Arc Behavior in Aluminum Alloy Plasma-MIG Hybrid Welding

Literature Overview

Published in the Journal of Welding in 2022 by Han Jiao and colleagues from Inner Mongolia University of Technology, this paper investigates the arc behavior in aluminum alloy plasma-MIG hybrid welding, focusing on the interaction between the plasma arc and the MIG arc during the pulse current cycle. The study examines how the MIG arc voltage, arc position, and arc stability vary during the base current period and the pulse current rise period of the hybrid welding process. The research was supported by the National Natural Science Foundation of China and Inner Mongolia Provincial Science and Technology programs.

Experimental Configuration and Process Parameters

The study investigates two MIG welding current levels to determine how current magnitude affects the arc interaction phenomena:

The plasma arc current is held constant at 130 A across both configurations, while the MIG welding current is varied to examine the threshold at which the arc interaction phenomena cease.

Arc Behavior During the Base Current Period

At the lower MIG current level of 180 A (base current 95 A), the MIG arc exhibits several distinctive behaviors during the base current period:

• The MIG arc ignites near the plasma arc with low impedance, indicating that the plasma arc creates a favorable ionization environment for the MIG arc.
• The MIG arc voltage during the base current period is lower than that of standalone MIG welding, confirming the reduced arc impedance in the hybrid configuration.
• The charge flow effect causes the MIG arc to deflect toward the plasma arc, creating an asymmetric arc configuration.
• The primary ionization medium of the MIG arc in the direction of deflection toward the plasma arc is argon gas, which is the shielding gas used in the process.

These observations indicate that the plasma arc significantly modifies the electrical and thermal environment in which the MIG arc operates. The plasma arc, with its concentrated energy and high temperature, creates a pre-ionized channel that facilitates easier arc ignition and reduces the arc voltage required to maintain the arc.

Arc Behavior During the Pulse Current Rise Period

When the MIG arc is deflected toward the plasma arc during the base current period, thermal inertia causes the arc to maintain its deflected position during the pulse current rise phase and immediately after reaching the peak current. During this phase:

• The MIG arc voltage is higher than that of standalone MIG welding.
• The arc position remains deflected toward the plasma arc despite the increased current magnitude.

This behavior is attributed to the thermal inertia of the arc, which means that the arc cannot instantaneously reposition when the current changes. The arc position lags behind the current change, creating a transient period where the arc is deflected but the current is increasing, resulting in a higher arc voltage than would be expected for the given current level.

Arc Stability Analysis

The study identifies a clear relationship between MIG welding current and arc stability in the hybrid configuration:

At the lower current level, the arc deflection and voltage fluctuations reduce arc stability, which can lead to increased spatter, irregular bead profile, and potential weld defects. At the higher current level, the arc deflection phenomenon disappears entirely, and the arc stability improves to a level comparable to standalone MIG welding.

The disappearance of arc deflection at higher MIG current levels suggests that the MIG arc's own thermal and electromagnetic forces become dominant over the plasma arc's influence, preventing the charge flow effect from deflecting the MIG arc. This implies that there exists a threshold MIG current level above which the plasma-MIG hybrid welding process operates with minimal arc interaction effects.

Study Insights and Engineering Implications

This paper provides fundamental insight into the arc physics of plasma-MIG hybrid welding, a process that combines the deep penetration of plasma welding with the high deposition rate of MIG welding. The findings have direct implications for process parameter selection:

• For aluminum alloy welding, where arc stability is critical due to the oxide layer on aluminum and the sensitivity of the weld pool to arc disturbances, the MIG current level must be selected carefully to ensure stable arc behavior.
• The threshold current level at which arc deflection disappears (somewhere between 180 A and 240 A) provides a practical guideline for process parameter selection. Operating above this threshold ensures that the hybrid welding process behaves predictably with minimal arc interaction effects.
• The observation that the plasma arc creates a favorable ionization environment for the MIG arc suggests that the hybrid configuration may offer advantages in terms of arc ignition reliability and arc stability at lower current levels, even though the arc deflection phenomenon introduces some instability.

The study also highlights the importance of understanding arc behavior in hybrid welding processes. Unlike standalone welding processes where the arc behavior is well characterized, hybrid welding introduces complex interactions between multiple arc sources that can significantly affect weld quality. Process development for hybrid welding must account for these interactions, and the findings of this study provide a foundation for developing process windows that ensure stable and consistent hybrid welding performance.

The thermal inertia effect observed during the pulse current rise period is a particularly important finding for pulse welding applications. It demonstrates that the arc position does not respond instantaneously to current changes, which has implications for the timing and synchronization of pulse parameters in hybrid welding processes. Process engineers must account for this lag when designing pulse waveforms for hybrid welding applications.