The dissertation generalizes the scientific and applied understanding of modern technological processes of Wire Arc Additive Manufacturing (WAAM) of large-scale functional products from High-Strength Low-Alloy (HSLA) steels of the MoNiVa type. The use of WAAM technology allows for the creation of metal structures of complex geometric shapes with a high deposition rate and minimal material costs, opening broad prospects for the digital production of critical components for the heavy machinery, transportation, and energy sectors.
The main thermohydrodynamic and rheological problems concerning the industrial implementation of layer-by-layer 3D printing of these steels are identified. It has been established that the elevated kinematic viscosity of the melt impairs the metal's ability for free lateral spreading and reduces wetting efficiency. Under conditions of free spatial shape formation, this leads to non-stationary fluctuations and spatial wandering of the molten pool, causing the formation of internal defects (local interlayer lack of fusion, macroporosity), a significant increase in the waviness of the side surfaces, and metal sagging with the risk of sudden part collapse. Accordingly, the relevant scientific and applied task of developing a methodology for the cyber-physical and thermodynamic control of WAAM structures' macrogeometry to stabilize the process and ensure near-net-shape formation has been solved.
Using a complex of computational-analytical and experimental studies, the patterns of the combined influence of the active shielding environment and arc discharge energetics on the rheology of the molten pool were determined. For the first time for the conditions of WAAM of multi-layer structures from MoNiVa-type steels, the role of the shielding gas environment as an active technological factor in shape formation control was formalized. It was established that the targeted regulation of the carbon dioxide concentration in argon inverses the temperature coefficient of surface tension, activating centripetal Marangoni convection and precisely altering the layer's macrogeometry. Furthermore, the patterns of the scale effect of gas activity were refined: it was shown that at low heat inputs, premature crystallization of the melt due to the endothermic dissociation of CO2 molecules limits the convective redistribution of the metal, causing a non-linear degradation of the cross-sectional area.
During the investigation of cyber-physical mass transfer mechanisms, an effect defined in the work as the "WAAM energy paradox" was recorded and mathematically described for the first time. It is proven that the synergetic loop of the power source adaptively reacts to arc contraction by forced kinematic throttling of the wire feed, as a result of which regimes with the lowest linear heat input paradoxically generate the highest specific volumetric energy capacity.
To overcome rheological barriers and the negative impact of refractory molybdenum and vanadium-based oxide films, a mechanism for improving shape formation stability by applying the discrete-pulse CMT Cycle Step mode is substantiated. It is established that the pulsed nature of mass transfer and high-energy current peaks create a powerful electrodynamic effect that dynamically disperses the oxide shells and stabilizes the macrogeometry. Methods for predicting geometry have been further developed through the creation of multifactorial topological models that proved the phenomenon of spatial stability of discrete mass transfer, where the layer height acts as a stable technological constant, independent of fluctuations in gas composition.
For practical use in industrial conditions, a rational technological regime for the wire arc additive manufacturing of MoNiVa-type steels is recommended, which involves the application of the discrete CMT Cycle Step mode in a shielding environment with 5–10% CO2 at a linear heat input of 310–330 J/mm. It has been proven that such a combination ensures stable shape formation with an effective width coefficient of over 95%, the absence of recorded macroporosity in the studied cross-sections, and a significant reduction in allowances for subsequent machining.
Based on the developed and validated predictive models, an autonomous computational algorithm in the form of a Python module was created, intended as a technological basis for integration into industrial CAM systems. The practical value of the module lies in the automated calculation of the basic deposition trajectory parameters, in particular, the tool hatch distance and the Z-axis increment. This makes it possible to formalize the selection of technological parameters, minimize the dependence of the result on the subjective experience of the operator, significantly reduce the number of trial depositions, and increase the overall reproducibility of the WAAM structures design process.
