CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of Flux on AC TIG Welding of Magnesium Alloys
Literature Overview
This research, published in the Journal of Welding in 2007 by Huang Yong, Fan Ding, Yang Peng, and Lin Tao from the State Key Laboratory of Non-ferrous Metal Materials at Lanzhou University of Technology, investigates the effect of flux on alternating current gas tungsten arc (AC TIG) welding of magnesium alloys. The work was supported by the Non-ferrous Metal Materials State Key Laboratory Open Fund (SKL04002) and the Ministry of Education Doctoral Program Special Fund (20040731001). Magnesium alloys are increasingly used in lightweight structural applications, and developing reliable welding methods is essential for their widespread adoption.
AC TIG Welding Principles for Magnesium Alloys
AC TIG welding is particularly suitable for magnesium alloys because the alternating polarity provides both cleaning and penetration actions. During the positive half-cycle (electrode positive), the cathodic cleaning action removes the oxide layer (MgO) from the weld pool surface, which is critical for achieving sound welds. During the negative half-cycle (electrode negative), the arc provides deep penetration. The balance between cleaning and penetration is controlled by the balance ratio (the ratio of electrode positive time to total cycle time).
| AC Parameter | Typical Value | Function |
|---|---|---|
| Balance Ratio | 30–50% electrode positive | Controls cleaning vs. penetration balance |
| Frequency | 50–100 Hz | Standard line frequency or higher for stability |
| Current | 100–250 A | Determines penetration and weld width |
| Travel Speed | 5–20 cm/min | Controls heat input and weld geometry |
| Shielding Gas | Ar or Ar/He mixture | Protects weld pool from atmospheric contamination |
The addition of flux in AC TIG welding of magnesium alloys is a technique borrowed from flux-cored welding but adapted for arc processes. The flux serves multiple functions: it provides additional cleaning action, modifies the weld pool fluidity, reduces surface tension, and can act as a release agent for trapped gases. However, flux can also introduce contamination if not carefully controlled.
Flux Composition and Effects
The researchers investigated various flux compositions, including chloride-based, fluoride-based, and mixed fluxes. The following table summarizes the effects of different flux types on weld quality:
| Flux Type | Composition | Effect on Weld |
|---|---|---|
| Chloride-based | MgCl2, KCl, NaCl | Good cleaning action; may increase porosity |
| Fluoride-based | CaF2, NaF | Moderate cleaning; lower porosity risk |
| Mixed (Cl+F) | MgCl2 + CaF2 | Balanced cleaning and gas release |
| No flux (baseline) | — | Reference for comparison |
The study found that chloride-based fluxes provide the most effective cleaning action but also increase the risk of porosity due to hydrogen absorption from moisture in the flux. Fluoride-based fluxes offer a more moderate cleaning effect with lower porosity risk but may require higher balance ratios to achieve adequate oxide removal. The mixed flux composition provides a compromise between cleaning effectiveness and porosity control.
The researchers also examined the effect of flux application method. Flux can be applied as a powder coating on the workpiece surface, as a flux-containing electrode, or as a flux wire fed into the arc. Each method has advantages and disadvantages:
- Surface coating: Simple application; risk of uneven distribution; may cause spatter.
- Flux-containing electrode: Consistent flux delivery; more complex equipment; higher cost.
- Flux wire: Flexible application; requires wire feeder; potential for flux burnout.
Weld Pool Dynamics and Microstructure
The presence of flux modifies the weld pool dynamics by changing the surface tension and fluidity of the molten magnesium. The flux decomposes at the arc temperature, releasing active species that interact with the oxide film and the liquid metal. This interaction affects the weld pool shape, penetration depth, and solidification pattern. The researchers observed that flux-assisted AC TIG welds exhibit a slightly wider weld bead with reduced penetration compared to flux-free welds, likely due to the flux modifying the arc behavior and heat distribution.
The microstructure of flux-assisted welds shows a finer grain structure in the weld metal, attributed to the nucleation effect of flux decomposition products. The interdendritic Mg17Al12 phase is more uniformly distributed, which can improve the mechanical properties. However, excessive flux can lead to the formation of flux inclusions in the weld metal, which act as stress concentrators and reduce fatigue strength.
Engineering Practice and Reflections
The use of flux in AC TIG welding of magnesium alloys is a promising technique for improving weld quality, particularly for thicker sections where deep penetration and adequate cleaning are required. However, the flux must be carefully selected and applied to avoid introducing new defects. The study provides a systematic evaluation of flux effects that can guide welding procedure development for magnesium alloy fabrication. The key insight is that flux is not merely an additive but a process variable that interacts with the arc, the weld pool, and the solidifying metal. Engineers must consider the flux composition, application method, and process parameters as an integrated system to achieve optimal weld quality. This work contributes to the growing body of knowledge on magnesium alloy welding and supports the development of lightweight structural components for automotive and aerospace applications.