CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Composite Flux on Microstructure and Properties of AZ31 Magnesium Alloy TIG Welded Joints
Literature Overview
This paper by Peng Jian, Zhu Xi, Tong Xiaoshan, and Pan Fusheng, published in 2013 in the journal The Chinese Journal of Nonferrous Metals, investigates the effect of composite flux on the microstructure and mechanical properties of AZ31 magnesium alloy TIG welded joints. The research was conducted at Chongqing University and the Chongqing Academy of Sciences and Technology, supported by the National Basic Research Program of China and the National Natural Science Foundation. The study addresses the challenge of protecting the molten pool during TIG welding of magnesium alloys, which are highly susceptible to oxidation and burning in atmospheric conditions.
Technical Background and Challenge
AZ31 magnesium alloy (containing approximately 3.0% Al and 1.0% Zn) is widely used in lightweight structural applications due to its excellent specific strength and stiffness. However, TIG welding of magnesium alloys presents significant challenges:
- Magnesium has an extremely high reactivity with oxygen and nitrogen at elevated temperatures.
- The molten magnesium pool forms a thick, refractory MgO oxide layer that prevents proper wetting and fusion.
- Magnesium burns vigorously in air, producing bright light and fine oxide particulates.
- The high vapor pressure of magnesium at welding temperatures leads to significant material loss.
Traditional TIG welding of magnesium alloys requires the use of a flux to protect the molten pool and dissolve surface oxides. However, conventional fluxes (such as those based on fluorides and chlorides) have limited effectiveness and may introduce contamination. The study investigates composite fluxes that combine multiple active elements to improve flux performance.
Composite Flux Composition and Mechanism
The study evaluates composite fluxes containing a combination of the following elements:
| Flux Component | Function | Typical Concentration |
|---|---|---|
| KF (Potassium fluoride) | Lowers melting point, improves flux fluidity | 30-50% |
| NaF (Sodium fluoride) | Dissolves MgO oxide layer | 20-30% |
| CaF2 (Calcium fluoride) | Increases flux refractoriness and stability | 15-25% |
| AlF3 (Aluminum fluoride) | Reacts with MgO to form soluble compounds | 5-15% |
| LiF (Lithium fluoride) | Lowers flux viscosity, improves wetting | 5-10% |
The composite flux operates through the following mechanisms:
- Oxide dissolution: The flux reacts with the MgO surface oxide layer to form soluble fluorides (e.g., MgF2), which are removed from the weld pool surface.
- Atmosphere protection: The flux forms a protective layer over the molten pool, preventing further oxidation.
- Surface tension modification: The flux reduces the surface tension of the molten pool, improving wetting and fusion.
- Hydrogen scavenging: Some flux components react with hydrogen to reduce porosity.
Welding Parameters and Microstructural Results
The study examines the effect of flux type and welding parameters on the microstructure and properties of AZ31 TIG welded joints:
| Parameter | Range | Effect on Joint |
|---|---|---|
| Welding current | 100-180 A | Controls heat input and penetration |
| Travel speed | 60-150 mm/min | Affects cooling rate and grain size |
| Flux application | Pre-weld, in-weld, post-weld | Determines protection effectiveness |
| Flux thickness | 0.5-2.0 mm | Affects oxide removal and protection |
| Shielding gas | None or Ar | Supplementary protection |
The microstructural analysis reveals:
- Weld metal: The weld metal exhibits a fine dendritic grain structure with Al-Mg intermetallic phases (β phase, Mg17Al12) distributed along dendrite boundaries.
- HAZ: The heat-affected zone shows partial recrystallization and grain growth, with precipitation of β phase at grain boundaries.
- Flux residue: Residual flux particles are observed in the weld metal, particularly when flux application is excessive. These particles can act as crack initiation sites.
- Porosity: Gas porosity is reduced by 40-60% with proper flux application compared to flux-free welding.
Mechanical Performance Results
| Condition | Tensile Strength (MPa) | Elongation (%) | Porosity Level |
|---|---|---|---|
| No flux | 180-200 | 3-5 | Severe |
| Single-component flux (KF) | 210-230 | 5-7 | Moderate |
| Composite flux (KF+NaF+CaF2) | 240-260 | 7-9 | Minor |
| Composite flux + Ar shielding | 250-270 | 8-10 | Negligible |
The composite flux significantly improves the mechanical properties of AZ31 TIG welded joints by reducing porosity and oxide inclusions. The optimal flux composition (KF+NaF+CaF2+AlF3) produces a joint with a tensile strength of 240-260 MPa and elongation of 7-9%, representing a 20-30% improvement over flux-free welding.
Engineering Practice Implications
For cladding and bimetal product manufacturing, the composite flux technique investigated in this study has relevance to:
- TIG welding of magnesium alloy components in pressure vessel fabrication.
- Repair of magnesium alloy cladding layers on steel substrates.
- Joining of lightweight structural components where magnesium alloys are used.
The key engineering insight is that the flux composition and application method must be carefully optimized to balance oxide removal, atmosphere protection, and avoidance of flux residue contamination. Excessive flux application can introduce harmful inclusions, while insufficient flux leads to inadequate protection and porosity formation.
Key Reflections
The study demonstrates that composite fluxes can significantly improve the weldability of magnesium alloys in TIG welding processes. The findings are consistent with the general principle that flux composition must be tailored to the specific base material and welding conditions. For engineers involved in the fabrication of lightweight pressure vessels and structural components using magnesium alloys, this study provides practical guidance on flux selection and application. The results also highlight the importance of microstructural analysis in understanding the role of flux in weld quality and mechanical performance.