TIG Welding Arc Behavior Analysis at Different Current Densities

Literature Overview

This 2017 study published in the Transactions of the China Welding Institution by Zhang Xiaohong, Chen Jingqing, Zhang Kang, Ma Pengzhao, and Chen Hui from Southwest Jiaotong University investigates the arc behavior in TIG welding across a range of current densities. Funded by the National Natural Science Foundation of China (grant 51504198), the research provides detailed measurements of arc voltage, arc shape, and arc root characteristics as functions of welding current. Understanding arc behavior at different current levels is fundamental to process optimization, particularly for cladding and overlay welding where arc parameters directly influence dilution, penetration, and bead geometry.

Core Technical Points

The researchers systematically varied the welding current from 60 A to 200 A and measured the corresponding arc characteristics using high-speed imaging, arc voltage waveform analysis, and optical emission spectroscopy. The key observations include:

• Arc voltage increases approximately linearly with current, from about 8.5 V at 60 A to 12.2 V at 200 A, with an average arc resistance of approximately 0.055 Ω.
• The arc root width increases from 2.8 mm at 60 A to 5.6 mm at 200 A, following a square root relationship with current.
• Arc length stability decreases at higher currents, with increased voltage fluctuation amplitude above 150 A.
• The arc column temperature, estimated from optical emission spectroscopy, increases from approximately 8000 K at 60 A to 10500 K at 200 A.

Interpretation of Technical Significance

The linear relationship between arc voltage and current is consistent with the classical arc resistance model, where arc resistance remains approximately constant over the measured current range. However, the increasing voltage fluctuation at higher currents indicates a transition toward less stable arc behavior, which has direct implications for weld quality.

From a cladding perspective, the arc root width is a critical parameter because it determines the heat input distribution on the base metal surface. A wider arc root at higher currents leads to greater lateral heat spread, which increases dilution in overlay welding. For example, when overlaying 316 stainless steel on carbon steel, the dilution rate can increase from 5 to 8 percent at 100 A to 12 to 18 percent at 200 A, which may compromise the corrosion resistance of the overlay layer.

The arc temperature data is also significant. Higher arc temperatures at elevated currents increase the vaporization rate of alloying elements from the base metal, particularly in nickel-based alloy cladding where chromium and molybdenum loss to the atmosphere can degrade the overlay composition. This effect is particularly pronounced when using unshielded or partially shielded configurations.

Engineering Practice Implications

For pressure vessel overlay welding operations, this study provides valuable guidance for selecting appropriate current levels:

1. Low current range (60 to 100 A): Suitable for root pass overlay on thin-walled components, where minimal dilution and precise bead placement are required. The stable arc behavior at these currents facilitates consistent bead geometry.
2. Medium current range (100 to 150 A): The optimal range for most overlay welding applications, providing adequate penetration and bead width without excessive dilution. This range is commonly used for 316L stainless steel overlay on carbon steel pressure vessel components.
3. High current range (150 to 200 A): Generally not recommended for overlay welding due to excessive dilution and arc instability. May be appropriate for base metal welding or for thick overlay deposits where dilution is less critical.

The voltage fluctuation data is directly relevant to welding procedure specification (WPS) development. When writing a WPS under NB/T 47014 or ASME IX, the current range should be specified with consideration for arc stability. Excessive voltage fluctuation can lead to porosity, spatter, and inconsistent bead geometry, all of which are detrimental to overlay quality.

Key Questions and Reflections

An important question not fully addressed in this study is the interaction between current density and electrode condition. Tungsten electrode erosion increases with current, and the electrode tip geometry changes over time, which can alter arc behavior. In overlay welding, where long weld lengths are common, electrode replacement intervals and tip dressing procedures must be carefully controlled to maintain consistent arc characteristics.

Another consideration is the effect of current density on the weld pool dynamics. Higher currents produce deeper, narrower weld pools with increased fluid flow velocity, which can affect the solidification pattern and grain structure of the overlay deposit. This is particularly important for preventing hot cracking in nickel-based alloy overlays, where columnar grain structures can promote crack propagation along grain boundaries.

Study Insights and Summary

This research provides a comprehensive characterization of TIG arc behavior across a practical current range, offering engineers a solid foundation for process parameter selection. The findings directly support the development of welding procedure specifications for overlay welding applications, particularly in terms of current selection to balance penetration, dilution, and arc stability. The data on arc root width and voltage fluctuation are particularly valuable for predicting weld geometry and quality. For the pressure vessel fabrication industry, where overlay welding is used to achieve corrosion resistance or wear resistance on pressure boundary components, understanding the fundamental arc behavior at different current levels is essential for producing reliable, code-compliant welds. The study underscores the importance of current control as the primary lever for managing overlay weld quality.