CLADDING TECHNOLOGY SHANXI CO., LTDMechanism of Flux Action During TIG Welding of ZA Alloys
Literature Overview
This 2006 paper in Shandong Metallurgy by Chen Libo and Liu Xiuzhong of Shandong University provides a preliminary investigation into the mechanism by which flux acts during TIG welding of ZA (zinc-aluminum) alloys. ZA alloys (such as ZA-8, ZA-12, ZA-15, ZA-22) are high-pressure die-casting alloys containing 8–27% aluminum in a zinc matrix. They offer excellent fluidity, dimensional accuracy, and wear resistance, making them suitable for automotive, plumbing, and industrial components. However, welding ZA alloys is notoriously difficult due to zinc's low boiling point (907°C), high vapor pressure, and tendency to cause zinc fume embrittlement in the weld metal.
Technical Background
The fundamental challenges in welding ZA alloys include:
- Zinc evaporation — Zinc boils at 907°C, well below the melting point of the alloy (390–420°C). During welding, zinc preferentially evaporates, creating porosity and changing the local composition.
- Fume toxicity — Zinc oxide fumes cause metal fume fever and require strict ventilation controls.
- Hot cracking — The wide freezing range and zinc evaporation promote solidification cracking.
- Oxidation — Zinc and aluminum readily form oxides that interfere with weld fusion.
- Low melting point — Excessive heat input causes distortion and potential melting of adjacent sections.
The role of flux in mitigating these problems has not been extensively studied for TIG welding, as flux-cored processes are more commonly associated with flux application. This paper represents an innovative approach to introducing flux into a TIG welding environment.
Flux Composition and Application Method
| Flux Component | Function | Typical Composition |
|---|---|---|
| NaF (sodium fluoride) | Fluxing agent, lowers surface tension | 20–30% |
| CaF₂ (calcium fluoride) | Stabilizer, reduces zinc evaporation | 30–40% |
| SiO₂ (silicon dioxide) | Refractory support, absorbs oxides | 20–25% |
| Al₂O₃ (aluminum oxide) | Refractory, protects molten pool | 10–15% |
| LiF (lithium fluoride) | Flux activator, improves wetting | 5–10% |
Application Methods Evaluated
- Pre-applied flux wire — A flux-filled wire placed in the root gap, melted during welding to release flux over the molten pool.
- Flux paste application — A paste of flux powder in a binder applied to the joint before welding.
- Flux ring — A ring of flux material surrounding the weld area, providing continuous flux supply.
- Flux-coated backing — Flux applied to the backing side of the joint to protect the root.
Mechanism of Flux Action
The authors identified several mechanisms through which flux improves weld quality in ZA alloy TIG welding:
1. Zinc Evaporation Suppression
Flux forms a protective layer over the molten pool, reducing the partial pressure of zinc vapor at the pool surface. This is analogous to the mechanism in flux-cored welding but adapted for TIG. The CaF₂ component is particularly effective as it has a high melting point (1418°C) and forms a stable barrier layer.
2. Oxide Removal and Prevention
The NaF and LiF components react with zinc and aluminum oxides to form soluble fluorides that float to the surface of the molten pool, preventing oxide inclusions in the solidified weld metal.
3. Wetting Improvement
Flux reduces the surface tension of the molten pool, improving wetting of the base metal and promoting better fusion. This is critical for ZA alloys where the wide freezing range can lead to poor fusion if surface tension is too high.
4. Crystal Structure Modification
Flux elements (particularly Li and Na) act as grain refiners in the solidifying weld metal, promoting finer grain structure and reducing hot cracking susceptibility.
Welding Parameters and Results
| Parameter | Without Flux | With Flux |
|---|---|---|
| Welding current | 80–120 A | 80–120 A |
| Arc voltage | 12–16 V | 12–16 V |
| Travel speed | 100–150 mm/min | 100–150 mm/min |
| Shielding gas | Ar (99.99%) | Ar (99.99%) |
| Preheat | 100°C | 100°C |
| Zinc content in weld (wt%) | 62–68% | 72–78% |
| Porosity density | High (30–50%) | Low (<5%) |
| Hot cracks | 2–5 per weld | 0–1 per weld |
| Tensile strength | 180–220 MPa | 240–280 MPa |
| Elongation | 3–5% | 8–12% |
Comparison with Other Welding Methods for ZA Alloys
| Method | Flux Use | Zinc Loss | Crack Resistance | Equipment Cost |
|---|---|---|---|---|
| TIG (no flux) | No | High | Poor | Low |
| TIG (with flux) | Yes | Moderate | Good | Low |
| MIG | No (standard) | High | Poor | Medium |
| Flux-cored arc | Yes (core) | Moderate | Good | Medium |
| Oxy-acetylene | No | Very high | Very poor | Low |
| Friction stir | No | Low | Excellent | High |
| Ultrasonic | No | Very low | Excellent | High |
Engineering Applications and Limitations
The flux-assisted TIG method is most applicable to:
- Repair welding of ZA alloy components where friction stir or ultrasonic welding is impractical
- Welding of ZA alloy castings to steel brackets or housings
- Joining of ZA alloy pipes and fittings in industrial systems
- Field repair where portable equipment is required
Limitations include:
- Flux residue requires post-weld cleaning, which can be labor-intensive
- Flux application is less precise than gas shielding alone, potentially causing spatter
- The technique requires procedure qualification for each specific alloy grade and thickness
- Ventilation requirements remain stringent due to zinc fume generation
Study Insights and Reflections
This work represents an important contribution to the limited body of knowledge on ZA alloy welding. The concept of introducing flux into a TIG welding environment is conceptually simple but practically challenging, as it requires careful control of flux quantity, placement, and melting sequence relative to the arc. The demonstrated improvement in zinc retention (from 62–68% to 72–78% in the weld metal) is significant, as zinc content directly correlates with the mechanical properties and corrosion resistance of ZA alloys. For engineers working with ZA alloy components in automotive and industrial applications, this technique provides an additional tool for situations where solid-state joining methods are not available. The work also raises important questions about long-term joint integrity — specifically, whether flux residues trapped in the weld microstructure could affect corrosion behavior or fatigue performance over extended service life. Further investigation into these aspects would be valuable for establishing full confidence in the technique for critical applications.