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Continuous contact transfer mode in bypass coupling triple-wire indirect arc welding (BCTW-GIA) is proposed to pursue an optional way to ensure the high deposition rate and obtain stable depositing process. The arc behaviors, droplet transfer, and process stability with different distance between the tips (d1) of the main wire and side wires and the feeding speed of main wire (FSM) were studied. With the increase d1 and FSM, the droplet transfer mode of main wire varied from explosive transfer to short-circuit transfer to bridge transfer. When the d1 was greater than 4 mm, the continuous contact transfer bypass coupling triple-wire indirect arc welding can be realized and the welding process stability was improved. With the d1 adjusted from 0 to 6 mm, the standard deviation of the current dropped from 123.8 to 53.8 and its variation coefficient from 117.33 to 39.24% at FSM = 1 m/min. The weld penetration and weld width reduced with the FSM increasing, while the weld reinforcement gradually increased. [ABSTRACT FROM AUTHOR]

Aluminum-steel hybrid structures have great application prospects in lightweight vehicles. In this study, AA6061-T6 and SS304 with 3 mm thickness and 10 mm lap width were welded-brazed by Cold Metal Transfer process. Both electric parameters and droplets transfer were monitored. An improved discretization method was employed to calculate the equivalent welding parameters. Macro and microstructures were characterized by OM, SEM with EDS and XRD, and the tensile-shear strength, Vickers and nanohardness were measured to evaluate the mechanical properties of the joints. CMT welding process is a short-circuit transfer mode with one pulse one droplet. With the increase in heat input, welding cycle and interfacial layer thickness increased. Al9Fe2Si2 and FeAl5 or FeAl3 were found in the interfacial reaction zone. The intermetallic compounds whose hardness is 1.7 times of SS304 are the main reason for the fracture failure to appear at the interfacial layer. [ABSTRACT FROM AUTHOR]

In the last decade, wire + arc additive manufacturing (WAAM), which is one of the most promising metal additive manufacturing technologies, has been attracting high interest from both academia and industry. WAAM systems are increasingly employed in the industry and academia, but there are still several challenges and barriers to process stability control. The process stability is highly dependent on how the molten feed wire is added into the melt pool, which is known as the droplet transfer mode. To ensure a stable WAAM deposition process, it is essential to maintain the transfer mode in a suitable stable status. Without an effective transfer mode control method, the operators need to determine and control the transfer mode based on their experience using manual adjustment, which is difficult to achieve in a long period of production process. In this paper, a deep learning-based technology was proposed for the control of the droplet transfer mode based on the data collected from the WAAM process. A long short term memory neural network was applied as the core transfer mode classification model. A time-series data, arc voltage, was collected and statistical and frequency features were extracted, which included 11 relevant features, as the inputs of the classification model. Then, the distance between the melted wire and the melt pool was adjusted based on the determined transfer mode to keep a suitable stability of the process. A case study was used to evaluate the proposed approach and to show its merit. The proposed approach was compared to three commonly used machine learning algorithms, k-nearest neighbours, support vector machine, and decision tree. The proposed method obtained the highest accuracy in determining the transfer mode, which was over 91%. The performance of the proposed approach was also evaluated by the single-pass and oscillated wall building. The proposed deep learning based approach improved the process stability in real-time, which resulted in better deposition qualities, in terms of geometry size and processing cleanliness compared to without control. Furthermore, this data-driven method could be applied to other WAAM processes and materials. [ABSTRACT FROM AUTHOR]

A numerical model of the metal transfer process was developed using the thermomagnetohydrodynamic equations and the phase-field method to investigate the influence of pulse frequency on the metal transfer mode, dynamics, and thermal behavior in the pulsed wire-arc additive manufacturing (WAAM) process. The control of droplet transfer mode, dynamics, and thermal behavior is essential in WAAM; otherwise, several potential defects such as high residual stresses and distortion, poor dimensional accuracy, and surface quality may occur due to uneven heat input condition and process instability. Therefore, in this study, eight sets of pulse frequencies ranging from 50 to 225 Hz, in steps of 25 Hz, using identical power source parameters, such as pulse duty cycle and average current of 25.4% and 152 A, respectively, were considered and compared for a nearly square current waveform. The results reveal that only the current pulses with a medium frequency regime (100-175 Hz) achieve the one-droplet-per-pulse mode of metal transfer. Moreover, an increase in pulse frequency leads to a shorter necking length of the pendent droplet and a significantly lower average speed and temperature of the detached droplet. The results for the heat flux analysis indicate that Joule heating and arc heating decrease due to the increase in pulse frequency, whereas the sheath heating remains almost constant using different pulse frequencies. The proposed numerical scheme provides a detailed understanding of controlling and tailoring the different metal transfer modes and their metal transfer stability during WAAM, which benefits further process optimization and control. [ABSTRACT FROM AUTHOR]

Real-time electric signal, matter transfer mode and welding pool behavior were obtained to investigate the effect of wires’ filling rate on arc stability and joints’ appearance during underwater wet flux-cored arc welding (FCAW). The electric signal results showed that arc stability first decreased and then increased rapidly because the raise of filling rate affected the number of charged particles and the electrical conductivity of welding arc atmosphere. Two typical transfer modes, globular repelled transfer mode and surface tension transfer mode, were observed in this study. The ratio of surface tension transition could be increased by adding wires’ filling rate. Meanwhile, the geometry of molten pool was changed and the distance between droplets to welding pool reduced as the filling rate increased. The fusion line became more regular and the radius of curvature increased under the effect of bubbles in the molten pool. As the filling rate improving, more slags on the welds surface were acquired and the welds were much flatter and smoother.

The twin-wire arc additive manufacturing (TWAAM) technology is an effective method for the in-situ fabrication of Ti 2 AlNb-based alloys relative to conventional cast and wrought routes. However, composition segregation is still challenging in the field of in-situ TWAAM. To homogenize the chemical composition, here we proposed a novel single droplet pre-alloying transfer mode of TWAAM based on the original droplet transfer mode of double wires directly fed into the molten pool. The designed nominal composition of Ti-25Al-23Nb was set as the target alloy. The as-printed parts were investigated systematically. The results showed that this unique droplet transfer mode could not only improve the forming accuracy but also effectively mitigate the composition segregation. In addition, the inconsistency of microstructure caused by the composition segregation was successfully solved by this mode and appears a fully lamellar microstructure. The improvement of composition uniformity played a positive role in enhancing the mechanical properties of the alloy, in which the tensile strength of room and high temperature were improved by 76.8% and 74.1% respectively compared with our previous research. This work is of great significance to alleviate the composition segregation of intermetallics and further promote the in-situ TWAAM fabrication of Ti 2 AlNb-based alloys. • The composition segregation that challenges the Ti 2 AlNb-based alloy in-situ fabricated by twin-wire arc additive manufacturing (TWAAM) was successfully mitigated by a novel single droplet pre-alloying transfer mode. • This single droplet pre-alloying transfer mode also promotes the forming stability and accuracy during the TWAAM process. • The improvement of composition uniformity played a positive role in solving the inconsistency of microstructure and enhancing the mechanical properties. [ABSTRACT FROM AUTHOR]

Real-time electric signal, matter transfer mode and welding pool behavior were obtained to investigate the effect of wires' filling rate on arc stability and joints' appearance during underwater wet flux-cored arc welding (FCAW). The electric signal results showed that arc stability first decreased and then increased rapidly because the raise of filling rate affected the number of charged particles and the electrical conductivity of welding arc atmosphere. Two typical transfer modes, globular repelled transfer mode and surface tension transfer mode, were observed in this study. The ratio of surface tension transition could be increased by adding wires' filling rate. Meanwhile, the geometry of molten pool was changed and the distance between droplets to welding pool reduced as the filling rate increased. The fusion line became more regular and the radius of curvature increased under the effect of bubbles in the molten pool. As the filling rate improving, more slags on the welds surface were acquired and the welds were much flatter and smoother. [ABSTRACT FROM AUTHOR]