘magnet crush’, while in peel tests, only ‘nugget pullout’ fracture mode was observed. The different fracture modes under different loading conditions were attributed to the mechanical locking effects present under shear testing but not under peel testing. For ‘nugget pullout’ mode fracture, the metallurgical quality of joints is the controlling factor of fracture forces; for ‘nugget through’ and ‘magnet crush’ mode fractures, the controlling factors are the size and strength of the nugget and base magnet. Certain nugget penetration should be achieved to avoid ‘nugget pullout’ fracture, while excessive growth of nugget should be limited to avoid ‘magnet crush’ fracture by appropriately adjusting process parameters. [Copyright &y& Elsevier]

A method based on Kalman filtering is proposed for weld defect detection in real-time radiographic NDT of spiral pipes. The existence of the image noises and the inhomogeneity of the background contrast induce numerous false alarms. In this paper, the trajectory continuity of the defects in the image sequence is detected by Kalman filtering for the identification of true defects. Potential defect regions without continuous motion are considered as false alarms and are eliminated. Experiments are performed to demonstrate the adaptability of the proposed method. The robustness of the method is also verified under unstable detection velocity. [ABSTRACT FROM AUTHOR]

Abstract: An effective and adaptive method is proposed to automatically detect weld defects using defect tracking in real-time radiographic image sequence of a moving weld. Firstly, a defect segmentation algorithm with low threshold is used to segment all of the potential weld defects in each image of the sequence. Then the modified Hough transform is employed to track the center of gravity of potential defects in image sequence, and the potential defects that cannot be tracked are eliminated as false defects. Experiment results show that the proposed method can detect weld defects with high certainty and avoid false alarms caused by the noise. [Copyright &y& Elsevier]

• Obvious cracks are found at the Ti/Al interface on both ends with the increase of the number of deposition layers. • The intermetallic compounds thickness for 5 deposit layers is 2.67 times that for 1 deposit layer. • The residual stress at the Ti/Al interface is concentrated at both start and the finish ends of the deposit layer. • The residual stress and intermetallic compounds both contribute to the cracking at the Ti/Al interface. In this paper, different number of AA2024 layers was deposited on Ti6Al4V substrate by using L-DED technology as a preliminary study for the additive manufacturing of Ti-Al dissimilar metal structures. Obvious cracks were found at both ends of the Ti/Al interface when the number of deposition layers was increased to five. To investigate the cracking mechanism, the microstructure at the Ti/Al interface were investigated at first. Continuous intermetallic compounds (IMCs) were formed at the Ti/Al interface. The IMCs were smooth on the side adjacent to Ti6Al4V, and were irregular on the side adjacent to AA2024. The thickness of IMCs increased with the increase of the number of deposition layers. The IMCs thickness for 5 deposit layers was 2.67 times that for 1 deposit layer. Then, the temperature and stress were computed with finite element method. The results showed that the residual stress at the Ti/Al interface was concentrated at both start and finish ends of the deposit layer, and the residual stress increased with the increase of the number of deposition layers. It can be concluded that the cracking at the Ti/Al interface resulted from the increases of the concentrated residual stress and the thickness of IMCs with the increase of the number of deposition layers. [ABSTRACT FROM AUTHOR]

Thousands of spherical fuel elements are transported pneumatically in pipelines in a pebble bed reactor every day. The motion of the fuel element has a strong influence on the efficiency and reliability of the reactor. The motion is determined by the force condition of the fuel element which is determined by the characteristics of the flow field in turn. Thus, it is of great significance to analyze the characteristics of the flow field and the force conditions of the fuel element. In this paper, we carry out simulations of the flow field with a structured mesh based on CFD (computational fluid dynamics). We analyze the effect of the motion and position of the fuel element on the flow field. Then we accomplish the force analysis of the fuel element in the flow, and acquire the fitting formulae of the force and torque on the fuel element. The experiment was performed on the experimental platform. The experimental results are in agreement with the theoretical analysis and verify its accuracy. This research may provide an important rationale for the dynamic and kinematic analysis of the fuel element pneumatic transportation as well as the basis for the stable operation of the reactor. [ABSTRACT FROM AUTHOR]

Pebble-bed high temperature gas-cooled reactors have many advantages such as inherent safety and high efficiency, so it has been considered as a preferred candidate for the Generation IV Nuclear Power Plants. Every day thousands of spherical fuel pebbles are transported pneumatically in the pipes outside the core of a pebble-bed reactor. During the transportation, each pebble will be lifted perpendicularly in a pipe of which the length is tens of meters. The lifting process affects the operation efficiency of the reactor directly, thus it is of great significance to the stability and reliability of the reactor. A pattern widely used nowadays is to lift a single fuel pebble each time, namely at any time there is at most one pebble lifted in a pipe. The efficiency of the pattern is low and it cannot meet the operational requirements, especially during loading and emptying the core. This paper introduces a new transportation pattern named two fuel pebbles lifted together. On this basis we obtain the force conditions of the pebbles. Then the dynamic characteristics of the pebbles are analyzed. The research shows that by controlling the time interval between transporting the two pebbles into the lifting pipe, the collisions between the pebbles can be avoided. In addition the transportation reliability and efficiency can be ensured. The experiment validates that the transportation pattern can meet the operational requirements of the reactor. The research is significant to improving the efficiency and reliability of the transportation of the fuel pebbles and provides an important basis for the design of the reactor. [ABSTRACT FROM AUTHOR]

The high-temperature oxidation behavior of the DED repaired IC10 DS superalloy is studied by selecting five representative zones, i.e., substrate, HAZ, and the bottom, middle, and top zones of the deposition layers. Results show that the substrate has the best oxidation resistance, while the oxidation resistance of the HAZ is the worst. For the deposit, the oxidation resistance decreases from the bottom to the top zones. The oxidation films consist of the internal continuous film of Al 2 O 3 and the external granular Cr 2 O 3 and TaO 2. The high-temperature oxidation resistance is mainly dominated by the fraction of γ′ phases. • The high-temperature oxidation resistance in HAZ is weaker than that in the substrate, and decreases along the BD. • The oxidation films consist of an internal continuous Al 2 O 3 film and external Cr 2 O 3 and TaO 2 particles. • The high-temperature oxidation resistance is more sensitive to fraction of γ′ phases than to the size of γ′ phases. [ABSTRACT FROM AUTHOR]

Abstract: The separating rings of a clutch in a heavy load vehicle are frequently replaced after a certain service period due to wear. However, the wear mechanism of the separating rings has not been well studied. In the present study, a worn separating ring is examined with scanning electron microscope, the metallographic structures are observed and the micro-hardness is measured. Results showed that the hardness of the slot surface is relatively low because the surface layer is mainly composed of austenite but not the preferred martensite; under the action of impact forces generated during the separation process of the ring, plastic furrows and pits are produced on the slot surface by particle abrasion. Based on these results, it is concluded that the wear failure of the separating ring is mainly caused by particle abrasion. [Copyright &y& Elsevier]

• Inconel 617 Ni-based superalloy sheets were joined using a laser-welding process under four different welding positions. • When using high-heat input parameters, the welding position has an obvious effect on the laser weld's quality. • Different welding positions generated different levels of hydrostatic melt pressure on the keyhole surface. Inconel 617 Ni-based superalloy sheets were joined using a laser-welding process. The welding experiments were conducted on four different welding positions (i.e., flat, horizontal, vertical-up, and vertical-down), using two sets of welding parameters with different heat inputs. Then, the geometry, internal defects, mechanical properties, and molten-pool dynamics of the laser joints were evaluated. The results showed that, when using high-heat input parameters, the welding position has an obvious effect on the laser weld quality. The top surface of the flat weld and the root surface of the vertical-down weld are both slightly concave. Moreover, the horizontal weld and vertical-down weld have serious keyhole-induced porosity defects, which lead to relatively worse mechanical properties. The analysis shows that the stability of keyhole differs for different welding positions; keyhole-induced porosity is easily generated in the horizontal and vertical-down positions. Moreover, solid metal blocks escape routes of the bubbles, resulting in more serious porosity for these welding positions. However, when using low-heat input parameters, the welding position has less influence on the laser weld quality because there is less molten metal, which makes the keyhole more stable in different welding positions. [ABSTRACT FROM AUTHOR]

• Superalloy K648 was manufactured by extreme high-speed laser metal deposition. • The microstructure features were revealed for K648 superalloy made by EHLMD. • The yield and ultimate strengths were tested for K648 superalloy made by EHLMD. Nickel-based superalloys have been widely used in manufacturing turbine blades, vanes, and discs of aircrafts and power generators that serve in challenging environment. In order to meet the requirements for rapid manufacturing of these large-scale and high-performance components, the extreme high-speed laser metal deposition (EHLMD) technology have attracted great attention in recent years. The EHLMD technology can significantly improve the efficiency compared with conventional laser metal deposition (LMD). Meanwhile, the solidification condition and the resultant microstructures and mechanical properties can also be dramatically changed. In this study, a high chromium superalloy (K648) has been additively manufacturing by using EHLMD for the first time. The key process parameters and the resulted microstructure and mechanical properties are investigated. The results have shown that the most important factors affecting the height, width, and depth of the single-track deposit of EHLMD K648 superalloy are powder feeding rate, laser power, and scanning speed, respectively. The height and the width of the deposits are the largest when the laser power, scanning speed, and the powder feeding rate are 1400 W, 25 m/min, and 30 g/min. There exist some incomplete fusions in the interlayer region and some pores in the interior region of the multi-track-multi-layer deposits. Columnar grains grow in the build direction within each layer, while equiaxed grains tend to form in the interlayer region. The average value of microhardness of EHLMD K648 superalloy is about 298.1 HV. The ultimate strength of EHLMD K648 superalloy is slightly lower than that of the forging GH648 superalloy but higher than that of the conventional LMD K648 superalloy. [ABSTRACT FROM AUTHOR]

• The K648 superalloys were additively manufactured by two kinds laser metal deposition processes. • The EHLMD K648 superalloy has a smaller grain size than that of the LMD K648 superalloy. • The oxidation process is mainly controlled by the outward diffusion of oxide-forming elements and the inward penetration of oxygen. In this paper, the high-temperature oxidation behaviors at 900 °C of the conventional laser metal deposition (LMD) and extreme high-speed laser metal deposition (EHLMD) high chromium K648 superalloys were comprehensive studied. The experimental results reveal that the oxidation resistance of the LMD K648 superalloy is better than the EHLMD K648 superalloy. The oxidation films of LMD and EHLMD K648 superalloy consists of continuous Cr 2 O 3 film and an internal wedge-shaped Al 2 O 3. The oxidation process is mainly controlled by the outward diffusion of oxide-forming elements and the inward penetration of oxygen. [ABSTRACT FROM AUTHOR]

Hot cracking is a severe defect that occurs during the repair or manufacturing of directionally solidified superalloy by laser metal deposition. Understanding the cracking behavior and mechanism is vital to avoid these defects. In this study, a directionally solidified superalloy, IC10, was investigated by laser re-melting. Hot cracking not only occurred at high angle grain boundaries, but also the low angle grain boundaries. A critical angle, below which the hot cracking may not occur, exists within the scope of low angle grain boundaries. Hot cracking can be ascribed to the coupled effects of a liquid film, stress concentration, and particles enriched with Cr and Mo. The formation of liquid film can be ascribed to the liquation of low melting point structures. The stress concentration around the grain boundaries provides the driving force for crack initiation and propagation. The liquid feeding can be inhibited by particles enriched with Cr and Mo, which promote the initiation of cracks. These findings provide technical support for achieving successful repair and additive manufacturing strategy of the non-weldable directionally solidified superalloys. • Hot cracking is the coupled effects of a liquid film, stress concentration and particles enriched with Cr and Mo. • Misorientation angle of dendrites affects the grain boundary energy, which decides the stability of liquid film. • The strain localization factor at liquid film of substrate is larger than that of re-melted zone. • Particles enriched with Cr and Mo can promote the hot cracking by inhibiting the liquid feeding. • The appropriate laser power and scanning speed are about 400 W and 8 mm/s, respectively. [ABSTRACT FROM AUTHOR]