Lightweight manufacturing reduces energy consumption and protects the environment; therefore, it offers an important direction of development in engineering. Variable polarity plasma arc (VPPA) welding is a light gauge aluminum and magnesium alloy joining technology capable of the highly efficient welding of medium-thickness metals via the keyhole effect, and without the complex pre-processing required by traditional methods. However, the stability of the keyhole weld pool is easily lost, resulting in a much narrower parameter window than that of conventional methods. Here, through keyhole morphology analysis and x-ray in situ imaging experiments, we reveal the material flow behavior, the mechanism behind the stability of the keyhole weld pool, and the reason for the very narrow process parameter window in VPPA welding processes. We discovered that the polarity pressure difference of the plasma arc, which is induced by the keyhole boundary shape and plasma arc intensity, drives the flow pattern to the top side of the keyhole, which is beneficial for keyhole filling and the formation of a stable weld bead. The influence of the difference in plasma arc pressure and the keyhole boundary on the flow field revealed in this study may guide the optimization process of light metal joining to achieve the highly efficient and defect-free manufacturing of large and complex structures.

A variable-polarity plasma arc (VPPA) is an effective energy source for the welding of thick aluminum alloy plates. However, the mechanisms influencing the fluid flow and the inhomogeneous distribution associated with thick-plate VPPA welding remain unclear, restricting the application of this technology in welding of thick aluminum alloys. Here, the relationship between the microstructure of the weld bead, energy transfer, and fluid flow is clarified by combining in situ three-dimensional x-ray imaging and multi-physics modeling. We find that heat conduction at the keyhole wall is the main factor influencing the morphology of the weld pool. The plasma arc pressure hinders the upward flow of liquid metal, while shear forces promote this flow. This causes the metal close to the weld pool surface to flow slowly, while that inside the weld pool has much higher velocity. It is also concluded that the large crystal size observed in the lower layer of the weld is partly caused by heat treatment from the upper layer of the thick plate. An eddy with a high flow velocity to the rear of the weld pool destroys the crystal-growth process, and this is considered to be one of the reasons for fine crystals appearing in the upper part of the weld. The mechanisms revealed here will help us to guide the use of VPPA technology in the production of stable, high-quality welding of thick aluminum alloys.

A novel bypass coupling variable polarity plasma arc was proposed to achieve the accurate adjusting of heat and mass transfer in the welding and additive manufacturing of aluminum alloy. However, the physical characteristics and decoupled transfer behavior remain unclear, restricting its application and development. A three-dimensional model of the bypass coupling variable polarity plasma arc was built based on Kirchhoff’s law, the main arc and the bypass arc are coupled by an electromagnetic field. The model of current attachment on the tungsten electrode surface is included for simulating different heating processes of the EP and EN phases in the coupling arc. The distribution of temperature field, flow field, and current density of the bypass coupling variable polarity plasma arc was studied by the three-dimensional numerical model. The heat input on the base metal under different current conditions is quantified. To verify the model, the arc voltages are compared and the results in simulation and experiment agree with each other well. The results show that the radius of the bypass coupling arc with or without bypass current action on the base metal is different, and the flow vector of the bypass coupling arc plasma with bypass current is larger than the arc without bypass current. By comparing the heat transfer on the electrodes’ boundary under different current conditions, it is found that increasing the bypass current results in the rise in heat input on the base metal. Therefore, it is concluded that using bypass current is unable to completely decouple the wire melting and the heat input of the base metal. The decoupled degree of heat transfer is one of the important factors for accurate control in the manufacturing process with this coupling arc.

Online monitoring and diagnosis of welding quality is essential for intelligent welding manufacturing. The recognition performance of penetration for aluminum alloy in gas tungsten arc welding (GTAW) still needs to be improved to meet the strict industry demands. This paper proposed a novel recognition method, time-frequency image based convolution neural network (TF-CNN), for GTAW penetration recognition. Time-frequency images were calculated from arc sound signals using short time Fourier transform and applied to analyze the non-stationarity of arc sound. The logarithm of time-frequency image was taken to construct the appropriate input matrix of CNN, which was optimized to improve its recognition performance, including the activation function, learning rate and architecture of network. The experimental results show that the proposed TF-CNN achieved an excellent recognition performance with 98.2% recognition accuracy and 0.21 accuracy variance for GTAW seam penetration recognition and outperformed the traditional methods. This paper provides some guidance for the application of CNN to other monitoring signals of intelligent manufacturing.

The bypass hybrid variable polarity plasma arc (VPPA) has the specific ability that could control the current through the wire and base metal independently. It offers a unique advantage for additive manufacturing for adjusting the heat input of wire and base metal freely. The deposition process of molten metal and the forming mechanism of the deposition layer are significantly different with the traditional wire and arc additive manufacturing (WAAM). Therefore, this paper investigates the effects of main current and bypass current on the morphology characteristics of the deposition layer, explores the surface and internal material flow of the molten metal, and constructs a multi-layer morphological model of the deposition layer under this process. The deposition process was observed by a high-speed camera, and the molten metal flow was tracked by slag and copper. It is found that the bypass hybrid VPPA can adjust the morphology of the deposition layer by changing the combination of the main and bypass current. The plasma arc establishes the deposition channel of the molten metal, and the surface flow in the deposition channel tends to stagnate further away from the arc and the mass transfer is in the form of internal filling. The depositing metal is not affected by the hybrid arc in solidification, and the surface tension plays a dominant role in forming. The flow pattern leads to an arch shape cross section of the deposition layer. The work lays a foundation for finely controlling and predicting the morphology of the deposition layer in WAAM.