CLADDING TECHNOLOGY SHANXI CO., LTDAC TIG Welding of AZ31B Magnesium Alloy Thin Plate — Microstructure and Properties
Literature Overview
Published in 2013 in Ordnance Materials Science and Engineering, this study by Wu Xiao-jun, Wang Huai-jian, and Bai Li from Chongqing Vocational and Technical College of Industry investigates the weld microstructure and mechanical properties of AZ31B magnesium alloy thin sheets joined by alternating current (AC) TIG welding. The research was funded under the college-level project GZY201106-YK. While the institutional level of the research is modest, the technical content is significant given the extreme sensitivity of magnesium alloys to welding thermal input and the practical importance of thin-sheet Mg alloy fabrication for lightweight structural applications.
Core Technical Content
AZ31B is a wrought magnesium alloy containing approximately 3 wt% aluminum and 1 wt% zinc. In the as-received condition, it exhibits a typical extruded microstructure with elongated grains aligned along the extrusion direction, intergranular precipitates of Mg₁₇Al₁₂, and a yield strength in the range of 130–160 MPa with elongation of 10–15%. AC TIG welding is selected for magnesium alloys because the AC cycle provides alternating cathodic and anodic effects: the cathodic half-cycle provides cleaning of the oxide layer (cathodic cleaning), while the anodic half-cycle provides deeper penetration. The balance between these two effects is governed by the AC balance ratio.
Key Welding Parameters and Their Effects
| Parameter | Typical Value for Thin AZ31B | Technical Rationale |
|---|---|---|
| AC balance ratio | 60:40 to 70:30 (cathode:anode) | Optimizes cleaning vs. penetration |
| Current | 80–150 A | Low to moderate for thin gauge |
| Travel speed | 300–800 mm/min | High speed to limit HAZ width |
| Shielding gas | 100% Ar or 95% Ar/5% He | Ar for cleaning; He addition for penetration |
| Plate thickness | 1.0–2.0 mm | Thin plate regime |
| Interpass temperature | <150°C | Prevents grain coarsening and hot cracking |
Microstructural Evolution Analysis
The weld zone microstructure of AZ31B welded by AC TIG typically exhibits the following characteristics:
- Fusion zone: Equiaxed grains with fine dendritic structure. The Mg₁₇Al₁₂ phase dissolves during melting and reprecipitates during solidification, often appearing as intergranular or intragranular particles. Grain sizes are typically 50–200 μm, significantly finer than the base metal due to the high cooling rate in thin plates.
- Heat-affected zone (HAZ): This is the critical region for property assessment. The HAZ in AZ31B exhibits a two-stage transformation:
- Near the fusion boundary: Overheating leads to grain coarsening and coarsening of Mg₁₇Al₁₂ precipitates, resulting in softening.
- Further from the fusion line: Moderate heating causes partial dissolution and reprecipitation of Mg₁₇Al₁₂, potentially leading to localized strengthening.
- Base metal: Remains essentially unaffected except for residual stress introduction.
Mechanical Property Assessment
| Region | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Relative to Base |
|---|---|---|---|---|
| Base metal | 230–260 | 140–160 | 12–15 | 100% |
| Fusion zone | 210–240 | 130–150 | 14–18 | ~90–95% |
| HAZ (worst) | 180–210 | 110–130 | 10–14 | ~75–85% |
The HAZ softening is the primary concern. In magnesium alloys, unlike in aluminum alloys, the precipitate strengthening mechanism is more thermally sensitive because Mg₁₇Al₁₂ dissolves at relatively low temperatures (~450°C) and requires controlled aging to reprecipitate effectively. The welding thermal cycle, with peak temperatures approaching but not exceeding the melting point (~650°C), causes significant precipitate coarsening in the HAZ.
Engineering Practice Implications
For engineers involved in bimetal product manufacturing, the findings from this study have several transferable implications:
- Thermal management in dissimilar metal welding: When welding magnesium alloy components to steel or aluminum substrates (as in bimetal composite structures), the thermal sensitivity of the magnesium side dictates the overall process parameters. This directly parallels the challenge of welding titanium to steel or nickel alloys to carbon steel in cladding applications.
- Interpass temperature control: The strict interpass temperature limit (<150°C) for AZ31B mirrors the requirements for austenitic stainless steel overlay welding, where interpass temperatures above 150–200°C can promote sensitization.
- Shielding gas purity: Magnesium alloys are extremely susceptible to atmospheric contamination. In practice, this translates to the same rigor required for titanium welding—gas flow rates of 15–20 L/min, trailing shields, and strict gas purity monitoring.
Key Reflections
The study confirms that AC TIG welding of thin AZ31B plates is feasible with acceptable property retention when parameters are carefully optimized. However, the HAZ softening of 15–25% relative to base metal remains a fundamental limitation that cannot be eliminated by parameter optimization alone—it is inherent to the precipitate-strengthened microstructure of AZ31B. For engineering applications requiring full-strength welds, post-weld heat treatment (solution treatment at 415°C/4h followed by aging at 175°C/6h) is essential, as is standard practice for wrought magnesium alloys.
This work, while focused on a specific alloy and thickness range, reinforces a universal principle in welding engineering: the thermal cycle imposed on the base material is the primary determinant of weld joint quality, and process parameter selection must always be subordinate to the metallurgical sensitivity of the material being joined.