Dual TIG Active Arc Additive Manufacturing Method and Process

Literature Overview

This 2024 paper by Zhang Jia, Shao Peize, Wang Xinxin, Huang Jiankang, and Fan Ding, published in the Chinese Journal of Welding, proposes a novel additive manufacturing method using a dual TIG active arc as the heat source. The technique combines two TIG torches with the addition of a small amount of oxygen to the shielding gas, utilizing 1.2 mm SUS304 austenitic stainless steel wire to deposit thin-walled components. The study investigates the effects of oxygen content, current distribution between the two torches, arc travel speed, and wire feed speed on deposited layer geometry and mechanical properties. This work represents a significant advancement in arc-wire additive manufacturing, which is increasingly relevant to large-scale component fabrication in the oil and gas, energy, and aerospace industries.

Core Technical Findings

The dual TIG active arc process employs two TIG torches positioned in a tandem configuration, with one torch serving as the primary heat source and the other as a secondary heat source. The addition of oxygen to the argon shielding gas creates an "active arc" that reduces the surface tension of the molten pool, enhancing wetting and spreading characteristics. The key process parameters and their effects are summarized below.

The deposition efficiency achieved with the dual TIG active arc process reaches 2.7 kg/h at equivalent current levels, representing a substantial improvement over single TIG arc deposition. The deposited metal microstructure consists of columnar dendritic crystals oriented perpendicular to the deposition direction, a typical microstructure for arc additive manufacturing of austenitic stainless steels. The introduction of oxygen has minimal effect on the microstructure morphology but causes a slight decrease in tensile strength and elongation of the deposited metal.

Process Mechanism and Parameter Interactions

The dual torch configuration provides several advantages over single torch processes. The front torch provides the primary heat input and wire melting, while the rear torch provides additional heat input to the already-deposited material, promoting better fusion between layers and reducing porosity. The active arc, created by the addition of oxygen, reduces the surface tension of the molten pool through the formation of surface-active oxides. This reduction in surface tension enhances the wetting of the base metal and previously deposited layers, resulting in improved layer adhesion and reduced spatter.

The current distribution between the two torches is a critical process parameter. When the rear torch current is low, the front torch provides most of the heat, resulting in a narrower but taller deposit. As the rear torch current increases, the additional heat input from the rear torch widens the molten pool, increasing the layer width while reducing the height. However, beyond an optimal point, excessive rear torch current causes excessive melting of the previously deposited material, leading to a decrease in width and increase in height as the molten pool becomes deeper and more concentrated. This non-monotonic behavior underscores the complexity of dual torch process optimization and the need for systematic parameter studies.

Mechanical Properties and Microstructural Analysis

The deposited metal microstructure consists of columnar dendrites with a primary dendrite arm spacing that increases with increasing heat input. The grain growth is driven by the thermal cycling inherent in the additive process, where each new layer is deposited on top of a partially solidified layer. The oxygen addition does not significantly alter the grain morphology but may influence the secondary dendrite arm spacing and the distribution of intermetallic phases. The slight decrease in tensile strength and elongation with oxygen addition is attributed to the formation of fine oxide inclusions that can act as crack initiation sites, although the effect is modest within the studied oxygen concentration range.

These values are representative of SUS304 austenitic stainless steel deposited by arc additive manufacturing and are consistent with the expected properties of the alloy. The slight property degradation with oxygen addition is an important consideration for applications where mechanical properties are critical, such as pressure boundary components in pipelines governed by ASME B31.3 or API 5L.

Integration with Engineering Practice

The dual TIG active arc additive manufacturing method is particularly relevant to large-scale component fabrication where traditional manufacturing methods are impractical or uneconomical. In the oil and gas industry, large-diameter pipe spools, pipe supports, and structural components can be fabricated using arc additive manufacturing, reducing material waste and enabling rapid production of complex geometries. The high deposition efficiency of 2.7 kg/h achieved in this study is competitive with other arc-based additive manufacturing methods and positions the technique as a viable alternative for production-scale applications.

For pipe manufacturing specifically, the technique could be applied to the fabrication of pipe fittings such as elbows, tees, and reducers, where complex geometries require extensive machining from forgings or rolled plates. The ability to deposit material layer by layer with controlled geometry could significantly reduce material costs and production time. However, the mechanical properties of the deposited metal must be qualified for the intended application, and the microstructural uniformity across large deposits must be verified through systematic testing.

Key Questions and Reflections

Several important questions arise from this study. First, how does the dual TIG active arc process perform on different base metals, such as carbon steel, low-alloy steel, or nickel-based alloys? Second, what is the effect of layer thickness and deposition strategy on the mechanical properties and residual stress distribution in thick-walled components? Third, can the process be scaled to larger torch configurations and higher currents for even greater deposition rates? Fourth, what is the long-term performance of components fabricated by this method under cyclic loading or corrosive environments?

These questions highlight the need for further research into the process's applicability to different materials and geometries, as well as the development of process qualification procedures that meet industry standards. The study provides a strong foundation for process development, but practical implementation requires additional work on process optimization, quality control, and standards compliance.

Study Insights and Implications

This study demonstrates that the dual TIG active arc additive manufacturing method is a promising technique for high-efficiency, high-quality material deposition. The combination of dual torch configuration and active arc chemistry provides a synergistic improvement in deposition efficiency and layer geometry, with only a modest impact on mechanical properties. For engineers involved in pipe and fitting manufacturing, this technique offers a pathway to more economical and flexible production methods, particularly for complex geometries and low-volume, high-value components. The research also illustrates the potential of process innovation to address manufacturing challenges that have persisted for decades, such as the high cost of fitting fabrication and the difficulty of producing complex geometries with traditional methods.