Yongquan Han

This study investigates the influence of varying positive–negative polarity ratios (EP:EN) on melt pool evolution during alternating current CMT (VP-CMT) arc additive manufacturing through a combined experimental and numerical approach. A multi-layer single-track droplet-melt pool coupling model was established, revealing the regulatory mechanisms governing melt pool flow, temperature distribution, and dimensional changes. These are driven by differences in arc morphology, heat input, and mechanical forces during EP and EN phases. Results indicate that molten pool flow is primarily governed by wire feed, retraction, and Marangoni forces. During the EP phase, arc divergence and elevated heat input result in significantly higher flow velocities than in the EN phase. Molten pool length increases with rising EP proportion, exhibiting periodic dynamic variations. Lateral flow intensity intensifies as EP ratio increases, directly influencing cladding layer morphology. This study provides theoretical basis for optimising additive manufacturing quality by adjusting the EP:EN ratio.

In the current numerical simulation study of high-strength steel welding, ignoring the phase transformation plasticity effect in the coupling analysis led to a significant deviation between the simulated value of residual stress and the experimentally measured value. To investigate the influence mechanism of the Welding Residual Stresses (WRSs) of 30MnCrNiMo armor steel, the transformation plasticity (TP) coefficient (7.81 × 10⁻⁵ MPa⁻¹) was measured via a Gleeble 3500, and a Finite Element Model (FEM) of thermal–metallurgical–mechanical coupling considering yield strength, volumetric strain and TP behavior in Solid-State Phase Transformation (SSPT) was developed. The results show that the volume expansion during the SSPT is the main factor for the shift in WRS from tensile to compressive. In contrast, the TP effect reduces the peak longitudinal tensile stress in the Heat-Affected Zone (HAZ) by 51 MPa. It also ultimately neutralizes the compressive component in this region. When the martensite fraction ranges from 0.12 to 0.45, transformation plastic strain becomes the dominant factor, leading to a characteristic evolution of longitudinal stress that initially decreases and subsequently increases. The FEM incorporating the TP effect successfully captures the dual reversals of residual stress in the HAZ. The average relative error between the simulated longitudinal stress and the experimental data obtained via X-ray diffraction (cosα method) is 8.8%. The TP coefficient database and the developed multi-field coupling model markedly enhance the predictive accuracy for WRS in 30MnCrNiMo steel, offering a robust theoretical foundation for the design of stress corrosion resistance and the service life assessment of welded joints in armored vehicles.

In this study, simulations and analyses of arc characteristics in EP (positive polarity) and EN (negative polarity) stages (including the arc polarity transition stage) of variable polarity cold metal transition (VP-CMT) during arc additive manufacturing of aluminum alloys are carried out. Temperature field, potential field, and current density distribution of arc plasma at different stages are systematically investigated by establishing a numerical model of arc heat–force coupling in combination with single-layer single-pass additive manufacturing experiments. The results indicate that the arc’s high-temperature zone in EP stage shows the wider distribution range, with enhanced heat transfer efficiency, reaching a surface temperature of up to 11,555.8 K at 2 mm from the substrate. In contrast, the arc during the EN stage demonstrates a more concentrated high-temperature zone, attributed to a more pronounced electromagnetic contraction effect, resulting in reduced heat input and a lower peak substrate temperature in comparison with EP stage. As revealed by analysis of potential and current density distribution, the arc in EP stage shows the “bell-shaped” expansion pattern with widely distributed current density, whereas the EN stage arc displays a “wrapped” contraction pattern with a more concentrated current density. The transition from EN to EP stage exhibits greater arc stability than the reverse transition. Moreover, electrode spacing significantly influences arc characteristics; a reduction in spacing leads to a more focused high-temperature zone and a substantial increase in peak current density. This study elucidates the dynamic variations in heat transfer behavior between the EP and EN stages, offering a theoretical foundation for optimizing process parameters in aluminum alloy arc additive manufacturing.

In the plasma-gas metal arc welding (GMAW) hybrid welding of aluminum alloy, the weld penetration under different plasma welding currents was compared. When the plasma welding current was lower than 150 A, the hybrid welding penetration was greater than the sum of the single plasma welding and the single GMAW penetration. When the plasma welding current reached 130 A, with the increase of the plasma welding current, the advantage of the weld penetration of the hybrid welding was gradually weakened compared with that of the single welding. The above change mechanism is revealed from the perspectives of heat source energy distribution and molten pool stress. The results show that when the plasma welding current is lower than 110 A, the area of the high-temperature region in the arc zone of GMAW in the hybrid welding increases compared with that in the single GMAW. The trend of arc energy transfer to the penetration direction increases under the action of arc pressure, arc shear force, electromagnetic force, and droplet impact. With the increase of plasma welding current, the high-temperature area of the GMAW arc first increases and then decreases. When the plasma current reaches 130 A, the effect of arc pressure, arc shear force, electromagnetic force, and droplet impact on increasing the penetration begins to weaken.