Arc Pressure Analysis in Alternating Polarity TIG Welding - Literature Study Note

Literature Overview

The paper by Cheng Lin, Hu Shengsun, and Wang Zhijiang, published in the Welding Journal (Vol. 35, No. 11, 2014, pp. 101–104), presents an experimental study of the arc pressure distribution in alternating polarity TIG (VPTIG) welding, specifically for horizontal welding (transverse position). The authors used pressure sensors to measure the arc pressure under different conditions and analyzed the radial distribution pattern. The study compares DC TIG welding with VPTIG welding at different reverse polarity duty cycles (5% and 15% of the cycle period with the tungsten electrode positive). The key finding is that the arc pressure radial distribution follows a double-sided exponential distribution, and that the arc pressure decreases as the reverse polarity duty cycle increases. The classification code TG444 confirms the focus on arc welding process technology. This work is significant because the arc pressure is a critical parameter that affects weld pool dynamics, penetration profile, and weld geometry, particularly in horizontal and overhead positions where gravitational forces compete with the arc force.

Experimental Methodology

The experimental setup consisted of the following components:

The pressure sensor was positioned on the workpiece surface at various radial distances from the arc center to capture the spatial distribution of the arc pressure. The sensor signal was recorded in real-time and analyzed to extract the mean pressure profile and the temporal fluctuations. The experimental conditions were controlled to ensure repeatability, with the arc length maintained at a constant value and the gas flow rate set to a standard value.

Arc Pressure Distribution Analysis

The key finding of the paper is that the arc pressure radial distribution follows a double-sided exponential distribution, which can be expressed as:

P(r) = P₀ × exp(-α × r)

where P(r) is the pressure at radial distance r from the arc center, P₀ is the peak pressure at the arc center, and α is the decay constant that characterizes the rate of pressure decrease with distance.

This distribution pattern indicates that the arc pressure is highest at the arc center and decreases exponentially in both directions along the radial axis. The decay rate α is influenced by the welding current, the arc length, and the reverse polarity duty cycle.

Effect of Reverse Polarity Duty Cycle

The paper reports that as the reverse polarity duty cycle increases from 0% (DC TIG) to 5% and then to 15%, the arc pressure decreases. This is attributed to two mechanisms:

1. Change in electron emission mechanism: During the reverse polarity period (tungsten positive), the electron emission from the tungsten electrode is suppressed because the tungsten is not the cathode. The electron emission shifts to the workpiece (anode), which changes the current density distribution and the arc geometry.
2. Arc divergence: The reverse polarity period causes the arc to diverge (spread out) rather than concentrate, which reduces the peak current density and the associated arc pressure.

The paper also reports that the integrated arc force (the integral of pressure over the area) is approximately proportional to the square of the welding current. This relationship is consistent with the theoretical prediction that the arc force is proportional to the square of the current density, which in turn is proportional to the square of the current.

Physical Interpretation

The reduction in arc pressure with increasing reverse polarity duty cycle has important implications for weld pool dynamics. In horizontal welding, the arc pressure acts to push the weld pool downward, counteracting the gravitational force that tends to cause the weld pool to sag. A lower arc pressure means that the weld pool is more susceptible to gravitational sagging, which can result in an uneven weld bead and potential defects such as undercut on the upper edge and excess metal on the lower edge.

However, the reverse polarity period also provides a cathodic cleaning effect on the tungsten electrode, which removes oxide buildup and maintains a stable arc.