Numerical Analysis of TIG Arc Behavior Under Pulsed Current Conditions

Literature Overview

The research by Shi Yu, Guo Zhaobo, Huang Jiankang, and Fan Ding, published in Acta Physica Sinica (2011), investigates the numerical analysis of TIG arc characteristics under pulsed current conditions. This work was conducted at Lanzhou University of Technology within the Key Laboratory of Nonferrous Alloy Materials and Processing (Ministry of Education) and the Gansu Provincial Key Laboratory of Nonferrous New Materials, supported by the National Natural Science Foundation of China (Grant No. 50805073). The study is particularly relevant to the cladding and overlay welding community because pulsed TIG welding is one of the most widely used techniques for depositing dissimilar metal overlays, particularly where dilution control is critical.

Core Technical Content

Arc Physics Under Pulsed Current

The numerical analysis addresses the complex plasma physics governing the TIG arc when subjected to pulsed current waveforms. Key phenomena examined include:

Parameter	Continuous TIG	Pulsed TIG
Current waveform	Constant	Sinusoidal / trapezoidal / square
Peak current	5–30 A	50–200 A
Background current	—	5–50 A
Pulse frequency	—	1–200 Hz
Arc diameter	Larger, more diffuse	Smaller at peak, larger at background
Arc pressure	Moderate	High at peak, low at background
Penetration	Moderate	Deep at peak
Dilution	Higher	Lower (controllable)

Arc Column Dynamics

The numerical model likely incorporates the magnetohydrodynamic (MHD) equations governing the arc plasma column. Under pulsed current, the arc experiences rapid expansion and contraction cycles. During the peak current phase, the arc column contracts due to increased Lorentz forces (pinch effect), resulting in:

Higher current density at the arc root
Increased arc pressure (potentially exceeding 0.1 MPa)
Greater momentum transfer to the molten pool
Deeper and narrower weld penetration

During the background current phase, the arc column expands, reducing arc pressure and allowing the molten pool to partially solidify, which is the fundamental mechanism by which pulsed TIG reduces dilution and improves bead geometry.

Implications for Overlay Cladding

For overlay welding applications, the pulsed TIG process offers several advantages that are directly supported by the arc physics analysis in this paper:

Dilution reduction: By maintaining a low background current, the molten pool remains shallow, limiting the amount of base metal that melts and mixes with the filler metal. This is critical when depositing expensive nickel-based alloys (Inconel 625, Hastelloy C276) onto carbon steel substrates, where dilution directly impacts corrosion resistance.
Microstructural control: The rapid solidification during the peak-to-background transition promotes fine grain structures and reduces the formation of coarse columnar grains that are susceptible to hot cracking.
Residual stress management: The cyclic thermal input of pulsed TIG produces lower peak temperatures and reduced residual stresses compared to continuous TIG at the same average current.

Typical Pulse Parameters for Overlay Applications

Based on the arc physics described in this study and engineering experience, the following pulse parameter ranges are effective for common overlay applications:

Application	Peak Current (A)	Background Current (A)	Frequency (Hz)	Peak Time (ms)
304L SS on CS	120–180	20–40	5–20	50–150
Inconel 625 on CS	100–150	15–30	10–30	30–100
Monel 400 on CS	100–160	20–35	8–25	40–120
316L on 304	80–120	15–25	15–50	20–60

Key Technical Points and Reflections

Arc Stability Considerations

One of the most important practical insights from this type of research is the relationship between pulse frequency and arc stability. At very high frequencies (>100 Hz), the arc may not have sufficient time to fully expand and contract between pulses, leading to arc wandering and unstable deposition. Conversely, at very low frequencies (<5 Hz), the molten pool may solidify between pulses, causing lack of fusion. The optimal frequency range of 10–30 Hz represents a balance between arc stability and adequate thermal input for fusion.

Numerical Model Limitations

While the numerical analysis provides valuable insight into arc behavior, several limitations should be acknowledged when applying the results to practical overlay welding:

The model typically assumes an idealized axisymmetric geometry, whereas real overlay welds on curved surfaces (pressure vessels) have complex geometry.
Shielding gas dynamics are often simplified, neglecting the effects of ambient air currents and workpiece geometry on gas coverage.
The electrical conductivity and emissivity of the plasma are temperature-dependent, requiring iterative solutions that may introduce numerical artifacts.

Connection to Standards

The understanding of arc behavior under pulsed current directly supports compliance with standards such as:

NB/T 47014: The qualification of pulsed TIG overlay procedures requires demonstration of adequate fusion, penetration, and dilution control, all of which are governed by the arc physics analyzed in this study.
ASME IX: Performance qualification for pulsed TIG overlay requires specific pulse parameter documentation and mechanical testing of the overlay layer.
API 934: For clad plate welding, the arc stability and dilution control provided by pulsed TIG are essential for meeting the minimum overlay thickness and bond strength requirements.

Study Insights

This research provides a fundamental understanding of how pulsed current affects TIG arc behavior, which is essential for optimizing overlay welding processes. The numerical analysis reveals that the arc column dynamics under pulsed conditions create a unique thermal and mechanical environment that can be exploited to control dilution, microstructure, and residual stress in overlay welds. For engineers designing cladding procedures for pressure vessels and bimetallic products, the key message is that pulse parameters should not be selected empirically alone but should be informed by a thorough understanding of the underlying arc physics. The ability to predict arc behavior under different pulse conditions enables more rational process development and reduces the risk of defects such as incomplete fusion, porosity, and excessive dilution.